Accepted Manuscript Structure and nonlinear optical property analysis of L-Methioninium Oxalate: a DFT approach G. Edwin Sheela, D. Manimaran, I. Hubert Joe, Sherifa Rahim, V. Bena Jothy PII: DOI: Reference:
S1386-1425(15)00190-0 http://dx.doi.org/10.1016/j.saa.2015.02.038 SAA 13328
To appear in:
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy
Received Date: Revised Date: Accepted Date:
5 November 2014 2 February 2015 6 February 2015
Please cite this article as: G. Edwin Sheela, D. Manimaran, I. Hubert Joe, S. Rahim, V. Bena Jothy, Structure and nonlinear optical property analysis of L-Methioninium Oxalate: a DFT approach, Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy (2015), doi: http://dx.doi.org/10.1016/j.saa.2015.02.038
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Structure and nonlinear optical property analysis of L-Methioninium Oxalate: a DFT approach G. Edwin Sheelaa,d, D. Manimaranb, I. Hubert Joeb*, Sherifa Rahimc, V. Bena Jothyd a
Department of Physics, Muslim Arts College, Thiruvithancode-629 174, Tamil Nadu, India Centre for Molecular and Biophysics Research, Department of Physics, Mar Ivanios College, Thiruvananthapuram-695 015, Kerala, India c Department of Chemistry, St. John's College, Anchal, Kollam-691 306, Kerala, India d Department of Physics, Women’s Christian College, Nagercoil-629 001, Tamil Nadu, India * E-Mail: [email protected]
b
Abstract Infrared and FT-Raman spectra of the nonlinear optical material L-Methioninium Oxalate were recorded and analyzed. The optimized geometry, first-order hyperpolarizability and harmonic vibrational wavenumbers were calculated with the help of density functional theory method. The detailed interpretation of the vibrational spectra was carried out with the aid of normal coordinate analysis following the scaled quantum mechanical force field methodology. Stability of the molecule arising from hyperconjugative interactions leading to its nonlinear optical activity and charge delocalization were analyzed using natural bond orbital technique. Mulliken atomic charge and molecular electrostatic potential are also predicted. HOMO-LUMO energy gap value suggests the possibility of charge transfer within the molecule. The thermodynamic properties at different temperatures are also calculated.
Keywords: FTIR; FT-Raman; DFT; NLO; HOMO-LUMO
*Corresponding author: Telephone: +91 471 2531053; Fax Number: +91 471 2530023 E-mail: [email protected] (Dr. I. Hubert Joe)
1
1. Introduction Recent research on effective nonlinear optical (NLO) materials reveals that amino acid derivatives are valuable materials with favorable optical, thermal and mechanical properties. The importance of amino acids in NLO application is due to the fact that all the amino acids have chiral symmetry and crystallize non-centro symmetric space groups. Organic systems with nonlinear optical properties have been the subject of intense research owing to their potential use in a variety of technologies including telecommunications, optical information processing and storage devices [1, 2]. Edsall et al. [3] were the first to make a study of the behaviours of amino acids. Koleva [4] has explained the linear dichroism spectral study for the methionine compound in the solid state. Lima Jr. et al. [5] investigated the Raman spectra of crystalline L-Methionine at low and high temperature. Studies of some amino acid complexes with inorganic acids have been reported by Rajkumar and Ramakrishnan [6]. Briget Mary et al. [7] have reported the vibrational spectral analysis of amino acids such as DL-Valine and DL-Methionine. The structural investigation of LMethioninium Oxalate (LMO) has not been reported so far. The present work deals with vibrational spectral investigation of LMO using normal coordinate analysis (NCA) with the aid of density functional theory (DFT) calculations to study the optimized geometry, bonding features, nature of hydrogen bonding, frontier orbital energy and nonlinear optical property of the crystal. 2. Experimental details L-Methionine and oxalic acid were taken in 1:1 equimolar ratio and dissolved in triple distilled water to get a saturated aqueous solution, which was then allowed to evaporate slowly at room temperature. The crystals of L-Methioninium Oxalate were obtained within two weeks. Repeated recrystallization yielded good quality crystals. The powder X-ray diffraction (XRD) measurements were carried out with Cu Kα radiation using a Rigaku powder diffractometer equipped with a rotating anode scanning (0.01 step in 2θ) over the angular range 10-70° at room temperature generating X-ray by 45kV and 30mA power settings. Monochromatic X-rays of λ=1.5406Å Kα1 line from a Cu target were made to fall on the prepared samples. The diffraction pattern was obtained by varying the scattering angle 2θ from 10° to 70° in step size of 0.02. The FT-IR spectrum of LMO was recorded by AT FT-IR spectrometer in the region 4000-400 cm-1, with the sample in KBr pellet method. The FT-Raman spectrum was recorded in the region 3500-50 cm-1 by WITec Raman microscope spectrometer. 2
3. Computational details DFT calculations of LMO were performed using Gaussian 09 program package [8] at the B3LYP/6-311++G(d,p) level basis set [9-11]. The natural bond orbital (NBO) calculations were performed using NBO 3.1 program [12] implemented in the Gaussian 09 program. Normal coordinate analysis was performed including the calculation of vibrational
modes
and
potential
energy
distribution (PED)
in
local
symmetry
coordinates. These calculations were done with the Molvib7 written by Sundius [13, 14]. 4. Results and discussion 4.1 XRD analysis The experimental powder X-ray diffraction pattern of LMO crystal is shown in figure 1. XRD peaks at specified 2θ locations have been indexed using the CRYSFIRE software [15] to generate the theoretical hkl values from the experimental diffraction pattern. The calculated unit cell parameters of LMO are a=13.481Ǻ, b=18.775Ǻ, c=4.969Ǻ, α=90.00°, β=90.57°, =90.00° which confirms the formation of the title compound in Monoclinic crystal system.
Position for Figure 1
4.2 Optimized geometry The optimized structural parameters are listed in table 1. The molecular structure of LMO is shown in figure 2.
Position for Table 1
Position for Figure 2
The minimum energy obtained for structure optimization of LMO with B3LYP/6311++G(d,p) basis set is -1178.87 a.u. The calculated bond lengths of N11-H19, N11-H20 and N11-H28 are found to be 1.0223, 1.0222 and 1.1041Ǻ, respectively. The elongation of N11-H28 (1.1041Ǻ) reveals the possibility of N-H...O hydrogen bonding. The bond length of O21…H28 is calculated to be 1.5009Ǻ, this value significantly less than the van der Waals radii [16, 17] of O and H atoms, which indicates the presence of N-H…O hydrogen bonding. The bond angle between N11-H28…O21 is 166.80°, which is well within the angle limit, as interaction path is necessarily linear also indicates the possibility of intramolecular charge transfer interaction. The elongation of O21-C22 (1.2759Ǻ) also shows the formation of N-H…O hydrogen bonding. The calculated bond lengths of C4-H15, C4-H14 are 1.0899 and 1.0933Ǻ.
3
The shortening of bond length C4-H15 represents the presence of improper C-H…O hydrogen bonding. 4.3 NBO analysis The natural bond orbital (NBO) analysis was performed at the B3LYP/6-311++G(d,p) level basis set. The stabilizing interactions between filled and unoccupied orbitals and destabilizing interactions between filled orbitals can be obtained from this analysis [18-20]. The various second order interactions between the filled and unoccupied orbitals are investigated using DFT level computation which gives a measure of the delocalization or hyperconjugation. NBO analysis provides the most accurate possible ‘natural Lewis structure’ picture of j, because all orbital details are mathematically chosen to include the highest possible percentage of the electron density. Delocalization of electron density between occupied Lewis-type (bond or lone pair) NBO orbitals and formally unoccupied (antibond or Rydberg) non-Lewis NBO orbitals corresponds to a stabilizing donor-acceptor interaction. The NBO method also gives information about interactions in both filled and virtual orbital spaces that could enhance the analysis of intra- and intermolecular interactions [18, 21]. The interactions due to electron delocalization are generally analyzed by selecting a number of bonding and antibonding NBO’s. For each donor NBO (i) and accepter NBO (j) the stabilization energy E(2) is associated with i→j delocalization is given by the following equation(1), E(2) = ΔEij = qiF(i,j)2 / Σj-Σi
(1)
where, qi is the donor orbital occupancy, Σi and Σj are diagonal elements and F(i,j) is the off diagonal NBO Fock matrix element. The second-order perturbation theory analysis of Fock matrix in NBO basis is given in table 2. In NBO analysis large E(2) value shows the intensive interaction between electron-donors and electron-acceptors and greater the extent of conjugation of the whole system [22]. The intramolecular hyperconjugative interactions are formed by the orbital overlap between lone pair of O21 and *(C22-O27) bond orbital which results intramolecular charge transfer interaction causing stabilization of the system. These interactions are observed with increase of electron density in the antibonding orbital and weaken the respective bonds. Hence the enhanced LP3O21 NBO bonding with *(C22-O27) resulting in an enormous stabilization 40.72 kcal/mol with ED 0.8404e. The increased electron density 0.9456e at O27 atom leads to the elongation of O21-C22 bond length and a lowering of O21-C22 stretching wavenumber causing red-shift. The charge transfer interaction between the lone pair of O9→σ*(C4-H15) gives the guidance of C-H...O improper hydrogen 4
bonding. The lowering of N-H stretching wavenumber confirms N-H…O hydrogen bonding due to the interaction between the lone pair of O21 and σ*(N11-H28). Thus the NBO analysis is confirmed by N-H…O and C-H…O hydrogen bonding.
Position for Table 2
4.4 Vibrational Spectral analysis The vibrational band assignments have been performed based on normal coordinate analysis. Internal coordinates of LMO have been constructed according to Pulay’s recommendations [23]. The calculated wavenumbers were selectively scaled according to the scaled quantum mechanical (SQM) method recommended by Rauhut and Pulay [24] using a set of 16 transferable scale factors with the root mean square (RMS) wavenumber error 13.5 cm-1, which is in the reasonable limit for proper assignment. The vibrational assignments based on SQM force field computations are shown in table 3. The observed FT-IR and Raman spectra and simulated theoretical spectra calculated at B3LYP/6-311++G(d,p) level are shown in figures 3 and 4. The calculated and observed vibrational wavenumbers and their assignments are presented in table 3. The detailed analyses of vibrational wavenumbers for various functional groups are discussed below
Position for Table 3
Position for Figure 3
Position for Figure 4
4.4.1 Hydroxyl vibrations Free O-H hydroxyl stretching vibration is generally occurs in the region 3700-3500 cm-1. In bonded form a broad and intense band appears in the region 3550-3200 cm-1 [25, 26]. The band is broader and its intensity enhancement describes the involvement in an intermolecular hydrogen bonding. The observed broad intense IR band at 3418 cm -1 corresponds to O-H stretching mode, which is calculated at 3418 cm-1 (PED 91%). The broadening of OH stretching wavenumber is due to the formation of intermolecular O-H…O hydrogen bonding in solid phase. 4.4.2 NH3+ group vibrations NH3+ asymmetric and symmetric bands are expected to occur at 3330 cm-1 and 3080 cm1
, respectively [27]. The observed strong band at 3083 cm-1 in IR is assigned to asymmetric
stretching mode. The weak band at 3025 cm-1 in Raman is assigned to symmetric stretching 5
mode and the corresponding calculated value at 3026 cm-1 (PED 69%). Red-shifting of N-H stretching vibrations indicates the presence of N-H…O hydrogen bonding. NH3+ asymmetric and symmetric bending vibrations normally appear in the region 1660-1610 cm-1 and 15501485 cm-1, respectively [28]. NH3+ asymmetric bending vibrations are observed as strong band at 1622 cm-1 in IR. NH3+ rocking mode predicted to occur around 1000 cm-1 [29], this rocking mode is observed at 1110 cm-1 in IR spectrum. 4.4.3 Methyl group vibrations Methyl asymmetric and symmetric stretching vibrations are expected to appear in the region 2972-2952 cm-1 and 2882-2862 cm-1, respectively [30]. The observed strong IR band at 2968 cm-1, and Raman band at 2969 cm-1 are assigned to the asymmetric stretching vibration. The simultaneous activation of both IR and Raman is responsible for the large hyperpolarizability of the molecule [31, 32]. Methyl asymmetric deformations normally appear in the region 1470-1450 cm-1 [30]. The band is observed as strong in IR at 1412 cm-1 corresponds to asymmetric deformation mode. The bands at 945 cm-1 in Raman and at 905 cm-1 in IR are assigned to methyl rocking vibrations. The calculated wavenumbers supported the assignments 944 (PED 77%), 905 cm-1 (PED 65%). 4.4.4 Methylene group vibrations CH2 asymmetric and symmetric stretching modes are expected to occur at 2935 and 2865 cm-1, respectively [30]. Methylene asymmetric stretching vibration is observed at 2919 cm-1 in IR and at 2931 cm-1 in Raman. Shortening of C4-H15 bond length (1.0922 Å) along with the blue-shift of C-H stretching wavenumber indicates the presence of improper C-H…O hydrogen bonding. CH2 scissoring deformations are expected to occur in the region 14651445 cm-1 [30]. The IR band at 1439 cm-1 and Raman band at 1429 cm-1 are assigned to the scissoring deformation mode of vibration of Me-I. CH2 wagging is usually occurs in the region 1382-1170 cm-1 [33]. A medium band is observed at 1284 cm-1 in IR and a weak band at 1281 cm-1 in Raman can be attributed to CH2 wagging vibration. CH2 twisting deformations are expected over a region of wavenumbers 1295-1063 cm-1 [34]. The observed band at 1186 cm-1 in IR is assigned to methylene twisting mode of vibration. CH2 rocking deformations are expected to occur in the region 1174-724 cm-1 [34]. The IR band at 1034 cm-1 and Raman band at 1017 cm-1 are assigned to the rocking mode of vibration. 4.4.5 Methine vibrations
6
The hydrocarbon C-H stretching generally occurs in the region 3100-2800 cm-1 [29] and the deformation lies in the region at 1350-1315 cm-1 in IR, respectively [34]. The observed strong shoulder band at 2856 cm-1 in IR and weak band at 2821 cm-1 in Raman are assigned to methine stretching mode. The band at 1331 cm-1 in IR is assigned to the rocking mode of vibration. 4.4.6 Carbonyl group vibrations Carbonyl vibrations are expected to occur in the region 1760-1730 cm-1 [26,34]. The intense IR band is observed at 1716 cm-1 corresponds to C=O stretching mode. C-O stretching mode and O-H bending modes are not independent vibrational modes because they couple with the vibrations of adjacent groups. The carboxylic acid C-O stretching and O-H in-plane bending modes are expected in the region 1440-1210 cm-1 [27]. The observed bands at 1227 cm-1 (IR) and 1225 cm-1 (Raman) are assigned to C-O stretching mode. 4.4.7 Carboxylate vibrations The ionized carboxyl group give rise to asymmetric stretching around 1600-1570 cm-1 and symmetric stretching around 1415-1400 cm-1, respectively [26, 33]. The strong band at 1505 cm-1 in IR and weak band at 1486 cm-1 in Raman is assigned to carboxylate (COO-) symmetric stretching mode. COO- deformation mode appears as medium band in IR spectrum at 497 cm-1, which may be responsible for the intramolecular charge transfer from a proton donor to a proton acceptor through the hydrogen bonds. Rocking, wagging and scissoring in-plane and out-of-plane deformation modes of COO- ionized carboxylic group are expected at 502, 577 and 665 cm-1, respectively [29]. In LMO, wagging mode is observed at 683 cm-1 in Raman and rocking mode at 548 cm-1 in IR. 4.4.8 Skeletal vibrations Skeletal stretching vibrations, the C-C and C-N stretching modes lie in the region 1260-700 cm-1 and the deformation bands occur below 600 cm-1 [25]. The medium band at 976 cm-1 (IR) and 888 cm-1 (Raman) is assigned to C-N stretching mode. The bands at 841, 716 cm-1 in IR and at 743 cm-1 in Raman are indicating the C-C stretching mode. The weak bands at 589 cm-1 in IR and at 588 cm-1 in Raman are referred to C-S stretching mode. The deformation mode of C-C-S is observed at 388 cm-1 in Raman. The bands at 388 cm-1 and at 310 cm-1 in Raman are assigned to C-C-S, C-C-N bending modes of vibration. 4.4.9 Comparison of vibrational assignments of LMO with the similar molecules
7
The observed vibrational bands of LMO are compared (Table S1: Supplementary Information) with reported results of DL- Methionine Dihydrogen Phosphate (DL-MDP) [6], DL-Methionine DL-Methioninium Perchlorate Monohydrate (DL-MMPM) [7] and DLMethionine DL-Methioninium Picrate (DL-MMP) [35] molecules. The NH3+ asymmetric stretching bands are observed as strong broad IR band at 3086 cm-1 in DL-MMPM and as medium IR band at 3089 cm-1 in DL-MMP. Similarly, NH3+ symmetric stretching IR band is observed at 3036 cm-1 in DL-MMP. The CH3 asymmetric stretching bands are appeared in Raman for DL-MMPM and DL-MDP are 2966 cm-1 and 2978 cm-1, respectively. In DLMMP, an IR bands appear at 2976 and 2988 cm-1 corresponds to CH3 asymmetric stretching mode. The C=O stretching IR bands are observed at 1711 cm-1 in DL-MDP, 1740 cm-1 in DLMMP and strong band at 1716 cm-1 in LMO. This shifted in C=O stretching vibrations indicate the existence of inter- and intramolecular interactions in the molecules. The vibrational assignment of LMO is good agreement with the similar molecules. 4.5 Nonlinear property analysis The first-order hyperpolarizability (β), dipole moment (µ) and polarizability (α) were calculated using B3LYP/6-311++G(d,p) basis set on the basis of the finite field approach. The complete equations for calculating the magnitude of total static dipole moment, polarizability and first-order hyperpolarizability using the x,y,z components from Gaussian’09 output are as follows µtot = [µx2+µy2+µz2]1/2
(2)
αtot = 1/3[αxx+αyy+αzz]
(3)
βtot=[(βxxx+βxyy+βxzz)2+(βyyy+βyzz+βyxx)2+(βzzz+βzxx+βzyy)2]1/2
(4)
B3LYP/6-311++G(d,p) level results of the dipole moment µi (i=x,y,z), polarizability and the first-order hyperpolarizability for LMO are given in table 4. The calculated dipole moment of LMO is 1.52 Debye. The calculated polarizability αtot is equal to 37.27x10-24 e.s.u. The calculated polarizability ‘α’ have non zero values and was dominated by the diagnol components. The first-order hyperpolarizability of LMO is found to be 3.84x10-29 e.s.u, which is 48 times that of urea. The hyperpolarizability dominated by the longitudinal components of βxxx, βyyy, βxzz and βyzz. These high values of particular components indicate on a substantial delocalization of charges in these directions [36, 37].
Position for Table 4
4.6 Frontier molecular orbital energies analysis 8
The energies of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) are computed at B3LYP/6-311++G(d,p) level. HOMO and LUMO orbitals are shown in figure 5. Generally the energy values of LUMO, HOMO and their energy gap reflect the chemical activity of the molecule. HOMO as an electron donor represents the ability to donate an electron, while LUMO as an electron acceptor represent the ability to receive an electron [22]. The energy gap value reflects the NLO properties of the molecule. In LMO, the HOMO is located on the carboxylate group and the LUMO is mainly spread on the amino group. The energies of the HOMO and LUMO based on the optimized structure are computed. HOMO energy LUMO energy
= -11.35eV = -5.12eV
HOMO- LUMO energy gap = 6.23eV It can be seen that the compound having higher βtot value corresponds to the low HOMOLUMO energy gap. This result indicates that intermolecular hydrogen bonds have a substantial influence on the first-order hyperpolarizability. The HOMO-LUMO energy gap reveals the intramolecular charge transfer (ICT) interaction occurs within the molecule. This result supports that LMO crystal can be used for nonlinear optical applications.
Position of Figure 5
4.7 Molecular electrostatic potential surface analysis Molecular electrostatic potential (MEP) is a plot of electrostatic potential mapped on to the constant electron density surface. MEP displays molecular size, shape and electrostatic potential value. Electrostatic potential correlates with dipole moment, electronegativity, partial charges and site of chemical reactivity of the molecule. It provides a visual method to understand the relative polarity of a molecule. The negative electrostatic potential corresponds to an attraction of the proton by the concentrated electron density in the molecule. The positive electrostatic potential corresponds to repulsion of the proton by the atomic nuclei in regions where low electron density exist and the nuclear charge is incompletely shielded. By definition, electron density isosurface is a surface on which molecule's electron density has a particular value and that encloses a specified fraction of the molecule's electron probability density [38]. The electrostatic potential values are represented by different colors. The positive, negative and neutral electrostatic potential regions of molecules are shown in terms of color grading. Potential increases in the order red < orange
S1386-1425(15)00190-0 http://dx.doi.org/10.1016/j.saa.2015.02.038 SAA 13328
To appear in:
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy
Received Date: Revised Date: Accepted Date:
5 November 2014 2 February 2015 6 February 2015
Please cite this article as: G. Edwin Sheela, D. Manimaran, I. Hubert Joe, S. Rahim, V. Bena Jothy, Structure and nonlinear optical property analysis of L-Methioninium Oxalate: a DFT approach, Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy (2015), doi: http://dx.doi.org/10.1016/j.saa.2015.02.038
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Structure and nonlinear optical property analysis of L-Methioninium Oxalate: a DFT approach G. Edwin Sheelaa,d, D. Manimaranb, I. Hubert Joeb*, Sherifa Rahimc, V. Bena Jothyd a
Department of Physics, Muslim Arts College, Thiruvithancode-629 174, Tamil Nadu, India Centre for Molecular and Biophysics Research, Department of Physics, Mar Ivanios College, Thiruvananthapuram-695 015, Kerala, India c Department of Chemistry, St. John's College, Anchal, Kollam-691 306, Kerala, India d Department of Physics, Women’s Christian College, Nagercoil-629 001, Tamil Nadu, India * E-Mail: [email protected]
b
Abstract Infrared and FT-Raman spectra of the nonlinear optical material L-Methioninium Oxalate were recorded and analyzed. The optimized geometry, first-order hyperpolarizability and harmonic vibrational wavenumbers were calculated with the help of density functional theory method. The detailed interpretation of the vibrational spectra was carried out with the aid of normal coordinate analysis following the scaled quantum mechanical force field methodology. Stability of the molecule arising from hyperconjugative interactions leading to its nonlinear optical activity and charge delocalization were analyzed using natural bond orbital technique. Mulliken atomic charge and molecular electrostatic potential are also predicted. HOMO-LUMO energy gap value suggests the possibility of charge transfer within the molecule. The thermodynamic properties at different temperatures are also calculated.
Keywords: FTIR; FT-Raman; DFT; NLO; HOMO-LUMO
*Corresponding author: Telephone: +91 471 2531053; Fax Number: +91 471 2530023 E-mail: [email protected] (Dr. I. Hubert Joe)
1
1. Introduction Recent research on effective nonlinear optical (NLO) materials reveals that amino acid derivatives are valuable materials with favorable optical, thermal and mechanical properties. The importance of amino acids in NLO application is due to the fact that all the amino acids have chiral symmetry and crystallize non-centro symmetric space groups. Organic systems with nonlinear optical properties have been the subject of intense research owing to their potential use in a variety of technologies including telecommunications, optical information processing and storage devices [1, 2]. Edsall et al. [3] were the first to make a study of the behaviours of amino acids. Koleva [4] has explained the linear dichroism spectral study for the methionine compound in the solid state. Lima Jr. et al. [5] investigated the Raman spectra of crystalline L-Methionine at low and high temperature. Studies of some amino acid complexes with inorganic acids have been reported by Rajkumar and Ramakrishnan [6]. Briget Mary et al. [7] have reported the vibrational spectral analysis of amino acids such as DL-Valine and DL-Methionine. The structural investigation of LMethioninium Oxalate (LMO) has not been reported so far. The present work deals with vibrational spectral investigation of LMO using normal coordinate analysis (NCA) with the aid of density functional theory (DFT) calculations to study the optimized geometry, bonding features, nature of hydrogen bonding, frontier orbital energy and nonlinear optical property of the crystal. 2. Experimental details L-Methionine and oxalic acid were taken in 1:1 equimolar ratio and dissolved in triple distilled water to get a saturated aqueous solution, which was then allowed to evaporate slowly at room temperature. The crystals of L-Methioninium Oxalate were obtained within two weeks. Repeated recrystallization yielded good quality crystals. The powder X-ray diffraction (XRD) measurements were carried out with Cu Kα radiation using a Rigaku powder diffractometer equipped with a rotating anode scanning (0.01 step in 2θ) over the angular range 10-70° at room temperature generating X-ray by 45kV and 30mA power settings. Monochromatic X-rays of λ=1.5406Å Kα1 line from a Cu target were made to fall on the prepared samples. The diffraction pattern was obtained by varying the scattering angle 2θ from 10° to 70° in step size of 0.02. The FT-IR spectrum of LMO was recorded by AT FT-IR spectrometer in the region 4000-400 cm-1, with the sample in KBr pellet method. The FT-Raman spectrum was recorded in the region 3500-50 cm-1 by WITec Raman microscope spectrometer. 2
3. Computational details DFT calculations of LMO were performed using Gaussian 09 program package [8] at the B3LYP/6-311++G(d,p) level basis set [9-11]. The natural bond orbital (NBO) calculations were performed using NBO 3.1 program [12] implemented in the Gaussian 09 program. Normal coordinate analysis was performed including the calculation of vibrational
modes
and
potential
energy
distribution (PED)
in
local
symmetry
coordinates. These calculations were done with the Molvib7 written by Sundius [13, 14]. 4. Results and discussion 4.1 XRD analysis The experimental powder X-ray diffraction pattern of LMO crystal is shown in figure 1. XRD peaks at specified 2θ locations have been indexed using the CRYSFIRE software [15] to generate the theoretical hkl values from the experimental diffraction pattern. The calculated unit cell parameters of LMO are a=13.481Ǻ, b=18.775Ǻ, c=4.969Ǻ, α=90.00°, β=90.57°, =90.00° which confirms the formation of the title compound in Monoclinic crystal system.
Position for Figure 1
4.2 Optimized geometry The optimized structural parameters are listed in table 1. The molecular structure of LMO is shown in figure 2.
Position for Table 1
Position for Figure 2
The minimum energy obtained for structure optimization of LMO with B3LYP/6311++G(d,p) basis set is -1178.87 a.u. The calculated bond lengths of N11-H19, N11-H20 and N11-H28 are found to be 1.0223, 1.0222 and 1.1041Ǻ, respectively. The elongation of N11-H28 (1.1041Ǻ) reveals the possibility of N-H...O hydrogen bonding. The bond length of O21…H28 is calculated to be 1.5009Ǻ, this value significantly less than the van der Waals radii [16, 17] of O and H atoms, which indicates the presence of N-H…O hydrogen bonding. The bond angle between N11-H28…O21 is 166.80°, which is well within the angle limit, as interaction path is necessarily linear also indicates the possibility of intramolecular charge transfer interaction. The elongation of O21-C22 (1.2759Ǻ) also shows the formation of N-H…O hydrogen bonding. The calculated bond lengths of C4-H15, C4-H14 are 1.0899 and 1.0933Ǻ.
3
The shortening of bond length C4-H15 represents the presence of improper C-H…O hydrogen bonding. 4.3 NBO analysis The natural bond orbital (NBO) analysis was performed at the B3LYP/6-311++G(d,p) level basis set. The stabilizing interactions between filled and unoccupied orbitals and destabilizing interactions between filled orbitals can be obtained from this analysis [18-20]. The various second order interactions between the filled and unoccupied orbitals are investigated using DFT level computation which gives a measure of the delocalization or hyperconjugation. NBO analysis provides the most accurate possible ‘natural Lewis structure’ picture of j, because all orbital details are mathematically chosen to include the highest possible percentage of the electron density. Delocalization of electron density between occupied Lewis-type (bond or lone pair) NBO orbitals and formally unoccupied (antibond or Rydberg) non-Lewis NBO orbitals corresponds to a stabilizing donor-acceptor interaction. The NBO method also gives information about interactions in both filled and virtual orbital spaces that could enhance the analysis of intra- and intermolecular interactions [18, 21]. The interactions due to electron delocalization are generally analyzed by selecting a number of bonding and antibonding NBO’s. For each donor NBO (i) and accepter NBO (j) the stabilization energy E(2) is associated with i→j delocalization is given by the following equation(1), E(2) = ΔEij = qiF(i,j)2 / Σj-Σi
(1)
where, qi is the donor orbital occupancy, Σi and Σj are diagonal elements and F(i,j) is the off diagonal NBO Fock matrix element. The second-order perturbation theory analysis of Fock matrix in NBO basis is given in table 2. In NBO analysis large E(2) value shows the intensive interaction between electron-donors and electron-acceptors and greater the extent of conjugation of the whole system [22]. The intramolecular hyperconjugative interactions are formed by the orbital overlap between lone pair of O21 and *(C22-O27) bond orbital which results intramolecular charge transfer interaction causing stabilization of the system. These interactions are observed with increase of electron density in the antibonding orbital and weaken the respective bonds. Hence the enhanced LP3O21 NBO bonding with *(C22-O27) resulting in an enormous stabilization 40.72 kcal/mol with ED 0.8404e. The increased electron density 0.9456e at O27 atom leads to the elongation of O21-C22 bond length and a lowering of O21-C22 stretching wavenumber causing red-shift. The charge transfer interaction between the lone pair of O9→σ*(C4-H15) gives the guidance of C-H...O improper hydrogen 4
bonding. The lowering of N-H stretching wavenumber confirms N-H…O hydrogen bonding due to the interaction between the lone pair of O21 and σ*(N11-H28). Thus the NBO analysis is confirmed by N-H…O and C-H…O hydrogen bonding.
Position for Table 2
4.4 Vibrational Spectral analysis The vibrational band assignments have been performed based on normal coordinate analysis. Internal coordinates of LMO have been constructed according to Pulay’s recommendations [23]. The calculated wavenumbers were selectively scaled according to the scaled quantum mechanical (SQM) method recommended by Rauhut and Pulay [24] using a set of 16 transferable scale factors with the root mean square (RMS) wavenumber error 13.5 cm-1, which is in the reasonable limit for proper assignment. The vibrational assignments based on SQM force field computations are shown in table 3. The observed FT-IR and Raman spectra and simulated theoretical spectra calculated at B3LYP/6-311++G(d,p) level are shown in figures 3 and 4. The calculated and observed vibrational wavenumbers and their assignments are presented in table 3. The detailed analyses of vibrational wavenumbers for various functional groups are discussed below
Position for Table 3
Position for Figure 3
Position for Figure 4
4.4.1 Hydroxyl vibrations Free O-H hydroxyl stretching vibration is generally occurs in the region 3700-3500 cm-1. In bonded form a broad and intense band appears in the region 3550-3200 cm-1 [25, 26]. The band is broader and its intensity enhancement describes the involvement in an intermolecular hydrogen bonding. The observed broad intense IR band at 3418 cm -1 corresponds to O-H stretching mode, which is calculated at 3418 cm-1 (PED 91%). The broadening of OH stretching wavenumber is due to the formation of intermolecular O-H…O hydrogen bonding in solid phase. 4.4.2 NH3+ group vibrations NH3+ asymmetric and symmetric bands are expected to occur at 3330 cm-1 and 3080 cm1
, respectively [27]. The observed strong band at 3083 cm-1 in IR is assigned to asymmetric
stretching mode. The weak band at 3025 cm-1 in Raman is assigned to symmetric stretching 5
mode and the corresponding calculated value at 3026 cm-1 (PED 69%). Red-shifting of N-H stretching vibrations indicates the presence of N-H…O hydrogen bonding. NH3+ asymmetric and symmetric bending vibrations normally appear in the region 1660-1610 cm-1 and 15501485 cm-1, respectively [28]. NH3+ asymmetric bending vibrations are observed as strong band at 1622 cm-1 in IR. NH3+ rocking mode predicted to occur around 1000 cm-1 [29], this rocking mode is observed at 1110 cm-1 in IR spectrum. 4.4.3 Methyl group vibrations Methyl asymmetric and symmetric stretching vibrations are expected to appear in the region 2972-2952 cm-1 and 2882-2862 cm-1, respectively [30]. The observed strong IR band at 2968 cm-1, and Raman band at 2969 cm-1 are assigned to the asymmetric stretching vibration. The simultaneous activation of both IR and Raman is responsible for the large hyperpolarizability of the molecule [31, 32]. Methyl asymmetric deformations normally appear in the region 1470-1450 cm-1 [30]. The band is observed as strong in IR at 1412 cm-1 corresponds to asymmetric deformation mode. The bands at 945 cm-1 in Raman and at 905 cm-1 in IR are assigned to methyl rocking vibrations. The calculated wavenumbers supported the assignments 944 (PED 77%), 905 cm-1 (PED 65%). 4.4.4 Methylene group vibrations CH2 asymmetric and symmetric stretching modes are expected to occur at 2935 and 2865 cm-1, respectively [30]. Methylene asymmetric stretching vibration is observed at 2919 cm-1 in IR and at 2931 cm-1 in Raman. Shortening of C4-H15 bond length (1.0922 Å) along with the blue-shift of C-H stretching wavenumber indicates the presence of improper C-H…O hydrogen bonding. CH2 scissoring deformations are expected to occur in the region 14651445 cm-1 [30]. The IR band at 1439 cm-1 and Raman band at 1429 cm-1 are assigned to the scissoring deformation mode of vibration of Me-I. CH2 wagging is usually occurs in the region 1382-1170 cm-1 [33]. A medium band is observed at 1284 cm-1 in IR and a weak band at 1281 cm-1 in Raman can be attributed to CH2 wagging vibration. CH2 twisting deformations are expected over a region of wavenumbers 1295-1063 cm-1 [34]. The observed band at 1186 cm-1 in IR is assigned to methylene twisting mode of vibration. CH2 rocking deformations are expected to occur in the region 1174-724 cm-1 [34]. The IR band at 1034 cm-1 and Raman band at 1017 cm-1 are assigned to the rocking mode of vibration. 4.4.5 Methine vibrations
6
The hydrocarbon C-H stretching generally occurs in the region 3100-2800 cm-1 [29] and the deformation lies in the region at 1350-1315 cm-1 in IR, respectively [34]. The observed strong shoulder band at 2856 cm-1 in IR and weak band at 2821 cm-1 in Raman are assigned to methine stretching mode. The band at 1331 cm-1 in IR is assigned to the rocking mode of vibration. 4.4.6 Carbonyl group vibrations Carbonyl vibrations are expected to occur in the region 1760-1730 cm-1 [26,34]. The intense IR band is observed at 1716 cm-1 corresponds to C=O stretching mode. C-O stretching mode and O-H bending modes are not independent vibrational modes because they couple with the vibrations of adjacent groups. The carboxylic acid C-O stretching and O-H in-plane bending modes are expected in the region 1440-1210 cm-1 [27]. The observed bands at 1227 cm-1 (IR) and 1225 cm-1 (Raman) are assigned to C-O stretching mode. 4.4.7 Carboxylate vibrations The ionized carboxyl group give rise to asymmetric stretching around 1600-1570 cm-1 and symmetric stretching around 1415-1400 cm-1, respectively [26, 33]. The strong band at 1505 cm-1 in IR and weak band at 1486 cm-1 in Raman is assigned to carboxylate (COO-) symmetric stretching mode. COO- deformation mode appears as medium band in IR spectrum at 497 cm-1, which may be responsible for the intramolecular charge transfer from a proton donor to a proton acceptor through the hydrogen bonds. Rocking, wagging and scissoring in-plane and out-of-plane deformation modes of COO- ionized carboxylic group are expected at 502, 577 and 665 cm-1, respectively [29]. In LMO, wagging mode is observed at 683 cm-1 in Raman and rocking mode at 548 cm-1 in IR. 4.4.8 Skeletal vibrations Skeletal stretching vibrations, the C-C and C-N stretching modes lie in the region 1260-700 cm-1 and the deformation bands occur below 600 cm-1 [25]. The medium band at 976 cm-1 (IR) and 888 cm-1 (Raman) is assigned to C-N stretching mode. The bands at 841, 716 cm-1 in IR and at 743 cm-1 in Raman are indicating the C-C stretching mode. The weak bands at 589 cm-1 in IR and at 588 cm-1 in Raman are referred to C-S stretching mode. The deformation mode of C-C-S is observed at 388 cm-1 in Raman. The bands at 388 cm-1 and at 310 cm-1 in Raman are assigned to C-C-S, C-C-N bending modes of vibration. 4.4.9 Comparison of vibrational assignments of LMO with the similar molecules
7
The observed vibrational bands of LMO are compared (Table S1: Supplementary Information) with reported results of DL- Methionine Dihydrogen Phosphate (DL-MDP) [6], DL-Methionine DL-Methioninium Perchlorate Monohydrate (DL-MMPM) [7] and DLMethionine DL-Methioninium Picrate (DL-MMP) [35] molecules. The NH3+ asymmetric stretching bands are observed as strong broad IR band at 3086 cm-1 in DL-MMPM and as medium IR band at 3089 cm-1 in DL-MMP. Similarly, NH3+ symmetric stretching IR band is observed at 3036 cm-1 in DL-MMP. The CH3 asymmetric stretching bands are appeared in Raman for DL-MMPM and DL-MDP are 2966 cm-1 and 2978 cm-1, respectively. In DLMMP, an IR bands appear at 2976 and 2988 cm-1 corresponds to CH3 asymmetric stretching mode. The C=O stretching IR bands are observed at 1711 cm-1 in DL-MDP, 1740 cm-1 in DLMMP and strong band at 1716 cm-1 in LMO. This shifted in C=O stretching vibrations indicate the existence of inter- and intramolecular interactions in the molecules. The vibrational assignment of LMO is good agreement with the similar molecules. 4.5 Nonlinear property analysis The first-order hyperpolarizability (β), dipole moment (µ) and polarizability (α) were calculated using B3LYP/6-311++G(d,p) basis set on the basis of the finite field approach. The complete equations for calculating the magnitude of total static dipole moment, polarizability and first-order hyperpolarizability using the x,y,z components from Gaussian’09 output are as follows µtot = [µx2+µy2+µz2]1/2
(2)
αtot = 1/3[αxx+αyy+αzz]
(3)
βtot=[(βxxx+βxyy+βxzz)2+(βyyy+βyzz+βyxx)2+(βzzz+βzxx+βzyy)2]1/2
(4)
B3LYP/6-311++G(d,p) level results of the dipole moment µi (i=x,y,z), polarizability and the first-order hyperpolarizability for LMO are given in table 4. The calculated dipole moment of LMO is 1.52 Debye. The calculated polarizability αtot is equal to 37.27x10-24 e.s.u. The calculated polarizability ‘α’ have non zero values and was dominated by the diagnol components. The first-order hyperpolarizability of LMO is found to be 3.84x10-29 e.s.u, which is 48 times that of urea. The hyperpolarizability dominated by the longitudinal components of βxxx, βyyy, βxzz and βyzz. These high values of particular components indicate on a substantial delocalization of charges in these directions [36, 37].
Position for Table 4
4.6 Frontier molecular orbital energies analysis 8
The energies of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) are computed at B3LYP/6-311++G(d,p) level. HOMO and LUMO orbitals are shown in figure 5. Generally the energy values of LUMO, HOMO and their energy gap reflect the chemical activity of the molecule. HOMO as an electron donor represents the ability to donate an electron, while LUMO as an electron acceptor represent the ability to receive an electron [22]. The energy gap value reflects the NLO properties of the molecule. In LMO, the HOMO is located on the carboxylate group and the LUMO is mainly spread on the amino group. The energies of the HOMO and LUMO based on the optimized structure are computed. HOMO energy LUMO energy
= -11.35eV = -5.12eV
HOMO- LUMO energy gap = 6.23eV It can be seen that the compound having higher βtot value corresponds to the low HOMOLUMO energy gap. This result indicates that intermolecular hydrogen bonds have a substantial influence on the first-order hyperpolarizability. The HOMO-LUMO energy gap reveals the intramolecular charge transfer (ICT) interaction occurs within the molecule. This result supports that LMO crystal can be used for nonlinear optical applications.
Position of Figure 5
4.7 Molecular electrostatic potential surface analysis Molecular electrostatic potential (MEP) is a plot of electrostatic potential mapped on to the constant electron density surface. MEP displays molecular size, shape and electrostatic potential value. Electrostatic potential correlates with dipole moment, electronegativity, partial charges and site of chemical reactivity of the molecule. It provides a visual method to understand the relative polarity of a molecule. The negative electrostatic potential corresponds to an attraction of the proton by the concentrated electron density in the molecule. The positive electrostatic potential corresponds to repulsion of the proton by the atomic nuclei in regions where low electron density exist and the nuclear charge is incompletely shielded. By definition, electron density isosurface is a surface on which molecule's electron density has a particular value and that encloses a specified fraction of the molecule's electron probability density [38]. The electrostatic potential values are represented by different colors. The positive, negative and neutral electrostatic potential regions of molecules are shown in terms of color grading. Potential increases in the order red < orange
