Research Journal of Recent Sciences ______ ______________________________ ______ ____ ___ ISSN 2277 - 2502 Vol. 2 ( ISC - 2012 ), 150 - 157 (201 3 ) Res.J. Recent .Sci. International Science Congress Association 150 Electronic structure, Non - linear properties and Vibrational analysis of ortho, meta and para - Hydroxybenzaldehyde by Density Functional Theory Hriday N. Mishra , Srivastava Rajesh Kumar, Narayan Vijay, Chand Satish, Sachan Alok Kumar, Shukla Vikas Kuamr, Prasad Onkar and Sinha Leena Physics Department, University of Lucknow, Lucknow, INDIA Available online at: www.isca.in Received 1 st December 2012, revised 26 th December 2012, accepted 19 th January 201 3 Abstract The p resent communication is aimed at comparing the molecular structural properties, vibrational and energetic data of ortho, meta and para hydroxybenzaldehyde , in gas phase, due to their commercial importance. The ground state properties of the title molecules have been calculated employing DFT/ B3LYP level of theory using the 6 - 311++G(d,p) basis set. The mean polarizability of all the three isomers are found to be nearly same in the range 88.415 to 90.933/a.u., but the dipole moment for ortho and meta hydroxyb enzaldehyde are calculated to be 5.0201 and 4.9101 Debye whereas the dipole moment for para hydroxybenzaldehyde has slightly lower value at 3.4655 Debye. The first static hyperpolarizability of ‘p’ - hydroxybenzaldehyde is found to be 1.5 times higher to th at of ‘m’ - hydroxybenzaldehyde ad 5 times higher tha ‘o’ - hydroxybenzaldehyde . MESP surfaces have also been drawn and compared. In order to obtain a complete description of molecular dynamics, vibrational wavenumber calculation along with the normal mode analysis, have been carried out at the DFT level. The calculated spectra of the molecules agree well with the experimental data. Keywords: Polarizability, first static hyperpolarizability, hydroxybenzaldehyde , IR spectra . Introduction Benzaldehyde, the simplest representative of the aromatic aldehydes is a key intermediate for the processing of perfume and flavouring compounds and in the preparation of certain aniline dyes. Benzaldehyde can have carcinostatic or antitumor properties 1 - 3 . Hydroxybenzaldeh yde, derivative of benzaldehyde, are used primarily as chemical intermediates for a variety of products. Out of the three isomers ortho, para and meta - Hydroxybenzaldehyde, otho - Hydroxybenzaldehyde (or salic ylaldehyde) is used in the manufacture of coumar in. Coumarin is an important commercia l chem ical used in soaps, flavors ad fragraces, ad electroplatig. Recently it has been shown that the incorporation of the benzaldehyde increases the activity of the benzaldehyde - thiosemicarbazone which exhibits high anti - trypanosomal potential 4 . Although much work has been done on benzaldehyde and its derivatives 5 - 9 , however a comprehensive comparative study of ortho, para and meta - hydroxybenzaldehyde on electronic structure, non - linear properties along with the detailed potential energy distribution of normal modes of vibratio ns has not been reported so far. The present communication is aimed at comparing the molecular structural properties, vibratioal ad eergetic data of ‘o’ - hydroxybenzaldehyde , ‘p’ - hydroxybenzaldehyde ad ‘m’ - hydroxybenzaldehyde , in gas phase, due to their commercial importance. The structure and the ground state energy of the molecules under investigation has been analyzed employing DFT / B3LYP level. The optimized geometry and their properties such as equilibrium energy, frontier orbital energy gap, dipol e moment and vibrational frequencies along with the electrostatic potential maps have also been used to understand the activity of the isomers of Hydroxybenzaldehyde. Methodology Structure and Spectra: The optimized molecular structures of ‘o’ - hydroxybe nzaldehyde , ‘p’ - hydroxybenzaldehyde ad ‘m’ - hydroxybenzaldehyde are given in figure 1. The theoretically calculated IR spectra have been given in figure 2. The calculated IR spectra of the molecules agree well with the experimental spectral data reported b y the NIST web book 10 . Computational Details: The molecules under investigation have been analyzed with density functional theory (DFT) 11 , employing Becke ’ s three parameter hybrid exchange functionals 12 with Lee - Yang - Parr correlation functionals (B3LYP) 13 ,14 . All the calculations were performed using the Gaussian 09 program 15 . As the DFT hybrid B3LYP functional tends to overestimate the fundamental normal modes of vibration, a scaling factor of 0.9679 16 has been applied. By combining the results of the Gau ssview ’ s program package 17 with symmetry consideration, vibrational frequency assignments were made with a high degree of accuracy. For the precise vibrational assignments, the normal modes have also been analyzed using the VEDA 4 program 18 . The DFT was a lso used to calculate the dipole moment, mean polarizability ‹α› ad the total first static hyperpolarizability β 19,20 . Following Buckingham ’ s definitions 21 , the total dipole Research Journal of Recent Sciences ______ _ _ _______________________________ ______________ _ ________ ISSN 2277 - 2502 Vol. 2 ( ISC - 2012 ), 150 - 157 (201 3 ) Res.J.Recent.Sci. International Science Congress Association 151 moment and the mean polarizability in a Cartesian frame is defined by μ = (μ 2 x + μ 2 y + μ 2 z ) 1/2 ‹α› = 1/3[ α xx + α yy + α zz ] The total itrisic hyperpolarizability β TOTAL 22 is defined as β TOTAL = [β 2 x + β 2 y + β 2 z ] 1/2 = [(β xxx + β xyy + β xzz ) 2 + (β yyy + β yzz + β yxx ) 2 + [(β zzz + β zxx + β zyy ) 2 ] 1/2 The β compoets of Gaussia output are reported in atomic units. Figure - 1 Optimized structures of (a)‘o’ Hydroxybezaldehyde (b) ‘m’ Hydroxybezaldehyde (c) ‘p’ Hydroxybezaldehyde Figure - 2 Theoretical IR spectra of ortho, meta and para Hydroxybenzaldehyde Research Journal of Recent Sciences ______ _ _ _______________________________ ______________ _ ________ ISSN 2277 - 2502 Vol. 2 ( ISC - 2012 ), 150 - 157 (201 3 ) Res.J.Recent.Sci. International Science Congress Association 152 Results and Discussion Mole cular geometry optimization and energies: The structures of ‘o’ - hydroxybenzaldehyde , ‘p’ - hydroxybenzaldehyde ad ‘m’ - hydroxybenzaldehyde have been optimized to compare the variation in electronic and non - linear properties on substitution of hydroxyl (OH) group at ortho, para and meta positions. The equilibrium geometry optimization for three isomers has been achieved by energy minimization, using DFT at the B3LYP level, employing the split valence basis set 6 - 311++G(d,p). The optimized molecular structures with numbering scheme of three molecules are shown in fig ure 1. The ground state optimized parameters are reported in table 1. As the calculated vibrational spectra have no imaginary frequency, the optimized geometry is confirmed to be located at the loca l minima on potential energy surface. The C - H/C - C bond lengths vary in the range 1.083 - 1.086 /1.386 - 1.481 , 1.083 - 1.086 /1.391 - 1.400 and 1.083 - 1.086 /1.385 - 1.402 (standard value 1.10 /1.40 ) in ortho, meta and para hydroxybenzaldehyde respectively. The C - O/C=O bond lengths for three molecules are calculated to be 1.365 /1.214 , 1.366 /1.210 and 1.360 /1.213 which are close to standard value 1.359 /1.208 23 . Electronic properties: The most important orbitals in a molecule ar e the frontier molecular orbitals, called highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO). These orbitals determine the way the molecule interacts with other species. The frontier orbital gap helps characterize the chemical reactivity and kinetic stability of the molecule. A molecule with a small frontier orbital gap is more polarizable and is generally associated with a high chemical reactivity, low kinetic stability and is also termed as soft molecule 24 . The fronti er orbital gaps of ‘o’ - hydroxybenzaldehyde / ‘m’ - hydroxybezaldehyde/ ‘p’ - hydroxybenzaldehyde are to be 0.17723/0.17229/0.18304 a.u. which clearly shows that ‘m’ - hydroxybenzzaldehyde is most reactive among the three isomers. The 3D plot of HOMO, LUMO and ME SP are shown in figure 3 and 4 respectively. The HOMO in three molecules is distributed over entire molecule except the carbon and hydrogen atom of aldehyde group. The LUMO’s show more ati - bodig character tha the HOMO’s. The oxygen atom of the hydroxyl group contribute to the LUMO, oly i case of ‘o’ - hydroxybenzaldehyde . The importance of MESP is that it shows the size, shape as well as positive, negative and neutral electrostatic potential in terms of colour grading. It is also very useful to correlat e the molecular structure with its physiochemical property relationship 25 - 29 . The MESP i case of ‘o’ - hydroxybenzaldehyde shows three distinct electron rich sites including the aromatic ring, the MESP of meta and para show two strong electronegative, where as a small negative potential at the aromatic ring site. Table 1 Parameters corresponding to optimized geometry of Ortho - meta - and para - Hydroxybenzaldehyde at B3LYP/6 - 311++G(d,p) level of theory Parameter ortho meta para Ground state energy (in Hartre e) Dipole moment (in Debye) Frontier orbital energy gap (in Hartree) Polarizability/a.u. - 420.915 5.0201 0.1772 88.4153 - 420.916 4.9101 0.1723 89.357 - 420.919 3.4655 0.1830 90.933 Figure - 3 Homo Lumo orbitals ad eergy gap of ‘ o’, m’ ad ‘p’ Hydroxybezaldehyde Research Journal of Recent Sciences ______ _ _ _______________________________ ______________ _ ________ ISSN 2277 - 2502 Vol. 2 ( ISC - 2012 ), 150 - 157 (201 3 ) Res.J.Recent.Sci. International Science Congress Association 153 Figure - 4 MESP map of ‘o’, m’ ad ‘p’ Hydroxybezaldehyde Electric moments: The dipole moment in a molecule is another important electronic property that results from non - uniform distribution of charges on the various atoms in a molecule. It is mainly used to study the intermolecular interactions involving the van der Waal type dipole - dipole forces, etc., because higher the dipole moment, stronger will be the intermolecular interactions. The calculated dipole moment for three molecules is given in table 1. Table 1 show that the calculated value of dipole momet i case of ‘o’ - hydroxybenzaldehyde ad ‘m’ are early equal ad quite higher tha ‘p’ - hydroxybenzaldehyde . The determination of electric polarizability and hype rpolarizability is of basic importance to study the phenomenon induced by intermolecular interactions, simulation studies and nonlinear optical effects. The values of polarizability and hyperpolarizability calculated at the same level of theory and the sam e basis set for the title molecules, can provide a reasonable comparison of these quantities, in the absence of experimental data. Although the mea polarizability of ‘o’, ‘m’ - ad ‘p’ hydroxybenzaldehyde is found to be almost same the total intrinsic hype rpolarizability β TOTAL of para isomer is fairly larger as compared to the other two counterparts (table 2). Table 2 All β components and β t otal of Hydroxybenzaldehyde at B3LYP/6 - 311++G(d,p) β compoets Ortho Meta Para β XXX - 149.5 340.2 100. 2 β XXY - 36.1 - 33.4 - 31.2 β XYY 90.5 129.6 - 98.2 β YYY 216.5 - 179.8 864.3 β XXZ 0 0 0 β XYZ 0 0 0 β YYZ 0 0 0 β XZZ - 47.7 - 14.5 15.8 β YZZ - 58.5 - 63.1 - 4.8 β ZZZ 0 0 0 β TOTAL 162.0 532.6 828.5 Vibrational assignments: The experimental and comp uted vibrational wave numbers and the detailed description of each normal mode of vibration of three molecules, carried out in terms of their contribution to the total potential energy are given in table 3, 4 and 5. The calculated harmonic wavenumbers are usually higher than the corresponding experimental quantities because of the combination of electron correlation effects and basis set deficiencies. These discrepancies are taken care of either by computing anharmonic corrections explicitly or by introduci ng scalar field or even by direct scaling of the calculated wavenumbers with a proper scaling factor 30, 31 . The vibrational wavenumbers are calibrated accordingly with scaling factor 0.9679 for DFT at B3LYP. The vibrational assignments have been done on th e basis of relative intensities, line shape, the VEDA 4 program and the animation option of Gaussview 5.0. C=O and C - O vibrations: The appearance of a strong band in IR spectra around 1650 - 1800 cm - 1 in aromatic compound shows the presence of C=O stretchin g motion. The C=O stretch of aldehyde group i ‘o’ - Hydroxybezaldehyde/ ‘m’ - Hydroxybenzaldehyde / ‘p’ - Hydroxybenzaldehyde is calculated at to be at 1697/1713/1703 cm - 1 and is assigned well with the experimental IR peak at 1672/1728/1779 cm - 1 . The other str ong band observed at 1227/1290/1241 cm - 1 is due to C - O stretching vibration of the (C - OH) bond whose general position is 1000 - 1200 cm - 1 . This band is calculated at 1224/1289/1250 cm - 1 for ‘o’ - hydroxybenzaldehyde / ‘m’ - hydroxybenzaldehyde / ‘p’ - hydroxybenza ldehyde. OH vibrations: The strong band calculated at 3710 /3711/3705 cm - 1 i IR spectra of ‘o’ - hydroxybezaldehyde/ ‘m’ - hydroxybezaldehyde/ ‘p’ - hydroxybenzaldehyde shows the presence of OH group. These bands contribute 100% to the total PED (experimenta l value 3892/3202 for ‘o’ - hydroxybenzaldehyde / ‘m’ - hydroxybenzaldehyde). The small discrepancy between the calculated and the observed wavenumber may be due to the intermolecular hydrogen bond. The bands observed at 1178, 1149/1290,1178,1155/1520 cm - 1 in IR spectra of ‘o’ - hydroxybezaldehyde/ ‘m’ - Research Journal of Recent Sciences ______ _ _ _______________________________ ______________ _ ________ ISSN 2277 - 2502 Vol. 2 ( ISC - 2012 ), 150 - 157 (201 3 ) Res.J.Recent.Sci. International Science Congress Association 154 hydroxybezaldehyde/ ‘p’ - hydroxybenzaldehyde are due to H - O - C in - plane bending and are calculated at 1183,1150/1284,1164,1147/1490 cm - 1 as mixed modes. Ring modes: The phenyl ring spectral region predominantly in volves the C - H, C - C and C=C stretching, and C - C - C, H - C - C and bending vibrations. Very intense band are found in the range 3100 - 3000 cm - 1 which is, in general, observed in the case of aromatic compounds due to aromatic C - H stretching vibrations. The C - C str etching modes are observed as mixed modes in the wavenumber range 1600 - cm - 1 to 1000 cm - 1 for three molecules and are in good agreement with general appearance of C - C stretching modes. The modes appearing below 1000 cm - 1 are mixed modes. The torsional mode s appear in general in the low wavenumber regions. Table - 3 Theoretical and Experimental wave - numbers (in cm - 1 ) of ‘o’ - Hydroxybenzaldehyde Calc. unsc. wave no. Calc. sc. wave no. *Exp. Wave no. Assignment of dominant modes in order of decreasing potential energy distribution (PED) 3833 3710 3892 ν(O7 - H14)(100) 3198 3095 ν(C1 - H10)(52)+ν(C2 - H11)(37)+ν(C3 - H11)(11) 3187 3085 ν(C2 - H11)(54)+ν(C3 - H12)(31)+ν(C1 - H10)(13) 3176 3074 3074 ν(C3 - H12)(49)+ν(C1 - H10)(35)+ν(C2 - H11)(10) 3152 3051 ν(C4 - H13)(90) 2972 2 877 2847 ν(C8 - H15)(100) 1753 1697 1672 ν(C8 - O9)(87) 1644 1591 1580 ν(C4 - C5)(36)+ν(C2 - C1)(22) 1627 1575 1551 ν(C3 - C1)(26)+ν(C6 - C2)(14)+β(C6 - C2 - C1)(10)+β(C3 - C1 - C2)(11)+β(C5 - C4 - C3)(10) 1520 1471 1488 β(H10 - C1 - C2)(17)+β(H11 - C2 - C6)(18)+β(H13 - C4 - C5)(24) 148 8 1440 1459 ν(C6 - C20)(12)+β(H10 - C1 - C3)(20)+β(H12 - C4 - C5)(21) 1430 1384 1381 β(H15 - C8 - O9)(81) 1357 1313 1323 ν(C2 - C1)(20)+ν(C4 - C3)(18)+β(H14 - C7 — C5)(20)+β(H11 - C2 - C6)(12) 1328 1285 1280 ν(C6 - C2)(16)+β(H11 - C2 - C6)(15)+β(H13 - C4 - C5)(12) 1265 1224 1227 ν(C6 - C2) (16)+ν(O7 - C5)(38) 1222 1183 1178 ν(C2 - C1)(17)+ν(C8 - C6)(30)+β(C6 - C2 - C1)(11)+β(H14 - O7 - C5)(15)+β(H10 - C1 - C2)(12) 1188 1150 1149 β(H12 - C3 - C1)(11)+ν(C5 - C4)(10)+β(H14 - O7 - C5)(27)+β(H13 - C4 - C5)(25) 1181 1143 1115 β(H13 - C4 - C5)(12)+β(H10 - C1 - C3)(24)+β(H11 - C2 - C6)(13) +β(H12 - C3 - C4)(30) 1107 1071 ν(C5 - C4)(11)+β(C6 - C2 - C1)(18)+β(H14 - C7 - C5)(10)+β(H10 - C1 - C3)(11) +β(H12 - C3 - C4)(12) 1058 1024 1033 ν(C4 - C3)(14)+ν(C3 - C1)(40)+β(H11 - C2 - C6)(12)+β(H133 - C4 - C5)(20) 1028 995 1009 ω(C - H)ring(80) 993 961 980 ω(C - H)ring(91) 960 929 9 46 ω(C - H)rig (85)+ρ(C5 - C4 - C3 - C1)(12) 856 828 883 ν(O7 - C5)(16)+ν(O8 - C6)(14)+β(C3 - C1 - C2)(24)+β(C5 - C4 - C3)(10) 851 824 859 ω(C - H)rig (76)+ρ(C5 - C4 - C3 - C1)(10) 815 789 ν(C6 - C2)(11)+β(C6 - C2 - C1)(10)+β(O9 - C8 - C6)(21)+β(C3 - C1 - C2)(10) 765 740 757 ρ(H10 - C1 - C3 - C4) (34)+ρ(H12 - C3 - C4 - C5)(27)+ρ(H13 - C4 - C5 - C6)(19) 701 678 665 ρ(H13 - C4 - C5 - C6)(13)+ρ(C5 - C4 - C3 - C1)(18)+ρ(C6 - C2 - C3 - C1)(19)+ρ(O7 - C5 - C6 - C8)(23) 641 620 641 β(O9 - C8 - C6)(23)+β(C4 - C3 - C1)(34)+β(C5 - C4 - C3)(17) 556 538 ν(C7 - C5)(13)+β(O9 - C8 - C6)(25)+β(C5 - C4 - C3)(16)+β(07 - C5 - C4)(19) 534 517 ρ(C6 - C2 - C1 - C3)(28)+ρ(O7 - C5 - C4 - C3)(18)+ρ(O7 - C5 - C6 - C6)(19) Research Journal of Recent Sciences ______ _ _ _______________________________ ______________ _ ________ ISSN 2277 - 2502 Vol. 2 ( ISC - 2012 ), 150 - 157 (201 3 ) Res.J.Recent.Sci. International Science Congress Association 155 Table - 4 Theoretical and Experimental wave - numbers (in cm - 1 ) of ‘m’ - Hydroxybenzaldehyde Calc. unsc. wave no. Calc. sc. wave no. *Exp. wave no. Assignment of dominant modes in order of decreasing potential energy distribution (PED) 3834 3711 3203 ν(O4 - H12)(100) 3203 3100 2973 ν(C3 - H11)(99) 3189 3087 ν(C8 - H15)(75)+ν(C6 - H13)(15)+ν(C2 - H10)(10) 3170 3068 ν(C6 - H13)(79)+ν(C8 - H15)(17) 3153 3052 ν(C2 - H10)(91) 2893 2800 2737 ν(C 7 - H14)(99) 1770 1713 1728 ν(C7 - O9)(89) 1648 1595 ν(C1 - C3)(36)+ν(C6 - C8)(10)+ν(C8 - C2)(10)+β(H12 - O4 - C1)(15) 1627 1574 1584 ν(C8 - C2)(18)+ν(C6 - C5)(30)+β(C2 - C8 - C6)(11)+β(C1 - C3 - C5)(10) 1513 1464 1504 β(H11 - C3 - C5)(14)+β(H13 - C6 - C5)(16)+β(H15 - C8 - C6)(27) 1501 1 452 ν(C1 - C3)(11)+ν(C8 - C6)(11)+ν(C5 - C3)(18)+β(H10 - C2 - C8)(17) 1419 1373 1365 β(H14 - C7 - O9)(78) 1361 1317 ν(C1 - C3)(13)+ν(C2 - C8)(20)+ν(C8 - C2)(18)+ν(C5 - C3)(21) 1327 1284 1290 ν(O4 - C1)(11)+β(C2 - C8 - C6)(10)+β(H12 - C4 - C1)(17)+β(H10 - C2 - C8)(17) +β(H11 - C3 - C5)(11)+β (H13 - C6 - C8)(20) 1285 1244 1250 ν(C6 - H5)(11)+ν(C4 - C1)(24)+β(H11 - C3 - C5)(11)+β(H13 - C6 - C8)(12) 1203 1164 1178 ν(C1 - C3)(13)+β(H12 - O4 - C1)(38)+β(H15 - C8 - C6)(29) 1185 1147 1155 ν(C8 - C6)(10)+β(H12 - O4 - C1)(19)+β(H10 - C2 - C8)(23)+β(H15 - C8 - C6)(18) 1153 1116 ν(O4 - C1)( 12)+ν(C7 - C5)(23)+β(H11 - C3 - C5)(31)+β(H13 - C6 - C8)(13) 1108 1072 1086 ν(C8 - C6)(12)+ν(C8 - C2)(23)+β(H13 - C6 - C8)(31)+β(H10 - C2 - C8)(13) 1023 990 1000 ω(C - H)rig(65)+ρ(O9 - C7 - C5 - C3)(32) 1012 980 β(C6 - C5 - C3)(32)+β(C6 - C8 - C2)(13)+β(C2 - C1 - C3)(14)+ν(C1 - C3)(14)+ν(C6 - C5) (10) 979 948 948 β(C8 - C6 - C5)(17)+ν(C7 - C5)(15)+ν(O4 - C1)(13)+β(H11 - C3 C5)(11) 964 933 897 ω(C - H)rig(72)+ ρ(C2 - C8 - C6 - C5)(13) 906 877 874 ω(C - H)ring(71) 883 855 ω(C - H)ring(89) 778 753 782 ρ(H15 - C8 - C6 - C5)(28)+ρ(H10 - C2 - C8 - C6)(25)+ρ(H13 - C6 - C8 - C2)(21) 764 739 707 β(C8 - C6 - C5)(25) +β(O9 - C7 - C5)(15)+ ν(O4 - C1)(10) 662 641 656 ρ(C1 - C2 - C3 - C6)(29)+ρ(C2 - C8 - C6 - C5)(29)+ρ(C6 - C8 - C5 - C3)(18) 657 636 β(C6 - C5 - C3)(36)+ β(C2 - C8 - C6)(29)+β(O9 - C7 - C5)(18) 555 537 ρ(O4 - C1 - C2 - H10)(54)+ρ(C3 - C5 - C7 - O9)(18) 540 523 β(C5 - C6 - C8)(4 2)+ β(C2 - C1 - C3)(33) 457 442 β(C7 - C5 - C6)(39)+ β(O4 - C1 - C2)(37) Research Journal of Recent Sciences ______ _ _ _______________________________ ______________ _ ________ ISSN 2277 - 2502 Vol. 2 ( ISC - 2012 ), 150 - 157 (201 3 ) Res.J.Recent.Sci. International Science Congress Association 156 Table - 5 Theoretical and Experimental wave - numbers (in cm - 1 ) of ‘p’ - Hydroxybenzaldehyde Calc. unsc. wave no. Calc. sc. wave no. * Exp. wave no. Assignment of dominant modes in order of decrea sing potential energy distribution (PED) 3828 3705 ν(O8 - H15)(100) 3199 3096 ν(C1 - H10)(94) 3194 3091 ν(C5 - H12)(95) 3163 3061 ν(C3 - H12)(95) 3153 3052 3047 ν(C7 - H14)(96) 2884 2791 ν(C6 - H13)(99) 1759 1703 1779 ν(O9 - C6)(85) 1642 1589 1596 ν(C5 - H7)( 30)+ν(C1 - C2)(20) 1625 1573 ν(C1 - C2)(25)+ν(C4 - C3)(22) 1539 1490 1520 β(H11 - C3 - C4)(14)+β(H12 - C5 - C7)(14)+β(H15 - O8 - C2)(12)+β(H14 - C7 - C5)(17) +β(C1 - C2 - C7)(13) 1470 1423 1419 ν(C3 - C1)(15)+ν(C5 - C7)(11)+β(H10 - C1 - C3)(12) 1418 1372 1388 β(H13 - C6 - O9)(73) 1372 13 28 1317 ν(C3 - C1)(12)+ν(C5 - C7)(19)+ν(C4 - C3)(12)+ν(C7 - C2)(13)+β(H15 - O8 - C2)(20) +β(H11 - C3 - C4)(10) 1328 1285 1287 ν(C4 - C3)(12)+β(H11 - C3 - C4)(17)+β(H12 - C5 - C7)(17)+β(H14 - C7 - C5)(16) 1291 1250 1241 ν(C2 - O8)(52)+ν(C3 - C1)(10) 1233 1193 1216 ν(C6 - C4)(32)+ β(H10 - C1 - C3)(12)+ β(C4 – C3 - C1)(10) 1193 1155 1165 β(H15 - O8 - C2)(55)+ β(H14 - C7 - C5)(13)+ν(C7 - C2)(12) 1177 1139 β(H10 - C1 - C3)(19)+ β(H11 - C3 - C4)(19)+ β(H12 - C5 - C7)(20) 1123 1087 1109 ν(C3 - C1)(14)+ν(C5 - C7)(10)+ β(H10 - C1 - C3)(18)+ β(H11 - C3 - C4)(12)+ β(H12 - C5 - C7)(21)+ β(H 14 - C7 - C5)(12) 1024 991 ρ(H13 - C6 - C4 - C3)(74)+ρ(O9 - C6 - C4 - C3)(10) 1023 990 β(C3 - C1 - C2)(41)+β(C4 - C5 - C7)(39) 979 948 ω(C - H)ring(84) 951 920 ω(C - H)rig(63)+ρ(C4 - C3 - C1 - C2)(15) 865 837 835 β(C1 - C2 - C7)(19)+β(C4 - C5 - C7)(19)+ν(C4 - C5)(18) 836 809 785 ω(C - H)rin g(88) 828 801 719 ω(C - H)ring(80) 794 769 ν(C8 - C2)(15)+β(O9 - C6 - C4)(18)+β(C1 - C2 - C7)(10)+β(C4 - C3 - C1)(25) 690 668 ρ(C3 - C1 - C2 - C7)(12)+ ρ(C5 - C7 - C2 - C1)(16)+ ρ(C4 - C3 - C2 - C1)(34)+ ρ(O8 - C2 - C7 - C5)(16) 653 632 648 β(C2 - C1 - C3)(32)+β(C4 - C5 - C7)(40) 614 594 602 β(C5 - C7 - C2)(11)+β(O9 - C6 - C4)(37)+β(C1 - C2 - C7)(11) 513 497 511 ρ(H11 - C3 - C4 - C5)(12)+ρ(C3 - C1 - C2 - C7)(11)+ρ(O8 - C2 - C1 - C3)(41) Note :Abbreviations used here have following meaning -  : stretchig; β: i plae bedig; ρ : torsio; ω: waggig . * http://webbook.nist.gov /chemistry/form - ser.html Conclusion In the present work we have calculated the geometric parameters, the vibrational frequencies, frontier orbital band gap, MESP surfaces and the non - liear optical properties of ‘o’, ‘m’ ad ‘p’ Hydroxybezaldehyde usig DFT/ B3LYP method. The higher frontier orbital energy gap and the lower dipole momet values make the ‘p’ - Hydroxybenzaldehyde less reactive and less polar, hence most stable among the three isomers. A good agreement between experimental and calculated nor mal modes of vibrations has been observed. NLO behavior of the title molecules were investigated by the determination of the polarizability and the first hyperpolarizability using the DFT/B3LYP/6 - 311G(d,p) method. The polarizability values are almost the same but the para isomer has significantly higher values for the total hyperpolarizability. Acknowledgements - The Authors (OP and LS) are grateful to University Grants commission for financial support and Prof. M.H. Jamroz for providing his VEDA 4 softwar e. References 1. Andersen A , Final report on the safety assessment of benzaldehyde , Int J. Toxicol, (25 Suppl 1) , 11 - 27, (2006) 2. 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