Structure and vibrational frequencies of 6,7-dimethoxy-1,4-dihydro-1,3-quinoxalinedione based on density functional theory calculations: The role of π-electron conjugation and back-donation

Spectrochimica Acta Part A 77 (2010) 238–247

Contents lists available at ScienceDirect

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa

Structure and vibrational frequencies of 6,7-dimethoxy-1,4-dihydro-1,3-quinoxalinedione based on density functional theory calculations: The role of ␲-electron conjugation and back-donation V. Krishnakumar a , N. Prabavathi b,∗ a b

Department of Physics, Periyar University, Salem 636 011, India Department of Physics, Sri Sarada College for Women (Autonomous), Salem 636 016, India

a r t i c l e

i n f o

Article history: Received 13 January 2010 Received in revised form 27 April 2010 Accepted 15 May 2010 Keywords: 6,7-Dimethoxy-1,4-dihydro-1,3quinoxalinedione DFT calculations Conformational stability Vibrational frequencies

a b s t r a c t This work deals with the vibrational spectroscopy of 6,7-dimethoxy-1,4-dihydro-1,3-quinoxalinedione by means of quantum chemical calculations. The mid and far FT-IR and FT-Raman spectra are recorded in the condensed State. The fundamental vibrational frequencies and intensity of vibrational bands are evaluated using density functional theory (DFT) with the standard B3LYP/6-31G* method and basis set combination and is scaled using various scale factors which yields a good agreement between observed and calculated frequencies. The vibrational spectrum is interpreted with the aid of normal coordinate analysis based on scaled density functional force field. The results of the calculations are applied to simulate infrared and Raman spectra of the title compounds, which showed excellent agreement with the observed spectra. The infrared unscaled frequencies and intensities are used to disentangle the role played by back-donation in the title compound. For this purpose five other molecules are considered as references: ethane, dimethyl ether, anisole, p-nitro-anisole, and p-hydroxyanisole, in which backdonation has already been ascertained also experimentally. From the study of infrared intensities it is shown that no back-donation of electrons from the oxygen lone pairs takes place, independently of the conformation of the methoxy-group. © 2010 Elsevier B.V. All rights reserved.

1. Introduction It is known that ␴-bonded molecules containing methoxy groups can give rise to a back-donation of electrons, that is the oxygen releases part of the electronic charge of its lone pairs and injects it in the ␴* orbitals of the adjacent C–H bonds [1]. As a consequence, the charge on the hydrogen decreases, the bond lengthens and its strength weakens. Experimental and theoretical evidences suggest that the process is mostly effective with the hydrogen atoms in trans conformation with respect to the lone pair orbitals. The vibrational frequencies and the infrared intensities of the C–H modes involved in back-donation are strongly affected. If the intensity is interpreted in the light of the ECCF model (Equilibrium Charge and Charge Fluxes) [2,3] these changes due to back-donation cause the softening in the frequency of one of the C–H stretching modes (trans to the lone pairs) and an increase in its intensity. The interpretation of the infrared spectrum in this region is not always straightforward. Cases have been reported in which a C–H

∗ Corresponding author. Tel.: +91 427 4264245. E-mail address: [email protected] (N. Prabavathi). 1386-1425/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.saa.2010.05.015

stretching has low frequency, but no back-donation seems to take place (e.g. the 2846 cm−1 mode of methyl acetate) [4]. Other evidence has been obtained from a detailed and careful analysis of isolated C–H stretching frequencies  (C–H), the only C–H stretching mode of a molecule in which all hydrogens but one are substituted by deuterium atoms [5]. Data relative to dimethyl ether prove that back-donation affects hydrogen atoms which are in trans position with respect to the lone pairs, that is out of the C–O–C plane, as shown by the following experimental frequency and intensity data [6]. 1 = (C–H; out-of-plane) = 2883 cm−1 A1 = 47.5 km/mol 2 = (C–H; in-plane) = 2985 cm−1 A2 = 25.8A3 km/mol. The corresponding values for ethane are: 3 = (C–H; ethane) = 2950 cm−1 A3 = 27.3A3 A1 is much larger than A2 , while A2 is comparable with A3 and 1 is almost 70 cm−1 lower than ethane 3 .

V. Krishnakumar, N. Prabavathi / Spectrochimica Acta Part A 77 (2010) 238–247

239

tion operating at 200 mW power. The reported wave numbers are expected to be accurate within ±1 cm−1 . 3. Computational details

Fig. 1. Possible resonance structures for methoxybenzene (or) anisole.

In the case of alkyl–aryl ethers, like anisole (methoxybenzene), the situation is more complex because the oxygen lone pairs may be in conjugation with the pz orbitals of the aromatic system and may not be anymore available for back-donation. In terms of structural formulae the situation can be sketched as in Fig. 1. The effect of increased conjugation should be observed in the strengthening (and shortening) of the aryl–O bond and in the intensity increase of the Raman-active vibrations with large contribution of the collective C–C stretching generally described with the 6 coordinate (i.e. the Effective Conjugation Coordinate of the ECC model) [6–8]. For anisole the two effects, i.e. back-donation and conjugation, can be competitive and their relative weight in determining the electronic properties of the molecule can be conformation dependent [6]. The consequences of the extent of conformational dependence have to be found also in the values of the molecular nonlinear optical properties (hyperpolarizabilities at various orders) which are at present the center of great interest in photonics and molecular electronics. This study aims at reaching a knowledge of general validity which may guide molecular engineering of systems with improved nonlinear optical properties [6,9]. In order to understand the physics behind all the problems outlined above, we have performed ab initio calculations for 6,7-dimethoxy-1,4-dihydro-1,3-quinoxalinedione and compared to that of similar compounds like anisole, nitroanizole, and hydroxyanizole, the results obtained are presented and discussed in this article. Moreover, quinoxaline derivatives are an important class of benzoheterocycles which has received much attention in recent years owing to their both biological properties and pharmaceutical applications. These derivatives are particularly interesting since some of them showed antimicrobial [10–18], anticancer [19–34], antimalarial [35–40], antiinflammatory [41,42], antinociceptive [43–45], antitubercular [46–48], anthelmintic [49,50], antidiabetic [51] and antiepileptic [52,53] properties. Particularly, 6,7-dimethoxy-1,4-dihydro-2,3-quinoxalinedione finds application in chemistry of co-ordination polymers also [54].

The molecular geometry optimization, energy and vibrational frequency calculations are carried out with Gaussian 98w software package [55] using the B3LYP functionals [56,57] combined with standard 6-31G* basis set. The Cartesian representation of the theoretical force constants have been computed at optimized geometry having CS point group symmetry. Scaling of the force field is performed according to the SQM procedure [58,59] using selective scaling in the natural internal coordinate representation [60,61]. Transformation of the force field and subsequent normal coordinate analysis including the least square refinement of the scaling factors, calculation of the potential energy distribution (PED) and the prediction of IR and Raman intensities are done on a PC with the Molvib program written by Sundius [62,63]. For the plots of simulated IR and Raman spectra, pure Lorentzian band shapes are used with a bandwidth (FWHM) of 10 cm−1. The symmetry of the molecule is also helpful in making vibrational assignments. The symmetries of the vibrational modes are determined by using the standard procedure [64] of decomposing the traces of the symmetry operation into the irreducible representation. The symmetry analysis for the vibrational modes of the title compound is presented in order to describe the basis for the assignments. By combining the results of the Gauss view program [65] with symmetry considerations, vibrational frequency assignments are made with a high degree of confidence. There is always some ambiguity in defining internal coordinates. However, the defined coordinate form complete set and matches quite well with the motions observed using the Gauss view program. 3.1. Prediction of Raman intensities The Raman activities (Ai ) calculated with the Gaussian 98 program and adjusted during the scaling procedure with Molvib were subsequently converted to relative Raman intensities (Ii ) using the following relationship derived from the theory of Raman scattering [66–68]. Ii =

f (0 − i )4 Ai i [1 − exp(−hci /kT )]

(1)

where 0 is the exciting frequency (in cm−1 units) i is vibrational wave number of the ith normal mode, h, c and k are fundamental constants (i.e. Plank’s constant, velocity of light and Boltzmann’s constant) and f is a suitably chosen common normalization factor for all peak intensities. 4. Results and discussion

2. Experimental details

4.1. Molecular geometry

The fine polycrystalline sample of 6,7-dimethoxy-1,4-dihydro1,3-quinoxalinedione is obtained from Sigma–Aldrich Company, USA and used as such for the spectral measurements. The room temperature Fourier transform IR spectrum of title compound is measured in the 4000–500 cm−1 region at a resolution of ±1 cm−1 using BRUKER IFS-66 V Fourier transform spectrometer equipped with an MCT detector, a KBr beam splitter and globar source. Boxcar abodization is made for the 250 averaged interferograms collected for the sample. The far IR spectrum is recorded on the same instrument using polyethylene pellet technique. The FT-Raman spectrum is recorded on a BRUKER IFS-66 V model interferometer equipped with an FRA-106 FT-Raman accessory. The spectrum is recorded in the region 3500–100 cm−1 stokes region using the 1064 nm line of Nd: YAG laser for the excita-

Actually two internally rotating groups are present in the molecule: the methyl group CH3 , which can rotate about the bond O–C (O16 –C17 , O21 –C22 for the methyl group attached to O16 and O21 respectively; see Fig. 2(a) for the numbering of the atoms) and the methoxy group O–CH3 , which can rotate about C–O (C6 –O16 , C7 –O21 for the methoxy groups O16 –CH3 and O21 –CH3 , respectively). During the O–CH3 rotation, the overlapping between the lone pairs and the ring pz orbitals changes, reaching a maximum when C17 (for the methyl group attached toO16 ) and C22 (for the methyl group attached to O21 ) lies in the plane of the ring ( = 0◦ ,  being the dihedral angle (C17 –O16 –C6 –C5 for the methyl group attached to O16 and C22 –O21 –C7 –C8 for the methyl group attached to O21 )). On the other hand, for every  it is possible to have hydrogens trans to the lone pairs by a rotation of the methyl group

240

V. Krishnakumar, N. Prabavathi / Spectrochimica Acta Part A 77 (2010) 238–247

Fig. 2. (a) Optimized geometry of 6,7-dimethoxy-1,4-dihydro-1,3-quinoxalinedione. (b)–(e) Different conformers of 6,7-dimethoxy-1,4-dihydro-1,3-quinoxalinedione with CS symmetry.

alone [6]. Therefore we can in principle expect a dependence of the properties of anisole on the value of . Electron diffraction studies [69] show that at 50 ◦ C C17 and C22 lies in the plane of the ring ( = 0◦ ) while at 255 ◦ C there is a deviation from planarity of ( = 0◦ ). Crystal structures of many other unhindered methoxy substituted aromatic molecules are nearly planar (mean dihedral angle  ≈ 6◦ ) [70]. For  = 0◦ a complete geometry optimization was performed within the CS point group symmetry. In order to find the most optimized geometry, the molecular energies are calculated for the molecule under CS , C2 and C2V symmetries. The calculated molecular energies are listed in Table 1 and from the data furnished in the

table it is evident that the global minimum on the potential energy surface of the molecule is produced under CS symmetry. However, there are many more possible conformers with this CS symmetry, which may not all be minima on the PES. This is further minimized Table 1 Calculated molecular energies of 6,7-dimethoxy-1,4-dihydro-1,3-quinoxalinedione based on DFT (B3LYP/6-31G*) level of theory. Symmetry C2V C2 CS

Calculated energies (in Hatrees) −797.375207 −797.374950 −797.482290

V. Krishnakumar, N. Prabavathi / Spectrochimica Acta Part A 77 (2010) 238–247

241

Table 2 Optimized geometrical parameters of 6,7-dimethoxy-1,4-dihydro-1,3quinoxalinedione obtained by B3LYP/6-31G* density functional calculation. Bond length

Value (Å)

Bond angle

Value (◦ )

N1 –C2 C2 –C3 C3 –N4 N4 –C10 C10 –C5 C5 –C6 C6 –C7 C7 –C8 C8 –C9 C9 –C10 N1 –H11 C2 –O12 C3 –O13 N4 –H14 C5 –H15 C6 –O16 C7 –O21 C8 –H26 O16 –C17 O21 –C22 C22 –H24(25,23) C17 –H18(19,20)

1.38 1.53 1.38 1.39 1.40 1.39 1.42 1.39 1.40 1.39 1.01 1.21 1.21 1.01 1.08 1.36 1.36 1.08 1.41 1.41 1.090 1.096

N1 –C2 –C3 C2 –C3 –N4 C3 –N4 –C10 C10 –C5 –C6 C5 –C6 –C7 C6 –C7 –C8 C7 –C8 –C9 C9 –N1 –C2 C5 –C6 –O16 C6 –O16 –C17 C8 –C7 –O21 C7 –O21 –C22 Torsion angle H11 –N1 –C9 –C8 O12 –C2 –N1 –C9 O13 –C3 –C2 –N1 H14 –N4 –C3 –C2 H15 –C5 –C10 –N4 O16 –C6 –C5 –C10 C17 –O16 –C6 –C5 O21 –C7 –C6 –C5 C22 –O21 –C7 –C8 H26 –C8 –C7 –C6

121.5 115.3 126.4 120.5 119.5 119.5 120.5 126.4 124.9 118.0 124.9 118.0 Value (◦ ) 0.0 180.0 180.0 180.0 0.0 180.0 0.0 180.0 0.0 180.0

Fig. 3. Comparison of observed and calculated FT-IR spectra of 6,7-dimethoxy-1,4dihydro-1,3-quinoxalinedione (a) observed, (b) calculated with B3LYP/6-31G*.

For numbering of atoms refer Fig. 2(a).

using the following expression: by considering syn and anti forms of the methyl groups with respect to the ring. The values of calculated molecular energies are given along with the figures (Fig. 2(a)–(e)). The energy obtained in this study is minimum for the most optimized geometry Fig. 2(a), in which both the methyl groups are in antiform (i.e. the hydrogen atom in the plane is in the antiposition with respect to the ring). The calculated optimized geometrical parameters obtained in this study are presented in Table 2. Calculated bond lengths are as in the aromatic systems. Bond length alternation is observed in the ring, with bonds C2 –C3, C6 –C7 , C8 –C9 , C10 –C5 slightly longer than the adjacent ones. The geometrical changes (i.e. shortening or strengthening) are observed in the aryl–O bonds C6 –O16 (for first methyl group) and C7 –O21 (for first methyl group) because of the increased effectiveness of ␲ conjugation [7,8].

   RMS = 

 1  calc exp 2 i − i (n − 1) n

i

The RMS error between unscaled (B3LYP/6-311+G**) and experimental frequencies is found to be 61 cm−1 . This is quite obvious since the frequencies calculated on the basis of quantum mechanical force fields usually differ appreciably from observed frequencies. This is partly due to the neglect of anharmonicity and partly due to the approximate nature of the quantum mechanical methods. In order to reproduce the observed frequencies, refinement of scaling factors are applied and optimized via least square refinement algorithm which resulted in a weighted RMS deviation of 9.61 cm−1 between the experimental and scaled frequencies. 5.1. C–H vibrations

5. Analysis of vibrational spectra Detailed description of vibrational modes can be given by means of normal coordinate analysis (NCA). For this purpose, the full set of 95 standard internal coordinates (containing 23 redundancies) is defined. From these, non-redundant set of local symmetry coordinates are constructed by suitable linear combinations of internal coordinates following the recommendation of Fogarasi et al. [60,61] and are summarized in Table 3 . The theoretically calculated DFT force fields are transformed in this latter set of vibrational coordinates and used in all subsequent calculations. The 72 normal modes are distributed among the symmetry species as 3N−6 = 47A (in-plane) + 25A (out-of-plane) and in agreement with CS symmetry. The detailed vibrational assignments of fundamental modes of the title compound along with calculated IR, Raman intensities and normal mode description (characterized by TED) are reported in Table 4 . For visual comparison, the observed and simulated FT-IR and FT-Raman spectra of the compound are produced in common frequency scales in Figs. 3 and 4. Root mean square (RMS) value is obtained in the study

Aromatic compounds commonly exhibit multiple weak bands in the region 3100–3000 cm−1 due to aromatic C–H stretching vibrations [71]. The calculated frequencies which lay in this region, i.e. 3095, 3092 cm−1 are attributed to C–H stretching vibrations of the title compound and their experimental counter parts are appeared both in IR and Raman spectra at 3116, 3047 cm−1 and 3090, 3060 cm−1 , respectively. Thus there is a good agreement between the experimental and calculated frequencies. The in-plane C–H bending vibrations appear in the range 1000–1300 cm−1 in the substituted benzenes and the out-of-plane bending vibrations occur in the frequency range 750–1000 cm−1 [72]. Accordingly, the calculated frequencies assigned to two C–H in-plane bending vibrations are at 1269, 1210 cm−1 , while the observed IR and Raman frequencies are identified at 1309, 1226 cm−1 in both. The calculated and observed frequencies assigned to two C–H out-of-plane bending vibrations are at 830, 816 cm−1 and 829, 817 cm−1 , respectively. Thus all the C–H vibrations of the title compound are well identified in the recorded spectra, within their characteristic region and their corresponding calculated frequencies using 6-31G* basis set are very well coincide with the experimental values. It is also found that the bands are not affected appreciably by the nature of the substituents.

242

V. Krishnakumar, N. Prabavathi / Spectrochimica Acta Part A 77 (2010) 238–247

Table 3 Definition of local symmetry coordinates and diagonal force constants of 6,7-dimethoxy-1,4-dihydro-1,3-quinoxalinedione. Symmetry coordinatesa

Descriptionb

In-plane vibrations S1 = r8 26

CH stretch

Diagonal force constantsc 3.50

S2 = r5 15

CH stretch

S3 = r2 12

C O stretch

12.63

S4 = r3 13

C O stretch

12.63

S5 = r7 16

C–O stretch

4.75

S6 = r7 21

C–O stretch

4.99

S7 = r16 17

C–O stretch

4.86

S8 = r21 22

C–O stretch

4.87

S9 = r1 11

N–H stretch

6.56

S10 = r4 14

N–H stretch

6.56

S11 = r3 2

C–C stretch

13.54

S12 = r5 10

C–C stretch

8.53

S13 = r6 5

C–C stretch

7.40

S14 = r7 6

C–C stretch

8.81

S15 = r8 7

C–C stretch

8.10

S16 = r9 8

C–C stretch

7.78

S17 = r9 10

C–C stretch

16.86

S18 = r2 1

C–N stretch

14.83

S19 = r9 1

C–N stretch

10.70

S20 = r3 4

C–N stretch

5.45

S21 = r10 4

C–N stretch

10.78

S22 = r17 19 + r17 20 + r17 18

CH3 ss

5.02

S23 = r22 24 + r22 25 + r22 23

CH3 ss

5.02

S24 = 2r17 19 − r17 20 − r17 18

CH3 ips

4.77

S25 = 2r22 24 − r22 25 − r22 23

CH3 ips

4.77

S26 = −ˇ16 17 19 − ˇ16 17 20 − ˇ16 17 18 + ˇ20 17 19 + ˇ20 17 18 + ˇ19 17 18

CH3 sb

0.67

S27 = −ˇ21 22 24 − ˇ21 22 25 − ˇ21 22 23 + ˇ25 22 24 + ˇ25 22 23 − ˇ24 22 23

CH3 sb

0.67

S28 = −ˇ20 17 19 − ˇ20 17 18 + 2ˇ19 17 18

CH3 ipb

0.58

S29 = −ˇ25 22 24 − ˇ25 22 23 + 2ˇ24 22 23

CH3 ipb

0.58

S30 = 2ˇ16 17 20 − ˇ16 17 19 − ˇ16 17 18

CH3 ipr

0.86

S31 = 2ˇ21 22 25 − ˇ21 22 45 − ˇ21 22 23

CH3 ipr

S32 = ˇ1 2 3 − ˇ2 3 4 + ˇ3 4 10 − ˇ4 10 9 + ˇ10 9 1 + ˇ9 1 2

Ring1 def

44.56

S33 = 2ˇ1 2 3 − ˇ2 3 4 − ˇ3 4 10 + 2ˇ4 10 9 − ˇ10 9 1 − ˇ9 1 2

Ring1 def

91.55

S34 = ˇ2 3 4 − ˇ3 4 10 + ˇ10 9 1 − ˇ9 1 2

Ring1 def

2.89

S35 = ˇ8 7 6 − ˇ7 6 5 + ˇ6 5 10 − ˇ5 10 9 + ˇ10 9 8 − ˇ9 8 7

Ring2 def

3.77

S36 = 2ˇ8 7 6 − ˇ7 6 5 − ˇ6 5 10 + 2ˇ5 10 9 − ˇ10 9 8 − ˇ9 8 7

Ring2 def

6.32

S37 = ˇ7 6 5 − ˇ6 5 10 + ˇ10 9 8 − ˇ9 8 7

Ring2 def

2.46

S38 = ˇ10 5 15 − ˇ6 5 15

C–H def

2.34

S39 = ˇ7 8 26 − ˇ9 8 26

C–H def

4.80

S40 = ˇ1 2 12 − ˇ3 2 12

C O def

1.32

S41 = ˇ2 3 13 − ˇ4 3 13

C O def

1.32

S42 = ˇ5 6 16 − ˇ7 6 16

C–O def

2.75

S43 = ˇ6 7 21 − ˇ8 7 21

C–O def

5.85

S44 = ˇ9 1 11 − ˇ2 1 11

N–H def

1.78

S45 = ˇ3 4 14 − ˇ10 4 14

N–H def

1.78

S46 = ˇ17 16 6

COC def

1.75

S47 = ˇ22 21 7

COC def

1.77

Out-of-plane vibrations S48 =  26 8 7 9

3.51

0.86

C–H wagg

1.47

S49 =  15 5 6 10

C–H wagg

1.45

S50 =  11 1 9 2

N–H wagg

0.21

S51 =  14 4 3 10

N–H wagg

0.21

S52 =  13 3 2 4

C O wagg

0.71

S53 =  12 2 1 3

C O wagg

0.71

S54 =  16 6 5 7

C–O wagg

2.30

S55 =  21 7 6 8

C–O wagg

2.31

S56 =  17 16 6 5(7)

COC tors

0.06

S57 =  22 21 7 6(9)

COC tors

0.10

S58 =  6 16 17 18(19,20)

O–CH3 tors

0.13

S59 =  7 21 22 23(24,25)

O–CH3 tors

0.13

S60 =  17 20 −  17 18

CH3 ops

4.55

S61 =  22 25 −  22 23

CH3 ops

4.55

S62 = ˇ20 17 19 − ˇ20 17 18

CH3 opb

0.62

V. Krishnakumar, N. Prabavathi / Spectrochimica Acta Part A 77 (2010) 238–247

243

Table 3 (Continued ) Symmetry coordinatesa

Descriptionb

Diagonal force constantsc

S63 = ˇ25 22 24 − ˇ25 22 23

CH3 opb

0.62

S64 = ˇ16 17 19 − ˇ16 17 18

CH3 opr

0.92

S65 = ˇ21 22 24 − ˇ21 22 23

CH3 opr

0.93

S66 =  1 2 3 4 −  2 3 4 10 +  3 4 10 9 −  4 10 9 1 +  10 9 1 2 −  9 1 2 3

Ring1 tors

0.16

S67 =  1 2 3 4 −  3 4 10 9 +  4 10 9 1 −  9 1 2 3

Ring1 tors

0.18

S68 = − 1 2 3 4 + 2 2 3 4 10 −  3 4 10 9 −  4 10 9 1 + 2 10 9 1 2 −  9 1 2 3

Ring1 tors

0.19

S69 =  8 7 6 5 −  7 6 5 10 +  6 5 10 9 −  5 10 9 8 +  10 9 8 7 −  9 8 7 6

Ring2 tors

0.24

S70 =  8 7 6 5 −  6 5 10 9 +  5 10 9 8 −  9 8 7 6

Ring2 tors

0.48

S71 = − 8 7 6 5 + 2 7 6 5 10 −  6 5 10 9 −  5 10 9 8 + 2 10 9 8 7 −  9 8 7 6

Ring2 tors

0.28

S72 =  4 10 9 8 −  5 10 9 1

Butt tors

1.08

For numbering of atoms refer Fig. 2(a). a Definitions are made in terms of the standard valence coordinates: ri,j is the bond length between atoms i and j; ˇi,j,k is the valence angle between atoms i, j, k where j is the central atom;  i,j,k,l is the out-of-plane angle between the i–j bond and the plane defined by the j, k, l atoms;  i,j,k,l is the torsion (dihedral) angle between the plane defined by i, j, k and j, k, l atoms. b Stretch, def, wagg and tors mean stretching, deformation, wagging and torsion motions, respectively and ss, symmetric stretching; sb, symmetric bending; ipb, in-planebending; opb, out-of-plane-bending; ipr, in-plane-rotation; opr, out-of-plane-rotation. c Stretching force constants are given in mdyn Å−1 , bending and torsional force constants are given in mdyn Å.

5.2. C–N and N–H vibrations

5.3. Methoxy group vibrations

The identification of C–N vibration is a difficult task, since the mixing of vibrations is possible in their region. However, with the help of the force field calculations, the C–N vibrations are identified and assigned. The IR bands appearing at 1190, 1222, 1252, 1286 cm−1 are assigned to C–N stretching vibrations [73] and their calculated values are found to be 1117, 1193, 1255, 1324 cm−1 , respectively. The IR bands observed at 3447, 3427 cm−1 [74], are assigned to N–H stretching, where as N–H in-plane bending and out-plane bending vibrational bands are identified at 1397, 14,461 cm−1 and 681, 617 cm−1 , respectively. The band due to N–H out-plane bending vibration is also observed in Raman spectra at 620 cm−1 and these N–H groups are intra-molecularly bonded to the C O groups that present in the compound.

The compound selected for the present study has two substituted methoxy (O–CH3 ) group. For the assignments of CH3 group frequencies, one can expect that nine fundamentals can be associated to CH3 group, namely CH3 ss, symmetric stretch; CH3 ips, in-plane stretch (i.e. in-plane hydrogen stretching mode); CH3 ipb, in-plane bending, (i.e. hydrogen deformation mode); CH3 sb, symmetric bending; CH3 ipr, in-plane rocking; CH3 opr, out-of plane rocking; CH3 ops, out-of-plane stretch; CH3 opb, out-of plane bending modes and tCH3 twisting modes. Among which the CH3 ops, out-of-plane stretch and CH3 opb, out-of plane bending modes of CH3 group would be expected to be depolarized for A symmetry species. In addition to this tOC wagging and C–O–C in-plane bending also exist.

Fig. 4. Comparison of observed and calculated FT-Raman spectra of 6,7-dimethoxy-1,4-dihydro-1,3-quinoxalinedione (a) observed, (b) calculated with B3LYP/6-31G*.

244

V. Krishnakumar, N. Prabavathi / Spectrochimica Acta Part A 77 (2010) 238–247

Table 4 Detailed assignment of fundamental vibrations of 6,7-dimethoxy-1,4-dihydro-1,3-quinoxalinedione by normal mode analysis based on SQM force field calculations. Sl. no

Symmetry species CS



Ramanb activity

Characterization of normal modes with PED (%)c

0.6 53.0 6.6 3.4 55.9

205.0 9.0 27.5 96.8 206.4

3007

12.1

78.7

NH (95) NH (98) CH (67), bCH (25) CH (68), bCH (26) CH3 ips (62), CH3 ss (21), CH3 ops (16) CH3 ips (63), CH3 ss (21), CH3 ops (16) CH3 ss (51), CH3 ips (37), CH3 ops (12) CH3 ss (51), CH3 ips (37), CH3 ops (12) CH3 ops (70), CH3 ss (27) CH3 ops (70), CH3 ss (28) CODOU (51), bring1 (24), CC (15), CN (5) CODOU (84), bring1 (13) CC (40), bring2 (34), bring1 (14) CC (57), bring2 (11), CH (9), CN (8) CC (30), bring2 (18), CN (11), bCH (9), bHN (8), bring1 (7) CH3 opb (23), CC (21), bCH (10), bring2 (10), CH (9), bCOSIN (8) CH3 opb (82), CH3 opr (8) CC (17), CH3 opb (49), bCOSIN (6), bCH (6), CH (6), bring1 (5) CH3 ipb (90), CH3 ipr (7) CH3 ipb (90), CH3 ipr (7) CH3 sb (41), bHN (17), bring1 (15), CC (13), CN (5) CH3 sb (72), bHN (10), CN (6) bHN (37), CH3 sb (25), bring1 (8), CC (7), CN (5) bHN (27), CC (21), bring2 (18), CN (8), CH3 sb (6), bCH (5) CC (40), CH (14), bring1 (12), bHN (11) CN (52), CC (23), bring1 (9) CC (28), bring2 (22), bring1 (16), CN (8), CH (7), COSIN (7) bCH (45), CH (245), CC (12), CN (6) CN (49), CC (33), CH3 opr (48), COSIN (9), CH3 ipr (7), CN (7), CC (7) CH3 opr (38), bring1 (14), CH (10), CC (10), CH3 ipr (5) bCH (41), CH (22), bring1 (13), CC (8), CH3 opr (5) CN (41), bring1 (31), bCODOU (6) CN (24), bCODOU (11), bring1 (11), CH3 ipr (11), bCH (10), bring2 (8) CH3 ipr (73), CH3 opr (12) CH3 ipr (54), bring1 (8), CN (6), bCH (6), bring2 (6) COSIN (68), CC (12), CH (5) COSIN (42), bring1 (18), bring2 (18), CN (6), bCOSIN (5) bring1 (42), CN (23), CC (15), COSIN (6) bring1 (82), CN (9) gCH (68), tring2 (17), tring1 (6) gCH (56), tring2 (33) gCODOU (62), tring1 (27), gHN (10) COSIN (26), bring1 (18), bring2 (16), CC (11), bCOSIN (9), CH (8) COSIN (29), CC (27), bring2 (17), CN (8), bring1 (5) tring2 (64), gCH (22), tOC (6) bring2 (61), CN (15), bring1 (7), bCODOU (7) gHN (72), gCODOU (16), tring1 (11)

Observed wavenumbers (cm−1 )

Calculated wavenumbers (cm−1 ) using B3LYP/6-31G* force field

IR

Unscaled

Scaled

3581 3580 3225 3223 3165

3438 3437 3095 3091 3007

3165

Raman

IRa Intensity

1. 2. 3. 4. 5.

A A A A A

3447 3427 3116 3047 3003

6.

A

2958

7.

A

2946

3078

2936

54.1

146.2

8.



2929

3077

2936

53.4

75.0

9. 10. 11.

A A A

2912 2902 1814

3022 3020 1814

2881 2881 1785

28.0 39.4 990.6

101.0 41.1 144.1

12. 13. 14. 15.

A A A A

1711 1680 1634 1528

1811 1681 1663 1578

1779 1703 1637 1558

115.8 1.4 23.8 167.3

32.7 134.6 106.6 47.0

16.

A

1550

1550

1545

93.4

7.6

17. 18.



A A

1543 1541

1535 1534

1538 1535

9.2 195.4

17.9 5.2

19. 20. 21.

A A A

1513 1474 1463

1519 1518 1506

1494 1492 1467

12.6 10.7 2.2

25.9 25.9 100.5

22. 23.

A A

1456 1446

1494 1474

1454 1448

5.5 43.8

8.1 64.5

24.

A

1397

1400

1420

1384

67.3

138.8

25.

A

1386

1386

1380

1370

53.3

233.3

26. 27.

A A

1286 1276

1362 1310

1324 1293

20.8 66.5

101.0 58.7

28. 29. 30.

A A A

1309 1252 1248

1309 1292 1248

1269 1255 1238

86.9 583.4 42.3

31.6 211.7 4.4

31.

A

1215

1244

1231

21.1

4.2

32.

A



1226

1225

1210

15.4

4.8

33. 34.

A A

1222 1190

1220 1189

1193 1177

40.6 25.4

1.6 8.1

35. 36.

A A

1183 1164

1185 1183

1175 1174

3.3 12.5

1.1 8.7

37. 38.

A A

1036 1008

1085 1049

1036 1021

22.9 141.9

1.9 0.5

39.

A

850

934

908

12.1

4.1

40. 41. 42. 43. 44.



A A A A A

843 829 817 791 786

817

849 830 818 791 783

838 830 816 786 779

3.1 57.8 2.0 0.1 8.1

3.7 0.9 6.5 0.0 23.6

45.

A

720

720

728

719

17.2

5.6

46. 47.



A A

696 690

696

692 690

693 692

0.0 11.8

0.1 1.4

48.

A

680

680

681

195.2

0.7

A

– 3090 3060

2960

1660 1630 1525

1475

1309 1252 1248

1225

1035 1007

V. Krishnakumar, N. Prabavathi / Spectrochimica Acta Part A 77 (2010) 238–247

245

Table 4 (Continued ) Observed wavenumbers (cm−1 )

Calculated wavenumbers (cm−1 ) using B3LYP/6-31G* force field

IR

Unscaled

Sl. no

Symmetry species CS

49.

A

679

50. 51. 52.

A A A

620 595 579

53.

A

54.

Raman

IRa Intensity

Ramanb activity

Scaled

679

677

0.0

0.0

620 595 579

617 595 571

0.2 13.1 31.4

4.7 1.6 12.0

534

533

532

0.2

1.4

A

460

460

465

0.0

1.5

55. 56.



A A

445 431

445

445 431

444 433

0.6 0.7

1.3 2.1

57.

A

431

430

430

433

2.8

4.6

58. 59.

A A

404 353

402

404 353

402 369

8.1 0.3

0.1 7.2

60.

A

339

340

339

336

0.0

19.8

61.

A

325

324

324

0.0

0.1

62. 63.

A A

301 275

300 272

304 278

0.3 0.1

0.5 0.8

64.

A

246

244

243

0.0

0.7

65.

A

211

210

209

0.0

0.0

66.

A

187

184

186

4.6

3.6

67. 68.

A A

168 152

167 153

164 148

2.5 0.0

1.6 0.4

69.

A

102

105

95

0.0

0.6

70.

A

86

90

83

12.4

0.5

71.

A

56

61

56

1.1

0.1

72.



31

55

52

0.2

0.5

A

620 579

300

210

150

Characterization of normal modes with PED (%)c

tring2 (62), tring1 (15), gCH (10), gCOSIN (7) gHN (86), gCH (6) bCODOU (75), bring1 (7), CN (6) bCODOU (40), bring1 (20), CN (18), bring2 (12) bring2 (44), bring1 (22), COSIN (13), bCOSIN (6) gCOSIN (35), tring2 (30), gCH (15), tOC (7), tButt (6) bring1 (88), CN (7) gCOSIN (37), tring2 (23), tring1 (22) bCOC (60), CN (13), bring1 (12), COSIN (6), CC (5) gCODOU (64), tring1 (21), gHN (11) bring1 (67), bCODOU (16), CC (10), CN (6) bCOSIN (35), COSIN (16), bring1 (16), CC (11), CN (9), bring2 (8) tButt (23), tring1 (20), tCH3 (19), gCOSIN (14), gHN (11), tring2 (11) bring2 (47), bring1 (22), CN (14) tCH3 (65), tring1 (17), tring2 (6), gHN (5) tCH3 (29), tring1 (27), tButt (16), gHN (14), tring2 (7) tring1 (44), gHN (23), tring2 (12), gCOSIN (11), CH3 (5) bCOSIN (34), COSIN (31), bCOC (16), CN (6), bring2 (5) bring1 (48), CN (30), COSIN (5) tring1 (28), tring2 (19), tOC (16), gCOSIN (16), gHN (14) tOC (37), tCH3 (26), tring1 (25), gHN (10) tring1 (53), gHN (26), tOC (12), tCH3 (8) tOC (39), tCH3 (34), tring1 (18), gHN (6) tring2 (34), tOC (17), tring2 (13), gHN (12), tButt (10), tCH3 (7)

Abbreviations used: b, bending; ␻, wagging; t, torsion; trig, trigonal; ss, symmetric stretching; sb, symmetric bending; ipb, in-plane-bending; opb, out-of-plane-bending; ipr, in-plane-rotation; opr, out-of-plane-rotation. a Relative absorption intensities normalized with highest peak absorption. b Relative Raman intensities calculated by Eq. (1) and normalized to 100. c For the notations used see Table 3.

For the title compound the peaks identified in IR at 3003, 2946 and 2912 cm−1 are ascribed to CH3 ips, CH3 ss, and CH3 ops vibrations [75] of the methyl group attached to O16 , where as for the methyl group attached to O21 , they are observed at 2958, 2926 and 2902 cm−1 , respectively. Their calculated values are at 3007,

2936, 2881 cm−1 and 3007, 2936, 2881 cm−1 respectively for the O16 –CH3 and O21 –CH3 groups. Infrared bands established at 1550, 1513, 1463, 1248 and 1183 cm−1 have been attributed to CH3 opb, CH3 ipb, CH3 sb, CH3 opr, and CH3 ipr vibrational modes of the methyl group attached to O16 and for the methyl group attached to O21 the

Table 5 Frequencies and infrared intensities (calculated with B3LYP/6-31G*) per methyl CH stretching vibrations of methyl groups in dimethyl ether, ethane, anisole, p-nitro-anisole, p-hydroxyanisole and the title compound. CH stretching Dimethyl ethera frequency

CH stretching Ethanea frequency

 (cm−1 ) I (km/mol) 1 1 2 3 3 Itot a b

3137 3153 3194 3277 3280

27.496 31.622 89.394 26.361 22.624 197.497

Anisolea

p-hydroxyanisolea , b

p-nitroanisolea , b

Title compound

 (cm−1 ) I (km/mol)  (cm−1 ) I (km/mol)  (cm−1 ) I (km/mol)  (cm−1 ) I (km/mol)  (cm−1 ) I (km/mol) 1 2 3

3179 3258 3258

36.121 50.021 50.021

136.163

Theoretically predicted (unscaled frequencies) parameters from Ref. [6]. Theoretically predicted (unscaled frequencies) parameters from Ref. [75].

3179 3244 3301

49.688 57.896 43.049

150.633

3191 3262 3318

45.207 45.671 32.191

123.069

3016 3071 3155

56.809 50.732 29.510

137.051

3165 3078 3022

39.761 70.435 13.092

123.291

246

V. Krishnakumar, N. Prabavathi / Spectrochimica Acta Part A 77 (2010) 238–247

peaks are established at 1543, 1474, 1456, 1215 and 1164 cm−1 , respectively. Calculated values for the corresponding modes are represented in Table 4 along with detailed potential energy distribution. The calculated frequency values for tOC wagging and O–CH3 torsional modes of the two groups are at 61, 105 and 243, 278 cm−1 [75]. The infrared unscaled frequencies and intensities are used to disentangle the role played by back-donation in the title compound. For this purpose five other molecules were considered as references: ethane (CH3 –CH3 ), with all the C–H bonds equivalent and unperturbed by any kind of intra-molecular interaction, dimethyl ether (CH3 –O–CH3 ) [1,5], anisole, p-nitro-anisole, and p-hydroxyanisole, in which back-donation has already been ascertained also experimentally [6,76]. The three methyl C–H stretching frequencies and infrared intensities are given for dimethyl ether (I), ethane (II). anisole (III), p-nitro-anisole (IV), p-hydroxyanisole (V) and the title compound (VI) in Table 5. For I, II and VI the intensities listed refer to the contribution of each methyl group (intensity additively is assumed). It is immediately evident from Table 5 that Itot for III, IV, V, and VI is very similar to the intensity per methyl group for ethane. The total infrared intensity Itot in dimethyl ether (197.50 km/mol per methyl group) is much larger than in ethane and this is the evidence of the presence of back-donation from the oxygen lone pairs in II. The total CH stretching intensities for the compounds studied are in the following order Itot (III) ≺ Itot (II) ≺ Itot (IV) ≺ Itot (VI) ≺ Itot (V) ≺ Itot (I); the low value of Itot in VI (title compound) allows us to conclude that no back-donation occurs in this molecule or conjugation effect is greater in this molecule, while the experimental spectroscopic evidence of back-donation in dimethyl ether is well reproduced by calculations. The strong band observed in the region 1740–1720 cm−1 [77] is due to C O stretching vibrations. In the present study the C O stretching vibrational bans are identified at 1711, 1814 cm−1 in IR spectrum. The stretching frequencies due to C–O bands are observed in IR spectrum at 1036, 1008, 786, 720 cm−1 . It is found that, because of the increased conjugation effect there is strengthening (or shortening) of the aryl–O bonds C6 –O16 (for first methyl group) and C7 –O21 (for first methyl group) [7,8], this is consistent with what already discussed above and therefore the peaks observed at 786, 720 cm−1 are unambiguously assigned to C6 –O16 and C7 –O21 bonds. The in-plane and out-of-plane bending vibrations of C O, C–O are also assigned and presented in Table 4 along with potential energy distribution. Thus, a comparison of the overall trends of calculated and experimental values is physically meaningful, as to calculated infrared and Raman intensities, the general experience shows that the calculated numbers are indeed directly comparable with the experimental values.

6. Conclusions The FT-IR and FT-Raman spectra have been recorded and the detailed vibrational assignment is presented for the title compound. The equilibrium geometry, harmonic vibrational frequencies, IR and Raman spectra are determined and analysed by B3LYP/6-31G* level of theory. The difference between the observed and calculated wave numbers is very small for most of fundamentals. Good agreement between the simulated and observed spectra is established for the investigated compounds. First, in this work we hope to have settled the problem of the vibrational assignment of the rather unusual and unexpected medium-strong infrared and Raman active vibrational transitions observed in the lower energy range of the C–H stretching modes, when the methyl group is attached to an oxygen atom. The assignment of this transition is not just a mere spectroscopic game, but has relevance in the under-

standing of the electronic properties of prototypical molecules in the structure-properties studies. Our calculations suggest that the conjugation by the lone pairs with the ␲ electrons of the ring is the dominant effect. Acknowledgements The authors are thankful to Sophisticated Analytical Instrumentation Facility (SAIF), IIT Madras, Chennai and Nehru Memorial College, Puthanampatti, Trichirappalli, India for providing spectral measurements. References [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]

D.C. McKean, Chem. Soc. Rev. 7 (1978) 399–422. M. Gussoni, C. Castiglioni, G. Zerbi, J. Phys. Chem. 88 (1984) 600–604. M. Gussoni, J. Mol. Struct. 141 (1986) 63–92. V. Hernandez, C. Castiglioni, G. Zerbi, J. Mol. Struct. 324 (1994) 189–198. A. Kindness, D.C. McKean, D. Stewart, J. Mol. Struct. 224 (1990) 363–384. M. Rumi, G. Zerbi, J. Mol. Struct. 509 (1999) 11–28. M. Gussoni, C. Castiglioni, G. Zerbi, in: R.J.H. Clark, R.E. Hester (Eds.), Spectroscopy of Advanced Materials, Wiley, New York, 1991, p. 251. C. Castiglioni, M. Del Zoppo, G. Zerbi, J. Raman Spectrosc. 24 (1993) 485–494. A. Gussoni, G. Zerbi, J.J.G.S. van Es, H.A.M. Biemans, E.W. Meijer, Synth. Met. 80 (1996) 201–204. S.A. Kotharkar, D.B. Shinde, Bioorg. Med. Chem. Lett. 16 (2006) 6181–6184. S.A. El-Hawash, N.S. Habib, M.A. Kassem, Arch. Pharm. (Weinheim) 339 (2006) 564–571. S.A. Khan, K. Saleem, Z. Khan, Eur. J. Med. Chem. 42 (2007) 103–108. P. Corona, G. Vitale, M. Loriga, G. Paglietti, P. La Colla, G. Collu, G. Sanna, R. Loddo, Eur. J. Med. Chem. 41 (2006) 1102–1107. V.K. Tandon, D.B. Yadav, H.K. Maurya, A.K. Chaturvedi, P.K. Shukla, Bioorg. Med. Chem. 14 (2006) 6120–6126. M. Abid, A. Azam, Bioorg. Med. Chem. Lett. 16 (2006) 2812–2816. H.M. Refaat, A.A. Moneer, O.M. Khalil, Arch. Pharm. Res. 27 (2004) 1093–1098. A. Carta, M. Loriga, S. Zanetti, L.A. Sechi, IL Farmaco 58 (2003) 1251–1255. S.H. Kim, T.E. Kim, Y. Kurasawa, J. Korean Chem. Soc. 45 (2001) 325–333. B. Zarranz, I. Aldana, Monge A. Jaso, Bioorg. Med. Chem. 12 (2004) 3711–3721. L. Yan, F.W. Liu, G.F. Dai, H.M. Liu, Bioorg. Med. Chem. Lett. 17 (2007) 609–612. F. Grande, F. Aiello, O.D. Grazia, A. Brizzi, A. Garofalo, N. Neamati, Bioorg. Med. Chem. 15 (2007) 288–294. O. Lenzi, V. Colotta, D. Catarzi, F. Varano, G. Filacchioni, C. Martini, L. Trincavelli, O. Ciampi, K. Varani, F. Marighetti, E. Morizzo, S. Moro, J. Med. Chem. 49 (2006) 3916–3925. J.K. Jung, E.K. Jung, K. Nam-Goong, J.S. Cho, H.M. Kim, S.G. Park, Y.A. Yoo, J.H. Kwon, H.S. Lee, Arch. Pharm. Res. 9 (2006) 276–281. A. Carta, M. Loriga, S. Piras, G. Paglietti, P. La Colla, B. Busonera, G. Collu, R. Loddo, Med. Chem. 2 (2006) 113–122. E. Novellino, B. Cosimelli, M. Ehlardo, G. Greco, M. Iandanza, A. Lavecchia, M.G. Rimoli, A. Sala, A. Da Settimo, G. Primofiore, F. Da Settimo, S. Taliani, C. La Motta, K.N. Klotz, D. Tuscano, M.L. Trincavelli, C. Martini, J. Med. Chem. 48 (2005) 8253–8260. C.H. Zhao, Y. Chen, J. Ding, W.H. Duan, Yao Xue Xue Bao 40 (2005) 814–819. H.U. Gali-Muhtasib, M. Diab-Assaf, M.J. Haddadin, Cancer Chemother. Pharmacol. 55 (2005) 369–378. C.H. Liu, B. Wang, W.Z. Li, L.H. Yun, Y. Liu, R.B. Su, J. Li, H. Liu, Bioorg. Med. Chem. 12 (2004) 4701–4707. V. Colotta, D. Catarzi, F. Varano, F.R. Calabri, O. Lenzi, G. Filacchioni, C. Martini, L. Trincavelli, J. Med. Chem. 47 (2004) 3580–3590. S. Piras, M. Loriga, G. Paglietti, IL Farmaco 59 (2004) 185–194. B. Zarranz, A. Jaso, I. Aldana, A. Monge, Bioorg. Med. Chem. 12 (2004) 3711–3721. M. Loriga, S. Piras, G. Paglietti, M.P. Costi, A. Venturelli, IL Farmaco 58 (2003) 51–61. N.A. Vekariya, M.D. Khunt, A.R. Parikh, Ind. J. Chem. 42 (2003) 421–424. S. Piras, M. Loriga, G. Paglietti, IL Farmaco 57 (2002) 1–8. B. Zarranz, A. Jaso, L.M. Lima, I. Aldana, A. Monge, S. Maurel, M. Sauvain, J. Brazilian, Pharm. Sci. 42 (2006) 357–361. B. Zarranz, A. Jaso, I. Aldana, A. Monge, S. Maure, E. Deharo, V. Jullian, M. Sauvain, Arzneimittelforschung 55 (2005) 754–761. J. Guillon, P. Grellier, M. Labaied, P. Sonnet, J.M. Leger, R. Deprez-Poulain, I. Forfar-Bares, P. Dallemagne, N. Lemaitre, F. Pehourcq, J. Rochette, C. Sergheraert, C. Jarry, J. Med. Chem. 47 (2004) 1997–2083. B. Zarranz, I. Aldana, A. Jaso, A. Monge, Bioorg. Med. Chem. 11 (2003) 2149–2156. L.E. Seitz, W.J. Suling, R.C. Reynolds, J. Med. Chem. 45 (2002) 5604–5606. C.N. Rangisetty, A.L. Gupta, P. Prasad, N. Srinivas, P. Sridhar, A. Veeranjaneyulu Parimoo, J. Pharm. Pharmacol. 53 (2001) 1409–1414. F. Lieu, C. Ouellet, E.H. Ruediger, M. Belema, Y. Qiu, X. Yang, J. Banville, J.R. Burke, K.R. Gregor, J.F. MacMaster, A. Martel, K.W. McIntyre, M.A. Pattoli, F.C. Zusi, D. Vyas, Bioorg. Med. Chem. Lett. 17 (2007) 1233–1237.

V. Krishnakumar, N. Prabavathi / Spectrochimica Acta Part A 77 (2010) 238–247 [42] S.K. Singh, V. Saibaba, V. Ravikumar, S.V. Rudrawar, P. Daga, C.S. Rao, V. Akhila, P. Hegde, Y.K. Rao, Bioorg. Med. Chem. 12 (2004) 1881–1893. [43] M.M. Ismail, Y.A. Ammar, M.K. Ibrahim, H.S. El-Zahaby, S.S. Mahmoud, Arzneimittelforschung 55 (2005) 738–743. [44] J.M. Vierfond, L. Legendre, C. Martin, P. Rinjard, M. Miocque, Eur. J. Med. Chem. 25 (1990) 251–255. [45] A. Carta, S. Piras, G. Loriga, G. Paglietti, Mini Rev. Med. Chem. 6 (2006) 1179–1200. [46] M.A. Ortega, I. Aldana, A. Jaso, A. Monge, M.E. Montoya, Y. Sainz, B. Zarranz, Arzneim. Forsch. Drug Res. 52 (2002) 113–119. [47] O.A. Aldana, I. Monge, A. Zarranz, Eur. J. Med. Chem. 38 (2003) 791–800. [48] L.A. Zanetti, P. Sechi, S. Molicotti, A. Cannas, A. Bua, A. Deriu, G. Paglietti, Int. J. Antimicrob. Agents 25 (2005) 179–181. [49] G. Sun, N.J. Uretsky, L.J. Wallace, G. Shams, D.M. Weinstein, D.D. Miller, J. Med. Chem. 39 (1996) 4430–4438. [50] M.H. Fisher, A. Lusi, J.R. Egerton, J. Pharm. Sci. 66 (1977) 1349–1352. [51] R.H. Bahekar, M.R. Jain, A.A. Gupta, A. Goel, P.A. Jadav, D.N. Patel, V.M. Prajapati, P.R. Patel, Arch. Pharm. 340 (2007) 359–366. [52] C.F. Bigge, T.C. Malone, P.A. Boxer, C.B. Nelson, D.F. Ortwine, R.M. Schelkun, D.M. Retz, L.J. Lescosky, S.A. Borosky, M.G. Vartanian, J. Med. Chem. 38 (1995) 3720–3740. [53] G. Ferreri, A. Chimirri, E. Russo, R. Gitto, P. Gareri, A. De Sarro, G. De Sarro, Pharmacol. Biochem. Behav. 77 (2004) 85–94. [54] K.F. Konidaris, S.P. Perlepes, G. Aromi, S.J. Teat, A. Escuer, E. Manessi-Zoupa, Inorg. Chem. Commun. 11 (2008) 186–191. [55] M.J. Frisch, G.W. Trucks, H.B. Schlegel, G.E. Scuseria, M.A. Robb, J.R. Cheeseman, V.G. Zakrzewski, J.A. Montgomery, R.E. Stratmann, J.C. Burant, S. Dapprich, J.M. Millam, A.D. Daniels, K.N. Kudin, M.C. Strain, O. Farkas, J. Tomasi, V. Barone, M. Cossi, R. Cammi, B. Mennucci, C. Pomelli, C. Adamo, S. Clifford, J. Ochterski, G.A. Petersson, P.Y. Ayala, Q. Cui, K. Morokuma, D.K. Malick, A.D. Rabuck, K. Raghavachari, J.B. Foresman, J. Cioslowski, J.V. Ortiz, B.B. Stefanov, G. Liu, A. Liashenko, P. Piskorz, I. Komaromi, R. Gomperts, R.L. Martin, D.J. Fox, T. Keith, M.A. Al-Laham, C.Y. Peng, A. Nanayakkara, C. Gonzalez, M. Challacombe, P.M.W. Gill, B.G. Johnson, W. Chen, M.W. Wong, J.L. Andres, M. Head-Gordon, E.S. Replogle, J.A. Pople, Gaussian 98, Revision A.3, Gaussian, Inc., Pittsburgh, PA, 1998. [56] A.D. Becke, J. Chem. Phys. 98 (1993) 5648–5652.

247

[57] C. Lee, W. Yang, R.C. Parr, Phys. Rev. B 37 (1988) 785–789. [58] P. Pulay, G. Fogarasi, G. Pongor, J.E. Boggs, A. Vargha, J. Am. Chem. Soc. 105 (1983) 7037–7047. [59] G. Rauhut, P. Pulay, J. Phys. Chem. 99 (1995) 3093. [60] G. Fogarasi, P. Pulay, in: J.R. Durig (Ed.), Vibrational Spectra and Structure, vol. 14, Elsevier, Amsterdam, 1985, p. 125. [61] G. Fogarasi, X. Zhou, P.W. Taylor, P. Pulay, J. Am. Chem. Soc. 114 (1992) 8191–8201. [62] T. Sundius, J. Mol. Struct. 218 (1990) 321–326. [63] (a) T. Sundius, Vib. Spectrosc. 29 (2002) 89–95; (b) Molvib (V 7.0): Calculation of Harmonic Force Fields and Vibrational Modes of Molecules, QCPE Program No. 807 (2002). [64] F.A. Cotton, Chemical Applications of Group Theory, Wiley Interscience, New York, 1971. [65] A. Frisch, A.B. Nielson, A.J. Holder, Gaussview Users Manual, Gaussian Inc., Pittsburgh, PA, 2000. [66] P.L. Polavarapu, J. Phys. Chem. 94 (1990) 8106–8112. [67] G. Keresztury, BT Raman spectroscopy. Theory, in: J.M. Chalmers, P.R. Griffiths (Eds.), Handbook of Vibrational Spectroscopy, vol. 1, John Wiley & Sons Lt., 2002, pp. 71–87. [68] G. Keresztury, S. Holly, J. Varga, G. Besenyei, A.Y. Wang, J.R. Durig, Spectrochim. Acta 49A (1993) 2007–2017. [69] H.M. Seip, R. Seip, Acta Chem. Scand. 27 (1973) 4024–4027. [70] G.M. Anderson III, P.A. Kollman, L.N. Domelsmith, K.N. Houk, J. Am. Chem. Soc. 101 (1979) 2344–2352. [71] J. Mohan, Organic Spectroscopy—Principles and Applications, Second ed., Narosa Publishing House, New Delhi, 2001. [72] C.P.D. Dwivedi, S.N. Sharma, Indian J. Pure Appl. Phys. 11 (1973) 447. [73] L.J. Bellamy, Advances in Infrared Group Frequencies, Barnes and Noble Inc., USA, 1968. [74] F. Billes, H. Endrei, G. Keresztury, J. Mol. Struct. (Theochem.) 530 (2000) 183–200. [75] V. Krishnakumar, V. Balachandran, Spectrochim. Acta Part A 63 (2006) 464– 476. [76] V. Krishnakumar, N. Prabavathi, Spectrochim. Acta Part A 74 (2009) 154–161. [77] Socrates George, Infrared and Raman Characteristic Group Frequencies—Tables and Charts, third ed., Wiley, New York, 2001.