indian journal of science...the ft-ir and ft-raman spectra of aciretin were recorded in the region...

Experimental and theoretical investigations of spectroscopic properties of acitretin Publication History Received: 19 January 2015 Accepted: 02 March 2015 Published: 11 March 2015 Citation Swarnalatha N, Gunasekaran S, Muthu S, Rajesh P. Experimental and theoretical investigations of spectroscopic properties of acitretin. Indian Journal of Science, 2015, 14(41), 43-62

Indian Journal of Science ANALYSIS International Journal for Science ISSN 2319 – 7730 EISSN 2319 – 7749 © 2015 Discovery Publication. All Rights Reserved

Experimental and theoretical investigations of spectroscopic properties of acitretin N.Swarnalatha a, S. Gunasekaran b, S. Muthu c, P.Rajeshd

a Department of Physics, SCSVMV University, Enathur, Kanchipuram 631561, Tamil Nadu,

India. b Research and Development, St. Peter’s Institute of Higher Education and Research,

St. Peter’s University, Avadi, Chennai 600054, Tamil Nadu, India c Department of Physics, Sri Venkateswara College of Engineering, Sriperumbudur 602117,

Tamil Nadu, India. dSpectrophysics Research Laboratory, Pachiayappa’s College, Chennai 600 054, India

ABSTRACT

The FT-IR and FT-Raman spectra of aciretin were recorded in the region 4000–400 cm-1 and 4000–100 cm-1, respectively. The geometrical structure, harmonic vibrational frequency, infrared intensity, Raman activities and bonding features of this compound were carried out by DFT methods with 6-31G (d, p) basis set. The calculated HOMO and LUMO energies show that, the charge transfer occurs within the molecule. The charge delocalizations of these molecules have been analyzed using NBO analysis. The other molecular properties like Mulliken population analysis, molecular electrostatic potential (MEP) and thermodynamic properties of the title compound at the different temperatures have been calculated. Finally, the calculated results were applied to simulate infrared and Raman spectra of the title compound which show good agreement with observed spectra.

Key words: DFT, FT-IR, FT-Raman, NBO Corresponding author: [email protected]

1.1 Introduction

Is a drug used in the treatment of psoriasis. Is a yellow crystalline powder which is soluble in acetone and alcohol. Its molecular formula is C21H26O3 whose molecular mass is 326.188 g. World Health Organization identified that the manifestations of psoriasis are not limited to the skin, diseases may complicatemoderate to severe psoriasis. In particular, the relative risks of ischemic heart disease, stroke, hypertension, dyslipidaemia, diabetes in people with psoriasis.UVB radiation is beneficial for the treatment of psoriasis. Patients with recalcitrant disease, however, are slow to respond to UVB phototherapy with and without the use of coal tars or emollients [1]. Exterminate and, more recently, have proved useful, but clinical improvement is slow when they are used as monotherapy in plaque psoriasis. A randomized, double-blind comparison study was carried out by Tanew et al[2].Vibrational spectroscopic analysis has enormous contribution towards the studies of structure and physio-chemical properties of molecular systems [3–6]. 1.2 Experimental process

The compound in the solid form was procured from Sigma Aldrich Company (USA) with a stated purity of 98% and used as such without further purification. The FT-Raman spectrum of the acitretin was recorded in the region 4000 cm-1 to 100 cm-1 in pure mode using Nd: YAG Laser of 100 mW with 2 cm-1 resolution on a BRUCKER RFS 27 at SAIF, IIT, Chennai, India. The FT-IR spectrum of the sample was recorded in the region 4000 cm-1 – 400 cm-1 in

evacuation mode using KBr pellet technique with 1.0 cm-1 resolution on a PERKIN ELMER FT-IR spectrophotometer. 1.3 Quantum Chemical calculations

To obtain the optimized structure of the title molecule, without any constraint on the geometry the energy of the title molecule was minimized, whole intra molecular forces were brought to zero.The optimized structure of the title compound, corresponding energy and vibrational harmonic frequencies were calculated by using DFT with B3LYP 6-31 G(d,p) basis set using GAUSSIAN 03W program package[7].The geometry was optimized at B3LYP level by using 6-31G(d,p) basis set. The frequency calculation delivered the fundamental vibrational frequencies, optimized geometrical parameters, energy, and thermodynamical data such as entropy, heat capacity and enthalpy. Also IR and Raman spectra were simulated. Mulliken population analysis determined the distribution of electrons into the molecular orbitals. The symmetries of the vibrational modes were determined by using the standard procedure [8] of decomposing the traces of the symmetry operation in to the irreducible representations. By combining the result of the Gaussview program [9] with symmetry considerations, vibrational frequency assignments were performed. Calculation of the potential energy distribution (PED) and the prediction of IR and Raman intensities were done with Veda 4 program [10].The electronic properties such as HOMO and LUMO energies were determined. The NBO analysis and MEP calculations were performed with the arguslab software on the title molecule. NBO to give clear evidence stabilization originating from hyperconjugation of various intramolecular interactions [11-13]. 1.4 Prediction of Raman Intensities

The Raman activities (Si) calculated by Gaussian 03 program have been suitably adjusted by the scaling procedure and subsequently converted to relative Raman intensities (Ii) using the following relationship derived from the basic theory of Raman scattering [14],

Ii =f(0-i)4 Si

i [1 - exp (hci/kbT)] Where υ0 is the exciting frequency (in cm−1), υi is the vibrational wave number of the i th normal mode, h, c and kb are universal constants, and f is the suitably chosen common scaling factor for all the peak intensities. For the plots of simulated IR and Raman spectra, pure Lorentzian band shapes are used with full width at half maximum of 10 cm−1[15 ]. 1.5 Results and discussions 1.5.1 Structural Analysis

Molecular geometry is determined by the quantum mechanical behavior of the electrons. Using the valence bond approximation, this can be understood by the type of bonds between the atoms that make up the molecule. The geometry can also be understood by molecular orbital theory where the electrons are delocalized.The molecular structure along with numbering of atoms of was obtained from Gaussian 03 and GAUSSVIEW programs as shown in Fig.1. The most optimized structural parameters (bond length and bond angle) calculated by DFT (B3LYP) with 6-31G(d,p) basis By considering the relaxation of all geometries, which correspond to true energy minima as revealed by the lack of imaginary frequencies in the vibrational mode calculation. The optimized bond lengths and bond angles of the title molecules were calculated

using DFT(B3LYP) with 6-31G(d,p) basis set were listed in Table 1 in accordance with numbering of the atoms. All groups of the molecule is non-planar therefore C1 point group symmetry is used for computation. The bond lengths of C1-C2,C3-C4,C7-C8 and C9 –C14 lies within the same range as 1.4 Ao to 1.776 Ao. And C2-C3, C4-C5 and C8-C9 lies within the range 1.3Ao.The bond length of O23-H31 is .971 Ao. This decrease in bond length may be due to the fact that the electronegativity of oxygen atom with neighboring atoms. In spiteof the differences, the calculated geometries represent a good approximation and they are the bases for calculating vibrational frequencies and thermodynamical properties. Table 1.

Optimized geometrical parameters of acitretin molecules, bond length(Å),

and bond angle(˚). Parameters B3LYP/6-31G(d,p) R(C1-C2) 1.467 R(C1-O10) 1.221 R(C1-O11) 1.365 R(C2-C3) 1.363 R(C2-H25) 1.086 R(C3-C4) 1.453 R(C3-C13) 1.509 R(C4-C5) 1.36 R(C4-H26) 1.089 R(C5-C6) 1.438 R(C5-H27) 1.086 R(C6-C7) 1.364 R(C6-H28) 1.089 R(C7-C8) 1.466 R(C7-C12) 1.511 R(C8-C9) 1.352 R(C8-H29) 1.088 R(C9-C14) 1.473 R(C9-H30) 1.089 R(O11-H31) 0.971 R(C12-H32) 1.097 R(C12-H33) 1.097 R(C12-H34) 1.09 R(C13-H35) 1.096 R(C13-H36) 1.096 R(C13-H37) 1.087 R(C14-C15) 1.422 R(C14-C19) 1.416 R(C15-C16) 1.406

R(C15-C22) 1.515 R(C16-C17) 1.407 R(C16-C21) 1.512 R(C17-C18) 1.396 R(C17-O23) 1.369 R(C18-C19) 1.394 R(C18-H38) 1.084 R(C19-C20) 1.513 R(C20-H39) 1.097 R(C20-H40) 1.095 R(C20-H41) 1.093 R(C21-42) 1.098 R(C21-H43) 1.089 R(C21-H44) 1.094 R(C22-H45) 1.098 R(C22-H46) 1.09 R(C22-H47) 1.093 R(O23-C24) 1.418 R(C24-H48) 1.091 R(C24-H49) 1.097 R(C24-H50) 1.097 A(C2-C1-O10) 129 A(C2-C1-O11) 110.3 A(C1-C2-C3) 127.2 A(C1-C2-H25) 114.2 A(C10-C1-O11) 120.7 A(C1-O11-H31) 105.3 A(C3-C2-H25) 118.6 A(C2-C3-C4) 117.5 A(C2-C3-C13) 123.7 A(C4-C3-C13) 118.8 A(C3-C4-C5) 125.9 A(C3-C4-H26) 116 A(C3-C13-H35) 110.2 A(C3-C13-H36) 110.2 A(C3-C13-H37) 111.3 A(C5-C4-H26) 118.1 A(C4-C5-C6) 123.2 A(C4-C5-H27) 118.7 A(C6-C5-H27) 118.1 A(C5-C6-C7) 127.3 A(C5-C6-H28) 115.2 A(C7-C6-H28) 117.5

A(C6-C7-C8) 122.1 A(C6-C7-C12) 123.5 A(C8-C7-C12) 114.4 A(C7-C8-C9) 126.8 A(C7-C8-H29) 114.9 A(C7-C12-H32) 110.8 A(C7-C12-H33) 110.1 A(C7-C12-H34) 113.5 A(C9-C8-H29) 118.3 A(C8-C9-C14) 127.3 A(C8-C9-H30) 117.4 A(C14-C9-H30) 115.2 A(C9-C14-C15) 123.7 A(C9-C14-C19) 117.3 A(H32-C12-H33) 106.6 A(H32-C12-H34) 108 A(H33-C12-H34) 107.6 A(H35-C13-H36) 107 A(H35-C13-H37) 109 A(H36-C13-H37) 109.1 A(C15-C14-C19) 119 A(C14-C15-C16) 120.6 A(C14-C15-C22) 121.6 A(C14-C19-C18) 119.9 A(C14-C19-C20) 121.5 A(C16-C15-C22) 117.7 A(C15-C16-C17) 119 A(C15-C16-C21) 121.5 A(C15-C22-H45) 112.1 A(C15-C22-H46) 111.6 A(C15-C22-H47) 110.8 A(C17-C16-C21) 119.5 A(C16-C17-C18) 120.6 A(C16-C17-O23) 116.2 A(C16-C21-H42) 111.6 A(C16-C21-H43) 110.8 A(C16-C21-H44) 111.5 A(C18-C17-O23) 123.2 A(C17-C18-C19) 120.8 A(C17-C18-H38) 120.5 A(C17-O23-C24) 118.8 A(C19-C18-H38) 118.7 A(C18-C19-C20) 118.7

A(C19-C20-H39) 111.9 A(C19-C20-H40) 111.8 A(C19-C20-H41) 110.8 A(H39-C20-H40) 106.9 A(H39-C20-H41) 107.3 A(H40-C20-H41) 107.9 A(H42-C21-H43) 107.5 A(H42-C21-H44) 107 A(H43-C21-H44) 108.3 A(H45-C22-H46) 107.5 A(H45-C22-H47) 106.6 A(H46-C22-H47) 108 A(O23-C24-H48) 105.9 A(O23-C24-H49) 111.8 A(O23-C24-H50) 111.8 A(H48-C24-H49) 109.1 A(H48-C24-H50) 109.2 A(H49-C24-H50) 109

Fig 1.Atom numbering system adopted in this study for acetretin.

1.5.2 Vibrational spectral analysis Vibrational spectral assignments are performed on the experimentally recorded FT-IR

and FT-Raman spectra based on the theoretically predicted wavenumbers by density functional B3LYP/6-31 G(d,p) method and are collected in Table 2. None of the predicted vibrational spectra has imaginary frequencies. These discrepancies are corrected by implementing the scaling calculations on the wavenumbers with the factor 0.967. The title molecule has 50 atoms and has 144 fundamental modes of vibrations. This molecule belongs to C1 symmetry. Theoretically computed frequencies of both infrared and Raman frequencies together with experimental data of the title molecule are presented in Table 2. The potential energy distribution

was calculated by Veda 4 program [16, 17]. The observed experimental FT-IR and FT-Raman spectra along with the theoretical spectra are shown in Figs. 2 and 3. C-H vibrations

The hetero aromatic structure shows the presence of C-H stretching vibrations in the region 3100–3000 cm-1, which is the characteristic region for the ready identification of acitretin vibrations [18]. In this region, the bands are not affected, appreciably by the Nature of the substituents. In the FT-Raman spectrum of acitretin is observed in the ranging from 3000 to 3200 cm-1are assigned to C-H stretching vibrations. This mode is calculated in the range 3010 to 3215 cm-1 with B3LYP / 6-31G (d,p) method with the PED contribution of almost above 95%. As expected these modes are pure stretching modes as it evident from PED column in Table 2. The C-H in-plane bending frequencies appear in the range 1300–1000 cm-1 and are very useful for characterization purpose [19]. In this present work, the in-plane bending vibrations were observed at 1355, 1290 ,1235,1214 cm-1 in FT-IR spectrum and at 1355, 1294, 1240, and 1210 cm-1 in FT-Raman spectrum . The PED of vibrations shows that they are not in pure modes. The theoretically scaled vibrations with B3LYP/6-31G (d,p) method, also shows good agreement with experimentally recorded data.

Fig 2.Experimental and calculated FT-IR spectrum of acitretin.

Fig 3.Experimental and calculated FT-Raman spectrum of acitretin.

Table 2.

Calculated scaled IR wavenumbers, relative intensities for acitretin using B3LYP/6-31G(d,p) basis set.

Experimental Calculated Frequencies (cm-1) Vibrational assignments PED(%) FT-IR FT-Raman B3LYP IR Inten IntRaman 3802 -

3765 116.15 0.98

γ OH(100)

3210 3200

3215 17.63 0.30

γ CH(100) 3173 3185

3199 28.81 0.11

γ CH(100)

3182 3183

3195 2.52 0.26

γ CH(100) 3180 3180

3193 5.81 0.20

γ CH(100)

3157 3160

3167 41.48 0.23

γ CH(100) 3162 3163

3165 5.46 0.27

γ CH(97)

3160 3160

3161 4.66 0.18

γ CH(95) 3150 3155

3156 29.23 0.07

γ CH(95)

3152 3153

3153 6.83 0.23

γ CH(95) 3151 3151

3152 11.61 0.04

γ CH(80)

3150 3150

3150 3.09 0.06

γ CH(86) 3144 3143

3146 33.56 0.65

γ CH(91)

3120 3122

3125 22.82 0.21

γ CH(80) 3102 3100

3102 20.59 0.26

γ CH(70)

3094 3095

3096 15.52 0.16

γ CH(99) 3093 3093

3093 28.63 0.36

γ CH(85)

3091 3090

3091 18.71 0.15

γ CH(95) 3080 3083

3084 16.19 0.39

γ CH(100)

3075 3075

3078 39.43 0.20

γ CH(100) 3044 3043

3045 16.27 0.32

γ CH(99)

3040 3037

3037 36.89 0.94

γ CH(100) 3041 3036

3036 24.56 0.67

γ CH(100)

3040 3035

3035 18.31 0.29

γ CH(86) 3034 3034

3034 26.39 0.24

γ CH(90)

2993 2986

2990 78.61 0.39

γ CH3(90) 1782 1767

1791 435.55 1.54

γ OC(80)+γ CC(10)

1693 1695

1697 2.78 1.50

γ CC(50)+β HCC(15) 1662 1663

1664 267.63 2.31

γ CC(65)

1655 1655

1650 101.06 37.28

γ CC(50)+β HCC(20) 1630 1635

1641 75.54 4.32

γ CC(42)

1624 1626

1621 689.46 100.00

γ CC(45) - 1610

1615 61.69 13.08

γ CC(56)

- 1535

1529 29.16 0.70

β HCH(10) - 1525

1530 29.16 0.70

β HCH(22)

1525 1515

1520 76.07 0.09

β HCH(52)+τ HCOC(15) 1515 -

1518 6.73 0.05

β HCH(75)+τ HCCC(20)

- -

1517 22.44 0.05

β HCH(87)+τ HCCC(15) - 1516

1516 23.59 0.31

β HCH(40)

1512 1510

1510 12.53 0.05

β HCH(66) 1515 1505

1504 3.81 0.10

β HCH(70)

1502 1503

1503 45.80 1.92

β HCH(70)+τ HCCC(15) 1501 1500

1500 7.17 0.08


1497 -

1497 21.25 0.14

β HCH(45)

-

1496 116.68 0.59

β HCH(96)+τ HCCC(15) 1496 1495

1496 11.49 0.08

β CH3(47)

1493 1493

1493 10.30 0.62

β CH3(56) 1484 1480

1484 8.26 0.14

γ CC(25)

1445 1445

1444 36.34 2.01

γ CC(35) 1435 1436

1439 105.91 0.76

γ CC(10)+β HCC(41)+β HCH(25)

1434 1434

1432 2.49 0.21

β HCH(92) 1433 1430

1430 2.30 0.13

β HCH(76)

1424 1425

1428 0.71 0.05

β HCH(90) 1415 1420

1421 6.60 0.04

β HCH(66)

1403 1405

1407 15.95 0.79

β HCH(50) 1394 1396

1397 119.05 3.98

β HCC(40)+β HCH(25)

1352 1353

1358 3.00 2.58

γ CC(22)+β HCC(36) 1355 1355

1356 15.73 0.99

β HCC(60)

1345 1347

1346 8.17 3.36

β HOC(30)+β HCC(30) - 1341

1342 65.88 0.10

γ CC(30)+β HCC(15)

- -

1330 350.29 8.27

γ CC(25)+γ OC(17)+ β CCC(15) 1315 1320

1324 39.26 5.46

γ CC(30)

1290 1294

1299 2.82 0.08

β HCC(62) - 1258

1261 17.98 0.04

γ CC(11)+β HOC(12)+β HCC(61)

- 1256

1256 27.28 0.07

β HCC(35) 1235 1240

1246 1.10 2.46

γ CC(14)+β HCC(16)

1214 1210

1218 16.51 0.09

β HCH(15)+τ HCOC(62) 1210 1208

1209 45.88 14.28

γ CC(22)+β HCC(20)

- 1207

1207 25.75 1.88

γ CC(47) - 1175

1177 1.20 0.02

γ CC(32)

1155 1160

1162 213.41 0.01

γ OC(48)+γ CC(16) 1135 1140

1143 1070.92 7.78

γ OC(40)+β HOC(26)

1105 1110

1114 8.33 0.26

γ CC(29)+τ HCCC (14) 1100 1072

1072 4.21 0.03


- -

1071 2.15 0.04

β HCH(23) 1063 1062

1061 9.05 0.06

β HCH(11)+τ HCCC (53)

- 1060

1060 3.88 0.02

β HCH(13)+τ HCCC(63) 1056 1055

1055 8.44 1.76


1050 1054

1054 11.12 1.02

τ HCCC(39) - 1043

1045 0.80 0.01

γ OC(17)+τ HCCC (32)

1041 1042

1042 0.66 0.02

τ HCCC(56) 1020 -

1024 42.18 3.40

τ HCCC(49)

1009 1010

1016 11.39 0.10

γ OC(11) - 1006

1007 3.88 2.50

τ HCCC(57)

1000 1003

1004 21.62 0.67

τ HCCC(75) 982 975

980 3.30 0.53

γ CC(48)+γ OC(14)

962 965

963 2.53 0.02

γ CC(22)

920

928 2.26 0.12

τ HCCC(71) 906 905

910 8.98 0.07

τ HCCC(44)

- -

900 1.08 0.03

γ CC(13)+τ HCCC (14) 876 875

877 2.90 1.93

τ HCCC(63)

865 862

861 9.00 0.10

γ CC(17) 859 858

853 26.99 0.21

τ HCCC(65)

851 850

850 2.61 0.05

τ HCCC(57) 845 846

847 5.63 0.02

γ CC(44)

755 756

754 33.76 0.00

ϒ OCOC(77) 745 -

748 4.14 0.00

τ CCCC(13)

726 720

727 1.01 0.01

τ CCCC(13) 720 -

699 26.24 0.09

γ OC(16)+β OCO(31)+β CCC(13)

653 652

654 8.67 0.01

ϒ OCCC(27)+ϒ CCCC(13) - 635

636 2.87 0.01

β OCC(13)+β CCC(12)

- 630

632 14.55 0.04

β CCC(14) 602 600

604 89.17 0.05

τ HOCC(92)

583 586

589 27.10 0.09

β OCO(22) - -

570 13.15 0.02

γ CC(32)+γ OC(11)+β CCC(15)

555 -

550 3.59 0.47

ϒ CCCC(46) 550 551

549 0.65 0.01

τ HCCC(28)+ϒ CCCC(40)

532 534

532 10.14 0.04

ϒ CCCC(16) 517 510

516 2.64 0.06

β HCH(55)+ϒ CCCC(13)

485 486

487 1.97 0.11

β CCC(39) - -

460 7.72 0.07

β CCC(25)

430 428

435 9.82 0.00

β CCC(14)+β COC(10) 400 403

405 14.58 0.64

ϒ CCCC(27)

- 401

402 3.62 0.15

β CCC(34)+β COC(25) 385 384

386 0.89 0.00

β CCC(47)+τ HCCC(11)

380 382

381 6.16 0.00

β OCC(31)+β CCC(13) - 340

345 0.54 0.25

β CCC(29)+τ CCCC(10)

- 335

331 0.05 0.02

β CCC(14) - 320

322 1.09 0.02

β CCC(30)+ϒ CCCC(11)

- 300

305 11.76 0.39

τ CCCC(13)+ϒ CCCC(57) - 290

292 11.80 1.08

β CCC(14)

- 268

264 11.70 0.33

β CCC(12) - 264

263 0.18 0.00

τ HCOC(23)

- 257

256 2.95 0.01

β HCC(35)+τ CCCC(51) - 226

227 0.07 0.12

β OCC(17)+β COC(15)+τ HCCC(12)

- 210

215 1.84 0.03

γ CH(99)+β OCC(23)+β COC(14) - -

203 0.13 0.02

τ HCCC(26)+ϒ OCCC(17)

- 196

198 1.43 0.06

β CCC(27)+β OCC(11) - 175

177 0.45 0.00

τ HCCC(18)+τ CCCC(17)+ϒ CCCC(24)

- 174

173 0.06 0.08

τ CCCC(25)

-

153 1.50 0.11

γ CH(76)+τ HCCC(29) - 143

145 3.25 0.33

τ HCCC(26)+τ CCCC(12)

- 130

131 12.93 2.09

τ CCCC(38) - 127

128 1.05 0.29

τ HCCC(36)

- 114

116 0.61 0.02

γ OC(48)+γ CC(14)+β CCC(10)+τ HCCC(29) - 106

109 1.68 0.08

τ HCCC(38)+τ CCCC(17)+τ COCC(13)

- 104

104 0.09 0.00

τ HCCC (23) - 80

83 3.35 0.04

τ COCC(38)

- 73

73 0.20 0.06

β CCC(13)+τ OCCC(30)+τ COCC(11) - 70

72 0.82 0.00

β CCC(18)+τ CCCC(11)+τ OCCC(24)

- -

47 1.13 0.01

τ CCCC(27)+τ OCCC(10) - -

44 0.60 0.05

τ CCCC(42)

- -

26 0.08 0.01

τ CCCC(59) - - 24 0.09 0.02 β CCC(28)+τ CCCC(21)

IR int b

IR intensity (Kmmol -1 )

Raman intensity (arb.units)

γ - stretching, β - in plane bending, ϒ - out of plane bending, τ - torsion a Scaling factor: 0.967 for DFT (B3LYP)/6-31G(d,p) b Relative absorption intenditied normalised with highest peak absorption = 100. c Relative Raman intensities normalized to 100.

C=O vibrations and C-O vibrations

The C=O stretch of carboxylic acids in identical to the C=O stretch in ketones, which is expected in the region 1740– 1660 cm-1[20]. The band is reasonably easy to be recognized due to its high intensity. In the present study the band observed at 1782 cm-1 in FT-IR and 1767 cm-1 in FT-Raman is assigned for C=O vibration and it coincides with calculated value as 1791 cm-1 with PED contribution of 80%. This multiple bonded group is highly polar and therefore gives rise to an intense infrared absorption band. In the C-O band, the absorption is sensitive for both the carbon and oxygen atoms of carbonyl group. We can find that the C-O vibration has been affected by the neighboring molecular interactions. Normally it occurs in the region 1260–1000 cm-1 [21].However, these bands overlap with other bands that are due to aromatic vibrations causing that their undisputed assignment is often difficult. In the present case, the bands at 1155, and 1135 cm-1 in FT-IR spectrum and 1160, 1140 cm-1 in FT-Raman spectrumare assigned to C-O vibrations which agrees with calculated values at 1162 and 1143 cm-1 respectively. These assignments are in good agreement with the literature value. C-C vibrations

The carbon–carbon stretching modes of the phenyl group are expected in the range from 1650 to 1200 cm-1. The actual position of these modes is determined not so much by the nature

of the substituent but by the form of substitution around the ring [22]. In general, the bands are of variable intensity and are observed at 1693, 1662, 1493, 1484, 1445, cm-1 in FT-IR spectrum and 1695, 1663, 1493, 1480, 1445cm-1 in FT-Raman spectrum respectively. The theoretically computed values of C-C vibrations are 1697, 1664,1493,1484,1444 cm-1 with almost 70% of PED contribution. CH3 vibrations

Two asymmetric and one symmetric stretching vibrations of CH3 group are usually observed in the range 2990–2950 cm−1 [23]. In the present case of our molecule, the asymmetric stretching vibrations of CH3 group have been identified at 2993 cm-1 by B3LYP method. The in-plane bending vibration of the CH3 group is identified at 1495 and 1493 cm-1 in FT-Raman spectrum and 1496 and 1493 cm-1 in FTIR spectrum. The computed wavenumbers at 1496 and 1493 cm-1 in B3LYP method is assigned to in-plane bending of the CH3 group. 1.6 Theoretical calculations 1.6.1 NBO analysis

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 calculated to evaluate the donor–acceptor interactions in NBO analysis [24]. The interactions result in 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 E2 associated with the delocalization i→j is estimated

as 퐸 = ∆퐸 = ( )

Where qi is the donor orbital occupancy, εj and εi are diagonal elements and Fij is the off diagonal NBO Fock matrix element. NBO analysis gives a convenient basis for investigating charge transfer or conjugative interaction in molecular systems. Some electron donor orbital, acceptor orbital and the interacting stabilization energies resulting from the second-order micro-disturbance theory are reported [25].The larger the E(2) 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 orbital and formally unoccupied (antibond or Rydgberg) non-Lewis NBO orbital correspond to a stabilizing donor–acceptor interaction. NBO analysis was performed on the title molecule at B3LYP/6-31G (d,p) level in order to elucidate the intramolecular, hybridization and delocalization of electron density in the title molecule and were presented in Table 3.The atoms of π(C2-C3) to π* (C1-C10) with stabilization energy of 0.27kj/mol and from π(C15-C16) to π* (C17-C18) antibonding orbitals with stabilization energy of 0.26kj/mol. From σ (C18-C19) to σ* (C17-O23) with the stabilization energy as 1.06kj/mol. From σ (C17-H28) to σ* (C17-H28) with stabilization energy of 0.67 kj/mol. The magnitude of energy transferred from LP (2) of O10→σ* (C1-O11) had the stabilization energies 0.66kj/mol. Similarly the electron donated from LP (2) of O23→ (C17-C18) leads to the stabilization energy of 0.38 kj/mol.

Table 3

Second order perturbation theory analysis of Fock matrix in NBO basis for acitretin using DFT by B3LYP/6-31G(d,p) density functional calculations.

Donar Type Acceptor Type E(2)a (kj/mol) E(j) - E(i)b (a.u) F (i,j)c (a.u)

C 1 - C 2 σ C 3 - C 4 σ* 4.34 1.1 0.062

C 1 - C 2 σ O 11 - H 31 σ* 2.82 1.07 0.049

C 1 - O 11 σ C 2 - C 3 σ* 0.55 1.63 0.027

C 2 - C 3 π C 1 - O 10 π* 21.73 0.27 0.071

C 2 - C 3 π C 4 - C 5 π* 10.66 0.32 0.052

C 3 - C 4 σ C 13 - H 37 σ* 0.74 1.16 0.026

C 3 - C 13 σ C 2 - C 3 σ* 3.31 1.29 0.058

C 4 - H 26 σ C 3 - C 13 σ* 2.93 0.96 0.047

C 5 - C 6 σ C 4 - C 5 σ* 2.41 1.3 0.05

C 6 - C 7 π C 12 - H 33 σ* 2.27 0.68 0.036

C 7 - C 8 σ C 12 - H 34 σ* 1.02 1.07 0.03

C 8 - C 9 π C 6 - C 7 π* 8.08 0.33 0.046

C 8 - H 29 σ C 6 - C 7 π* 0.9 0.58 0.021

C 9 - C 14 σ C 15 - C 16 σ* 2.46 1.19 0.048

C 9 - H 30 σ C 14 - C 19 π* 0.83 0.57 0.021

C 12 - H 32 σ C 6 - C 7 π* 4.26 0.56 0.045

C 13 - H 37 σ C 3 - C 4 σ* 5.15 0.91 0.061

C 14 - C 19 σ C 8 - C 9 σ* 0.83 1.33 0.03

C 15 - C 16 π C 14 - C 19 π* 18.15 0.28 0.064

C 15 - C 16 π C 17 - C 18 π* 22.84 0.26 0.07

C 15 - C 22 σ C 22 - H 47 σ* 0.64 1.04 0.023

C 16 - C 21 σ C 14 - C 15 σ* 2.88 1.19 0.052

C 17 - C 18 π C 15 - C 16 π* 17.58 0.29 0.064

C 18 - C 19 σ C 17 - C 18 σ* 2.37 1.22 0.048

C 18 - C 19 σ C 17 - O 23 σ* 3.72 1.06 0.056

C 19 - C 20 σ C 20 - H 41 σ* 0.56 1.04 0.022

C 20 - H 39 σ C 14 - C 19 σ* 1.89 1.06 0.04

C 20 - H 41 σ C 19 - C 20 σ* 0.54 0.93 0.02

C 21 - H 42 σ C 15 - C 16 σ* 1.92 1.06 0.04

C 21 - H 44 σ C 16 - C 17 σ* 4.15 1.05 0.059

C 21 - H 44 σ C 16 - C 21 σ* 0.67 0.93 0.022

C 22 - H 47 σ C 14 - C 15 σ* 2.05 1.06 0.042

C 22 - H 47 σ C 15 - C 16 π* 3.04 0.54 0.04

O 10 LP ( 2) C 1 - O 11 σ* 30.27 0.66 0.129

O 10 LP ( 2) C 13 - H 37 σ* 23.83 0.82 0.128

O 23 LP ( 2) C 17 - C 18 π* 12.23 0.38 0.065

C 1 - O 10 π* C 2 - C 3 π* 25.03 0.05 0.067

C 4 - C 5 π* C 6 - C 7 π* 43.2 0.01 0.056

C 8 - C 9 π* C 22 - H 46 σ* 0.56 0.4 0.053

C 14 - C 19 π* C 9 - H 30 σ* 0.72 0.4 0.034

C 14 - C 19 π* C 20 - H 39 σ* 1.26 0.38 0.043

C 14 - C 19 π* C 20 - H 41 σ* 1.19 0.38 0.042

C 15 - C 16 π* C 21 - H 42 σ* 1.01 0.39 0.041

C 15 - C 16 π* C 21 - H 43 σ* 1.01 0.38 0.041

C 17 - C 18 π* C 14 - C 19 π* 168.68 0.02 0.079

C 17 - C 18 π* O 23 - C 24 σ* 0.92 0.29 0.033

1.6.2 Molecular electrostatic potential (MEP) Molecular electrostatic potential(MEP) at a point in the space around a molecule gives an

indication of the net electrostatic effect produced at that point by the total charge distribution (electron + nuclei) of the molecule and correlates with dipole moments, electron negativity ,partial charges and chemical reactivity of the molecule [26,27]. It provides a visual method to understand the relative polarity of the molecule as shown in Fig. 4. Different values of the electrostatic potential are represented by different colors; red represents the regions of the most negative electrostatic potential, white represents the regions of the most positive electrostatic potential and blue represents the region of zero potential. The potential increases in the order red<green<blue<pink<white. It can be seen that the negative regions are mainly over the O10, O11, O23 atoms.Negative (red color) regions of MEP are related to electrophilic reactivity and the positive ones (white color) to nucleophilic reactivity. The negative electrostatic potential corresponds to an attraction of the proton by the aggregate electron density in the molecule (shades of red), while the positive electrostatic potential corresponds to the repulsion of the protons by the atomic nuclei (shades of white). According to these calculated results, the MEP map shows that the negative potential sites are on oxygen atoms and the positive potential sides as well are around the hydrogen atoms. The predominance of light green region MEP surface corresponds to a potential half way between the two extremes red and dark blue color. Total electron density and MEP surfaces of the molecules under investigation are constructed by using B3LYP/6-31G(d,p) method. The MEP map shows that the negative potential sites are on electro negative atoms as well as the positive potential sites are around the hydrogen atom. These sites give information about the region from where the compound can have non-covalent interactions.

Fig 4.The total electron density surface mapped with electrostatic potential of acitretin.

1.6.3 Homo ‒Lumo energy gap

The concepts of aromaticity, HOMO, and LUMO are of fundamental importance in understanding the chemical stability and reactivity of many organic molecules. Molecular orbital and their properties like energy is very useful to the physicists and chemists and their frontier electron density is used for predicting the most reactive position in p electron system and also

explains several types of reaction in conjugated systems . Both the HOMO and LUMO are the main orbitals which take part in chemical stability. HOMO represents the ability to donate an electron, LUMO as an electron acceptor, represents the ability to obtain an electron. Moreover, the Eigen values of the HOMOs (p donor) and the LUMOs (p acceptor) and their energy gap reflects the chemical activity. Recently, the energy gap between HOMO and LUMO has been used to prove the bioactivity from intra-molecular charge transfer (ICT) [28,29]. The conjugated molecules are characterized by HOMO–LUMO separation, which is the result of a significant degree of intermolecular charge transfer (ICT) from the end-capping electron-donor to the efficient electron acceptor group through p conjugated path. The strong charge transfer interaction through p conjugated bridge results in substantial ground state donor–acceptor mixing and the appearance of a charge transfer band in the electronic absorption spectrum. Therefore, an electron density (ED) transfer occurs from the more aromatic part of the p conjugated system in the electron-donor side to electron-withdrawing part. The calculated energy values for HOMO and LUMO energies are -5.124 eV and -2.076 eV respectively. The difference in energies of HOMO and LUMO is 3.0781 eV respectively. The 3D plots of HOMO and LUMO of acitretin are shown in Fig.5.

Fig 5.The HOMO, LUMO orbitals of acitretin using density functional calculations

1.6.4 Mulliken atomic charges The Mulliken atomic charge distribution brings important role in the application of

quantum chemical calculations to molecular system [30,31]. The charge distribution over the atom suggests the formation of donor and acceptor pairs involving the charge transfer in the molecule. The Mulliken population analysis in the acitretin molecule was calculated using B3LYP level with 6-31G (d,p) basis set is listed in table 4 . The calculated charges of O10, O11, O23 atoms are -0.9723,-0.9198,-1.1198 respectively. The neighboring atom C1 is connected with these oxygen atoms has the high positive charge 1.8358. This distribution may be the reason for the stability of the molecule under consideration. Table 4.

Mulliken atomic charges of acitretincalculated by B3LYP/6-31G(d,p) basis set.

Atom with numbering B3LYP/6-31G(d,p)

C1 1.835824 C2 -0.853968 C3 0.719541 C 4 -0.458271 C 5 0.414384

C6 -0.446817 C7 0.445333 C8 -0.184662 C9 0.211172 O10 -0.972302 O11 -0.919804 C 12 -0.002635 C13 -0.056364 C14 -0.221021 C15 0.187965 C16 -0.161681 C17 0.769745 C18 -0.278476 C19 0.128326 C20 0.064985 C21 0.06649 C22 0.033522 O23 -1.119852 C24 0.597147 H25 0.033675 H26 -0.002819 H27 0.024969 H28 -0.001811 H29 -0.005757 H30 0.004168 H31 0.321452 H32 0.002599 H33 -0.016482 H34 -0.007664 H35 0.000165 H36 -0.009432 H37 0.068439 H38 0.016833 H39 -0.017609 H40 -0.013352 H41 -0.023859 H42 -0.030875 H43 0.018896 H44 -0.029629 H45 -0.020696 H46 0.027164 H47 -0.022339 H48 -0.027772

H49 -0.036749 H50 -0.050096

1.6.5 Thermodynamic Features

On the basis of vibrational analysis and statistical thermodynamic, the standard thermodynamic functions : heat capacity (C0

p,m), entropy (S0m) and enthalpy(H0

m) were calculated using perl script [32] and are listed in Table 5.The thermodynamic values increase with temperature from 100 to 1000K and even after ionization , there is an increment in all values , which is attributed to equation among entropies, heat capacities, enthalpy changes and temperatures were fitted by quadratic formulas the enhancement of the molecular vibration as the temperature increases. The correlation and the corresponding fitting factors (R2) for these thermodynamic properties are 0.99996, 0.99971 and 0.99957 respectively. The correlations between these thermodynamic properties and temperatures are shown in Fig.6. are shown as follows: Cp,m

o =295.80984+ 1.6759 -4.1059610-4 T2 (R2= 0.9998) Sm

o =51.62438+ 1.40268T -5.39605 10-4 T2 (R2= 0.99971) Hm

o =-16.63424 + 0.18824 T + 4.06225 10-4 T2 (R2= 0.99957) These equations could be used for further studies on the title compound. For instance, when we investigate the interaction between the title compound and another compound, thermodynamic properties could be obtained from these equations and then used to calculate the change of Gibbs-free energy of the reaction, which will help to judge the spontaneity of the reaction.

Fig. 6.Correlation graph between thermodynamic parameters and temperature.

1.7 Conclusion

The spectral studies such as FT-IR, FT-Raman for acitretin was carried out with quantum chemical computations. A complete vibrational and molecular structure analysis has been performed based on the quantum mechanical approach by B3LYP calculations with 6-31G(d,p) basis set. The difference between the observed and scaled wave number values of the most of the fundamental is very small. The complete vibrational assignment with PED was calculated. Mulliken population analysis was carried out. NBO analysis was made and it indicated the intra molecular charge transfer between the bonding and antibonding orbitals. The calculated HOMO and LUMO energies were used to analyze the charge transfer within the molecule. The predicted

MEP figure revealed the negative regions of the molecule, was subjected to the electrophilic attack of this compound. The theoretically constructed FT-IR and FT-Raman spectra had good correlation with experimentally observed FT-IR and FT-Raman spectra. The thermodynamic properties to the title compound were calculated for different temperatures, and the correlations among the properties and temperatures were obtained.

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