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Fourier transform-infrared and Raman spectra, ab initio calculations and assignments for 6-methyl-4-bromomethylcoumarin
Spectrochimica Acta Part A 64 (2006) 301–307
Fourier transform-infrared and Raman spectra, ab initio calculations and assignments for 6-methyl-4-bromomethylcoumarin Veenasangeeta Sortur a , Jayashree Yenagi a , J. Tonannavar a,∗ , V.B. Jadhav b , M.V. Kulkarni b a
Department of Physics, Karnatak University, Pawate Nagar, Dharwad 580003, India b Department of Chemistry, Karnatak University, Dharwad 580003, India Received 4 June 2005; received in revised form 6 July 2005; accepted 9 July 2005
Abstract Fourier transform-infrared (4000–400 cm−1 ) and Raman (3500–50 cm−1 ) spectral measurements have been made for 6-methyl-4bromomethylcoumarin. Equilibrium structures, harmonic vibrational frequencies, infrared intensities, and depolarization ratios have been computed at RHF/6-31G* and B3LYP/6-31G* levels of theory. Twisting CH2 Br moiety in the geometry optimization leads to the most stable conformer lacking symmetry (C1 ). This is reflected in the richness of bands in the experimental spectra. A complete assignments of the bands, aided by the ab initio calculations, has been proposed for the 6-methyl-4-bromomethylcoumarin. Due to lack of symmetry, several normal vibrations have been found to be mixed ones. © 2005 Elsevier B.V. All rights reserved. Keywords: Ab initio; DFT; 6-Methyl-4-bromomethylcoumarin; Infrared; Raman
1. Introduction Coumarin derivatives are associated with the class of naturally occurring lactones that are found in different food sources, such as fruits, herbs, and vegetables [1]. They are of great interest owing to their vital and diverse role in the fields of biology, medicine, industry, botany and chemistry [2–18]. The parent coumarin and its some derivatives have been subjected to numerous spectroscopic studies [19–21]. Specifically, Thomas Wolff and Helmut Gorner have investigated the photodimerization of the parent coumarin and a series of 6-alkyl substituted coumarins in solution by the time resolved UV–vis absorption spectroscopy [22]. Substitution and solvent effects on the photophysical behavior of the coumarin and its derivatives (that includes methyl, methoxy, chlorine and their combinations) have been reported by Macanita et al. [23]. While the literature is abound with photochemical and photophysical studies of coumarins, it seems that not much work has been reported on the vibrational spectroscopy ∗
Corresponding author. Tel.: +91 9448375426; fax: +91 944836771275. E-mail address: [email protected] (J. Tonannavar).
1386-1425/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.saa.2005.07.024
of coumarin derivatives. However, the vibrational spectra of dihydro-, 6-methyl-, and 7-methylcoumarins have been studied [24]. Probably the first report on the Raman spectrum of the coumarin in the crystalline state is due to Venkateshwaran [25]. This spectrum shows a large number of intense bands including in those regions where they are expected to be weak even in liquid phase. This was followed by a short supplementary report on the low frequency Raman bands assigned to lattice vibrations. Of the substituted coumarins, 4-bromomethylcoumarins have served as fluorescent labels for t-RNA [26,27]. Chlorinated derivatives have been found to be antibacterial [28] and photographic [29] developers. 6-methyl-4bromomethylcoumarin whose vibrational spectroscopy has been reported in the present paper, also belongs to the class of 4-bromomethylcoumarins that serves as useful precursor in the synthesis of 4-aryloxymethylcoumarins [30], 4-dichloro acetamidomethylcoumarins [31] and 4-2-benzo furaxylcoumarins [32]. Electronic structure methods, namely, ab initio HartreeFock self consistent field method (HF) and density functional methods (DFT), are increasingly used by spectroscopists
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for modeling molecular properties that includes equilibrium structures, vibrational frequencies and intensities. The HF and DFT methods differ, among others, as to their treatment of electron correlation. The DFT methods have become popular in making quantitative predictions more accurately by taking into account electron correlation via variety of functionals, which is ignored in a direct way by the HF method. Further, both the methods give rise to systematic errors which arise from finite basis sets and neglect of anharmonicity, leading to 10–15% deviations of computed vibrational frequencies from experimental values. However, good agreement between theory and experiment is achieved through scaling of the computed frequencies. We report in this paper, the infrared and Raman spectral measurements, assignments and electronic structure calculations for 6-methyl-4-bromomethylcoumarin. The electronic structure calculations include restricted Hartree-Fock (RHF) and DFT’s B3LYP (Becke’s three parameter hybrid functional combined with Lee-Yang-Parr correlation functional) methods combined with 6-31G* basis set. Geometry optimizations at the two levels of theory have predicted, among others, C1 conformer in the ground electronic state to be the most stable structure. A complete assignment of the experimental spectra, aided by the computed spectroscopic data and normal modes, has been proposed for 6-methyl-4bromomethylcoumarin.
2. Experimental 6-Methyl-4-bromomethylcoumarin was prepared by the reaction of p-cresol with 4-bromoethyl acetoacetate at ice bath temperatures using sulphuric acid as the cyclising agent [33]. It is a colorless crystalline compound obtained as white silky needles from acetic acid. The Fourier transform-infrared (FT-IR) spectrum was recorded in KBr pellet on the Nicolet’s Impact 410 FTspectrometer with DTGS detector. The signals were collected for 300 scans at the interval of 4 cm−1 . The Raman spectrum of the powder sample was recorded on Bruker IFs 100/s FT-Raman spectrometer with Nd:YAG laser source giving 1064 nm as exciting line at 75 mw power. A total of 300 scans were collected at the interval of 4 cm−1 .
0.8929(RHF) and 0.9614(B3LYP) as recommended by the Gaussian and Scott and Radom [35,36]. 4. Results and discussion 4.1. Geometry optimization Fig. 1 shows the structure of 6-methyl-4-bromomethylcoumarin. Predicting infrared and Raman spectra by electronic structure calculations involves two steps: to find equilibrium structures through the optimization, followed by frequency calculations corresponding to the equilibrium structures. We also note that the conformational form of the 6-methyl-4-bromomethylcoumarin involving CH2 Br can make a difference both to the molecular symmetry and the vibrational structure. Thus, the Berny optimization [37] was performed by restricting the CH2 Br moiety to the molecular plane for initial dihedral angles of 0◦ and 180◦ . The Cs structure was predicted for two optimized angles 0◦ and 180◦ corresponding to the same dihedral angles, respectively. However, the frequency job corresponding to the 180◦ dihedral angle produced one small negative frequency at 121 cm−1 . The negative frequency suggests that there exists a first order saddle point, that is, a transition structure linking two minima on the potential energy surface (PES). In order to locate the two minima on the PES the normal mode associated with the negative frequency was examined. The normal mode indicated the torsional displacement of CH2 Br. This is clearly an out-of-plane distortion of the molecular structure. Accordingly the molecular geometry was “relaxed” along the normal mode and the optimization was performed for, say, intermediate dihedral angles 30◦ , 60◦ , 90◦ , 120◦ , and 150◦ . Five structures were predicted to be of C1 symmetry of which the conformer corresponding to the dihedral angle 150◦ (the optimized angle is 110.6827◦ ) has the lowest energy of 3146.7587 hartrees. The other minimum being at the same energy on the PES corresponds to the dihedral angle of 210◦ . Thus, this C1 conformer lies lower in energy than Cs (dihedral angle 0◦ ) by about 7.99 KJ/mol (RHF/6-31G* ) and 7.59 KJ/mol (B3LYP/6-31G* )
3. Computational methods The ab initio quantum chemical calculations presented here were carried out using the Gaussian03W suite of programs [34] on a P4 PC machine. We performed geometry optimizations and computed harmonic vibrational frequencies, IR intensities and Raman activities, force constants, and depolarization ratios for the ground electronic state at RHF/631G* and B3LYP/6-31G* levels of theory. In order to rectify the systematic errors due to finite basis sets and neglect of anharmonicity we have scaled the computed frequencies with
Fig. 1. Molecular structure of 6-methyl-4-bromomethylcoumarin.
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Table 1 Optimized parameters of 6-methyl-4-bromomethylcoumarin Parameter
Fig. 2. Fourier transform-infrared spectrum of 6-methyl-4-bromomethylcoumarin.
The computed geometrical parameters, for the sake of brevity, are presented in Table 1 only for the C1 structure. We note that the bond lengths and bond angles remain the same for the ring R1 of the molecule indicating no effect of CH2 Br moiety’s rotation with respect to the ring R2 . But we notice small changes in the ring R2 . The bond length C14 Br22 is increased by about 1.3% from Cs to C1 structure. The neighboring bond lengths, C14 H21 , C16 H23 , C4 C14 , C10 C4 are decreased by about 1%. A similar discussion holds good for the bond angles as well. It may thus be said that the changes in the bond lengths and bond angles are consistent with the conformational change from Cs to C1. 4.2. Assignments 6-Methyl-4-bromomethylcoumarin has 63 normal modes. The normal modes evidently do not possess symmetry properties corresponding to the C1 equilibrium conformation. However, the normal modes distribute as 41A + 22A corresponding to the Cs equilibrium conformation. In either case all the modes are both infrared and Raman active. The experimental spectra are highly rich of bands since the molecule lacks symmetry. The experimental infrared spectrum is shown in Fig. 2 and the Raman spectrum in Fig. 3. The
Fig. 3. Raman spectrum of 6-methyl-4-bromomethylcoumarin.
Bond lengths (A◦ ) C(9) C(10) C(5) C(6) C(8) C(7) C(4) C(3) C(10) C(5) C(10) C(4) C(9) C(8) C(7) C(6) C(3) C(2) C(5) H(15) C(8) H(11) C(7) H(17) C(3) H(13) C(9) O(1) O(1) C(2) C(2) O(12) C(4) C(14) C(6) C(16) C(14) Br(22) C(14) H(22) C(14) H(23) C(16) H(18) C(16) H(19) C(16) H(20) Bond angles (◦ ) C(9) C(10)–C(5) C(9) C(10)–C(4) C(9) C(10)–C(8) C(10) C(5)–C(6) C(9) C (8)–C (7) C(10) C(4)–C(3) C(8) C(7)–C(6) C(4) C(3)–C(2) C(5) C(6)–C(7) C(5) C(10) C(4) C(10) C(9)–O(1) O(1) C(9) C(8) O(1) C(2) C(3) C(10) C(5) H(15) C(6) C(5) H(15) C(9) C(8) H(11) C(7) C(8) H(11) C(6) C(7) H(17) C(2) C(3) H(13) C(9) O(1) C(2) C(10) C(4) C(14) C(3) C(4) C(14) C(8) C(7) H(17) C(4) C(3) H(13) C(5) C(6) C(16) C(7) C(6) C(16) O(1) C(2) O(12) C(3) C(2) O(12) C(4) C(14) Br(22) C(4) C(14) H(21) C(4) C(14) H(23) C(6) C(16) H(18) C(6) C(16) H(19) C(6) C(16) H(20)
RHF/6-31G* 1.385 1.378 1.375 1.357 1.404 1.465 1.387 1.399 1.467 1.073 1.073 1.076 1.073 1.355 1.346 1.184 1.499 1.511 1.963 1.077 1.075 1.083 1.086 1.086 118.2 117.1 121.2 121.7 119.2 119.1 121.5 122.3 118.1 124.6 122.0 116.7 116.3 119.2 118.9 118.9 121.8 119.4 114.9 123.3 121.2 119.7 119.1 122.8 121.5 120.4 119.3 124.4 112.2 112.2 111.1 111.3 111.2 111.1
B3LYP/6-31G* 1.406 1.391 1.387 1.357 1.409 1.455 1.397 1.409 1.458 1.085 1.085 1.087 1.085 1.367 1.390 1.208 1.496 1.512 1.994 1.089 1.088 1.094 1.097 1.097 118.1 117.3 121.4 121.8 119.3 119.2 121.5 122.9 118.2 124.6 122.2 116.7 115.9 119.1 119.1 118.8 121.8 119.4 115.0 122.4 121.2 119.5 119.1 121.9 121.3 120.5 118.4 125.7 112.3 112.5 111.7 111.4 111.4 111.4
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4.2.1. C H vibrations: In the IR spectrum, two weak bands at 3064 and 3172 cm−1 , and a strong band at 3092 cm−1 are assigned to C H stretching vibrations. A medium strong IR band at 3043 cm−1 and a weak Raman band at 3033 cm−1 are assigned to C H stretching vibration. The corresponding computed band at 3066 cm−1 band also shows clear C H stretching vibration. The bands corresponding to both outof-plane and in-plane C H deformations are observed in the region 750–1300 cm−1 . But all these vibrations are not pure vibrational modes in the sense that the analysis of the contributions from the internal coordinates shows that, apart from C H contribution of the rings, there is a contribution from the C X vibrations as well. For example, in the Raman spectrum four weak bands at 920, 904, 1056, and 1134 cm−1 show C H vibrations of the rings along with C H vibrations of CH2 Br group. In all these vibrations, C H vibrations of CH2 Br group dominate over the C H vibrations of the rings. The 904 cm−1 band shows C O vibrations along with C H vibrations of the ring as well as with C H vibrations of CH2 Br group. The same is the case with a weak IR band at 1039 cm−1 where C H vibrations of the rings are vibrating along with vibrations of CH3 group. But in the case of a medium strong IR band at 1172 cm−1 , C H vibrations of the rings dominate over the C H vibrations of CH2 Br group. This band corresponds to the band at 1168 cm−1 in naphthalene [40] and 1178 cm−1 in 6-methylcoumarin [24].
Fig. 4. Computed infrared spectrum of 6-methyl-4-bromomethylcoumarin in the region 1700–50 cm−1 : (a) C1 symmetry and (b) Cs symmetry.
computed spectra, one for C1 conformation and one for Cs are shown in Fig. 4 with bands in the range of 1700–50 cm−1 . These spectra are obtained by fitting vibrational frequencies and infrared intensities data of B3LYP/6-31G* level to the lorentzian function. The B3LYP/6-31G* frequencies corresponding to the C1 equilibrium conformation are nearer to the experimental ones except in the C H region where DFT methods are known to overestimate C H frequencies [38]. But the RHF/6-31G* method gives good agreement with CH3 stretching frequencies. Over-all the RMS value for RHF/631G* is 42 and, 35 cm−1 for B3LYP/6-31G* . Therefore, the following assignments are discussed using B3LYP/6-31G* frequencies with occasional references to RHF/6-31G* frequencies. However, Table 2 presents the assignments for 6-methyl-4-bromomethylcoumarin in terms of both RHF/631G* and B3LYP/6-31G* spectra. Further, as the normal modes lack symmetry properties, the assignments on the basis of the computed nuclear displacements were by no means straightforward. We therefore used freq = internal job in the Gaussian03W package, giving contributions of internal coordinates in a particular normal mode. All the assignments were subsequently confirmed in the MOLDEN visualization package [39].
4.2.2. C C vibrations and C O vibrations C C vibrations are observed in the region 1300– 1650 cm−1 . In the IR spectrum, two weak bands at 1320 and 1619 cm−1 , a strong band at 1492 cm−1 , a medium strong band at 1360 cm−1 and a very strong band at 1569 cm−1 with their corresponding computed bands are all mixed vibrational modes. In all these modes, the contribution of C C stretching vibrations are present along with the rings’ deformation. This kind of mixing of modes is a consequence of the lowering of symmetry and it has been observed in mono-substituted benzenes, nitrobenzene, phenol, benzaldehyde, azobenzenes, naphthalene, coumarin [41–46]. All C C in-plane-bending modes are observed in the region below 1000 cm−1 and C C out-of-plane bending modes in the region below 700 cm−1 . Again in these regions we see some of the modes are mixed vibrational modes in agreement with the reported results on the parent coumarin [46]. For example, a very weak IR band at 474 cm−1 corresponds to a predicted band at 504 cm−1 is a mixed mode of skeletal deformation and C O deformation. This is in agreement with the assignment of a band at 492 cm−1 in the parent coumarin [46]. In the lower region, a band at 296 cm−1 in the computed spectrum with no corresponding experimental band is assigned to a mixed mode of CH2 Br deformation and skeletal deformation. This band corresponds to a band at 300 cm−1 in the computed spectrum of the parent coumarin [46]. It is important to note that this band is also missing in the experimental spectrum of the parent coumarin. The stretching frequency of carbonyl group
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Table 2 Experimental and computed frequencies (cm−1 ) with assignments of 6-methyl-4-bromomethylcoumarin Experimental
Computational
IR
Raman
RHF/6-31G
3172vw 3092s 3064w – 3043ms 2981w 2956w 2919ms 2864w 1728vs 1619 w 1610s 1569vs 1492s – – 1450s 1435s 1418s 1360ms 1320w 1282w 1235w 1222ms 1203s 1172ms – 1129s 1054w 1039w 1015vw – 952s 919vw 900s 881ms – 820vs 758vw 721ms 674ms 635w 581s 565w 530ms 521w 499vw 487w 474vw 459vw 444vw 419ms – – – – – – – – – – –
– 3089w 3066s 3050w 3033w 2983w 2954w 2921m 2862w 1699s 1621m 1610m 1571vs 1494m – – – – 1425w 1380m 1321m 1282w 1238w 1220m 1194s 1174m – 1134w 1056w – – – – 920w 904w 875w – 827w 759w 715w 675w 636w – 567m 528w 516w – 484m – – – 430m 362m 316w – 216m – 169m – 96vs 63s – –
3048 3039 3035 3022 3004 2958 2933 2910 2862 1812 1655 1627 1582 1492 1468 1461 1456 1416 1402 1360 1276 1261 1248 1232 1215 1164 1161 1131 1101 1051 1036 985 981 941 900 898 884 853 837 809 750 719 695 669 615 562 559 519 505 473 417 402 350 340 294 200 197 173 150 80 58 56 53
*
Assignments INT
B3LYP/6-31G
0.47 2.91 8.70 1.77 14.09 8.86 21.09 27.32 39.61 959.86 55.72 12.20 60.92 30.41 23.76 13.79 4.75 37.72 4.99 58.00 41.59 28.29 88.42 89.74 49.21 5.56 12.37 14.41 10.79 2.41 6.04 2.50 1.95 41.94 9.63 8.42 18.41 8.95 46.11 4.61 1.38 27.32 1.80 12.76 10.06 0.19 29.23 6.63 5.50 3.40 0.21 1.42 6.15 3.45 0.91 0.78 3.67 2.78 1.29 1.30 0.19 2.47 0.36
3104 3010 3089 3077 3066 3011 3010 2978 2928 1767 1614 1604 1555 1480 1467 1457 1451 1414 1391 1361 1318 1269 1244 1223 1209 1181 1144 1123 1108 1039 1036 995 932 914 901 865 860 840 809 803 736 700 688 660 614 559 550 517 504 471 414 403 350 339 296 211 196 174 150 81 61 57 53
*
INT 8.61 3.15 5.60 1.46 11.99 5.00 13.57 17.30 31.12 623.16 35.15 6.67 76.54 36.89 6.90 5.32 8.65 33.87 0.50 29.25 11.27 11.65 35.69 35.99 24.29 21.97 31.73 6.69 37.76 6.75 4.83 4.81 0.83 46.45 1.55 13.70 8.16 11.79 30.35 6.04 0.32 1.31 15.62 5.45 9.08 6.14 18.57 8.27 4.85 2.48 0.03 0.94 4.48 2.08 0.51 0.51 2.54 2.18 0.82 1.06 0.55 1.69 0.29
ν CH ν CH ν CH ν CH2 Br asym ν CH ν CH2 Br sym ν CH2 (CH3 ) ν CH3 (asym) ν CH3 (sym) νC O ν CC + Skel def ν CC ν CC + Skel def Skel def + ν CC CH3 def + Skel def CH3 def C CH2 def (asym) (CH2 Br) (C CH3 ) def + Skel def + ν CC (C CH3 ) def Skel def + ν CC ν CC + CH3 def CH def CH def CH2 Br wag + Skel def CH2 Br def CH def + CH2 Br def CH def + ν CC + CH2 Br def CH def + CH2 Br def CH2 Br def + CH def + ν CC ν CC + CH def + CH3 def CH3 rock CH3 rock CH def CH2 Br def + CH def CH2 Br def + CH def + ν CO Skel def CH def + CH2 Br def CH2 Br def + CH def Skel def Skel def Skel def Skel def CH2 Br def + ν CO + Skel def Skel def + CH2 Br def Skel def + CO def Skel def Skel def + ν (C Br) Skel def + CO def Skel def + CO def Skel def Skel def Skel def Skel def + CO def + (C CH3 ) def CH3 rock + Skel def + CO def CH2 Br def + Skel def CH2 Br def + Skel def Skel def + CH2 Br def Skel def + CH3 def Skel def CH3 tor + Skel def Skel def + CH2 Br def CH3 def CH3 tor
INT, IR intensity (KM/mol); vs, very strong; s, strong; m, medium; w, weak; vw, very weak; Skel def, skeletal deformation; wag, wagging; tor, torsion; rock, rocking; ν, stretching.
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(C O) is not at all affected by the rotation of bromomethyl group and it is observed as a very strong IR band at 1728 cm−1 and a Raman band at 1699 cm−1 corresponding to the computed band at 1767 cm−1 . 4.2.3. C X vibrations (X = CH3 , CH2 Br) In the Raman spectrum, a weak band at 2862 cm−1 and a medium strong band at 2921 cm−1 are assigned to CH3 stretching vibrations. These two bands are more accurately predicted by RHF/6-31G* level at 2862 and 2910 cm−1 , respectively. In the computed spectrum of B3LYP/6-31G* level, they are predicted at 2928 and 2978 cm−1 , respectively. Two weak Raman bands at 2983 and 3050 cm−1 are assigned to the stretching frequencies of CH2 Br group. These two bands correspond to the bands at 3011 and 3077 cm−1 in the computed spectrum. A strong IR band at 1203 cm−1 with a corresponding Raman band at 1194 cm−1 is assigned to CH2 Br deformation vibration. The corresponding computed band is 1209 cm−1 . C Br stretching along with the skeletal deformation is observed as a very weak IR band at 499 cm−1 with a corresponding computed band at 550 cm−1 . The C CH3 deformations are expected in the region 1370–1465 cm−1 . So we have two strong IR bands at 1435 and 1450 cm−1 assigned to C CH3 deformation along with skeletal deformation and C CH2 deformation, respectively. Similarly, a medium strong IR band at 1222 cm−1 with a corresponding Raman band at 1220 cm−1 , is assigned to CH2 wagging along with skeletal deformation. A predicted band at 1457 cm−1 with no corresponding experimental band is assigned to CH3 deformation vibration. But a strong IR band at 1418 cm−1 is assigned to C CH3 deformation and corresponds to the computed band at 1391 cm−1 . In the lower region below 200 cm−1 we see that both CH3 and CH2 Br groups dominate the normal modes of vibrations. In their work on the Raman spectrum of the parent coumarin, Venkateswaran and Girijavallabhan [47] have observed seven low frequency lattice lines at 29, 39, 54, 67, 92, 112 and 146 cm−1 . However, we have observed two bands at 63 and 96 cm−1 of which 96 cm−1 is a very strong Raman band as in coumarin. In the computed spectrum, it is assigned to CH3 torsion vibration and the ring deformation vibration lying at 81 cm−1 . Similarly, the band at 63 cm−1 corresponds to a predicted band at 61 cm−1 and is assigned to the ring deformation and CH2 Br deformation vibrations. Two calculated bands at 53 and 57 cm−1 with no corresponding experimental bands are assigned to CH3 torsion and CH3 deformation, respectively. All the assignments are presented in Table 2.
5. Conclusions The infrared and Raman spectra of 6-methyl-4-bromomethylcoumarin have been measured and assigned on the basis of spectral correlations and electronic structure calculations at RHF/6-31G* and B3LYP/6-31G* levels. The geometry optimizations at different dihedral angles of CH2 Br
have suggested that the C1 conformer is the most favored one. The proposed assignments are in agreement with literature results of the similar systems. The level B3LYP/6-31G* has performed better than RHF/6-31G* and the vibrational harmonic frequencies obtained with it corresponding to the C1 structure are more nearer to experimental frequencies. Acknowledgements We are greatly indebted to Dr. S. Umapathy, Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore, for initiating us into quantum chemical calculations and arranging for Raman spectral measurements. We thank Professor B.G. Mulimani, Programme coordinator, UGC’s DSA program (Phase III) for his unwavering support and help in implementing Gaussian package. One of us, VS, acknowledges the Karnatak University for the award of Research Studentship. References [1] H.E. Kleiner, S.V. Vulimiri, L. Miller, W.H. Johnson, C.P. Whitman, J. DiGiovanni, Carcinogenesis 22 (2001) 73 (and all references therein). [2] K.Y. Anklekar, Synthetic studies in nitrogen and oxygen heterocycles of structural and biological interest, Ph.D. Thesis, Department of Chemistry, Karnatak University, Dharwad, 1996 (and all the references therein). [3] K. Kumaki, S. Hata, K. Mizerno, S. Tomooka, Chem. Phasrm. Bull. 17 (1969) 1751. [4] M.F. Walter, F. Karlheinz, S. Niederreither, G. Uray, W. Stadlbauer, J. Mol. Struct. 477 (1999) 209. [5] S.S. Saleem, G. Aruldhas, Ind. J. Pure Appl. Phys. 19 (1981) 1081. [6] W.W. Mantulin, Pill-Soon Song, J. Am. Chem. Soc. 95 (1973) 5122. [7] Pill-Soon Song, W.H. Gordon III, J. Phys. Chem. 74 (1970) 4234. [8] Pill-Soon Song, M.L. Harter, T.A. Moore, W.C. Herndon, PhotoChem. PhotoBiol(GB) 14 (1971) 521. [9] M.L. Dhar, A.C. Jain, Curr. Sci. 41 (1972) 177. [10] P.K. Jesthi, M.K. Rout, J. Ind. Chem. Soc. 47 (1970) 1211. [11] Pill-Soon Song, Chen-AN Chin, Iwao Yamazaki, Hiroaki Baba, Int. J. Quant. Chem.: Quant. Biol.y Symp. 2 (1975) 1. [12] V.G. Lake, J.G. Evans, Food Chem. Toxicol. 31 (1993) 963. [13] J.C. Marr, L.M. McDowell, M.A. Quilliam, Nat. Toxins. 2 (1994) 302. [14] Kavita Dorai, Arvind, Anil Kumar, arXiv: quant-Ph/9906027v2, 5 Feb 2000. [15] W.L. Stanley, Leonard Jurd, J. Arg. Food Chem. 19 (1971) 1106. [16] Aoife Lacy, Richard O’Kennedy, Curr. Pharm. Des. 10 (2004) 3797. [17] L. Musajo, F. Bordin, G. Caporale, S. Marciani, G. Rigatti, Photochem. Photobiol. 6 (1967) 711. [18] L. Musajo, G. Rodighiero, Photochem. Photobiol. 11 (1970) 27. [19] S.A. Tuccio, K.H. Drexhage, G.A. Reynolds, Opt. Commn. (the Netherlands) 7 (1973) 248. [20] R.S. Rasmussen, R.R. Brattain, J. Am. Chem. Soc. 71 (1949) 1073. [21] R.N. Jones, C.L. Angell, T. Ito, R.J.D. Smith, Can. J. Chem. 37 (1959) 2007. [22] Tomas Wolff, Helmut Gorner, Phys. Chem. Chem. Phys. 6 (2004) 368. [23] J. Sergio Seixas de Melo, R.S. Becker, A.L. Macanita, J. Phys. Chem. 98 (1994) 6054.
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Fourier transform-infrared and Raman spectra, ab initio calculations and assignments for 6-methyl-4-bromomethylcoumarin Veenasangeeta Sortur a , Jayashree Yenagi a , J. Tonannavar a,∗ , V.B. Jadhav b , M.V. Kulkarni b a
Department of Physics, Karnatak University, Pawate Nagar, Dharwad 580003, India b Department of Chemistry, Karnatak University, Dharwad 580003, India Received 4 June 2005; received in revised form 6 July 2005; accepted 9 July 2005
Abstract Fourier transform-infrared (4000–400 cm−1 ) and Raman (3500–50 cm−1 ) spectral measurements have been made for 6-methyl-4bromomethylcoumarin. Equilibrium structures, harmonic vibrational frequencies, infrared intensities, and depolarization ratios have been computed at RHF/6-31G* and B3LYP/6-31G* levels of theory. Twisting CH2 Br moiety in the geometry optimization leads to the most stable conformer lacking symmetry (C1 ). This is reflected in the richness of bands in the experimental spectra. A complete assignments of the bands, aided by the ab initio calculations, has been proposed for the 6-methyl-4-bromomethylcoumarin. Due to lack of symmetry, several normal vibrations have been found to be mixed ones. © 2005 Elsevier B.V. All rights reserved. Keywords: Ab initio; DFT; 6-Methyl-4-bromomethylcoumarin; Infrared; Raman
1. Introduction Coumarin derivatives are associated with the class of naturally occurring lactones that are found in different food sources, such as fruits, herbs, and vegetables [1]. They are of great interest owing to their vital and diverse role in the fields of biology, medicine, industry, botany and chemistry [2–18]. The parent coumarin and its some derivatives have been subjected to numerous spectroscopic studies [19–21]. Specifically, Thomas Wolff and Helmut Gorner have investigated the photodimerization of the parent coumarin and a series of 6-alkyl substituted coumarins in solution by the time resolved UV–vis absorption spectroscopy [22]. Substitution and solvent effects on the photophysical behavior of the coumarin and its derivatives (that includes methyl, methoxy, chlorine and their combinations) have been reported by Macanita et al. [23]. While the literature is abound with photochemical and photophysical studies of coumarins, it seems that not much work has been reported on the vibrational spectroscopy ∗
Corresponding author. Tel.: +91 9448375426; fax: +91 944836771275. E-mail address: [email protected] (J. Tonannavar).
1386-1425/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.saa.2005.07.024
of coumarin derivatives. However, the vibrational spectra of dihydro-, 6-methyl-, and 7-methylcoumarins have been studied [24]. Probably the first report on the Raman spectrum of the coumarin in the crystalline state is due to Venkateshwaran [25]. This spectrum shows a large number of intense bands including in those regions where they are expected to be weak even in liquid phase. This was followed by a short supplementary report on the low frequency Raman bands assigned to lattice vibrations. Of the substituted coumarins, 4-bromomethylcoumarins have served as fluorescent labels for t-RNA [26,27]. Chlorinated derivatives have been found to be antibacterial [28] and photographic [29] developers. 6-methyl-4bromomethylcoumarin whose vibrational spectroscopy has been reported in the present paper, also belongs to the class of 4-bromomethylcoumarins that serves as useful precursor in the synthesis of 4-aryloxymethylcoumarins [30], 4-dichloro acetamidomethylcoumarins [31] and 4-2-benzo furaxylcoumarins [32]. Electronic structure methods, namely, ab initio HartreeFock self consistent field method (HF) and density functional methods (DFT), are increasingly used by spectroscopists
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for modeling molecular properties that includes equilibrium structures, vibrational frequencies and intensities. The HF and DFT methods differ, among others, as to their treatment of electron correlation. The DFT methods have become popular in making quantitative predictions more accurately by taking into account electron correlation via variety of functionals, which is ignored in a direct way by the HF method. Further, both the methods give rise to systematic errors which arise from finite basis sets and neglect of anharmonicity, leading to 10–15% deviations of computed vibrational frequencies from experimental values. However, good agreement between theory and experiment is achieved through scaling of the computed frequencies. We report in this paper, the infrared and Raman spectral measurements, assignments and electronic structure calculations for 6-methyl-4-bromomethylcoumarin. The electronic structure calculations include restricted Hartree-Fock (RHF) and DFT’s B3LYP (Becke’s three parameter hybrid functional combined with Lee-Yang-Parr correlation functional) methods combined with 6-31G* basis set. Geometry optimizations at the two levels of theory have predicted, among others, C1 conformer in the ground electronic state to be the most stable structure. A complete assignment of the experimental spectra, aided by the computed spectroscopic data and normal modes, has been proposed for 6-methyl-4bromomethylcoumarin.
2. Experimental 6-Methyl-4-bromomethylcoumarin was prepared by the reaction of p-cresol with 4-bromoethyl acetoacetate at ice bath temperatures using sulphuric acid as the cyclising agent [33]. It is a colorless crystalline compound obtained as white silky needles from acetic acid. The Fourier transform-infrared (FT-IR) spectrum was recorded in KBr pellet on the Nicolet’s Impact 410 FTspectrometer with DTGS detector. The signals were collected for 300 scans at the interval of 4 cm−1 . The Raman spectrum of the powder sample was recorded on Bruker IFs 100/s FT-Raman spectrometer with Nd:YAG laser source giving 1064 nm as exciting line at 75 mw power. A total of 300 scans were collected at the interval of 4 cm−1 .
0.8929(RHF) and 0.9614(B3LYP) as recommended by the Gaussian and Scott and Radom [35,36]. 4. Results and discussion 4.1. Geometry optimization Fig. 1 shows the structure of 6-methyl-4-bromomethylcoumarin. Predicting infrared and Raman spectra by electronic structure calculations involves two steps: to find equilibrium structures through the optimization, followed by frequency calculations corresponding to the equilibrium structures. We also note that the conformational form of the 6-methyl-4-bromomethylcoumarin involving CH2 Br can make a difference both to the molecular symmetry and the vibrational structure. Thus, the Berny optimization [37] was performed by restricting the CH2 Br moiety to the molecular plane for initial dihedral angles of 0◦ and 180◦ . The Cs structure was predicted for two optimized angles 0◦ and 180◦ corresponding to the same dihedral angles, respectively. However, the frequency job corresponding to the 180◦ dihedral angle produced one small negative frequency at 121 cm−1 . The negative frequency suggests that there exists a first order saddle point, that is, a transition structure linking two minima on the potential energy surface (PES). In order to locate the two minima on the PES the normal mode associated with the negative frequency was examined. The normal mode indicated the torsional displacement of CH2 Br. This is clearly an out-of-plane distortion of the molecular structure. Accordingly the molecular geometry was “relaxed” along the normal mode and the optimization was performed for, say, intermediate dihedral angles 30◦ , 60◦ , 90◦ , 120◦ , and 150◦ . Five structures were predicted to be of C1 symmetry of which the conformer corresponding to the dihedral angle 150◦ (the optimized angle is 110.6827◦ ) has the lowest energy of 3146.7587 hartrees. The other minimum being at the same energy on the PES corresponds to the dihedral angle of 210◦ . Thus, this C1 conformer lies lower in energy than Cs (dihedral angle 0◦ ) by about 7.99 KJ/mol (RHF/6-31G* ) and 7.59 KJ/mol (B3LYP/6-31G* )
3. Computational methods The ab initio quantum chemical calculations presented here were carried out using the Gaussian03W suite of programs [34] on a P4 PC machine. We performed geometry optimizations and computed harmonic vibrational frequencies, IR intensities and Raman activities, force constants, and depolarization ratios for the ground electronic state at RHF/631G* and B3LYP/6-31G* levels of theory. In order to rectify the systematic errors due to finite basis sets and neglect of anharmonicity we have scaled the computed frequencies with
Fig. 1. Molecular structure of 6-methyl-4-bromomethylcoumarin.
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Table 1 Optimized parameters of 6-methyl-4-bromomethylcoumarin Parameter
Fig. 2. Fourier transform-infrared spectrum of 6-methyl-4-bromomethylcoumarin.
The computed geometrical parameters, for the sake of brevity, are presented in Table 1 only for the C1 structure. We note that the bond lengths and bond angles remain the same for the ring R1 of the molecule indicating no effect of CH2 Br moiety’s rotation with respect to the ring R2 . But we notice small changes in the ring R2 . The bond length C14 Br22 is increased by about 1.3% from Cs to C1 structure. The neighboring bond lengths, C14 H21 , C16 H23 , C4 C14 , C10 C4 are decreased by about 1%. A similar discussion holds good for the bond angles as well. It may thus be said that the changes in the bond lengths and bond angles are consistent with the conformational change from Cs to C1. 4.2. Assignments 6-Methyl-4-bromomethylcoumarin has 63 normal modes. The normal modes evidently do not possess symmetry properties corresponding to the C1 equilibrium conformation. However, the normal modes distribute as 41A + 22A corresponding to the Cs equilibrium conformation. In either case all the modes are both infrared and Raman active. The experimental spectra are highly rich of bands since the molecule lacks symmetry. The experimental infrared spectrum is shown in Fig. 2 and the Raman spectrum in Fig. 3. The
Fig. 3. Raman spectrum of 6-methyl-4-bromomethylcoumarin.
Bond lengths (A◦ ) C(9) C(10) C(5) C(6) C(8) C(7) C(4) C(3) C(10) C(5) C(10) C(4) C(9) C(8) C(7) C(6) C(3) C(2) C(5) H(15) C(8) H(11) C(7) H(17) C(3) H(13) C(9) O(1) O(1) C(2) C(2) O(12) C(4) C(14) C(6) C(16) C(14) Br(22) C(14) H(22) C(14) H(23) C(16) H(18) C(16) H(19) C(16) H(20) Bond angles (◦ ) C(9) C(10)–C(5) C(9) C(10)–C(4) C(9) C(10)–C(8) C(10) C(5)–C(6) C(9) C (8)–C (7) C(10) C(4)–C(3) C(8) C(7)–C(6) C(4) C(3)–C(2) C(5) C(6)–C(7) C(5) C(10) C(4) C(10) C(9)–O(1) O(1) C(9) C(8) O(1) C(2) C(3) C(10) C(5) H(15) C(6) C(5) H(15) C(9) C(8) H(11) C(7) C(8) H(11) C(6) C(7) H(17) C(2) C(3) H(13) C(9) O(1) C(2) C(10) C(4) C(14) C(3) C(4) C(14) C(8) C(7) H(17) C(4) C(3) H(13) C(5) C(6) C(16) C(7) C(6) C(16) O(1) C(2) O(12) C(3) C(2) O(12) C(4) C(14) Br(22) C(4) C(14) H(21) C(4) C(14) H(23) C(6) C(16) H(18) C(6) C(16) H(19) C(6) C(16) H(20)
RHF/6-31G* 1.385 1.378 1.375 1.357 1.404 1.465 1.387 1.399 1.467 1.073 1.073 1.076 1.073 1.355 1.346 1.184 1.499 1.511 1.963 1.077 1.075 1.083 1.086 1.086 118.2 117.1 121.2 121.7 119.2 119.1 121.5 122.3 118.1 124.6 122.0 116.7 116.3 119.2 118.9 118.9 121.8 119.4 114.9 123.3 121.2 119.7 119.1 122.8 121.5 120.4 119.3 124.4 112.2 112.2 111.1 111.3 111.2 111.1
B3LYP/6-31G* 1.406 1.391 1.387 1.357 1.409 1.455 1.397 1.409 1.458 1.085 1.085 1.087 1.085 1.367 1.390 1.208 1.496 1.512 1.994 1.089 1.088 1.094 1.097 1.097 118.1 117.3 121.4 121.8 119.3 119.2 121.5 122.9 118.2 124.6 122.2 116.7 115.9 119.1 119.1 118.8 121.8 119.4 115.0 122.4 121.2 119.5 119.1 121.9 121.3 120.5 118.4 125.7 112.3 112.5 111.7 111.4 111.4 111.4
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4.2.1. C H vibrations: In the IR spectrum, two weak bands at 3064 and 3172 cm−1 , and a strong band at 3092 cm−1 are assigned to C H stretching vibrations. A medium strong IR band at 3043 cm−1 and a weak Raman band at 3033 cm−1 are assigned to C H stretching vibration. The corresponding computed band at 3066 cm−1 band also shows clear C H stretching vibration. The bands corresponding to both outof-plane and in-plane C H deformations are observed in the region 750–1300 cm−1 . But all these vibrations are not pure vibrational modes in the sense that the analysis of the contributions from the internal coordinates shows that, apart from C H contribution of the rings, there is a contribution from the C X vibrations as well. For example, in the Raman spectrum four weak bands at 920, 904, 1056, and 1134 cm−1 show C H vibrations of the rings along with C H vibrations of CH2 Br group. In all these vibrations, C H vibrations of CH2 Br group dominate over the C H vibrations of the rings. The 904 cm−1 band shows C O vibrations along with C H vibrations of the ring as well as with C H vibrations of CH2 Br group. The same is the case with a weak IR band at 1039 cm−1 where C H vibrations of the rings are vibrating along with vibrations of CH3 group. But in the case of a medium strong IR band at 1172 cm−1 , C H vibrations of the rings dominate over the C H vibrations of CH2 Br group. This band corresponds to the band at 1168 cm−1 in naphthalene [40] and 1178 cm−1 in 6-methylcoumarin [24].
Fig. 4. Computed infrared spectrum of 6-methyl-4-bromomethylcoumarin in the region 1700–50 cm−1 : (a) C1 symmetry and (b) Cs symmetry.
computed spectra, one for C1 conformation and one for Cs are shown in Fig. 4 with bands in the range of 1700–50 cm−1 . These spectra are obtained by fitting vibrational frequencies and infrared intensities data of B3LYP/6-31G* level to the lorentzian function. The B3LYP/6-31G* frequencies corresponding to the C1 equilibrium conformation are nearer to the experimental ones except in the C H region where DFT methods are known to overestimate C H frequencies [38]. But the RHF/6-31G* method gives good agreement with CH3 stretching frequencies. Over-all the RMS value for RHF/631G* is 42 and, 35 cm−1 for B3LYP/6-31G* . Therefore, the following assignments are discussed using B3LYP/6-31G* frequencies with occasional references to RHF/6-31G* frequencies. However, Table 2 presents the assignments for 6-methyl-4-bromomethylcoumarin in terms of both RHF/631G* and B3LYP/6-31G* spectra. Further, as the normal modes lack symmetry properties, the assignments on the basis of the computed nuclear displacements were by no means straightforward. We therefore used freq = internal job in the Gaussian03W package, giving contributions of internal coordinates in a particular normal mode. All the assignments were subsequently confirmed in the MOLDEN visualization package [39].
4.2.2. C C vibrations and C O vibrations C C vibrations are observed in the region 1300– 1650 cm−1 . In the IR spectrum, two weak bands at 1320 and 1619 cm−1 , a strong band at 1492 cm−1 , a medium strong band at 1360 cm−1 and a very strong band at 1569 cm−1 with their corresponding computed bands are all mixed vibrational modes. In all these modes, the contribution of C C stretching vibrations are present along with the rings’ deformation. This kind of mixing of modes is a consequence of the lowering of symmetry and it has been observed in mono-substituted benzenes, nitrobenzene, phenol, benzaldehyde, azobenzenes, naphthalene, coumarin [41–46]. All C C in-plane-bending modes are observed in the region below 1000 cm−1 and C C out-of-plane bending modes in the region below 700 cm−1 . Again in these regions we see some of the modes are mixed vibrational modes in agreement with the reported results on the parent coumarin [46]. For example, a very weak IR band at 474 cm−1 corresponds to a predicted band at 504 cm−1 is a mixed mode of skeletal deformation and C O deformation. This is in agreement with the assignment of a band at 492 cm−1 in the parent coumarin [46]. In the lower region, a band at 296 cm−1 in the computed spectrum with no corresponding experimental band is assigned to a mixed mode of CH2 Br deformation and skeletal deformation. This band corresponds to a band at 300 cm−1 in the computed spectrum of the parent coumarin [46]. It is important to note that this band is also missing in the experimental spectrum of the parent coumarin. The stretching frequency of carbonyl group
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Table 2 Experimental and computed frequencies (cm−1 ) with assignments of 6-methyl-4-bromomethylcoumarin Experimental
Computational
IR
Raman
RHF/6-31G
3172vw 3092s 3064w – 3043ms 2981w 2956w 2919ms 2864w 1728vs 1619 w 1610s 1569vs 1492s – – 1450s 1435s 1418s 1360ms 1320w 1282w 1235w 1222ms 1203s 1172ms – 1129s 1054w 1039w 1015vw – 952s 919vw 900s 881ms – 820vs 758vw 721ms 674ms 635w 581s 565w 530ms 521w 499vw 487w 474vw 459vw 444vw 419ms – – – – – – – – – – –
– 3089w 3066s 3050w 3033w 2983w 2954w 2921m 2862w 1699s 1621m 1610m 1571vs 1494m – – – – 1425w 1380m 1321m 1282w 1238w 1220m 1194s 1174m – 1134w 1056w – – – – 920w 904w 875w – 827w 759w 715w 675w 636w – 567m 528w 516w – 484m – – – 430m 362m 316w – 216m – 169m – 96vs 63s – –
3048 3039 3035 3022 3004 2958 2933 2910 2862 1812 1655 1627 1582 1492 1468 1461 1456 1416 1402 1360 1276 1261 1248 1232 1215 1164 1161 1131 1101 1051 1036 985 981 941 900 898 884 853 837 809 750 719 695 669 615 562 559 519 505 473 417 402 350 340 294 200 197 173 150 80 58 56 53
*
Assignments INT
B3LYP/6-31G
0.47 2.91 8.70 1.77 14.09 8.86 21.09 27.32 39.61 959.86 55.72 12.20 60.92 30.41 23.76 13.79 4.75 37.72 4.99 58.00 41.59 28.29 88.42 89.74 49.21 5.56 12.37 14.41 10.79 2.41 6.04 2.50 1.95 41.94 9.63 8.42 18.41 8.95 46.11 4.61 1.38 27.32 1.80 12.76 10.06 0.19 29.23 6.63 5.50 3.40 0.21 1.42 6.15 3.45 0.91 0.78 3.67 2.78 1.29 1.30 0.19 2.47 0.36
3104 3010 3089 3077 3066 3011 3010 2978 2928 1767 1614 1604 1555 1480 1467 1457 1451 1414 1391 1361 1318 1269 1244 1223 1209 1181 1144 1123 1108 1039 1036 995 932 914 901 865 860 840 809 803 736 700 688 660 614 559 550 517 504 471 414 403 350 339 296 211 196 174 150 81 61 57 53
*
INT 8.61 3.15 5.60 1.46 11.99 5.00 13.57 17.30 31.12 623.16 35.15 6.67 76.54 36.89 6.90 5.32 8.65 33.87 0.50 29.25 11.27 11.65 35.69 35.99 24.29 21.97 31.73 6.69 37.76 6.75 4.83 4.81 0.83 46.45 1.55 13.70 8.16 11.79 30.35 6.04 0.32 1.31 15.62 5.45 9.08 6.14 18.57 8.27 4.85 2.48 0.03 0.94 4.48 2.08 0.51 0.51 2.54 2.18 0.82 1.06 0.55 1.69 0.29
ν CH ν CH ν CH ν CH2 Br asym ν CH ν CH2 Br sym ν CH2 (CH3 ) ν CH3 (asym) ν CH3 (sym) νC O ν CC + Skel def ν CC ν CC + Skel def Skel def + ν CC CH3 def + Skel def CH3 def C CH2 def (asym) (CH2 Br) (C CH3 ) def + Skel def + ν CC (C CH3 ) def Skel def + ν CC ν CC + CH3 def CH def CH def CH2 Br wag + Skel def CH2 Br def CH def + CH2 Br def CH def + ν CC + CH2 Br def CH def + CH2 Br def CH2 Br def + CH def + ν CC ν CC + CH def + CH3 def CH3 rock CH3 rock CH def CH2 Br def + CH def CH2 Br def + CH def + ν CO Skel def CH def + CH2 Br def CH2 Br def + CH def Skel def Skel def Skel def Skel def CH2 Br def + ν CO + Skel def Skel def + CH2 Br def Skel def + CO def Skel def Skel def + ν (C Br) Skel def + CO def Skel def + CO def Skel def Skel def Skel def Skel def + CO def + (C CH3 ) def CH3 rock + Skel def + CO def CH2 Br def + Skel def CH2 Br def + Skel def Skel def + CH2 Br def Skel def + CH3 def Skel def CH3 tor + Skel def Skel def + CH2 Br def CH3 def CH3 tor
INT, IR intensity (KM/mol); vs, very strong; s, strong; m, medium; w, weak; vw, very weak; Skel def, skeletal deformation; wag, wagging; tor, torsion; rock, rocking; ν, stretching.
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(C O) is not at all affected by the rotation of bromomethyl group and it is observed as a very strong IR band at 1728 cm−1 and a Raman band at 1699 cm−1 corresponding to the computed band at 1767 cm−1 . 4.2.3. C X vibrations (X = CH3 , CH2 Br) In the Raman spectrum, a weak band at 2862 cm−1 and a medium strong band at 2921 cm−1 are assigned to CH3 stretching vibrations. These two bands are more accurately predicted by RHF/6-31G* level at 2862 and 2910 cm−1 , respectively. In the computed spectrum of B3LYP/6-31G* level, they are predicted at 2928 and 2978 cm−1 , respectively. Two weak Raman bands at 2983 and 3050 cm−1 are assigned to the stretching frequencies of CH2 Br group. These two bands correspond to the bands at 3011 and 3077 cm−1 in the computed spectrum. A strong IR band at 1203 cm−1 with a corresponding Raman band at 1194 cm−1 is assigned to CH2 Br deformation vibration. The corresponding computed band is 1209 cm−1 . C Br stretching along with the skeletal deformation is observed as a very weak IR band at 499 cm−1 with a corresponding computed band at 550 cm−1 . The C CH3 deformations are expected in the region 1370–1465 cm−1 . So we have two strong IR bands at 1435 and 1450 cm−1 assigned to C CH3 deformation along with skeletal deformation and C CH2 deformation, respectively. Similarly, a medium strong IR band at 1222 cm−1 with a corresponding Raman band at 1220 cm−1 , is assigned to CH2 wagging along with skeletal deformation. A predicted band at 1457 cm−1 with no corresponding experimental band is assigned to CH3 deformation vibration. But a strong IR band at 1418 cm−1 is assigned to C CH3 deformation and corresponds to the computed band at 1391 cm−1 . In the lower region below 200 cm−1 we see that both CH3 and CH2 Br groups dominate the normal modes of vibrations. In their work on the Raman spectrum of the parent coumarin, Venkateswaran and Girijavallabhan [47] have observed seven low frequency lattice lines at 29, 39, 54, 67, 92, 112 and 146 cm−1 . However, we have observed two bands at 63 and 96 cm−1 of which 96 cm−1 is a very strong Raman band as in coumarin. In the computed spectrum, it is assigned to CH3 torsion vibration and the ring deformation vibration lying at 81 cm−1 . Similarly, the band at 63 cm−1 corresponds to a predicted band at 61 cm−1 and is assigned to the ring deformation and CH2 Br deformation vibrations. Two calculated bands at 53 and 57 cm−1 with no corresponding experimental bands are assigned to CH3 torsion and CH3 deformation, respectively. All the assignments are presented in Table 2.
5. Conclusions The infrared and Raman spectra of 6-methyl-4-bromomethylcoumarin have been measured and assigned on the basis of spectral correlations and electronic structure calculations at RHF/6-31G* and B3LYP/6-31G* levels. The geometry optimizations at different dihedral angles of CH2 Br
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