Molecular structure, normal coordinate analysis, harmonic vibrational frequencies, NBO, HOMO–LUMO analysis and detonation properties of (S)-2-(2-oxopyrrolidin-1-yl) butanamide by density functional methods

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 118 (2014) 702–715

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Molecular structure, normal coordinate analysis, harmonic vibrational frequencies, NBO, HOMO–LUMO analysis and detonation properties of (S)-2-(2-oxopyrrolidin-1-yl) butanamide by density functional methods S. Renuga a,b, S. Muthu c,⇑ a b c

Department of Physics, Anna University, Chennai, Tamil Nadu, India Department of Physics, Indira Institute of Engineering and Technology, Thiruvallur, Tamil Nadu, India Department of Physics, Sri Venkateswara College of Engg., Sriperumbudur 602105, Tamil Nadu, India

h i g h l i g h t s

g r a p h i c a l a b s t r a c t

 FT-IR and FT-Raman investigations of

A complete vibrational analysis of (S)-2-(2-oxopyrrolidin-1-yl) butanamide is performed by combining the experimental and theoretical information using Pulay’s density functional theory (DFT) based on scaled quantum chemical approach. The calculated HOMO and LUMO energies show that charge transfer occur within the molecule. Comparison of simulated spectra with the experimental spectra provides important information about the ability of the computational method to describe the vibrational modes.

(S)-2-(2-oxopyrrolidin-1-yl) butanamide were carried out.  Geometrical parameters and vibrational frequencies of (S)-2-(2oxopyrrolidin-1-yl) butanamide were calculated.  The complete assignments are performed on the basis of the potential energy distribution (PED).  Hyperpolarizability, HOMO and LUMO energies were calculated.

a r t i c l e

i n f o

Article history: Received 15 August 2013 Received in revised form 4 September 2013 Accepted 7 September 2013 Available online 19 September 2013 Keywords: FTIR FT-Raman DFT NBO PED

a b s t r a c t Density functional theory (DFT) computations have become an efficient tool in the prediction of molecular structure, harmonic force fields, vibrational wave numbers as well as the IR and Raman intensities of pharmaceutically important molecule. In this work, we report harmonic vibrational frequencies, molecular structure, NBO and HOMO, LUMO analysis and detonation properties of (S)-2-(2-oxopyrrolidin-1-yl) butanamide. The solid phase FT-IR and FT-Raman spectra of (S)-2-(2-oxopyrrolidin-1-yl) butanamide were recorded in the region 4000–450 cm1 and 4000–50 cm1 respectively. Harmonic frequencies of this compound were determined and analyzed by DFT utilizing 6-31G(d,p), 6-31+G(d,p) basis sets. The assignments of the vibrational spectra have been carried out with the help of Normal Coordinate Analysis (NCA) following the Scaled Quantum Mechanical Force Field Methodology (SQMFF). The calculated infrared and Raman spectra of the title compounds were also stimulated utilizing the scaled force fields and the computed dipole derivatives for IR intensities and polarizability derivatives for Raman intensities. The change in electron density (ED) in the r* and p* antibonding orbital’s and stabilization energies E(2) have been calculated by Natural Bond Orbital (NBO) analysis to give clear evidence of stabilization originating in the hyperconjugation of hydrogen-bonded interaction. Heat of formation (HOF) and calculated density were estimated to evaluate detonation properties using Kamlet–Jacobs equations. The linear polarizability (a) and the first

⇑ Corresponding author. Tel.: +91 9443690138; fax: +91 4427162462. E-mail address: [email protected] (S. Muthu). 1386-1425/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.saa.2013.09.055

S. Renuga, S. Muthu / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 118 (2014) 702–715

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order hyperpolarizability (b) values of the investigated molecule have been computed using DFT calculations. The calculated HOMO and LUMO energies show that charge transfer occurs within the molecule. The observed and calculated wave numbers are found to be in good agreement. On the basis of vibrational analyses, the thermodynamic properties of title compound at different temperatures have been calculated. Ó 2013 Elsevier B.V. All rights reserved.

Introduction (S)-2-(2-oxopyrrolidin-1-yl) butanamide is a Pyrrolidone derivative and belongs to a class of antiepileptics. It is a white crystalline powder used as an anticonvulsant an antiepileptic drug (AED), was first approved as an adjunctive therapy for the treatment of partial epilepsy in adults. It is currently being used in the treatment of multiple seizure disorders, including generalized tonic–clonic; absence; myoclonic; especially juvenile myoclonic; Lennox–Gastaut syndrome; and refractory epilepsy in children and adults. There is preliminary evidence about the efficacy of (S)-2-(2-oxopyrrolidin-1-yl) butanamide in the treatment of different psychiatric disorders, including anxiety, panic, stress, mood and bipolar, autism, and Tourette’s syndrome [1], It is also used in veterinary medicine for similar purposes. It is generally well tolerated, but may cause drowsiness, weakness, unsteady gait, coordination problems, headache, pain, forgetfulness, agitation, dizziness, mood changes, and nervousness, loss of appetite, vomiting, diarrhea, constipation, and changes in skin pigmentation [2]. Serious side effects may include depression, hallucinations, suicidal thoughts, seizures that are worse or different, fever, sore throat, signs of infection, double vision, itching, rash, swelling of the face. A study published in 2005 suggests that the addition of pyridoxine (vitamin B6) may curtail some of the psychiatric symptoms [3]. Drugs used to specifically facilitate learning or memory, particularly to prevent the cognitive deficits associated with dementias. These drugs act by a variety of mechanisms. While no potent nootropic drugs have yet been accepted for general use, several are being actively investigated. There are only a few papers published reporting therapeutic drug monitoring methods of this compound. Three of them employed HPLC with UV-detection, and two methods were using GC with NPD-detection, Microemulsion electro kinetic chromatography with UV-detection was utilized in one method [4–8]. Two methods facilitating chiral separation of the S- and R-enantiomer of title

molecule utilizing GC–MS and the other HPLC–UV were published [9,10]. Density functional methods have been increasingly used by spectroscopist for modeling molecular properties that includes equilibrium structures, vibrational frequencies and intensities. In the present study FT-IR, FT-Raman spectral investigation of title compound has been performed using Density Functional Theory (DFT). A complete vibrational analysis of the molecule was performed by combining the experimental and theoretical information using Pulay’s DFT based scaled quantum mechanical (SQM) approach. The change in electron density (ED) in the r* and p* antibonding orbital’s and stabilization energies E(2) have been calculated by NBO analysis to acquire clear evidence of stabilization originating in the hyper conjugation of hydrogen-bonded interaction. Heat of formation (HOF) and calculated density were estimated to evaluate detonation properties using Kamlet–Jacobs equations. The linear polarizability (a) and the first order hyperpolarizability (b) values of the investigated molecule have been computed using DFT calculations. In addition, HOMO, LUMO analysis has been used to elucidate the information regarding charge transfer within the molecule.

Experimental details The compound under investigation of (S)-2-(2-oxopyrrolidin-1yl) butanamide was purchased from sigma chemical company, USA with stated purity 99% and used as such without further purification. It is used to record the FTIR and FT-Raman spectra. The FTIR spectrum of (S)-2-(2-oxopyrrolidin-1-yl) butanamide has been recorded in the region 450–4000 cm1 on a Perkin Elmer FTIR BX spectrometer calibrated using polystyrene bands. The FT Raman spectrum has been recorded in the region 4000–50 cm1 in pure mode using Nd–Yag laser of 200 mw, the spectra was recorded using BRUKER RFS 27spectrophotometer with calibrated using

Fig. 1. Optimized structure of (S)-2-(2-oxopyrrolidin-1-yl) butanamide.

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Table 1 The optimized structural parameters for (S)-2-(2-oxopyrrolidin-1-yl) butanamide computed at B3LYP method using 6-31G(d,p), 6-31+G(d,p) basis sets.

a

Bondlength (Å)

DFT/6-31G(d,p)

DFT/6-31+G(d,p)

Experimentala

C11AH26 C11AH25 C10AH24 C10AH23 C9AH22 C9AH21 N6AH20 N6AH19 C4AH18 C4AH17 C4AH16 C3AH15 C3AH14 C2AH13 C2AN7 C8AO12 N7AC11 C10AC11 C9AC10 C8AC9 N7AC8 C1AN6 C1AO5 C3AC4 C2AC3 C1AC2

1.093 1.099 1.092 1.094 1.092 1.097 1.015 1.01 1.096 1.093 1.093 1.094 1.097 1.095 1.463 1.23 1.465 1.544 1.537 1.525 1.368 1.361 1.225 1.532 1.53 1.55

1.092 1.099 1.093 1.095 1.093 1.097 1.015 1.011 1.096 1.094 1.095 1.095 1.097 1.095 1.468 1.233 1.468 1.544 1.539 1.523 1.368 1.361 1.227 1.533 1.531 1.551

1.113 1.113 1.113 1.113 1.113 1.113 1.012 1.012 1.113 1.113 1.113 1.113 1.113 1.113 1.45 1.208 1.45 1.523 1.501 1.509 1.369 1.369 1.208 1.523 1.523 1.509

Bond angle (°) H26AC11AC10 H26AC11AN7 C10AC11AN7 H24AC10AH23 H24AC10AC11 H24AC10AC9 H23AC10AC11 H23AC10AC9 C11AC10AC9 H22AC9AH21 H22AC9AC10 H22AC9AC8 H21AC9AC10 H21AC9AC8 C10AC9AC8 O12AC8AC9 O12AC8AN7 C9AC8AN7 C2AN7AC11 C2AN7AC8 C11AN7AC8 H20AN6AH18 H20AN6AC1 H19AN6AC1 H18AC4AH17 H18AC4AH16 H18AC4AC3 H17AC4AH16 H17AC4AC3 H16AC4AC3 H15AC3AH14 H15AC3AC4 H15AC3AC2 H14AC3AC4 H14AC3AC2 C4AC3AC2 H13AC2AN7 H13AC2AC3 H13AC2AC1 N7AC2AC3 N7AC2AC1 C3AC2AC1 N6AC1AO5 N6AC1AC2 O5AC1AC2

112.5 111.0 102.7 107.8 112.0 113.5 109.4 110.0 104.0 107.2 114.7 110.7 112.4 107.5 104.3 126.7 125.1 108.1 124.2 120.7 113.0 119.6 119.4 117.5 108.4 107.7 111.4 107.8 110.8 110.6 108.0 109.1 109.0 110.0 108.3 112.3 106.0 109.9 108.3 112.7 108.6 112.0 124.0 113.4 122.6

112.6 110.9 102.8 107.9 111.9 113.4 109.5 110.1 104.0 107.2 114.6 110.5 112.4 107.6 104.3 126.7 125.0 108.3 124.2 120.9 112.9 119.5 120.2 118.1 108.4 107.7 111.4 107.6 111.1 110.5 107.9 109.3 109.4 109.8 108.0 112.4 105.0 109.5 108.1 112.8 108.7 112.4 123.5 113.7 122.7

112.6 112.6 104.0 108.7 110.2 110.2 109.7 109.7 108.3 105.4 113.3 113.3 111.1 111.1 102.9 124.5 124.5 111.0 124.5 124.5 111.0 120.0 120.0 120.0 109.5 109.5 109.5 109.4 109.4 109.5 109.5 109.5 109.5 109.4 109.4 109.5 109.5 109.5 109.5 109.4 109.4 109.5 120.0 120.0 120.0

polystyrene bands. Liquid nitrogen cooled Ge diode was used as a detector. Spectra were collected for samples with 1000 scan accumulated for over 30 min duration. The spectral resolution after apodization was 4 cm1. A correction according to the fourth power scattering factor was performed, but no instrumental correction was done. The spectral measurements are carried out at Sophisticated Analytical Instruments Facility (SAIF), Indian Institute of Technology (IIT), Chennai. Computational details The molecular geometry, harmonic force field and vibrational frequencies were computed using the B3 [11] exchange functional combined with the LYP [12] correlation functional resulting in the B3LYP density functional, using 6-31G(d,p)6-31+G(d,p) basis sets with the Gaussian 03 software system [13]. The transformation of force field from Cartesian to symmetry coordinate, the scaling, the subsequent normal coordinate analysis, calculation of potential energy distribution (PED) and IR and Raman intensities were done using MOLVIB program written by Sundius [14,15]. The NBO calculations [16] were performed using NBO 3.1 program as implemented in the Gaussian 03W package at the DFT/B3LYP level in order to understand the intra-molecular delocalization or hyperconjugation. Predictions of Raman intensities

Taken from Refs. [19,20].

The Raman activities (Si) calculated by Gaussian 03 program have been suitably adjusted by the scaling procedure with MOLVIB and subsequently converted to relative Raman intensities (Ii) using the following relationship derived from the basic theory of Raman scattering [17,18].

Ii ¼

f ðt0  ti Þ4 Si ti ½1  expðhcti=kb TÞ

where t0 is the exciting frequency (in cm1), ti is the vibrational wave number of the ith 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 cm1. Results and discussion Optimized geometries The optimized molecular structure of the title compound with numbering scheme for the atoms is presented in Fig. 1. The most optimized structural parameters of (S)-2-(2-oxopyrrolidin-1-yl) butanamide are calculated by B3LYP level with 6-31G(d,p) 6311+G(d,p) basis set and presented in Table 1. The results are compared with the experimental X-ray diffraction data [19,20]. From the single crystal XRD data, it is found that the compound has the following dimensions; a = 6.272 Å; b = 7.993 Å; c = 9.199 Å; a = 90°; b = 108°; c = 90° The values for all the bond lengths and bond angles have small deviation with the experimental results are calculated by 6-31G(d,p)6-31+G(d,p) basis sets. This is due to the theoretical calculations are performed for an isolated molecule in gaseous phase and the experimental results are for a molecule in a solid state. This molecule has twelve CAH bond lengths, six CAC bond lengths, two CAO bond lengths, two NAH bond lengths, two CAN bond lengths and two NAC bond lengths. It is observed that the calculated CAC bond distances are higher than the CAH bond lengths, they are found to be slight difference at all levels of

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S. Renuga, S. Muthu / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 118 (2014) 702–715 Table 2 Second order perturbation theory analysis of Fock matrix in NBO basis. Donor (i)

a b c

Type

ED (e)

Acceptor (j)

Type 

ED (e)

E(2)

a

(kJ/mol)

E(j)–E(i)b (a.u.)

F(i, j)c (a.u.)

C1AC2

r

1.97213

N6AH19 N7AC8 C1AO5 C2AC3

r r r r*

0.02307 0.08618 0.01570 0.02909

3.46 1.63 1.10 0.92

1.05 1.12 1.31 1.02

0.054 0.039 0.034 0.027

C1AO5

p

1.98837

C2AN7 C2AH13

r r

0.03674 0.01869

1.73 1.06

0.71 0.74

0.031 0.025

C1AN6

r

1.99208

C4AH16

r

0.03433

1.34

1.17

0.036

C2AN7

r

1.97987

C1AO5 C8AC9

p r

0.28695 0.06109

1.30 1.52

0.78 1.15

0.031 0.038

C2AH13

r

1.96167

C1AO5 C1AN6 N7AC11

p r r

0.28695 0.06863 0.02418

3.91 1.85 5.11

0.54 0.97 0.85

0.044 0.038 0.059

C3AC4

r

1.98655

N6AH20

r

0.28772

1.67

1.11

0.041

C3AH15

r

1.97792

C2AN7 C4AH18

r r

0.03674 0.03159

1.82 2.61

0.87 0.83

0.036 0.042

C4AH16

r

1.98346

C2AC3 C3AC4

r r

0.02909 0.01241

2.94 1.17

0.89 0.86

0.046 0.028

C4AH17

r

1.83672

C3AH14 N6AH19

r r

0.01340 0.02307

2.72 3.04

0.85 0.89

0.045 0.048

C4AH18

r

1.83533

C3AH15 N6AH20

r r

0.01339 0.28772

2.83 124.20

0.85 0.95

0.045 0.316

N6AH20

r

1.95497

C1AO5 C4AH16

r r

0.01570 0.03433

3.22 13.14

1.33 1.02

0.059 0.104

C9AC10

r

1.98084

C8AO12 C11AH26

r r

0.01924 0.02201

4.60 1.01

1.27 0.99

0.068 0.028

O5

LP2

1.86827

C1AC2 C1AN6

r r

0.07346 0.06863

19.97 24.56

0.63 0.70

0.101 0.019

N6

LP1

1.69116

C1AO5 C4AH17 C4AH18

p r r

0.28695 0.03227 0.03159

63.26 7.20 7.12

0.28 0.57 0.57

0.119 0.061 0.061

N7

LP1

1.68128

C8AO12 C11AH25

p r

0.27507 0.02952

53.47 7.92

0.29 0.63

0.113 0.068

O12

LP2

1.85740

N7AC8 C8AC9

r r

0.08618 0.06109

26.53 20.30

0.71 0.64

0.124 0.103

E(2) means energy of hyper conjugative interaction (stabilization energy). Energy difference between donor and acceptor i and j NBO orbitals. F(i,j) is the Fock matrix element between i and j NBO orbitals.

calculations. The NAC bond distance calculated by DFT methods are higher than the experimental result but NAH bond length is very small compare than other bond lengths. The CAO bond length of 1.230 Å and CAN of 1.468 Å is slightly larger than the experimental value and this is caused by the inter-molecular hydrogen bonding in crystalline state. Hence the O5 and O12 act as lone pair donor. The very high bond length of strong bond is found to be C10AC11 as shown in Table 1. From Table 1, the largest deviation for the bond angles from the experimental values are found for CACAC bonds by 4°. The HACAC bond angle values lies between 1° and 3° .These deviations are small. The bond angles obtained by the two basis sets are very similar. The Very largest deviation for the bond angle from experimental value found for N6AC1AC2 bond by 7° due to butterfly mode of vibration. The calculated bond angles using both basis sets for H26AC11AC10, H15AC3AC4, H15AC3AC2, H14AC3AC4, and H13AC2AC3 are in good agreement with the experimental ones. The variation in bond angle depends on the electro negativity of the central atom, the presence of lone pair of electrons and the conjugation of the double bonds. The dipole moment of a bond depends on the differences in electro negativity between the two atoms in the bond. The dipole moment of a molecule is then the resultant vector of all the bond dipoles. The larger the dipole

moment, the stronger will be the intermolecular interaction. Dipole moment increase can be attributed to hyper-conjugated structures, thereby shortening the bond lengths. It is mainly used to study the intermolecular interactions involving the van der Waal type dipole–dipole forces, etc., because, larger the dipole moment, stronger will be the intermolecular interactions. The dipole moment of (S)-2-(2-oxopyrrolidin-1-yl) butanamide is 7.1423D computed by B3LYP/6-31+G(d,p) method. NBO analysis NBO analysis provides the most accurate possible ‘natural Lewis structure’ picture of u, because all the orbital details are mathematically chosen to include the highest possible percentage of the electron density. A useful aspect of the NBO method is that it gives information about interactions in both filled and virtual orbital spaces that could enhance the analysis of intra and intermolecular interactions. The second-order Fock matrix was carried out to evaluate the donor–acceptor interactions in NBO analysis [21]. The interactions’ result is the loss of occupancy from the localized NBO of the idealized Lewis structure into an empty non-Lewis orbital. For each donor (i) and acceptor (j), the stabilization energy E(2) associated with the delocalization i ? j is estimated as

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Table 3 Definition of internal coordinates of (S)-2-(2-oxopyrrolidin-1-yl) butanamide. No (i)

Symbol

Type

Definition

Stretching 1–6 ri 7–18 Ri 19–20 gi ui 21–24 25–26 Qi

CAC CAH C@O CAN NAH

C1AC2, C2AC3, C3AC4, C8AC9, C9AC10, C10AC11 C2AH13, C4AH16, C4AH17, C4AH18, C3AH14, C3AH15, C9AH21, C9AH22, C10AH23, C10AH24, C11AH25, C11AH26 C1AO5, C8AO12 C11AN7, C8AN7, C1AN6, C2AN7 N6AH19, N6AH20

In plane bending 27–31 bi 32–50 bi

Ring CACAH

51–55 56–58 59–60 61–62

CACAN NACAH CANAH NACAO

N7AC8AC9, C8AC9AC10, C9AC10AC11, C10AC11AN7, C11AN7AC8 C1AC2AH13, C3AC2AH13, C2AC3AH15, C2AC3AH14, C4AC3AH14, C4AC3AH15, C3AC4AH16, C3AC4AH17, C3AC4AH18, C8AC9AH21, C8AC9AH22, C9AC10AH23, C9AC10AH24, C10AC9AH21, C10AC9AH22, C10AC11AH25, C10AC11AH26, C11AC10AH24, C11AC10AH25 C1AC2AN7, C3AC2AN7, C2AC1AN6, C10AC11AN7, C9AC8AN7 N7AC2AH13, N7AC11AH25, N7AC11AH26 C1AN6AH19, C1AN6AH20 N7AC8AO12, N6AC1AO5.

bi bi di di

Out of plane bending 63–69 xi HACACAC 70–71 72–73 74–79 Torsion 80–84 85–88 89–90 91–96 97–98 99– 102 103– 104

xi xi xi

OACACAN CANACAC HACACAH

H13AC2AC3AC1, H14AC3AC2AC4, H15AC3AC2AC4, H21AC9AC8AC10, H22AC9AC8AC10, H23AC10AC9AC11, H24AC10AC9AC11 O5AC1AC2AN6, O12AC8AC9AN7 C2AN7AC11AC8, C1AC2AC3AN7 H16AC4AC3AH17, H16AC4AH17AH18, H17AC4AH16AH19, H17AC4AC3AH16, H18AC4AH16AH17, H19AN6AC1AH20

si si si si si si

sRing 1 sCACH2 sC@O sCACH3 sCANH2 sCAC

N7AC8AC9AC10, C8AC9AC10AC11, C9AC10AC11AN7, C10AC11AN7AC8, C11AN7AC8AC9 H23AC10AC9AH21, H24AC10AC9AH22, H25AC11AC10AH23, H26AC11AC10AH24 O12AC8AN7AC11, C10AC9AC8AO12 H18AC4AC3AH14, H18AC4AC3AH15, H17AC4AC3AH14, H17AC4AC3AH15, H16AC4AC3AH14, H16AC4AAC3AH15 H19AN6AC1AC2, H20AN6AC1AO5 N7AC2AC3AC4, H13AC2AC3AC4, H21AC9AC8AO12, H22AC9AC8AN7

si

sButterfly

C2AC1AN6AH19, O5AC1AN6AH20

E2 ¼ DEij ¼

qi ðF ij Þ2 ej  ei

where qi is the donor orbital occupancy, ej and ei are diagonal elements and Fij is the off diagonal NBO Fock matrix element. NBO analysis provides an efficient method for studying intra and intermolecular bonding and interaction among bonds, and also provides a convenient basis for investigation charge transfer or conjugative interactions in molecular system [22]. Another useful aspect of 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. In Table 2, the perturbation energies of significant donor–acceptor interactions are presented. The intramolecular hyper conjugative interactions are formed by the orbital overlap between r and p (CAC, C@O, CAH and NAH) and r and p (CAC, C@O, CAH and NAH) bond orbital’s which results ICT causing stabilization of the system. These interactions are observed as increase in electron density (ED) in CAC, CAH, CAO and NAH antibonding orbital that weakens the respective bonds. The electron density of two conjugated single as well as double bond of title compound (1.9e) clearly demonstrate strong delocalization for title molecule. The strong intramolecular hyper conjugative interaction of the r and p electrons of CAH to the anti NAH bond of the ring leads to stabilization of some part of the ring as evident from Table 2. For example the intramolecular hyper conjugative interaction of r (C2AH13) distribute to r*(C1AN6), p*(C1AO5), r*(N7AC11) leading to stabilization of 5.11 kJ/mol. The important interaction energy, related to the resonance in the molecule, is the electrons donating from LP (2) O5, LP (2) O12 to the antibonding acceptor r*(C1AC2), r*(C1AN6), r*(N7AC8), r*(C8AC9) has moderate stabilization energy of 19.97, 24.56, 26.53 and 20.30 kJ/mol respectively. Furthermore, the most important interaction is the electrons donating from LP (1) N6, LP (1) N7 to the antibonding acceptor p*(C1AO5),

p*(C8AO12) has strong stabilization energy of 63.26, 53.47 kJ/mol as shown in Table 2 which quantity extend the intermolecular hydrogen bonding. Vibrational analysis (S)-2-(2-oxopyrrolidin-1-yl) butanamide consists of 26 atoms, which has 72 normal modes of vibration. These modes of (S)-2(2-oxopyrrolidin-1-yl) butanamide have been assigned according to the detailed vibrations of the individual atoms. Internal valence coordinates of the title compound has been defined in Table 3 and are summarized in Table 4. Figs. 2 and 3 are present for the observed FT-IR and FT-Raman and theoretical spectra of the molecule respectively. The experimental FT-IR and FT-Raman together with the calculated wave numbers are tabulated (Table 5). NH vibrations The asymmetric and symmetric stretching modes of NH2 group of primary amides are expected in the range 3540–3480 cm1 and 3420–3380 cm1 respectively [23]. The observed bands at 3574, 3174 cm1 in IR spectrum are assigned for NH2 asymmetric and symmetric stretching with 100% of PED contribution. NH2 rocking and NH2 twisting modes observed at 1207, 349 cm1 in Raman spectrum. In the present study, the CANH2 in plane bending observed at 1338, 1289 cm1 in Raman spectrum and CANH2 out of plane deformation vibration is identified at 758, 747 cm1 in IR and Raman spectrum respectively. Theoretical calculated values of NAH vibrations by B3LYP/6-31G(d,p) in high frequency region are reliable with experimental results and these assignments are in good agreement with the literature data [24]. Methyl & methylene group vibrations For the assignments of CH3 group frequencies, nine fundamentals can be associated to each CH3 group namely, CH3 symmetrical

S. Renuga, S. Muthu / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 118 (2014) 702–715 Table 4 Definition of local symmetry coordinates of (S)-2-(2-oxopyrrolidin-1-yl) butanamide. No

Symbol

1–6 7 8 9 10 11–14

CAC CAH CH3ss CH3ips CH3ops CH2ss

15–18 19–20 21–24 25 26 27 28 29–38

39–41 42–43 44 45 46 47 48–54 55–56 57–58 59–64 65 66 67 68 69 70 71 72

707

the CAH stretching vibrations of title compound with the literature data [29].

Definition

r1, r2, r3, r4, r5, r6 R7 p (R8 + R9 + R10)/ 3 p (2R8R9R10)/ 6 p (R9R10)/ 2 p p p (R11 + R12)/ 2, (R13 + R14)/ 2, (R15 + R16)/ 2, p (R17 + R18)/ 2 p p p CH2ass (R11R12)/ 2, (R13R14)/ 2, (R15R16)/ 2, p (R17R18)/ 2 C@O g19, g20 CAN /21, /22, /23, /24 p NH2ss (Q25 + Q26)/ 2 p NH2ips (Q25Q26)/ 2 R1bend1 b27 + a(b28 + b29) + b(b30 + b31) R1bend2 (ab)(b28b29) + (1a)(b30b31) p p p bCCH (b32b33)/ 2, (b34b35)/ 2, (b36b37)/ 2, p p (b38b39)/ 2, (b40b41)/ 2, p p p (b42b43)/ 2, (b44b45)/ 2, (b46b47)/ 2, p (b48b49)/ 2, b50 p p bCCN (b51b52)/ 2, b53, (b54b55)/ 2 p bNCH b56, (b57b58)/ 2 p dCANH2twist (d59 + d60)/ 2 p dCANH2rock (d59d60)/ 2 p dNC@Otwist (d61 + d62)/ 2 p dNC@Orock (d61d62)/ 2 CC wag x63, x64, x65, x66, x67, x68, x69 C@Owag x70, x71 CANwag x72, x73 CAH2wag x74, x75, x76, x77, x78, x79 sring 1 b(s80 + s84) + a(s81 + s83) + s82 sring 1 (ab)(s84s80) + (1a)(s83s81) sCACH2 (s85 + s86 + s87 + s88)/2 p sC@O (s89s90)/ 2 p sCACH3 (s91 + s92 + s93 + s94 + s95 + s96)/ 6 p sCANH2 (s97s98)/ 2 sCAC (s99 + s100 + s101 + s102)/2 p sButterfly (s103s104)/ 2

stretch, CH3 in-plane stretch (i.e., in-plane hydrogen stretching modes), CH3 in-plane bending (i.e., hydrogen deformation modes), CH3 symmetrical bending, CH3 in-plane rocking, CH3 out-of-plane rocking and CH3 twisting. In addition to that, CH3 out-of-plane stretch and CH3 out-of-plane bending modes of the CH3 group would be expected to be depolarized for symmetry species. The CAH stretching in CH3 occurs at lower frequencies than those of aromatic ring (3100–3000 cm1). The vibrations of methyl group in this title molecule are observed in the typical range reported earlier [25–27]. In the present study in plane and out of plane CH3 stretching vibrations observed at 2998, 2831 cm1 in FT-IR spectrum and the bands identified at 2990, 2903 cm1 in Raman spectrum with maximum contribution of PED. The Raman bands observed at 1468 cm1 assigned to CH3 in plane bending vibrations and one band observed at 1014 cm1 in both spectra. The computed values identified at 260, 187 cm1 are assigned to CH3 out of plane bending vibration and the CH3 torsion mode respectively. For the assignments of CH2 group frequencies, the symmetric and asymmetric CH2 stretching vibrations are observed as strong intense bands at 2982, 2954, 2923 and 2910 cm1 in FT-IR and the very strong bands at 2946, 2941 and 2910 cm1 in Raman spectrum. The CH2 wagging vibration are observed at 1452, 1428 cm1 in FT-IR spectra and at 1455 cm1 in Raman Spectrum. The CH2 torsion modes are observed at 512, 482 cm1 in FT-IR and at 544, 453 cm1 in Raman spectrum respectively [28]. The CAH stretching modes usually appear with strong Raman intensity and are highly polarized. In the FT-IR and FT-Raman the bands observed at 3004 cm1 and 3011, 3016 cm1 respectively are assigned to

C@O vibrations The C@O stretching vibrations give raise to the characteristics in IR and Raman spectra, and the intensity of these bands may increase owing to the formation of hydrogen bonds. The interaction of carbonyl group with other groups present in the system did not produce such as drastic and characteristic changes in the frequency of C@O stretch as did by interaction of NAH stretch. The carbon–oxygen double bond is formed by pp–pp between carbon and oxygen. Because of the different electro negativities of carbon and oxygen atoms, the bonding electrons are not equally distributed between the two atoms. Also, the lone pair of electrons on oxygen determines the nature of the carbonyl group. The position of the C@O stretching vibration is very sensitive to various factors such as the physical state, electronic effects by substituents, ring strains [30]. Normally carbonyl group vibrations occur in the region 1760–1730 cm1 [31,32]. In this study, the C@O stretching vibration of title compound is observed at 1694 cm1 in the FT-IR and 1713, 1690 cm1 in the FT-Raman spectrum with maximum contribution of PED. The C@O stretching vibration coupled with CAC stretching modes identified at the region 1090–1053 cm1 in both spectra in our compound. The very strong FT Raman band at 142 cm1 is assigned to butterfly vibration. The calculated frequency correlates well with the experimental data. The in-plane and out of- plane bending vibrations of C@O group are identified and presented in Table 5. CAC vibrations The ring carbon–carbon stretching vibrations occur in the region 1430–1625 cm1. In general, the bands are of variable intensity and are observed at 1625–1590, 1575–1590, 1470–1540, 1430–1465 and 1280–1380 cm1 from the frequency ranges given by Varsanyi [33] for the five bands in the region. In the present work, the frequencies observed in FT-IR spectrum at 1484, 1367 cm1 and 1317 cm1 have been assigned to CAC stretching vibrations. The corresponding vibration appears in the FT-Raman spectrum at 1573, 1490, 1414 cm1and 1383 cm1. The NACH2 stretching mode is coupled with CAC stretching as it is evident from Table 5. The CAC bending modes are observed at lower region 740–620 cm1 with the literature data [34]. The CAC and CCC vibrations are pulled considerably to the lower region and are purely due to the substitutions. CAN vibrations The identification of CAN vibrations is a difficult task, since the mixing of vibration is possible in this region. However with the help of force field calculations, the CAN vibrations are identified and assigned in this study. Silverstein [35] assigned CAN stretching absorption in the region 1382–1266 cm1 for aromatic amines. In 3,5-dibromopyridine the band observed at 1410 cm1 in IR and 1412 cm1 in Raman is assigned to CAN stretching mode [36]. Pyrimidine absorbs strongly in the region 1600–1500 cm1 due to the CAC and CAN ring stretching vibration [37]. In benzotriazole, the CAN stretching bands are found to be observed at 1387 cm1 and 1370 cm1. In our present study, the bands observed at 1452 cm1 in FT-IR spectrum is assigned for CAN stretching vibration. CAN stretching coupled with other modes of vibrations is observed at the lower region 1200–1123 cm1 in both spectra as shown in Table 5. The slight shift in wave number is due to the fact that force constants of the CAN band increased due to resonance with the ring. Accordingly the FTIR bands observed at 963, 933 cm1 and band at 935 cm1 in the Raman spectrum are assigned to CAN bending modes of vibrations.

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Fig. 2. FTIR spectra of title compound (A) experimental, (B) B3LYP/6-31G(d,p) and (C) B3LYP/6-31+G(d,p).

Fig. 3. FT-Raman spectra of title compound (A) experimental (B) B3LYP/6-31G(d,p) and (C) B3LYP/6-31+G(d,p).

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Table 5 The observed FT-IR, FT-Raman and calculated wave numbers using B3LYP method with 6-31G(d,p), 6-31+G(d,p) basis sets and probable assignments for (S)-2-(2-oxopyrrolidin-1yl) butanamide. Mode no

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70

Observed (cm1)

Calculated (cm1)

DFT/6-31G(d,p)

Ft-IR

Ft-Raman

6-31G(d,p)a

6-31+G(d,p)a

IIR

3574vs 3174vs – – 3004w 2998vw – 2982s 2954s – – 2923s 2910s 2831w – 1694w – 1484w – – 1452 ms 1428vw – – 1367vw – 1317vw – – 1266vw 1264w – – – 1186s – 1146s – – 1066 ms – 1014 ms 963s 933s 911s 907vw 856vs 842 ms 814s 793vs 758s 701s – 620w 586s 567vs – 512 ms 482vw – – – – – – – – – – –

– – 3016vs 3011s – – 2990s – – 2946vs 2941vs – 2910vs 2903 ms 1713 ms 1690s 1573s 1490s 1468 ms 1455s – – 1414vs 1383s – 1338s – 1300vw 1289vs – – 1245w 1233vs 1207s – 1172vw – 1123vs 1090s – 1053vs 1014s – 935vs – – 855s – – – 747vs 705s 638vs – – – 544s – – 453s 381s 349s – – 184s – – 142vs 122s 76s

3543 3378 3013 3011 3002 2997 2989 2980 2957 2955 2940 2930 2928 2907 1729 1697 1570 1489 1467 1455 1454 1435 1426 1397 1371 1356 1344 1313 1296 1268 1266 1246 1233 1202 1185 1180 1145 1129 1094 1062 1052 1014 996 985 911 903 879 838 833 791 734 714 657 623 599 567 547 512 481 447 320 296 291 260 194 187 157 143 114 87

3553 3395 3016 3012 3007 3000 2997 2983 2964 2958 2946 2935 2931 2915 1702 1673 1571 1486 1461 1453 1450 1434 1422 1398 1369 1355 1344 1313 1296 1268 1265 1249 1232 1204 1183 1181 1146 1128 1089 1060 1053 1014 997 986 911 905 880 839 834 791 735 710 649 623 601 561 549 512 461 444 322 296 290 257 187 182 154 143 110 85

206 255 75 74 36 132 49 30 18 70 39 79 84 98 1052 735 473 25 20 11 23 10 5 337 10 75 39 19 17 150 76 247 44 16 128 6 15 5 12 9 4 8 2 14 23 12 6 33 8 6 17 22 14 76 67 114 16 117 547 94 9 4 35 14 23 7 20 6 2 4

b

DFT/6-31+G(d,p)

IRamanc

IIR

b

97 118 39 109 120 88 70 74 62 78 128 77 131 80 3 8 6 10 14 20 12 13 22 5 3 5 4 1 6 6 5 2 2 15 2 9 1 5 2 2 1 5 8 3 5 5 3 5 5 1 2 12 3 2 2 2 2 5 1 2 3 1 1 0 0 0 1 0 1 1

297 305 86 99 60 55 106 85 124 189 51 100 136 130 1707 1170 579 28 36 34 32 14 6 417 19 91 41 24 24 226 104 338 59 28 128 31 24 7 11 11 9 10 4 18 26 21 6 41 9 14 15 21 13 70 70 75 18 77 565 334 12 5 42 17 18 14 27 10 4 5

Vibrational assignments (>10% PED)

IRamanc 109 167 141 45 152 177 55 106 85 124 189 145 199 133 7 19 6 7 9 11 8 13 16 8 1 8 5 2 4 2 6 3 2 12 4 6 1 5 3 3 2 7 11 3 5 8 5 5 8 2 2 16 2 2 3 2 2 6 1 2 3 3 2 0 0 1 1 0 1 2

NH2ass(100) NH2ss(100) cCH(99) cCH(98) cCH(98) CH3ips(85) CH3ops(81) CH2ass(80) CH2ss(81) CH2ass(100) CH2ss(100) CH2ass(72) CH2ss(72) CH3ss(84) cC@O(75) cC@O(94) R1bend 2(73) + cCC(22) cCC(80) CH3ipb(72) CH2wag(31) cCN(89) + CH2wag(11) CH2wag(43) cCC(78) cCC(55) cCC(42) CANH2ipb(85) + cCC(11) cCC(68) cCC(73) CANH2ipb(68) cCN(89) + Chipb(11) cCN(82) + CH2twi(20) R1bend1(64) HCCHwag(26) + sCH2(11) NH2rock(64) + CNopb(18) CH3rock(29) cCN(81) + CCipb(15) cCN(74) + bCCN(13) cCN(82) + cC@O(40) cC@O(39) + cCC(12) cC@O(31) + cCC(12) cC@O(40) + cCC(9) cCC(25) + CH3opb(24) bCN(81) bCN(68) cCC(20) + sCH2(7) sCH2(51) + bCH2(8) bCN(22) + CNCCwag(11) bCN(61) + cCC(24) + CH2roc(12) CNCCwag(27) + sRing(17) cCC(36) bCC(51) + CANH2opb(49) bCC(48) bCC(61) bCC(25) bC@O(24) + bCC(12) bC@O(61) sCH2(26) + bCCO(5) sCH2(40) + OCCNwag(7) sCH2(45) sCH2(49) bCC(17) NH2 twi(48) + bCN(29) cCC(34) sCH3(29) HCCC(11) sCH3(89) sC@O(19) Butterfly(58) R1bend1s(38) sCH2(17) (continued on next page)

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Table 5 (continued) Mode no

71 72

Observed (cm1)

Calculated (cm1)

DFT/6-31G(d,p)

Ft-IR

Ft-Raman

6-31G(d,p)a

IIR

– –

– –

64 53

6-31+G(d,p)a 63 56

b

DFT/6-31+G(d,p)

IRamanc

16 13

IIR

1 1

b

26 19

Vibrational assignments (>10% PED)

IRamanc 1 2

sRing(47) sRing(23) + HCCHwag(8)

Abbreviations: s – strong; vs – very strong; ms – medium strong; w – weak; vw – very weak; c – stretching; b – bending; s – Torsion; sci – scissoring; wag – wagging; twi – twisting; rok – rocking, sdefor; as – asymmetric stretching; ss – symmetric stretching; ipb – in plane bending; opb – out of plane bending; ips – in plane stretching; Ops – out of plane stretching. a Scaling factor: 0.961 for B3LYP/6-31G(d,p) and 0.964 for B3LYP/6-31+G(d,p). b Relative absorption intensities normalized with highest peak absorption equal to 100. c Relative Raman intensities normalized to 100.

Table 6 HOMO–LUMO energy value and related properties of title compound calculated by B3LYP method. Parameters

B3LYP/6-31G(d,p) (a.u.)

B3LYP/6-31+G(d,p) (a.u.)

EHOMO (a.u.) ELUMO (a.u.) DEHOMO–LUMO gap (a.u.) EHOMO1 (a.u.) ELUMO+1 (a.u.) DEHOMO1LUMO+1 gap (a.u.) Electronegativity (v) Global softness (S) Global Hardness (g) Chemical potential (l) Global electrophilicity (x)

0.23264 0.01047 0.22217 0.25554 0.03208 0.22346 0.11108 4.11336 0.12155 0.11108 2.9996  103

0.24372 0.00710 0.23662 0.26856 0.00433 0.26423 0.12541 4.13188 0.12101 0.12541 3.8064  103

HOMO–LUMO energy gap The HOMO (highest occupied molecular orbital)–LUMO (lowest unoccupied molecular orbital) energy gap of (S)-2-(2-oxopyrrolidin-1-yl) butanamide has been calculated at the B3LYP/631G(d,p). Many organic molecules containing p conjugated electrons are characterized hyperpolarizabilities and were analyzed by means of vibrational spectroscopy [38,39]. These orbitals determine the way the molecule interacts with other species. Fig. S1 (Supplementary material) shows the distributions and energy levels of the HOMO1, HOMO, LUMO and LUMO+1 orbital’s computed at the B3LYP/6-31G(d,p) level for the title compound. HOMO is mainly localized on the carbon atoms of pyrrole ring and strongly delocalized on methyl group. LUMO is mainly delocalized on CH3 and CH2 group and slightly delocalized on CN group. HOMO1 is delocalized over the pyrrole ring and LUMO+1 is localized on NH2, C@O and CAN atoms of pyrrole ring. The value of the energy separation between the HOMO and LUMO is 0.22217 a.u. 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 group. The higher the energy of HOMO, the easier it is for HOMO to donate electrons whereas it is easier for LUMO to accept electrons when the energy of LUMO is low. The calculated energy values are presented in Table 6. This reveals the chemical reactivity of title compound and proves the occurrence of eventual charge transfer within the molecule. Global and local reactivity descriptors Global chemical reactivity descriptors of compounds such as hardness, chemical potential, softness, electro negativity and electro Philicity index as well as local reactivity has been defined [40– 42]. Pauling introduced the concept of electro negativity as the power of an atom in a compound to attract electrons. Hardness

(g), chemical potential (l) electronegativity (v) and softness are defined follows.

!   1 @2E 1 @U g¼ ¼ 2 2 @N 2 @N v ðrÞ v ðrÞ 

 @E @N v ðrÞ

l¼



v ¼ l ¼ 

 @E @N v ðrÞ

where E and v(r) are electronic energy, external potential of an Nelectron system respectively. Softness is a property of compound that measures the extent of chemical reactivity. It is the reciprocal of hardness

S¼

1 2g

Using Koop man’s theorem for closed-shell compounds g, l and v can be defined as,

l¼

ðI  AÞ 2

v¼

ðI þ AÞ 2

where A and I are the ionization potential and electron affinity of the compounds respectively. Electron affinity refers to the capability of legend to accept precisely one electron from a donor. However in many kinds of bonding viz. covalent hydrogen bonding, partial charge transfer takes places. Recently Parr et al. [40] have defined a new descriptor to quantity the global electro philic power of the compound as electro philicity index (x), which defines a quantitative classification of global electro philic nature of a compound. Parr et al. have proposed electrophilicity index (x) as a measure of energy lowering due to maximal electron flow between donor and acceptor. They defined electrophilicity index (x) as follows.

x¼

l2 2g

Considering the chemical hardness, large HOMO–LUMO gap means a hard molecule and small HOMO–LUMO gap means a soft molecule. One can also relate the stability of the molecule to hardness, which means that the molecule with least HOMO–LUMO gap means it is more reactive. The usefulness of this new reactivity quantity has been recently demonstrated in understanding the toxicity of various pollutants in terms of their reactivity and site selectivity [43]. The calculated value of electro Philicity index describes the biological activity of (S)-2-(2-oxopyrrolidin-1-yl) butanamide. All the calculated values of hardness, potential, softness and electro philicity index are shown in Table 6.

S. Renuga, S. Muthu / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 118 (2014) 702–715

Total, sum of alpha plus beta electrons DOS In the boundary region, neighboring orbitals may show quasi degenerate energy levels. In such cases, consideration of only the HOMO and LUMO may not yield a realistic description of the frontier orbitals. For this reason, the total (TDOS), sum of a and b electron density of states [44,45], in terms of Mulliken population analysis were calculated and created by convoluting the molecular orbital information with Gaussian curves of unit height and full width at half maximum (FWHM) of 0.3 eV by using the GaussSum 2.2 program [46]. The TDOS, abDOS of the (S)-2-(2-oxopyrrolidin1-yl) butanamide are plotted in Figs. 4 and 5. They provide a pictorial representation of MO (molecule orbital) compositions and their contributions to chemical bonding. The most important application of the DOS plots is to demonstrate MO compositions and their contributions to the chemical bonding through the positive and negative charges provide abDOS, TDOS diagrams. The abDOS shows the bonding, sum of positive and negative electron with nature of the interaction of the two orbitals, atoms or groups. In this case, the title molecule consists of 46 a-electrons and 46-b electrons, totally 92 electrons are occupied in density of states. The way we designate a pictorial representation for cat ions and anions is essentially similar to that for neutral atoms in their ground state. Because of the short range of absorption, alphas are not, in general, dangerous to life unless the source is ingested or inhaled, in which case they become extremely dangerous [47]. A positive value of the abDOS indicates a bonding interaction, negative value means that there is an anti-bonding interaction and zero value indicates nonbonding interactions [48].

Mulliken and Natural atomic charges The atomic charge in molecules is fundamental to chemistry. For instance, atomic charge has been used to describe the processes of electro negativity equalization and charge transfer in chemical reactions [49,50]. Mulliken and Natural atomic charges calculated at the B3LYP/6-31(d,p) method is collected in Table 7. It is worthy to mention that C8 atoms of title compound exhibit positive charge, while C1, C2, C3, C4, C9, C10 and C11 atoms exhibit

711

negative charges. Nitrogen has a maximum negative charge value of both MPA and NPA respectively. The charge on H19 has the maximum magnitude of 0.376 among the hydrogen atoms present in the molecule at B3LYP/6-31G(d,p) level of theory. However all the hydrogen atoms exhibit a net positive charge. These magnitudes are changing between 0.8802 and 0.376 both Mulliken and Natural population analysis. The Histogram of atomic charges plotted at 6-31G(d,p) level has been shown in Fig. S2 (Supplementary material). The presence of large negative charge on N and O atom and net positive charge on H atom may suggest the formation of intramolecular interaction in solid forms [51].

Other molecular properties Nonlinear optical effects The polarizability a and the hyper polarizability b and the electric dipole moment l of title compound are calculated by finite field method using B3LYP/6-31G(d,p) basis set available in DFT package. To calculate all the electric dipole moments and the first hyperpolarizabilities for the isolated molecule, the origin of the Cartesian coordinate system (x, y, z) = (0, 0, 0) was chosen at own center of mass of (S)-2-(2-oxopyrrolidin-1-yl) butanamide. The first order hyperpolarizability (b) of title molecule along with related properties (l, a, and ao) are calculated and it is based on the finite-field approach. In the presence of an applied electric field, the energy of a system is a function of the electric field. Polarizabilities and hyperpolarizabilities characterize the response of a system in an applied electric field [52]. They determine not only the strength of molecular interactions (long-range inter induction, dispersion force, etc.) as well as the cross sections of different scattering and collision process but also the nonlinear optical properties (NLO) of the system [53,54]. First hyperpolarizability is a third rank tensor that can be described by 3  3  3 matrix. The 27 components of the 3D matrix can be reduced to 10 components due to the Klein man symmetry [55]. It can be given in the lower tetrahedral format. It is obvious that the lower part of the 3  3  3 matrixes is a tetrahedral. The components of b are defined as the coefficients in the Taylor series expansion of the

Fig. 4. The calculated TDOS diagram of title compound.

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Fig. 5. The sum of alpha plus beta electrons DOS diagram of title compound.

Table 7 Mulliken and Natural population analysis of title compound calculated by B3LYP/6-31G(d,p) method. Atom no

MPA

NPA

C1 C2 C3 C4 O5 N6 N7 C8 C9 C10 C11 O12 H13 H14 H15 H16 H17 H18 H19 H20 H21 H22 H23 H24 H25 H26

0.04227 0.35026 0.22649 0.59904 0.30884 0.43856 0.336334 0.147355 0.31569 0.39377 0.24634 0.33596 0.26837 0.185864 0.185507 0.186184 0.1495 0.171439 0.308119 0.297411 0.186683 0.1868 0.151107 0.158089 0.195365 0.143078

0.63071 0.10395 0.3768 0.60592 0.60826 0.88023 0.51012 0.6964 0.46668 0.39819 0.16804 0.61628 0.26842 0.21736 0.21592 0.20266 0.29201 0.29684 0.37623 0.29535 0.22782 0.23018 0.20088 0.20897 0.19304 0.18169

energy in the external electric field. When the external electric field is weak and homogeneous, this expansion becomes

1 1 E ¼ E0  la F a  aab F a F b  babc F a F b F c þ . . . 2 6 where E0 is the energy of the unperturbed molecules, Fa the field at the origin la and aab babc are the components of dipole moments, polarizability and the first hyperpolarizabilities, respectively. The total static dipole moments l, the mean polarizability, a0, the

anisotropy of the polarizabilities a and the mean first hyper-polarizability b, using the x, y and z components they are defined as: The total static dipole moment is 1=2

l ¼ ðl2X þ l2Y þ l2Z Þ

The isotropic polarizability is

a0 ¼

ðaXX þ aYY þ aZZ Þ 3

The polarizability anisotropy invariant is

a ¼ 21=2 ½ðaXX  aYY Þ2 þ ðaYY  aZZ Þ2 þ ðaZZ  aXX Þ2 þ 6a2XX 

1=2

And the average hyperpolarizability is

b0 ¼ ðb2X þ b2Y 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 Since the value of the polarizability (a0) and hyperpolarizability (b0) components of Gaussian 03W output are reported in atomic units (a.u.). The calculated values have been converted into electrostatic units (esu) (a: 1 a.u. = 0.1482  1024 esu, b: 1 a.u. = 8.6393  1030 esu). In our present study, the total static dipole moment, polarizabilities and first order hyper-polarizabilities of (S)-2-(2-oxopyrrolidin-1-yl) butanamide were calculated. Table 8 lists the values of the electric dipole moment (Debye) and dipole moment components, polarizabilities and hyperpolarizabilities of the (S)-2-(2-oxopyrrolidin-1-yl) butanamide. In addition to the isotropic polarizabilities and polarizabilities anisotropy invariant were also calculated. The DFT/6-31G(d,p)calculated first hyperpolarizability of title molecule is 1.3470  1030 esu. The calculated values of l, a and b for the title compound are 2.5550 D, 5.5036  1023 esu and 1.3470  1030 esu which are lesser than those of urea (l, a

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S. Renuga, S. Muthu / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 118 (2014) 702–715 Table 8 The calculated electric dipole moments (Debye), polarizability (1024 esu), b components and btot (1030 esu) value of (S)-2-(2-oxopyrrolidin-1-yl) butanamide by B3LYP/6-31G(d,p) method. Parameters

B3LYP/6-31G(d,p)

Parameters

B3LYP/6-31G(d,p)

lx ly lz l axx axy ayy axz ayz azz a0 a (esu)

0.27341 2.00602 1.55871 2.5550 106.9664 14.8270 98.7185 0.6285 3.5774 92.7352 99.4773 5.5036  1023

bxxx bxxy bxyy byyy bxxz bxyz byyz bxzz byzz bzzz btot (esu)

44.2557 9.7911 54.0333 9.4572 14.0731 12.7374 12.1144 35.5815 90.3420 116.9757 1.3470  1030

Table 9 The detonation properties of (S)-2-(2-oxopyrrolidin-1-yl) butanamide. Detonation properties

B3LYP/6-31G(d,p)

Detonation velocity (km/s) Detonation pressure (Gpa) Packed density (g/cm3) Volume (cm3/mol)

2.6869 23.0998 1.168 147.5

Table 10 Thermodynamic properties for obtained by B3LYP/6-31G(d,p) method.

and b of urea are 1.373 D, 3.8312  1024 esu and 0.37289  1030 esu) obtained by B3LYP/6-31G(d) method [56]. Theoretically, the first-order hyperpolarizability of title molecule is of 3 times magnitude of urea. The results indicate that the title molecule is good candidate of NLO material.

Temp (K)

S0m (J/mol K)

C0p;m (J/mol K)

H0m (kJ/mol)

100 200 298.15 300 400 500 600 700 800 900 1000

308.83 388.94 456.7 457.94 523.54 586.67 646.84 703.77 757.45 808.04 855.77

92.45 144.4 199.5 200.58 257.69 308.63 351.43 387.01 416.83 442.09 463.66

6.03 17.88 34.72 35.09 58.03 86.42 119.48 156.46 196.69 239.68 284.99

Molecular electrostatic potential analysis The molecular electrostatic potential, v(r), at a given point r (x, y, z) in the vicinity of a molecule, is defined in terms of the interaction energy between the electrical charge generated from the molecule electrons and nuclei and a positive test charge (a proton) located at r. The molecular electrostatic potential (MEP) is related to the electronic density and is a very useful descriptor for determining sites for electrophilic attack and nucleophilic reactions as well as hydrogen-bonding interactions [57,58]. To predict reactive sites for electrophilic and nucleophilic attack for the title molecule, MEP was calculated at the B3LYP/6-31G(d,p) optimized geometry. The negative (red) regions of MEP were related to electrophilic reactivity and the positive (black) regions to nucleophilic reactivity. The negative regions are mainly localized on the methyl, methylene atom. Also, a negative electrostatic potential region is observed around the nitrogen atom (N6atom). A maximum positive region is localized on the hydrogen atoms indicating a possible site for nucleophilic attack. The total electron density and MESP surfaces of the molecules under investigation are constructed by using B3LYP/6-31G(d,p) method. These pictures illustrate an electrostatic potential model of the compounds, computed at the 0.002 a.u. isodensity surface. The MEP mapped surface of the compounds and electrostatic potential contour map for positive and negative potentials are shown in Figs. S3 and S4 (Supplementary material) respectively. The MEP map shows that the negative potential sites are on electronegative atoms as well as the positive potential sites are around the hydrogen atoms. These sites give information about the region from where the compound could have noncovalent interactions. Detonation properties Detonation velocity (D) and pressure (P) are the most important targets of scaling the detonation characteristics of energetic materials. Kamlet–Jacobs (K–J) equations [59,60] have been used to calculate D and P to quantitatively evaluate high energy density compound in molecular design. For the explosives with CHNO elements, these parameters can be calculated using the following Kamlet–Jacobs (K–J) equations:

D ¼ 1:01  U0:5 ð1:0 þ 1:3qÞ

ðaÞ

U ¼ NM0:5 Q 0:5 P ¼ 1:558  Uq2

ðbÞ

where each term in Eqs. (a) and (b) is defined as follows: P, detonation pressure (GPa); D, the detonation velocity (km/s); q, the packed density (g/cm3); U, the characteristics value of explosives; N, the moles of gas produced by per gram of explosives; M, an average molar weight of detonation products; and Q, the estimated heat of detonation (cal/g) defined as the difference between the heats of formation of the products and reactants of the most exothermic reactants. The density is obtained according to the method given by Politzer [61], in which the electrostatic potential is considered. According to the criteria as an high energy density material [62] i.e., D P 9 km/s and P P 40 GPa. From Table 9, the detonation properties of compound are not as large as those of HMX and RDX. (S)-2-(2-oxopyrrolidin-1-yl) butanamide is a potential low energy density material and worth further investigations of the biological activity. Thermodynamic properties The total energy of a molecule is the sum of translational, rotational, vibrational and electronic energies, i.e., E = Et + Er + Ev + Ee. The statistical thermo chemical analysis of (S)-2-(2-oxopyrrolidin-1-yl) butanamide is carried out considering the molecule to be at room temperature of 298.15 K and one atmospheric pressure. On the basis of vibrational analysis and statistical thermodynamics, the standard thermodynamic functions: heat capacity (C0p;m ), entropy (S0m ), and enthalpy (H0m ) were obtained and listed in Table 10. As is evident from Table 10, all the values of C0p;m , S0m , and H0m increases with the increase of temperature from 100 K to1000 K, which is attributed to the enhancement of the molecular vibration while the temperature increases because at a constant pressure (db = 1 atm) values of C0p;m , S0m , and H0m are equal to the quantity of temperature [63]. The correlations between these thermodynamic properties and temperatures T are fitted by quadratic formulas as follows and corresponding fitting factors (R2) for these thermodynamic properties are found to be 1, 0.999 and 0.999 for heat capacity, entropy, and enthalpy, respectively. The temperature dependence correlation graphs are shown in Fig. 6. Notice:

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S. Renuga, S. Muthu / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 118 (2014) 702–715

ular charge distribution and is given as a vector in three dimensions. Therefore, it can be used an illustrator to depict the charge movement across the molecule. Direction of the dipole moment vector in a molecule depends on the centers of negative and positive charges. Dipole moments are strictly identified for neutral molecules. For charged systems, its value depends on the choice of origin and molecular orientation. As a result the dipole moment of (S)-2-(2-oxopyrrolidin-1-yl) butanamide was observed theoretically 2.735 D with B3LYP/6-31G(d,p) method. Conclusion Molecular modeling of (S)-2-(2-oxopyrrolidin-1-yl) butanamide is done by B3LYP density functional theory calculations. The calculated geometry of the molecule agrees well with the experimental values. NBO analysis clearly demonstrates the intra-molecular interaction and the charge transfer reaction of the methyl groups attached to the pyrrol ring. A complete vibrational investigation of the title compound has been performed using FT-IR and Raman spectroscopic techniques. The simulated FT-IR and Raman spectra of the title compound show good agreement with the observed spectra. Mulliken atomic charge analysis shows that charge transfer occurring within the molecule. The lowering of HOMO and LUMO energy gap supports the bioactivity of the molecule. Thermodynamic properties in the range from 100 K to 1000 K are obtained. The gradients of Cp,m, Sm, Hm, and vibrational intensity increases with increase of temperature. Furthermore, the thermodynamic, non-linear optical, first-order hyperpolarizabilities and total dipole moment properties of the compound have been calculated in order to get insight into the compound. We hope the results will be of assistance in the quest of experimental and theoretical evidence for the title compound in reaction intermediates, non-linear optical and photoelectric material. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.saa.2013.09.055. References

Fig. 6. Correlation graphs of Thermodynamic properties at different temperatures for (S)-2-(2-oxopyrrolidin-1-yl) butanamide.

all the thermodynamic calculations were done in gas phase and they could not be used in solution. Scale factors have been recommended [64] for an accurate prediction in determining the zeropoint vibration energies, heat capacities, entropies, enthalpies.

C0p;m ¼ 0:166 þ 0:009T þ 0:000T 2  104 ; ðR2 ¼ 1Þ S0m ¼ 6:161 þ 0:044T þ 0:000T 2  104 ; ðR2 ¼ 0:999Þ H0m ¼ 4:709 þ 0:044T þ 0:000T 2  104 ; ðR2 ¼ 0:999Þ All these thermodynamic data provide helpful information for further study on (S)-2-(2-oxopyrrolidin-1-yl) butanamide. They can be used to compute the other thermodynamic parameters according to relationships of thermodynamic functions and to determine the directions of chemical reactions according to the second law of thermodynamics. Dipole moment reflects the molec-

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