Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 129 (2014) 491–498
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Synthesis, spectral, SHG efficiency and computational studies of some newly synthesized unsymmetrical azines of 4-biphenylcarboxaldehyde R. Arulmani, K.R. Sankaran ⇑ Department of Chemistry, Annamalai University, Annamalainagar 608 002, Tamil Nadu, India
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
A series of novel unsymmetrical
azines 2–8 were synthesized and analyzed by some spectral studies. Hyperpolarizability, dipole moment, HOMO–LUMO energies were computed. Computed IR frequencies are compared with the observed values.
a r t i c l e
i n f o
Article history: Received 26 October 2013 Received in revised form 28 February 2014 Accepted 18 March 2014 Available online 1 April 2014 Keywords: Unsymmetrical azines HOMO–LUMO NBO Hyperpolarizability NLO
a b s t r a c t A series of novel unsymmetrical azines 2–8 are prepared and characterized by FT-IR, 1H, 13C NMR, Mass and UV spectral studies. The Gaussian-03 B3LYP/6-311+G(d,p) calculations on these azines are used to evaluate the heat of formation of the different conformers, identify the stable conformation, to determine dipole moment (l), polarizability (a0), first hyperpolarizability (btot), selected geometrical parameters, MEP surface, frontier molecular orbital energies (HOMO–LUMO) and their energy gap. The l, a0, btot values clearly depict that the unsymmetrical azine 8 is found to have a good NLO property compared to other azines 1–7. The SHG measurement of unsymmetrical azine 8 was performed by Kurtz and Perry powder method and the results indicated that the azine 8 is having comparable efficiency as that of potassium dihydrogen phosphate (KDP) crystal. The natural bond orbital (NBO) analysis of the unsymmetrical azines 2–8 are also made using B3LYP/6-31G(d,p) basis set. Ó 2014 Elsevier B.V. All rights reserved.
Introduction Azines, condensation products of hydrazine and carbonyl compounds are designated as symmetrical or asymmetrical azines, based on the two carbonyl compounds from which the azine derived are the same or different. Azines are both chemically and biologically important molecules with potent physical properties [1–9]. Azines with donor and acceptor groups at the ends of ⇑ Corresponding author. Tel.: +91 4144 238601; fax: +91 4144 238145. E-mail address: [email protected] (K.R. Sankaran). http://dx.doi.org/10.1016/j.saa.2014.03.093 1386-1425/Ó 2014 Elsevier B.V. All rights reserved.
p-conjugated backbone behave as novel nonlinear optical (NLO) materials. This property of azine is attributed to the development of light based technologies for communication and computing [10]. Organic molecules with conjugated p-electron system are known to exhibit extremely large optical nonlinear responses in terms of their molecular hyperpolarizabilities, with application in many current areas of interest, such as second harmonic generator (SHG) and linear-electro optimization (LEO) [11–13]. Only a limited work has been reported in the synthesis and biological aspects of azines derived from several hetero aromatic and bicyclic carbonyl compounds. The present investigation is focussed on the
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synthesis and theoretical investigation of the molecular structures and their NBO analysis of newly synthesized azines derived from 4-biphenylcarboxaldehyde having extended conjugation. HOMO– LUMO energies, dipole moments, polarizabilities and first hyperpolarizabilities were determined by density functional theory (DFT) method. SHG efficiency of unsymmetrical azine 8 was determined by Kurtz and Perry powder technique. Experimental details Materials 4-Biphenylcarboxaldehyde was purchased from Sigma–Aldrich and is used as such. All the reagents and solvents were of laboratory grade. Methods All the melting points were performed in open capillary tube melting point apparatus and are uncorrected. The IR spectra were recorded in potassium bromide (KBr) disk on a Nicolet Avatar 360 FT-IR spectrometer and the wave numbers are given in cm1. The 1 H and 13C NMR and 2D NMR spectra were recorded at 400, 500 and 100, 125 MHz on a Bruker spectrometer in CDCl3, TMS was used as an internal standard and the chemical shift values (d) are given in parts per million (ppm). The Mass spectra were performed using Varian Saturn 2200 GC–MS spectrometer. The UV–visible spectra of the azines are recorded in Shimadzu UV-1800 UV–visible spectrophotometer using N,N-dimethylformamide as solvent at ambient room temperature. NLO technique In order to confirm the second order nonliner optical properties of the material, the second harmonic generation (SHG) test on the powder sample of a representative azine 8 was performed by Kurtz and Perry powder SHG method [14]. A Q-switched Nd:YAG laser wavelength 1064 nm was used with input radiation 2.2 mJ/pulse. A small portion of the azine 8 was powdered to a uniform particle size of about 125–150 nm and then packed in a capillary of uniform bore and exposed to laser radiations. The output from the sample was monochromated to collect only the second harmonic (532 nm) and the intensity was measured using a photomultiplier tube. Synthesis of unsymmetrical azines 2–8 Synthesis of (1E,2E)-1-benzylidene-2(biphenyl-4-ylmethylidene) hydrazine 2 About 0.01 mol of 4-biphenylcarboxaldehyde and 5 mL of hydrazine hydrate (0.01 mol) was taken in a stoppered conical flask. To this mixture few drops of acetic acid was added. The reaction mixture was stirred well for about half an hour. The hydrazine was separated as white solid and it was stirred with benzaldehyde (0.005 mol). The mixture was cooled. Yellow solid separated out and it was filtered, washed and recrystallized from ethanol. The other unsymmetrical azines 3–8 are synthesized in the similar manner. All the synthesized azines were characterized by FT-IR, 1 H, 13C NMR, Mass and UV spectral analysis. The reaction pathway has been summarized in Scheme 1. Yield 70%; m.p. 140 °C; yellow powder; molecular formula: C20H16N2. FT-IR (KBr, cm1): 3050 (CH stretching); 1618 (CH@N). 1 H NMR (400 MHz, CDCl3, ppm): d = 8.71 (s, CH, 11H), 8.70 (s, CH, 21H); 7.38–7.48 (aromatic protons). 13C NMR (100 MHz, CDCl3,
ppm): d = 162.14 (C11); 161.83 (C21); 127.19–143.99 (aromatic carbons). HRMS (m/z): 284.13. UV–Vis (DMF, nm): kmax = 325.5. (1E,2E)-1-(4-methylbenzylidene)-2(biphenyl-4-ylmethylidene) hydrazine 3 Yield 70%; m.p. 186 °C; yellow powder; molecular formula: C21H18N2. FT-IR (KBr, cm1): 3029 (CH stretching); 1620 (CH@N). 1 H NMR (500 MHz, CDCl3, ppm): d = 8.74 (s, CH, 11H); 8.70 (s, CH, 21H), 7.30–7.95 (aromatic protons). 13C NMR (125 MHz, CDCl3, ppm): d = 162.20 (C11); 161.43 (C21); 127.16–143.85 (aromatic carbons). HRMS (m/z): 298.15. UV–Vis (DMF, nm): kmax = 326.0. (1E,2E)-1-(4-methoxybenzylidene)-2(biphenyl-4-y1methylidene) hydrazine 4 Yield 70%; m.p. 144 °C; yellow powder; molecular formula: C21H18N2O1. FT-IR (KBr, cm1): 3060 (CH stretching); 1617 (CH@N). 1H NMR (500 MHz, CDCl3, ppm): d = 8.73 (s, CH, 11H); 8.68 (s, CH, 21H); 7.01–7.94 (aromatic protons). 13C NMR (125 MHz, CDCl3, ppm): d = 162.82 (C11); 161.07 (C21); 114.31– 162.19 (aromatic carbons). HRMS (m/z): 314.14. UV–Vis (DMF, nm): kmax = 333.5. (1E,2E)-1-(4-chlorobenzylidene)-2(biphenyl-4-y1methylidene) hydrazine 5 Yield 70%; m.p. 170 °C; yellow powder; molecular formula: C20H15ClN2. FT-IR (KBr, cm1): 3057 (CH stretching); 1620 (CH@N). 1 H NMR (500 MHz, CDCl3, ppm): d = 8.72 (s, CH, 11H); 8.67 (s, CH, 21H); 7.42–7.95 (aromatic protons). 13C NMR (125 MHz, CDCl3, ppm): d = 162.11 (C11); 160.70 (C21); 127.16–144.10 (aromatic carbons). HRMS (m/z): 318.09. UV–Vis (DMF, nm): kmax = 325.5. (1E,2E)-1-(4-bromobenzylidene)-2(biphenyl-4-y1methylidene) hydrazine 6 Yield 70%; m.p. 174 °C; pale yellow powder; molecular formula: C20H15BrN2. FT-IR (KBr, cm1): 3057 (CH stretching); 1621 (CH@N). 1 H NMR (500 MHz, CDCl3, ppm): d = 8.72 (s, CH, 11H); 8.66 (s, CH, 21H); 7.17–7.95 (aromatic protons). 13C NMR (125 MHz, CDCl3, ppm): d = 162.14 (C11); 160.79 (C21); 125.66–144.12 (aromatic carbons). HRMS (m/z): 362.04. UV–Vis (DMF, nm): kmax = 328.5. (1E,2E)-1-(4-fluorobenzylidene)-2(biphenyl-4-y1methylidene) hydrazine 7 Yield 70%; m.p. 164 °C; pale yellow powder; molecular formula: C20H15FN2. FT-IR (KBr, cm1): 3059 (CH stretching); 1624 (CH@N). 1 H NMR (400 MHz, CDCl3, ppm): d = 8.67 (s, CH, 11H); 8.73 (s, CH, 21H); 7.15–7.93 (aromatic protons). 13C NMR (100 MHz, CDCl3, ppm): d = 163.39 (C11); 160.92 (C21); 116.06–161.95 (aromatic carbons). HRMS (m/z): 302.12. UV–Vis (DMF, nm): kmax = 328.0. (1E,2E)-1-(4-nitrobenzylidene)-2(biphenyl-4-y1methylidene) hydrazine 8 Yield 70%; m.p. 198 °C; yellow solid; molecular formula: C20H15N3O2. FT-IR (KBr, cm1): 3060 (CH stretching); 1617 (CH@N). 1H NMR (500 MHz, CDCl3, ppm): 8.76 (s, CH, 11H); 8.75 (s, CH, 21H); 7.43–7.97 (aromatic protons). 13C NMR (125 MHz, CDCl3, ppm): d = 162.11 (C11); 160.70 (C21); 124.07–161.83 (aromatic carbons). HRMS (m/z): 329.12. UV–Vis (DMF, nm): kmax = 345.0. Results and discussion The FT-IR spectra and high resolution 1H and 13C NMR spectra of (1E,2E)-1-benzylidene-2(biphenyl-4-y1methylidene)hydrazine 2, (1E,2E)-1-(4-methylbenzylidene)-2(biphenyl-4-y1methylidene) hydrazine 3, (1E,2E)-1-(4-methoxybenzylidene)-2(biphenyl-4-
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Scheme 1. Synthetic route of unsymmetrical azines 2–8.
y1methylidene)hydrazine 4, (1E,2E)-1-(4-chlorobenzylidene-2(biphenyl-4-y1methylidene)-hydrazine 5, (1E,2E)-1-(4-bromobenzylidene-2 (biphenyl-4-y1methylidene)hydrazine 6, (1E,2E)-1-(4-fluorobenzylidene)-2(biphenyl-4-y1methylidene)hydrazine 7 and (1E,2E)-1-(4-nitrobenzylidene)-2(biphenyl-4-y1methylidene)hydrazine 8 have been recorded and analyzed. Spectral studies FT-IR spectral studies of 2–8 The sharp peaks around 1600 cm1 in the FT-IR spectra are exhibited to vC@N mode. Aromatic C@C stretching vibrations are seen around 1500 and 1450 cm1. The peaks around 1069– 1005 cm1 are due to the N–N stretching mode. Aromatic CAH out-of-plane bending vibrations appeared around 840 and 757 cm1. Two very strong adsorption bands at 1515 and 1337 cm1 in the FT-IR spectrum of unsymmetrical azine 8 are attributed to antisymmetric and symmetric NO2 stretching vibrations respectively. Asymmetric bending vibration for methyl group appeared at 1300 cm1 for azine 3. FT-IR spectral data of 2–8 are listed in Table 1. NMR spectral studies 1 H NMR spectral analysis of unsymmetrical azine 2. The signals in the 1H NMR spectra were assigned based on their positions integrals and multiplicities and confirmed by the correlations observed in the COSY spectra. 1H and 13C NMR spectra of 2 are shown in Figs. S1 and S2. Two high frequency singlets at 8.72 and 8.70 ppm were assigned to the protons H(11) and H(21). A doublet observed at
7.92 ppm is assigned to the protons in the biphenyl ring i.e., H(13) and H(17). A doublet of doublet at 7.86 ppm (integral corresponds to two protons) is assigned to the ortho protons of the benzylidene ring [H(23) and H(27)]. The two doublets at 7.69 and 7.64 ppm are assigned to the protons in the biphenyl ring H(14) and H(16) and H(152) and H(156), respectively. For the remaining aromatic protons signals are observed in the range 7.38–7.48 ppm. They were assigned to H(24), H(26); H(153), H(155) and H(154) and H(25) based on the correlations observed in the 1H-1H COSY spectrum. In a similar manner assignments were done for other hydrazines 3–8. 1H–1H COSY spectra of 2 are shown in Fig. S3 (a and b). 13 C NMR spectral studies of 2. The assignment of signals in the 13C NMR spectrum of 2 is made as follows. The two high frequency signals at 162.14and 161.83 ppm are due to azomethine carbons C(11) and C(21) respectively. The high intense signals at 128.62 and 128.84 ppm were due to ortho [C(23) and C(27)] and meta [C(24) and C(26)] carbons of the benzylidene ring. The remaining high intense signals at 129.08, 127.52, 127.19 and 128.91 ppm were assigned to biphenyl ring carbons (ortho with respect to azomethine carbon C(11) i.e., C(13) and C(17); meta [C(14) and C(16)]) and remaining carbons C(152), C(156), C(153) and C(155), respectively. These assignments were further confirmed from the correlations observed in the 1H-13C COSY spectra of 2 are shown in Fig. S4 (a and b). The 13C NMR spectrum further reveals four signals for quaternary carbons [C(12), C(22), C(15) and C(151)] at 133.05, 134.15, 143.99, 140.30, ppm which can easily be distinguished from other carbons based on small intensities. The signals at 133.05 and 134.15 ppm are assigned to the ipso carbons C(12)
Table 1 IR spectral data (cm1) of unsymmetrical azines 2–8. Assignments
2
3
4
5
6
7
8
vC@N
1618
1620
1617
vC@C
1483
1483
1410 1248
1620 1591 1484
1621 1585 1483 1402
1623 1599 1515 1485
vNAN
1005 957 834 757 – –
1175 1006 816 763 – 1300
1170 1028 834 764 – 1303
1089 1008 827 761 – –
1069 1008 827 761 – –
1624 1599 1505 1228 1406 1153
Aromatic CH out-of-plane bending vibration
vNO2 vCH3
833 761 – –
1185 1337 840 769 1515 1337 –
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Table 2 H chemical shifts (ppm) and coupling constants (Hz) of unsymmetrical azines 2–8.
1
Compds.
H(21)
H(23) and H(27)
H(24) and H(26)
H(11)
H(13) and H(17)
H(14) and H(16)
H(152) and H(156)
H(153) and H(155)
H(154)
H(25)
H(26)
2
8.70 (s) 8.70 (s) 8.68 (s) 8.67 (s) 8.66 (s) 8.73 (s) 8.75 (s)
7.86 (dd, 5.60, 7.20) 7.79 (d, 8.00)
7.47–7.48
7.92 (d, 8.50)
7.69 (d, 8.50)
7.64 (d, 7.50)
7.47–7.48
–
7.95 (d, 8.00)
7.72 (d, 8.50)
7.67 (d, 7.50)
7.50 (t)
–
2.47
7.01 (d, 8.50)
7.94 (d, 8.50)
7.71 (d, 8.00)
7.67 (d, 7.50)
7.50
7.38 (t) 7.41 (t) 7.41
7.45
7.84 (d, 9.00)
–
7.82 (d, 8.50)
7.46 (d, 8.50)
7.95 (d, 8.50)
7.72 (d, 8.00)
7.67 (d, 7.50)
7.50 (t)
–
7.62 (d, 8.50)
7.95 (d, 8.00)
7.72 (d, 8.00)
7.17 (d, 7.00)
7.50 (t)
–
–
7.87–7.82
7.15
7.93
7.71–7.68
7.66–7.63
7.48
–
–
8.05 (d, 8.50)
8.34 (d, 9.00)
7.97 (dd, 8.00)
7.73
7.68 (d, 7.50)
7.51 (d, 7.50)
7.42 (t) 7.42 (t) 7.39 (t) 7.43 (t)
3.90 (s) –
7.76 (d, 8.00)
8.72 (s) 8.74 (s) 8.73 (s) 8.72 (s) 8.72 (s) 8.67 (s) 8.76 (s)
–
–
3 4 5 6 7 8
7.30 (d, 7.50)
Values within parentheses are observed coupling constants in Hz.
and C(22) and the remaining signals at 140.30 and 143.99 ppm are assigned to C(151) and C(15), respectively. In a similar manner assignments were made for other azines 3–8. The 1H and 13C chemical shifts obtained in this manner were listed in Tables 2 and 3, respectively.
first hyperpolarizability (b) were calculated by finite field approach using B3LYP/6-31G basis set. Highest occupied molecular orbital (HOMO), lowest unoccupied molecular orbital (LUMO) energies and other thermodynamic properties of the stable conformer were determined by DFT method.
Electronic spectral studies The ultraviolet absorption spectra are recorded for all the newly synthesized unsymmetrical azines in dimethylformamide in the region 260–400 nm. The maximum absorption lays around 325.5, 326.0, 333.5, 325.5, 328.5, 328.0 and 345.0 nm for the azines 2– 8, respectively. All the absorption bands are due to the n-p transition. The higher kmax is observed for 8 when compared to other unsymmetrical azines 2–7 due to the presence of nitro group in 8. UV–Vis spectrum of 2 is shown in Fig. S5.
Experimental and theoretical IR spectral analysis of 2 Many quantum chemical computations are used in vibrational analysis of many compounds [16,17]. The IR frequencies of 2 were determined theoretically by DFT using B3LYP/6-31G(d,p) method and the data were displayed in Table 6 along with the observed values. The vibrational frequencies obtained from the computational studies are scaled with proper factor for B3LYP/6-31G(d,p) method i.e., 0.9679 [18]. There is a close agreement between the calculated IR frequencies with the experimental ones.
Computational studies There are four possible conformations for the azine 2 and these conformations are illustrated in Fig. 1. In order to decide the favoured conformation computational calculations were performed according to DFT method using B3LYP/6-311+G(d,p) basis set available in Gaussian-03 package [15]. The conformer in which heat of formation is minimum is predicted to be the stable conformer and the values of the conformers other than stable conformer are reported as relative values with respect to the favoured conformer. The relative heat of formations thus determined in this manner for the unsymmetrical azines 2–8 were displayed in Table 4. Among the four conformers of azines 2–8, the conformer A is found to be the stable conformer. The structure of stable conformer along with numbering of atoms adopted in this study is given in Fig. 2. The bond lengths, bond angles and torsional angles of the stable conformer of all the azines 2–8 are given in Table 5. Further, vibrational frequencies are also computed using B3LYP/ 6-31G(d,p). The total static dipole moments, polarizability (a) and
NLO analysis First polarizability can be described by a (3 3 3) matrix, the 27 components of the 3D matrix can be reduced to 10 components due to their Kleinman symmetry [19]. The total static dipole moment (l), the mean polarizability (a0), the anisotropy of the polarizability (Da) and the mean first hyperpolarizability (btot) are related directly to the nonlinear optical efficiency of structures. The calculated values of l, a0, btot by finite field approach are given in Table 7 along the corresponding components using the x, y, z components are defined as 1=2
l ¼ ðl2x þ l2y þ l2z Þ
ð1Þ
Da ¼ 21=2 ½ðaxx ayy Þ2 þ ðayy azz Þ2 þ ðazz axx Þ2 þ 6a2 xx
1=2
ð2Þ The a is a second rank tensor property called dipole polarizability, the mean polarizability hai is evaluated using
Table 3 C Chemical shifts (ppm) of unsymmetrical azines 2–8.
13
Compds.
C(11)
C(21)
C(12)
C(15)
C(151)
C(22)
C(25)
C(17) and C(13)
C(14) and C(16)
C(152) and C(156)
C(153) and C(155)
C(154)
C(23) and C(27)
C(24) and C(26)
C(26)
2 3 4 5 6 7 8
162.14 162.20 161.82 162.11 162.14 163.39 160.44
161.83 161.43 161.07 160.70 160.79 160.92 159.32
133.05 133.15 133.24 132.92 133.09 133.00 132.57
143.99 143.85 143.75 144.10 144.12 144.04 144.58
140.30 140.45 140.34 140.24 140.23 140.30 140.05
134.15 131.44 126.88 132.67 132.91 161.83 149.19
131.22 141.75 162.19 129.08 125.66 161.95 161.83
129.08 129.00 130.32 129.72 129.94 128.94 129.39
127.52 127.47 127.46 127.51 127.52 127.53 127.61
127.19 127.16 127.15 127.16 127.16 127.19 127.20
128.91 128.91 128.92 129.11 129.12 129.08 128.99
127.94 127.88 127.85 127.95 127.95 127.94 128.10
128.62 128.61 127.46 128.93 128.92 130.56 129.15
128.84 129.58 114.31 129.14 132.10 116.06 124.07
– 21.65 55.42 – – – –
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Table 4 Relative heat of formation values (kJ mol1) of various conformers for unsymmetrical azines 2–8. Compds.
2 3 4 5 6 7 8
Conformers A
B
C
D
0 0 0 0 0 0 0
20.85 21.98 21.72 21.68 21.81 22.31 22.27
21.43 21.89 21.68 22.19 22.19 21.72 21.72
33.28 34.20 34.37 34.12 34.24 34.12 34.49
btot ¼ ðb2x þ b2y þ b2z Þ
1=2
ð5Þ
The complete equation for calculating the magnitude of first hyperpolarizability from GAUSSIAN-03W output is given as follows:
btot ¼ ½ðbxxx þ bxyy þ bxzz Þ2 þ ðbyyy þ byzz þ byxx Þ2 þ ðbzzz þ bzxx þ bzyy Þ2
1=2
ð6Þ
Fig. 1. Possible conformations of azines 2–8.
hai ¼
1 ðaxx þ ayy þ azz Þ 3
ð3Þ
The components of the first hyperpolarizability can be calculated using the following equation
bi ¼ biii þ
1X ðb þ bjij þ bjji Þ 3 i–j ijj
ð4Þ
Using the x, y and z components the magnitude of the first hyperpolarizabilty tensor can be calculated by
To calculate all the electric dipole moments and the first hyperpolarizabilities, the origin of the Cartesian coordinate system (x, y, z) = (0, 0, 0) was chosen at own centre of mass of each compound. The l values of 2–8 are 0.34, 0.40, 1.94, 2.64, 2.51, 1.90, 6.67 Debye, respectively. The highest value of dipole moment (lx) is observed for 8 compared to other azines. The unsymmetrical azines 2–8 are found to be polar molecules having non-zero dipole moment values. These result in large microscopic first hyperpolarizabilties and hence have a good microscopic NLO behaviour. The calculated polarizabilities aij have non-zero values and dominated by the diagonal components (axx, ayy, azz). The b value of components 2–8 indicate that the azine 8 which has electronwithdrawing group (ANO2) is found to have greater molecular asymmetry than all other azines. Delocalization of charges in particular direction is indicated by large values of those particular components of polarizability and hyperpolarizability. Among the azines, the azine 8 was found to have a good NLO behaviour that is indicated by its high first hyperpolarizability value of 188 107 esu. The NLO behavioural analysis of azine 8 also has been studied experimentally by SHG test. A second harmonic signal of 24 mV was obtained. The standard potassium dihydrogen phosphate (KDP) crystal gave an SHG signal of 24.4 mV for the same input energy. From the obtained results it was evident that the relative SHG efficiency of sample was almost equal to that of well known KDP crystal. From that it is clear that the azine 8 is NLO active in nature. HOMO–LUMO energies The NLO response can be qualitatively understood by examining the energetics of frontier molecular orbital (HOMO–LUMO) of azines 2–8 (Table 8).
Fig. 2. Optimized geometry of the most favourable conformation of 2 along with numbering.
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Table 5 Selected geometric parameters [bond lengths (Å), bond angles (°) and torsional angles (°)] in unsymmetrical azines 2–8 by DFT/B3LYP/6-31G(d,p) method. Geometric parameters
Compounds 2
3
4
5
6
7
8
Bond length C17AC12 C13AC12 C12AC11 C11AN1 N1AN2 C21AN2 C21AC22 C22AC23 C22AC27 C23AC24 C24AC25 C25AC26 C26AC27 C25AX28
1.40 1.41 1.46 1.28 1.39 1.28 1.46 1.40 1.41 1.39 1.39 1.40 1.39 –
1.40 1.40 1.46 1.28 1.39 1.28 1.46 1.40 1.41 1.39 1.40 1.40 1.38 1.51
1.40 1.40 1.46 1.28 1.39 1.29 1.46 1.40 1.41 1.39 1.40 1.41 1.38 1.36
1.40 1.40 1.46 1.28 1.39 1.28 1.46 1.40 1.40 1.39 1.39 1.39 1.39 1.34
1.40 1.40 1.46 1.28 1.39 1.28 1.46 1.40 1.40 1.39 1.39 1.40 1.40 1.92
1.40 1.40 1.46 1.28 1.39 1.28 1.46 1.40 1.41 1.39 1.39 1.39 1.39 1.35
1.40 1.41 1.46 1.29 1.39 1.28 1.46 1.40 1.41 1.39 1.39 1.39 1.38 1.48
Bond angle C17AC12AC11 C13AC12AC11 C11AN1AN2 N2AC21AC22 C22AC23AC24 C24AC25AC26 C25AC26AC27 C22AC27AC26 C26AC25AX28
119.3 122.3 112.7 122.6 120.2 119.9 119.9 120.7 –
119.5 122.1 112.6 122.5 120.4 118.1 120.9 120.8 120.1
119.5 122.2 112.7 122.6 120.8 119.7 119.4 121.6 124.6
122.1 119.5 112.5 122.2 121.1 121.3 119.3 120.8 119.4
119.5 122.1 112.4 122.2 120.8 121.3 118.9 121.1 119.4
119.5 122.1 112.5 122.4 120.7 122.2 118.4 121.1 118.9
119.4 122.1 112.3 121.8 120.5 122.0 118.5 120.5 118.9
Torsional angle C17AC12AC11AN1 C11AN1AN2AC21 C13AC12AC11AN1 N2AC21AC22AC27 C22AC23AC24AC25 C23AC24AC25AC26 C24AC25AC26AC27 C23AC24AC25AX28 C27AC26AC25AX28 C25
0.0 179.6 0.4 179.8 0.0 0.0 0.0 – – –
179.7 179.7 0.2 180.0 0.1 0.1 0.1 179.5 179.5 1.5
179.6 179.4 0.4 179.7 0.0 0.0 0.0 180.0 180.0 1.4
179.7 179.5 0.1 179.8 0.0 0.0 0.0 180.0 180.0 1.2
179.7 179.7 0.2 179.8 0.0 0.0 0.0 179.9 180.0 1.9
179.7 179.6 0.2 179.9 0.0 0.0 0.0 180.0 180.0 1.3
179.8 179.7 0.2 179.9 0.0 0.0 0.0 180.0 180.0 1.5
X = CH3, OCH3, Cl, Br, F, NO2.
From the HOMO–LUMO analysis it is clear that the intramolecular charge transfer (ICT) must occur within the molecule. The HOMO–LUMO energy gap of azine 2 is relatively higher than that of azines 3–8. This shows the lower btot value of azine 2 than those of azines 3–8. It is evident that there should be an inverse relationship between HOMO–LUMO energy gap and the first order hyperpolarizability [20]. This fact is supported by UV–visible spectral study which visualizes a large kmax for azine 8 which has lower energy gap. The HOMO pictures of all the azines except the azine 8
Table 6 Experimental and computed [B3LYP/6-31G(d,p) method] vibrational frequencies of unsymmetrical azine 2. Observed vibrational frequency (cm1)
Calculated vibrational frequency (cm1) (correction factor: 0.9679)
IR intensity
3050.08 2933.01 1667.15 1618.56 1483.25 1444.25 1301.38 1214.04 1005.20 957.99 834.57 757.36 690.38
2999.76 3105.55 1621.45 1660.64 1550.98 1485.09 1295.14 1208.76 992.18 847.08 752.75 686.89 679.55
37.90 24.01 27.26 275.23 14.03 25.64 28.46 22.83 10.03 12.22 13.92 24.31 25.80
reveal that all atoms get involved in the formation of the HOMO orbitals. In nitro substituted benzylidene hydrazine 8, the pz orbitals of C(24) and C(26) are not participated, however the pz orbitals of nitro group are well participated in the formation of HOMO orbitals. In benzylidene hydrazines 2, 6 and 7, all the pz orbitals mainly from the biphenyl ring atoms and side chain atoms are involved in the formation of LUMO orbitals except pz orbitals of carbon atoms C(155) and C(153). In azines 3 and 4, the pz orbitals of carbon atoms C(155), C(153), C(24) and C(26) are not involved in the formation of LUMO orbitals. In nitro substituted benzylidene hydrazine 8, the pz orbitals of carbon atoms C(155), C(154), C(153), C(152), C(156), C(151), C(16) and C(14) are not involved in the formation of LUMO orbital. The HOMO–LUMO pictures of the azine 2 is given in Fig. S6.
NBO analysis NBO analysis have been performed on the molecules at the DFT/ B3LYP/6-31G(d,p) level in order to elucidate the intermolecular, rehybridization and delocalization of electron density within the molecule. The second order Fock matrix was carried out to evaluate the donor–acceptor interactions in the NBO analysis and the values are displayed in Table 9. The interactions result is a loss of occupancy from the localized NBO of the idealized Lewis structure into an empty non-Lewis orbital. For each donor (i) and acceptor (j), the stabilization energy E(2) associated with the delocalization i ? j is estimated as
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Table 7 The electric dipole moment l (D), the mean polarizability hai (1024 esu) and the first hyperpolarizability btot (1033 esu) of unsymmetrical azines 2–8 by B3LYP/6-31G method.
axx axy ayy axz ayz azz hai bxxx bxxy bxyy byyy bxxz bxyz byyz bxzz byzz bzzz btotal107
2
3
4
5
6
7
8
83.89 1.661 30.383 0.1062 0.037 12.771 42.346 21151.69 1058.521 9.875 121.121 33.666 348.413 79.831 169.336 27.469 3.044 22
89.215 1.324 31.475 0.06815 0.0969 14.137 44.94 5260.812 564.819 198.337 47.358 100.527 311.354 100.527 93.239 140.600 85.854 1.5
90.708 1.898 31.599 1.6318 0.1777 14.775 45.69 9854.304 928.171 175.596 164.721 82.382 264.445 82.382 160.066 7.438 388.759 5.2
92.157 1.0403 30.893 0.2167 0.0724 13.462 45.504 23575.36 442.0606 107.042 48.839 93.564 328.324 59.613 139.33 15.918 11.951 27.2
95.461 0.822 31.506 0.2560 0.1032 14.235 47.07 20726.51 384.175 225.932 79.262 105.577 291.957 18.448 81.7465 36.402 13.050 20
84.852 1.5954 30.330 0.1180 0.0763 12.913 42.70 15279.32 740.200 276.019 53.111 18.016 383.168 64.369 103.334 23.31 1.1161 11
91.959 2.1680 31.117 0.0051 0.5410 16.086 46.39 61939.95 1087.504 351.931 137.822 9.464 263.825 76.310 188.27 3.09043 13.998 188
a = 1 a.u = 0.1482 1024 esu. b = 1 a.u = 8.6393 1033 esu.
Table 8 HOMO–LUMO energies (eV) and dipole moments l(D) of unsymmetrical azines 2–8. Compds.
2 3 4 5 6 7 8
HOMO–LUMO energies
Dipole moment
HOMO
LUMO
Energy gap (DE)
ltot
0.224 0.218 5.714 0.224 0.224 0.222 0.234
0.084 0.078 2.0136 0.086 0.086 0.082 0.111
3.81 3.81 3.70 3.75 3.75 3.81 3.35
0.35 0.41 1.94 2.64 2.52 1.91 6.68
2
Eð2Þ ¼ DEij ¼ qi
Fði; jÞ ei ej
electron donors and electron acceptors, i.e., the more donating tendency from electron donors to electron acceptors and the greater the extent of charge transfer or conjugation of the whole system. From Table 9, it is seen that the primary delocalization occurs from the biphenyl ring to benzylidene ring only rather than vice versa. Delocalization energy corresponding to the transfer of electrons from bonding orbital of N2AC21 bond to the antibonding orbital of N1AC11 (57 kJ mol1) is higher when compared to reverse transfer (50 kJ mol1). Therefore electrons flow occurs from biphenyl ring to the benzene ring side. The energy (E(2)) due to the transfer of electron from bonding orbital C15AC16 to antibonding orbital C12AC17 is found to be very high (E(2) = 93 kJ mol1). The highest interaction is found in the azine 8 which has highest E(2) value of 95.29 kJ mol 1.
ð7Þ
where qi is the donor orbital occupancy, ei and ej are diagonal elements and F(i,j) is the off diagonal NBO Fock matrix element. The larger the E(2) value, the more intensive is the interaction between
Thermodynamic properties Thermodynamic parameters have been calculated by using DFT method at B3LYP/6-31G(d,p) level [21] and the parameters are given in Table 10.
Table 9 Delocalization energies (E(2), kJ mol1) for unsymmetrical azines 2–8 by NBO analysis. Donor
Acceptor
2
3
4
5
6
7
8
N1AC11 N2AC21 C12AC17 C12AC17 C12AC17 C22AC27 C22AC27 C22AC27 C13AC14 C13AC14 C15AC16 C15AC16 C151AC156 C151AC156 C152AC153 C152AC153 C154AC155 C154AC155 C23AC24 C23AC24 C25AC26 C25AC26
N2AC21 N1AC11 N1AC11 C13AC14 C15AC16 N2AC21 C23AC24 C25AC26 C12AC17 C15AC16 C12AC17 C13AC14 C152AC153 C154AC155 C151AC156 C154AC155 C151AC156 C152AC153 C22AC27 C25AC26 C22AC27 C23AC24
54.88 54.63 80.76 79.42 81.51 79.88 79.42 81.81 77.49 86.28 93.57 72.38 81.47 87.79 87.79 83.90 84.65 85.36 80.51 87.71 87.67 76.70
54.17 55.14 80.30 83.52 81.93 81.85 81.76 75.90 77.58 86.12 93.15 72.51 81.47 87.96 84.44 83.86 84.48 85.36 76.95 94.16 94.16 72.38
54.76 54.59 80.80 79.38 81.55 80.38 82.27 81.34 86.20 86.20 93.53 72.43 81.47 20.97 20.16 83.86 20.22 20.39 76.95 93.70 89.43 73.31
55.89 53.67 81.72 79.46 80.88 78.04 80.30 92.90 77.41 86.45 93.86 72.18 81.47 87.50 84.44 83.94 84.90 76.91 78.37 86.70 78.46 74.98
55.89 50.24 81.85 79.38 80.80 78.00 79.84 93.61 77.37 86.08 94.37 72.38 81.47 87.79 84.36 83.98 84.61 85.24 78.75 85.41 77.95 75.48
55.09 54.21 81.14 79.38 81.30 79.96 85.45 83.35 77.49 86.28 93.82 72.34 81.47 87.67 84.44 83.90 84.78 87.08 73.31 93.78 88.04 71.30
57.48 52.50 82.98 79.46 80.00 74.23 79.00 95.96 77.28 86.75 95.29 71.81 81.43 87.08 84.44 84.02 85.28 85.20 81.09 88.34 76.49 80.05
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Table 10 Theoretically calculated thermodynamic properties of unsymmetrical azines 2–8. Parameters
2
3
4
5
6
7
8
Total energy Zero-point vibrational energy (kcal mol1) Rotational constants (GHz)
880.96 192.007 1.666 0.067 0.065 141.791 42.831 35.040 63.920
920.251 209.407 1.644 0.057 0.056 141.342 42.974 35.356 63.011
995.444 212.200 1.535 0.049 0.048 138.956 43.130 35.706 60.120
1340.564 185.903 1.654 0.048 0.047 145.706 43.167 35.692 66.846
3452.073 185.641 1.646 0.037 0.036 148.815 43.553 36.215 69.047
980.199 186.799 1.661 0.056 0.054 142.288 43.014 35.399 62.875
1085.382 191.465 1.459 0.044 0.043 149.564 43.269 35.981 70.314
Total entropy (cal mol1 K1) Translational entropy (cal mol1 K1) Rotational entropy (cal mol1 K1) Vibrational entropy (cal mol1 K1)
The total entropy of 8 is found to be higher than those of 2–7. It is evident that among all azines that the azine with electron-withdrawing substituent in the phenyl ring with lower energy gap between HOMO and LUMO orbitals (azine 8) is found to have higher entropy.
Acknowledgment The authors thank the NMR Research Centre, Indian Institute of Technology, Chennai for recording the NMR spectra. Appendix A. Supplementary material
MEP surfaces Three dimensional distribution of molecular electrostatic potential (MEP) is highly useful in predicting the reactive behaviour of the molecule. The MEP surface is on overlaying of the eletrostatic potential on to the isoelectron density surface. This is a valuable tool for describing over all molecule charge distribution as well as anticipating sites of electrophilic addition. In all azines region of negative charges (red colour) is seen around the electronegative nitrogens N(1) and N(2). Region of negative charge (red colour) is also seen around the electro negative oxygen O(38) in azine 4 and O(39) and O(40) in azine 8. The red region is susceptible for electrophillic attack. Blue colour represents strongly positive region and the predominant green region in the MEP surfaces corresponds to a potential halfway between the two extremes red and blue region in azines 2–8. The MEP surface picture of 2 is given in Fig. S7.
Conclusion In this work a series of novel unsymmetrical azines 2–8 are prepared and characterized by FT-IR, 1H, 13C NMR, Mass and UV spectral studies. The Gaussian-03 B3LYP/6-311+G(d,p) calculations on these azines are used to evaluate the heat of formation of the different conformers, to identify the stable conformation, to determine the dipole moment, polarizability, first hyperpolarizability, bond lengths, bond angles and torsional angles, MEP surface, HOMO and LUMO energies. All the azines are found to be polar molecules having non-zero dipole moment components. These result in large microscopic first hyperpolarizabilities and hence have a good microscopic NLO behaviour. Among the azines 2–8, azine 8 is found to have a good NLO behaviour that is indicated by its high first hyperpolarizability value. The NLO behavioural analysis of azine 8 has been studied experimentally by SHG test. From the UV studies it is found that the azine 8 is high kmax compared to other azines. The thermodynamic properties for all the azines are also calculated. The total entropy for 8 is found to be higher than those of 2–7.
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Synthesis, spectral, SHG efficiency and computational studies of some newly synthesized unsymmetrical azines of 4-biphenylcarboxaldehyde R. Arulmani, K.R. Sankaran ⇑ Department of Chemistry, Annamalai University, Annamalainagar 608 002, Tamil Nadu, India
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
A series of novel unsymmetrical
azines 2–8 were synthesized and analyzed by some spectral studies. Hyperpolarizability, dipole moment, HOMO–LUMO energies were computed. Computed IR frequencies are compared with the observed values.
a r t i c l e
i n f o
Article history: Received 26 October 2013 Received in revised form 28 February 2014 Accepted 18 March 2014 Available online 1 April 2014 Keywords: Unsymmetrical azines HOMO–LUMO NBO Hyperpolarizability NLO
a b s t r a c t A series of novel unsymmetrical azines 2–8 are prepared and characterized by FT-IR, 1H, 13C NMR, Mass and UV spectral studies. The Gaussian-03 B3LYP/6-311+G(d,p) calculations on these azines are used to evaluate the heat of formation of the different conformers, identify the stable conformation, to determine dipole moment (l), polarizability (a0), first hyperpolarizability (btot), selected geometrical parameters, MEP surface, frontier molecular orbital energies (HOMO–LUMO) and their energy gap. The l, a0, btot values clearly depict that the unsymmetrical azine 8 is found to have a good NLO property compared to other azines 1–7. The SHG measurement of unsymmetrical azine 8 was performed by Kurtz and Perry powder method and the results indicated that the azine 8 is having comparable efficiency as that of potassium dihydrogen phosphate (KDP) crystal. The natural bond orbital (NBO) analysis of the unsymmetrical azines 2–8 are also made using B3LYP/6-31G(d,p) basis set. Ó 2014 Elsevier B.V. All rights reserved.
Introduction Azines, condensation products of hydrazine and carbonyl compounds are designated as symmetrical or asymmetrical azines, based on the two carbonyl compounds from which the azine derived are the same or different. Azines are both chemically and biologically important molecules with potent physical properties [1–9]. Azines with donor and acceptor groups at the ends of ⇑ Corresponding author. Tel.: +91 4144 238601; fax: +91 4144 238145. E-mail address: [email protected] (K.R. Sankaran). http://dx.doi.org/10.1016/j.saa.2014.03.093 1386-1425/Ó 2014 Elsevier B.V. All rights reserved.
p-conjugated backbone behave as novel nonlinear optical (NLO) materials. This property of azine is attributed to the development of light based technologies for communication and computing [10]. Organic molecules with conjugated p-electron system are known to exhibit extremely large optical nonlinear responses in terms of their molecular hyperpolarizabilities, with application in many current areas of interest, such as second harmonic generator (SHG) and linear-electro optimization (LEO) [11–13]. Only a limited work has been reported in the synthesis and biological aspects of azines derived from several hetero aromatic and bicyclic carbonyl compounds. The present investigation is focussed on the
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synthesis and theoretical investigation of the molecular structures and their NBO analysis of newly synthesized azines derived from 4-biphenylcarboxaldehyde having extended conjugation. HOMO– LUMO energies, dipole moments, polarizabilities and first hyperpolarizabilities were determined by density functional theory (DFT) method. SHG efficiency of unsymmetrical azine 8 was determined by Kurtz and Perry powder technique. Experimental details Materials 4-Biphenylcarboxaldehyde was purchased from Sigma–Aldrich and is used as such. All the reagents and solvents were of laboratory grade. Methods All the melting points were performed in open capillary tube melting point apparatus and are uncorrected. The IR spectra were recorded in potassium bromide (KBr) disk on a Nicolet Avatar 360 FT-IR spectrometer and the wave numbers are given in cm1. The 1 H and 13C NMR and 2D NMR spectra were recorded at 400, 500 and 100, 125 MHz on a Bruker spectrometer in CDCl3, TMS was used as an internal standard and the chemical shift values (d) are given in parts per million (ppm). The Mass spectra were performed using Varian Saturn 2200 GC–MS spectrometer. The UV–visible spectra of the azines are recorded in Shimadzu UV-1800 UV–visible spectrophotometer using N,N-dimethylformamide as solvent at ambient room temperature. NLO technique In order to confirm the second order nonliner optical properties of the material, the second harmonic generation (SHG) test on the powder sample of a representative azine 8 was performed by Kurtz and Perry powder SHG method [14]. A Q-switched Nd:YAG laser wavelength 1064 nm was used with input radiation 2.2 mJ/pulse. A small portion of the azine 8 was powdered to a uniform particle size of about 125–150 nm and then packed in a capillary of uniform bore and exposed to laser radiations. The output from the sample was monochromated to collect only the second harmonic (532 nm) and the intensity was measured using a photomultiplier tube. Synthesis of unsymmetrical azines 2–8 Synthesis of (1E,2E)-1-benzylidene-2(biphenyl-4-ylmethylidene) hydrazine 2 About 0.01 mol of 4-biphenylcarboxaldehyde and 5 mL of hydrazine hydrate (0.01 mol) was taken in a stoppered conical flask. To this mixture few drops of acetic acid was added. The reaction mixture was stirred well for about half an hour. The hydrazine was separated as white solid and it was stirred with benzaldehyde (0.005 mol). The mixture was cooled. Yellow solid separated out and it was filtered, washed and recrystallized from ethanol. The other unsymmetrical azines 3–8 are synthesized in the similar manner. All the synthesized azines were characterized by FT-IR, 1 H, 13C NMR, Mass and UV spectral analysis. The reaction pathway has been summarized in Scheme 1. Yield 70%; m.p. 140 °C; yellow powder; molecular formula: C20H16N2. FT-IR (KBr, cm1): 3050 (CH stretching); 1618 (CH@N). 1 H NMR (400 MHz, CDCl3, ppm): d = 8.71 (s, CH, 11H), 8.70 (s, CH, 21H); 7.38–7.48 (aromatic protons). 13C NMR (100 MHz, CDCl3,
ppm): d = 162.14 (C11); 161.83 (C21); 127.19–143.99 (aromatic carbons). HRMS (m/z): 284.13. UV–Vis (DMF, nm): kmax = 325.5. (1E,2E)-1-(4-methylbenzylidene)-2(biphenyl-4-ylmethylidene) hydrazine 3 Yield 70%; m.p. 186 °C; yellow powder; molecular formula: C21H18N2. FT-IR (KBr, cm1): 3029 (CH stretching); 1620 (CH@N). 1 H NMR (500 MHz, CDCl3, ppm): d = 8.74 (s, CH, 11H); 8.70 (s, CH, 21H), 7.30–7.95 (aromatic protons). 13C NMR (125 MHz, CDCl3, ppm): d = 162.20 (C11); 161.43 (C21); 127.16–143.85 (aromatic carbons). HRMS (m/z): 298.15. UV–Vis (DMF, nm): kmax = 326.0. (1E,2E)-1-(4-methoxybenzylidene)-2(biphenyl-4-y1methylidene) hydrazine 4 Yield 70%; m.p. 144 °C; yellow powder; molecular formula: C21H18N2O1. FT-IR (KBr, cm1): 3060 (CH stretching); 1617 (CH@N). 1H NMR (500 MHz, CDCl3, ppm): d = 8.73 (s, CH, 11H); 8.68 (s, CH, 21H); 7.01–7.94 (aromatic protons). 13C NMR (125 MHz, CDCl3, ppm): d = 162.82 (C11); 161.07 (C21); 114.31– 162.19 (aromatic carbons). HRMS (m/z): 314.14. UV–Vis (DMF, nm): kmax = 333.5. (1E,2E)-1-(4-chlorobenzylidene)-2(biphenyl-4-y1methylidene) hydrazine 5 Yield 70%; m.p. 170 °C; yellow powder; molecular formula: C20H15ClN2. FT-IR (KBr, cm1): 3057 (CH stretching); 1620 (CH@N). 1 H NMR (500 MHz, CDCl3, ppm): d = 8.72 (s, CH, 11H); 8.67 (s, CH, 21H); 7.42–7.95 (aromatic protons). 13C NMR (125 MHz, CDCl3, ppm): d = 162.11 (C11); 160.70 (C21); 127.16–144.10 (aromatic carbons). HRMS (m/z): 318.09. UV–Vis (DMF, nm): kmax = 325.5. (1E,2E)-1-(4-bromobenzylidene)-2(biphenyl-4-y1methylidene) hydrazine 6 Yield 70%; m.p. 174 °C; pale yellow powder; molecular formula: C20H15BrN2. FT-IR (KBr, cm1): 3057 (CH stretching); 1621 (CH@N). 1 H NMR (500 MHz, CDCl3, ppm): d = 8.72 (s, CH, 11H); 8.66 (s, CH, 21H); 7.17–7.95 (aromatic protons). 13C NMR (125 MHz, CDCl3, ppm): d = 162.14 (C11); 160.79 (C21); 125.66–144.12 (aromatic carbons). HRMS (m/z): 362.04. UV–Vis (DMF, nm): kmax = 328.5. (1E,2E)-1-(4-fluorobenzylidene)-2(biphenyl-4-y1methylidene) hydrazine 7 Yield 70%; m.p. 164 °C; pale yellow powder; molecular formula: C20H15FN2. FT-IR (KBr, cm1): 3059 (CH stretching); 1624 (CH@N). 1 H NMR (400 MHz, CDCl3, ppm): d = 8.67 (s, CH, 11H); 8.73 (s, CH, 21H); 7.15–7.93 (aromatic protons). 13C NMR (100 MHz, CDCl3, ppm): d = 163.39 (C11); 160.92 (C21); 116.06–161.95 (aromatic carbons). HRMS (m/z): 302.12. UV–Vis (DMF, nm): kmax = 328.0. (1E,2E)-1-(4-nitrobenzylidene)-2(biphenyl-4-y1methylidene) hydrazine 8 Yield 70%; m.p. 198 °C; yellow solid; molecular formula: C20H15N3O2. FT-IR (KBr, cm1): 3060 (CH stretching); 1617 (CH@N). 1H NMR (500 MHz, CDCl3, ppm): 8.76 (s, CH, 11H); 8.75 (s, CH, 21H); 7.43–7.97 (aromatic protons). 13C NMR (125 MHz, CDCl3, ppm): d = 162.11 (C11); 160.70 (C21); 124.07–161.83 (aromatic carbons). HRMS (m/z): 329.12. UV–Vis (DMF, nm): kmax = 345.0. Results and discussion The FT-IR spectra and high resolution 1H and 13C NMR spectra of (1E,2E)-1-benzylidene-2(biphenyl-4-y1methylidene)hydrazine 2, (1E,2E)-1-(4-methylbenzylidene)-2(biphenyl-4-y1methylidene) hydrazine 3, (1E,2E)-1-(4-methoxybenzylidene)-2(biphenyl-4-
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Scheme 1. Synthetic route of unsymmetrical azines 2–8.
y1methylidene)hydrazine 4, (1E,2E)-1-(4-chlorobenzylidene-2(biphenyl-4-y1methylidene)-hydrazine 5, (1E,2E)-1-(4-bromobenzylidene-2 (biphenyl-4-y1methylidene)hydrazine 6, (1E,2E)-1-(4-fluorobenzylidene)-2(biphenyl-4-y1methylidene)hydrazine 7 and (1E,2E)-1-(4-nitrobenzylidene)-2(biphenyl-4-y1methylidene)hydrazine 8 have been recorded and analyzed. Spectral studies FT-IR spectral studies of 2–8 The sharp peaks around 1600 cm1 in the FT-IR spectra are exhibited to vC@N mode. Aromatic C@C stretching vibrations are seen around 1500 and 1450 cm1. The peaks around 1069– 1005 cm1 are due to the N–N stretching mode. Aromatic CAH out-of-plane bending vibrations appeared around 840 and 757 cm1. Two very strong adsorption bands at 1515 and 1337 cm1 in the FT-IR spectrum of unsymmetrical azine 8 are attributed to antisymmetric and symmetric NO2 stretching vibrations respectively. Asymmetric bending vibration for methyl group appeared at 1300 cm1 for azine 3. FT-IR spectral data of 2–8 are listed in Table 1. NMR spectral studies 1 H NMR spectral analysis of unsymmetrical azine 2. The signals in the 1H NMR spectra were assigned based on their positions integrals and multiplicities and confirmed by the correlations observed in the COSY spectra. 1H and 13C NMR spectra of 2 are shown in Figs. S1 and S2. Two high frequency singlets at 8.72 and 8.70 ppm were assigned to the protons H(11) and H(21). A doublet observed at
7.92 ppm is assigned to the protons in the biphenyl ring i.e., H(13) and H(17). A doublet of doublet at 7.86 ppm (integral corresponds to two protons) is assigned to the ortho protons of the benzylidene ring [H(23) and H(27)]. The two doublets at 7.69 and 7.64 ppm are assigned to the protons in the biphenyl ring H(14) and H(16) and H(152) and H(156), respectively. For the remaining aromatic protons signals are observed in the range 7.38–7.48 ppm. They were assigned to H(24), H(26); H(153), H(155) and H(154) and H(25) based on the correlations observed in the 1H-1H COSY spectrum. In a similar manner assignments were done for other hydrazines 3–8. 1H–1H COSY spectra of 2 are shown in Fig. S3 (a and b). 13 C NMR spectral studies of 2. The assignment of signals in the 13C NMR spectrum of 2 is made as follows. The two high frequency signals at 162.14and 161.83 ppm are due to azomethine carbons C(11) and C(21) respectively. The high intense signals at 128.62 and 128.84 ppm were due to ortho [C(23) and C(27)] and meta [C(24) and C(26)] carbons of the benzylidene ring. The remaining high intense signals at 129.08, 127.52, 127.19 and 128.91 ppm were assigned to biphenyl ring carbons (ortho with respect to azomethine carbon C(11) i.e., C(13) and C(17); meta [C(14) and C(16)]) and remaining carbons C(152), C(156), C(153) and C(155), respectively. These assignments were further confirmed from the correlations observed in the 1H-13C COSY spectra of 2 are shown in Fig. S4 (a and b). The 13C NMR spectrum further reveals four signals for quaternary carbons [C(12), C(22), C(15) and C(151)] at 133.05, 134.15, 143.99, 140.30, ppm which can easily be distinguished from other carbons based on small intensities. The signals at 133.05 and 134.15 ppm are assigned to the ipso carbons C(12)
Table 1 IR spectral data (cm1) of unsymmetrical azines 2–8. Assignments
2
3
4
5
6
7
8
vC@N
1618
1620
1617
vC@C
1483
1483
1410 1248
1620 1591 1484
1621 1585 1483 1402
1623 1599 1515 1485
vNAN
1005 957 834 757 – –
1175 1006 816 763 – 1300
1170 1028 834 764 – 1303
1089 1008 827 761 – –
1069 1008 827 761 – –
1624 1599 1505 1228 1406 1153
Aromatic CH out-of-plane bending vibration
vNO2 vCH3
833 761 – –
1185 1337 840 769 1515 1337 –
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Table 2 H chemical shifts (ppm) and coupling constants (Hz) of unsymmetrical azines 2–8.
1
Compds.
H(21)
H(23) and H(27)
H(24) and H(26)
H(11)
H(13) and H(17)
H(14) and H(16)
H(152) and H(156)
H(153) and H(155)
H(154)
H(25)
H(26)
2
8.70 (s) 8.70 (s) 8.68 (s) 8.67 (s) 8.66 (s) 8.73 (s) 8.75 (s)
7.86 (dd, 5.60, 7.20) 7.79 (d, 8.00)
7.47–7.48
7.92 (d, 8.50)
7.69 (d, 8.50)
7.64 (d, 7.50)
7.47–7.48
–
7.95 (d, 8.00)
7.72 (d, 8.50)
7.67 (d, 7.50)
7.50 (t)
–
2.47
7.01 (d, 8.50)
7.94 (d, 8.50)
7.71 (d, 8.00)
7.67 (d, 7.50)
7.50
7.38 (t) 7.41 (t) 7.41
7.45
7.84 (d, 9.00)
–
7.82 (d, 8.50)
7.46 (d, 8.50)
7.95 (d, 8.50)
7.72 (d, 8.00)
7.67 (d, 7.50)
7.50 (t)
–
7.62 (d, 8.50)
7.95 (d, 8.00)
7.72 (d, 8.00)
7.17 (d, 7.00)
7.50 (t)
–
–
7.87–7.82
7.15
7.93
7.71–7.68
7.66–7.63
7.48
–
–
8.05 (d, 8.50)
8.34 (d, 9.00)
7.97 (dd, 8.00)
7.73
7.68 (d, 7.50)
7.51 (d, 7.50)
7.42 (t) 7.42 (t) 7.39 (t) 7.43 (t)
3.90 (s) –
7.76 (d, 8.00)
8.72 (s) 8.74 (s) 8.73 (s) 8.72 (s) 8.72 (s) 8.67 (s) 8.76 (s)
–
–
3 4 5 6 7 8
7.30 (d, 7.50)
Values within parentheses are observed coupling constants in Hz.
and C(22) and the remaining signals at 140.30 and 143.99 ppm are assigned to C(151) and C(15), respectively. In a similar manner assignments were made for other azines 3–8. The 1H and 13C chemical shifts obtained in this manner were listed in Tables 2 and 3, respectively.
first hyperpolarizability (b) were calculated by finite field approach using B3LYP/6-31G basis set. Highest occupied molecular orbital (HOMO), lowest unoccupied molecular orbital (LUMO) energies and other thermodynamic properties of the stable conformer were determined by DFT method.
Electronic spectral studies The ultraviolet absorption spectra are recorded for all the newly synthesized unsymmetrical azines in dimethylformamide in the region 260–400 nm. The maximum absorption lays around 325.5, 326.0, 333.5, 325.5, 328.5, 328.0 and 345.0 nm for the azines 2– 8, respectively. All the absorption bands are due to the n-p transition. The higher kmax is observed for 8 when compared to other unsymmetrical azines 2–7 due to the presence of nitro group in 8. UV–Vis spectrum of 2 is shown in Fig. S5.
Experimental and theoretical IR spectral analysis of 2 Many quantum chemical computations are used in vibrational analysis of many compounds [16,17]. The IR frequencies of 2 were determined theoretically by DFT using B3LYP/6-31G(d,p) method and the data were displayed in Table 6 along with the observed values. The vibrational frequencies obtained from the computational studies are scaled with proper factor for B3LYP/6-31G(d,p) method i.e., 0.9679 [18]. There is a close agreement between the calculated IR frequencies with the experimental ones.
Computational studies There are four possible conformations for the azine 2 and these conformations are illustrated in Fig. 1. In order to decide the favoured conformation computational calculations were performed according to DFT method using B3LYP/6-311+G(d,p) basis set available in Gaussian-03 package [15]. The conformer in which heat of formation is minimum is predicted to be the stable conformer and the values of the conformers other than stable conformer are reported as relative values with respect to the favoured conformer. The relative heat of formations thus determined in this manner for the unsymmetrical azines 2–8 were displayed in Table 4. Among the four conformers of azines 2–8, the conformer A is found to be the stable conformer. The structure of stable conformer along with numbering of atoms adopted in this study is given in Fig. 2. The bond lengths, bond angles and torsional angles of the stable conformer of all the azines 2–8 are given in Table 5. Further, vibrational frequencies are also computed using B3LYP/ 6-31G(d,p). The total static dipole moments, polarizability (a) and
NLO analysis First polarizability can be described by a (3 3 3) matrix, the 27 components of the 3D matrix can be reduced to 10 components due to their Kleinman symmetry [19]. The total static dipole moment (l), the mean polarizability (a0), the anisotropy of the polarizability (Da) and the mean first hyperpolarizability (btot) are related directly to the nonlinear optical efficiency of structures. The calculated values of l, a0, btot by finite field approach are given in Table 7 along the corresponding components using the x, y, z components are defined as 1=2
l ¼ ðl2x þ l2y þ l2z Þ
ð1Þ
Da ¼ 21=2 ½ðaxx ayy Þ2 þ ðayy azz Þ2 þ ðazz axx Þ2 þ 6a2 xx
1=2
ð2Þ The a is a second rank tensor property called dipole polarizability, the mean polarizability hai is evaluated using
Table 3 C Chemical shifts (ppm) of unsymmetrical azines 2–8.
13
Compds.
C(11)
C(21)
C(12)
C(15)
C(151)
C(22)
C(25)
C(17) and C(13)
C(14) and C(16)
C(152) and C(156)
C(153) and C(155)
C(154)
C(23) and C(27)
C(24) and C(26)
C(26)
2 3 4 5 6 7 8
162.14 162.20 161.82 162.11 162.14 163.39 160.44
161.83 161.43 161.07 160.70 160.79 160.92 159.32
133.05 133.15 133.24 132.92 133.09 133.00 132.57
143.99 143.85 143.75 144.10 144.12 144.04 144.58
140.30 140.45 140.34 140.24 140.23 140.30 140.05
134.15 131.44 126.88 132.67 132.91 161.83 149.19
131.22 141.75 162.19 129.08 125.66 161.95 161.83
129.08 129.00 130.32 129.72 129.94 128.94 129.39
127.52 127.47 127.46 127.51 127.52 127.53 127.61
127.19 127.16 127.15 127.16 127.16 127.19 127.20
128.91 128.91 128.92 129.11 129.12 129.08 128.99
127.94 127.88 127.85 127.95 127.95 127.94 128.10
128.62 128.61 127.46 128.93 128.92 130.56 129.15
128.84 129.58 114.31 129.14 132.10 116.06 124.07
– 21.65 55.42 – – – –
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Table 4 Relative heat of formation values (kJ mol1) of various conformers for unsymmetrical azines 2–8. Compds.
2 3 4 5 6 7 8
Conformers A
B
C
D
0 0 0 0 0 0 0
20.85 21.98 21.72 21.68 21.81 22.31 22.27
21.43 21.89 21.68 22.19 22.19 21.72 21.72
33.28 34.20 34.37 34.12 34.24 34.12 34.49
btot ¼ ðb2x þ b2y þ b2z Þ
1=2
ð5Þ
The complete equation for calculating the magnitude of first hyperpolarizability from GAUSSIAN-03W output is given as follows:
btot ¼ ½ðbxxx þ bxyy þ bxzz Þ2 þ ðbyyy þ byzz þ byxx Þ2 þ ðbzzz þ bzxx þ bzyy Þ2
1=2
ð6Þ
Fig. 1. Possible conformations of azines 2–8.
hai ¼
1 ðaxx þ ayy þ azz Þ 3
ð3Þ
The components of the first hyperpolarizability can be calculated using the following equation
bi ¼ biii þ
1X ðb þ bjij þ bjji Þ 3 i–j ijj
ð4Þ
Using the x, y and z components the magnitude of the first hyperpolarizabilty tensor can be calculated by
To calculate all the electric dipole moments and the first hyperpolarizabilities, the origin of the Cartesian coordinate system (x, y, z) = (0, 0, 0) was chosen at own centre of mass of each compound. The l values of 2–8 are 0.34, 0.40, 1.94, 2.64, 2.51, 1.90, 6.67 Debye, respectively. The highest value of dipole moment (lx) is observed for 8 compared to other azines. The unsymmetrical azines 2–8 are found to be polar molecules having non-zero dipole moment values. These result in large microscopic first hyperpolarizabilties and hence have a good microscopic NLO behaviour. The calculated polarizabilities aij have non-zero values and dominated by the diagonal components (axx, ayy, azz). The b value of components 2–8 indicate that the azine 8 which has electronwithdrawing group (ANO2) is found to have greater molecular asymmetry than all other azines. Delocalization of charges in particular direction is indicated by large values of those particular components of polarizability and hyperpolarizability. Among the azines, the azine 8 was found to have a good NLO behaviour that is indicated by its high first hyperpolarizability value of 188 107 esu. The NLO behavioural analysis of azine 8 also has been studied experimentally by SHG test. A second harmonic signal of 24 mV was obtained. The standard potassium dihydrogen phosphate (KDP) crystal gave an SHG signal of 24.4 mV for the same input energy. From the obtained results it was evident that the relative SHG efficiency of sample was almost equal to that of well known KDP crystal. From that it is clear that the azine 8 is NLO active in nature. HOMO–LUMO energies The NLO response can be qualitatively understood by examining the energetics of frontier molecular orbital (HOMO–LUMO) of azines 2–8 (Table 8).
Fig. 2. Optimized geometry of the most favourable conformation of 2 along with numbering.
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Table 5 Selected geometric parameters [bond lengths (Å), bond angles (°) and torsional angles (°)] in unsymmetrical azines 2–8 by DFT/B3LYP/6-31G(d,p) method. Geometric parameters
Compounds 2
3
4
5
6
7
8
Bond length C17AC12 C13AC12 C12AC11 C11AN1 N1AN2 C21AN2 C21AC22 C22AC23 C22AC27 C23AC24 C24AC25 C25AC26 C26AC27 C25AX28
1.40 1.41 1.46 1.28 1.39 1.28 1.46 1.40 1.41 1.39 1.39 1.40 1.39 –
1.40 1.40 1.46 1.28 1.39 1.28 1.46 1.40 1.41 1.39 1.40 1.40 1.38 1.51
1.40 1.40 1.46 1.28 1.39 1.29 1.46 1.40 1.41 1.39 1.40 1.41 1.38 1.36
1.40 1.40 1.46 1.28 1.39 1.28 1.46 1.40 1.40 1.39 1.39 1.39 1.39 1.34
1.40 1.40 1.46 1.28 1.39 1.28 1.46 1.40 1.40 1.39 1.39 1.40 1.40 1.92
1.40 1.40 1.46 1.28 1.39 1.28 1.46 1.40 1.41 1.39 1.39 1.39 1.39 1.35
1.40 1.41 1.46 1.29 1.39 1.28 1.46 1.40 1.41 1.39 1.39 1.39 1.38 1.48
Bond angle C17AC12AC11 C13AC12AC11 C11AN1AN2 N2AC21AC22 C22AC23AC24 C24AC25AC26 C25AC26AC27 C22AC27AC26 C26AC25AX28
119.3 122.3 112.7 122.6 120.2 119.9 119.9 120.7 –
119.5 122.1 112.6 122.5 120.4 118.1 120.9 120.8 120.1
119.5 122.2 112.7 122.6 120.8 119.7 119.4 121.6 124.6
122.1 119.5 112.5 122.2 121.1 121.3 119.3 120.8 119.4
119.5 122.1 112.4 122.2 120.8 121.3 118.9 121.1 119.4
119.5 122.1 112.5 122.4 120.7 122.2 118.4 121.1 118.9
119.4 122.1 112.3 121.8 120.5 122.0 118.5 120.5 118.9
Torsional angle C17AC12AC11AN1 C11AN1AN2AC21 C13AC12AC11AN1 N2AC21AC22AC27 C22AC23AC24AC25 C23AC24AC25AC26 C24AC25AC26AC27 C23AC24AC25AX28 C27AC26AC25AX28 C25
0.0 179.6 0.4 179.8 0.0 0.0 0.0 – – –
179.7 179.7 0.2 180.0 0.1 0.1 0.1 179.5 179.5 1.5
179.6 179.4 0.4 179.7 0.0 0.0 0.0 180.0 180.0 1.4
179.7 179.5 0.1 179.8 0.0 0.0 0.0 180.0 180.0 1.2
179.7 179.7 0.2 179.8 0.0 0.0 0.0 179.9 180.0 1.9
179.7 179.6 0.2 179.9 0.0 0.0 0.0 180.0 180.0 1.3
179.8 179.7 0.2 179.9 0.0 0.0 0.0 180.0 180.0 1.5
X = CH3, OCH3, Cl, Br, F, NO2.
From the HOMO–LUMO analysis it is clear that the intramolecular charge transfer (ICT) must occur within the molecule. The HOMO–LUMO energy gap of azine 2 is relatively higher than that of azines 3–8. This shows the lower btot value of azine 2 than those of azines 3–8. It is evident that there should be an inverse relationship between HOMO–LUMO energy gap and the first order hyperpolarizability [20]. This fact is supported by UV–visible spectral study which visualizes a large kmax for azine 8 which has lower energy gap. The HOMO pictures of all the azines except the azine 8
Table 6 Experimental and computed [B3LYP/6-31G(d,p) method] vibrational frequencies of unsymmetrical azine 2. Observed vibrational frequency (cm1)
Calculated vibrational frequency (cm1) (correction factor: 0.9679)
IR intensity
3050.08 2933.01 1667.15 1618.56 1483.25 1444.25 1301.38 1214.04 1005.20 957.99 834.57 757.36 690.38
2999.76 3105.55 1621.45 1660.64 1550.98 1485.09 1295.14 1208.76 992.18 847.08 752.75 686.89 679.55
37.90 24.01 27.26 275.23 14.03 25.64 28.46 22.83 10.03 12.22 13.92 24.31 25.80
reveal that all atoms get involved in the formation of the HOMO orbitals. In nitro substituted benzylidene hydrazine 8, the pz orbitals of C(24) and C(26) are not participated, however the pz orbitals of nitro group are well participated in the formation of HOMO orbitals. In benzylidene hydrazines 2, 6 and 7, all the pz orbitals mainly from the biphenyl ring atoms and side chain atoms are involved in the formation of LUMO orbitals except pz orbitals of carbon atoms C(155) and C(153). In azines 3 and 4, the pz orbitals of carbon atoms C(155), C(153), C(24) and C(26) are not involved in the formation of LUMO orbitals. In nitro substituted benzylidene hydrazine 8, the pz orbitals of carbon atoms C(155), C(154), C(153), C(152), C(156), C(151), C(16) and C(14) are not involved in the formation of LUMO orbital. The HOMO–LUMO pictures of the azine 2 is given in Fig. S6.
NBO analysis NBO analysis have been performed on the molecules at the DFT/ B3LYP/6-31G(d,p) level in order to elucidate the intermolecular, rehybridization and delocalization of electron density within the molecule. The second order Fock matrix was carried out to evaluate the donor–acceptor interactions in the NBO analysis and the values are displayed in Table 9. The interactions result is a loss of occupancy from the localized NBO of the idealized Lewis structure into an empty non-Lewis orbital. For each donor (i) and acceptor (j), the stabilization energy E(2) associated with the delocalization i ? j is estimated as
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Table 7 The electric dipole moment l (D), the mean polarizability hai (1024 esu) and the first hyperpolarizability btot (1033 esu) of unsymmetrical azines 2–8 by B3LYP/6-31G method.
axx axy ayy axz ayz azz hai bxxx bxxy bxyy byyy bxxz bxyz byyz bxzz byzz bzzz btotal107
2
3
4
5
6
7
8
83.89 1.661 30.383 0.1062 0.037 12.771 42.346 21151.69 1058.521 9.875 121.121 33.666 348.413 79.831 169.336 27.469 3.044 22
89.215 1.324 31.475 0.06815 0.0969 14.137 44.94 5260.812 564.819 198.337 47.358 100.527 311.354 100.527 93.239 140.600 85.854 1.5
90.708 1.898 31.599 1.6318 0.1777 14.775 45.69 9854.304 928.171 175.596 164.721 82.382 264.445 82.382 160.066 7.438 388.759 5.2
92.157 1.0403 30.893 0.2167 0.0724 13.462 45.504 23575.36 442.0606 107.042 48.839 93.564 328.324 59.613 139.33 15.918 11.951 27.2
95.461 0.822 31.506 0.2560 0.1032 14.235 47.07 20726.51 384.175 225.932 79.262 105.577 291.957 18.448 81.7465 36.402 13.050 20
84.852 1.5954 30.330 0.1180 0.0763 12.913 42.70 15279.32 740.200 276.019 53.111 18.016 383.168 64.369 103.334 23.31 1.1161 11
91.959 2.1680 31.117 0.0051 0.5410 16.086 46.39 61939.95 1087.504 351.931 137.822 9.464 263.825 76.310 188.27 3.09043 13.998 188
a = 1 a.u = 0.1482 1024 esu. b = 1 a.u = 8.6393 1033 esu.
Table 8 HOMO–LUMO energies (eV) and dipole moments l(D) of unsymmetrical azines 2–8. Compds.
2 3 4 5 6 7 8
HOMO–LUMO energies
Dipole moment
HOMO
LUMO
Energy gap (DE)
ltot
0.224 0.218 5.714 0.224 0.224 0.222 0.234
0.084 0.078 2.0136 0.086 0.086 0.082 0.111
3.81 3.81 3.70 3.75 3.75 3.81 3.35
0.35 0.41 1.94 2.64 2.52 1.91 6.68
2
Eð2Þ ¼ DEij ¼ qi
Fði; jÞ ei ej
electron donors and electron acceptors, i.e., the more donating tendency from electron donors to electron acceptors and the greater the extent of charge transfer or conjugation of the whole system. From Table 9, it is seen that the primary delocalization occurs from the biphenyl ring to benzylidene ring only rather than vice versa. Delocalization energy corresponding to the transfer of electrons from bonding orbital of N2AC21 bond to the antibonding orbital of N1AC11 (57 kJ mol1) is higher when compared to reverse transfer (50 kJ mol1). Therefore electrons flow occurs from biphenyl ring to the benzene ring side. The energy (E(2)) due to the transfer of electron from bonding orbital C15AC16 to antibonding orbital C12AC17 is found to be very high (E(2) = 93 kJ mol1). The highest interaction is found in the azine 8 which has highest E(2) value of 95.29 kJ mol 1.
ð7Þ
where qi is the donor orbital occupancy, ei and ej are diagonal elements and F(i,j) is the off diagonal NBO Fock matrix element. The larger the E(2) value, the more intensive is the interaction between
Thermodynamic properties Thermodynamic parameters have been calculated by using DFT method at B3LYP/6-31G(d,p) level [21] and the parameters are given in Table 10.
Table 9 Delocalization energies (E(2), kJ mol1) for unsymmetrical azines 2–8 by NBO analysis. Donor
Acceptor
2
3
4
5
6
7
8
N1AC11 N2AC21 C12AC17 C12AC17 C12AC17 C22AC27 C22AC27 C22AC27 C13AC14 C13AC14 C15AC16 C15AC16 C151AC156 C151AC156 C152AC153 C152AC153 C154AC155 C154AC155 C23AC24 C23AC24 C25AC26 C25AC26
N2AC21 N1AC11 N1AC11 C13AC14 C15AC16 N2AC21 C23AC24 C25AC26 C12AC17 C15AC16 C12AC17 C13AC14 C152AC153 C154AC155 C151AC156 C154AC155 C151AC156 C152AC153 C22AC27 C25AC26 C22AC27 C23AC24
54.88 54.63 80.76 79.42 81.51 79.88 79.42 81.81 77.49 86.28 93.57 72.38 81.47 87.79 87.79 83.90 84.65 85.36 80.51 87.71 87.67 76.70
54.17 55.14 80.30 83.52 81.93 81.85 81.76 75.90 77.58 86.12 93.15 72.51 81.47 87.96 84.44 83.86 84.48 85.36 76.95 94.16 94.16 72.38
54.76 54.59 80.80 79.38 81.55 80.38 82.27 81.34 86.20 86.20 93.53 72.43 81.47 20.97 20.16 83.86 20.22 20.39 76.95 93.70 89.43 73.31
55.89 53.67 81.72 79.46 80.88 78.04 80.30 92.90 77.41 86.45 93.86 72.18 81.47 87.50 84.44 83.94 84.90 76.91 78.37 86.70 78.46 74.98
55.89 50.24 81.85 79.38 80.80 78.00 79.84 93.61 77.37 86.08 94.37 72.38 81.47 87.79 84.36 83.98 84.61 85.24 78.75 85.41 77.95 75.48
55.09 54.21 81.14 79.38 81.30 79.96 85.45 83.35 77.49 86.28 93.82 72.34 81.47 87.67 84.44 83.90 84.78 87.08 73.31 93.78 88.04 71.30
57.48 52.50 82.98 79.46 80.00 74.23 79.00 95.96 77.28 86.75 95.29 71.81 81.43 87.08 84.44 84.02 85.28 85.20 81.09 88.34 76.49 80.05
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Table 10 Theoretically calculated thermodynamic properties of unsymmetrical azines 2–8. Parameters
2
3
4
5
6
7
8
Total energy Zero-point vibrational energy (kcal mol1) Rotational constants (GHz)
880.96 192.007 1.666 0.067 0.065 141.791 42.831 35.040 63.920
920.251 209.407 1.644 0.057 0.056 141.342 42.974 35.356 63.011
995.444 212.200 1.535 0.049 0.048 138.956 43.130 35.706 60.120
1340.564 185.903 1.654 0.048 0.047 145.706 43.167 35.692 66.846
3452.073 185.641 1.646 0.037 0.036 148.815 43.553 36.215 69.047
980.199 186.799 1.661 0.056 0.054 142.288 43.014 35.399 62.875
1085.382 191.465 1.459 0.044 0.043 149.564 43.269 35.981 70.314
Total entropy (cal mol1 K1) Translational entropy (cal mol1 K1) Rotational entropy (cal mol1 K1) Vibrational entropy (cal mol1 K1)
The total entropy of 8 is found to be higher than those of 2–7. It is evident that among all azines that the azine with electron-withdrawing substituent in the phenyl ring with lower energy gap between HOMO and LUMO orbitals (azine 8) is found to have higher entropy.
Acknowledgment The authors thank the NMR Research Centre, Indian Institute of Technology, Chennai for recording the NMR spectra. Appendix A. Supplementary material
MEP surfaces Three dimensional distribution of molecular electrostatic potential (MEP) is highly useful in predicting the reactive behaviour of the molecule. The MEP surface is on overlaying of the eletrostatic potential on to the isoelectron density surface. This is a valuable tool for describing over all molecule charge distribution as well as anticipating sites of electrophilic addition. In all azines region of negative charges (red colour) is seen around the electronegative nitrogens N(1) and N(2). Region of negative charge (red colour) is also seen around the electro negative oxygen O(38) in azine 4 and O(39) and O(40) in azine 8. The red region is susceptible for electrophillic attack. Blue colour represents strongly positive region and the predominant green region in the MEP surfaces corresponds to a potential halfway between the two extremes red and blue region in azines 2–8. The MEP surface picture of 2 is given in Fig. S7.
Conclusion In this work a series of novel unsymmetrical azines 2–8 are prepared and characterized by FT-IR, 1H, 13C NMR, Mass and UV spectral studies. The Gaussian-03 B3LYP/6-311+G(d,p) calculations on these azines are used to evaluate the heat of formation of the different conformers, to identify the stable conformation, to determine the dipole moment, polarizability, first hyperpolarizability, bond lengths, bond angles and torsional angles, MEP surface, HOMO and LUMO energies. All the azines are found to be polar molecules having non-zero dipole moment components. These result in large microscopic first hyperpolarizabilities and hence have a good microscopic NLO behaviour. Among the azines 2–8, azine 8 is found to have a good NLO behaviour that is indicated by its high first hyperpolarizability value. The NLO behavioural analysis of azine 8 has been studied experimentally by SHG test. From the UV studies it is found that the azine 8 is high kmax compared to other azines. The thermodynamic properties for all the azines are also calculated. The total entropy for 8 is found to be higher than those of 2–7.
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