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FT-IR, FT-Raman spectra and ab initio HF, DFT vibrational analysis of 2,3-difluoro phenol
Spectrochimica Acta Part A 68 (2007) 561–566
FT-IR, FT-Raman spectra and ab initio HF, DFT vibrational analysis of 2,3-difluoro phenol N. Sundaraganesan ∗ , B. Anand, C. Meganathan, B. Dominic Joshua Department of Physics (Engineering), Annamalai University, Annamalai Nagar 608 002, India Received 8 July 2006; received in revised form 12 December 2006; accepted 16 December 2006
Abstract The FT-IR and FT-Raman spectra of 2,3-difluoro phenol (2,3-DFP) has been recorded in the region 4000–400 and 4000–100 cm−1 , respectively. The optimized geometry, frequency and intensity of the vibrational bands of 2,3-DFP were obtained by the ab initio HF and density functional theory (DFT) levels of theory with complete relaxation in the potential energy surface using 6-311 + G(d,p) basis set. The harmonic vibrational frequencies were calculated and the scaled values have been compared with experimental FT-IR and FT-Raman spectra. The observed and the calculated frequencies are found to be in good agreement. The experimental spectra also coincide satisfactorily with those of theoretically constructed bar type spectrograms. © 2006 Elsevier B.V. All rights reserved. Keywords: FT-IR and FT-Raman spectra: ab initio HF and DFT; 2,3-Difluoro phenol; Vibrational analysis
1. Introduction Phenols are organic compounds that contain a hydroxyl group ( OH) bound directly to a carbon atom in the benzene ring. Unlike normal alcohols, phenols are acidic because of the influence of the aromatic ring. Phenols are made by fusing a sulphonic acid with sodium hydroxide to form the sodium salt of the phenol. The free phenol is liberated by adding sulphuric acid. It is used as anti-bacterial and anti-septic and also for the treatment of surgical instrument and bandaging materials [1]. Halogination of phenol derivatives produces active substances with substantially high anti-microbial effect. The degree of dissociation of haloginated phenol derivatives increases with the number of halogen atoms. The combination of alkylation and halogenations in particular, the later produces phenolic microbiocides which are used as active disinfectants and for the preservation of materials. Chlorophenol is an important group of industrial chemical which got variety of uses ranging from preparation of preservatives to insecticides. But they show high oral toxicity and were almost not biologically degradable [2]. However, with few exceptions, organic fluorine compounds which are physiologically inert, display insignificant toxicity.
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This is a consequence of the chemical stability of the C F bond and the increased stability of hydrogen and halogen bonds attached to a fluorinated carbon atom [3]. Low toxicity is an important factor in many applications like agricultural chemicals, pharmaceuticals, biocides and dyes. Due to low toxicity and wide ranging applications of fluorine derivatives of phenols, we have undertaken a thorough vibrational analysis for 2,3-difluoro phenol along with ab initio HF and density functional theory (DFT) calculations. The vibrational spectrum of phenol was extensively studied and analyzed [4–6]. Among all the halogenated phenols having the same halogen substituents, vibrational assignments for almost all the chlorinated phenols, such as isomers of monochloro–dichloro- and trichlorophenols and pentachlorophenols were reported by various earlier investigators [4,7–17]. The vibrational spectrum of 2,4,6-trichlorophenol has been reported by Faniran and Shurwell [18]. However, the force field calculations for this molecule have not been reported so far. Various spectroscopic studies of chloro and methyl phenols have been reported in literatures [19–21]. Recently, Singh and Rai have studied the infrared and the electronic absorption spectra of 4-chloro-2-methyl, 4-chloro-3-methyl and 6-chloro3-methyl phenols. A complete vibrational assignment of phenol and phenol-OD has been given by Evans [5]. The vibrational spectra of p-cresol and its deuterated derivatives have been studied by Jakobsen [22], who gave detailed interpretations of the
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vibrational bands. The assignment of the vibrational frequencies for substituted phenols becomes complicated problem because of the superposition of perhaps several vibrations due to fundamentals and due to substituents. However, a comparison of the spectra with that of the parent compound gives some definite clues about the nature of the molecular vibrations. To the best of our knowledge, neither quantum chemical calculations, nor the vibrational spectra of 2,3-DFP have been reported, as yet. Therefore, the present investigation was undertaken to study the vibrational spectra of the molecule completely and to identify the various normal modes with greater wavenumbers accurately. Ab initio HF and density functional theory calculations have been performed to support our wave number assignments. 2. Experimental The compound 2,3-DFP in the solid form was purchased from the Sigma–Aldrich Chemical Company (USA) with a stated purity of greater than 98% and it was used as such without further purification. The FT-Raman spectrum of 2,3-DFP 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 66 V spectrophotometer equipped with FRA 106 FT-Raman module accessory. The FT-IR spectrum of this com-
Fig. 1. FTIR spectrum of 2,3-difluoro phenol.
pound was recorded in the region 400–4000 cm−1 on IFS 66 V 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 Regional Sophisticated Instrumentation Centre (RSIC), IIT Chennai. 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 [23] program package, invoking gradient geometry optimization [24]. Initial geometry generated from standard geometrical parameters was minimized without any constraint in the potential energy surface at Hartree–Fock and B3LYP level, adopting the standard 6-311 + G(d,p) basis set. 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 2,3-DFP were used for harmonic vibrational frequency calculations resulting in IR and Raman frequencies together with intensities and Raman depolarization ratios. We have used the density functional theory [25] with the three-parameter hybrid functional (B3) [26] for the exchange part and the Lee–Yang–Parr (LYP) correlation function [27], 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 [28] with symmetry considerations, vibrational frequency assignments were made with a high degree of accuracy. 4. Results and discussion 4.1. Geometrical structure
Fig. 2. FT-Raman spectrum of 2,3-difluoro phenol.
The atom numbering scheme for the title molecule is given in Fig. 3. The optimized bond lengths and angles for 2,3-difluoro phenol at the HF/6-311 + G(d,p) and B3LYP/6-311 + G(d,p) levels are presented in Table 1 along with avilable X-ray data, viz., 2,6-dichloro phenol [29]. The structure of 2.3-difluoro phenol in the gas phase is unknown, whereas in 2,6-dichloro phenol crystal the geometry is probably distorted due to hydrogen bonding [29]. From the data shown in Table 1, it is seen that the both HF and B3LYP levels of theory in general slightly underestimates some bond lengths and overestimates few bond lengths, bond angles but B3LYP/6-311 + G(d,p) method yields bond length and bond angles in excellent agreement with X-ray data.
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Table 1 Geometrical parameters optimized in 2,3-difluoro phenol Parameter
HF/6-311 + G(d,p)
˚ Bond length (A) C1 C2 C1 C6 C1 O10 C2 C3 C2 F12 C3 C4 C3 F13 C4 C5 C4 H7 C5 C6 C5 H8 C6 H9 O10 H11
Fig. 3. Numbering system adopted in this study (2,3-difluoro phenol).
The various bond lengths are found to be almost the same at HF/6-311 + G(d,p) and B3LYP/6-311 + G(d,p) (Table 1). The ˚ is just 0.02 A ˚ lower than the C1 O10 bond distance of 1.360 A ˚ for 2,6-dichloro phenol reported experimental value of 1.380 A [29]; however, similar discrepancies were obtained in calculation with MP2 and B3LYP methods [30,31]. The optimized ˚ for HF/6C F bond lengths by two methods are 1.329 A ˚ for B3LYP/6-311 + G(d,p), which are 311 + G(d,p) and 1.356 A ˚ for in good agreement with those of reported values of 1.328 A 3,5-dichloro-2,4,6-trifluoropyridine [32].
Bond angle (◦ ) C2 C1 C6 C2 C1 O10 C6 C1 O10 C1 C2 C3 C1 C2 F12 C3 C2 F12 C2 C3 C4 C2 C3 F13 C4 C3 F13 C3 C4 C5 C3 C4 H7 C5 C4 H7 C4 C5 C6 C4 C5 H8 C6 C5 H8 C1 C6 C5 C1 C6 H9 C5 C6 H9 C1 O10 H11 a
1.381 1.386 1.341 1.375 1.329 1.375 1.318 1.386 1.073 1.382 1.075 1.074 0.943 118.9 121.3 119.7 120.8 118.6 120.5 120.9 118.6 120.6 118.3 119.4 122.2 121.3 119.3 119.4 119.7 118.7 121.6 111.2
B3LYP/ 6-311 + G(d,p) 1.394 1.394 1.360 1.387 1.356 1.385 1.346 1.395 1.082 1.392 1.083 1.083 0.965 118.9 121.2 119.9 120.9 118.3 120.8 120.7 118.6 120.7 118.5 119.3 122.2 121.3 119.3 119.4 119.8 118.6 121.6 109.5
Experimentala 2-6-dichloro phenol 1.410 1.370 1.380 1.380 – 1.410 – 1.420 – 1.400 – – – 118.0 118.0 124.0 121.0 – – 119.0 – – 120.0 – – 117.0 – – 123.0 – – –
The X-ray data from Ref. [29].
4.2. Vibrational assignments According to the theoretical calculations, 2,3-DFP has a planar structure of Cs point group symmetry. The molecule under investigation has 13 atoms and 33 normal modes of fundamental vibrations which span the irreduciable representations: 23A + 10A . All the 33 fundamental vibrations are active in both IR and Raman. The harmonic–vibrational frequencies calculated and observed FT-IR and FT-Raman frequencies for 2,3-DFP at HF and B3LYP levels using the triple split valence basis set along with diffuse and polarization functions, 6-311 + G(d,p) have been collected in Table 2. Comparison of the frequencies calculated at HF and 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 6-311 + G(d,p). Any way not withstanding the level of calculations, it is customary to scale down the calculated har-
monic frequencies in order to improve the agreement with the experiment. Vibrational frequencies calculated at B3LYP level were scaled by 0.96 [33], and those calculated at HF level were scaled by 0.89 [34]. The stick spectra of 2,3-DFP at HF and B3LYP levels using 6-311 + G(d,p) have been shown in Fig. 4. 4.3. OH vibrations The OH group gives rise to three vibrations stretching, in-plane bending and out-of-plane bending vibrations. In 2,6dichloro-6-nitrophenol [35], the strong and broad band observed at 3396 cm−1 in the solid phase infrared spectrum and the sharp and strong band observed at 3528 cm−1 in the solution spectrum are taken to represent the OH stretching vibration. Similarly, in our case also a strong and broad FT-IR band observed at 3372 cm−1 is assigned to OH stretching vibration. 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 [36]. A comparison of these band with literature data predict that there is negative deviation of ∼285 cm−1 may be due to fact that the presence of strong intramolecular hydrogen bonding. However,
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Table 2 Experimental, HF and B3LYP levels computed vibrational frequencies (cm−1 ) obtained for 2,3-difluoro phenol Experimentala FT-IR
HF/6-311 + G(d,p) FT-Raman
1250 m 1331 s 1279 s 1182 s 1150 m 1015 vs 867 w 815 m 1055 m 701 s
Computed
Corrected
4161 3370 3363 3342 1810 1804 1682 1643
3703 2999 2993 2974 1611 1605 1497 1462
3805 3212 3204 3187 1659 1647 1536 1515
3653 3083 3076 3060 1592 1581 1475 1455
A A A A A A A A
O C C C C C C C
1474 1415
1312 1259
1381 1338
1326 1284
A A
C O+ O H C C+ C O
1376 1313 1290 1197 1148 1121 1091 986 889 872
1225 1169 1148 1065 1021 997 971 877 792 776
1287 1246 1186 1178 1074 1034 970 880 828 777
1235 1196 1138 1131 1031 993 931 845 795 746
A A A A A A A A A A
 O H+ C H C F+ C O C F+ O H+ C H  C H+ O H C H  C H+ C F ␥C H ␥C H CCC trigonal bending ␥C F
785 754
699 671
699 694
671 666
A A
Ring breathing ␥ CCC
521 m
661 628 591
588 559 526
586 581 542
563 557 520
A A A
 CCC  CCC  CCC
506 m
550
489
510
490
A
␥ CCC
472 382 293 287 272 240 149
A
␥ CCC ␥O H  C O+ C F C F ␥C F ␥ CCC ␥C O
3040 w 1624 w
1310 m
1332 m 1280 w 1175 w 1155 w
825 m 1057 m 699 vs
914 w 572 w
505 w 492 w
577 m
491 w 356 w 314 m 292 w 272 m 170 vw
a b
Vibrational assignmentsb
Corrected
3089 s
1305 s
Species
Computed
3372 s, br 3062 m 3040 m 1625 m 1567 w 1526 vs 1478 vs
B3LYP/6-311 + G(d,p)
530 361 338 333 310 276 172
471 321 301 296 276 246 153
492 398 306 299 284 250 155
A A A A A A
H H H H C C C C
s: strong; vs: very strong; m: medium; w: weak; vw: very weak; sh: shoulder; br: broad. : stretching; s : symmetry stretching; as : asymmetry stretching; : in-plane-bending; ␥: out-of-plane bending; : wagging; : rocking; t: twisting; : torsion.
the calculated value by B3LYP/6-311 + G(d,p) level shows at 3653 cm−1 . The frequency due to OH in-plane bending vibration in phenols, in general, lies in the region 1150–1250 cm−1 and is not much affected due to hydrogen bonding unlike the stretching and out-of-plane bending frequencies. In almost all 1,2,3,5-tetrasubstituted benzene derivatives with one OH and two halogen substitutents, this vibration was found in a narrow region 1225–1252 cm−1 [36]. The medium strong FT-IR band at 1250 cm−1 is attributed to this vibration. The theoretically computed value at 1235 cm−1 by B3LYP/6-311 + G(d,p) method is exactly coincides with the experimental observations. The OH out-of-plane bending mode for the free molecule lies below 300 cm−1 and hence it is beyond the infrared spectral range of the present investigation. However, for the associated molecule [36] the OH out-of-plane bending mode lies in the region 517–710 cm−1 . In both intermolecular and intramolecular associations, the frequency is at a higher value than in
free OH. In the IR spectrum of solid p-nitrophenol this band is shifted to higher wavenumber (668 cm−1 ) due to intermolecular hydrogen bonding [37]. In our title molecule the FT-IR band at 701 cm−1 is assigned to OH out-of-plane bending vibration. 4.4. C O vibration The C O stretching vibrations of the light substituents (OH, NO2 ) modes in molecule 2,6-dichloro-4-nitrophenol [34] lie in the region 1095–1310 cm−1 . In our study the C O stretching vibration in 2,3-DFP has a main contribution in this mode with B3LYP/6-311 + G(d,p) predicted frequency of 1326 cm−1 (Table 2). This is in excellent agreement with very strong FT-IR band at 1305 cm−1 and medium FT-Raman band at 1310 cm−1 . The in-plane C O bending vibrational mode with the theoretical frequency of 293 cm−1 also shows excellent agreement with experimental FT-Raman frequency at 314 cm−1 . The above conclusions are in agreement with literature value [37].
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these vibrations are relatively at a higher frequency due to its coupling with the in-plane OH bending mode. The C C aromatic stretch, known as semicircle stretching, predicted at 1581 cm−1 is in excellent agreement with experimental observations of both in FT-IR and FT-Raman spectra. In the benzene, the fundamental (992 cm−1 ) and (1010 cm−1 ) represent the ring breathing mode and carbon trigonal mode. Under the Cs point group both the vibrations will have the same symmetry species A . As the energies of these vibrations are very close, there is an appreciable interaction between these vibrations and consequently their energies will be modified [43]. The ring breathing and trigonal bending modes of 2,3-DFP are assigned to 914 and 1055 cm−1 , respectively. However, the theoretically computed value at 671 and 795 cm−1 by B3LYP/6-311 + G(d,p) method does not coincides with experimental observations. Both the computed values shows negative deviations of ∼243 and ∼260 cm−1 , respectively, for ring breathing and trigonal bending modes. The theoretically calculated C C C out-of-plane and in-plane bending modes have been found to be consistent with the recorded spectral values. 4.7. C F stretching modes
Fig. 4. Comparison of corrected frequencies in cm−1 normalised IR intensities at each level of calculations considered.
4.5. C H vibrations The aromatic C H stretching vibrations are in general observed in the region 3000–3100 cm−1 and in the present study also absorptions in this region are attributed to the C H stretching vibrations. The expected three C H stretching vibrations correspond to the scaled vibration stretching modes of C4 H, C5 H and C6 H units. The vibrations assigned to aromatic C H stretch in the region 3060–3083 cm−1 [38] are in agreement with experimental assignment 3040–3089 cm−1 [39]. The three in-plane C H bending vibrations appear in the range 1000–1300 cm−1 in the substituted benzenes and the three out-of-plane bending vibrations occurs in the frequency range 750–1000 cm−1 [40]. The C H in-plane bending vibrations assigned in the region 1031–1235 cm−1 , even though found to be contaminated by OH in-plane bending are in the range found in literatures [41,42], while the experimental observations are at 1015–1182 cm−1 . The calculated frequencies 746–931 cm−1 for the C H out-of-plane bending falls in the FT-IR values of 815–867 cm−1 . 4.6. C C stretching vibrations The degenerate C C stretching modes of benzene are observable in the region 1365–1620 cm−1 . The non-degenerate Kekule vibration, lies in the region 1205–1280 cm−1 . But in phenols
Infrared spectra of a number of mono- and di-substituted fluorine derivatives have been studied by Narasimham et al. [44] and those of tri- and tetrafluorobenzene by Ferguson et al. [45]. They have assigned the frequency 1250 cm−1 to C F stretching mode of vibration. In analogy to these assignments, infrared frequency observed at 1235 cm−1 , which is strong in intensity, is assigned as C F stretching frequency for 1-fluoro-2,4-dinitro benzene [46], corresponding Raman frequency for the same mode is 1246 cm−1 . In the present investigation, we have assigned the strong bands at 1279 and 1331 cm−1 in FT-IR spectrum due to C F stretching mode. Their counter part in Raman spectrum is at 1280 and 1332 cm−1 . The C F in-plane bending frequency appears in the region 250–350 cm−1 [47]. In the present case, a band at 292 cm−1 in FT-Raman is assigned to C F in-plane bending mode. 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 2,3-difluoro phenol Parameters
HF/6-311 + G(d,p)
B3LYP/6-311 + G(d,p)
Total energy Zero-point energy
−503.3932635 59.52
−506.0886401 55.38
2.3597 1.8334 1.0318
2.3084 1.7930 1.0092
Rotational constants
Entropy Total Translational Rotational Vibrational
81.643 40.501 28.667 12.476
83.362 40.501 28.733 14.128
Dipole moment
1.605
1.479
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5. Other molecular properties Several calculated thermodynamic parameters are presented in Table 3. Scale factors have been recommended [48] 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 changes in the total entropy of 2,3-DFP at room temperature at different methods are also presented.
[17] [18] [19] [20] [21] [22] [23]
6. Conclusions The FT-IR and FT-Raman spectra have been recorded and the detailed vibrational assignment is presented for 2,3-DFP for the first time. The equilibrium geometries, harmonic vibrational frequencies and IR spectra of 2,3-DFP were determined and analysed both at HF and DFT B3LYP/6-311 + G(d,p) levels of theory. The difference between the corresponding wavenumbers (observed and calculated) is very small, for most of fundamentals. Therefore, the results presented in this work for 2,3-DFP indicate that this level of theory is reliable for prediction of both infrared and Raman spectra of the title compound. The optimized geometry parameters calculated at B3LYP/6-311 + G(d,p) are slightly larger than those calculated at HF/6-311 + G(d,p) level and the B3LYP calculated values coincides well compared with the available X-ray data on the whole. References [1] B.S. Bhal, A. Bhal, Advanced Organic Chemistry, fourth ed., S. Chand and Company, 1995. [2] W. Gerhavtz (Ed.), Ullmann’s Encyclopedia of Industrial Chemistry, vol. A19, fifth ed., VCH Publishers, USA, 1986. [3] W. Gerhavtz (Ed.), Ullmann’s Encyclopedia of Industrial Chemistry, vol. A11, fifth ed., VCH Publishers, USA, 1986. [4] G. Varsanyi, Assignments for Vibrational Spectra of Seven Hundred Benzene Derivatives, vols. 1 and 2, Adam Hilger, 1974. [5] J.C. Evans, Spectrochim. Acta A 16 (1960) 1382. [6] H.D. Bist, J.C.D. Brand, D.R. Williams, J. Mol. Spectrosc. 24 (1967) 402. [7] J.H.S. Green, D.J. Harrison, W. Kynaston, Spectrochim. Acta A 27 (1971) 2199. [8] S.L. Srivastava, Ind. J. Pure Appl. Phys. 8 (1970) 237. [9] T.S. Varadarajan, S. Pardhasaradhy, Ind. J. Pure Appl. Phys. 9 (1971) 401. [10] J.H.S. Green, D.J. Harrison, W. Kynaston, Spectrochim. Acta A 28 (1972) 33. [11] N.K. Sanyal, A.M. Pandey, Ind. J. Pure Appl. Phys. 11 (1973) 913. [12] S.M. Pandey, S.J. Singh, Ind. J. Phys. 48 (1974) 961. [13] P.K. Mallik, Ind. J. Phys. 48 (1974) 1089. [14] G.N.R. Tripathi, S. Sitaram, Ind. J. Pure Appl. Phys. 12 (1974) 529. [15] J.A. Faniran, H.F. Shurwell, J. Raman Spectrosc. 9 (1980) 73. [16] J.H.S. Green, D.J. Harrison, C.P. Stockley, J. Mol. Struct. 33 (1976) 307.
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FT-IR, FT-Raman spectra and ab initio HF, DFT vibrational analysis of 2,3-difluoro phenol N. Sundaraganesan ∗ , B. Anand, C. Meganathan, B. Dominic Joshua Department of Physics (Engineering), Annamalai University, Annamalai Nagar 608 002, India Received 8 July 2006; received in revised form 12 December 2006; accepted 16 December 2006
Abstract The FT-IR and FT-Raman spectra of 2,3-difluoro phenol (2,3-DFP) has been recorded in the region 4000–400 and 4000–100 cm−1 , respectively. The optimized geometry, frequency and intensity of the vibrational bands of 2,3-DFP were obtained by the ab initio HF and density functional theory (DFT) levels of theory with complete relaxation in the potential energy surface using 6-311 + G(d,p) basis set. The harmonic vibrational frequencies were calculated and the scaled values have been compared with experimental FT-IR and FT-Raman spectra. The observed and the calculated frequencies are found to be in good agreement. The experimental spectra also coincide satisfactorily with those of theoretically constructed bar type spectrograms. © 2006 Elsevier B.V. All rights reserved. Keywords: FT-IR and FT-Raman spectra: ab initio HF and DFT; 2,3-Difluoro phenol; Vibrational analysis
1. Introduction Phenols are organic compounds that contain a hydroxyl group ( OH) bound directly to a carbon atom in the benzene ring. Unlike normal alcohols, phenols are acidic because of the influence of the aromatic ring. Phenols are made by fusing a sulphonic acid with sodium hydroxide to form the sodium salt of the phenol. The free phenol is liberated by adding sulphuric acid. It is used as anti-bacterial and anti-septic and also for the treatment of surgical instrument and bandaging materials [1]. Halogination of phenol derivatives produces active substances with substantially high anti-microbial effect. The degree of dissociation of haloginated phenol derivatives increases with the number of halogen atoms. The combination of alkylation and halogenations in particular, the later produces phenolic microbiocides which are used as active disinfectants and for the preservation of materials. Chlorophenol is an important group of industrial chemical which got variety of uses ranging from preparation of preservatives to insecticides. But they show high oral toxicity and were almost not biologically degradable [2]. However, with few exceptions, organic fluorine compounds which are physiologically inert, display insignificant toxicity.
∗
Corresponding author. Tel.: +91 413222 1847. 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.2006.12.028
This is a consequence of the chemical stability of the C F bond and the increased stability of hydrogen and halogen bonds attached to a fluorinated carbon atom [3]. Low toxicity is an important factor in many applications like agricultural chemicals, pharmaceuticals, biocides and dyes. Due to low toxicity and wide ranging applications of fluorine derivatives of phenols, we have undertaken a thorough vibrational analysis for 2,3-difluoro phenol along with ab initio HF and density functional theory (DFT) calculations. The vibrational spectrum of phenol was extensively studied and analyzed [4–6]. Among all the halogenated phenols having the same halogen substituents, vibrational assignments for almost all the chlorinated phenols, such as isomers of monochloro–dichloro- and trichlorophenols and pentachlorophenols were reported by various earlier investigators [4,7–17]. The vibrational spectrum of 2,4,6-trichlorophenol has been reported by Faniran and Shurwell [18]. However, the force field calculations for this molecule have not been reported so far. Various spectroscopic studies of chloro and methyl phenols have been reported in literatures [19–21]. Recently, Singh and Rai have studied the infrared and the electronic absorption spectra of 4-chloro-2-methyl, 4-chloro-3-methyl and 6-chloro3-methyl phenols. A complete vibrational assignment of phenol and phenol-OD has been given by Evans [5]. The vibrational spectra of p-cresol and its deuterated derivatives have been studied by Jakobsen [22], who gave detailed interpretations of the
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vibrational bands. The assignment of the vibrational frequencies for substituted phenols becomes complicated problem because of the superposition of perhaps several vibrations due to fundamentals and due to substituents. However, a comparison of the spectra with that of the parent compound gives some definite clues about the nature of the molecular vibrations. To the best of our knowledge, neither quantum chemical calculations, nor the vibrational spectra of 2,3-DFP have been reported, as yet. Therefore, the present investigation was undertaken to study the vibrational spectra of the molecule completely and to identify the various normal modes with greater wavenumbers accurately. Ab initio HF and density functional theory calculations have been performed to support our wave number assignments. 2. Experimental The compound 2,3-DFP in the solid form was purchased from the Sigma–Aldrich Chemical Company (USA) with a stated purity of greater than 98% and it was used as such without further purification. The FT-Raman spectrum of 2,3-DFP 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 66 V spectrophotometer equipped with FRA 106 FT-Raman module accessory. The FT-IR spectrum of this com-
Fig. 1. FTIR spectrum of 2,3-difluoro phenol.
pound was recorded in the region 400–4000 cm−1 on IFS 66 V 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 Regional Sophisticated Instrumentation Centre (RSIC), IIT Chennai. 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 [23] program package, invoking gradient geometry optimization [24]. Initial geometry generated from standard geometrical parameters was minimized without any constraint in the potential energy surface at Hartree–Fock and B3LYP level, adopting the standard 6-311 + G(d,p) basis set. 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 2,3-DFP were used for harmonic vibrational frequency calculations resulting in IR and Raman frequencies together with intensities and Raman depolarization ratios. We have used the density functional theory [25] with the three-parameter hybrid functional (B3) [26] for the exchange part and the Lee–Yang–Parr (LYP) correlation function [27], 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 [28] with symmetry considerations, vibrational frequency assignments were made with a high degree of accuracy. 4. Results and discussion 4.1. Geometrical structure
Fig. 2. FT-Raman spectrum of 2,3-difluoro phenol.
The atom numbering scheme for the title molecule is given in Fig. 3. The optimized bond lengths and angles for 2,3-difluoro phenol at the HF/6-311 + G(d,p) and B3LYP/6-311 + G(d,p) levels are presented in Table 1 along with avilable X-ray data, viz., 2,6-dichloro phenol [29]. The structure of 2.3-difluoro phenol in the gas phase is unknown, whereas in 2,6-dichloro phenol crystal the geometry is probably distorted due to hydrogen bonding [29]. From the data shown in Table 1, it is seen that the both HF and B3LYP levels of theory in general slightly underestimates some bond lengths and overestimates few bond lengths, bond angles but B3LYP/6-311 + G(d,p) method yields bond length and bond angles in excellent agreement with X-ray data.
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Table 1 Geometrical parameters optimized in 2,3-difluoro phenol Parameter
HF/6-311 + G(d,p)
˚ Bond length (A) C1 C2 C1 C6 C1 O10 C2 C3 C2 F12 C3 C4 C3 F13 C4 C5 C4 H7 C5 C6 C5 H8 C6 H9 O10 H11
Fig. 3. Numbering system adopted in this study (2,3-difluoro phenol).
The various bond lengths are found to be almost the same at HF/6-311 + G(d,p) and B3LYP/6-311 + G(d,p) (Table 1). The ˚ is just 0.02 A ˚ lower than the C1 O10 bond distance of 1.360 A ˚ for 2,6-dichloro phenol reported experimental value of 1.380 A [29]; however, similar discrepancies were obtained in calculation with MP2 and B3LYP methods [30,31]. The optimized ˚ for HF/6C F bond lengths by two methods are 1.329 A ˚ for B3LYP/6-311 + G(d,p), which are 311 + G(d,p) and 1.356 A ˚ for in good agreement with those of reported values of 1.328 A 3,5-dichloro-2,4,6-trifluoropyridine [32].
Bond angle (◦ ) C2 C1 C6 C2 C1 O10 C6 C1 O10 C1 C2 C3 C1 C2 F12 C3 C2 F12 C2 C3 C4 C2 C3 F13 C4 C3 F13 C3 C4 C5 C3 C4 H7 C5 C4 H7 C4 C5 C6 C4 C5 H8 C6 C5 H8 C1 C6 C5 C1 C6 H9 C5 C6 H9 C1 O10 H11 a
1.381 1.386 1.341 1.375 1.329 1.375 1.318 1.386 1.073 1.382 1.075 1.074 0.943 118.9 121.3 119.7 120.8 118.6 120.5 120.9 118.6 120.6 118.3 119.4 122.2 121.3 119.3 119.4 119.7 118.7 121.6 111.2
B3LYP/ 6-311 + G(d,p) 1.394 1.394 1.360 1.387 1.356 1.385 1.346 1.395 1.082 1.392 1.083 1.083 0.965 118.9 121.2 119.9 120.9 118.3 120.8 120.7 118.6 120.7 118.5 119.3 122.2 121.3 119.3 119.4 119.8 118.6 121.6 109.5
Experimentala 2-6-dichloro phenol 1.410 1.370 1.380 1.380 – 1.410 – 1.420 – 1.400 – – – 118.0 118.0 124.0 121.0 – – 119.0 – – 120.0 – – 117.0 – – 123.0 – – –
The X-ray data from Ref. [29].
4.2. Vibrational assignments According to the theoretical calculations, 2,3-DFP has a planar structure of Cs point group symmetry. The molecule under investigation has 13 atoms and 33 normal modes of fundamental vibrations which span the irreduciable representations: 23A + 10A . All the 33 fundamental vibrations are active in both IR and Raman. The harmonic–vibrational frequencies calculated and observed FT-IR and FT-Raman frequencies for 2,3-DFP at HF and B3LYP levels using the triple split valence basis set along with diffuse and polarization functions, 6-311 + G(d,p) have been collected in Table 2. Comparison of the frequencies calculated at HF and 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 6-311 + G(d,p). Any way not withstanding the level of calculations, it is customary to scale down the calculated har-
monic frequencies in order to improve the agreement with the experiment. Vibrational frequencies calculated at B3LYP level were scaled by 0.96 [33], and those calculated at HF level were scaled by 0.89 [34]. The stick spectra of 2,3-DFP at HF and B3LYP levels using 6-311 + G(d,p) have been shown in Fig. 4. 4.3. OH vibrations The OH group gives rise to three vibrations stretching, in-plane bending and out-of-plane bending vibrations. In 2,6dichloro-6-nitrophenol [35], the strong and broad band observed at 3396 cm−1 in the solid phase infrared spectrum and the sharp and strong band observed at 3528 cm−1 in the solution spectrum are taken to represent the OH stretching vibration. Similarly, in our case also a strong and broad FT-IR band observed at 3372 cm−1 is assigned to OH stretching vibration. 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 [36]. A comparison of these band with literature data predict that there is negative deviation of ∼285 cm−1 may be due to fact that the presence of strong intramolecular hydrogen bonding. However,
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Table 2 Experimental, HF and B3LYP levels computed vibrational frequencies (cm−1 ) obtained for 2,3-difluoro phenol Experimentala FT-IR
HF/6-311 + G(d,p) FT-Raman
1250 m 1331 s 1279 s 1182 s 1150 m 1015 vs 867 w 815 m 1055 m 701 s
Computed
Corrected
4161 3370 3363 3342 1810 1804 1682 1643
3703 2999 2993 2974 1611 1605 1497 1462
3805 3212 3204 3187 1659 1647 1536 1515
3653 3083 3076 3060 1592 1581 1475 1455
A A A A A A A A
O C C C C C C C
1474 1415
1312 1259
1381 1338
1326 1284
A A
C O+ O H C C+ C O
1376 1313 1290 1197 1148 1121 1091 986 889 872
1225 1169 1148 1065 1021 997 971 877 792 776
1287 1246 1186 1178 1074 1034 970 880 828 777
1235 1196 1138 1131 1031 993 931 845 795 746
A A A A A A A A A A
 O H+ C H C F+ C O C F+ O H+ C H  C H+ O H C H  C H+ C F ␥C H ␥C H CCC trigonal bending ␥C F
785 754
699 671
699 694
671 666
A A
Ring breathing ␥ CCC
521 m
661 628 591
588 559 526
586 581 542
563 557 520
A A A
 CCC  CCC  CCC
506 m
550
489
510
490
A
␥ CCC
472 382 293 287 272 240 149
A
␥ CCC ␥O H  C O+ C F C F ␥C F ␥ CCC ␥C O
3040 w 1624 w
1310 m
1332 m 1280 w 1175 w 1155 w
825 m 1057 m 699 vs
914 w 572 w
505 w 492 w
577 m
491 w 356 w 314 m 292 w 272 m 170 vw
a b
Vibrational assignmentsb
Corrected
3089 s
1305 s
Species
Computed
3372 s, br 3062 m 3040 m 1625 m 1567 w 1526 vs 1478 vs
B3LYP/6-311 + G(d,p)
530 361 338 333 310 276 172
471 321 301 296 276 246 153
492 398 306 299 284 250 155
A A A A A A
H H H H C C C C
s: strong; vs: very strong; m: medium; w: weak; vw: very weak; sh: shoulder; br: broad. : stretching; s : symmetry stretching; as : asymmetry stretching; : in-plane-bending; ␥: out-of-plane bending; : wagging; : rocking; t: twisting; : torsion.
the calculated value by B3LYP/6-311 + G(d,p) level shows at 3653 cm−1 . The frequency due to OH in-plane bending vibration in phenols, in general, lies in the region 1150–1250 cm−1 and is not much affected due to hydrogen bonding unlike the stretching and out-of-plane bending frequencies. In almost all 1,2,3,5-tetrasubstituted benzene derivatives with one OH and two halogen substitutents, this vibration was found in a narrow region 1225–1252 cm−1 [36]. The medium strong FT-IR band at 1250 cm−1 is attributed to this vibration. The theoretically computed value at 1235 cm−1 by B3LYP/6-311 + G(d,p) method is exactly coincides with the experimental observations. The OH out-of-plane bending mode for the free molecule lies below 300 cm−1 and hence it is beyond the infrared spectral range of the present investigation. However, for the associated molecule [36] the OH out-of-plane bending mode lies in the region 517–710 cm−1 . In both intermolecular and intramolecular associations, the frequency is at a higher value than in
free OH. In the IR spectrum of solid p-nitrophenol this band is shifted to higher wavenumber (668 cm−1 ) due to intermolecular hydrogen bonding [37]. In our title molecule the FT-IR band at 701 cm−1 is assigned to OH out-of-plane bending vibration. 4.4. C O vibration The C O stretching vibrations of the light substituents (OH, NO2 ) modes in molecule 2,6-dichloro-4-nitrophenol [34] lie in the region 1095–1310 cm−1 . In our study the C O stretching vibration in 2,3-DFP has a main contribution in this mode with B3LYP/6-311 + G(d,p) predicted frequency of 1326 cm−1 (Table 2). This is in excellent agreement with very strong FT-IR band at 1305 cm−1 and medium FT-Raman band at 1310 cm−1 . The in-plane C O bending vibrational mode with the theoretical frequency of 293 cm−1 also shows excellent agreement with experimental FT-Raman frequency at 314 cm−1 . The above conclusions are in agreement with literature value [37].
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these vibrations are relatively at a higher frequency due to its coupling with the in-plane OH bending mode. The C C aromatic stretch, known as semicircle stretching, predicted at 1581 cm−1 is in excellent agreement with experimental observations of both in FT-IR and FT-Raman spectra. In the benzene, the fundamental (992 cm−1 ) and (1010 cm−1 ) represent the ring breathing mode and carbon trigonal mode. Under the Cs point group both the vibrations will have the same symmetry species A . As the energies of these vibrations are very close, there is an appreciable interaction between these vibrations and consequently their energies will be modified [43]. The ring breathing and trigonal bending modes of 2,3-DFP are assigned to 914 and 1055 cm−1 , respectively. However, the theoretically computed value at 671 and 795 cm−1 by B3LYP/6-311 + G(d,p) method does not coincides with experimental observations. Both the computed values shows negative deviations of ∼243 and ∼260 cm−1 , respectively, for ring breathing and trigonal bending modes. The theoretically calculated C C C out-of-plane and in-plane bending modes have been found to be consistent with the recorded spectral values. 4.7. C F stretching modes
Fig. 4. Comparison of corrected frequencies in cm−1 normalised IR intensities at each level of calculations considered.
4.5. C H vibrations The aromatic C H stretching vibrations are in general observed in the region 3000–3100 cm−1 and in the present study also absorptions in this region are attributed to the C H stretching vibrations. The expected three C H stretching vibrations correspond to the scaled vibration stretching modes of C4 H, C5 H and C6 H units. The vibrations assigned to aromatic C H stretch in the region 3060–3083 cm−1 [38] are in agreement with experimental assignment 3040–3089 cm−1 [39]. The three in-plane C H bending vibrations appear in the range 1000–1300 cm−1 in the substituted benzenes and the three out-of-plane bending vibrations occurs in the frequency range 750–1000 cm−1 [40]. The C H in-plane bending vibrations assigned in the region 1031–1235 cm−1 , even though found to be contaminated by OH in-plane bending are in the range found in literatures [41,42], while the experimental observations are at 1015–1182 cm−1 . The calculated frequencies 746–931 cm−1 for the C H out-of-plane bending falls in the FT-IR values of 815–867 cm−1 . 4.6. C C stretching vibrations The degenerate C C stretching modes of benzene are observable in the region 1365–1620 cm−1 . The non-degenerate Kekule vibration, lies in the region 1205–1280 cm−1 . But in phenols
Infrared spectra of a number of mono- and di-substituted fluorine derivatives have been studied by Narasimham et al. [44] and those of tri- and tetrafluorobenzene by Ferguson et al. [45]. They have assigned the frequency 1250 cm−1 to C F stretching mode of vibration. In analogy to these assignments, infrared frequency observed at 1235 cm−1 , which is strong in intensity, is assigned as C F stretching frequency for 1-fluoro-2,4-dinitro benzene [46], corresponding Raman frequency for the same mode is 1246 cm−1 . In the present investigation, we have assigned the strong bands at 1279 and 1331 cm−1 in FT-IR spectrum due to C F stretching mode. Their counter part in Raman spectrum is at 1280 and 1332 cm−1 . The C F in-plane bending frequency appears in the region 250–350 cm−1 [47]. In the present case, a band at 292 cm−1 in FT-Raman is assigned to C F in-plane bending mode. 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 2,3-difluoro phenol Parameters
HF/6-311 + G(d,p)
B3LYP/6-311 + G(d,p)
Total energy Zero-point energy
−503.3932635 59.52
−506.0886401 55.38
2.3597 1.8334 1.0318
2.3084 1.7930 1.0092
Rotational constants
Entropy Total Translational Rotational Vibrational
81.643 40.501 28.667 12.476
83.362 40.501 28.733 14.128
Dipole moment
1.605
1.479
566
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5. Other molecular properties Several calculated thermodynamic parameters are presented in Table 3. Scale factors have been recommended [48] 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 changes in the total entropy of 2,3-DFP at room temperature at different methods are also presented.
[17] [18] [19] [20] [21] [22] [23]
6. Conclusions The FT-IR and FT-Raman spectra have been recorded and the detailed vibrational assignment is presented for 2,3-DFP for the first time. The equilibrium geometries, harmonic vibrational frequencies and IR spectra of 2,3-DFP were determined and analysed both at HF and DFT B3LYP/6-311 + G(d,p) levels of theory. The difference between the corresponding wavenumbers (observed and calculated) is very small, for most of fundamentals. Therefore, the results presented in this work for 2,3-DFP indicate that this level of theory is reliable for prediction of both infrared and Raman spectra of the title compound. The optimized geometry parameters calculated at B3LYP/6-311 + G(d,p) are slightly larger than those calculated at HF/6-311 + G(d,p) level and the B3LYP calculated values coincides well compared with the available X-ray data on the whole. References [1] B.S. Bhal, A. Bhal, Advanced Organic Chemistry, fourth ed., S. Chand and Company, 1995. [2] W. Gerhavtz (Ed.), Ullmann’s Encyclopedia of Industrial Chemistry, vol. A19, fifth ed., VCH Publishers, USA, 1986. [3] W. Gerhavtz (Ed.), Ullmann’s Encyclopedia of Industrial Chemistry, vol. A11, fifth ed., VCH Publishers, USA, 1986. [4] G. Varsanyi, Assignments for Vibrational Spectra of Seven Hundred Benzene Derivatives, vols. 1 and 2, Adam Hilger, 1974. [5] J.C. Evans, Spectrochim. Acta A 16 (1960) 1382. [6] H.D. Bist, J.C.D. Brand, D.R. Williams, J. Mol. Spectrosc. 24 (1967) 402. [7] J.H.S. Green, D.J. Harrison, W. Kynaston, Spectrochim. Acta A 27 (1971) 2199. [8] S.L. Srivastava, Ind. J. Pure Appl. Phys. 8 (1970) 237. [9] T.S. Varadarajan, S. Pardhasaradhy, Ind. J. Pure Appl. Phys. 9 (1971) 401. [10] J.H.S. Green, D.J. Harrison, W. Kynaston, Spectrochim. Acta A 28 (1972) 33. [11] N.K. Sanyal, A.M. Pandey, Ind. J. Pure Appl. Phys. 11 (1973) 913. [12] S.M. Pandey, S.J. Singh, Ind. J. Phys. 48 (1974) 961. [13] P.K. Mallik, Ind. J. Phys. 48 (1974) 1089. [14] G.N.R. Tripathi, S. Sitaram, Ind. J. Pure Appl. Phys. 12 (1974) 529. [15] J.A. Faniran, H.F. Shurwell, J. Raman Spectrosc. 9 (1980) 73. [16] J.H.S. Green, D.J. Harrison, C.P. Stockley, J. Mol. Struct. 33 (1976) 307.
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