2-Bromohydroquinone: Structures, vibrational assignments and RHF, B- and B3-based density functional calculations

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Spectrochimica Acta Part A 69 (2008) 926–932

2-Bromohydroquinone: Structures, vibrational assignments and RHF, B- and B3-based density functional calculations Anuradha Ramoji, Jayashree Yenagi, J. Tonannavar ∗ Department of Physics, Karnatak University, Dharwad 580003, India Received 30 December 2006; received in revised form 19 May 2007; accepted 25 May 2007

Abstract Vibrational spectral measurements, namely, infrared (4000–400 cm−1 ) and Raman (3500–50 cm−1 ) spectra have been made for 2Bromohydroquinone. Optimized geometrical structures, harmonic vibrational frequencies and intensities have been computed by the ab initio (RHF), B-based (BLYP, BP86) and B3-based (B3P86, B3LYP, B3PW91) density functional methods using 6-31G(d) basis set. A complete assignment of the observed spectra has been proposed. Coupling of vibrations has been determined by calculating potential energy distributions (PEDs) at BP86/6-31G(d) level of theory. In the computed equilibrium geometries by all the levels, the bond lengths and bond angles show changes in the neighborhood of Bromine. Similarly, the vibrational spectra exhibit some marked spectral features unlike in hydroquinone and phenol. On the other hand, the infrared spectrum shows a clear evidence of O H· · ·O bonding near 3200 cm−1 as in hydroquinone. Evaluation of the theoretical methods demonstrates that all the levels but the RHF have reproduced frequencies fairly accurately in the 2000–500 cm−1 ; below 500 cm−1 the RHF has performed reasonably well. © 2007 Elsevier B.V. All rights reserved. Keywords: 2-Bromohydroquinone; Infrared; Raman; ab initio/DFT; Assignments

1. Introduction Hydroquinone (HQ) has been a widely studied molecular system because of its diverse commercial and technological applications [1–10]. It is one of the three isomers of the dihydroxybenzene the para-hydroquinone the other two being, ortho-hydroquinone (pyrocatechol) and meta-hydroquinone (resorcinol). There are trans and cis conformers for HQ: the trans conformer is more stable than the cis with the energy difference of 0.12 kcal/mol [3]. The last two decades have seen intense studies on the spectroscopy and dynamics of weakly bound complexes of HQ in both the solid and vapor phases [1–15]. Solid HQ exists in three different polymorphs ␣-HQ, ␤-HQ and ␥-HQ. ␤-HQ is known to form a cage-like structure (clatharate host structure), a property desirable for the organic inclusion compounds [1]. All the three polymorphs of HQ have been found to have trans and cis conformers depending on the associated steric hindrance and weak hydrogen bonding [11,12]. The spectroscopy of HQ monomer and dimer have been

∗

Corresponding author. E-mail address: [email protected] (J. Tonannavar).

1386-1425/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.saa.2007.05.056

investigated by both the theoretical and experimental methods [8]. Of the HQ derivatives, recently, the intramolecular hydrogen bonding in tetrafluorohydroquinone has been studied from gas-phase electron diffraction [16]. 2-Bromohydroquinone is another derivative of HQ, used as a building block for the synthesis of biologically active marine metabolites; it is also used in nonlinear optics as a monomer [12–18]. In the present work we report vibrational assignments for 2-Bromohydroquinone based on IR and Raman spectral measurements, aided by the electronic structure calculations at ab initio (RHF), B-based (BLYP, BP86,) and B3-based (B3P86, B3LYP, B3PW91) density functionals with 6-31G(d) levels of theory. In the assignments, coupling of vibrations has been determined by calculating potential energy distributions (PEDs) at BP86/6-31G(d) level of theory. Further, equilibrium structures for the trans structure of 2-Bromohydroquinone have been calculated at all levels. Among all the levels, B3-based DFT levels have reproduced reasonably good geometries as compared to experiment. The vibrational spectra show features different from the spectra in Hydroquinone and phenol: the ring stretching and in-plane vibrational frequencies are increased; and C O stretching frequencies are decreased by about 30 cm−1 . The O H· · ·O bonding has been observed near 3200 cm−1 as

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in hydroquinone; the downshifted O H frequencies have not been accounted for owing to the non-inclusion of the bonding effect in the theoretical levels. Of all the theoretical levels, the BP86/6-31G(d) spectrum agrees well with the experimental spectrum. 2. Experimental details The solid 2-Bromohydroquinone was obtained from Aldrich Chemical Co. and used as received. The FT-Infrared spectrum of the sample in KBr was measured on Nicolet 5700 spectrometer. The spectrometer had a ETC Ever-GloTM mid IR source, Thermo electric cooled Deuterated Lanthanum Triglycine Sulfate detector (DLaTGS) and a Ge-on-KBr beam splitter. The signals were collected for 100 scans with a scan interval of 1 cm−1 and at optical resolution of 0.4 cm−1 . The FT-Raman spectral measurements were made on Bruker RFS 100/S FTRaman spectrometer with diode-pumped air-cooled cw Nd-YAG laser source giving 1064 nm as exciting line at 75 mW power. The interferometer consisted of a quartz beam splitter and a liquid nitrogen pre-cooled high sensitivity Ge-diode as the detector. Holographic notch filters were used to suppress the Rayleigh scattering. The sample was packed in a glass tube of about 5 mm diameter and excited in the 180◦ geometry. The spectrum was measured with a total of 300 scans at the interval of 4 cm−1 and optical resolution of 0.1 cm−1 . 3. Computational details All the ab initio and density functional theory (DFT) calculations were performed on 2-Bromohydroquinone using the Gaussian 03W suite of programs [19]. Despite its limitation with respect to the neglect of electron correlation, the RHF method is reasonably good at computing geometries and vibrational frequencies of stable molecules. On the other hand, a variety of functionals are available that take into account of exchange and correlation factors in a satisfactory way. The BLYP and BP86 are gradient corrected functionals; the B3P86, B3LYP, B3PW91 are hybrid functionals. By combining these methods with 6-31G(d) basis set, we computed equilibrium geometries and harmonic vibrational spectra. The potential energy distributions (PEDs) were also calculated from BP86/6-31G(d) results [20]. 4. Results and discussions 4.1. Geometry optimization The molecular structure is shown in Fig. 1. Geometry optimization was performed on the compound yielding the CS symmetry. Table 1 presents the optimized parameters calculated at all the six levels of theory. The geometrical parameters of HQ and Bromobenzene deduced from the microwave spectroscopy studies [2,21] are listed for comparison. From the Table 1, it is apparent that all the levels are found to predict the C H bond lengths longer than the experimental values except by the RHF method. The B3-based DFT methods are found to predict

Fig. 1. Molecular structure of 2-Bromohydroquinone.

more accurate values. For each level of calculation, the bond lengths C3-H11, C6-H9 and C5-H12 and their corresponding errors are found to be in the increasing order, which may be attributed to the heavy bromine substitution. The O H bond lengths are longer than the experimental values due to all the methods except the RHF. Larger magnitudes of errors for the length O7-H13 than those for O8-H14 are again indicative of the effect of bromine substitution. Both C1 O7 and C4 O8 bond lengths are predicted to be longer and shorter, respectively by the BLYP while they are shorter as for the remaining levels. Again the relative error for the bond length, C4 O8, which is closer to Br atom, is higher. Here the performance of B-based DFT methods is observed to be better. As HQ has higher symmetry, C1 C2, C2 C3, C3 C4 are exactly equal to the equivalent bonds C4 C5, C5 C6 and C1 C6, respectively [2]. When Br is substituted, the B- and B3-based DFT methods predict the bond lengths C1 C2 and C2 C3 longer and shorter, respectively than their equivalent bond lengths C4 C5 and C5 C6, while the bond lengths C4 C3 and C1 C6 hardly show any change. Again B3-based methods yield accurate ring bond lengths. The bond lengths C1 C2 and C2 C3 linked to C Br bond show more deviations than the bond length C4 C3. This is reflected also in their respective equivalent bonds. Again as for the RHF method, all the C C bond lengths are shorter than the experimental values. The predicted C Br bond length values due to all the methods are higher than the experimental value of Bromobenzene [21]. The B- and B3-based DFT methods calculate smaller value of COH angle near Br (C1 O7 H13), whereas the B3-based DFT methods and RHF compute higher value of the other COH angle (C4 O8 H14). Comparison of relative errors again show that the B3-based DFT methods have performed better. The two CCH angles (C2 C3 H11 and C4 C3 H11) formed between the ring and the C3 H11 bond are greater than the CCH angles (C5 C6 H9 and C1 C6 H9) formed by its equivalent C6 H9 bond. The angle O7 C1 C2 (near Br) is computed to be greater than O8 C4 C5 angle. The trend is identical for all the six levels. The O7 C1 C2 angles are observed to have more deviations. The C1 C2 C3 angles are calculated to be longer by all the methods. Also they are larger than the angle C4 C5 C6 predicted by all the methods. The angle C2 C3 C4 is smaller than the equivalent angle C1 C6 C5; the angle C2 C1 C6 is

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Table 1 Comparison of optimized parameters of 2-Bromohydroquinone Bond lengths (A◦ )

Expta

B3P86

B3PW91

BP86

BLYP

B3LYP

RHF

(O8 (O8 (C4 (C4 (C3 (C3 (C2 (C2 (C1 (C1 (C6 (C6 (C5 (O7

1.377 0.960 1.395 1.394 1.394 1.080 1.394 1.867b 1.395 1.377 1.394 1.080 1.077 0.960

1.363 0.967 1.393 1.396 1.387 1.084 1.399 1.901 1.394 1.354 1.389 1.085 1.088 0.974

1.364 0.967 1.395 1.397 1.388 1.084 1.400 1.904 1.396 1.355 1.390 1.086 1.089 0.973

1.377 0.980 1.407 1.408 1.397 1.093 1.412 1.924 1.407 1.367 1.400 1.094 1.097 0.988

1.385 0.981 1.409 1.410 1.399 1.091 1.413 1.944 1.409 1.374 1.402 1.093 1.095 0.987

1.370 0.969 1.397 1.399 1.390 1.084 1.402 1.920 1.398 1.361 1.392 1.085 1.088 0.974

1.355 0.947 1.382 1.387 1.383 1.072 1.386 1.903 1.387 1.347 1.381 1.074 1.077 0.949

C4) H14) C3) C5) C2) H11) C1) Br10) C6) O7) C5) H9) H12) H13)

Bond angles (◦ )

Expta

BP386

B3PW91

BP86

BLYP

B3LYP

RHF

(C4 (O8 (O8 (C3 (C4 (C4 (C2 (C3 (C3 (C1 (C2 (C2 (C6 (C1 (C1 (C5 (C4 (C4 (C6 (C1

108.4 – 123.0 120.1 120.0 120.3 – 119.9 121.3b 121.3b 120.1 123.0 – 120.0 120.3 – 119.9 119.0 – 108.4

108.84 117.28 123.17 119.55 119.12 119.85 121.04 122.36 119.76 117.88 117.56 123.78 118.67 120.97 118.20 120.83 120.45 120.06 119.50 108.00

108.83 117.28 123.20 119.52 119.17 119.82 121.01 122.32 119.69 117.99 117.55 123.84 118.61 121.00 118.21 120.79 120.44 120.09 119.47 108.04

107.96 117.08 123.31 119.62 118.91 119.93 121.15 122.62 119.88 117.50 117.41 123.78 118.81 120.97 118.22 120.81 120.47 120.05 119.49 106.91

108.10 117.00 123.26 119.74 118.88 119.88 121.24 122.54 119.68 117.77 117.52 123.99 118.49 120.94 118.23 120.83 120.37 120.03 119.60 107.25

108.89 117.21 123.15 119.64 119.08 119.82 121.10 122.34 119.58 118.08 117.61 123.97 118.42 120.95 118.21 120.84 120.38 120.03 119.59 108.21

110.77 117.52 123.03 119.45 119.50 119.72 120.78 121.91 118.90 119.19 117.76 124.37 117.86 121.03 118.23 120.74 120.34 120.15 119.51 110.88

a b

O8 C4 C4 C4 C3 C3 C3 C2 C2 C2 C1 C1 C1 C6 C6 C6 C5 C5 C5 O7

H14) C3) C5) C5) C2) H11) H11) C1) Br10) Br10) C6) O7) O7) C5) H9) H9) C6) H12) H12) H13)

Ref. [2]. Ref. [21].

shorter than its experimental value and also than its equivalent angle C3 C4 C5 predicted at all the six levels. Again the CCC angles near Br (C2 C1 C6 and C3 C2 C1) show large deviations; similarly, the calculated C3 C2 Br10 angle is less than the experimental value. 4.2. Vibrational analysis 2-BHQ has 25 in-plane and 11 out-plane-modes; all the modes are both IR and Raman active. The experimental IR and calculated spectra are shown in Fig. 2; the experimental Raman spectrum is shown in Fig. 3. Table 2 presents the observed and calculated frequencies with assignments. The calculated frequencies have been scaled using appropriate scaling factors for all the methods [22]. 4.2.1. C–H vibrations 4.2.1.1. Stretching vibrations. Three medium strong IR bands observed at 3043, 3061 and 3112 cm−1 are assigned to C H

Fig. 2. (a) Experimental and (b) computed infrared spectra with BP86/6-31G(d) method for 2-Bromohydroquinone.

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Table 2 Experimental and calculated vibrational frequencies (cm−1 ) for 2-Bromohydroquinone IR

Raman

B3LYP

B3P86

BLYP

B3PW91

BP86

RHF

Assignments (% PED)

3259 vs 3209 vs 3112 ms 3061 ms 3043 ms 1618 m 1597 s 1519 s 1454 vs 1372 s 1275 s 1230 vs 1201 vs 1136 w 1124 m 1037 s 931 w 892 s 876 m 856 s 818 s 807 s 774 vs 699 w 679 w 572 m 542 w 526 w 449 w – – – – – – –

– – – 3066 m – 1624 m 1603 m – 1453 vw 1345 vw 1278 s 1235 w 1209 w – 1130 w 1043 w – 900 s – – 816 w – 780 m 704 w 685 s 583 w 542 w 492 m – 409 w 381 m 314 w 275 w 202 w 179 vs 116 vs

3613 3528 3114 3095 3056 1612 1583 1488 1440 1340 1302 1270 1224 1177 1162 1110 1004 891 868 882 818 787 707 688 588 516 475 455 410 386 375 316 268 197 181 139

3631 3524 3108 3091 3052 1622 1589 1493 1438 1348 1309 1278 1225 1173 1157 1105 1002 885 875 872 810 787 704 684 584 514 471 453 419 385 374 320 270 197 180 139

3577 3468 3138 3116 3075 1597 1572 1482 1442 1357 1309 1262 1225 1186 1168 1114 1001 936 919 891 827 805 713 706 597 529 486 469 443 394 379 345 271 202 184 142

3634 3533 3109 3092 3053 1621 1587 1492 1438 1346 1308 1277 1225 1173 1159 1107 1003 887 874 873 811 786 704 683 584 513 471 452 416 385 374 319 269 197 180 138

3600 3461 3129 3110 3069 1610 1580 1488 1441 1371 1315 1275 1228 1184 1163 1110 999 921 921 874 813 803 706 699 590 526 481 473 445 392 377 348 274 202 182 141

3689 3662 3058 3042 3004 1637 1608 1502 1446 1318 1280 1251 1213 1181 1115 1059 1015 954 900 871 831 763 717 669 588 504 463 455 377 377 306 261 221 192 179 138

␯ OH (100) ␯ OH (100) ␯ CH (99) ␯ CH (96) ␯ CH (96) ␯ CC (55), ␯ CO (5) ␯ CC (63), ␤ OH (14) ␯ CC (32), ␯ CO (26), ␤CH (6) ␤ OH (31), ␯ CC (24), ␤ CH (6) ␯ CC (62), ␤ OH (17) ␯ CC (38), ␤ OH (26), ␯ CO (22), ␤ CH (7) ␯ CO (47), ␯ CC (12), ␤ OH (8), ␤ CH (5) ␯ CO (35), ␤ OH (17), ␤ CH (13) ␤ OH (36), ␤ CH (21), ␯ CC (14) ␤ OH (43), ␤ CH (25), ␯ CO (11), ␯ CC (9) ␤ CH (56), ␯ CC (18) ␯ CC (18), ␤ CH (17), ␯ CBr (10) ␯ CC (29), ␯ CO (24), ␯ CBr (16) ␥ CH (89) ␥ CH (86) ␥ CH (81) ␯ CO (37), ␤ CC (19), ␯ CC (6) ␥ CC (48), ␥ CH (8) ␤ CC (51), ␯ CBr (18) ␥ CH (37), ␥ CC (20) ␤ CO (58), ␤ CBr (8), ␯ CB (7) ␤ CC (29), ␤ CO (19), ␯ CBr (8), ␯ CO (6) ␥ OH (86) ␥ OH (55), ␥ CC (14) ␤ CO (53), ␯ CBr (17) ␥ CO (56), ␥ CH (6), ␥ OH (97) ␯ CBr (54), ␤ CO (10), ␤ CC (7) ␤ CBr (80) ␥ CBr (41), wag CO (30) ␥ CO (28), ␥ CC (23), ␥ CBr (6)

vs, Very strong; s, strong; ms, medium strong; m, medium; w, weak; vw, very weak; ␯, stretching; ␤, in-plane deformation; ␥, out-of-plane deformation; wag, wagging.

Fig. 3. Experimental FT-Raman spectrum of 2-Bromohydroquinone.

stretching vibrations as in HQ. These bands have appeared as shoulders to the broad and intense band due to the O H· · ·O bonding of which more will be discussed later. A medium strong Raman band corresponding to the second IR band is observed at 3066 cm−1 . The respective PEDs clearly show that the normal modes are pure C H stretching vibrations. When compared with the computed frequencies, the RHF underestimates (3004, 3042 and 3058 cm−1 ) all the three frequencies with error in the range 0.7–2%. The BP86 (3069, 3110, 3129 cm−1 ) and BLYP (3075, 3116, 3138 cm−1 ) overestimate all the frequencies with an error of 1%. The B3P86 overestimates first two frequencies (3052, 3091 cm−1 ) with an error less than 1% and underestimates the last frequency (3108 cm−1 ) by 1%. The B3PW91 overestimates the first two frequencies (3053, 3092 cm−1 ) by 0.3 and 0.9%, respectively while the third one is quite accurately calculated (3109 cm−1 ) within 0.1%. All the three frequencies are much more overestimated by B3LYP (3056, 3095, 3114 cm−1 ) than any other by 0.41, 1.0 and 0.1%, respectively.

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4.2.1.2. In-plane bending vibrations. These are coupled with the ring and C–X vibrations. A strong IR band observed at 1037 cm−1 with a corresponding weak Raman band at 1043 cm−1 is assigned to C H in-plane bending vibration. Another weak IR band at 931 cm−1 is also assigned to C H inplane bending vibration which is coupled with the ring as well as C Br vibrations. No band corresponding to this vibration is observed in the Raman Spectrum. All the levels overestimate these two frequencies. First band is accurately calculated by RHF (∼2%) while remaining levels show larger errors (∼7%). Second band is calculated accurately by BP86 (∼7%) whereas others show error in the range 8–9%. 4.2.1.3. Out-of-plane bending vibrations. A weak IR band at 679 cm−1 , two strong bands at 818 cm−1 , 856 cm−1 and a medium strong band at 876 cm−1 are readily assigned to the outof-plane bending of C–H vibrations. There is a strong Raman band at 685 cm−1 and a weak band at 816 cm−1 corresponding to the first two IR bands (679 cm−1 and 818 cm−1 ). PED contributions for 818, 856 and 876 cm−1 bands show only C–H vibrations, while there is a contribution of other internal coordinates in the case of hydroquinone and phenol [1,23]. First band (679 cm−1 ) is underestimated by all the levels with deviations around 12% to 17%. The BLYP values are more reasonable than by the BP86 method. Second band (818 cm−1 ) is overestimated by BLYP (1.1%) and RHF (1.6%) whereas other methods under estimate (0.9–5.7%) except B3LYP which predicts the exact value. The 856 cm−1 frequency is overestimated by all levels (1.8–3%). RHF, BP86 and BLYP overestimate by 2.7–5.1% the third band 876 cm−1 , whereas other levels predict the same band smaller than the experimental by 0.2–1.5%. 4.2.2. C–C vibrations 4.2.2.1. Stretching vibrations. Strong bands at 892, 1275, 1372, 1519 and 1597 cm−1 and a medium strong band at 1618 cm−1 are observed in the infrared spectrum with corresponding bands except for the fourth one are observed in the Raman spectrum at 900, 1278, 1345, 1603, and 1624 cm−1 , respectively. These are assigned as C C stretching vibrations coupled with the vibrations of the substituents. In HQ, a very strong Raman band at 855 cm−1 is assigned to the C C stretching coupled to the C O stretching and in-plane bending vibrations. This band is shifted to 900 cm−1 in the Raman spectrum, which may be due to the coupling between C C and C Br stretching vibrations. The C C stretching coupled with the in-plane bending of C H is observed at 1096 cm−1 in the infrared spectrum of HQ and is shifted to 1275 cm−1 which may be due to the additional coupling of in-plane bending of O H and C O stretching vibrations. The 1372 cm−1 band corresponds to the C C stretching coupled with the in-plane O H bending vibrations. The bands corresponding to this mode are observed at 1210 and 1070 cm−1 in HQ and phenol, respectively [1,23]. The bands which are assigned to C C stretching vibration coupled with in-plane C H bending are observed at 1179 and 1363 cm−1 in phenol and HQ, respectively. With the substitution of Bromine in 2-BHQ, the contribution due to the in-plane OH bending vibration becomes

totally insignificant while C H in-plane bending shows a little effect. PED values indicate a considerable C O stretching vibration with the shift of 56 cm−1 in the infrared spectrum. Kubinyi et al. and Lampert et al. [1,23] have assigned the band around 1600 in phenol as coupling of C C stretch with C H in-plane bending vibrations. In HQ, there is no shift in the band position but C C and O H bendings also get coupled here. In 2-BHQ, again bending of O H bonds show up without any considerable shift in the band position. The C C and C H in-plane vibrations are coupled with C C stretch in phenol and HQ (1600 cm−1 ) [1] while C O stretching gets coupled with C C stretch in 2-BHQ. The 892 cm−1 band is overestimated by BLYP, BP86 and RHF with 3–6% of error and under estimated by B3-based methods. The 1275 cm−1 band is overestimated by all the levels within 1% of error. The 1372 cm−1 and 1519 cm−1 are best calculated by BLYP (1357 cm−1 ) with an error of 0.1% while underestimated by the remaining levels. 1597 cm−1 is underestimated by all the levels with an approximate error of 1% except RHF which shows less than 1% of error. 1618 cm−1 is best calculated by B3PW91 (1612 cm−1 ) while remaining B3- and B-based DFT methods estimate slightly smaller values (∼1%). RHF overestimates this frequency by more than 1% of error. 4.2.2.2. In-plane bending vibrations. Two weak IR bands are observed at 542 cm−1 and 699 cm−1 . Based on the PED contributions these are assigned to the C C in-plane bending vibration along with minor contributions due to the substituents. The bands due to these modes appear in the Raman spectrum at 542 cm−1 and 704 cm−1 , respectively. The C C in-plane bending vibration coupled with in-plane C O bending is observed at 492 cm−1 in the Raman spectrum of HQ [1]. In 2-BHQ additional coupling of small percentage of C Br and C O stretchings give rise to a large shift (50 cm−1 ) in the IR band at 542 cm−1 . Due to the coupling of C O in-plane bending and C Br stretching, again a large shift from HQ (649 cm−1 ) to 2-BHQ (704 cm−1 ) is observed in the Raman spectrum. All the levels underestimate the first frequency with the deviations in the range 10–15% in which the BLYP gives the frequencies with the lowest error. Except BLYP (0.6%), all the levels overestimate the second frequency (0.4–0.5%) in which the BP86 shows the best performance. 4.2.2.3. Out-of-plane bending vibrations. A strong IR band at 774 cm−1 results mainly due to the C C out-of-plane bending vibration weakly coupled with the C H out-of-plane bending vibration. The same mode gives rise to the Raman band at 780 cm−1 . The Raman active band corresponding to this mode is observed at 704 cm−1 in HQ. Computed frequencies at all the six levels are less than that of the experimental ones: the Band B3-based DFT methods compute the frequencies with errors between 8 and 9% while the RHF calculates the frequency with the error less than 8%. 4.2.3. C–X vibrations: (X = O, Br, O–H) 4.2.3.1. C–O vibrations. 4.2.3.1.1. Stretching vibrations. Three strong IR bands at 807 cm−1 , 1201 cm−1 and 1230 cm−1 are assigned to C O

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stretching vibration with minor contributions from the ring and O H vibrations. While there is no Raman band corresponding to the first IR frequency, the bands corresponding to the last two frequencies also appear in the Raman spectrum at 1209 cm−1 and 1235 cm−1 , respectively. Kubinyi et al. [1] have assigned these modes to 1246 cm−1 and 1261 cm−1 , respectively in HQ. All the levels over estimate these frequencies. The RHF calculates accurate value of both the frequencies with nearly 1% of error and the remaining levels calculate these bands with the errors in the range 2–4%. 4.2.3.1.2. In-plane bending vibrations. The 409 cm−1 and 583 cm−1 bands in the Raman spectrum correspond to in-plane C–O bending vibration coupled with vibrations of the substituents. A single band corresponding to the second Raman band appears in the IR spectrum at 572 cm−1 . The C–Br stretching and in-plane bending vibration also have small contributions to the mode. The corresponding bands in HQ are 386 cm−1 and 460 cm−1 , respectively. Both the Raman bands in 2-BHQ are underestimated by the six levels. The B-based DFT methods compute the two frequencies more accurately with an error 1% for the first frequency and with 9% for the second frequency. 4.2.3.1.3. Out-of-plane bending vibrations. The C O outof-plane bending vibrations may be assigned to the Raman band at 381 cm−1 ; no corresponding IR band is observed. The calculated frequencies are smaller than the experimental values. The B-based DFT methods calculate the frequency accurately with less than 1% of error while the B3 based methods calculate it with slightly larger error (∼2%). Another band at 116 cm−1 is also assigned to the out-of-plane C O bending vibration with significant contribution from the ring deformation and small contribution from C Br out-of-plane bending vibration. Again no IR band of this mode is observed. All the levels calculate higher values than 116 cm−1 with 18–20% of errors. The bands corresponding to 116 cm−1 and 381 cm−1 are observed in the IR spectrum of hydroquinone (␤-HQ) at 421 cm−1 and 201 cm−1 , respectively by Kubinyi et al. [1] and they are assigned mainly to IR active out-of-plane C C bending vibrations. This change of modes of vibration from ␤-HQ to 2-BHQ is an evidence of the presence of heavy Bromine substitution [24,25]. 4.2.3.2. C–Br vibrations. The 275 cm−1 , 202 cm−1 and 179 cm−1 bands appearing in the Raman spectrum are assigned to the stretching, in-plane bending and out-of-plane C Br bending vibrations. The C O vibrations are also mixed with the first and the third vibrations. The bands 275 and 202 cm−1 are best calculated by the BLYP and BP86 methods with less than 1.5% of error. The band 179 cm−1 is calculated fairly accurately again by the B3-based DFT methods with 1% of error while RHF gives the exact value. 4.2.3.3. O H vibrations. 4.2.3.3.1. Stretching vibrations. Very strong and broad IR bands (width ∼300 cm−1 ) at 3209 cm−1 and 3259 cm−1 are assigned solely to the O–H stretching vibrations as the PEDs show 100% contributions. Normally, free O H stretching vibra-

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tions appear around 3600 cm−1 as in, for example, phenol [23]. Since the bands have been downshifted apparently caused by the O H· · ·O bonding, we look for the second signature; the O H in-plane bending vibrational frequency is increased by about 30 cm−1 [26]. The similar phenomenon has been observed in HQ too, where the appearance of the two bands at 3203 and 3154 cm−1 have been attributed to the O H· · ·O bonding. All the levels overestimate these two frequencies. All the DFT methods predict the two frequencies with an error of 10–11%. It is not surprising that this large error is due to the non-inclusion of the O H· · ·O bonding in the calculations of harmonic frequencies. 4.2.3.3.2. In-plane bending vibrations. A medium, a weak and a strong bands observed at 1124 cm−1 , 1136 cm−1 and 1454 cm−1 , respectively in the IR spectrum are assigned to O H in-plane bending vibration mixed with the ring and C O vibrations. The Raman bands, corresponding to the first and third IR bands, are 1130 cm−1 and 1453 cm−1 , respectively. All the levels overestimate the first two IR frequencies (2–4%); the RHF which calculates the first frequency smaller than the experimental one by 1% while the third frequency is underestimated by all the levels (less than 1%). 4.2.3.3.3. Out-of-plane bending vibrations. A weak Raman active band at 314 cm−1 corresponds to the O H out-ofplane bending vibration while there is no corresponding band in the IR spectrum. This band is calculated better by B3-based DFT methods (0.6–1.6%) while B-based methods predict still larger values (∼10%). The RHF method underestimates this frequency with a large error (17%). Two weak IR bands at 449 cm−1 and 526 cm−1 are assigned to O H out-of-plane bending vibrations. These two bands are underestimated by all the methods. The B-based DFT methods estimate these two frequencies more accurately (∼1 and 7.5%, respectively) while the B3-based DFT methods show larger errors (more than 7 and 10%, respectively). Again a medium strong Raman band at 492 cm−1 corresponds to the latter IR band. 5. Conclusions The IR and Raman spectral measurements have been made for 2-Bromohydroquinone and assigned, as aided by the RHF/631G(d), B-based and B3-DFT/6-31G(d) levels of calculation. The coupling of vibrations has been determined from PEDs calculated using BP86/6-31G(d) results. The computed geometries show changes in the bond lengths and bond angles caused by the presence of Bromine. All the observed spectral features have been explained. The O H· · ·O bonding has been observed near 3200 cm−1 as in hydroquinone. As this bonding effect has not been incorporated into the theoretical calculations, the O H stretching frequencies are overestimated by all the levels. In assessing the performance of all the levels, it has been seen that the observed frequencies in the 2000–500 cm−1 are reproduced reasonably well by the B-based (BLYP, BP86) and B3-based (B3P86, B3LYP, B3PW91) methods. The calculated frequencies deviate from the observed values by a factor of two to three below 500 cm−1 , where the RHF frequencies show less deviations.

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