Experimental and theoretical vibrational spectroscopic and HOMO

Indian Journal of Chemistry Vol. 48A, September 2009, pp. 1219-1227

Experimental and theoretical vibrational spectroscopic and HOMO, LUMO studies of 1,3-dimethylbarbituric acid S Chandra, H Saleem, N Sundaraganesan* & S Sebastian Department of Physics (Engg.), Annamalai University, Annamalai Nagar 608 002, Tamil Nadu, India Email: [email protected] Received 30 March 2009; revised and accepted 20 August 2009 The Fourier transform gas phase (5000-400 cm-1), solid phase (4000-400 cm-1) infrared spectra as well as Raman spectra (3500-50 cm-1) of 1,3-dimethylbarbituric acid have been recorded. Quantum chemical calculations of energies, geometrical structure and vibrational wave numbers of 1,3-dimethylbarbituric acid have been made by density functional method with 6-31G(d,p) as basis set. The best level of theory in order to reproduce the experimental wave numbers is B3LYP method with the 6-31G(d,p) basis set. A detailed interpretation of the infrared and Raman spectra of 1,3-DMBA is also reported. The calculated HOMO and LUMO energies show that charge transfer occurs within the molecule. The theoretical FT-IR and FT-Raman spectra for the title molecule have been constructed. Keywords: Theoretical chemistry, Vibrational spectroscopy, IR spectroscopy, Raman spectroscopy

Barbituric acid is an important compound with a heterocyclic structure and possibility of existing in several tautomeric forms because of mobility of the hydrogen atoms in its molecules. Some experimental investigations of this compound and its derivatives mainly tuned to its application in medicine and as dye in some branches of chemical industry are available1,2. Barbituric acid derivatives possess a rather broad spectrum of therapeutic activity. In particular, drugs belonging to this class of compounds have been used for more than a century as hypnotics and anticonvulsants. New pharmacological properties of some barbituric acid derivatives established in recent years significantly expanded the application range of these compounds3. Rukhadze et al.4 studied the binding constants between barbiturates and spherical or helical micelles by the method of micellar liquid chromatography. Л- Л interactions in the self-assembly of melamine and barbituric acid derivatives had been studied by Wensheng et al.5 A semiempirical MO study of tautomerism and the electronic structure of barbituric acid was studied by Kakkar et al.6 It has been found that the triketo form is most stable, followed by the 4-hydroxytautomer. Reactions of 5-dihydrocotarnyl-1,3-dimethylbarbituric acid and other cotarnine derivatives with 1,3-dimethylbarbituric acid and X-ray diffraction analysis of a 5,5 spiro derivative of

1,3-dimethylbarbituric acid have been studied by Krasnov et al.7 Ramondo et al.8 have studied the effects of intermolecular hydrogen bonding on the molecular properties of barbituric acid by B3LYP method level. The theoretical spectra of all investigated monomers have been compared with their FT-IR spectra in argon and nitrogen8. To the best of our knowledge, the quantum chemical calculations for 1,3-DMBA have not been reported so far. Therefore, the present investigation using DFT method of calculations along with experimental measurements of FT-IR gas phase, FT-IR (solid phase) and FT-Raman spectra was undertaken to study the vibrational spectra of this molecule completely and to identify the various normal modes with greater wave number accuracy for the first time. HOMO and LUMO analysis have been used to elucidate the information regarding charge transfer within the molecule. Methodology FT-IR and FT-Raman measurements

The compound 1,3-dimethylbarbituric acid (1,3-DMBA) in solid form was purchased from Sigma-Aldrich Chemical Company, USA, with a stated purity of greater than 99% and was used as such without further purification. The FT-Raman spectrum of 1,3-DMBA in solid form has been recorded using 1064 nm line of Nd:YAG laser as

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INDIAN J CHEM, SEC A, SEPTEMBER 2009

excitation wavelength in the region 50-3500 cm-1 on a Brucker (model IFS 66 V) spectrophotometer. The FT-IR spectrum of this compound was recorded in the region 400-4000 cm-1 on IFS 66V spectrophotometer using KBr pellet technique. The FT-IR gas phase spectrum was recorded by Fourier Transform Infrared instrument (Brucker, Vector). The spectra was recorded at room temperature, with spectral resolution of 2.0 cm-1. The observed experimental FT-IR and FT-Raman spectra of the title compound along with scaled FT-IR and FT-Raman spectra are shown in Figs 1-3. The spectral measurements were carried out at Sree Chitra Tirunal Institute for Medical Sciences and Technology, Poojappura, Thiruvanathapuram, Kerala, India.

(B3LYP) level. Polarization functions have been added for the better treatment of the methyl, carboxylic acid and carbonyl groups. Optimized structural parameters were used in the vibrational frequency calculations at DFT level to characterize all stationary points as minima. Using GAUSSVIEW molecular visualization program11 along with available related molecules, the vibrational frequency assignments were made with a high degree of accuracy. Prediction of Raman intensities

The Raman activities (Si) calculated with Gaussian 03 program converted to relative Raman intensities (Ii) using the following relationship derived from the intensity theory of Raman scattering12, 13,

Computational details

The DFT (B3LYP) calculations were performed using GAUSSIAN 03W9 program package without any constraint on the geometry10. Geometries of the model 1,3-DMBA, were first optimized with full relaxation on the potential energy surfaces at HF/6-31G(d,p) level and the resultant geometries were used as inputs for further calculations at DFT

Fig. 2  FT-IR spectrum of 1,3-dimethylbarbituric acid.

Fig. 1  Comparison of (a) experimental (FT-IR gas phase) and (b) theoretical FT-IR spectra for 1,3-dimethylbarbituric acid.

Fig. 3  Comparison of (a) experimental and (b) theoretical FT-Raman spectra of 1,3- dimethylbarbituric acid.

CHANDRA et al.: VIBRATIONAL SPECTROSCOPIC STUDIES ON 1,3-DIMETHYLBARBITURIC ACID

f (vo − vi ) 4 Si Ii = vi [1 − exp(−hcvi / kt )] where νo is the exciting frequency in cm−1, νi the vibrational wave number of the ith normal mode, h, c and k are fundamental constants, and f is a suitably chosen common normalization factor for all peak intensities. For simulation, calculated FT-Raman spectra have been plotted using pure Lorentizian band shape with a bandwidth of (FWHM) of 10 cm-1 as shown in Fig. 3. Results and Discussion Geometric structure

The crystal structure parameters of 1,3-dimethyl-barbituric acid (1,3-DMBA) was reported by Bertolasi et al.14 The structure is orthorhombic of the space group Fdd2, a = 15.642(3), b = 29.006(6), c = 6.556(1) Å with Z = 16. 1,3-DMBA does not enolize in crystals with formation of infinite H-bonded chains. Conversely, it forms crystals that completely lack traditional H-bond donors and hence are held together by C-H…OH-bonds and Cδ+=0δCδ+ interaction of a putative donor-acceptor nature14. The optimized geometric parameters (bond lengths and bond angles) were calculated by B3LYP method with 6-31G(d,p) as basis sets (Table 1) in accordance with the atom numbering scheme given in Fig. 4. Literature survey reveals that to the best of our knowledge, only hydrogen bonding and other molecular interactions studies of 1,3-DMBA has been reported so far14. Therefore, we could not compare the above calculations with experimental data. In our study the C-N-C bond angles are slightly shorter and longer than the C-C-C or N-C-C bond angles while the C-N bond distances are predicted to be slightly shorter than C-C bond distances. The presence of methyl and methylene groups in the ortho and para position respectively leads to redistribution of electron cloud in the ring, for example the lengthening of N2-C8 and N4-C13 is about 1.470Å by B3LYP method when compared with all other N-C bond, which is less than 1.41Å. The C1=O7, C3=O12 and C5=O17 bond length is 1.217 Å and clearly shows that carbonyl group is not affected by the other group present in the molecule. Based on the above comparison, although there are some differences between the theoretical and experimental values, the optimized structural

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parameters can well reproduce the experimental ones and are basis for the thereafter discussions. Vibrational assignments

The spectral assignments have been made on the recorded FT-IR(solid phase), FT-IR gas phase and FT-Raman spectra based on the theoretically predicted wave numbers by ab initio HF and density functional B3LYP/6-31G(d,p) method. None of the predicted vibrational frequencies have any imaginary frequency, implying that the optimized geometry is located at the local minimum point on the potential energy surface. We know that ab initio HF and DFT potentials systematically overestimate the vibrational wave numbers15. These discrepancies are corrected either by computing anharmonic corrections explicitly or by introducing a scaled field16 or directly scaling the calculated wave numbers with the proper factor17. The scaling factor of 0.9668 is used for B3LYP method. After scaling with a scaling factor, the deviation from the experimental value is less than 10 cm-1 with a few exceptions. The title molecule belongs to Cs point group. There are 51 normal modes of fundamental vibrations which span the irreducible representations: 32A′ + 19A″ and all the 51 fundamental vibrations are active in both IR and Raman. If we assume C2V symmetry, there is one imaginary frequency corresponding to CH3 torsional

Fig. 4  Numbering system adopted in this study for 1,3-dimethylbarbituric acid.

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INDIAN J CHEM, SEC A, SEPTEMBER 2009 Table 1  Geometrical parameters, bond length (Å), bond angle (°) and dihedral angle (°) optimized for 1,3-dimethylbarbituric acid by B3LYP/6-31G(d,p) method Bond length (Å)

C1-N2 C1-C6 C1-O7 N2-C3 N2-C8 C3-N4 C3-O12 N4-C5 N4-C13 C5-C6 C5-O17 C6-H18 C6-H19 C8-H9 C8-H10 C8-H11 C13-H14 C13-H15 C13-H16

1.392 1.512 1.217 1.401 1.470 1.401 1.217 1.392 1.470 1.512 1.217 1.095 1.095 1.091 1.091 1.087 1.091 1.091 1.087

Bond angle (o) N2-C1-C6 N2-C1-O7 C6-C1-O7 C1-N2-C3 C1-N2-C8 C3-N2-C8 N2-C3-N4 N2-C3-O12 N4-C3-O12 C3-N4-C5 C3-N4-C13 C5-N4-C13 N4-C5-C6 N4-C5-O17 C6-C5-O17 C1-C6-C5 C1-C6-H18 C1-C6-H19 C5-C6-H18 C5-C6-H19 H18-C6-H19 N2-C8-H9 N2-C8-H10 N2-C8-H11 H9-C8-H10 H9-C8-H11 H10-C8-H11 N4-C13-H14 N4-C13-H15 N4-C13-H16 H14-C13-H15 H14-C13-H16 H15-C13-H16

mode. Comparison of the frequencies calculated at DFT method using 6-31G(d,p) basis set with experimental values reveals that the B3LYP method show very good agreement with experimental observation due to inclusion of electron correlation for this method. Vibrational frequencies of some modes are given in Table 2. Data for others are as supplementary data. C=O vibrations

The carbonyl stretching frequency has been most extensively studied by infrared spectroscopy18. This multiple bonded group is highly polar and therefore gives rise to an intense infrared absorption band in the

116.3 122.3 121.3 125.0 119.1 115.8 118.1 120.9 120.9 125.0 115.8 119.1 116.3 122.3 121.3 118.9 107.9 107.9 107.9 107.9 105.2 109.9 109.9 107.1 108.5 110.6 110.6 109.9 109.9 107.1 108.5 110.6 110.6

Dihedral angle (o) C6-C1-N2-C3 C6-C1-N2-C8 O7-C1-N2-C3 O7-C1-N2-C8 N2-C1-C6-C5 N2-C1-C6-H18 N2-C1-C6-H19 O7-C1-C6-C5 O7-C1-C6-H18 O7-C1-C6-H19 C1-N2-C3-N4 C1-N2-C3-O12 C8-N2-C3-N4 C8-N2-C3-O12 C1-N2-C8-H9 C1-N2-C8-H10 C1-N2-C8-H11 C3-N2-C8-H9 C3-N2-C8-H10 C3-N2-C8-H11 N2-C3-N4-C5 N2-C3-N4-C13 O12-C3-N4-C5 O12-C3-N4-C13 C3-N4-C5-C6 C3-N4-C5-O17 C13-N4-C5-C6 C13-N4-C5-O17 C3-N4-C13-H14 C3-N4-C13-H15 C3-N4-C13-H16 C5-N4-C13-H14 C5-N4-C13-H15 C5-N4-C13-H16 N4-C5-C6-C1 N4-C5-C6-H18 N4-C5-C6-H19 O17-C5-C6-C1 O17-C5-C6-H18 O17-C5-C6-H19

0.03 179.98 180.04 -0.01 0.09 -123.22 123.44 -179.92 56.77 -56.57 -0.07 179.95 179.98 -0.01 -120.20 120.35 0.08 59.76 -59.69 -179.97 -0.03 -179.95 179.96 0.04 0.15 -179.89 -179.93 0.02 59.69 -59.76 179.96 -120.24 120.31 0.04 -0.17 123.14 -123.52 179.87 -56.82 56.52

region 1700-1800 cm-1. The carbon – oxygen double bond is formed by pπ – pπ bonding between carbon and oxygen. Because of the different electronegativities of carbon and oxygen atoms, the bonding electrons are not equally distributed between the two atoms. The following two resonance forms contribute to the bonding of the carbonyl group, > C = O ↔ C+ - O-. The lone pair of electrons on oxygen also determines the nature of the carbonyl group18. In our present study one can expect, three C=O stretching vibrations corresponding to C1=O, C3=O and C5=O modes. The experimental frequency observed as very strong to medium bands in both FT-Raman and FT-IR gas phase spectrum at 1735,

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1700 and 1713 cm-1 assigned to C=O stretching vibrations show good agreement with B3LYP method scaled values at 1762, 1723 and 1705 cm-1 (mode 9-11). The FT-IR spectral measurements showing strong bands at 1755, 1744 and 1722 cm-1 are assigned to C=O stretching vibrations. The C=O in plane bending vibrations computed by B3LYP method also show good agreement with recorded FT-IR and FT-Raman spectrum as shown in Table 2. The C=O out-of-plane bending vibration mode with theoretical frequency of 122 cm-1 by B3LYP method deviate by ~30 cm-1 when compared with experimental FT-Raman value. This may be due to mixing of γC=O vibration with out-of-plane vibration of N-C-N. Methylene group vibrations

For the assignments of CH2 group frequencies, basically six fundamentals can be associated to each CH2 group, namely, CH2 asymmetric stretch, CH2 symmetric stretch, CH2 scissoring and CH2 rocking modes which belong to polarized in-plane vibrations. In addition, the CH2 wagging and CH2 twisting bending modes of CH2 group would be expected to be depolarized for out-of-plane symmetry species. The C-H stretching of the methylene groups are at lower frequencies than those of the aromatic C-H ring stretching. The asymmetric CH2 stretching vibrations

are generally observed in the region 3000-2900 cm-1, while the CH2 symmetric stretch will appear19,20 between 2900-2800 cm-1. We calculated the asymmetric and symmetric CH2 stretching vibrations of the methylene group at 2994 and 2961 cm-1 by B3LYP/6-31G(d,p) method. The recorded spectrum does not show such kind of vibration, since both CH3 and CH2 stretching vibrations occur in the same region. In the present study, the CH2 bending modes follow, in decreasing wave number, the general order CH2 scissoring > CH2 wagging > CH2 twist > CH2 rock. Since the bending modes involving the hydrogen atom attached to the central carbon atom falls in the 1450-875 cm-1 range, there is extensive vibrational coupling of these modes with CH2 deformations, particularly with the CH2 twist. It may be noted that both δCH2 and ρCH2 were sensitive to the molecular conformation. For cyclohexane21, the CH2 scissoring mode has been assigned to the medium intensity IR band at about 1450 cm-1. In our study the weak band observed in FT-Raman spectrum at 1363 cm-1 is assigned to CH2 scissoring vibrations coupled with C-N stretching and CH3 wagging modes as shown in Table 2 (modes 18, 20). The CH2 twists observed to be weak FT-IR band at 1143 cm-1 show good agreement with computed wave number at 1168 cm-1 by B3LYP/6-31G(d,p) method. The CH2

Table 2  Observed FT-IR(gas phase), FT-Raman and calculated wave number for 1,3-dimethylbarbituric acid using B3LYP/6-31G(d,p) methods Mode no.

Observed wave number (cm-1)

Species FT-IR solid

FT-IR (gas phase)

FT-Raman

Theoretical wave number (cm-1) B3LYP/ 6-31G(d,p)

IRint (Km mol-1)

Vib. assign.

Sact (Å4/amu)

1 A’ 3097w 3030vw 3044w 3080 0.25 150.80 υasym CH3 9 A’ 1755s 1735vs 1762 3.78 58.83 υ C=O 10 A’ 1744s 1713vs 1723 375.48 20.89 υ C=O 11 A’ 1712s 1700m 1705 634.43 21.40 υ C=O δ CH2 13 A’ 1449ms 1457 7.80 13.63 16 A’ 1424m 1420 282.88 14.91 υ C-N + υ C-C 25 A’ 1125 35.54 4.51 υ N-CH3 26 A’’ 1115 0.86 12.03 ρ CH3 33 A’’ 753w 764w 721 19.06 γ N-C-N 3.58 A’’ 34 694w 695w 686 1.38 γ C-C-C 1.62 35 A’ 669m 665 5.13 14.43 β N-C-N 37 A’ 628m 630m 634vs 599 7.54 6.29 Ring breathing 42 A’ 381 31.23 2.03 β C-N-C 49 A’’ 77m 74 1.71 1.08 τ CH3 50 A’’ 63 0.00 2.39 τ CH3 A’' 51 10 8.73 τ CH2 0.73 w-weak; vw- very weak; s-strong; vs-very strong; m-medium; br, sh- broad, shoulder; υ - stretching; υsym – symmetric stretching; υasy- asymmetric stretching; β- in- plane bending; γ- out-of –plane bending; ω – wagging; t- twisting; δ –scissoring; τ- torsion; ρ-rocking.

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wagging, rocking vibrations computed by B3LYP method also show good agreement with recorded spectral data. Methyl group vibrations

The title molecule 1,3-DMBA, possesses two CH3 groups in the first and third position of the ring. For the assignments of CH3 group frequencies one can expect nine fundamentals to be associated with each CH3 group, viz., the symmetrical stretching in CH3 (CH3 sym. stretch) and asymmetrical stretching (CH3 asym. stretch) in-plane stretching modes (i.e. in-plane hydrogen stretching mode) the symmetrical (CH3 sym. deform) and asymmetrical (CH3 asym. deform) deformation modes; the in-plane rocking (CH3 ipr); out-of- plane rocking (CH3 opr) and twisting (tCH3) bending modes. The C-H stretching in CH3 group occurs at lower frequencies than those of the aromatic ring (3000-3100 cm-1). The asymmetric C-H stretching mode of CH3 group is expected in the region around 2980 cm-1 while the symmetric18,22-24 mode is expected around 2870 cm-1. In our present study, two weak bands appearing at 3044 and 3009 cm-1 in FT-Raman are assigned to CH3 asymmetric stretching vibration. The theoretically scaled value predicted by B3LYP/6-31G(d,p) method at 3080-3031 cm-1 (modes 1-4) shows very good agreement with experimental observation, although there is a slight deviation of about ~80 cm-1 when compared with literature data. This may be due to influence of C=O group in the adjacent positions. The CH3 symmetric stretching mode is observed as a very weak band in FT-IR gas phase at 2968 cm-1. The FT-Raman spectrum also shows the same kind of vibration as strong bands at 2961 and 2927 cm-1, assigned to CH3 symmetric stretching vibration and is well correlated with B3LYP/6-31G(d,p) method at 2963 and 2964 cm-1(modes 7 and 8). For the methyl substituted benzene derivatives the asymmetric and symmetric deformation vibrations25,26 of methyl group normally appear in the region 14651440 cm-1 and 1390-1370 cm-1. Based on the above literature data, in our present study the weak band observed in FT-IR at 1363 cm-1 is assigned to CH3 symmertic deformation vibration. The theoretically computed value by B3LYP/6-31G(d,p) method at 1356 cm-1 (mode 19) shows excellent agreement with literature value as well as experimental observations. The theoretically computed value by B3LYP/ 6-31G(d,p) method at 1449 cm-1 (mode 14), assigned

to CH3 asymmetric deformation vibration in CH3 group, is also in good agreement with the above literature data. However, these vibrations are absent both in FT-IR as well as FT-Raman spectra. The rocking vibrations of CH3 group in 1,3-DMBA appear as independent vibrations. These modes usually appears 27 in the region 1070-1010 cm-1. The theoretically computed value by B3LYP/6-31G(d,p) method at 1114, 1115 cm-1(modes 27 and 26) are assigned to CH3 rocking vibration but these vibrations are not observed in either FT-IR or FT-Raman spectra. As CH3 torsional mode is expected below 100 cm-1, the computed bands at 87 and 74 cm-1 in 1,3-DMBA are assigned to this mode; for the same vibration the spectral measurements show a medium FT-Raman band at 77 cm-1. C–N vibrations

The identification of C-N and C =N vibrations is a very difficult task, since the mixing of several bands are possible in this region. Silverstein et al.27 assigned C–N stretching absorption in the region 1266–1382 cm-1 for aromatic amines. In benzamide the band observed at 1368 cm-1 is assigned to be due to C–N stretching. In benzotriazole, the C–N stretching bands are found to be at 1307 and 1382 cm-1. In our title molecule, three C-N stretching vibrations are possible and the bands are observed in the range 1375-1267 cm-1 in FT-IR and 1386-1213 cm-1 in FT-Raman spectrum. These have been assigned to C-N stretching vibration. The theoretically computed value of C-N stretching vibration also falls in the region 1420-1225 cm-1 (modes 16-24) by B3LYP/6-31G(d,p) method. Theoretically computed N-C-N and N-CH3 in-plane and out-of-plane bending vibrations also shows good agreement with experimental observations. Ring vibrations

Many ring modes are affected by the substitution in the ring of 1,3-DMBA. In the present study, the bands observed at 1375 and 1386, 1363 cm-1 in infrared and Raman spectra for 1,3-DMBA have been assigned to the ring C-C stretching vibrations and these vibrations are mixed with C-N stretching vibrations. The theoretically computed values in the range 1420-1356 cm-1 show an excellent agreement with experimental data by B3LYP/6-31G(d,p) method (modes 16-19). The very strong band at 634 cm-1 observed in FT-Raman spectrum, the medium band at 628 cm-1 in FT-IR solid phase and medium band at 630 cm-1 in


FT-IR gas phase spectra of 1,3-DMBA, correspond to ring breathing vibration. The theoretically scaled value for the above vibration at 599 cm-1 by B3LYP/6-31G(d,p) (mode 37) coincides well with experimental observation. The theoretically predicated intensities by B3LYP method also show very strong intensities. The in-plane deformation vibration is at higher frequencies than the out-of-plane vibrations. Shimanouchi et al.28 have reported the frequency of these vibrations for different benzene derivatives as results of normal coordinate analysis. The bands observed at 938 and 694 cm-1 are assigned to C-C-C deformation vibrations of title compound. The theoretically computed values at 918, 686 and 611 cm-1 by B3LYP/6-31G(d,p) (modes 31, 34 and 35) method are in excellent agreement with experimental data.

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methyl and carbonyl group. Moreover, these orbital significantly overlap in their position for 1,3-DMBA. The atomic orbital compositions of the frontier molecular orbital are sketched in Fig. 5. The HOMO-LUMO energy gap of 1,3-DMBA calculated at the B3LYP/6-31(d,p) level (Fig. 5) reveals that the energy gap reflects the chemical activity of the molecule. LUMO as an electron acceptor represents the ability to obtain an electron, and HOMO represents the ability to donate an electron. Thermodynamic properties

Entropy of the title compound is presented in (Table 3). Scale factors have been recommended31 for an accurate prediction in determining the zero-point vibration energies (ZPVE), and the entropy, Svib(T).

HOMO and LUMO analysis

Many organic molecules, containing conjugated л electrons characterized by large values of molecular first hyperpolarizabilities, were analyzed by means of vibrational spectroscopy29,30. In most cases, even in the absence of inversion symmetry, the strongest bands in the Raman spectrum are weak in the IR spectrum and vice versa. But the intramolecular charge transfer from the donor to acceptor group through a single–double bond conjugated path can induce large variations of both the molecular dipole moment and the molecular polarizability, making IR and Raman activity strong at the same time. The experimental spectroscopic behavior described above is well accounted for by ab initio calculations in л conjugated systems that predict exceptionally large Raman and infrared intensities for the some normal modes29. For 1,3-DMBA, the corresponding bands in FT-IR and Raman spectra show that the relative intensities in IR and Raman spectra are comparable resulting from the electron cloud movement through л conjugated frame work from electron donor to electron acceptor groups. Analysis of the wave function indicates that the electron absorption corresponds to the transition from the ground to the first excited state and is mainly described by oneelectron excitation from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO). The LUMO, i.e., of л nature, (i.e., heterocyclic ring) is delocalized over the entire C-C and C-N bond. In contrast, the HOMO is located over methyl and carbonyl groups, and consequently the HOMO → LUMO transition implies an electron density transfer to heterocyclic ring from

Fig. 5  The atomic orbital compositions of the frontier molecular orbital for 1,3- dimethylbarbituric acid.

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Table 3  Theoretically computed energies (a.u.), zero-point vibrational energies (kcal mol−1), rotational constants (GHz), entropies (cal mol−1 K−1) and dipole moment (D) for 1,3-dimethylbarbituric acid

FT-IR and FT-Raman spectrum coincides with the experimentally observed spectra. HOMO, LUMO energies and HOMO-LUMO energy gaps has been also discussed.

Parameters

HF/6-31G(d,p)

B3LYP/6-31(d,p)

Total energy (au)

-565.433

-568.694

Zero-point energy (kcal mol−1)

99.488

92.3092

Supplementary Data Vibrational frequencies for all modes, 1-51, may be obtained from the authors on request.

1.66139

1.62406

References

1.13751

1.11324

0.68366

0.66869

1 Wurthner F, Yao S, Heise B & Tshierske C, Chem Comun, (2001) 212260. 2 Rezende M C, Campodonico P & Abuin E, Spectrochim Acta, 57 (2001)1183. 3 Arzamastsev A P, Luttseva T Y & Sadchikova N P, Pharm Chem, J, 35 (2001) 47. 4 Rukhadze M D, Sebiskveradze M V, Okudzhava V M & Khim A, J Phy Chem, 53Z (1998) 595. 5 Wensheng Y, Yueshun J, Jiaqi Z, Nan L, Siguang C & Tiejin L I, Sci China, 44B (2001) 479. 6 Kakkar R & Katoch V, Proc Indian Acad Sci (Chem Sci), 110 (1998) 535. 7 Kransnov K A, Kartsev V G & Khrustaley V N, Russ Chem Bull, Int Edn, 51 (2002) 1540. 8 Ramondo F, Pieretti A, Gontrani L & Bencivenni L, Chem Phys, 24 (2001) 293. 9 Gaussion 03 Program, 2004, (Gaussian Inc., Wallingford CT). 10 Schlegel H B, J Comput Chem, 3 (1982) 214. 11 Frisch A, Nielson A B & Holder A J, GAUSSVIEW User Manual, (Gaussian Inc,Pittsburgh, PA) 2000. 12 Keresztury G, Holly S, Varga J, Besenyei G, Wang A Y & Durig J R, Spectrochim Acta, 49A (1993) 2007. 13 Keresztury G & Chalmers J M, Griffith P R, Raman Spectroscopy: Theory in Hand Book of Vibrational Spectroscopy, Vol 1, (John Wiley, New York) 2002. 14 Bertolasi V, Gilli P, Ferretti V & Gilli G, New J Chem, 25 (2001) 408. 15 Sundaraganesan N, Anand B, Dominic Joshua B, Spectrochim Acta 65A (2006) 1053. 16 Pulay P, Fogarasi G, Ponger G, Boggs J E & Vargha A, Am Chem Soc, 105(1983) 7037. 17 Scott A P & Radom L, J Phys Chem, 100 (1996) 16502. 18 Socrates G, Infrared and Raman Characteristic Group Frequencies, Tables and Charts, 3rd Edn, (Wiley, Chichester) 2001. 19 Sajan D, Binoy J, Pradeep B , Krishna K V, Kartha V B, Joe I H & Jayakumar V S, Spectrochim Acta, 60A (2004) 173. 20 Furič K, Mohaček V, Bonifačič M & Ŝtefanič I, J Mol Struct, 267(1992) 39. 21 Sundaraganesan N, Anand B , Meganathan C , Joshua D B & Saleem H, Spectrochim Acta, 69A (2008) 198. 22 Reddy B V & Ramana Rao G, Vibr Spectr, 6(1994) 231. 23 Smith B, Infrared Spectral Interpretation. A Systematic

Rotational constants (GHz)

−1

−1

Entropy (cal mol K ) Total

102.865

98.632

Translational

41.045

41.045

Rotational

29.899

29.965

Vibrational Dipole moment (D)

31.921

27.622

0.5703

0.7483

The variations in the ZPVE’s seem to be insignificant. The total energies and the changes in the total entropy of 1,3-DMBA at room temperature at different methods are also presented. Dipole moment is a measure of the asymmetry in the molecular charge distribution and is given as a vector in the three dimensions. The values of dipole moments and energies for 1,3-DMBA molecule were also calculated (Table 3). According to HF and DFT(B3LYP) calculations, the largest dipole moment and the lowest energy were observed for B3LYP/ 6-31G(d,p). Conclusions Vibrational spectral analysis of 1,3-DMBA has been carried out using FT-IR (gas phase), FT-IR (solid phase) and Raman spectroscopy. Assignments of the vibrational spectra have been facilitated by DFT calculation. Good correlation is found between the computed and experimental wave numbers. The best fittings between calculated and measured vibrational frequencies have been achieved by B3LYP theoretical level. At this level, the deviation between calculated and experimental values are quite small for a given type of vibration. Therefore, this study confirms that the theoretical calculation of vibrational frequencies for 1,3-DMBA is quite useful for the vibrational assignments and for predicting new vibrational frequencies. The theoretically constructed

CHANDRA et al.: VIBRATIONAL SPECTROSCOPIC STUDIES ON 1,3-DIMETHYLBARBITURIC ACID Approach, (CRP Press, Washington, DC) 1999. 24 Colthup N B, Daly L H & Wiberly S E, Introduction to Infrared and Raman Spectroscopy, (Academic Press, New York) 1990. 25 Varsanyi G, Vibrational Spectra of Benzene Derivatives, (Academic Press, New York) 1969. 26 Tocon L I, Wooley M S, Otero J C & Marcos J I, J Mol Struct, 410 (1997) 447. 27 Silverstein M, Basseler G C & Morill C, Spectrometric

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Identification of Organic Compounds, (Wiley, New York) 1981. Shimanouchi T, Kakiuti Y & Gamo I, J Chem Phys, 25(1956)1245. Sundaraganesan N, Elango G, Meganathan C, Karthikeiyan B & Kurt M, Mole Sim, 35 (2009) 705. Vijayakumar T, Joe I H, Nair C P R & Jayakumar V S, Chem Phys, 343(2008) 83. Palafox M A, Int J Quant Chem, 77 (2000) 661.