Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 137 (2015) 1078–1085
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
A combined experimental and theoretical analysis on molecular structure and vibrational spectra of 2,4-dihydroxybenzoic acid Yaping Tao, Ligang Han, Yunxia Han, Zhaojun Liu ⇑ College of Physics and Electronic Information, Luoyang Normal University, Luoyang 471022, China
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
FT-IR and FT-Raman spectra of 2,4-
dihydroxybenzoic acid have been recorded and analyzed. Different conformers with their relative energies of 2,4dihydroxybenzoic acid have been calculated. The complete vibrational assignments of were made by combining experimental and theoretical data using PED analysis. HOMO, LUMO energies and MEP distribution of 2,4-dihydroxybenzoic acid were performed.
a r t i c l e
i n f o
Article history: Received 3 May 2014 Received in revised form 24 July 2014 Accepted 31 August 2014
Keywords: 2,4-Dihydroxybenzoic acid DFT FT-IR FT-Raman Vibrational analysis
a b s t r a c t The FT-IR and FT-Raman spectra of 2,4-dihydroxybenzoic acid (2,4-DHBA) in the solid phase have been recorded in the regions 4000–400 cm1 and 3700–100 cm1, respectively. The total energies of sixteen stable conformers for 2,4-DHBA have been calculated by density functional theory (DFT) using the B3LYP method with 6-311++G (d, p) basis set, and the C1 conformer with the lowest energy was obtained, the geometrical parameters between X-ray experiment diffraction and DFT calculation show good consistency. Furthermore, the vibrational frequencies of 2,4-DHBA were computed, and the detailed analysis of vibrational spectra was made on the basis of the potential energy distribution (PED) by combining experimental with theoretical data. In addition, frontier molecular orbitals, atomic charge distribution and molecular electrostatic potential (MEP) were also given. Ó 2014 Elsevier B.V. All rights reserved.
Introduction 2,4-Dihydroxybenzoic acid, also known as b-resorcylic acid, is one of the hydroxybenzoic acid derivatives. Hydroxybenzoic acids are commonly used in pharmaceutical and perfumery industry because of their activities in the biological system [1]. Dihydroxybenzoic acids are the important materials in the production of disinfectants and antipyretic drugs [2–4], and in the synthesis of
⇑ Corresponding author. Tel.: +86 379 65515016. E-mail address: [email protected] (Z. Liu). http://dx.doi.org/10.1016/j.saa.2014.08.151 1386-1425/Ó 2014 Elsevier B.V. All rights reserved.
organic compounds such as resins, polyesters, plasticizers, dyestuff and rubber chemicals. As an intermediate, 2,4-DHBA plays an important role in the production of cosmetics and dyes [5,6]. It can be also used as antimicrobial wash for reducing escherichia coli O157:H7 on apples. In addition, it is very efficient to reduce oxidative damage [7] associated with various diseases such as cancer, cataracts, arthritis, and diabetes. Meanwhile, as a precursor to the quinones which has an deleterious effect on environment, it can often cause water pollution [8]. So far, several researches have investigated the structure of 2,4DHBA. In 2007, Parkin et al. [9] made an effort on its crystal structure and hydrogen bonding by multiple temperature single-crystal
Y. Tao et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 137 (2015) 1078–1085
1079
Fig. 1. Conformers of 2,4-DHBA.
X-ray diffraction study. Recently, Braun and co-workers [10] studied its crystallization and compared with 2,5-dihydroxybenzoic acid. To the best of our knowledge, no detailed study on vibrational spectra for 2,4-DHBA has been reported up to now. Literature survey shows that the vibrational assignments for the similar molecule like 2,3-dihydroxybenzoic acid [11] and 4-dihydroxybenzoic acid [12] have been made. However, the position of hydroxyl groups and the hydrogen bonds strongly affect the vibrational spectra, the highly accurate calculations on the structure of 2,4DHBA and the detailed assignment on its vibrational bands are still desired. Density functional theory (DFT) is a reliable and accurate method for predicting molecular structure, vibrational frequencies, Raman activities, infrared intensities and charge distribution of the organic molecule [13–16]. In this study, quantum chemical
calculation was used to gain a better understanding on the vibrational spectra of 2,4-DHBA. Firstly, the total energy of various conformers of 2,4-DHBA was calculated, and the most stable conformer was found at the B3LYP/6-311++G (d, p) level. Secondly, the complete molecular geometry description and the normal coordinate analysis were given using the method suggested by Pulay et al. [17–19]. Also, the HOMO–LUMO analysis, atomic charge distribution and molecular electrostatic potential (MEP) were studied theoretically on the same basis set.
Experimental details The fine solid sample of 2,4-DHBA was purchased from Beijing Chemical Factory, and used without further purification. The FT-IR
1080
Y. Tao et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 137 (2015) 1078–1085
spectrum was collected from 4000 to 400 cm1 on a Nicolet Nexus FT-IR spectrometer. Nujol mull method was used in the sample preparation. The FT-Raman spectrum was recorded in the range of 3700–100 cm1 on a Raman module of the Nexus spectrometer with 1064 nm excitation from an Nd:YAG laser. The laser power on the sample was about 400 mw. Both IR and Raman spectra were accumulated with a 2 cm1 resolution at room temperature. Computational details Geometry optimization, energy and vibrational wavenumber computation were performed using Gaussian 09 w [20] program for 2,4-DHBA at B3LYP [21,22] level with the standard 6-311++G (d, p) basis set. B3LYP is a popular and relatively efficient density functional method for the determination of energies and geometries and has been proven to be useful to investigate interactions and conformational preferences for the various molecules [23– 25]. A comparison between the calculated and the measured frequencies usually shows that the calculated data are slightly greater than the measured frequencies. To improve the agreement, the computed harmonic frequencies are usually scaled for comparison. Many studies [26,27] show that the scaled quantum mechanical (SQM) method provides calculated frequencies well matched to the experimental. In this paper, the scaling calculation and potential energy distribution (PED) for each normal mode were done with the MOLVIB program (Version 7.0-G77) by Sundius [28,29]. Raman activities (Si) obtained from Gaussian 09 output were converted to relative Raman intensity (Ii) using the following relation from the basic theory of Raman scattering [30,31]:
Ii ¼
f ðm0 mi Þ4 Si mi ½1 exp ðhcmi =kT Þ
In the above formula, m0 is the laser excitation wavenumber (in cm1), mi is the vibrational wavenumber (in cm1) of the ith normal mode; c, h, k and T are the speed of light, Planck’s constant, Boltzmann’s constant and temperature (in K), respectively. f is a suitably chosen scale factor for all the band intensities. Simulation of calculated IR and Raman spectra have been plotted using pure Lorentzian band shapes with a bandwidth (FWHM) of 10 cm1. Results and discussion Molecular structure description In order to obtain the most stable geometry of 2,4-DHBA, the total energy calculations were carried out using B3LYP/6-311++G (d, p) level for various possible conformers without imaginary
Fig. 2. Molecular model of 2,4-DHBA along with numbering of atoms.
frequency, and shown in Fig. 1. It was clear from Table S1 that the C1 conformer of 2,4-DHBA has produced the global energy minimum. The total energies and energy differences with respect to C1 conformer were also listed in Table S1. The discussion below refers only to this C1 conformer. The molecular models along with numbering of atoms on the monomer and dimer were shown in Figs. 2 and 3, respectively. The total energies of the monomer and dimer were about 1500007.06 and 3000071.92 kJ/mol, respectively. The molecules in dimer are bound together via doubly hydrogen bonded, so the energy of dimer was not twice of its monomer structure owing to the effect of hydrogen bonds. The interaction energy of the formation of the intermolecular hydrogen bond dimer (DE = Edimer 2 Emonomer) [32] was 57.7980619 kJ/ mol. This is very close to the energy of hydrogen bond. Also, the geometrical parameters for monomer and dimmer were depicted in Table 1. For the purpose of comparison, the corresponding experimental parameters obtained from X-ray diffraction were provided. Root mean square deviation (RMSD), as showed in Table 1, has been calculated separately for bond lengths and bond angles. From the structural data given in Table 1, we can find that most of the optimized bond lengths of dimer were matched to the experimental values than monomer values, the
Fig. 3. Dimer of 2,4-DHBA.
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Y. Tao et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 137 (2015) 1078–1085 Table 1 The selected experimental and theoretical geometric parameters for 2,4-DHBA. Bond length (Å) Parameters
a
Bond angle (°)
Cal.
Exp.
Monomer
Dimer
C6AC1 C1AC2 C2AC3 C3AC4 C4AC5 C5AC6 C6AC7 C5AO8 C3AO9 C7AO10 C7AO11 C1AH12 C2AH13 C4AH14 O8AH15 O9AH16 O11AH17 O10AH15
1.409 1.378 1.406 1.389 1.397 1.419 1.456 1.341 1.360 1.229 1.350 1.082 1.082 1.085 0.984 0.964 0.969 1.747
1.407 1.379 1.407 1.388 1.395 1.422 1.457 1.342 1.360 1.249 1.320 1.082 1.085 1.082 0.981 0.963 0.998 1.751
RMSD
0.0148
0.0078
a
1.413 1.382 1.411 1.397 1.396 1.416 1.458 1.363 1.349 1.253 1.320 1.082 1.082 1.084 0.985 0.976 1.003
Parameters
C5AC6AC1 C6AC1AC2 C1AC2AC3 C2AC3AC4 C3AC4AC5 C4AC5AC6 C1AC6AC7 C5AC6AC7 C6AC5AO8 C4AC5AO8 C2AC3AO9 C4AC3AO9 C6AC7AO10 C6AC7AO11 O10AC7AO11 C6AC1AH12 C2AC1AH12 C3AC2AH13 C1AC2AH13 C5AC4AH14 C3AC4AH14 C5AO8AH15 C3AO9AH16 C7AO11AH17 RMSD
Dihedral angle (°) Cal.
Exp.
Monomer
Dimer
118.70 121.61 118.91 121.04 120.03 119.71 122.30 119.00 122.75 117.54 116.77 122.18 124.54 114.95 120.51 118.50 119.88 119.27 121.82 118.43 121.54 107.73 110.12 106.79 1.35
118.58 121.49 119.06 121.00 119.94 119.94 121.05 120.37 122.69 117.37 121.87 117.13 122.23 116.07 121.70 118.44 120.08 120.33 120.61 119.61 120.45 107.76 110.01 110.50 1.57
Inter-molecular bond, angles and dihedral angles Bond Value(Å) Bond angle length
Value (°)
Dihedral angle value (°)
O10AH34 H17AO27
126.88 126.88
C7AC11AO27AC24 C23AC24AO27AH17 O28AC24AO27AH17
1.673 1.673
C7AO10AH34 C24AO27AH17
a
Parameters
Cal. Monomer
Dimer
118.43 121.44 119.10 120.91 119.50 120.65 120.59 120.96 122.00 117.41 117.00 122.10 122.60 115.51 122.00 118.80 119.82 119.30 121.60 119.70 120.90 107.90 111.50 111.60
C4AC5AC6AC1 C5AC6AC1AC2 C6AC1AC2AC3 C1AC2AC3AC4 C2AC3AC4AC5 C3AC4AC5AC6 C4AC5AC6AC7 C3AC4AC5AO8 C1AC2AC3AO9 C1AC6AC7AO10 C5AC6AC7AO11 C3AC2AC1AH12 C6AC1AC2AH13 O9AC3AC4AH14 C6AC5AO8AH15 C2AC3AO9AH16 C6AC7AO11AH17 O10AC7AO11AH17
0.00 0.00 0.00 0.00 0.00 0.00 180.00 180.00 180.00 180.00 180.00 180.00 180.00 0.00 0.00 180.00 180.00 0.00
0.00 0.00 0.00 0.01 0.02 0.01 179.93 179.98 179.99 179.97 179.93 180.00 179.98 0.00 0.04 0.01 179.99 0.05
0.03 179.98 0.02
C7AO10AO28AC24 O11AC7AO10AH34 C6AC7AO10AH34
0.03 0.02 179.98
Ref. [9].
biggest bond difference between the experimental and calculated bond length was 0.0341 Å of O11AH17 in monomer, while it was 0.005 Å in dimer structure. The CAC bond lengths of the ring are varying in the range 1.389–1.419 Å in monomer and 1.379– 1.422 Å in dimer. The six-member ring in benzene is a perfect hexagon, the phenyl ring of 2,4-DHBA appears little distorted, due to the substitutions of the hydroxyl group and carboxyl group. The computational result showed that the torsional angles of C5AC6AC1AC2, C3AC4AC5AC6, O9AC3AC4AH14 and O10AC7A O11AH17 are nearly 0.0°or 180.0°, indicating the optimized structure of 2,4-DHBA is planar. Intermolecular hydrogen bonds are responsible for the stability and geometry of a predominant conformation [33]. Depending on our calculation, it showed that some bonds are affected by the dimerization. The shortening of the single C7AO11 bond upon dimerization is due to the redistribution of partial charges on the O11 atom as the unpaired electron is significantly delocalized and thereby the C7AO11 bond shows considerable double bond character. Similar effect can also be seen in bond angle C7AO11AH17 with an increase of 4.81°. Intermolecular hydrogen bonds O15AH34, H17AO32 are predicted as 1.673 Å, which are well within the range
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
A combined experimental and theoretical analysis on molecular structure and vibrational spectra of 2,4-dihydroxybenzoic acid Yaping Tao, Ligang Han, Yunxia Han, Zhaojun Liu ⇑ College of Physics and Electronic Information, Luoyang Normal University, Luoyang 471022, China
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
FT-IR and FT-Raman spectra of 2,4-
dihydroxybenzoic acid have been recorded and analyzed. Different conformers with their relative energies of 2,4dihydroxybenzoic acid have been calculated. The complete vibrational assignments of were made by combining experimental and theoretical data using PED analysis. HOMO, LUMO energies and MEP distribution of 2,4-dihydroxybenzoic acid were performed.
a r t i c l e
i n f o
Article history: Received 3 May 2014 Received in revised form 24 July 2014 Accepted 31 August 2014
Keywords: 2,4-Dihydroxybenzoic acid DFT FT-IR FT-Raman Vibrational analysis
a b s t r a c t The FT-IR and FT-Raman spectra of 2,4-dihydroxybenzoic acid (2,4-DHBA) in the solid phase have been recorded in the regions 4000–400 cm1 and 3700–100 cm1, respectively. The total energies of sixteen stable conformers for 2,4-DHBA have been calculated by density functional theory (DFT) using the B3LYP method with 6-311++G (d, p) basis set, and the C1 conformer with the lowest energy was obtained, the geometrical parameters between X-ray experiment diffraction and DFT calculation show good consistency. Furthermore, the vibrational frequencies of 2,4-DHBA were computed, and the detailed analysis of vibrational spectra was made on the basis of the potential energy distribution (PED) by combining experimental with theoretical data. In addition, frontier molecular orbitals, atomic charge distribution and molecular electrostatic potential (MEP) were also given. Ó 2014 Elsevier B.V. All rights reserved.
Introduction 2,4-Dihydroxybenzoic acid, also known as b-resorcylic acid, is one of the hydroxybenzoic acid derivatives. Hydroxybenzoic acids are commonly used in pharmaceutical and perfumery industry because of their activities in the biological system [1]. Dihydroxybenzoic acids are the important materials in the production of disinfectants and antipyretic drugs [2–4], and in the synthesis of
⇑ Corresponding author. Tel.: +86 379 65515016. E-mail address: [email protected] (Z. Liu). http://dx.doi.org/10.1016/j.saa.2014.08.151 1386-1425/Ó 2014 Elsevier B.V. All rights reserved.
organic compounds such as resins, polyesters, plasticizers, dyestuff and rubber chemicals. As an intermediate, 2,4-DHBA plays an important role in the production of cosmetics and dyes [5,6]. It can be also used as antimicrobial wash for reducing escherichia coli O157:H7 on apples. In addition, it is very efficient to reduce oxidative damage [7] associated with various diseases such as cancer, cataracts, arthritis, and diabetes. Meanwhile, as a precursor to the quinones which has an deleterious effect on environment, it can often cause water pollution [8]. So far, several researches have investigated the structure of 2,4DHBA. In 2007, Parkin et al. [9] made an effort on its crystal structure and hydrogen bonding by multiple temperature single-crystal
Y. Tao et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 137 (2015) 1078–1085
1079
Fig. 1. Conformers of 2,4-DHBA.
X-ray diffraction study. Recently, Braun and co-workers [10] studied its crystallization and compared with 2,5-dihydroxybenzoic acid. To the best of our knowledge, no detailed study on vibrational spectra for 2,4-DHBA has been reported up to now. Literature survey shows that the vibrational assignments for the similar molecule like 2,3-dihydroxybenzoic acid [11] and 4-dihydroxybenzoic acid [12] have been made. However, the position of hydroxyl groups and the hydrogen bonds strongly affect the vibrational spectra, the highly accurate calculations on the structure of 2,4DHBA and the detailed assignment on its vibrational bands are still desired. Density functional theory (DFT) is a reliable and accurate method for predicting molecular structure, vibrational frequencies, Raman activities, infrared intensities and charge distribution of the organic molecule [13–16]. In this study, quantum chemical
calculation was used to gain a better understanding on the vibrational spectra of 2,4-DHBA. Firstly, the total energy of various conformers of 2,4-DHBA was calculated, and the most stable conformer was found at the B3LYP/6-311++G (d, p) level. Secondly, the complete molecular geometry description and the normal coordinate analysis were given using the method suggested by Pulay et al. [17–19]. Also, the HOMO–LUMO analysis, atomic charge distribution and molecular electrostatic potential (MEP) were studied theoretically on the same basis set.
Experimental details The fine solid sample of 2,4-DHBA was purchased from Beijing Chemical Factory, and used without further purification. The FT-IR
1080
Y. Tao et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 137 (2015) 1078–1085
spectrum was collected from 4000 to 400 cm1 on a Nicolet Nexus FT-IR spectrometer. Nujol mull method was used in the sample preparation. The FT-Raman spectrum was recorded in the range of 3700–100 cm1 on a Raman module of the Nexus spectrometer with 1064 nm excitation from an Nd:YAG laser. The laser power on the sample was about 400 mw. Both IR and Raman spectra were accumulated with a 2 cm1 resolution at room temperature. Computational details Geometry optimization, energy and vibrational wavenumber computation were performed using Gaussian 09 w [20] program for 2,4-DHBA at B3LYP [21,22] level with the standard 6-311++G (d, p) basis set. B3LYP is a popular and relatively efficient density functional method for the determination of energies and geometries and has been proven to be useful to investigate interactions and conformational preferences for the various molecules [23– 25]. A comparison between the calculated and the measured frequencies usually shows that the calculated data are slightly greater than the measured frequencies. To improve the agreement, the computed harmonic frequencies are usually scaled for comparison. Many studies [26,27] show that the scaled quantum mechanical (SQM) method provides calculated frequencies well matched to the experimental. In this paper, the scaling calculation and potential energy distribution (PED) for each normal mode were done with the MOLVIB program (Version 7.0-G77) by Sundius [28,29]. Raman activities (Si) obtained from Gaussian 09 output were converted to relative Raman intensity (Ii) using the following relation from the basic theory of Raman scattering [30,31]:
Ii ¼
f ðm0 mi Þ4 Si mi ½1 exp ðhcmi =kT Þ
In the above formula, m0 is the laser excitation wavenumber (in cm1), mi is the vibrational wavenumber (in cm1) of the ith normal mode; c, h, k and T are the speed of light, Planck’s constant, Boltzmann’s constant and temperature (in K), respectively. f is a suitably chosen scale factor for all the band intensities. Simulation of calculated IR and Raman spectra have been plotted using pure Lorentzian band shapes with a bandwidth (FWHM) of 10 cm1. Results and discussion Molecular structure description In order to obtain the most stable geometry of 2,4-DHBA, the total energy calculations were carried out using B3LYP/6-311++G (d, p) level for various possible conformers without imaginary
Fig. 2. Molecular model of 2,4-DHBA along with numbering of atoms.
frequency, and shown in Fig. 1. It was clear from Table S1 that the C1 conformer of 2,4-DHBA has produced the global energy minimum. The total energies and energy differences with respect to C1 conformer were also listed in Table S1. The discussion below refers only to this C1 conformer. The molecular models along with numbering of atoms on the monomer and dimer were shown in Figs. 2 and 3, respectively. The total energies of the monomer and dimer were about 1500007.06 and 3000071.92 kJ/mol, respectively. The molecules in dimer are bound together via doubly hydrogen bonded, so the energy of dimer was not twice of its monomer structure owing to the effect of hydrogen bonds. The interaction energy of the formation of the intermolecular hydrogen bond dimer (DE = Edimer 2 Emonomer) [32] was 57.7980619 kJ/ mol. This is very close to the energy of hydrogen bond. Also, the geometrical parameters for monomer and dimmer were depicted in Table 1. For the purpose of comparison, the corresponding experimental parameters obtained from X-ray diffraction were provided. Root mean square deviation (RMSD), as showed in Table 1, has been calculated separately for bond lengths and bond angles. From the structural data given in Table 1, we can find that most of the optimized bond lengths of dimer were matched to the experimental values than monomer values, the
Fig. 3. Dimer of 2,4-DHBA.
1081
Y. Tao et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 137 (2015) 1078–1085 Table 1 The selected experimental and theoretical geometric parameters for 2,4-DHBA. Bond length (Å) Parameters
a
Bond angle (°)
Cal.
Exp.
Monomer
Dimer
C6AC1 C1AC2 C2AC3 C3AC4 C4AC5 C5AC6 C6AC7 C5AO8 C3AO9 C7AO10 C7AO11 C1AH12 C2AH13 C4AH14 O8AH15 O9AH16 O11AH17 O10AH15
1.409 1.378 1.406 1.389 1.397 1.419 1.456 1.341 1.360 1.229 1.350 1.082 1.082 1.085 0.984 0.964 0.969 1.747
1.407 1.379 1.407 1.388 1.395 1.422 1.457 1.342 1.360 1.249 1.320 1.082 1.085 1.082 0.981 0.963 0.998 1.751
RMSD
0.0148
0.0078
a
1.413 1.382 1.411 1.397 1.396 1.416 1.458 1.363 1.349 1.253 1.320 1.082 1.082 1.084 0.985 0.976 1.003
Parameters
C5AC6AC1 C6AC1AC2 C1AC2AC3 C2AC3AC4 C3AC4AC5 C4AC5AC6 C1AC6AC7 C5AC6AC7 C6AC5AO8 C4AC5AO8 C2AC3AO9 C4AC3AO9 C6AC7AO10 C6AC7AO11 O10AC7AO11 C6AC1AH12 C2AC1AH12 C3AC2AH13 C1AC2AH13 C5AC4AH14 C3AC4AH14 C5AO8AH15 C3AO9AH16 C7AO11AH17 RMSD
Dihedral angle (°) Cal.
Exp.
Monomer
Dimer
118.70 121.61 118.91 121.04 120.03 119.71 122.30 119.00 122.75 117.54 116.77 122.18 124.54 114.95 120.51 118.50 119.88 119.27 121.82 118.43 121.54 107.73 110.12 106.79 1.35
118.58 121.49 119.06 121.00 119.94 119.94 121.05 120.37 122.69 117.37 121.87 117.13 122.23 116.07 121.70 118.44 120.08 120.33 120.61 119.61 120.45 107.76 110.01 110.50 1.57
Inter-molecular bond, angles and dihedral angles Bond Value(Å) Bond angle length
Value (°)
Dihedral angle value (°)
O10AH34 H17AO27
126.88 126.88
C7AC11AO27AC24 C23AC24AO27AH17 O28AC24AO27AH17
1.673 1.673
C7AO10AH34 C24AO27AH17
a
Parameters
Cal. Monomer
Dimer
118.43 121.44 119.10 120.91 119.50 120.65 120.59 120.96 122.00 117.41 117.00 122.10 122.60 115.51 122.00 118.80 119.82 119.30 121.60 119.70 120.90 107.90 111.50 111.60
C4AC5AC6AC1 C5AC6AC1AC2 C6AC1AC2AC3 C1AC2AC3AC4 C2AC3AC4AC5 C3AC4AC5AC6 C4AC5AC6AC7 C3AC4AC5AO8 C1AC2AC3AO9 C1AC6AC7AO10 C5AC6AC7AO11 C3AC2AC1AH12 C6AC1AC2AH13 O9AC3AC4AH14 C6AC5AO8AH15 C2AC3AO9AH16 C6AC7AO11AH17 O10AC7AO11AH17
0.00 0.00 0.00 0.00 0.00 0.00 180.00 180.00 180.00 180.00 180.00 180.00 180.00 0.00 0.00 180.00 180.00 0.00
0.00 0.00 0.00 0.01 0.02 0.01 179.93 179.98 179.99 179.97 179.93 180.00 179.98 0.00 0.04 0.01 179.99 0.05
0.03 179.98 0.02
C7AO10AO28AC24 O11AC7AO10AH34 C6AC7AO10AH34
0.03 0.02 179.98
Ref. [9].
biggest bond difference between the experimental and calculated bond length was 0.0341 Å of O11AH17 in monomer, while it was 0.005 Å in dimer structure. The CAC bond lengths of the ring are varying in the range 1.389–1.419 Å in monomer and 1.379– 1.422 Å in dimer. The six-member ring in benzene is a perfect hexagon, the phenyl ring of 2,4-DHBA appears little distorted, due to the substitutions of the hydroxyl group and carboxyl group. The computational result showed that the torsional angles of C5AC6AC1AC2, C3AC4AC5AC6, O9AC3AC4AH14 and O10AC7A O11AH17 are nearly 0.0°or 180.0°, indicating the optimized structure of 2,4-DHBA is planar. Intermolecular hydrogen bonds are responsible for the stability and geometry of a predominant conformation [33]. Depending on our calculation, it showed that some bonds are affected by the dimerization. The shortening of the single C7AO11 bond upon dimerization is due to the redistribution of partial charges on the O11 atom as the unpaired electron is significantly delocalized and thereby the C7AO11 bond shows considerable double bond character. Similar effect can also be seen in bond angle C7AO11AH17 with an increase of 4.81°. Intermolecular hydrogen bonds O15AH34, H17AO32 are predicted as 1.673 Å, which are well within the range
