Journal of Molecular Structure 1005 (2011) 202–213

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Journal of Molecular Structure

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Rotational isomers, vibrational assignments, HOMO–LUMO, NLO propertiesand molecular electrostatic potential surface of N-(2 bromoethyl) phthalimide

T. Karthick a, V. Balachandran b,⇑, S. Perumal c, A. Nataraj d

a Department of Physics, Vivekanandha College for Women, Tiruchengode 637 205, Indiab Department of Physics, AA Government Arts College, Musiri, Tiruchirappalli 621 201, Indiac Department of Physics, ST Hindu College, Nagercoil 629 002, Indiad Department of Physics, Thanthai Hans Roever College, Perambalur 621 212, India

a r t i c l e i n f o a b s t r a c t

Article history:Received 24 July 2011Received in revised form 26 August 2011Accepted 26 August 2011Available online 16 September 2011

Keywords:IsomersN-(2 bromoethyl) phthalimideDFTVibrational spectraHOMO–LUMOHyperpolarizability

0022-2860/$ - see front matter Crown Copyright � 2doi:10.1016/j.molstruc.2011.08.051

⇑ Corresponding author. Tel.: +91 431 2432454; faxE-mail address: brsbala66@gmail.com (V. Balachan

The stable isomer of N-(2 bromoethyl) phthalimide (NBEP) is determined. FT-IR (4000–400 cm�1) andFT-Raman (3500–100 cm�1) spectra were recorded on the solid phase of the molecule. Optimized geo-metrical parameters, vibrational wavenumbers of the stable isomer of NBEP are predicted by DFT/B3LYP/6-311++G(d,p). Reliable vibrational assignments associated with this molecule is made on thebasis of total energy distribution (TED) results. The non-linear optical properties such as dipole moment,polarizability and first order hyperpolarizability of the title molecule are determined. Density plots overthe highest occupied molecular orbitals (HOMO) and lowest unoccupied molecular orbitals (LUMO)energy surface directly identifies the donor and acceptor atoms in the molecule. It also provides informa-tion about the charge transfer within the molecule. To obtain chemical reactivity of the molecule, themolecular electrostatic potential (MEP) surface map is plotted over the optimized geometry of the mol-ecule. Furthermore, reactive electrophilic and nucleophilic sites in the MEP surface are compared withthe fitting point charges to electrostatic potential.

Crown Copyright � 2011 Published by Elsevier B.V. All rights reserved.

1. Introduction

Phthalimide and its N-substituted phthalimide analogues hasbeen the subject of today’s research for many reasons. Owing totheir intense applications in pharmacy, medicinal chemistry andindustry, vibrational analysis of N-substituted phthalimide deriva-tives have been investigated earlier. Krishnakumar et al. [1,2] re-ported the vibrational assignments of N-bromophthalimide andN-hydroxyphthalimide based on DFT theory calculations respec-tively. In pharmaceutical, they are being used as an intermediatematerial. In medical field, phthalimide analogues are being usedin the synthesis of antimicrobial activity, androgens and otheragents for treating tumor necrosis factor [1].

Apart from their well recognized applications in pharmaceutical,phthalimide analogues play a vital role as anti-convulsant,anti-inflammatory, analgesic, hypolipidimic and immunomodula-tory activities [3] in medicinal chemistry. Certain phthalimidederivatives are used as herbicides and for reducing bacterialcontamination. In industry, they are being widely used in theproduction of pesticides, dyes, plastics, high performance ionexchange resins, surfactants. In particular, N-(2 bromoethyl) phthal-imide (abbreviated as NBEP) is being most extensively used as an

011 Published by Elsevier B.V. All

: +91 4326 262630.dran).

intermediate material for the synthesis of a drug [4]. The under-standing of molecular geometry, vibrational analysis as well as delo-calization of charges, reactive electrophilic and nucleophilic sites ofthe compound have special attention in the synthesis of a drug and itis the subject of today’s experimental and theoretical studies.

Hence in the present study, the energies of different possiblerotational isomers are determined. The optimized geometricalparameters, vibrational assignments, certain non-linear opticalproperties (such as, dipole moment, polarizability, and hyperpolar-izability) and HOMO–LUMO energy gap for the title molecule areanalyzed. Moreover, molecular electrostatic potential (MEP) surfaceis plotted over the optimized geometry to elucidate the reactivity ofNBEP molecule. To the best of our knowledge, neither vibrationalassignments nor hyperpolarizability, HOMO–LUMO energy calcula-tions have ever been made on the basis of N-(2 bromoethyl) phthal-imide. This inadequacy observed in the literature encouraged us tomake vibrational spectroscopic studies, non-linear optical proper-ties and HOMO–LUMO energy calculations on NBEP.

2. Methodology

2.1. General

The sample NBEP in the solid form was purchased from theLancaster Chemical Company, (UK) with a purity of greater than

rights reserved.

T. Karthick et al. / Journal of Molecular Structure 1005 (2011) 202–213 203

98% and it was used as such without further purification. The FT-IRspectrum of NBEP is recorded in the wavenumber region 400–4000 cm�1 on a NEXUS 670 spectrophotometer equipped with anMCT detector, a KBr pellet technique. The FT-Raman spectrum ofNBEP is recorded in the wavenumber region 100–3500 cm�1 on aNEXUS 670 spectrophotometer equipped with Raman moduleaccessory. The Nd:YAG laser of wavelength 1064 nm operating at1.5 W power is used as an excitation source.

2.2. Computational details

In order to obtain stable isomer, the self consistent field (SCF)energy calculation is performed on the possible rotational isomersof NBEP using B3LYP method with basic function set6-311++G(d,p) for better results. In the present study, the entireoptimized geometrical and vibrational parameters for the stableisomer of NBEP are predicted by means of Density FunctionalB3LYP method with internally stored standard 6-311++G(d,p) basisset level in Gaussian 03 W software package [5]. B3LYP representsBecke’s three-parameter hybrid functional method [6] with Lee–Yang–Parr’s correlation functional (LYP) [7,8]. In the calculations,the electronic structure of NBEP in the ground state is optimizedby assuming C1 point group symmetry and the charge of each pointis taken as zero and the spin multiplicity is taken as one. Because ofthe neglect of anharmonicity effect at the beginning of calculation,the predicted vibrational wavenumbers at B3LYP/6-311++G(d,p)method are found to be in disagreement with experimental wave-numbers. In order to bring the calculated wavenumber closer tothat of experimental, two different empirical scaling factors areintroduced. Reliable vibrational mode assignments made in thiswork are performed on the basis of total energy distribution(TED) results obtained from MOLVIB program (version V7.0-G77)written by Sundius [9–11].

The non-linear optical properties such as dipole moment,polarizability, and first order hyperpolarizability of NBEP are com-puted with the aid of B3LYP/6-311++G(d,p) levels of DFT theory.The frontier molecular orbital energies, energy gap between vari-ous occupied and unoccupied molecular orbitals of NBEP are alsocalculated in the same method. For obtaining chemical reactivityof the molecule, molecular electrostatic potential (MEP) surfaceis plotted over the optimized geometry of stable isomer of NBEPusing Gaussian 03W software package [5]. The fitting pointcharges to the electrostatic potential on each atom of the opti-mized geometry are compared with MEP surface. Moreover, cer-tain thermodynamic properties such as entropy, enthalpy,rotational, vibrational constants, and zero point vibrational energyof the title compound are also calculated at the same level of DFTtheory.

3. Results and discussion

3.1. Rotational isomers

To obtain stable isomer geometry, the self consistent field (SCF)energy calculation is performed on the nine possible isomers ofNBEP as shown in Fig. 1. The possibility of isomer geometry isfound by locating ACH2Br group of the title molecule in three dif-ferent orientations. The position of Br atom in different orienta-tions gives rise to nine possible isomers. The SCF energycalculation reveals that, the isomer ‘f’ acquires dominant stabilityamong others as shown in Table 1. It is also observed that, the en-ergy of the isomer ‘e’ is almost equal to that of ‘f’ isomer and con-former ‘g’ possesses least stability.

3.2. Optimized structure parameters

The optimized configuration corresponding to the stable isomerof NBEP given in Fig. 2 has 22 atoms which belong to N-substitutedheterocyclic aromatic organic compound. The optimized geometri-cal parameters of NBEP calculated at B3LYP/6-311++G(d,p) levelare presented in Table 2. Since the exact crystal structure of the ti-tle compound is not available till now, the optimized geometricalparameters are compared with the available experimental data ofsimilar kind of molecules [1]. It is observed that, CAC bondlengths in the optimized geometry of NBEP fall in the range:1.3852–1.3990 Å for six member ring. Introduction of substituentin the N-substituted phthalimide ring leads to significant variationin charge distribution in the molecule. Consequently, this greatlyaffects the molecular geometrical parameters of the five memberring. Hence, the CAC bond lengths of the five member ring suchas C1AC8 (1.4920 Å), C3AC9 (1.4925 Å) are distorted and appearsomewhat longer than that of aromatic ring. The calculated CACbond lengths of six and five member ring of NBEP are found tobe in good agreement with the experimental bond lengths ofphthalimide and N-bromophthalimide [1].

Due to the influence of bromoethyl substituent onN-substituted phthalimide, the interatomic distance N2AC11

(1.4536 Å) is slightly differ from N2AC3 (1.4097 Å) and C1AN2

(1.4022 Å). The average CAH bond length in the aromatic ring cal-culated by B3LYP/6-311++G(d,p) is �1.0836 Å whereas outside thering; the average CAH bond length (1.0902 Å) is found to be largewhen compared to aromatic CAH. It is also observed that, C14ABr17

bond length is calculated as 1.9762 Å. The C@O bond lengths suchas C1AO10 and C3AO18 calculated by B3LYP/6-311++G(d,p) are verycloser to the experimental data.

Due to the symmetry of the benzene ring, its bond angles are al-most equal. Therefore, the average bond angle corresponding toaromatic CACAH calculated at B3LYP/6-311++G(d,p) is 120.44�.In the five member ring, the average CACAO bond angle(C8AC1AO10, C9AC3AO18) calculated is 128.76�. As far as the titlemolecule is concerned, variation in bond angle is not appreciable.In other words, the substituent does not affect bond angle ofphthalimide ring significantly. From the theoretical values, wefound most of the optimized bond lengths are in good agreementwith experimental bond lengths but bond angles are slightly longerand shorter than that of experimental values.

3.3. Normal coordinate analysis

There are 60 fundamental modes of vibrations associated withthis title molecule. In agreement with C1 symmetry, all the 60vibrations are distributed as 41 in-plane and 19 out-of-planevibrations of same symmetry species. A detailed description ofthe vibrational modes can be given by means of normal coordinateanalysis (NCA). For this purpose, the full sets of 83 standard inter-nal coordinates [containing 23 redundancies] are defined as givenin the Table 3. From the full set of 83 internal co-ordinates, a non-redundant set of local symmetry coordinates are constructed bymeans of suitable linear combinations of internal coordinatesfollowed by the recommendations of Fogarasi and Pulay [12,13].The local symmetry co-ordinates corresponding to probabledegrees of freedom of NBEP are presented in Table 4.

3.4. Hyperpolarizability and HOMO–LUMO energy gap

Based on the finite-field approach, the non-linear opticalparameters such as dipole moment, polarizability, anisotropypolarizability and first order hyperpolarizability of NBEP moleculeare calculated using B3LYP method with basic function set 6-311++G(d,p) for more reliability. The numerical values of above

Fig. 1. Possible rotational isomer geometry of N-(2 bromoethyl) phthalimide.

Table 1Calculated energies and energy difference for Possible rotational isomers of N-(2bromoethyl) phthalimide by B3LYP/6-311++G(d,p) method.

Rotational isomers Energy (kJ/mol) Energy differencesa (kJ/mol)

a �8310551.436 �9.616b �8308726.681 �1834.371c �8310554.170 �6.882d �8310532.639 �28.413e �8310561.050 �0.002f �8310561.052 0.000g �8308720.962 �1840.09h �8310554.115 �6.937i �8310551.381 �9.671

a Energies of the other eight possible isomers relative to the most stable (f)conformer. Fig. 2. Stable isomer geometry of N-(2 bromoethyl) phthalimide.

204 T. Karthick et al. / Journal of Molecular Structure 1005 (2011) 202–213

mentioned parameters are listed in Table 5. In the presence of anexternal electric field (E), the energy of the system is a functionof the electric field. First hyperpolarizability is a third-rank tensorthat can be described by a 3 � 3 � 3 matrix. The 27 components ofthe 3D matrix can be reduced to 10 components because of theKleinman symmetry [14]. The components of b are defined as thecoefficients in the Taylor series expansion of energy in an externalelectric field.

When an external electric field is weak and homogeneous,Taylor series expansion becomes

E ¼ E0 � liFi

1!� aijFiFj

2!�

bijkFiFjfk

3!�

cijklFiFjFkFl

4!þ � � �

where E is the energy of the unperturbed molecules, Fi is the field atorigin and li, aij, bijk and c ijkl are the components of dipole moment,polarizability, first hyperpolarizabilities and the second hyperpolar-izabilities, respectively.

The total static dipole moment l, mean polarizability a0, anisot-ropy of the polarizability Da and first hyperpolarizability btot usingx, y and z mponents, are defined as

l ¼ ðl2x þ l2

y þ l2z Þ

1=2

a0 ¼ðaxx þ ayy þ azzÞ

3

Table 2Optimized bond lengths and bond angles of N-(2 bromoethyl) phthalimide obtained at B3LYP/6-311++G(d,p) method.

Parametersa Bond length (Å) Experimentalb Parametersa Bond angle (�) Experimentalb

Ph N-BrPh Ph N-BrPh

C1AN2 1.4022 1.40 1.35 N2AC1AC8 105.74 108.23 108.23C1AC8 1.4920 1.45 1.53 N2AC1AO10 125.01 125.80 125.94C1AO10 1.4206 1.42 1.43 C8AC1AO10 129.25 125.85 125.91N2AC3 1.4097 1.40 1.35 C1AN2AC3 112.13 110.37 110.34N2AC11 1.4536 C1AN2AC11 124.02C3AC9 1.4925 C3AN2AC11 123.80C3AO18 1.4268 1.43 1.43 N2AC3AC9 105.42C4AC5 1.3990 N2AC3AO18 125.35 125.80 125.80C4AC9 1.3852 1.38 1.34 C9AC3AO18 129.22C4AH19 1.0833 1.09 1.07 C5AC4AC9 117.40C5AC6 1.3989 1.39 1.41 C5AC4AH19 121.63 121.34 121.35C5AH20 1.0839 1.10 1.06 C9AC4AH19 120.97 121.34 121.35C6AC7 1.3989 1.40 1.41 C4AC5AC6 121.08C6AH21 1.0839 C4AC5AH20 119.58C7AC8 1.3853 1.38 1.34 C6AC5AH20 119.35C7AH22 1.0833 1.09 1.06 C5AC6AC7 121.07C8AC9 1.3938 1.44 1.35 C5AC6AH21 119.35C11AH12 1.0962 C7AC6AH21 119.58C11AH13 1.0903 C6AC7AC8 117.40 117.27 117.37C11AC14 1.5222 C6AC7AH22 121.60C14AH15 1.0861 C8AC7AH22 121.00C14AH16 1.0880 C1AC8AC7 130.20C14ABr17 1.9762 C1AC8AC9 108.27

C7AC8AC9 121.53 122.91 123.15C3AC9AC4 130.07 130.54 130.42C3AC9AC8 108.41C4AC9AC8 121.52N2AC11AH12 108.79N2AC11AH13 106.62N2AC11AC14 114.19H12AC11AH13 108.58H12AC11AC14 107.37H13AC11AC14 111.18C11AC14AH15 111.97C11AC14AH16 110.03C11AC14ABr17 112.76H15AC14AH16 110.07H15AC14ABr17 105.98H16AC14ABr17 105.79

a For numbering of atom see Fig 1.b Ref. [1].

T. Karthick et al. / Journal of Molecular Structure 1005 (2011) 202–213 205

Da ¼ 2�1=2 ðaxx � ayyÞ2 þ ðayy � azzÞ2 þ ðazz � axxÞ2 þ 6a2xz

h i1=2

btot ¼ ðb2x þ b2

y þ b2z Þ

1=2

and

bx ¼ bxxx þ bxyy þ bxzz

by ¼ byyy þ bxxy þ byzz

bz ¼ bzzz þ bxxz þ byyz

The B3LYP/6-311++G(d,p) method calculated the first hyperpo-larizability of NBEP is 0.09636 � 10�31 esu. The total molecular di-pole moment (l), mean polarizability (a0) and anisotropypolarizability (Da) of NBEP molecule are collected in Table 5. Whilecomparing hyperpolarizability of molecules reported earlier[15,16], the numerical value of first hyperpolarizability of this mol-ecule is not appreciable. In other words, the title molecule NBEPhas less non-linear optical response. Hence we conclude that, thetitle molecule is not suitable for future non-linear optical studies.

Fig. 3 shows mapped isodensity surface plots of molecular orbi-tals from HOMO � 2 to LUMO + 2 of the title molecule in which allthe LUMO surfaces are well localized within the phthalimide ring.In other words all the ring component intramolecular interactions

mostly occurred in LUMO levels. It is also observed that, all the sur-faces shown in Fig. 3b, d, and f have no amplitude [17] on ethylgroup linked to the five member ring of NBEP. In contrast, all themapped HOMO surfaces shown in Fig. 3a, c, and e are well localizedon the ethyl group. In this case, the orbital overlapping within thephthalimide ring is absent except HOMO � 2. In HOMO � 2, C14 inethyl group is highly coupled with H15, H16 and Br17. The presenceof orbital overlapping within the molecule reveals that, there is anintramolecular interaction exists between bonding (p) and anti-bonding (p⁄) molecular orbitals.

The presence of intramolecular charge transfer from donor toacceptor group within molecule can also be identified by analyzingthe co-existence of IR and Raman activity [16] itself. It is also ob-served that in our title molecule the bands at 2975, 1725, 1433,732, 604 and 585 cm�1 in FT-IR spectrum have their counterpartsin FT-Raman at 2978, 1725, 1428, 730, 607 and 583 cm�1. Thisshows that the relative intensities in IR and Raman are comparableresulting from the electron cloud movement through single-doublebond p conjugated path from donor to acceptor groups. The anal-ysis of wave function indicates that, the electron absorption corre-sponding to the transition from the ground state to the first exitedstate is mainly described by one-electron excitation from the high-est occupied molecular orbital (HOMO) to the lowest unoccupiedmolecular orbital (LUMO). The HOMO, LUMO energies of stable iso-mer of NBEP calculated at B3LYP/6-311++G(d,p) level are pre-

Table 3Definition of internal coordinates associated with N-(2 bromoethyl) phthalimide.

No(i) Symbol Type aDefinition

Stretching1–4 ri CAH (aromatic) C4AH19, C5AH20, C6AH21, C7AH22

5–6 Ri C@O C1AO10, C3AO18

7–9 ti CAN C1AN2, C3AN2, C11AN2

10 Xi CABr C14ABr17

11 Pi CAC (ethylene) C11AC14

12–19 Pi CAC (aromatic) C4AC5, C5AC6, C6AC7, C7AC8, C8AC9, C9AC4, C9AC3, C8AC1

20–23 ri CAH (ethylene) C11AH12, C11AH13, C14AH15, C14AH16

In-plane bending24–31 bi CACAH C9AC4AH19, C5AC4AH19, C4AC5AH20, C6AC5AH20, C5AC6AH21, C7AC6AH21, C6AC7AH22, C8AC7AH22

32–33 /i CACAO C9AC3AO18, C8AC1AO10

34–35 pi NACAO N2AC3AO18, N2AC1AO10

36–37 ri CANAC C3AN2AC11, C1AN2AC11

38–43 bi t(Ring 1) C4AC5AC6, C5AC6AC7, C6AC7AC8, C7AC8AC9, C8AC9AC4, C9AC4AC5

44–48 bi t(Ring 2) C1AC8AC9, C8AC9AC3, C9AC3AN2, C3AN2AC1, N2AC1AC8

49–50 wi NACAH (ethylene) N2AC11AH12, N2AC11AH13

51–52 wi HACAH H12AC11AH13, H15AC14AH16

53–54 bi CACAH (ethylene) C11AC14AH15, C11AC14AH16

55–56 wi HACABr H15AC14ABr17, H16AC14ABr17

Out-of plane bending57–60 xi CarAH H19AC4AC5AC9, H20AC5AC6AC4, H21AC6AC7AC5, H22AC7AC8AC6

61–62 xi C@O O10AC1AC8AN2, O18AC3AC9AN2

63 qi CAN C11AN2AC3AC1

66–67 di t(N)ACH2 C1AN2AC11AH12, C1AN2AC11AH13, C3AN2AC11AH12, C3AN2AC11AH13

68–73 si t(Ring1) C4AC5AC6AC7, C5AC6AC7AC8, C6AC7AC8AC9, C7AC8AC9AC4, C8AC9AC4AC5, C9AC4AC5AC6

74–78 si t(Ring2) C8AC1AN2AC3, C1AN2AC3AC9, N2AC3AC9AC8, C3AC9AC8AC1, C9AC8AC1AN2

79–80 si Butterfly C7AC8AC9AC3, C4AC9AC8AC1

81–82 xi NACACAH N2AC11AC14AH15, N2AC11AC14AH16

83 xi NACACAX N2AC11AC14ABr17

a For numbering of atom see Fig 1.

206 T. Karthick et al. / Journal of Molecular Structure 1005 (2011) 202–213

sented in Table 6. The energy gap presented in Table 6 reflects thechemical activity of the molecule. HOMO represents the ability todonate an electron and LUMO represents the ability to accept anelectron. Among the six subsequent excited states calculated, thestrongest transition appears between HOMO ? LUMO orbitals.The numerical value of energy gap between HOMO–LUMO orbitalscalculated at B3LYP level is�0.18393 a.u. The energy gaps for otherpossible energy transitions are also presented in Table 6.

3.5. Molecular electrostatic potential

Molecular electrostatic potential (MEP) at a point in spacearound a molecule gives information about the net electrostatic ef-fect produced at that point by total charge distribution (elec-tron + proton) of the molecule [18]. Moreover, MEP surface helpsto predict the reactivity of wide variety of chemical systems inboth electrophilic and nucleophilic reactions, the study of biologi-cal recognition processes and hydrogen bonding interactions[19,20]. It also provides visual understanding of relative polarityof the molecule. An electron density isosurface mapped with elec-trostatic potential surface depicts the size, shape, charge densityand reactive sites of the molecules. The different values of the elec-trostatic potential at the surface are represented by different col-ors; red represents regions of most electro negative electrostaticpotential, blue represents regions of most positive electrostatic po-tential and green represents regions of zero potential. The electro-static potential increases in the order red < orange < yellow <green < blue [18].

To predict reactive sites for electrophilic and nucleophilic attackfor the investigated molecule, MEP surface is plotted over opti-mized geometry of stable isomer of NBEP at B3LYP/6-311++G(d,p) basis set. Fig. 4 (left) shows electrostatic potentialcontour map of NBEP along with the fitting point charges to theelectrostatic potential (see the right side of Fig. 4), where the elec-

tron density isosurface being 0.02 a.u. In the present study, the va-lue of point charges to the electrostatic potential are predictedwith the help of B3LYP level of the theory incorporating 6-311++G(d,p) basis set. As easily can be seen in Fig. 4, investigatedmolecule has several possible sites for electrophilic (the electro-philic sites are most electro negative and are represented as redcolor) and nucleophilic attack (the nucleophilic sites are most po-sitive and are represented as blue color). The fitting point charge tothe electrostatic potential indicates that, the atom O10 and O18 arethe most electronegative (�0.5606 e) atoms in NBEP molecule. Thecalculated point charge corresponding to N2 atom is �0.1479 e.But, the region very close to N2 appears blue color in MEP surface.This is because of the fact of that, N2 atom of NBEP is surroundedby most electropositive atoms (C3 = 0.4400, C3 = 0.3697 andC11 = 0.2021 e). From this figure, we also found that charges accu-mulated on hydrogen atoms of the ring and ethyl groups are lesspositive, while ring carbon atoms such as C9 (�0.0399 e), C4

(�0.1026 e), C5 (�0.1268 e) and C6 (�0.1324 e) are less negative.The uniform charge distributions on the ring components protectthe symmetry of six member ring from substituent. It is also seenin Fig. 4 that, the region of less electro positive (yellow color)envelops the p-system of benzene ring, which prevent the ringhydrogen atom from more electrophilic region.

3.6. Vibrational spectra

Usually harmonic wavenumbers derived from Gaussian 03 arefound to be in disagreement with that of experimental due to theneglect of anharmonicity effect at beginning of calculation, elec-tron correlation approximate treatment and basis set deficiencies,etc., [21]. Linear scaling procedure is adopted in this work toeliminate errors in the harmonic wavenumber output. After imple-menting scaling procedure, the theoretically computed wavenum-bers matched well with the experimental ones. Based on this

Table 4Definition of local symmetry coordinates for N-(2 bromoethyl) phthalimide.

No(i) Type aDefinition

Stretching1–4 CHar r1, r2, r3, r4

5–6 C@O R5, R6

7–9 CN t7, t8, t9

10 CABr X10

11 CAC (ethylene) P11

12–19 CCar P12, P13, P14, P15, P16, P17, P18, P19

20 (CH2)1 ss r20 + r21

21 (CH2)1 as r20 � r21

22 (CH2)2 ss r22 + r23

23 (CH2)2 as r20 � r21

In-plane bending24–27 bCH ðb24 � b25Þ=

ffiffiffi2p

; ðb26 � b27Þ=ffiffiffi2p

;

ðb28 � b29Þ=ffiffiffi2p

; ðb30 � b31Þ=ffiffiffi2p

28 bCO ð/32 � /33Þ=ffiffiffi2p

29 bCO ðp34 � p35Þ=ffiffiffi2p

30 bCN ðr36 � r37Þ=ffiffiffi2p

31 Ring 1 ðb38 � b39 þ b40 � b41 þ b42 � b43Þ=ffiffiffi6p

32 Ring 2 ð�b38 � b39 þ 2b40 � b41 þ b42 � 2b43Þ=ffiffiffiffiffiffi12p

33 Ring 3 ðb38 � b39 � b41 � b42Þ=ffiffiffi2p

34 Ring 4 b44 þ aðb45 þ b48Þ þ bðb46 þ b47Þ35 Ring 5 ða� bÞðb44 � b47Þ þ ð1� aÞðb45 � b46Þ36 CH2 scissoring ð2W49 �W50 �W51Þ=

ffiffiffi6p

37 CH2 scissoring ð2W53 �W54 �W52Þ=ffiffiffi6p

38 CH2 rocking ðW50 �W51Þ=ffiffiffi2p

49 CH2 rocking ðW54 �W52Þ=ffiffiffi2p

40 CH2 twisting ðW50 þW51Þ=ffiffiffi2p

41 CH2 twisting ðW54 þW52Þ=ffiffiffi2p

42 bCABr ðW55 �W56Þ=ffiffiffi2p

Out-off plane bending43–46 xCH x57, x58, x59, x60

47–48 xCO x61, x62

49 xCN x63

50–51 xNACAH ðx64 �x65Þ=ffiffiffi2p

; ðx66 �x67Þ=ffiffiffi2p

52 t Ring1 ðs68 � s69 þ s70 � s71 þ s72 þ s73Þ=ffiffiffi6p

53 t Ring2 ðs68 � s69 þ s70 � s71Þ=ffiffiffi2p

54 t Ring3 ð�s68 þ 2s69 � s70 � s71 þ 2s72 � s73Þ=ffiffiffiffiffiffi12p

55 t ring4 bðs74 þ s78Þ þ aðs75 þ s77Þ þ s76

56 t ring5 ða� bÞðs78 � s74Þ þ ð1� aÞðs77 � s75Þ57 Butterfly ðs79 � s80Þ=

ffiffiffi2p

58–59 CH2 wagging x81, x82

60 xCABr x83

a = cos 144�, b = cos 72�.a For numbering of atom see Fig. 1.

T. Karthick et al. / Journal of Molecular Structure 1005 (2011) 202–213 207

procedure, the theoretical harmonic wavenumbers have beenscaled by the scale factor of 0.958 and 0.983 in the region from3200 to 1700 cm�1 and lower than 1700 cm�1, respectively [22].For comparative purpose, scaled theoretical and experimental FT-IR, FT-Raman wavenumbers are summarized in Table 7 along withdetailed vibrational assignments and calculated IR intensities andRaman activities. Similarly, the experimental and calculated FT-IRand FT-Raman spectra are also presented in Figs. 5 and 6 for com-parative purpose. The title molecule NBEP has 22 atoms and pos-sesses 60 normal vibrational modes, where all such modes areassigned on the basis of total energy distribution results obtainedfrom MOLVIB output. The detailed assignments along with the per-centage of TED are summarized in Table 7, where the assignmentshaving <10% (TED percentage) are ignored.

3.6.1. Spectral region between 3200 and 3000 cm�1

The characteristic wavenumbers of this region belong to CAHstretching modes [23]. The theoretical description for this regionis somewhat difficult due to the weakness of CAH stretchingmodes. For NBEP, the very weak absorption band at 3095,

2975 cm�1 in FT-IR and the bands at 3090, 3068, 3020 and2978 cm�1 in FT-Raman are assigned to CAH stretching modes ofthe ring system. The scaled theoretical values at B3LYP/6-311++G(d,p) of ring CAH stretching modes coincide well with thatof experimental data as depicted in Table 7. For phthalimide com-pound, the bands at 3230, 3180, 3095 and 2950 cm�1 in FT-IR wereassigned to ring CAH stretching modes [1]. For N-bromophthali-mide the bands at 3198, 3058, 2936, 2870 cm�1 in FT-IR were as-signed to ring CAH stretching modes [1]. From these results, weconclude that the vibrational wavenumbers associated with sixmember ring of phthalimide are greatly affected by the nature ofsubstituents. The percentage of TED predicts that CAH modes ofNBEP are very pure since their percentage is almost 100%. How-ever, CAH stretching modes have also been satisfied harmonicoscillator equation [24]. According to this, vibrational wavenumberis proportional to the root of bond force constant and is inverselyproportional to the root of reduced masses of possible pairs ofthe molecule. As a result, large vibrational wavenumbers will beassociated with light atoms. Thus the CAH stretching modes ofNBEP appear in the range 3099–2724 cm�1 by B3LYP/6-311++G(d,p). Average force constants corresponding to CAH stretchingmodes calculated at B3LYP/6-311++G(d,p) is 6.3539 mdyne/Å andaverage reduced masses for CAH stretching modes of NBEP is1.08753 amu.

The ethyl group of the title molecule gives rise to four stretchingmodes and the couple of scissoring, wagging, rocking and twistingmodes. The peaks for all the four stretching modes are observed inthis title region. The band observed at 2913 cm�1 in FT-IR andbands at 2910 cm�1 in FT-Raman is assigned to asymmetric CAHstretching modes of the ethyl group of NBEP. The very strong bandat 2860 cm�1 and a weak band at 2720 cm�1 in FT-IR are desig-nated as symmetric CAH stretching modes.

3.6.2. Spectral region between 1900 and 1200 cm�1

As far as the title compound is concerned, the region 1900–1300 cm�1 has very strong and sharp peaks as shown in Figs. 5and 6. On the basis of TED, the prominent absorptions at 1780,1725 cm�1 in FT-IR and 1775, 1725 cm�1 in FT-Raman are assignedto C@O stretching modes. From this we found that, the intensity ofthese bands in FT-IR is very high. Similarly very large force con-stants have been reported against C@O stretching vibrations (Table7). Such intense infrared absorption bands are observed due to thefact that, the multiple bonded carbonyl group is highly polar [25].The pattern of band observed at 1615, 1328 and 1178 cm�1 in FT-IR and 1615, 1322 and 1180 cm�1 in FT-Raman are assigned as CACstretching vibrations of the aromatic system (also known as skele-tal vibrations) [26]. The scaled theoretical wavenumbers corre-sponding to skeletal CAC stretching vibrations at B3LYP/6-311++G(d,P) level show excellent agreement with those observedexperimentally.

Beside the CAC skeletal, multiple combinations or overtones areobserved in this region in relation to aromatic CAH in-plane bend-ing, CH2 in-plane scissoring and CH2 in-plane wagging of the ethylgroup. In the present study, the medium strong band at 1476 cm�1

in FT-IR and 1470, 1458 cm�1 in FT-Raman are assigned to CAH in-plane bending of the six member ring. The percentage of TED forthis mode is about 70%. Moreover, the skeletal CAC stretchingmodes are also taking a crucial part (13%) in these modes (m13,m14) since their TED contributions are 15 and 13%, respectively.For NBEP molecule, the assignment of the skeletal CAC stretchingmode in both FT-IR and FT-Raman is quite difficult, since thisband is frequently masked by the more intense bands at 1240–1100 cm�1 (FT-Raman) arising from the CH2 deformation(H15AC14AH16, H12AC11AH13) vibrations [27,28]. The theoreticallycalculated CH2 deformation modes have been consistent with therecorded spectral data (Table 7). One of the CH2 deformation

Table 5Electric dipole moment l (Debye), mean polarizability a0 (�10�22 esu), anisotropypolarizability Da (�10�25 esu) and first hyperpolarizability btot (�10�31 esu) for N-(2bromoethyl) phthalimide.

Parameters Values Parameters Values

lx �2.8466 bxxx 0.3498ly 0.7474 byyy �0.3726lz 0.2819 bzzz �0.6623l 2.9566 bxyy 0.4748axx �1.2842 bxxy 18.9358axy 17.5756 bxxz �0.6724axz 10.8284 bxzz 0.2631ayy �0.9857 byzz 1.8809ayz 28.3760 byyz �0.4663azz �1.0740 bxyz 1.1579a0 �1.1012 btot 0.09636Da 3.7969

208 T. Karthick et al. / Journal of Molecular Structure 1005 (2011) 202–213

modes called CH2 scissoring generates bands at 1433 and1400 cm�1 in FT-IR. In FT-IR spectrum, the bands at 1428 and1397 cm�1 are assigned to CH2 scissoring. The predicted valuesfor CH2 scissoring modes at 1435, 1404 cm�1 by B3LYP/6-311++G(d,p) are matched well with that of experimental as wellas the range given in the literature [1].

(a) HOMO

(c) HOMO−1

(e) HOMO−2

Fig. 3. Isodensity plots of the frontier molecula

The medium strong band at 1370 cm�1 in FT-IR is attributed toCH2 wagging (xCH2). The percentage of TED reveals that, all the in-plane CH2 deformations are pure modes except mode m20. Theassignment of CAN stretching modes is rather difficult task, sincethere are problems in identifying these wavenumbers from othervibrations. Silverstein et al. [29] assigned the CAN stretchingabsorption in the region 1382–1266 cm�1 for substituted hetero-cyclic aromatic compounds. In the present study, CAN stretchingmodes are assigned on the basis of TED. The observed strong bandat 1234 cm�1 in FT-IR and the weak bands at 1355, 1232 cm�1 inFT-Raman are assigned to CAN stretching modes. The wavenum-bers at 1361, 1238 and 1066 cm�1 calculated by B3LYP/6-311++G(d,p) show excellent agreement with the experimentaldata.

3.6.3. Spectral region between 1200 and 800 cm�1

Most of the in-plane and out-of-plane bending vibrations of ringCAH modes are reported in this region. The bands at 1476,1257 cm�1 in FT-IR and 1470, 1458, 1258 cm�1 in FT-Raman areassigned to aromatic CAH in-plane bending vibrations. CAH out-of plane vibrations of NBEP are mostly very pure modes. In thepresent study, the observed bands occur at 985, 875 cm�1 in FT-IR and 978, 870 cm�1 in FT-Raman are designated as ring CAH

(b) LUMO

(d) LUMO+1

(f) LUMO+2

r orbitals of N-(2 bromoethyl) phthalimide.

Table 6Selected HOMO, LUMO energies and their energy gap of N-(2 bromoethyl) phthalimide.

S. No Molecular orbitals Energy (a.u) Molecular orbital energy transitions Energy gap (a.u)

1 HOMO �0.27966 HOMO ? LUMO �0.181072 HOMO � 1 �0.28465 HOMO � 1 ? LUMO �0.186063 HOMO � 2 �0.28741 HOMO � 2 ? LUMO �0.188824 LUMO + 2 �0.02359 HOMO ? LUMO + 1 �0.219435 LUMO + 1 �0.06023 HOMO � 1 ? LUMO + 1 �0.224426 LUMO �0.09859 HOMO � 2 ? LUMO + 1 �0.227187 HOMO ? LUMO + 2 �0.256078 HOMO � 1 ? LUMO + 2 �0.261069 HOMO � 2 ? LUMO + 2 �0.26382

C3 0.4400

C1 0.3697

H15 0.2298

H16 0.2064

C11 0.2021

H22 0.1812

H19 0.1690

H21 0.1647

H20 0.1607

C8 0.0840

H13 0.0836

H12 0.0596

C9 −−0.0399

C4 −0.1026

Br17 −0.1102

C5 −0.1268

C6 −0.1324

N2 −0.1479

C7 −0.1502

C14 −0.4383

O18 −0.5419

O10 −0.5606

Fig. 4. Molecular electrostatic potential (MEP) surface (left) mapping and fitting point charges to the electrostatic potential (right) of N-(2 bromoethyl) phthalimide.

T. Karthick et al. / Journal of Molecular Structure 1005 (2011) 202–213 209

out-of plane bending vibrations. The theoretical scaled wavenum-bers of these modes are found to be an excellent agreement withthe experimental FT-IR and FT-Raman wavenumbers.

Like CAN stretching, the CAN bending modes are also coupledwith bCHeth and s ring vibrations. In FT-IR, band at 894 cm�1 inFT-IR is designated as CAN in-plane bending mode. The percentageof TED contributing to this mode is about 76%. In the present work,a weak intense band at 392 cm�1 in FT-Raman is assigned as CANout-of-plane vibration. The experimental counterpart belongs tocCN in FT-IR is not observed, because cCN vibration is observedonly in far IR region. The calculated theoretical wavenumber396 cm�1 depicted in Table 7 is an excellent agreement with theexperimental FT-Raman wavenumber.

For ethyl moieties of the compound, the peaks at 1205 cm�1 inFT-IR and 1205, 1076 cm�1 in FT-Raman are ascribed to ethyltwisting vibrations (@CH2). Ethyl rocking vibrations of the titlecompound display medium strong peak at 807 cm�1 in FT-IR andweak bands at 800, 316 cm�1 in FT-Raman. The wavenumber ofCH2 deformations based on B3LYP/6-311++G(d,p) show better re-sults when comparing experimental CH2 deformations. The C@O

in-plane bending modes are mixed with @CH2, and bCH. The bandsat 854, 746 cm�1 in FT-IR are assigned to C@O in-plane bendingmodes.

3.6.4. Spectral region below 800 cm�1

The region below 800 cm�1 is mostly dominated by in-planeand out-of-plane ring vibrations. In this region, we also expectC@O out-of-plane and CABr vibrations. Usually in-plane ringdeformation vibrations are at higher wavenumbers than out-of-plane vibrations [30]. In the present study, the bands observed at604, 523 cm�1 in FT-IR and 710, 700, 607, 525 and 406 cm�1 inFT-Raman are assigned to ring in-plane bending modes. The ringout-of-plane bending mode wavenumbers observed in FT-Ramanspectra are reported in Table 7.

The C@O out-of-plane vibrations are coupled with cCH and areobserved at 680 cm�1 in FT-IR and 677, 358 cm�1 in FT-Raman. Theheavier mass of bromine Br17 obviously makes CABr stretching toappear at longer wavelength region (200–480 cm�1) as reported byVarasanyi [31]. In the present study, stretching mode of CABr isnot observed in both FT-IR and FT-Raman. Hence the assignment

Table 7Observed and calculated vibrational wavenumbers (cm�1), force constants (mdyne/Å), reduced masses (amu) of N-(2 bromoethyl) phthalimide.

Vibrationalmode No.

Symmetryspecies

Experimentalwavenumbers(cm�1)

Unscaledfrequency(cm�1)

Scaledfrequency(cm�1)

Calculated IRintensities

CalculatedRamanactivity

Reducedmass(amu)

Forceconstant(mdyne/Å)

Vibrationalassignmenta (%TED)

FT-IR FT-Raman

m1 A 3095vw 3090w 3210 3099 8.8 280.7 1.0967 6.6199 tCH (98)m2 A – 3068ms 3197 3072 2.7 31.0 1.0939 6.5881 tCH (98)m3 A – 3020w 3186 3025 5.1 124.0 1.0906 6.5235 tCH (98)m4 A – – 3175 3010 1.1 47.6 1.1094 6.5897 tasCH2 (99)m5 A 2975s 2978ms 3156 2980 1.5 59.8 1.0870 6.4504 tCH (96)m6 A 2913vs 2910ms 3117 2917 0.7 35.9 1.0942 6.2630 tasCH2 (99)m7 A 2860vs – 3095 2866 11.6 160.1 1.0602 6.0219 tsCH2 (100)m8 A 2720w – 3029 2724 19.6 188.6 1.0682 5.7750 tsCH2 (100)m9 A 1780s 1775vs 1826 1782 102.6 122.1 12.4052 24.3600 tC@O (82)m10 A 1725vs 1725s 1771 1730 644.4 17.0 12.7363 23.5266 tC@O (79)m11 A 1615vw 1615s 1687 1619 16.1 35.2 6.9096 11.0373 tCC (63), bCH (20)m12 A – – 1628 1552 2.5 7.6 7.1850 11.4473 tCC (66), bCH (24)m13 A 1476ms 1470w 1516 1478 7.0 2.9 2.4088 3.1774 bCH (67), tCC (15)m14 A – 1458vw 1500 1462 0.0 2.3 2.5518 3.3522 bCH (70), tCC (13)m15 A 1433vs 1428ms 1485 1435 13.1 7.7 1.0993 1.4110 rCH2 (97)m16 A 1400ms 1397vw 1473 1404 34.6 16.4 1.1154 1.4259 rCH2 (93)m17 A 1370ms – 1427 1374 93.5 14.1 1.7537 2.0733 xCH2 (82)m18 A – 1355vw 1397 1361 259.7 32.4 2.6740 3.0763 tCN (56), bCH2

(29)m19 A 1328s 1322vw 1381 1330 25.6 1.1 5.9707 6.7068 tCC (60), tCN (31)m20 A – – 1343 1296 130.9 18.3 1.3923 1.4788 xCH2 (45), bCH2

(25)m21 A 1257w 1258w 1309 1262 1.8 0.2 1.5620 1.5777 bCH (68)m22 A 1234s 1232w 1278 1238 17.9 7.2 1.4004 1.3467 tCN (41), xCH2

(37)m23 A 1205vw 1205w 1228 1211 17.5 39.4 1.6654 1.4790 @CH2 (68), b ring

(13)m24 A 1178vw 1180w 1209 1185 13.7 38.4 2.7270 2.3481 tCC (41), bCH (24),

@CH2 (16)m25 A – – 1192 1169 4.6 4.9 2.1450 1.7878 bCH (61), tCC (16)m26 A – – 1180 1154 5.4 8.7 1.1787 0.9676 tCC (80)m27 A – 1076vw 1104 1079 9.9 1.5 2.0514 1.4731 @CH2 (56), b ring

(22)m28 A – – 1093 1066 52.6 3.8 1.9540 1.3762 tCN (40), bCH2

(32), b ring (17)m29 A 1017w – 1043 1023 13.2 6.0 2.4118 1.5466 tCC(52), bCH (32)m30 A – 1008s 1026 1012 16.2 33.1 2.9300 1.8169 tCC(52), bCH (23)m31 A 985w 978vw 1010 988 0.0 0.0 1.3171 0.7911 cCH (91)m32 A – – 996 968 2.6 0.2 1.4471 0.8234 cCH (85)m33 A 933w 938vw 957 940 23.6 1.2 2.3387 1.3296 tCC(59), @CH2

(16), cCH (12)m34 A 894w – 928 900 23.1 4.7 2.4992 1.2690 bCN (76), bCH2

(14)m35 A 875s 870w 902 878 0.0 0.0 1.3597 0.6598 cCH (92)m36 A 854w – 873 856 10.9 4.5 3.5593 1.5965 bC@O (41), bCH

(35), @CH2 (22)m37 A 807ms 800w 821 803 13.9 1.4 2.3793 1.0301 /CH2 (68), bCH

(22)m38 A 770w – 789 776 10.1 1.1 1.9301 0.7311 cCH (75), s ring

(15)m39 A 746w – 765 751 0.4 0.2 6.0737 2.2841 bC@O (51), bCH

(27), @CH2 (12)m40 A 732s 730s 749 735 85.1 0.2 2.3352 0.7312 cCH (51), cC@O

(26)m41 A – 710s 728 714 3.5 11.6 5.8637 1.7733 b ring (68), bCH

(11)m42 A – 700w 717 703 5.3 7.0 6.3856 1.8910 b ring (57), bCH

(13)m43 A 680w 677ms 692 679 0.1 0.2 4.9660 1.3476 cC@O (71), cCH

(16)m44 A 604s 607ms 617 604 5.0 11.4 8.3810 1.8179 b ring (67), tCC

(13)m45 A 585s 583ms 602 588 12.0 10.8 3.5888 0.6659 cCH2 (68)m46 A 523ms 525w 537 524 9.9 1.7 7.1823 1.2202 b ring (52), /CH2

(15), bC@O (11)m47 A – – 510 497 9.2 9.6 4.1630 0.6369 tCBr (58), b ring

(18)m48 A – – 464 452 0.0 0.1 4.8850 0.6187 s ring (61), cCN

(19), bCBr (12)m49 A – 406vw 417 408 1.5 0.9 3.3018 0.3384 b ring (58), bC@O

210 T. Karthick et al. / Journal of Molecular Structure 1005 (2011) 202–213

4000 3500 3000 2500 2000 1500 1000 500

Wavenumber (cm−1)

Wavenumber (cm−1)

Tra

nsm

itta

nce

(%)

Tra

nsm

itta

nce

(%)

(a)

(b)

4000 3000 2000 1500 1000 400

100

80

60

40

20

0

Fig. 5. Experimental (a) and calculated (b) FT-IR spectra of N-(2 bromoethyl) phthalimide.

Table 7 (continued)

Vibrationalmode No.

Symmetryspecies

Experimentalwavenumbers(cm�1)

Unscaledfrequency(cm�1)

Scaledfrequency(cm�1)

Calculated IRintensities

CalculatedRamanactivity

Reducedmass(amu)

Forceconstant(mdyne/Å)

Vibrationalassignmenta (%TED)

FT-IR FT-Raman

(13)m50 A – 392vw 405 396 7.4 2.7 3.9406 0.3653 cCN (46), s ring

(18)m51 A – 358s 369 361 21.0 1.7 10.4886 0.7537 cC@O (61), cCH

(18)m52 A – 316w 325 318 2.8 1.1 2.3719 0.1192 /CH2 (63), cCN

(20)m53 A – 295w 303 297 0.6 1.2 5.6378 0.2068 bCBr (51), b ring

(30)m54 A – 250w 258 253 1.2 0.5 6.9506 0.2087 b ring (51), cCN

(20), cCBr (11)m55 A – 204w 212 207 3.4 1.4 6.4072 0.1154 Butterfly (65)m56 A – 188w 195 191 2.6 0.3 3.7990 0.0538 cCBr (62), cC@O

(13),m57 A – 146w 153 149 0.0 0.9 5.0339 0.0567 s ring (52), bCH

(12), cCN (11)m58 A – 115w 120 117 3.5 1.6 8.5112 0.0379 s ring (58), cCBr

(18), bCN (10)m59 A – – 93 91 0.9 4.3 8.7043 0.0065 s ring (54), cC@O

(13)m60 A – – 76 75 0.6 4.4 9.7905 0.0045 s ring (56), bCN

(17), cC@O (13)

a ts, symmetry stretching; tas, asymmetry stretching; b, in-plane bending; c, out-of-plane bending; r, scissoring; x, wagging; @, twisting; /, rocking; s, torsion.

T. Karthick et al. / Journal of Molecular Structure 1005 (2011) 202–213 211

Ram

an in

tens

ity

Ram

an in

tens

ity

3500 3000 2500 2000 1500 1000 500

Wavenumber (cm−1)

Wavenumber (cm−1)

(a)

(b)

3500 3000 2500 2000 1500 1000 500 100

100

80

60

40

20

0

Fig. 6. Experimental (a) and calculated (b) FT-Raman spectra of N-(2 bromoethyl) phthalimide.

212 T. Karthick et al. / Journal of Molecular Structure 1005 (2011) 202–213

of CABr stretching has been made on the basis of TED. By greaterconfidence, the scaled theoretical wavenumber 497 cm�1 (modeno: m47) is assigned to CABr stretching vibrations. The TEDcontribution to this mode is about 58%.

The CABr in-plane (bCBr) and out-of-plane bending (cCBr)vibrations of NBEP molecule coupled with bring and cC@O modes.Hence in the present study, the bands at 295 cm�1 and 188 cm�1 inFT-Raman are assigned to CABr in-plane and out-of-plane bendingvibrations, respectively. The assigned experimental and calculatedvalues of bCBr and cCBr are good agreement with the literature gi-ven by Mooney [32]. The butterfly mode of NBEP contributes 65%at 204 cm�1 in FT-Raman spectra. The calculated value of thismode is excellent agreement with the experimental data.

Table 8Thermodynamic functions of N-(2 bromoethyl) phthalimide.

Parameter B3LYP/6-311G++(d,p)

Self consistent field energy �3165.4280 a.uZero point vibrational energy 102.08266 (kcal/mol)Rotational constants 1.01396 GHz

0.29530 GHz0.21476 GHz

Entropy 112.355 cal/mol KSpecific heat capacity at constant volume 44.212 cal/mol KTranslational energy 42.485 cal/mol KRotational energy 32.635 cal/mol KVibrational energy 37.235 cal/mol K

4. Other molecular parameters

In addition to vibrational assignments, hyperpolarizability andHOMO–LUMO calculations, several thermodynamic parametersare also calculated at B3LYP/6-311++G(d,p). The calculatedthermodynamic properties are presented in the Table 8. The selfconsistent field (SCF) energy, zero point vibrational energies(ZPVE), rotational constants and entropy SVib are calculated to theextent of accuracy and the variations in the ZPVEs seem to be insig-nificant. The total energies and change in total entropy of NBEP atroom temperature are only marginal.

5. Conclusion

In this work we have performed SCF energy calculations on pos-sible isomers of NBEP. The optimized geometrical parameters,vibrational wavenumbers, non-linear optical properties, HOMO–LUMO energy gap of NBEP molecule were predicted at DFT/B3LYP/6-311++G(d,p) method. All the normal mode frequenciesassociated with this molecule have been discussed and assignedon the basis of percentage of total energy distribution (TED). Scaledtheoretical wavenumbers are compared with experimental FT-IRand FT-Raman wavenumbers. From the hyperpolarizability calcu-lations, we found that the title molecule NBEP has less non-linearoptical response. The energy differences between various occupiedand unoccupied molecular orbital energies have been computed.The smallest energy gap of �0.18107 a.u is found between HOMO

T. Karthick et al. / Journal of Molecular Structure 1005 (2011) 202–213 213

(EHOMO = �0.27966 a.u) and LUMO (ELUMO = �0.09859 a.u). Theisodensity surface contour map of frontier orbitals of HOMO andLUMO indicates that there is a charge transfer occurring withinthe molecule. Molecular electrostatic potential surface map indi-cates that the most suitable atomic sites for electrophilic attackor for metal coordination are O10 and O18 atoms, while the mostprobable sites which could be involved in nucleophilic processare ethyl groups. The fitting point charges to the electrostatic po-tential on each atom of the title compound have been computedand are compared with MEP surface.

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