Spectroscopic and molecular structure (monomeric and dimeric structure) investigation of 2-[(2-hydroxyphenyl) carbonyloxy] benzoic acid by DFT method: A combined experimental and theoretical study

Journal of Molecular Structure 1038 (2013) 145–162

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Spectroscopic and molecular structure (monomeric and dimeric structure) investigation of 2-[(2-hydroxyphenyl) carbonyloxy] benzoic acid by DFT method: A combined experimental and theoretical study S. Muthu a, E. Isac Paulraj b,c,⇑ a b c

Department of Applied Physics, Sri Venkateswara College of Engineering, Sriperumbudur 602 105, Tamil Nadu, India Department of Physics, Manonmaniam Sundaranar University, Tirunelveli 627 012, Tamil Nadu, India Department of Applied Physics, Pallavan College of Engineering, Kancheepuram 631 502, Tamil Nadu, India

h i g h l i g h t s " FT-IR and FT-Raman spectra of 2-[(2-hydroxyphenyl) carbonyloxy] benzoic acid in the solid phase are recorded and analyzed. " The optimized geometry and vibrational wavenumbers are computed using DFT method for monomer and dimer. " The complete vibrational assignment and spectroscopic analysis have been carried out. " The first order hyperpolarizability and HOMO, LUMO energy gap are theoretically predicted. " The NBO analysis explained the intramolecular hydrogen bonding.

a r t i c l e

i n f o

Article history: Received 28 October 2012 Received in revised form 15 January 2013 Accepted 17 January 2013 Available online 25 January 2013 Keywords: DFT PES Hyperpolarizability NBO Fukui functions

a b s t r a c t The experimental and theoretical study on the structures and vibrations of 2-[(2-hydroxyphenyl) carbonyloxy] benzoic acid (abbreviated as HPCBA) are presented. The FT-IR and FT-Raman spectra of the title compound have been recorded in the region 4000–400 cm1 and 4000–100 cm1 respectively. The molecular structures, vibrational wavenumbers, infrared intensities, Raman activities were calculated using DFT (B3LYP) method with 6-31G(d,p) basis set. The most stable conformer of HPCBA is identified from the computational results. The assignments of the vibrational spectra have been carried out with the help of normal co-ordinate analysis (NCA) following the scaled quantum mechanical force field (SQMFF) methodology. Intermolecular hydrogen bonds are discussed in dimer structure of the molecule. The first order hyperpolarizability (b0) and related properties (b, a0 and Da) of HPCBA are calculated. The stability and charge delocalization of the molecule was studied by natural bond orbital (NBO) analysis. The molecule orbital contributions are studied by density of energy states (DOSs). UV–Visible spectrum of the compound was recorded in the region 200–400 nm and the electronic properties such as HOMO and LUMO energies were determined by time-dependent TD-DFT approach. Fukui functions, local softness and electrophilicity indices for selected atomic sites of the title compound are determined. Mulliken population analysis on atomic charges is also calculated. Thermodynamic properties (heat capacity, entropy and enthalpy) of the title compound at different temperatures are calculated. Ó 2013 Elsevier B.V. All rights reserved.

1. Introduction The generic name of 2-[(2-hydroxyphenyl) carbonyloxy] benzoic acid (HPCBA) is salsalate. It is a non-steroidal anti-inflammatory drug used for decades to relieve pain, tenderness, swelling,

⇑ Corresponding author at: Department of Applied Physics, Pallavan College of Engineering, Kancheepuram 631 502, Tamil Nadu, India. Tel.: +91 9442721543. E-mail address: [email protected] (E. Isac Paulraj). 0022-2860/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.molstruc.2013.01.043

and stiffness caused by rheumatoid arthritis has now been shown to reduce blood glucose levels in individuals with type 2 diabetes. Salsalate is in a class of drugs called salicylates. It has also been used as an alternative to narcotic pain medicine for people with spinal disc protrusion. The molecular formula for 2-[(2hydroxyphenyl) carbonyloxy] benzoic acid is C14H10O5. It is a white crystalline powder and soluble in organic solvents such as ethanol, DMSO and acetone. The salsalate and its derivatives were investigated by several authors. The hydrogen bonding in salicylsalicylic acid crystals was described in literature [1]. The Fluorimetric

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determination of salsalate in urine, serum and pharmaceutical preparations were reported by Pulgarin and Bermejo [2]. The simultaneous TG–DTA study of the thermal decomposition of 2-hydroxybenzoic acid, 2-carboxyphenyl ester has been studied [3]. Hemostatic effects of salsalate and changes in thyroid function tests during short-term salsalate were reported earlier [4,5]. Inspite of its importance for pharmaceutical purposes, the reported spectroscopical studies on HPCBA are scarce. The evolution of density function theory that includes electron correlation in an alternative way has affording opportunities of performing vibrational analysis of moderately large organic molecules. The results from DFT theory with results obtained from experiments has shown that the methods using B3LYP are the most promising in providing correct vibrational wave numbers. The kinetics and decomposition of 2-hydroxybenzoic acid have been studied with the help of thermogravimetry–differential thermal analysis (TG–DTA) and gas and liquid chromatography [6]. The hydrogen bonding in salsalate crystal was previously carried out and the position of hydroxyl group in ortho position on an aryl ring with possible interamolecular hydrogen bonds are discussed with the crystal data collected at low temperature [1]. From the crystal data, the structure of the molecule was determined and there is no information about the structural conformation and vibrational analysis of the molecule. A scan of literature survey reveals that to the best of our knowledge, no PES, DFT calculations and molecular interpretation of monomer and dimer conformer of 2-[(2-hydroxyphenyl) carbonyloxy] benzoic acid have been reported so far. This inadequacy observed in literature encouraged us to make this theoretical and experimental vibrational spectroscopic for the title molecule. Hence in this work, a systematic study on the optimized structure of the most stable conformer of the molecule, a detailed vibrational analysis based on FT-IR and FT-Raman spectra, hyperpolarizability calculations, inter and intra molecular charge transfer, and biological activity of the molecule based on its electrophilicity index have been carried out. The 2-[(2-hydroxyphenyl) carbonyloxy] benzoic acid [hereafter referred as HPCBA] consists two phenyl rings linked with carbonyl group, one carboxyl and one hydroxyl group. There are eight possible conformers for this molecule. The optimized geometry and vibrational wavenumbers are calculated only for the most stable conformer at DFT/B3LYP level of theory using 6-31G(d,p) basis set. The C1 form is the more stable conformer than the others. The vibrational wavenumbers of the C1 dimer conformer of HPCBA have also been calculated. These calculations are valuable for providing an insight into the vibrational spectrum and molecular parameters. A detailed interpretation of the vibrational spectra of HPCBA has been made on the basis of the calculated amount of potential energy distribution (PED). The results of the theoretical and spectroscopic studies are reported here. The stable position of HPCBA with respect to phenyl ring is obtained by performing the potential energy surface (PES) scan with B3LYP/6-31G(d,p) level of theory. The redistribution of electron density (ED) in various bonding, anti-bonding orbitals and E(2) energies had been calculated by natural bond orbital (NBO) analysis to give clear evidence of stabilization originating from the hyper conjugation of various intra-molecular interactions. By analyzing the total (TDOS), partial (PDOS), and overlap population (OPDOS) density of states, the molecular orbital compositions and their contributions to the chemical bonding are studied. The UV spectroscopic studies along with HOMO, LUMO analysis have been used to elucidate information regarding charge transfer within the molecule. Besides these, the dipole moment, nonlinear optical properties (NLO) and first order hyperpolarizability have also been studied. Moreover, the Mulliken population analyses of the title compound have been calculated and the calculated results have been reported. The standard thermodynamic functions like

heat capacity, entropy and enthalpy are reported on the basis of vibrational analyses and statistical thermodynamics. The experimental and theoretical results supported each other, and the calculations are valuable for providing a reliable insight into the vibrational spectra and molecular properties. 2. Experimental details The compound under investigation namely HPCBA is purchased from Sigma–Aldrich chemicals, USA with a stated purity of 99%. The FT-IR spectrum of HPCBA is recorded in Bruker IFS 66 V spectrometer in the range of 4000–400 cm1. The spectrum is recorded at room temperature with a scanning speed of 30 cm1 min1 and the spectral resolution of ±2 cm1. The FT-Raman spectrum of HPCBA is also recorded in the range of 4000–100 cm1 using the same instrument with FRA 106 Raman module equipped with Nd:YAG laser source operating at 1.064 lm line widths with 200 mW power. The frequencies of all sharp bands are accurate to ±1 cm1. The ultraviolet absorption spectrum of HPCBA solved in water is examined in the range 200–400 nm by using Cary 5E UV–Vis NIR recording spectrometer. All the spectral measurements are carried out at Indian Institute of Technology, Chennai, India. 3. Computational details The optimized molecular structure of HPCBA (a non planar molecule), belongs to C1 point group symmetry. The geometrical parameters for the most stable conformer C1 of monomer and dimer structure of title molecule in the ground state were optimized at DFT/B3LYP level theory using the 6-31G(d,p) basis set. The vibrational wavenumbers of the compound dimer with H-bonding between carboxyl groups of C1 conformation have also been calculated to improve the simulation of the theoretical spectra. The optimized geometrical parameters, fundamental vibrational frequencies are calculated theoretically using GAUSSIAN 09W package on the personal computer [7]. The theoretical results have enabled us to make the detailed assignments of the experimental IR and Raman spectra of the molecules. Scaling of force field is performed according to SQM procedure [8] using selective scaling in the natural internal coordinate representation [9,10]. Transformations of the force field and the subsequent normal coordinate analysis including the least square refinement of the scaling factors, calculation of the potential energy distribution (PED) and the prediction of IR and Raman intensities are done on a PC with the MOLVIB program (Version V7.0-G77) written by Sundius [11,12]. The symmetries of the vibrational modes are determined by using the standard procedure [13] of decomposing the traces of the symmetry operation into the irreducible representations. By combining the result of the GAUSSVIEW program [14] with symmetry considerations, vibrational frequency assignments are made with a high degree of confidence. The electronic transitions, vertical excitation energies, absorbance and oscillator strengths are computed with the time-dependent DFT (TD-DFT) method and compared with UV–Vis spectra. The total density of states (TDOS or DOS) the partial density of states (PDOS) and overlap population density of states (OPDOS) spectra are prepared by using the program GaussSum 2.2 [15] and the functional group contributions to the molecular orbitals are calculated. 3.1. Prediction of Raman intensities The Raman scattering activities (Si) calculated with the help of GAUSSIAN 09W program are converted to relative Raman intensities (Ii) using Raint program [16] by the following relationship derived from the basic theory of Raman scattering [17].

S. Muthu, E. Isac Paulraj / Journal of Molecular Structure 1038 (2013) 145–162

Ii ¼

mi

h

f ðm0  mi Þ4 Si  i mi 1  exp hc kT

In the above formula m0 is the laser exciting frequency in cm1 (in this work, we have used the excitation wave number m0 = 9398.5 cm1, which corresponds to the wavelength of 1064 nm of a Nd:YAG laser), mi is the vibrational wave number of the ith normal mode (cm1) and Si is the Raman scattering activity of the normal mode mi, f (is the constant equal to 1012) is the suitably chosen common normalization factor for all peak intensities. h, k, c and T are Planck constant, Boltzmann constant, speed of light and temperature in Kelvin, respectively. The time dependent DFT (TD-DFT) is proved to be a powerful and effective computational tool for the study of ground and excited state properties by comparison to the available experimental data [y, z]. Hence, we used TD-CAM-B3LYP to obtain wavelengths kmax and compare with the experimental UV absorption spectra of HPCBA. 4. Results and discussion 4.1. Structural analyses The atomic numbering scheme of all the eight conformers and dimer structure of the title compound are shown in Figs. 1 and 2, respectively. The optimized geometrical parameters are calculated by B3LYP with 6-31G(d,p) basis set. The crystal structure of the title compound was taken from Cambridge Crystallographic Data Center (CCDC 117404) [18]. The results of calculated geometrical parameters (bond lengths, bond angles) are compared with the experimental X-ray diffraction data. Since the bond lengths and bond angles obtained by B3LYP/6-31G(d,p) basis set are closer to experimental values [18], are listed in Table 1. Taking into account that the molecular geometry in the vapor phase may be different from the solid base, owing to extended hydrogen bonding and staking interactions there is reasonable agreement between the calculated and experimental geometric parameters. By comparing the theoretical and the experimental bond lengths, the theoretical parameters are slightly deviates from the experimental data. It is well known that DFT method predicts bond lengths that are symmetrically too long, particularly the CAH bond lengths [19]. This theoretical pattern also found for HPCBA molecule. Since, the large deviation from the experimental CAH bond lengths may arise from the low scattering factors of hydrogen atoms in the X-ray diffraction experiment. This overestimation is also verified in our calculation and presented in Table 1. The calculated bond distances of C6AH10, C1AH7, C2AH8, C3AH9, C19AH22, C21AH25 and C20AH24

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are 0.950 Å shows the difference as 0.155 Å than the X-ray data [18]. The computed bond lengths and bond angles of dimer conformer are slightly differ from the X-ray data. The variations are due to the intermolecular interactions with the carboxylic acid group. In the present case, the main differences occur for the carboxylic acid group, as expected both the monomer and dimer structures, since these are involved in hydrogen bonding interactions with neighboring molecules. For example, the C11AO12 and C11AO13 bond lengths in the carboxylic acid group are 1.229 Å and 1.312 Å. The C@O and CAO bond distances are found to be at 1.214 Å and 1.357 Å, respectively in X-ray pattern [18]. The theoretically calculated bond distances are similar for carboxylic acid group which are approximately 0.045 Å longer for OH than X-ray data. The above results reflected the intermolecular hydrogen bonding interactions. The obtained CAC bond lengths of the ring fall at 1.39 Å while the calculated results change from 1.391 Å to 1.408 Å for the monomer structure. The interaction arises through two stable hydrogen-bonded O12  H43AO42 and O13AH14  O41 contacts, which result in increased stabilization. The intermolecular hydrogen bonds are almost linear their bond lengths are 1.973 Å, and 1.963 Å for O12  H43 and H14  O41, respectively. The calculated bond angle for O12  H43AO42 is 176.72° and for O13AH14  O41 is 178.7°. The OAH distances in the groups involved in the hydrogen bonds are lengthened by 0.016 Å upon dimerization. The lengthening of the C11@O12 bond of HPCBA (0.026 Å) upon dimerization is due to the redistribution of partial charges on the H43 atom, as the unpaired electron is significantly delocalization and therefore the CAO bond shows considerable double bond character typical of a carbonyl group. The optimized bond angles CACAC for the phenyl ring fall in the range from 119.4° to 121.3°. 4.2. Potential energy surface scan analysis PES scan studies are carried out on HPCBA molecule with the B3LYP/6-31G(d,p) level of theory to arrive the reliable conformation about dihedral angle between ring and COH atoms (C4AC5AC11AO13), dihedral angle between the two ring systems (C3AC4AO15AC16) and dihedral angle between ring and OH (C17AC18AO27AH28). During the calculation, all the geometrical parameters are simultaneously relaxed while the C4AC5AC11AO13, C3AC4AO15AC16 and C17AC18AO27AH28 torsional angles are varied steps of 10° up to 360°. For this rotation minimum energy curve has been obtained at 260° for C3AC4AO15AC16 and 20° for C4AC5AC11AO13, 170° for C17AC18AO27AH28 as shown in Fig. 3. Therefore, in the present work we have focused on the most stable form of HPCBA molecule to clarify molecular structure assignments of vibrational spectra.

Fig. 1. Possible conformers of HPCBA.

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Fig. 2. Dimer form of HPCBA.

4.3. Vibrational analysis Detailed description of vibrational modes can be given by means of normal coordinate analysis (NCA). The molecule HPCBA consists of 29 atoms, so it has 81 normal vibrational modes of vibrations. For this purpose, nonredundant set of local symmetry coordinates are constructed by suitable linear combinations of internal coordinates following the recommendation of Fogarasi et al. [9,10]. From these, the full set of 109 standard internal coordinates (containing 28 redundancies) is defined in Table 2. To obtain the normal modes in a molecular coordinate system, local symmetry coordinates for HPCBA were recommended by Fogarasi and Pulay [9] and were presented in Table 3. The output files of the quantum chemical calculations contain the force constant matrix in Cartesian coordinates and in Hartree/Bohr2 units. The force constants are transformed to the force fields in the local symmetry co-ordinates. The force constants are refined to the experimental frequencies through optimization of scale factors. The scaling factors are crucial for IR spectral predictions. To calculate optimal scaling factors, k, we employed a least-square procedure using the below equation

Pall k¼

i

xtheory texpt i i

Pall  i

xtheory i

2

where xtheory and texpt are the ith theoretical harmonic and ith i i experimental fundamental frequencies (in cm1) respectively. Only single (uniform) scaling factors were calculated, without discrimination for different vibrations. The value obtained is 0.9615 for B3LYP/6-31G(d,p) basis set and it is very close to the recommended scaling factor of 0.961. The root mean square (RMS) value is obtained in the study using the following expression:

rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 1 Xn  calc RMS ¼ ti  texp i i n1 The RMS error between unscaled (B3LYP/6-31G(d,p)) and experimental frequencies are found to be 112 cm1 and 102 cm1 for FTIR and FT-Raman respectively. This is quite obvious since the frequencies calculated on the basis of quantum mechanical force fields usually differ appreciably from observed frequencies. This is partly due to the neglect of anharmonicity and partly

due to the approximate nature of the quantum mechanical methods. In order to reproduce the observed frequencies, refinement of scaling factors are applied and optimized via least square refinement algorithm which resulted in a weighted RMS deviation of 28.3 cm1 between the experimental and scaled frequencies. The scaled frequencies together with the definition of internal coordinates are listed in Table 4. For visual comparison, the observed and stimulated FT-IR and FT-Raman spectra of the title compound are presented in Figs. 4 and 5 respectively, which help to understand the observed spectral features. From the PED calculation it was clear that many vibrations have a high degree of mixing with other modes. Therefore, we provide the following tentative assignments for almost all intense spectral features in the vibrational spectra of the molecule. 4.3.1. OAH vibrations The OAH group gives rise to the three vibrations, viz. stretching, in-plane bending and out-of-plane bending vibrations. The OAH stretching vibrations are sensitive to hydrogen bonding [20]. The non-hydrogen-bonded hydroxyl group of phenols absorbs strongly in the 3700–3584 cm1 region [21]. Hydrogen bonding alters the frequencies of the stretching and bending vibration. The OAH stretching bands move to lower frequencies usually with increased intensity and band broadening in the hydrogen bonded species. Hydrogen bonding if present in five or six member ring system would reduce the OAH stretching band to 3200–3550 cm1 regions [22]. In the FT-Raman spectrum of title compound, a weak band observed at 3651 and 3479 cm1 are assigned to OAH stretching mode of vibration. The PED contribution of these modes is 100%. A comparison of these bands with literature data predict that there is a deviation which may be due to the fact that the presence of intramolecular hydrogen bonding in monomer and intermolecular hydrogen bonding in dimer. In general, for phenols the in-plane bending vibrations lay in the region 1150–1250 cm1 [23]. In our present study, the band observed in FT-IR at 1084 cm1and in FT-Raman at 1127, 1101 and 1060 cm1 are assigned to OAH in-plane bending vibration. The theoretically computed value at 1168, 1103 and 1091 cm1 for C1 conformer by BLYP/6-31G(d,p) method and at 1157, 1101 and 1080 cm1 for dimer by B3LYP/6-31G(d,p) method show good agreement with experimental observation. The position of band due OAH out-ofplane deformation vibration is dependent on the strength at hydro-

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S. Muthu, E. Isac Paulraj / Journal of Molecular Structure 1038 (2013) 145–162 Table 1 Optimized geometrical parameters of HPCBA at HF and B3LYP/6-31G(d,p) level. Bond lengtha

X-rayb (Å)

C1AC2 1.386 C1AC6 1.377 C1AH7 0.950 C2AC3 1.382 C2AH8 0.949 C3AC4 1.375 C3AH9 0.950 C4AC5 1.396 C4AO15 1.406 C5AC6 1.399 C5AC11 1.481 0.950 C6AH10 C11AO12 1.229 C11AO13 1.312 O13AH14 0.927 O15AC16 1.357 C16AC17 1.470 1.214 C16AO29 C17AC18 1.384 C17AC19 1.396 C18AC20 1.397 C18AO27 1.349 C19AC21 1.386 C19AH22 0.950 C20AC23 1.368 C20AH24 0.950 C21AC23 1.382 C21AH25 0.950 C23AH26 0.950 O27AH28 0.897 Intermolecular H bond lengths and angles O12  H43 H14  O41 O12  H43AO42 O13AH14  O41

Dihedral angle (°) C4AC3AC2AC1 C5AC4AC3AC2 C6AC1AC2AC3 H7AC1AC6AC5 H8AC2AC1AC6 H9AC3AC2AC1 H10AC6AC1AC2 C11AC5AC4AC3 O12AC11AC5AC4 O13AC11AC5AC4 H14AO13AC11AC5 O15AC4AC3AC2 C16AO15AC4AC3 a b

Calculated value (Å) C1

Dimer

1.397 1.391 1.100 1.392 1.101 1.403 1.101 1.408 1.386 1.404 1.465 1.103 1.238 1.365 0.972 1.389 1.468 1.227 1.410 1.401 1.406 1.370 1.392 1.105 1.390 1.099 1.397 1.100 1.101 0.970

1.397, 1.390, 1.100, 1.391, 1.101, 1.403, 1.101, 1.411, 1.386, 1.404, 1.468, 1.103, 1.244, 1.345, 0.988, 1.389, 1.470, 1.227, 1.410, 1.402, 1.405, 1.371, 1.392, 1.105, 1.390, 1.099, 1.396, 1.100, 1.101, 0.970,

1.396 1.390 1.101 1.391 1.102 1.404 1.102 1.410 1.387 1.405 1.468 1.102 1.244 1.346 0.987 1.390 1.469 1.228 1.411 1.402 1.405 1.371 1.393 1.106 1.391 1.099 1.396 1.101 1.102 0.971

1.973 1.963 176.7 178.7

0.5 0.2 0.4 179.7 180.0 179.5 179.9 178.9 172.2 9.2 179.0 177.0 108.1

0.5, 0.1, 0.3, 179.5, 180.0, 179.5, 179.9, 178.7, 171.8, 9.3, 178.9, 175.1, 61.8,

0.5 0.1 11.1 171.5 179.7 179.5 2.8 178.7 171.8 9.4 178.9 175.1 61.8

Bond anglea

X-rayb (Å)

Calculated value (Å) C1

Dimer

C2AC1AC6 C2AC1AH7 C1AC2AC3 C1AC2AH8 C6AC1AH7 C1AC6AC5 C1AC6AH10 C3AC2AH8 C2AC3AC4 C2AC3AH9 C4AC3AH9 C3AC4AC5 C3AC4AO15 C5AC4AO15 C4AC5AC6 C4AC5AC11 C4AO15AC16 C6AC5AC11 C5AC6AH10 C5AC11AO12 C5AC11AO13 O12AC11AO13 C11AO13AH14 O15AC16AC17 O15AC16AO29 C17AC16AO29 C16AC17AC18 C16AC17AC19 C18AC17AC19 C17AC18AC20 C17AC18AO27 C17AC19AC21 C17AC19AH22 C20AC18AO27 C18AC20AC23 C18AC20AH24 C18AO27AH28 C21AC19AH22 C19AC21AC23 C19AC21AH25 C23AC20AH24 C20AC23AC21 C20AC23AH26 C23AC21AH25 C21AC23AH26

119.6 120.2 120.3 119.8 120.1 121.1 119.5 119.8 119.7 120.0 120.1 121.3 117.8 120.7 117.8 122.3 115.9 119.8 119.3 123.3 113.6 123.0 106.0 113.2 122.1 124.6 118.4 122.7 118.8 120.4 119.5 120.5 119.7 119.9 119.6 120.1 107.3 112.4 119.4 120.7 120.2 121.0 119.4 120.2 119.1

120.1 120.1 120.4 120.0 119.8 120.6 120.7 119.6 119.5 120.9 119.6 120.7 121.8 117.2 118.7 123.8 120.0 117.5 118.7 127.6 116.7 115.7 108.7 111.4 118.6 130.0 120.7 120.3 119.0 120.3 117.5 120.5 118.6 122.2 119.6 119.8 108.2 120.9 120.1 119.9 120.6 120.4 119.6 120.0 119.9

120.1, 120.0, 120.4, 120.0, 119.9, 120.7, 120.7, 119.6, 119.6, 120.9, 119.5, 120.6, 121.3, 117.9, 118.6, 124.0, 119.5, 117.4, 118.6, 125.6, 117.3, 117.0, 110.1, 111.9, 118.1, 130.0, 120.4, 120.6, 118.9, 120.4, 117.5, 120.5, 118.9, 122.1, 119.6, 119.8, 108.1, 120.6, 120.1, 119.8, 120.6, 120.4, 119.6, 120.1, 120.0,

174.7 162.6 18.8 178.7 178.4 0.9 0.3 179.7 179.6 179.8 2.2 178.4 5.9

177.0, 162.2, 19.0, 178.9, 178.5, 0.3, 0.4, 179.6, 179.4, 179.7, 2.2, 178.8, 2.9,

C17AC16AO15AC4 C18AC17AC16AO15 C19AC17AC16AO15 C20AC18AC17AC16 C21AC19AC17AC16 H22AC19AC17AC16 C23AC20AC18AC17 H24AC20AC18AC17 H25AC21AC19AC17 H26AC23AC20AC18 O27AC18AC17AC16 H28AO27AC18AC17 O29AC16AO15AC4

120.2 120.1 120.3 120.1 119.9 120.7 120.8 119.6 119.6 121.0 119.5 120.6 121.5 117.7 118.6 123.9 119.8 117.4 118.6 127.7 113.5 116.9 110.0 111.8 118.7 130.0 120.3 120.5 119.0 120.4 117.6 120.4 118.9 122.1 119.6 119.8 108.1 120.7 120.1 119.8 120.6 120.4 119.5 120.0 120.0 177.0 162.3 19.0 178.9 178.5 0.2 0.4 179.6 179.4 179.7 2.2 178.8 2.9

For numbering of atoms refer Fig. 1. See Ref. [18].

gen bond [24]. The OAH out-of-plane deformation vibration of phenol lies in the region 290–320 cm1 for free OAH and in the region 517–710 cm1 for associated OAH [25]. The frequency increase with hydrogen bond strength because of the larger amount of energy required to twist the OAH bond out-of-plane [26]. In our case the band observed in Raman spectrum at 331and 303 cm1 are assigned as OAH out-of-plane deformation.

4.3.2. CAH vibrations In aromatic compounds, the CAH stretching wavenumbers appear in the range 3000–3100 cm1 which is the characteristic region for the ready identification of CAH stretching vibrations [27]. In this region, the bands are not affected appreciably by the nature of substituent. The CAH stretching modes usually appear with strong Raman intensity and are highly polarized.

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Fig. 3. PES scan for dihedral angle of HPCBA at B3LYP/6-31G(d,p) method.

Accordingly, in the present study, the four adjacent hydrogen atoms left around the ring one and ring two of HPCBA give rise to eight CAH stretching vibrations (mode nos. 79–72). In the FTIR and FT-Raman the bands observed from 3478 cm1 to 2993 cm1 and from 3435 cm1 to 3003 cm1 respectively are assigned to the CAH stretching vibrations of HPCBA. As expected, these eight modes are pure stretching modes as is evident from PED column; they almost contribute 99%. The in-plane

CAH bending vibrations normally appear in the range 1000– 1300 cm1 in the substituted benzenes and the out-of-plane bending vibrations occur in the region 750–1000 cm1 region [28,29]. In this study, the CAH in-plane-bending vibrations are assigned from 1319 cm1 to 1017 cm1 in FT-IR and from 1288 cm1 to 1013 cm1 in FT-Raman spectrum. The CAH outof-plane bending vibrations are observed at 966, 952, 888, 854 and 760 cm1 in FT-Raman spectrum.

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151

Table 2 Definition of internal coordinates of HPCBA. No.(i)

Symbol

Type

Definitiona

Stretching 1–14 15–22 23–28 29–30

Ri Ri Ri Ri

CAC CAH CAO OAH

C1AC2, C2AC3, C3AC4, C4AC5, C5AC6, C6AC1, C5AC11, C16AC17, C17AC18, C18AC20, C20AC23, C23AC21, C21AC19, C19AC17 C1AH7, C2AH8, C3AH9, C6AH10, C19AH22, C21AH25, C23AH26, C20AH24 C11AO12, C11AO13, C4AO15, C16AO15, C16AO29, C18AO27 O13AH14, O27AH28

In-plane bending 31–36 ci 37–42 ci 43–58 ci

Ring1 Ring2 CACAH

ci ci ci ci ci

CAC–O OACAO CACAC HAOAC CAOAC

C1AC2AC3, C2AC3AC4, C3AC4AC5, C4AC5AC6, C5AC6AC1, C6AC1AC2 C17AC18AC20, C18AC20AC23, C20AC23AC21, C23AC21AC19, C21AC19AC17, C19AC17AC18 H10AC6AC5, H10AC6AC1, H7AC1AC6, H7AC1AC2, H8AC2AC1, H8AC2AC3, H9AC3AC2, H9AC3AC4, H24AC20AC18, H24AC20AC23, H26AC23AC20, H26AC23AC21, H25AC21AC23, H25AC27AC19, H22AC19AC21, H22AC19AC17 O15AC4AC5, O15AC4AC3, O27AC18AC17, O27AC18AC20, O13AC11AC5, O12AC11AC5, O15AC16AC17, O29AC16AC17 O12AC11AO13, O15AC16AO29 C11AC5AC4, C11AC5AC6, C16AC17AC18, C16AC17AC19 H14AO13AC11, H28AO27AC18 C4AO15AC16

59–66 67–68 69–72 73–74 75

Out-of-plane bending 76–83 qi

a

84–85 86–87 88–89

qi qi qi

OACAC CACAC OACAO

H10AC6AC1AC5, H7AC1AC2AC6, H8AC2AC3AC1, H9AC3AC4AC2, H22AC19AC17AC21, H25AC21AC19AC23, H26AC23AC21AC20, H24AC20AC23AC18 O15AC4AC3AC5, O27AC18AC17AC20 C11AC5AC4AC6, C16AC17AC18AC19 C5AC11AO13AO12, C17AC16AO15AO29

Torsion 90–95 96–101 102–103 104–107 108 109

si si si si si si

t Ring1 t Ring2 CAOAC CO2 CACAO HAOAC

C1AC2AC3AC4, C2AC3AC4AC5, C3AC4AC5AC6, C4AC5AC6AC1, C5AC6AC1AC2, C6AC1AC2AC3 C17AC18AC20AC23, C18AC20AC23AC21, C20AC23AC21AC19, C23AC21AC19AC17, C21AC19AC17AC18, C19AC17AC18AC20 C4AO15AC16AC17, C4AO15AC16AO29 O13AC11AC5AC4, O12AC11AC5AC4, O29AC16AC17AC18, O15AC16AC17AC19 C5AC4AO15AC16 H28AO27AC18AC17

HACAC

For numbering of atom refer Fig. 1.

4.3.3. CAO vibration The absorption is sensitive for both the carbon and oxygen atoms of the carbonyl group. Both have the same while it vibrates. Normally, the CAO stretching vibrations occur in the region 1260– 1000 cm1 [30]. In the present study, the CAO stretching vibration is assigned at 1263, 1244, 1168, 1138 and 1091 cm1 for the C1 conformer of the title molecule at B3LYP/6-31G(d,p) level. According to the literature, the CAO vibration is pushed to the lower region by the influence of other vibrations, because of the proximity. In HPCBA, the CAO in-plane bending vibration is found at 842 cm1 and 798 cm1 at B3LYP/6-31G(d,p) level, which is found mixed with the OAH deformation mode. A medium band is found at 432 and 422 cm1 for CAO out-of-plane bending vibration for a title molecule and it is also observed at 404 and 399 cm1 in FT-Raman spectrum. These assignments are agreement with the literature value [31]. The band appearing at 567 cm1 in infrared and 552 cm1 in Raman spectra was assigned to the rocking mode of the COO group in HPCBA. The assignments of the COO group vibrations are closely agree with the literature values. [32,33]. The wagging, COO mode is calculated as 752 cm1 by B3LYP/631G(d,p) method whereas the COO wagging is localized at 724 cm1 as a very weak band in the infrared spectra. 4.3.4. Phenyl ring vibrations The benzene ring possesses six stretching vibrations. The carbon–carbon stretching modes of the phenyl group are expected in the range from 1650 to 1200 cm1 [34]. The actual position of these mode are determined not so much by the nature of the substituents but by the form of substitution around the ring [35], although heavy halogens cause diminish the frequency [36]. In HPCBA the bands appeared at 1686, 1646 and 1623 cm1 in the infrared spectrum and the bands appeared at 1752, 1733 and 1676 cm1 in Raman spectrum is assigned to CAC stretching vibrations. These are all considered to be absolute modes according to the normal coordinate analysis. These assignments are confirmed

by the theoretical observation and PED calculation. The bands occurring at 665 and 567 cm1 in the infrared and at 701, 659 and 552 cm1 in Raman is assigned to the CCC in-plane bending modes of the title molecule. The CCC in-plane bending vibrations of HPCBA conformer C1 have been determined from the B3LYP/631G(d,p) method is 722, 688 and 590 cm1. The CCC out of plane bending modes of HPCBA is attributed to the Raman wavenumbers are 498, 404, 399, 295 and 253 cm1. All these assignments are agreed well with the reported literature. The CCC in-plane bending and out of plane vibrations are described as mixed modes as there are about 12–50% PED contributions mainly from C to H in-plane bending and out of plane bending vibrations, respectively. The animated vibrational modes (movies) of OH-stretching, CH-stretching, CH-out-of-plane bending and CO2 twisting are available online in Supplementary data (Fig. S1, Fig. S2, Fig. S3 and Fig. S4 respectively). 4.4. UV–Vis spectra analysis Ultraviolet spectra analyses of HPCBA have been investigated by experimental and theoretical calculation. Time-dependent density functional theory (TDDFT) [37] has recently emerged as a powerful tool for investigating the static and dynamic properties of the molecules in their excited states, allowing for the best compromise between accuracy and computational cost [38]. Therefore, in this study CAM-B3LYP [39] hybrid functional including long range corrected hybrids are used. CAM-B3LYP is a long-range corrected functional that uses a Coulomb attenuating method to combine the hybrid B3LYP method with a long-range correction by introducing two extra parameters, instead of the single parameter used by Tian [40]. The experimental spectra have been observed in the ethanol solvent. Therefore, a combination of the solvent model and the TDDFT method is required for the computation of the absorption spectra, to compare with the experimental results. Polarizable continuum models (PCMs) [41] have

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Table 3 Definition of local symmetry coordinates of HPCBA. No.(i)

Symbola

Definitionb

1–14 15–22 23–24 25–26 27–28 29–30 31 32 33 34 35 36 37–44 45–46 47–48 49–50 51–52 53–54 55 56–63 64–65 66–67 68–69 70 71 72 73 74 75 76–77 78–79 80 81

m CC m CH m (CO2ss) m (CO2as) m CO m OH

R1, R2, R3, R4, R5, R6, R7, R8, R9, R10, R11, R12, R13, R14 R15, R16, R17, R18, R19, R20, R21, R22 p p (R23 + R24)/ 2, (R26 + R27)/ 2 p p (R23  R24)/ 2, (R26  R27)/ 2 R25, R28 R29, R30 p (c31  c32 + c33  c34 + c35  c36)/ 6 p (c31  c32 + 2c33  c34  c35 + 2c36)/ 12 (c31  c32 + c34  c35)/2 p (c37  c38 + c39  c40 + c41  c42)/ 6 p (c37  c38 + 2c39  c40  c41 + 2c42)/ 12 (c37  c38 + c40  c41)/2 p p p p p p p p (c43  c44)/ 2, (c45  c46)/ 2, (c47  c48)/ 2, (c49  c50)/ 2, (c51  c52)/ 2, (c53  c54)/ 2, (c55  c56)/ 2, (c57  c58)/ 2 p p (c59  c60)/ 2, (c61  c62)/ 2 p p (c63  c64)/ 2, (c65  c66)/ 2 p p (2c67  c63  c64)/ 6, (2c68  c65  c66)/ 6 p p (c69  c70)/ 2, (c71  c72)/ 2 c73, c74 c75 q76, q77, q78, q79, q80, q81, q82, q83 q84, q85 q86, q87 q88, q89 p (s90  s91 + s92  s93 + s94  s95)/ 6 p (s90  s91 + 2s92  s93  s94 + 2s95)/ 12 (s90  s91 + s93  s94)/2 p (s96  s97 + s98  s99 + s100  s101)/ 6 p (s96  s97 + 2s98  s99  s100 + 2s101)/ 12 (s96  s97 + s99  s100)/2 s102, s103 (s104 + s105)/2, (s106 + s107)/2 s108 s109

trig1 asym1 sym1 trig2 asym2 sym2 CCH CCO CO2rock CO2sci CCC HOC COC x HCC x OCC x CCC x CO2wag s trig1 s asym1 s sym1 s trig2 s asym2 s sym2 s COC s CO2 s OCC s HOC b b b b b b b b b b b b b

Abbreviations: m, stretching; b, in plane bending; x, out of plane bending; s, torsion. a These symbols are used for description of the normal modes by PED in Table 4. b The internal coordinates used here are defined in table given in Table 2.

emerged in the last two decades as the most effective tools to treat bulk solvent effects for both the ground and excited states. In this work, the linear response (LR)-PCM, where random phase approximation excitation energies are computed directly, has been used [42]. Geometries of the ground states (S0) have been optimized and vertical excited energies, relevant to the absorption spectra for the molecules of interest, have been calculated at the optimized S0 geometries. Subsequently, calculations involving geometrical optimization of the lowest excited state (S1) have been performed, simultaneously yielding information relevant to the emission spectra. The 6-31++G(d,p) basis set [43] is used for all of the calculations, and all the computations have been performed using the GAUSSIAN 09W program [7]. The calculated frontier orbital energies, absorption wavelengths (k), oscillator strengths (f) and excitation energies (E) for gas phase and ethanol solvent are illustrated in Table 5. The major contributions of the transitions are designated with the aid of Swizard program [44]. Calculations of the molecular orbital geometry show that the visible absorption maxima of this molecule correspond to the electron transition between frontier orbitals such as translation from HOMO to LUMO. The experimental and theoretical UV vibrationally resolved spectra of the HPCBA are shown in Fig. 6. Experimentally determined maximum absorption values in ethanol are 40,538, 42,550, 44,014 cm1 (246, 235, 227 nm), and theoretically calculated values have been found to be 38,409, 40,397, 44,280 cm1 (260, 247, 226 nm). As can be seen, all calculations performed are small deviation. The deviations of the computed results from the experimental results are larger than 14 nm. In view of calculated absorption spectra, the maximum absorption wavelength corresponds to the electronic transition

from HOMO to LUMO with 20% contribution. This transition is predicted as p–p. The other wavelength, excitation energies, oscillator strength, calculated counterparts with major contributions and assignments can be seen in Table 5. Molecular orbitals; both the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) and their properties such as energy are very useful for physicists and chemists are the main orbital taking part in chemical reaction. While the energy of the HOMO is directly related to the ionization potential, LUMO energy is directly related to the electron affinity [45,46]. This is also used by the frontier electron density for predicting the most reactive position in p-electron systems and also explains several types of reaction in conjugated system [47]. The conjugated molecules are characterized by a small highest occupied molecular orbital–lowest unoccupied molecular orbital (HOMO–LUMO) separation, which is the result of a significant degree of intramolecular charge transfer from the end-capping electron-donor groups to the efficient electron-acceptor group through-p-conjugated path [48]. Surfaces for the frontier orbitals were drawn to understand the bonding scheme of present compound. The energy difference between HOMO and LUMO orbital which is called as energy gap is a critical parameter in determining molecular electrical transport properties because it is a measure of electron conductivity, calculated 5.0086 eV for the title molecule. The plots of MOs (HOMO and LUMO) are drawn and given in Fig. 7. All the HOMO and LUMO have nodes. The nodes in each HOMO and LUMO are placed symmetrically. The positive phase is red and the negative one is green. According to Fig. 7, the HOMO a charge density localized over the ring of the entire molecule, but the LUMO is characterized by a charge distribution on all struc-

Table 4 Comparison of the calculated and experimental (FT-IR and FT-Raman) vibrational spectra for C1 conformers of HPCBA by B3LYP/6-31G(d,p) basis set. Mode No.

Experimental

Theoretical monomer C1

Wavenumber (cm

)

Wavenumber (cm

1

)

FTIR

FTRaman

Unscaled

Scaled

3640s 3599vw 3478m 3368m 3225m – – 3188vw 3078m 2993m 2251s 2185vs 1796s 1686w 1646s 1623vw 1616vw 1590vw 1565m 1506vs 1444vs 1407s 1348vs 1319vs 1269s – – 1175vw – 1139m – – – 1084w 1040m 1017w – 981m 970vw 944vw 918vw 904vw 896vw 882vs 801s – 764m 738m 724vw – – –

3651w 3479vw 3435vw 3389s 3372w 3266vw 3094vw 3076vw 3043vw 3003w 2220vs 2131vs 1934vs 1752vs 1733vw 1676w 1637vs 1577vs 1533vw 1520s 1481vw 1445vw 1382vw 1348vs 1288vw 1249vw 1208vw 1171s 1127vw 1122vw 1115vw 1122vw 1101s 1060vs 1034m 1013m 1009vw 989s 966s – 952vw – 888m 854w 836m 821vw 760m 747vw – 722vw 714s 701vw

3809 3755 3240 3227 3219 3210 3207 3193 3191 3166 1838 1811 1662 1661 1643 1629 1545 1524 1495 1493 1395 1380 1356 1343 1314 1294 1269 1235 1215 1202 1191 1184 1147 1135 1112 1075 1066 1041 1001 984 979 959 908 876 858 850 830 804 782 764 760 751

3662 3610 3115 3102 3094 3086 3083 3070 3068 3043 1767 1741 1598 1596 1580 1566 1485 1465 1437 1435 1341 1327 1303 1291 1263 1244 1220 1187 1168 1155 1145 1138 1103 1091 1069 1034 1025 1001 962 946 941 922 873 842 825 817 798 773 752 735 731 722

Assignments (PED)a,d

Theoretical dimer IIR

b

0.080 0.111 0.003 0.009 0.009 0.027 0.025 0.016 0.006 0.022 0.432 0.523 0.053 0.145 0.071 0.014 0.031 0.056 0.144 0.096 0.234 0.048 0.025 0.158 0.027 0.037 0.469 1.000 0.131 0.055 0.055 0.022 0.327 0.053 0.031 0.006 0.090 0.447 0.001 0.000 0.003 0.001 0.012 0.021 0.017 0.019 0.004 0.001 0.006 0.093 0.060 0.034

IRaman

0.59 0.45 0.60 1.11 0.69 1.26 0.97 0.79 0.50 0.70 2.38 1.10 1.53 2.51 1.05 0.36 0.54 0.17 0.03 0.12 0.46 0.26 0.36 0.51 0.02 0.04 2.31 4.73 2.82 0.43 0.72 1.15 0.08 0.23 0.11 2.20 1.41 0.50 0.01 0.00 0.07 0.09 0.44 0.48 0.54 0.62 1.48 0.06 0.17 0.36 0.51 1.52

c

1

Wavenumber (cm

)

Unscaled

Scaled

3662 3662 3121 3121 3111 3111 3105 3105 3088 3088 3084 3084 3070 3070 3069 3069 3043 3043 2976 2864 1761 1760 1679 1628 1597 1597 1595 1594 1580 1579 1566 1566 1486 1486 1478 1468 1465 1447 1435 1435 1427 1416 1328 1328 1307 1306 1305 1301 1293 1293 1263 1263 1242 1241 1220 1220 1191 1188 1157 1157 1145 1145 1144 1143 1128 1126 1101 1101 1080 1079 1035 1035 1030 1030 1003 1003 977 960 960 951 951 938 938 932 927 927 864 864 844 844 824 823 813 813 808 805 772 771 769 757 753 752 736 736 730 730

3521 3521 3001 3001 2991 2991 2985 2985 2969 2969 2965 2965 2951 2951 2951 2951 2926 2926 2861 2753 1693 1692 1614 1565 1535 1535 1533 1532 1519 1518 1506 1506 1429 1429 1421 1411 1408 1391 1380 1380 1372 1361 1277 1277 1257 1256 1255 1251 1243 1243 1214 1214 1194 1193 1173 1173 1145 1142 1112 1112 1101 1101 1100 1099 1084 1083 1059 1059 1038 1037 995 995 990 990 964 964 939 923 923 914 914 902 902 896 891 891 831 831 811 811 792 791 782 782 777 774 742 741 739 728 724 723 708 708

mOH (100) mOH (100) mCH (98) mCH (98) mCH (98) mCH (99) mCH (99) mCH (99) mCH (99) mCH (99) mCO2as (56), mCO2ss (18), mCC (12) bCOC (40), bCOC (15), mCC (18), bCCC (16) bCCH (32), mCC (26), bCO2roc (12) mCC (42), bCCO (23), bCCH (10) mCC (54), bCCH (13), bCCO (12) mCC (70), bCCH (13) mCC (40), bCCO (18), bCCC (18), bR1asy (10) bCCH (39), mCC (43) bCCO (15), mCC (35), bCCH (23) bCCH (36), mCC (45), mCO (12) bCCC (24), mCC (27), bCCH (17), mCO2as (22), mCC (21), bCCH (14), bHOC (10) mCC (59), bCCO (11), bCCH (11) mCC (49), bCCC (17), bCCH (11), bCCO (10) bCCH (23), mCC (21), mCO (11) mCC (27), bCCO (24), bCO2roc (12), mCO (10) mCC (17), mCO (27), bCCH (19), bCCO (17) bCCO (31), bCCH (19), bCCC (14), mCC (14) mCO (38), mCC (22), bCCH (13), bHOC (11) bCCH (71), mCC (14) bCCH (66), mCO2as (13), mCC (15) mCC (25), bCCH (29), mCO (21) bCCH (20), mCC (25), bCCO (20), bCCC (11) mCC (32), bHOC (23), mCO (10) mCC (21), bR1trig (19), bCCC (14), bCCO (13), bCCH (12) bCCO (27), mCC (27), bR1trig (12), bCCH (10), bHOC (11) mCC (62), bCCH (18), bHOC (11) mCC (16), bCO2sci (20), bCO2roc (18), xHCC (17), bR2trig (15) xHCC (55), sR1trig (25) xHCC (38), bHOC (20) xHCC (73), sR2trig (13) xHCC (46), bHOC (17), bCCC (15) xHCC (61), xCCC (18), sR2sym (10) xHCC (19), bCOC (27), sR1trig (19), xCCC (16), sR1asy (15) mCO (14), bR1trig (18), bR2trig (15), bR1asy (12), mCC (11) xCCC (36), xHCC (27), sCOC (10), bCOC (11) xHCC (10), bCOC (33), bR1asy (13), mCC (13), mCO (12), bR1trig (11) bCOC (14), xHCC (22), xOCC (29), mCC (17) xCCC (36), sR1trig (21), xCO2wag (14), xOCC (13) xCCC (34), sCOC (23), xCO2wag (15), mCO2as (16), xCCC (31), xHCC (21), sR2trig (10) sCOC (11), sR1trig (14), bCCO (14), bCCC (11), mCC (12)

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81 80 79 78 77 76 75 74 73 72 71 70 69 68 67 66 65 64 63 62 61 60 59 58 57 56 55 54 53 52 51 50 49 48 47 46 45 44 43 42 41 40 39 38 37 36 35 34 33 32 31 30

1

154

Table 4 (continued) Mode No.

Experimental

Theoretical monomer C1

Wavenumber (cm

1

)

Wavenumber (cm

1

)

FTRaman

Unscaled

Scaled

29 28 27 26 25 24 23 22 21 20 19 18 17 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1

– 665w 617w 606vw 567s 547vw – – 514vw – 459vw – 412vw – – – – – – – – – – – – – – – –

667vw 659vw 648vw 604m 552w 541vw 530vw 524vw 509vw 498vw 451w 404vw 399vw 375vw 331vw – 303vw 295s 279vw 221vw 204s 178vw 170vs 113s – – – – –

716 695 675 636 614 606 574 560 537 521 480 449 439 404 398 391 361 296 253 245 221 178 129 122 90 59 37 33 15

689 668 649 612 590 582 551 539 516 501 461 432 422 389 382 375 347 285 244 236 213 171 124 117 86 57 35 32 14

Mean absolute deviation SD RMS r

53.17 84.60

46.88 71.89

IIR

0.059 0.016 0.034 0.050 0.074 0.011 0.004 0.006 0.008 0.003 0.021 0.010 0.010 0.047 0.121 0.009 0.001 0.004 0.001 0.002 0.004 0.001 0.001 0.000 0.004 0.001 0.001 0.003 0.001

IRaman

c

0.73 0.11 0.42 0.42 0.86 0.36 1.40 0.26 0.13 0.37 0.91 0.05 0.18 0.60 0.55 0.78 0.56 0.35 1.08 0.37 0.50 1.12 4.17 3.28 1.06 9.50 1.08 13.57 100.00

1

Wavenumber (cm

)

Unscaled

Scaled

679 677 666 666 654 653 642 636 583 583 559 553 549 540 518 518 511 508 469 467 434 433 428 427 406 402 394 386 384 379 366 355 290 288 279 274 241 241 205 203 187 184 152 145 121 118 109 97 79 72 64 59 55 38 37 33 25 17

702 702 653 651 640 640 629 628 617 611 560 560 537 532 528 519 498 498 491 488 451 449 417 416 411 411 390 386 379 371 369 364 352 341 279 277 268 263 232 232 197 195 180 177 146 139 116 113 105 93 76 69 62 57 53 37 36 32

sR1trig (24), sCOC (12), bCOC (11), sR2trig (11) sR2trig (23), sCOC (18), bCCC (11), xCO2wag (12), xOCC (10) sR1trig (35), bR2asy (14), mCC (10) bCOC (25), bCCO (12), sR1trig (16), xHCC (11), bR1sym (16) bCCC (23), bCCO (27), bR1sym (11), sR1trig (10) bR1asy (55), bCOC (7), bCO2roc (12) bR2sym (26), bCOC (22), mCC (10), xHCC (11) sR2asy (27), sR2trig (14), xCCC (14), xOCC (11) sR1asy (33), xOCC (24), xCCC (27), sCOC (20), sR1asy (15), bCCO (10) bCCO (58), sCOC (29) xCCC (36), sCOC (17), sR1asy (13), bCCO (11) xCCC (41), sCOC (11), bCCO (12) bCCO (57), xCCC (11), sCOC (10) sHOC (11), bCCO (26), bCOC (25), xCCC (12) bCCO (42), sCOC (26), bR1sym (10) bCCO (23), sCOC (24), bCO2roc (13), bHOC (10) xCCC (40), sCOC (10), sR2sym (12), bCOC (10), bCO2roc (15) xCCC (45), sCOC (19), bCCO (15) bCCC (31), sCOC (36), xCCC (13), xCCC (28), sCOC (28), bCCC (26) sCOC (27), bCCC (31), sCO2 (27) sCOC (48), sR1asy (17), xCCC (47), sR2asy (10), sCO2 (26), xCCC (23), sR1asy (21) sOCC (41), sCOC (36), sCO2 (17) sOCC (27), sCO2 (32) sCOC (36), bCCC (31), sOCC(11) sHOC (11), sR1asy (12), xCCC (40)

112.40 0.9929

a Abbreviations: m, stretching; b, in plane bending; x, out of plane bending; s, torsion; ip, in plane; op, out of plane; ss, symmetric stretching; as, asymmetric stretching; trig, trigonal deformation; wa, wagging; ro, rocking; sc, scissoring; tw, twisting; sym, symmetric deformation; asym, asymmetric deformation; vs very strong; s, strong; m, medium; w, weak; vw, very weak. b Relative absorption intensities normalized with highest peak absorption equal to 1. c Relative Raman intensities calculated by Eq. (1) and normalized to 100. d Only PED contributions P10% are listed.

S. Muthu, E. Isac Paulraj / Journal of Molecular Structure 1038 (2013) 145–162

FTIR

Assignments (PED)a,d

Theoretical dimer b

S. Muthu, E. Isac Paulraj / Journal of Molecular Structure 1038 (2013) 145–162

155

4.5. Total, partial, and overlap population density-of-states

Fig. 4. Experimental and theoretical FT-IR spectra of HPCBA.

Consideration of only the HOMO and LUMO may not yield a realistic description of the frontier orbitals, because in the boundary region, neighboring orbitals may show quasi degenerate energy levels. For this reason, the total (TDOS), partial (PDOS), and overlap population (OPDOS or COOP (Crystal Orbital Overlap Population)) density of states [49–51], in terms of Mulliken population analysis are calculated and created by convoluting the molecular orbital information with Gaussian curves of unit height and full width at half maximum (FWHM) of 0.3 eV by using the GaussSum 2.2 program [15]. Figs. 8–10 represent the TDOS, PDOS and OPDOS plot of HPCBA, respectively. The most important application of the DOS plots is to demonstrate MO compositions and their contributions to the chemical bonding through the OPDOS plots, which are also referred in the literature as COOP diagrams. The bonding, antibonding and nonbonding natures of the interaction of the two orbitals, atoms or groups are shown by OPDOS diagram. A positive value of the OPDOS indicates a bonding interaction (because of the positive overlap population), negative value means that there is an anti-bonding interaction (due to negative overlap population) and zero value indicates nonbonding interactions [52]. Additionally, the OPDOS diagrams allow us to determine and compare of the donor–acceptor properties of the ligands and ascertain the bonding and non-bonding. The calculated total electronic density of states (TDOSs) diagrams of the HPCBA is given in Fig. 8. The partial density of state plot (PDOS) mainly presents the composition of the fragment orbitals contributing to the molecular orbitals which is seen from Fig. 9. As seen Fig. 7, HOMO orbitals are localized on the ring and their contributions are about 62%. The LUMO orbitals are localized on the ring (91%) of the compound. Only based on the percentage shares of atomic orbitals or molecular fragments in the molecule is difficult to compare groups in terms of its bonding and anti-bonding properties. Thus the OPDOS diagram is shown Fig. 10 and some of orbitals of energy values of interaction between selected functional groups which are shown from figures easily, (CAO) atoms (red1 line) and (CAC) atoms (green line) are negative (anti-bonding interaction). As can be seen from the OPDOS plots for the HPCBA have anti-bonding character in frontier HOMO and LUMO molecular orbitals for phenyl and oxygen atoms. Also OPDOS shows the bonding character both HOMO and LUMO. 4.6. Chemical reactivity

Fig. 5. Experimental and theoretical FT-Raman spectra of HPCBA.

ture, expect H atoms. The observed transition from HOMO to LUMO is p ? p. Moreover lower in the HOMO and LUMO energy gap explains the eventual charge transfer interactions taking place within the molecule.

4.6.1. Global reactivity descriptors The energies of frontier molecular orbitals (EHOMO, ELUMO), energy band gap (EHOMO  ELUMO), electronegativity (v), chemical potential (l), global hardness (g), global softness (S) and global electrophilicity index (x) [53–57] of HPCBA have been listed in Table 6. On the basis of EHOMO and ELUMO, these are calculated using the below equations.

1 2

v ¼  ðELUMO þ EHOMO Þ 1 2

l ¼ v ¼ ðELUMO þ EHOMO Þ

HOMOenergy ¼ 6:3730 eV

1 2

g ¼ ðELUMO  EHOMO Þ LUMOenergy ¼ 1:3644 eV S¼

1 2g

HOMO  LUMOenergy gap ¼ 5:0086 eV However the value of chemical hardness, electronegativity, chemical potential and electrophilicity index are given in Table 6.

1 For interpretation of color in Figs. 7, 10 and 11, the reader is referred to the web version of this article.

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Table 5 Theoretical and experimental absorption wavelength k (nm), excitation energies, E (cm1) and oscillator strengths (f) of HPCBA in gas phase ethanol and DMSO solutions. Method

TD-CAM-B3LYP/631++G(d,p)

TD-B3LYP/631G(d,p) a

DMSO

Ethanol

Gas

Experimental

k (nm)

E (cm1)

F

k (nm)

E (cm1)

F

k (nm)

E (cm1)

F

k (nm)

E (cm1)

260.4 243.9

38,409 40,998

0.1748 0.0629

260.0 243.8

38,460 41,016

0.1706 0.0607

255.1 242.3

39,206 41,269

0.1335 0.0412

246 235

40,538 42,550

225.4

44,359

0.0326

225.5

44,347

0.0287

232.1

43,094

0.0067

227

44,014

278.5 267.1 252.3

35,907 37,439 39,635

0.0653 0.0834 0.0544

278.1 266.8 252.3

35,958 37,481 39,635

0.0637 0.0823 0.0521

268.9 259.7 254.1

37,189 38,506 39,355

0.0700 0.0520 0.0046

Gas major contributiona

H ? L (20%), H ? L + 1 (66%) H  2 ? L + 5 (14%), H  1 ? LUMO (65%), H  1 ? L + 1 (10%) H  4 ? LUMO (17%), H  4 ? L + 1 (43%) H ? L (86%) H ? L + 1 (79%) H  1 ? L (48%)

H: HOMO, L: LUMO.

Fig. 6. Experimental and theoretical UV–Vis spectra of HPCBA.

x¼

l2 2g

The usefulness of this new reactivity quantity has been recently demonstrated in understanding the toxicity of various pollutants in terms of their reactivity and site selectivity [58–60]. The calculated value of electrophilicity index describes the biological activity of HPCBA. 4.6.2. Local reactivity descriptors  0 Fukui functions (fkþ , fk , fk0 ), local softness (sþ k sk , sk ) and local  0 electrophilicity indices (xþ x , x ) [54,57] for selected atomic k k k sites in HPCBA have been listed in Table S1 (Supplementary material). Using Hirshfeld atomic charges of neutral, cation and anion states of HPCBA, Fukui functions are calculated using the following equations

fkþ ¼ ½qðn þ 1Þ  qðNÞ for nucleophilic attack fk ¼ ½qðn þ 1Þ  qðNÞ  qðN  1Þ for electrophilic attack fk0 ¼ 1=2½qðN þ 1Þ þ qðN  1Þ for radical attack Local softness and electrophilicity indices are calculated using the following equations

þ



0

sþk ¼ Sf k ; sk ¼ Sf k ; s0k ¼ Sf k

xþk ¼ xfkþ ; xk ¼ xfk ; x0k ¼ xfk0 where +, , 0 signs show nucleophilic, electrophilic and radical attack respectively. The maximum values of all the three local electrophilic reactivþ ity descriptors (fkþ , sþ k , xk ) at atom C21 indicate that this site is prone to nucleophilic attack. 4.7. Natural bond orbital (NBO) analysis NBO analysis provides the most accurate possible ‘natural Lewis structure’ picture of /, because all orbital details are mathematically chosen to include the highest possible percentage of the electron density. A useful aspect of the NBO method is that it gives information about interactions in both filled and virtual orbital spaces that could enhance the analysis of intra- and intermolecular interactions. The second-order Fock matrix was carried out to evaluate the donor–acceptor interactions in the NBO analysis [61]. 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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157

Fig. 7. Highest occupied and lowest unoccupied molecular orbitals of HPCBA obtained with B3LYP/6-31G(d,p) method.

Table 6 Calculated energy values of HPCBA by B3LYP/6-31G(d,p) method. Basic set

Conformer C1

SCF energy (a.u.) Dipole moment l (Debye) EHOMO (eV) ELUMO (eV) EHOMO–LUMO gap (eV) EHOMO1 (eV) ELUMO+1 (eV) EHOMO1–LUMO+1 gap (eV) Chemical hardness (g) Softness (S) Chemical potential (l) Electronegativity (v) Electrophilicity index (x)

915.67159416 3.9218 6.3730 1.3644 5.0086 6.8168 1.1410 5.6758 2.5043 0.3993 3.8687 3.8687 2.9882

E2 ¼ DEij ¼ qi

Fði; jÞ2 ej  ei

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.

Natural bond orbital analysis has been carried out to explain the charge transfer or delocalization of charge due to the intra-molecular interaction among bonds, and also provides a convenient basis for investigating charge transfer or conjugative interaction in molecular systems. Some electron donor orbital, acceptor orbital and the interacting stabilization energy resulting from the second-order micro disturbance theory is reported [62]. The larger the stabilization energy value, the more intensive is the interaction between electron donors and electron acceptors, i.e. the more donating tendency from electron donors to electron acceptors and the greater the extent of conjugation of the whole system. Delocalization of electron density between occupied Lewis-type (bond or lone pair) NBO orbitals and formally unoccupied (anti bond or Rydberg) non-Lewis NBO orbitals correspond to a stabilizing donor–acceptor interaction. The interamolecular interaction are formed by the orbital overlap between r(CAC) and r(CAC) and r(CAO) bond orbital which results interamolecular charge transfer (ICT) causing stabilization of the system. These interactions are observed as increase in electron density (ED) in CAC, and CAO anti-bonding orbital that weakens the respective bonds. The electron density of conjugated bond of phenyl ring (1.98e) clearly demonstrates strong delocalization.

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Fig. 8. The calculated TDOS diagrams of HPCBA.

Fig. 9. The calculated PDOS diagrams of HPCBA.

The strong intramolecular hyper conjugation interaction of the r electrons of CAC, CAH and OAH to the anti CAC and CAO bond of the ring as well as carbonyl acid group leads to stabilization of some part of the ring as evident from Table S2 (Supplementary material). There occurs a strong intramolecular hyperconjugative interaction of p electrons from C4AC5 to p(C11AO12) (ED ffi 0.309e and E(2) = 26.39 kJ mol1), from C19AC21 to p(C20AC23) (ED ffi 0.344e and E(2) = 25.42 kJ mol1) and from C20AC23 to p(C17AC18) (ED ffi 0.432e and E(2) = 23.92 kJ mol1). These interactions give rise to increasing the ED on p bonds leading to stabilization of HPCBA phenyl ring system with different stabilization energies. The most important highest energy, related to the molecule is electron

donating from p(C4AC5) to the antibonding acceptor p(C2AC3) and p(C1AC6) with stabilization energy 242.94 and 229.80 kJ mol1 respectively. Further in dimer from unit 1 to unit 2, LP (2) O12 conjugate with antibond i.e. r(O42AH43) leads to the stabilization energy of 17.08 kJ mol1. Similarly from unit 2 to unit 1, LP (2) O14 conjugates with antibond i.e. p(O13AH14) leads to the stabilization energy of 17.21 kJ mol1. Thus the nature and strength of the intermolecular hydrogen bonding can be explored by studying the changes in electron density in the vicinity of O  H hydrogen bonds. The NBO analysis of HPCBA clearly explains the evidences of the formation of strong H-bonded interaction between LP(O) and r OAH anti-bonding orbitals. According to Table S2 (Supplementary material), intermolecular

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159

Fig. 10. The OPDOS (or COOP) diagrams of HPCBA.

Fig. 11. Molecular electrostatic potential map calculated at B3LYP/6-31G(d,p) method.

CAO  H bond like weak interactions are exposed in the NBO analysis by the interaction between r(C11AO12) ? r(O42AH43) and r(O13AH14) ? r(C40AO41). The differences in E(2) energies are reasonably due to fact that the accrual of electron density in the OAH bond is not only drawn from the LP(O) of hydrogen-acceptor but also from the whole molecule. 4.8. Molecular electrostatic potential The molecular electrostatic potential, V(r), at a given point r (x, y, z) in the vicinity of a molecule, is defined in terms of the interaction energy between the electrical charge generated from the molecule electrons and nuclei and a positive test charge (a proton)

located at r. The molecular electrostatic potential (MEP) is related to the electronic density and is a very useful descriptor for determining sites for electrophilic attack and nucleophilic reactions as well as hydrogen-bonding interactions [63,64]. To predict reactive sites for electrophilic and nucleophilic attack for the title molecule, MEP was calculated at the B3LYP/6-31G(d,p) optimized geometry. The negative (red) regions of MEP were related to electrophilic reactivity and the positive (white) regions to nucleophilic reactivity shown in Fig. 11. The negative regions are mainly localized on the carbonyl oxygen atom. Also, a negative electrostatic potential region is observed around the O12, O15, O27 and O29 atoms. A maximum positive region is localized on the hydrogen atoms indicating a possible site for nucleophilic attack. The MEP map shows that the

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Fig. 12. Atomic charge distribution of HPCBA monomer (C1) and dimer.

Table 7 Thermodynamic properties at different temperatures at the B3LYP/6-31G(d,p) level for HPCBA molecule. T (K)

C 0p;m (cal mol1 K1)

S0m (cal mol1 K1)

DH0m (kcal mol1)

100 150 200 250 298.15 300 350 400 450 500 550 600 650 700 750 800

109.67 145.15 183.93 224.04 262.17 263.61 301.19 335.89 367.31 395.43 420.46 442.71 462.53 480.25 496.14 510.47

364.81 415.85 462.87 508.21 550.95 552.58 596.07 638.59 679.99 720.18 759.06 796.62 832.85 867.79 901.48 933.96

7.5 13.85 22.07 32.27 43.98 44.47 58.6 74.54 92.13 111.21 131.62 153.21 175.85 199.43 223.85 249.02

negative potential sites are on electronegative atoms as well as the positive potential sites are around the hydrogen atoms. The MEP provides a visual representation of the chemically active sites and comparative reactivity of the atoms.

5. Charge distribution Mulliken atomic charge calculation has an important role in the application of quantum chemical calculation to molecular system because of atomic charges effect dipole moment, molecular polarizability, electronic structure and a lot of properties of molecular systems [65]. The charge distributions over the atoms suggest the formation of donor and acceptor pairs involving the charge transfer in the molecule. The Mulliken population analysis in HPCBA molecule (monomer and dimer) is calculated using B3LYP

level with 6-31G(d,p) basis set are listed in Table S3 (Supplementary material). The plot of Mulliken atomic charges is shown in Fig. 12. The Mulliken charge analysis of HPCBA shows that presence of five oxygen atoms (O12 = 0.5489; O13 = 0.5078; O15 = 0.5388; O27 = 0.5266; O29 = 0.4518) imposes positive charges on C4, C11, H14, C18 and C16 atoms. However, C11, C5 and H14 possess positive charge due to large negative charge (0.5489) of atom O12. Moreover, the positive charge distribution observed on the remaining hydrogen atoms (H7, H8, H9, H10, H14, H22, H24, H25, H26 and H28). 6. Hyperpolarizability calculations The first order hyperpolarizability (btotal) of the title compound HEHMPT along with related properties (l, hai and Da) are calculated by using DFT-B3LYP method with 6-31G(d,p) basis set, based on the finite-field approach. In the presence of an applied electric field, the energy of a system is a function of the electric field. First order hyperpolarizability is a third rank tensor that can be described by a 3  3  3 array. The 27 components of the 3D matrix can be reduced to 10 components due to the Kleinman symmetry [66]. It can be given in the lower tetrahedral format. The components of btotal are defined as the coefficients in the Taylor series expansion of the energy in the external electric field. When the external electric field is weak and homogeneous, this expansion becomes:

E ¼ E0  la F a  1=2aab F a F b  1=6babc F a F b F c þ    where E0 is the energy of unperturbed molecule, Fa the field at the origin, la, aab, and babc are the components of dipole moment, polarizability and the first order hyperpolarizabilities, respectively. The total static dipole moment l, the mean dipole polarizability (a), the anisotropy of the polarizability Da and the total first order hyperpolarizability btotal, using x, y, z components they are defined as

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161

bx ¼ bxxx þ bxyy þ bxzz ; by ¼ byyy þ bxxy þ byzz ; bz ¼ bzzz þ bxxz þ byyz : The calculated dipole moment, polarizability and first order hyperpolarizability values obtained from B3LYP/6-31G(d,p) method are collected in Table S4 (Supplementary material). The total molecular dipole moment of HPCBA from B3LYP with 6-31G(d,p) basic set is 1.543 D, which is nearer to the value for urea (l = 1.3732 D). Similarly the first order hyperpolarizability of HPCBA is 3.3336  1030 which is nine times greater than the value of urea (b0 = 0.372  1030 esu). From the computation the high values of the hyperpolarizabilities of HPCBA are probably attributed to the charge-transfer existing between the phenyl rings within the molecular skeleton. So we conclude that the title compound is an attractive object for future studies of nonlinear optical properties. 7. Thermodynamic properties On the basis of vibrational analysis, the statically thermodynamic functions: heat capacity ðC 0p;m Þ, entropy ðS0m Þ, and enthalpy changes ðDH0m Þ for the title molecule are obtained from the theoretical harmonic frequencies and listed in Table 7. From Table 7, it can be observed that these thermodynamic functions are increasing with temperature ranging from 100 to 800 K due to the fact that the molecular vibrational intensities increase with temperature. The correlation equations between heat capacity, entropy, enthalpy changes and temperatures are fitted by quadratic formulas and the corresponding fitting factors (R2) for these thermodynamic properties are 0.9994, 0.9999 and 0.9997, respectively. The corresponding fitting equations are as follows and the correlation graphics of those shown in Fig. 13.

C 0p;m ¼ 5:6926 þ 1:0093T  4:7144  104 T 2 ðR2 ¼ 0:9994Þ S0m ¼ 265:2598 þ 1:0325T  2:4546  104 T 2 ðR2 ¼ 0:9999Þ H0m ¼ 6:1831 þ 0:0847T þ 2:9678  104 T 2 ðR2 ¼ 0:9997Þ All the thermodynamic data supply helpful information for the further study on the HPCBA. They can be used to compute the other thermodynamic energies according to relationships of thermodynamic functions and estimate directions of chemical reactions according to the second law of thermodynamics in thermochemical field [67]. Notice: all thermodynamic calculations were done in gas phase and they could not be used in solution. 8. Conclusions Fig. 13. Correlation graphs of thermodynamic properties at different temperature for HPCBA.

l ¼ ðl2x þ l2y þ l2z Þ1=2 ; hai ¼

axx þ ayy þ azz 3

;

Da ¼ 21=2 ½ðaxx  ayy Þ2 þ ðayy  azz Þ2 þ ðazz  axx Þ2 þ 6a2xx 1=2 ; btotal ¼ ðb2x þ b2y þ b2z Þ1=2 ; And

In the present work, the optimized molecular structure of the stable conformer, thermodynamic and electronic properties, vibrational frequencies, intensity of vibrations of the title compound have been calculated by DFT method using B3LYP/6-31G(d,p) basis set. The optimized geometric parameters (bond lengths, bond angles and dihedral angles) are theoretically determined and compared with the experimental results. The vibrational FT-IR and FT-Raman spectra of the HPCBA are recorded and on the basis of agreement between the calculated and experimental results, assignments of all the fundamental vibrational modes of the title compound were made unambiguously based on the results of the PED output obtained from normal coordinate analysis. Intermolecular and intramolecular hydrogen bonding between H and O is expected in C1 conformer. The hydrogen bonds are formed between

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the OH hydrogen atoms and the C@O oxygen atoms. The stable form of the title molecule was analyzed by the PES scan. The electronic properties are also calculated and compared with the experimental UV–Vis spectrum. The energies of important MOs and the kmax of the compound are also evaluated from TD-CAM-B3LYP method with 6-31++G(d,p) basis set. When all theoretical results scanned, they are showing good correlation with experimental data. The differences between the observed and scaled wavenumber values of most of the fundamentals are very small. Therefore, the assignments made at DFT level of theory with only reasonable deviations from the experimental values seem to be correct. The NBO analysis indicates the intramolecular charge transfer between the bonding and antibonding orbitals. Orbital energy interactions between selective functional groups are analyzed by density of energy states. Fukui functions, local softness and electrophilicity indices for selected atomic sites in HPCBA have been calculated. Thermodynamic properties in the range from 100 to 800 K are obtained. The gradients of C 0p and S0m to the temperature decreases, but that of DH0m increases, as the temperature increases. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.molstruc.2013. 01.043. These data include MOL files and InChiKeys of the most important compounds described in this article. References [1] [2] [3] [4] [5] [6] [7]

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