Molecular Geometry, Vibrational Assignments, HOMO-LUMO, Mulliken’s charge analysis and DFT Calculations of 2-(2-Phenylaminothiazole-5-oyl)1- methyl-6-methylbenzimidazole

 

S. Sangeetha¹*, T. F. Abbs Fen Reji²

¹Department of Chemistry, Sivanthi Adithanar College, Pillayarpuram-629501, Tamilnadu, India

²Department of Chemistry and Research Centre, Nesamony Memorial Christian College, Marthandam-629165, Tamilnadu, India

 

ABSTRACT:

The vibrational wave numbers of 2-(2-phenylaminothiazol-5-oyl)-1-methyl-6-methylbenzimidazole were calculated using B3LYP/6-31G basis set and calculations are used to assign vibrational bands obtained experimentally. The B3LYP method is able to predict vibrational frequencies and structural parameters. The optimized molecular geometry, bond lengths, bond angles, dihedral angles and harmonic vibrational wave numbers of the titled Compound have been investigated by Density Functional Theory (DFT) method. The geometries obtained from DFT method is found to be in good agreement with experimental data. The Mulliken population analysis on atomic charges has been computed using DFT calculations. Energetics of the Highest Occupied Molecular Orbital (HOMO) and Lowest unoccupied Molecular (LUMO) of the molecule were calculated using the Gaussian 09 software package.

 

 

 

INTRODUCTION:

Heterocyclic compounds have played an important role in pharmaceutical chemistry due to their biological activities¹. Benzimidazole shows several structural features and biological activities just an indoles². The literature survey shows several examples of compounds having benzimidazole derivatives possess anticancer³, anti-inflammatory⁴, antibacterial⁵ and anti-fungal⁶ activities.

 

Vibrational spectroscopy is used extensively in organic chemistry for the identification of functional groups of organic compounds, for studies on molecular confirmation, reaction kinetics etc^7-9.

 

 

 

Literature survey reveals that to the best of our knowledge, the results based on quantum chemical calculations, HOMO-LUMO analysis of 2-(2-phenylaminothiazole-5-oyl)-N-methyl-6-methylbenzimidazole have no reports. Here in we reported detailed interpretations of vibrational assignments which are acceptable and supportable.

 

Patil et al. reported the DFT study on dihydroxyphenylbenzothiazole by using B3LYP/6-31G(d)¹⁰. The main objective of this paper is to present, more accurate vibrational assignments, bond lengths, bond angles, atomic charges and HOMO-LUMO of 2-(2-phenylaminothiazole-5-oyl)-N-methyl-6-methylbenzimidazole using DFT/B3LYP method. The optimized geometry obtained from DFT calculation was then used to perform NBO analysis. A systematic study on vibrational spectra and structure of the compound is carried out. Numerous reports have been made citing the success of DFT compound to conventional methods in computing molecular and chemical properties such as geometries, vibrational frequencies thermodynamical properties¹¹

 

Experimental Details:

The regents and solvents used were purchased from Sigma Aldrich, Merck specialties Pvt. Ltd and Hi-media laboratories Pvt. Ltd. The compound 2-(2-phenylaminothiazole-5-oyl)-N-methyl-6-methylbenzimidazole was prepared according to the following method. A solution of 1-aryl-3-(N, N-dimethyl) thiourea (1mmol) in DMF(2ml) was added to a solution of 2-(2-bromoacetyl)-N-methyl (-6-methylbenzimidazole (0.254g, 1mmol) which was prepared from 2-(1-hydroxyethyl)benzimidazole in DMF (2ml).The reaction mixture was stirred well and triethyl amine (0.15 ml, 1mmol)was added. The reaction mixture was warmed at 80-85°C for 5 minutes. It was then cooled and poured in to ice-cold water with constant stirring. A yellow precipitate thus obtained was filtered, washed with water and dried. The crude product was crystallized from ethanol-water (2:1) to give yellow crystalline solid.

 

Computational Details:

The DFT computation of 2-(2-phenylaminothiazole-5-oyl)-N-methyl-6-methyl benzimidazole has been performed using Gaussian ’09 program package at the Becke-3Lee-Yang-Parr(B3LYP) level with standard 6-31G basis set. The optimized structural parameters are used in the vibrational frequency calculations at DFT level. At the optimized geometry of the title molecule no imaginary frequency modes are obtained, so there is a true minimum potential energy surface is found.

 

The assignments of the normal modes of vibration for the titled compound have been made by visual inspection of the individual mode using the Gauss view software¹². All the calculations were done for the optimized structures in gas phase. The methodology involved in density functional theory includes electron correlation and hence is the desired method for obtaining theoretical charge density in molecules. DFT, over the years has become a practical tool for calculating charge-density distributions. Since its adaptability to high- speed computers in easy.

 

RESULTS AND DISCUSSION:

Optimized Geometry:

Optimized geometry was subjected to optimization in the ground state. The optimized structural parameter was calculated by DFT/B3LYP -6-31G basis set is listed in Table-1 in accordance with the atom numbering scheme given in figure-1.

 

 

Figure 1: Optimized geometrical structure of 2-(2-phenylaminothiazol-5-oyl)-1-methyl-6- methylbenzimidazole.

 

Table.1: Optimized geometrical parameters of 2-(2-phenylaminothiazole-5-oyl)-N-methyl-6-methylbenzimidazole at B3LYP with 6-31G level.

Parameters	Bond lengths(Å)	Parameters	Bond angles(◦)	Parameters	Dihedral angle(◦)
	Calculated		Calculated		Calculated
N1-C2 C2-N3 N3-C4 C4-C5 C5-C6 C6-C7 N3-C8 N1-C9 C2-C10 C10-S11 S11-C12 C12-N13 N13-C14 C14-C15 C12-C16 C16-C17 C17-C18 C18-C19 C19-C20 S20-C21 C12-N22 C10-O23 C5-H24 C6-H25 C7-H26 N22-H27 C21-H28 C17-H29 C20-H30 C18-H31 C19-H32 N3-C33 C33-H34 C33-H35 C33-H36 C4-H37 C37-H38 C37-H39 C37-H40 C14-H41	1.3343 1.4000 2.5937 1.3984 1.4147 1.385 1.4004 1.3849 1.475 2.803 1.8384 1.3222 1.377 1.376 2.5129 1.408 1.394 1.400 1.399 1.3986 1.3594 1.2649 1.0860 1.084 1.0835 1.010 1.080 1.087 1.085 1.085 1.084 1.471 1.090 1.090 1.084 1.513 1.097 1.097 1.093 1.079	N1-C2-N3 C2-N3-C4 N3-C4-C5 C4-C5-C6 C5-C6-C7 C2-N3-C8 C2-N1-C9 N1-C2-C10 C2-C10-S11 C10-S11-C12 S11-C12-N13 C12-N13-C14 N13-C14-C15 S11-C12-C16 C12-C16-C17 C16-C17-C18 C17-C18-C19 C18-C19-C20 C19-C20-C21 S11-C12-N22 C2-C10-O23 C4-C5-H24 C5-C6-H25 C6-C7-H26 C12-N22-H27 C20-C21-H28 C16-C17-H29 C19-C20-H30 C17-CH18-H31 C18-C19-H32 C2-N3-C33 N3-C33-H34 N3-C33-H35 N3-C33-H36 N3-C33-H37 C4-C37-H38 C4-C37-H39 C4-C37-H40 N13-C14-H41	112.1955 119.77 138.34 123.63 120.70 106.56 105.98 124.42 155.14 113.69 114.55 112.13 117.50 144.94 141.37 120.22 119.29 121.25 119.09 120.20 117.84 119.04 122.55 115.83 115.83 121.47 119.82 119.91 119.48 120.28 125.79 110.31 110.30 109.01 102.44 112.43 112.43 109.77 120.02	N1-C2-N3-C4 C2-N3-C4-C5 N3-C4-C5-C6 C4-C5-C6-C7 N1-N2-N3-C8 N3-C2-N1-C9 C9-N1-C2-C10 N1-C2-C10-S11 C2-C10-S11-C12 C10-S11-C12-N13 S11-C12-N13-C14 C12-N13-C14-C15 C10-S11-C12-C16 S11-C12-C16-C17 C12-C16-C17-C18 C16-C17-C18-C19 C17-C18-C19-C20 C18-C19-C20-C21 C10-S11-C12-N22 N1-C2-C10-O23 N3-C4-C5-H24 C4-C5-C6-H25 C5-C6-C7-H26 S11-C12-N22-H27 C19-C20-C21-H28 C12-C16-C17-H29 C18-C19-C20-H30 C16-C17-C18-H31 C17-C18-C19-H32 N1-C2-N3-C33 C2-N3-C33-H34 C2-N3-C33-H35 C2-N3-C33-H36 C2-N3-C4-C37 N3-N4-C37-H38 N3-C4-C37-H39 N3-C4-C37-H40 C12-N13-Cl4-H41	-0.0 0 -0.0 0 -0.0 -0.0 179.98 0 -0.0 0 -0.0 -0.0 -179.98 0 179.99 0 0 0 -179.99 -179.97 179.98 -179.99 179.99 -0.0 -179.99 -0.0 -180.0 -180.0 -180.0 179.99 -59.42 59.26 179.9 179.99 -60.77 60.92 -179.92 -179.99

 

The molecule contains Benzimidazole ring, phenyl ring, amino group, methyl group. Geometry optimization can usually locate transition structures. The potential energy surface (PES) specifies hence the energy of a molecular system varies with small changes in the structure. A potential energy surface is a mathematical relationship linking molecular structure and the resultant energy. For a diatomic molecule, it is a two dimensional plot with the inter- nuclear separation Vs potential energy at that bond distance, producing a curve. For larger systems the surface has as many dimensions as there are degrees of freedom within the molecule. Generally a non-linear N atomic molecule has 3N-6 degrees of freedom. The title molecule has 41 atoms and it has 117 degrees of freedom. The optimized bond lengths and bond angles of the title compound are given in Table-1. From the experimental values of literature C-C single bond length is 1.4 A°_, C-O length is 1.26 A°_, C-N single bond length is 1.40 A°_, C-H bond length is 1.08 A°_, for benzimidazole ring in title compound and in phenyl ring the C-C bond length is 1. 39 A°_, C-H bond length is 1.08 A°_, C-N bond length 1.3 A°_. The C-C bonds in benzimidazole ring and phenyl ring are not of the same length. Substitution of methyl group leads to some changes of bond angles in the benzimidazole ring. The B3LYP bond lengths are close to the literature data due to slightly exaggerated electron correlation effect.

 

Vibrational Analysis:

In order to obtain the spectroscopic signature of the title compound we performed a frequency calculation analysis¹³. The scaling factor 0.96 is used for getting theoretical vibrational frequencies. After applying the scaling factor, the theoretically computed wave numbers are in good agreement with literature values. The stimulated IR spectrum of 2-(2-phenylaminothiazole-5-oyl)-N-methyl-6-methyl benzimidazole is shown in figure-2. 

 

 

Figure-2: Stimulated IR spectrum of 2-(2-phenylaminothiazole-5-oyl)-N-methyl-6-methyl benzimidazole.

C-H Vibrations:

The aromatic C-H stretching vibrations are normally found between 3100 to 3000 cm^{-1 14-16}. In this region the bands are not affected appreciably by the nature of the substituents. In benzimidazole ring the C-H stretching mode is observed at 3057cm^-1. Our theoretically computed wave number for benzimidazole ring falls within the range of 3092 cm^-1 to 3053 cm^-1. Thus there is a good agreement between theoretically computed values and literature values. The C-H in plane bending vibrations is observed in the region 1396 cm^-1-1320 cm^-1. The C-H out of plane bending vibrations of the title compound is well identified within their characteristic region 1027 cm^-1-886 cm^-1.

 

Ring Vibration:

The C=C vibration are more interesting if the double bonds are in conjugation with the ring. The ring carbon-carbon stretching vibrations occur in the region 1600-1300 cm^-1 and are due to the stretching and contraction of all the bands contained in the ring and also the interactions between these stretching vibration include ring breathing modes near 1000v cm^-1. In the present work the frequencies are observed at 1601 cm^-1. The ring puckering modes usually occur at 645 cm-1- 566 cm^-1.

 

Methyl group Vibration:

The C-H methyl group stretching vibrations are generally observed in the range 3580-3095 cm^-1 for the assignments of CH₃ group frequencies 9 fundamental vibrations can be associated to CH₃ group namely CH₃ sym stretching, CH₃ asymmetric stretching, CH₃ in plane scissoring, CH₃ out plane scissoring, CH₃ in plane bending, CH₃ out plane bending, CH₃ in plane twisting, CH₃ out plane twisting, CH₃ torsion modes, for the title compound the methyl in plane bending modes occur in the range 1411 cm^-1. The C-H out of plane bending occurs at 837 cm^-1 and 764 cm^-1. The assignments are in line with the literature values.

 

N-H Vibration:

The hetero aromatic molecule containing an N-H group and its stretching absorption occur in the region 3500-3220 cm^-1. Primary amines examined in dilute solution display two weak absorption bands one near 3500 cm^-1 and the other near 340 cm^-1. These bands represent respectively the asymmetric and symmetric N-H stretching modes¹⁷. The theoretically computed wave numbers for the above vibration are observed at 3484 cm^-1 represents the asymmetric N-H stretching mode. Primary aromatic amines normally absorb at 1615-1580 cm^-1. In the present work, the theoretical calculations indicate the scaled frequency value at 1601 cm^-1 is assigned to N- H in plane bending vibration. The presence of aromatic N-H out of plane bending vibration are appeared in the region 767-670 cm-1 for the title compound the theoretical calculation indicates the scaled frequency value at 647 cm-1 is assigned to N-H out of plane bending vibration.

 

Carbonyl group Vibrations:

The carbonyl group is important and its characteristic frequency has been extensively used to study a wide range of compounds. The C=O stretching vibration is observed in the region of 1850-1560 cm^{.-1 18} . The intensity of these bands can increase due to conjugation or formation of hydrogen bands. The lone pair of electron on oxygen also determines the nature of the carbonyl group. In our present study the theoretically computed wave number for C=O stretching vibration occur at 1556 cm^-1.

 

C-S Vibrations:

The C-S stretching vibration is expected in the region 710-690 cm^{-1 19}. In the present work, the frequency is observed at 687 cm^-1 which is in line with literature value.

 

C-N Vibration:

The identification of C-N vibrations is a difficult task²⁰, since the mixing of vibrations is possible in this region 1600-1500 cm^-1. In the present work the theoretical calculation (benzimidazole ring) indicates the scaled frequency at 1556 cm^-1 which is assigned to C-N vibration.

 

IR absorption frequency of 2-(2-phenylaminothiazole-5-oyl)-N-methyl-6-methylbenzimidazole

Modes	Exp IR frequency	Frequency  (unscaled)	Frequency (scaled)	Intensity	Vibrational Assignments
117 116 115 114 113 112 111 110 109 108 107 106 105 104 103 102 101 100 99 98 97 96 95 94 93 92 91 90 89 88 87 86 85 84 83 82 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 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	3447             3167                   1604       1580     1486             1418           1317                   1243   1104                 1081           892           858       757                             693	3621.79 3297.60 3270.30 3244.22 3238.44 3227.08 3214.52 3210.27 3199.40 3192.74 3178.10 3174.53 3128.93 3107.85 3095.92 3038.34 1665.02 1664.79 1661.45 1635.28 1618.15 1600.59 1583.17 1557.88 1553.61 1550.51 1546.87 1538.86 1536.34 1524.35 1524.18 1506.84 1503.88 1467.37 1447.47 1416.58 1398.39 1396.66 1373.95 1356.72 1329.76 1320.66 1305.88 1300.74 1280.03 1249.47 1239.60 1223.51 1216.92 1171.61 1163.95 1138.07 1133.27 1125.72 1122.0 1097.93 1082.38 1067.64 1039.59 1033.96 1009.20 1004.28 1001.24 978.91 947.67 930.25 921.25 870.61 867.25 843.05 834.45 820.37 804.29 794.51 780.66 776.19 721.40 714.71 672.65 671.31 657.97 653.76 643.95 621.43 614.18 602.63 598.12 588.76 541.38 527.44 507.42 506.75 494.84 428.95 390.54 357.37 342.18 332.01 316.06 303.84 298.39 278.49 274.15 234.26 218.10 205.77 167.30 123.35 121.23 119.27 96.26 67.96 47.14 44.58 37.59 24.49 16.35	3484.0 0 3172.20 3146.02 3120.90 3115.30 3104.40 3092.36 3088.27 3077.82 3071.41 3057.33 3053.89 3010.00 2988.90 2978.20 2922.88 1601.70 1601.50 1598.30 1573.13 1556.60 1539.70 1523.00 1498.68 1494.57 1491.59 1488.0 1480.38 1477.95 1466.42 1466.26 1449.58 1446.73 1411.60 1392.45 1362.74 1345.25 1343.58 1321.73 1305.16 1279.22 1270.47 1256.25 1251.31 1231.38 1201.99 1192.49 1177.01 1170.67 1127.08 1119.28 1094.82 1090.20 1082.94 1079.36 1056.20 1041.24 1027.06 1000.0 994.66 970.850 966.11 963.19 941.71 911.65 894.90 886.24 837.52 834.29 811.01 802.74 789.19 773.72 764.31 750.99 746.69 693.98 687.55 647.08 645.80 632.96 628.91 619.47 597.81 590.84 579.73 575.39 566.38 520.80 507.39 488.13 487.49 476.03 412.64 375.69 343.78 329.17 319.39 304.04 292.29 287.05 267.90 263.73 225.35 209.81 197.95 160.94 118.66 116.62 114.73 92.60 65.37 45.34 42.88 36.16 23.55 15.72	66.9156 9.0810 12.9602 14.7977 15.2929 36.1869 23.5866 28.4396 0.02415 14.2291 13.9492 7.2241 24.4197 16.4869 19.7193 32.3563 16.1779 6.7042 120.8584 9.4612 159.2258 511.3247 229.6870 47.8009 143.6085 80.5184 1.8989 904.3572 253.7118 38.3113 15.2308 363.3581 78.3816 0.3155 25.1985 55.5925 101.9328 145.6871 11.22 4.8989 21.9024 4.2873 43.1703 19.4752 101.9092 552.9616 31.9497 2.9575 4.5103 0.1420 99.3152 8.7425 24.6808 64.5515 3.3098 3.4626 0.7062 3.9370 3.0465 0.9468 10.3459 0.4111 0.3717 6.740 12.8352 0.1108 43.6075 0.5509 5.9584 0.6029 96.4128 30.3619 0.1207 90.3441 10.2044 25.4908 24.2454 3.9633 9.1954 7.9327 67.4585 0.8312 7.7683 8.01511 0.0896 33.3541 0.0896 13.5640 8.7584 20.8107 6.4325 4.9264 0.1951 0.0024 9.5536 3.0948 6.3524 0.0723 3.2719 6.7296 0.7149 0.0576 5.5732 0.2353 2.8753 13.3144 2.3508 0.4003 1.0591 1.2109 1.1046 0.4923 0.4315 0.4853 0.6302 0.2040 2.2726	N22-H27 asy.str. C14-H41 asy.str. C21-H28 asy.str. C33-H36 asy.str. C7-H26, C6-H25 asy.str. C18-H31, C28-H30, C19-H32 asy.str. C6-H25, C7-H26, C5-H24 asy.str. C18-H31, C17-H29 asy.str. C20-H30 asy.str. C5-H24, C6-H25 asy.str. C5-H24, C6-H25 asy.str. H34-H35 asy.str. H38-H39-H40 asy.str. H34-H35-H36 sym.str. C18-H31 bend(roc) H38-H39-H40 sym.str. Phenyl ring puckering, C-C sym.str. N22-H27 (bend), C-H bend(wagg.) C-H bend(wagg.) C-H bend(wagg.) C10-O(bend) N-H bend(roc.) N-H bend(roc.) C5-H24 bend(wagg.), C6-H25 bend(wagg.) Phenyl ring wagging Methyl group roc. Methyl group roc. N22-H27, C19-H32, C20-H30, C18-H31(opb) Methyl group vib., phenyl ring vib. Benzimidazole ring vib. Benzimidazole ring vib. C5-H24, C6-H25, C7-H26 bend(twist.) Methyl group vib. Methyl group sci. Methyl group twist. Benzimidazole ring vib., N22-H27 bend(wagg.) Phenyl ring vib. C5-H24, C7-H26, C6-H25 bend(wagg.) C-H in Phenyl ring twist. Phenyl ring bend(wagg.) Benzimidazole ring, Phenyl ring vib. Phenyl ring puckering C14-H37, N22-H27 bend(roc.) C5-H24, C6-H25, C7-H26 bend(wagg.) Phenyl ring vib., C6-H25 ipb, Methyl group vib. C14-H37 bend (wagg.), Phenyl ring vib. Phenyl ring vib. Methyl group vib. Phenyl ring vib. Methyl group vib. H37-C14bend(twist.)C5-H24, C6-H25 bend(wagg.) Phenyl ring opb, Methyl group twist. C19-H32, C18-H31, C20-H30 bend(roc.) Benzimidazole ring vib., C7-H26 bend(wagg.) Phenyl ring opb C15-C14-H37 bend(sciss.), Methyl group opb C15-C14-H37 bend(sciss.), Methyl group opb Methyl group bend(roc, ) Phenyl ring puckering C-H(wagg.) Phenyl ring only C-H (twist.) Phenyl ring only Phenyl ring puckering C-H( roc.) in Phenyl ring C7-H26, C6-H25, C5-H24(opb) C17-H29, C18-H31, C19-H32, C20-H30(opb) C14-H37(opb) C-H(tiwist.)in phenyl ring C-H(roc.)in phenyl ring C-H(wagg.)in phenyl ring C-H(sciss.)in phenyl ring Phenyl ring puckering Benzimidazole ring puckering, methyl group (opb) Phenyl ring puckering C20-H30, C19-H32, C18-H31(wagg.) C-O(twist.), N-C(twist.) C-H(twist.) in phenyl C20-H30, C19-H32, C18-H31(wagg.) Benzimidazole ring puckering C-H(twist.)in phenyl ring C-S (Sym.str.), phenyl ring vib. C15-S11(Sym.str.) C15-S11(Sym.str.), phenyl ring puckering C-H(roc.)in phenyl ring Whole molecule vib. Whole molecule vib. and phenyl ring puckering N13-C12-H27(opb) C-C(twist.), N-C(roc.) C15-S11(sym.str), benzimidazole ring vib. C33-H34-H35(roc.), C5-H24, C6-H24(sciss.) Methyl group(opb), phenyl ring puckering C15-S11(wagg.), N22-H27(roc.) N22-H27(twist.), C7-H36, C6-H25(Sciss.) Benzimidazole ring puckering N22-H27(roc.) Phenyl ring(bend) H29-H31(opb), phenyl ring vib. Phenyl ring vib., C10-O23(opb) Phenyl ring vib., C10-O23(opb), methyl group(roc.) Methyl group (opb) Benzimidazole ring puckering C10-O23(roc.) N22-H27, C2-N1, H24-C5, H25-C6(wagg.) N22-H27, C2-N1, H24-C5, H25-C6(wagg.) C37-H38, H39-H40(roc.) N22-H27(wagg.) C19-H32, C10-O23(twist) C37-H38(opb) C10-O23(roc.) C17-H29, C18-H31(wagg.) C10-O23(opb) Methyl group Vib., phenyl ring puckering Methyl group Vib., phenyl ring puckering C33-H34-H35-H36(twist.) C37-H389(opb) C37-H389(opb) C37-H389(opb) C37-H389(opb), phenyl ring vib. C33-H34(roc.) C33-H34(roc.)

 

Mulliken Atomic Charges:

Mulliken atomic charge calculation has an important role in the application of quantum chemical calculation of molecular system because of atomic charges effect dipole moment, molecular polarizability electronic structure and more a lot of properties of molecular systems. The bonding capability of a molecule depends on the chelating atoms. Atomic charge distributions were calculated by determining the electron population of each atom as defined by the basis set. To validate the reliability of our results the mulliken population analysis of 2-(2-phenylaminothiazole-5-oyl)-N-methyl-6-methyl benzimidazole has been calculated using B3LYP/6-31G basis set shown in Table 1. The mulliken atomic charge of sulfur carries positive charge (0.429). Nitrogen has a maximum negative charge value of about -0.436. However all the hydrogen atoms exhibit net positive charge. These magnitudes are changing between 0.119 and 0.339.It is worthy to mention that C₂, C₄, C₈, C₉, C₁₀, C₁₂, C₁₄, C₁₆ atoms exhibit positive charge, while C₅, C₇, C₁₅, C₁₇, C₁₈, C₁₉, C₂₀, C₂₁, C₃₃, C₃₇ atoms exhibit negative charge. The presence of large negative charge on nitrogen and oxygen atom and a net positive charge on hydrogen atom may suggest the formation of intramolecular interaction.

 

Mulliken population analysis of 2-(2-phenylaminothiazole-5-oyl)-N-methyl-6-methyl benzimidazole performed at B3LYP/6-31G set.

Atom	Mulliken atomic charges	Atom	Mulliken atomic charges
N1 C2 N3 C4 C5 C6 C7 C8 C9 C10 S11 C12 N13 C14 C15 C16 C17 C18 C19 C20	-0.473 0.456 -0.747 0.186 -0.189 -0.135 -0.789 0.289 0.085 0.325 0.429 0.172 -0.436 0.517 -0.392 0.313 -0.164 -0.135 -0.111 -0.154	C21 N22 O23 H24 H25 H26 H27 H28 H29 H30 H31 H32 C33 H34 H35 H36 C37 H38 H39 H40 C41	-0.294 -0.758 -0.463 0.119 0.126 0.144 0.339 0.192 0.124 0.132 0.130 0.127 -0.223 0.187 0.187 0.156 -0.482 0.159 0.159 0.151 0.215

 

 

Mulliken charge distribution of 2-(2-phenylaminothiazole-5-oyl)-N-methyl-6-methylbenzimidazole

 

HOMO-LUMO:

The most important orbital’s in a molecule are the frontier molecular orbital’s, called highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO). These orbital’s determine the way the molecule interacts with other species. The lowering of the HOMO-LUMO band gap is essentially a consequence of the large stabilization of the LUMO due to the strong electron- acceptor ability of the electron- acceptor group²¹. Analysis of wave function indicates that the electron absorption corresponds to the transition from the ground to the first excited state and in explained by one electron excitation from HOMO to LUMO. The frontier orbital gap helps to characterize the chemical reactivity optical polarizability and chemical hardness-softness of a molecule. The HOMO-LUMO energy gap is an important value for stability index²². This reflects the chemical activity of the molecule. The lowering of energy gap explains the eventual charge transfer interactions taking place within the molecule. The molecules having large energy gap are known as hard and molecules having small energy gap are known as soft molecules²³. The soft molecules are more polarisable than hard ones because they need small energy to excitation. According to koopman’s theorem²⁴, the HOMO energy is related to the ionization potential and the LUMO energy has been used to estimate the electron affinity. The average value of the HOMO and LUMO energies is related to the electronegativity defined by mulliken

 

ƞ= (Ɛ_HOMO+Ɛ_LUMO)/2

 

The calculated energy value of HOMO is -0.24488 and for the LUMO is -0.03399, the value of energy separation ∆E is +0.27887 a.u. Hence from the calculations we conclude that the molecule taken for investigation belongs to soft material.

 

 

HOMO

 

LUMO

 

 

 

Dipole Moment:

DFT predicts dipole moment and higher multipole moments. The dipole moment is the first derivative of the energy with respect to an applied electric field. It is a measure of the asymmetry in the given as a vector in three dimensions. The dipole moments are broken down in to X, Y and Z components. The total dipole moment of the title compound is found to be 3.2685 Debye.

 

CONCLUSION:

The geometry of 2-(2-phenylaminothiazole-5-oyl)-N-methyl-6-methylbenzimidazole was optimized in different levels with DFT-B3LYP method using 6-31G basis set. Using the optimized geometry the vibrational frequencies have been found to agree well with the literature reported values. Atomic charge distributions were calculated by determining the electron population of each atom. The lowering of HOMO and LUMO energy gap explains the charge transfer interactions that take place within the molecule.

 

ACKNOWLEDGEMENT:

T.F. Abbs Fen Reji thanks University Grants Commission, New Delhi for Financial Assistance in the form of Major Research project [F.No.41-229/2012 (SR)]. The authors thank NIIST, Trivandrum and CDRI, Lucknow for spectral and analytical data.

 

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Received on 18.09.2018         Modified on 18.10.2018

Accepted on 15.11.2018         © AJRC All right reserved