Vibrational spectroscopy investigation using ab initio and density functional theory analysis on the structure of 5-amino-o-cresol

Spectrochimica Acta Part A 68 (2007) 619–625

Vibrational spectroscopy investigation using ab initio and density functional theory analysis on the structure of 5-amino-o-cresol N. Sundaraganesan a,∗ , C. Meganathan a , H. Saleem a , B. Dominic Joshua b a

b

Department of Physics (Engineering), Annamalai University, Annamalai Nagar 608 002, Tamil Nadu, India Department of Physics, Sri Aravindar Arts and Science College, Akasampet, Vanur District 605 111, Tamil Nadu, India Received 11 March 2006; received in revised form 3 December 2006; accepted 16 December 2006

Abstract The Fourier transform Raman and Fourier transform infrared spectra of 5-amino-o-cresol (5AOC) were recorded in the solid phase. The equilibrium geometry, harmonic vibrational frequencies, infrared intensities and Raman scattering activities were calculated by HF and density functional B3LYP method with the 6-311G(d,p) basis set. The scaled theoretical wavenumbers showed very good agreement with the experimental values. The thermodynamic functions of the title compound were also performed at HF/6-311G(d,p) and B3LYP/6-311G(d,p) levels of theory. A detailed interpretation of the infrared and Raman spectra of 5-amino-o-cresol is reported. The theoretical spectrograms for FT-IR spectra of the title molecule have been constructed. © 2006 Elsevier B.V. All rights reserved. Keywords: FT-IR and FT-Raman spectra; Ab initio and DFT; 5-Amino-o-cresol; Vibrational analysis

1. Introduction The 5-amino-o-cresol is an important compound in the aspect of its known metabolitic properties. Some ingredients used in and for hair dyes such as phenolic compounds (phenol, thymol, cresol, p-tert-butylphenol, resorcinol) can cause allergic reactions like rashes, dermal inflammation, irritation and dermatitis or promote cancer [1,2]. Raman spectra of proteins give information not only on the structure of the main chain but also on the micro environments of side chains [3]. The important amino acid residues are tryptophan, tyrosine and phenylalanine which are used in the Raman spectroscopic studies of protein structure, because the side chain vibrations of these aromatic residues are strong in the Raman spectra of visible light excitation and further more, they can be selectively enhanced by ultra violet excitation [3–13]. From the knowledge of characteristic aromatic side chain, we can interpret the vibrational modes of Raman spectra of proteins. Harada and Takeuchi [11], have studied the vibrational modes of tryptophan side chain and tyrosine side chain. The simplest model of the tyrosine side chain is p-cresol (4-methylphenol). The Raman spectra of this compound and ∗

Corresponding author. E-mail address: sundaraganesan [email protected] (N. Sundaraganesan).

1386-1425/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.saa.2006.12.036

its three deuterated derivatives in the liquid state and the IR spectra in the liquid solution, vapour and solid states have reported partially by Jakobsen [14]. Davy-Dova et al. [7] calculated the normal frequencies of out-of-plane vibrations and assigned some of the infrared and Raman bands. Green et al. [10] proposed complete assignments of fundamental frequencies for non-deuterated p-cresol. We have also reported normal coordinate analysis of 6-amino-m-cresol using the classical method developed by Wilson to support the vibrational analysis [15]. Literature survey reveals that to the best of our knowledge, neither the complete Raman and IR spectra nor the force fields for 5-amino-o-cresol have been reported so far. Therefore, the present investigation was undertaken to study the vibrational spectra of this molecule completely and to identify the various normal modes with greater wavenumber accuracy. Ab initio HF and density functional theory (DFT) calculations have been performed to support our wavenumber assignments. Specific scale factors reported by Rastogi et al. [16] deduced from the benzene molecule have been employed to predict wavenumber of the ring modes of 5-amino-o-cresol. 2. Experimental The compound 5-amino-o-cresol (5-amino-2-methylphenol) in the solid form was purchased from the Sigma–Aldrich Chem-

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Fig. 1. FT-IR spectrum of 5-amino-o-cresol.

adopting the standard 6-311G(d,p) basis set. This geometry was then re-optimized again at B3LYP level, using basis set 6-311G(d,p), for better description of polar bonds of amino, hydroxial and methyl groups. The optimized structural parameters were used in the vibrational frequency calculations at the HF and DFT levels to characterize all stationary points as minima. Then vibrationally averaged nuclear positions of 5AOC were used for harmonic vibrational frequency calculations resulting in IR and Raman frequencies together with intensities and Raman depolarization ratios. We have utilised the gradient corrected density functional theory (DFT) [19] with the three-parameter hybrid functional (B3) [20] for the exchange part and the Lee-Yang-Parr (LYP) correlation function [21], accepted as a cost-effective approach, for the computation of molecular structure, vibrational frequencies and energies of optimized structures. Vibrational frequencies computed at DFT level have been adjudicated to be more reliable than those obtained by the computationally demanding Moller-Plesset perturbation methods. Density functional theory offers electron correlation frequently comparable to second-order Moller-Plesset theory (MP2). Finally, the calculated normal mode vibrational frequencies provide thermodynamic properties also through the principle of statistical mechanics. By combining the results of the GAUSSVIEW program [22] with symmetry considerations, vibrational frequency assignments were made with a high degree of accuracy. There is always some ambiguity in defining internal coordination. However, the defined coordinate form complete set and matches quite well with the motions observed using the GAUSSVIEW program. 4. Results and discussion

Fig. 2. FT-Raman spectrum of 5-amino-o-cresol.

4.1. Geometrical structure ical Company (USA) with a stated purity of greater than 98% and it was used as such without further purification. The FTRaman spectrum of 5AOC has been recorded using 1064 nm line of Nd:YAG laser as excitation wavelength in the region 100–4000 cm−1 on a Brucker model IFS 66V spectrophotometer equipped with FRA 106 FT-Raman module accessory. 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 spectrum was recorded at room temperature, with a scanning speed of 30 cm−1 min−1 and the spectral resolution of 2.0 cm−1 . The observed experimental FT-IR and FT-Raman spectra are shown in Figs. 1 and 2. The spectral measurements were carried out at Sophisticated Analytical Instrumentation Facility (SAIF), IIT, Chennai.

The atom numbering scheme for the title molecule is given in Fig. 3. The optimized bond lengths and angles for 5-aminoo-cresol at the HF/6-311G(d,p) and B3LYP/6-311G(d,p) levels are represented in Table 1 along with available experimental data, viz. o-cresol [23]. It is seen from Table 1, a general priority

3. Computational details The entire calculations were performed at Hartree-Fock (HF) and B3LYP levels on a Pentium IV/1.6 GHz personal computer using Gaussian 03W [17] program package, invoking gradient geometry optimization [18]. Initial geometry generated from standard geometrical parameters was minimized without any constraint in the potential energy surface at Hartree-Fock level,

Fig. 3. Numbering system adopted in this study (5-amino-o-cresol).

N. Sundaraganesan et al. / Spectrochimica Acta Part A 68 (2007) 619–625 Table 1 ˚ and Geometrical parameters optimized in 5-amino-o-cresol, bond length (A), angle (◦ ) Parameters

Experimentala

˚ Bond lengths (A) C1–C2 C1–C6 C1–O17 C2–C3 C2–C13 C3–C4 C3–H7 C4–C5 C4–H8 C5–C6 C5–N10 C6–H9 N10–H11 N10–H12 C13–H14 C13–H15 C13–H16 O17–H18 Bond angles (◦ ) C2–C1–C6 C2–C1–O17 C6–C1–O17 C1–C2–C3 C1–C2–C13 C3–C2–C13 C2–C3–C4 C2–C3–H7 C4–C3–H7 C3–C4–C5 C3–C4–H8 C5–C4–H8 C4–C5–C6 C4–C5–N10 C6–C5–N10 C1–C6–C5 C1–C6–H9 C5–C6–H9 C5–N10–H11 C5–N10–H12 H11–N10–H12 C2–C13–H14 C2–C13–H15 C2–C13–H16 H14–C13–H15 H14–C13–H16 H15–C13–H16 C1–O17–H18 a

1.406 1.398 1.370 1.399 1.505 1.399 1.087 1.396 1.085 1.398 – 1.088 – – 1.095 1.093 1.093 0.962 120.9 116.4 118.0 122.5 121.7 119.9 119.5 120.3 119.8

120.0 119.6 119.6

110.8 110.6 110.8

107.7

HF/6-311G (d,p) 1.394 1.382 1.351 1.382 1.509 1.387 1.077 1.386 1.076 1.393 1.395 1.078 0.996 0.996 1.086 1.086 1.084 0.940 121.7 117.0 121.3 116.6 120.6 122.8 122.8 118.7 118.5 119.7 120.2 120.2 118.6 121.2 120.1 120.5 119.7 119.8 114.1 114.4 111.1 111.4 111.4 110.4 107.1 108.2 108.2 110.7

B3LYP/6-311G (d,p) 1.403 1.392 1.371 1.394 1.506 1.392 1.086 1.400 1.085 1.402 1.399 1.088 1.010 1.010 1.095 1.095 1.092 0.962 121.7 116.7 121.6 116.7 120.5 122.8 122.7 118.5 118.8 120.0 120.2 119.9 118.4 121.1 120.4 120.6 119.7 119.7 114.5 115.0 111.4 111.5 111.5 110.7 106.6 108.2 108.2 109.1

Taken from Ref. [23].

for reproducing the experimental bond lengths taken from Ref. [23] is not present among HF and DFT-B3LYP levels. However, all the bond lengths and bond angles computed with the DFT-B3LYP levels shows excellent agreement with available experimental results when compared with HF levels. The reduction in the bond lengths, is more pronounced in B3LYP level compared to HF level. The C–CH3 bond length is slightly overestimated in both the levels whereas the C–O bond

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length is underestimated in HF levels and slightly overestimated in B3LYP levels. In aniline, the nitrogen atom is out of the ring plane for about 2–3◦ (tilt angle) and the angle between the plane defined by the amino group and the plane of the ring is 38 ± 4◦ [24]. Experimental data on the degree of non-planarity of the amino group is not available in the literature for p-methylaniline [25]. In 5amino-o-cresol also the tilt angle and the angle between the planes of amino group and the ring calculated with HF and DFT-B3LYP methods is quite similar to the experimental data on aniline and p-methylaniline [25]. 4.2. Vibrational assignments According to the theoretical calculations, 5-amino-o-cresol has a planar structure of Cs point group symmetry. The molecule has 18 atoms and 48 normal modes of fundamental vibrations which span the irreducible representations: 33A + 15A . All the 48 fundamental vibrations are active in both FT-IR and FTRaman. The harmonic-vibrational frequencies calculated for 5AOC at B3LYP levels using the triple split valence basis set along with diffused and polarization functions, 6-311G(d,p) have been collected in Table 3. The observed FT-IR and FT-Raman frequencies for various modes of vibrations are presented in Table 2. Comparison of the frequencies calculated at B3LYP with the experimental values (Table 2) reveals the overestimation of the calculated vibrational modes due to neglect of anharmonicity in real system. Inclusion of electron correlation in density functional theory to a certain extend makes the frequency values smaller in comparison with the HF frequency data. Reduction in the computed harmonic vibrations, though basis set sensitive are marginal as observed in the DFT values using 6-311G(d,p). Any way not withstanding the level of calculations, it is customary to scale down the calculated harmonic frequencies in order to improve the agreement with the experiment. In our study, we have followed two different scaling factors (i.e.) 0.9089 up to 800 cm−1 and 0.8992 beyond 800 cm−1 and those values are entered in column ‘a’ of Table 3. The ‘b’ column contains the values scaled down differently as indicated at the footnotes of Table 3. The stick spectra of 5AOC at B3LYP levels using 6-311G(d,p) have been shown in Fig. 4. 4.3. OH vibrations The OH group vibrations are likely to be most sensitive to the environment, so they show pronounced shifts in the spectra of the hydrogen-bonded species. In the case of unsubstituted phenol it has been shown that the frequency of OH stretching vibration in the gas phase is 3657 cm−1 [26]. In our case a strong band in FT-IR spectrum at 3387 cm−1 and a weak band in FT-Raman at 3390 cm−1 is assigned to OH stretching vibration. A comparison of these bands with literature data predict that there is negative deviation of ∼270 cm−1 may be due to fact that the presence of strong intra-molecular hydrogen bonding. However, the calculated value by B3LYP/6-311G(d,p) level shows at 3682 cm−1 (mode no. 48).

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Table 2 Experimental FT-IR and FT-Raman frequencies and assignments for 5-aminoo-cresol (cm−1 ) Species

FT-IR frequency and intensity

FT-Raman frequency and intensity

Vibrational assignments

A A A A A A A A

3387 s 3323 ms

3390 w 3326 w 3050 w 3036 w 2989 w 2948 w 2911 w 2863 w 2736 w 2606 vw

OH stretch NH sym. stretch CH stretch CH stretch CH stretch CH3 asym. stretch CH3 asym. stretch CH3 sym. stretch (1465 + 1252) (1524 + 1104) (2 × 931) NH2 scissoring C–C stretch C–C stretch C–C stretch C–C stretch CH3 asym. deform CH3 sym. deform C–NH2 stretch C–CH3 stretch CH ipb C–O stretch OH ipb CH ipb CH3 rocking CCC ipb CH opb CH opb CCC ipb Ring deform CH opb Ring breathing CCC opb NH2 wagging C–NH2 ipb CCC opb CCC ipb CCC opb NH2 rocking C–NH2 opb C–O opb CH3 torsion

A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A

3029 w 2943 sh 2903 vw 2855 vw 2729 vw 2620 vw 1860 w

1624 ms 1597 w 1560 w 1524 w 1465 ms 1430 ms 1378 ms 1306 ms 1252 vs 1205 vs 1173 vs 1119 w 1104 w 998 s 952 s 931 w 850 w

1473 w 1382 w 1317 ms 1259 w 1180 w 1130 w 1004 ms 959 vw

827 vw 812 w 759 vw 731 w 627 s 524 w 457 s

767 vs 743 vw 635 vw 581 ms 532 ms 461 ms 349 vw 252 ms 187 ms 133 s 94 vs

s, strong; m, medium; vs, very strong; w, weak; vw, very weak; ipb, in-plane bending; opb, out-of-plane bending.

The OH in-plane bending vibration in phenol, in general, lies in the region 1150–1250 cm−1 and is not much affected due to hydrogen bonding unlike the stretching and out-ofplane deformation frequencies. In 3-aminophenol this vibration was found in at 1178 cm−1 [27]. The very strong FT-IR frequency at 1173 cm−1 and a weak FT-Raman frequency at 1180 cm−1 respectively are attributed this vibration. The theoretically computed value at 1178 cm−1 by B3LYP/6-311G(d,p) method exactly coincides with experimental observations. The OH out-of-plane deformation vibration in phenol lies in the region 290–320 cm−1 for free OH and in the region 517–710 cm−1 for associated OH [28]. In both inter-molecular and intra-molecular associations, the frequency is at a higher

Fig. 4. Comparison of corrected frequencies in cm−1 normalised IR intensities at each level of calculations considered.

value than in free OH. The frequency increases with hydrogen bond strength because of the larger amount of energy required to twist the O–H bond out-of-plane [29]. In the Raman spectrum of 3-aminophenol [27] a band at 350 cm−1 was assigned to OH out-of-plane deformation. The calculated value of this vibration at 360 cm−1 (see Table 3, mode no. 8) shows excellent agreement with literature value. The calculated value at 360 cm−1 is missing in both FT-IR and FT-Raman. 4.4. CO vibration The CO stretching vibration in 5-amino-o-cresol has a main contribution in the mode no. 28, with B3LYP/6-311G(d,p) predicted frequency of 1212 cm−1 (Table 3). This is excellent agreement with very strong experimental frequency in FT-IR spectrum at 1205 cm−1 . The C–O out-of-plane bending vibration mode with the theoretical frequency of 237 cm−1 deviates positively by ∼104 cm−1 from experimental FT-Raman value. This may be due to mixing of CO vibration with CCC out-ofplane bending vibration. The above conclusions are in agreement with literature value [30]. 4.5. C–H vibrations Since 5-amino-o-cresol is a trisubstituted aromatic system, it has two adjacent and one isolated C–H moieties. The

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Table 3 Vibrational wavenumbers obtained for 5-amino-o-cresol at B3LYP/6-311G(d,p) Number

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48

Wavenumber

110 138 225 270 283 308 322 354 455 458 511 576 597 649 708 753 768 789 803 932 968 1016 1061 1110 1139 1194 1211 1247 1327 1343 1367 1417 1471 1478 1506 1557 1631 1659 1672 3022 3067 3100 3133 3152 3171 3558 3653 3835

a

110 138 225 270 283 308 322 354 456 459 512 577 598 650 709 754 769 790 772 896 931 977 1020 1067 1095 1148 1164 1199 1276 1291 1314 1362 1414 1421 1448 1497 1568 1595 1607 2905 2948 2980 3012 3030 3048 3420 3512 3687

b

127 154 237 280 292 316 329 360 456 459 510 572 592 641 698 741 755 775 788 912 946 992 1035 1081 1109 1162 1178 1212 1288 1304 1327 1374 1426 1433 1459 1508 1579 1605 1618 2906 2949 2980 3012 3030 3048 3417 3508 3682

IR intensity

Raman intensity

Rel

Abs

Rel

Abs

0 4 5 20 5 28 1 67 5 14 10 10 283 60 1 7 0 0 77 1 3 18 2 25 105 36 14 107 22 37 8 0 24 6 29 82 41 12 205 45 24 21 23 15 20 12 9 48

0 1 2 7 2 10 0 24 2 5 4 4 100 21 0 2 0 0 27 0 1 6 1 9 37 13 5 38 8 13 3 0 8 2 10 29 14 4 72 16 8 7 8 5 7 4 3 17

0 1 2 1 0 2 1 3 4 1 5 5 6 1 0 4 24 1 2 0 9 1 0 2 1 1 5 9 4 14 2 21 3 14 11 1 4 3 65 250 108 70 92 68 165 215 74 104

0 0 1 0 0 1 0 1 2 0 2 2 2 0 0 2 10 0 1 0 4 0 0 1 0 0 2 4 2 6 1 8 1 6 4 0 2 1 26 100 43 28 37 27 66 86 30 42

Vibrational assignments

τCH3 τCH3 γC–OH + γCCC γC–NH2 βCH3 + βOH γOH + γC–CH3 βC–NH2 + βOH γOH βCCC γCCC + γCH βCCC βCCC βC–NH2 ωNH2 + γCCC + γCH γCCC + γCH Ring deform Ring breathing γCH γCH γCH βCCC + CH3 deform CH3 deform CH2 twist in CH3 t NH2 + βCH βCH + βOH βCH + βOH βOH + βCH νC–O + βCH βCH + βOH + νCC νC–NH2 + νC–O βOH + νC–C CH3 sym. deform γCH3 + νC–C CH3 asym. deform νC–C νC–C + βCH3 νC–C δNH2 δNH2 + νC–C νs CH3 νas CH3 νas CH3 νCH νCH νCH νs NH2 νas NH2 νOH

˚ 4 amu−1 ). a: with the scale factor of 1.0013 for calculated wavenumbers lower Harmonic frequency (cm−1 ); IR intensities (km mol−1 ); Raman scattering activities (A than 800 cm−1 and the scale factor of 0.9613 for larger wavenumbers; b: νexp = 22.1 + 0.954 3ωcal.b3lyp , ν, stretching; νs , sym. stretching; νas , asym. stretching; β, in-plane-bending; γ, out-of-plane bending; δ, scissoring; ω, wagging; ρ, rocking; t, twisting; τ, torsion.

expected three C–H stretching vibrations correspond to 43, 44 and 45. The scaled vibration, column ‘b’ of 43, 44 and 45 (Table 3) corresponds to stretching modes of C3–H, C4–H and C6–H units. The vibrations 43–45 assigned to aromatic C–H stretch in the region 3047–3077 cm−1 [31] are in agreement with experimental assignment 3012–3048 cm−1 [32]. The C–H in-plane bending vibrations assigned in the region

1081–1288 cm−1 (mode nos. 24–26) even though found to be contaminated by –NH2 rocking and OH in-plane bending are in the range found in literature [33,34], while the experimental observation are at 1130–1259 cm−1 . The calculated frequencies 755–912 cm−1 (mode nos. 18–20) for the C–H out-of-plane bending falls in the FT-IR/FT-Raman values of 812–959 cm−1 .

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4.6. C–NH2 vibrations The scaled –NH2 symmetric and asymmetric stretches in the range 3417–3508 cm−1 (mode nos. 46 and 47) is in agreement with experimental value of ∼3323 cm−1 in FT-IR and 3326 cm−1 in FT-Raman. The asymmetric vibration calculated at ∼3508 cm−1 is missing in both FT-IR and FT-Raman. The computed –NH2 scissoring vibration at 1605 cm−1 is in excellent agreement with expected characteristic value, 1600 cm−1 [35,36]. This is also in very good agreement with recorded medium strong band in FT-Raman value at 1624 cm−1 (mode no. 38). The NH2 scissoring mode also contributes with C–C stretching mode at 1618 cm−1 (mode no. 39). The medium band FT-Raman value at 1317 cm−1 corresponding to C–NH2 moiety was calculated to be 1304 cm−1 (mode no. 30). The C–NH2 out-of-plane and in-plane bending vibrations at 280 and 592 cm−1 respectively, are also in good agreement with the assignment in the experimental data. The NH2 wagging computed at 641 cm−1 (mode no. 14) exactly matches with FT-Raman value at 635 cm−1 . The NH2 twisting vibration calculated to be 1081 cm−1 (mode no. 24) is missing in both FT-IR and FT-Raman spectra. 4.7. C–C vibrations The C–C aromatic stretch, known as semi-circle stretching, predicted at 1579 cm−1 is in excellent agreement with experimental observation of FT-IR value at 1560 cm−1 . The ring breathing mode at 755 cm−1 (mode no. 17) coincides satisfactorily with very strong Raman band at 767 cm−1 and a very weak band at 759 cm−1 in FT-IR, respectively [37]. The theoretically calculated C–C–C in-plane bending and out-of-plane bending modes have been found to be consistent with the recorded spectral values. 4.8. Methyl group vibrations The title molecule 5-amino-o-cresol, under consideration possess one CH3 group in second position of the ring. For the assignment of CH3 group frequencies one can expect that nine fundamentals can be associated viz. the symmetrical stretching in CH3 (CH3 sym. stretch) and asymmetrical stretching (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 (t CH3 ) bending modes. The CH stretchings at lower frequencies than those of the aromatic ring (3000 cm−1 ). For the CH3 compounds, the mode νs appears in the range 2860–2935 cm−1 , where as the νas modes appear in the region 2925–2985 cm−1 . The FT-IR/FT-Raman bands at 2943/2948 and 2903/2911 cm−1 have been assigned to asymmetric stretching vibration of CH3 group. The theoretically computed values of 2949 and 2980 cm−1 (mode nos. 41 and 42) shows an excellent agreement with experimental results. The FT-IR band at 2855 cm−1 and FT-Raman band at 2863 cm−1 have been assigned to CH3 symmetric stretch. The theoretically computed value

Table 4 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 5-amino-o-cresol Parameters

HF/6-311G(d,p)

B3LYP/6-311G(d,p)

Total energy Zero-point energy Rotational constants

−399.7225208 99.51 3.0586 1.3295 0.9334

−402.2497681 93.02 3.1000 1.3066 0.9252

87.473 40.337 28.802 18.334 2.330

89.603 40.337 28.841 20.425 2.757

Entropy Total Translational Rotational Vibrational Dipole moment

at 2906 cm−1 (mode no. 40) correlates with experimental data. The asymmetric and symmetric bendings are recorded in the 1455–1410 cm−1 region and about 1375 cm−1 respectively, while the rocking appear in the 1050–990 cm−1 . In the case of 5amino-o-cresol a medium strong band is observed at 1430 cm−1 in FT-IR spectrum corresponds to asymmetric CH3 deformation and correlated with the calculated frequency at 1433 cm−1 . This behaviour can be found in other fundamentals such as symmetric deformation and the rockings. The torsion vibrations are not observed in the FT-IR spectrum because these appear at very low frequency. The scaling procedure predicts that these vibrations could appear at about 127 and 154 cm−1 in 5-amino-o-cresol (mode nos. 1–2). The FT-Raman experimental observations at 94 cm−1 shows an excellent agreement with theoretical results. This means that the rotation of the methyl group is not significantly hindered. These assignments find support from the work of Singh and Prasad [38] and are within the frequency intervals given by Varsanyi [39]. 5. Other molecular properties Several calculated thermodynamic parameters are presented in Table 4. Scale factors have been recommended [40] for an accurate prediction in determining the zero-point vibration energies (ZPVE), and the entropy, Svib (T). The variations in the ZPVEs seem to be insignificant. The total energies and the change in the total entropy of 5AOC at room temperature at different methods are also presented. 6. Conclusion Attempts have been made in the present work for the proper frequency assignments for the compound 5AOC from the FT-IR and FT-Raman spectra. The equilibrium geometries and harmonic frequencies of 5AOC were determined and analysed at DFT level of theories utilizing 6-311G(d,p) basis set, giving allowance for the lone pairs through diffuse functions. The difference between the observed and scaled wavenumber values of most of the fundamentals is very small. Any discrepancy noted

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between the observed and the calculated frequencies may be due to the fact that the calculations have been actually done on a single molecule in the gaseous state contrary to the experimental values recorded in the presence of intermolecular interactions. Therefore, the assignments made at higher levels of theory with only reasonable deviations from the experimental values, seem to be correct. References [1] A.A. Shvedova, C. Kommineni, B.A. Jeffries, V. Castranove, Y.Y. Tyurina, V.A. Tyurin, J. Invest. Dermatol. 114 (2) (2000) 354. [2] K. Eggenreich, S. Golouch, B. T¨oscher, H. Beck, D. Kuehnelt, R. Wintersteiger, J. Boiochem. Biophys. Meth. 61 (2004) 23. [3] S.A. Asher, M. Ludwig, C.R. Johnson, J. Am. Chem. Soc. 108 (1986) 3186. [4] L.J. Bellamy, The Infrared Spectra of Complex Molecules, Chapmann and Hall, London, 1975. [5] K.J. Bellamy, Infrared Spectra of Organic Molecules, Chapmann and Hall, London, 1959. [6] R.A. Copeland, T.G. Spiro, Biochemistry 24 (1985) 4960. [7] N.J. Davy-Dova, I.A. Zhigunova, M.A. Ignaateva, 56 (1965) 1077; N.J. Davy-Dova, I.A. Zhigunova, M.A. Ignaateva, Opt. Spectrosc., 18 (1965) USSR 605. [8] J.R. Durig, G.C. James, T.J. Hizer, J. Raman Spectrosc. 21 (1990) 155. [9] J.C. Evans, Spectrochim. Acta 16 (1960) 428. [10] J.H.S. Green, D.J. Harrison, W. Kynatson, Spectrochim. Acta 27A (1971) 2199. [11] I. Harada, H. Takeuchi, in: R.J. Clark, R.E. Hester (Eds.), Advances in Spectroscopy, vol. 13, Wiley, Chichester, 1986, p. 113. [12] C.R. Johnson, M. Ludwig, S. D’Donell, S.A. Asher, J. Am. Chem. Soc. 106 (1986) 5008. [13] C.R. Johnson, M. Ludwig, S.A. Asher, J. Am. Chem. Soc. 108 (1986) 905. [14] R.J. Jakobsen, Spectrochim. Acta 21 (1965) 433. [15] S. Mohan, N. Sundaraganesan, Ind. J. Phys. 66(B) (2) (1992) 213. [16] K. Rastogi, M.A. Palafox, R.P. Tanwar, L. Mittal, Spectrochim. Acta A 58 (2002) 1989. [17] M.J. Frisch, G.W. Trucks, H.B. Schlegel, G.E. Scuseria, M.A. Robb, J.R. Cheeseman, J.A. Montgomery, Jr., T. Vreven, K.N. Kudin, J.C. Burant, J.M. Millam, S.S. Iyengar, J. Tomasi, V. Barone, B. Mennucci, M. Cossi, G. Scalmani, N. Rega, G.A. Petersson, H. Nakatsuji, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H.

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