V&ariuaal Specrroscqpy, 4 (1993) 321-334 Elsevier Science Publishers B.V.. Amsterdam

321

Normal coordinate and rotational barrier calculations on 1,2_dihydroxybenzene

F.J. Ramirez and J.T. Lopez Navarrete

Lkpartamento de Quhica Fiiica, Facultad de Ciencias, hive&dad de Milaga, 29071 Milaga (Spain)

(Received 12th June 1992)

Abstract

The infrared and Raman spectra of solid catechol were obtained and a new assignment of the observed frequencies is proposed on the basis of depolarization ratios measured from aqueous solution, force field and normal coordinate calculations, factor group analysis and previously reported data. In addition, a conformationai study invoking rotational barrier calculations was performed by using the AM1 semi-empirical method in order to establish the minimum energy geometry. The MNDO force field at this point was scaled and a good description of the vibrational specta was obtained.

Keywords: Infrared spectrometly; Raman spectrometry; Catechol; Dihydroxybenzene; Normal coordinate calcula- tions; Rotational barrier

In the last few years, studies on interpreting vibrational spectra of polyatomic molecules by means of semi-empirical methods have been per- formed in this laboratory [l-5]. These studies in many instances involved aromatic molecules such as pyrazine [1,2] or benzene derivatives [3,41, and they confirmed that the force field and normal coordinate calculations are useful tools for assign- ing the infrared and Raman spectra. The work reported represents the start of studies on poly- hydroxybenzenes, a series of interesting molecules from the structural and vibrational point of view.

The infrared and Raman spectra of catechol (1,2_dihydroxybenzene) have been reported previ- ously by different workers [6-101. Tentative as- signments were discussed based on the experi- mental results, but no data concerning theoretical calculations of the vibrational spectra have been published so far. On the other hand, the molecu-

correspondence to: F.J. Ramirez, Departamento de Qu’mica F&a, Facultad de Ciencias, Universidad de Mblaga, 29971 M6iaga (Spain).

lar structure of catechol in the solid state is known by x-ray diffraction [ll] and, more re- cently, ab initio calculations [12,13] in addition to microwave spectroscopy [14] have determined the geometry and conformations of the isolated molecule.

In this work, the MNDO semi-empirical method was used to calculate the harmonic force field for catechol, which was scaled in order to obtain the best set of theoretical frequencies and normal coordinates. Previously, the structure of this molecule was optimized to obtain the mini- mum energy geometry and the rotational barriers. The results were employed to assign the infrared and Raman spectra and to compare them with the reported assignments and, on the basis of that, some modifications are proposed.

EXPERIMENTAL

For recording the spectra catechol (Aldrich, 99% + 1 was used as received. Infrared spectra

0924-2031/93/$06.00 0 1993 - Eisevier Science Publishers B.V. Ail rights reserved

322 F.J. Ramirez and J. T. L&ez Navarrete / VT. Spectrosc. 4 (1993) 321-334

between 4000 and 400 cm- ’ were obtained from potassium bromide pellets on a Perkin-Elmer 1700X Fourier transform IR spectrometer purged with argon. In order to exclude the introduction of moisture with KBr, the infrared spectra of solid catechol was also recorded by means of the mulling procedure, using perfluorohydrocarbon as the mulling fluid; the spectrum obtained showed no differences from that recorded in KBr pellets. To increase the signal-to-noise ratio a minimum of 50 scans were accumulated in all instances, with a resolution of 2 cm-‘. Raman spectra of microcrystalline powder were recorded on a Jobin Yvon Ramanor UlOOO spectrometer using excitation radiation at 488.0 and 514.5 nm generated by a Spectra-Physics argon ion laser working at 300-500 mW. The measurements of the depolarization ratio of each Raman line were done in aqueous solution prepared with triply distilled water. The best resolution obtained was 1 cm-‘.

CALCULATIONS

To calculate the quadratic force field the MNDO semi-empirical method [15] was chosen, which has been used successfully elsewhere for similar problems [16]. The force constants were calculated by evaluating the first and second derivatives of the energy with respect to the Cartesian coordinates. Previously, it was necessary to find the molecular structure corresponding to a minimum in the potential energy surface, whereby a theoretical AM1 [17] study of the internal rotation in this molecule was made in order to obtain the most stable conformation for it. Because of the high speed of semi-empirical methods, all structural parameters were opti- mixed for every conformation, with the exception of the two C-O-H torsional angles.

The Cartesian force field was transformed with a set of local symmetry coordinates which were defined following Pulay et al.‘s coordinate de- scriptions [18]; then the force constants were scaled in order to fit the calculated frequencies to the experimental results and to obtain a better description of the normal vibrations. To do this, a

set of MNDO scaling factors were transferred from the benzene molecule [191 for modes that were analogous in catechol, and for other modes they were evaluated from the MNDO systematic behaviour as regards the predition of vibrational frequencies [20], by means of the equation

ci = (vch/vca~~2

The off-diagonal force constants were scaled with the factor (CiCjll/*, following Pulay et al’s method [21]. Frequencies and normal coordinates were determined by the Wilson FG matrix method [22]. All calculations were performed by a VAX 8530 computer.

RESULTS AND DISCUSSION

Intern421 rotation The internal rotation in catechol was studied

by using the semi-empirical AM1 method because it can be used to study systems when hydrogen bonds occur [23]. In order to obtain the full potential function, the calculations were carried out in steps of 10 Q for the two rotors, 8,(C,-C,- 01,-H,,) from 0 o to 180 o and 8,(C,-C,-O,,- H,,) from 0 o to 360 o (see Fig. 1). At every step

0 a

Fig. 1. Molecular structure of the catechol molecule with the atomic numbering used in this work.

F.J. Ramim and J.T. L.&z Navarrete/ VI. Spectrosc. 4 (1993) 321-334 323

of 8,8, the calculation was made by relaxing all the other geometrical parameters.

The internal rotation of the OH groups in catechol is expected to be largely determined by the intramolecular hydrogen bond HI*. . . O,, and by the steric hindrance of each OH group. The evolution of the AM1 relative energy of catechol as a function of the two torsional angles is given in Fig. 2a and b, and Fig. 3 shows the torsional potentials for the rotation of the two hydroxyl moieties. Table 1 gives the relative energies found for the most significant configurations for this molecule.

The absolute minimum is calculated for the planar C, conformer (Fig. 11, where the in- tramolecular hydrogen bond is possible; a sec- ondary energy minimum was found for the trans-

C,*, conformer (8, = 180 ‘, 8, = 0 ’ 1. The global energy maximum is obtained near to the conrota- tory orthogonal conformer at 8, = 110 O, 8, = 80 ‘, and a secondary local maximum is found near to the disrotatory orthogonal conformer at 8, = 110 O, 8, = 290 O. The high rotational barrier calculated between the two isomers (Fig. 3, Table 1) agrees with the structure of the isolated molecule [12-141; in addition, the results suggest that the disrotatory motion is more probable than the conrotatory motion for the transition from the ck to the trult~ conformer. On the other hand, the valte obtained for the H,, . . .O,, length, 2.264 A, is in the range of hydrogen- bonded H.. .O distances [24], in accord with the structure of catechol in condensed states and also with the previous ab initio data [12,13], and this

THETA 1

in catechol: (a) three-dimensional plot; (b) two-dimensional

FJ. Ram& and LT. L&a Navawete / V&. Spectrosc. 4 (1993) 321-334

TABLE 1

Significant configurations from catechol internal rotation cal- culation

Conformer A E (kcal mol-‘)

8, (“1 8, (“) (AMl)

0 0 0.00 0 180 4.06

180 0 3.49 0 90 1.79

90 0 4.33 90 90 7.10 90 270 5.60

Theoretical spectra and force field Force field calculation requires the molecular

geometry to be optimized for a point of minimum energy; therefore, using MNDO the ground-state geometry was calculated by energy minimization procedures in which all geometrical parameters were allowed to change unrestrictedly. The calcu- lated global equilibrium structure obtained for

Rg. 3. Calculated torsional potentials in catechol for the internal rotation of the two hydorxyl groups. o = 19~; l = 19,.

proves the ability of AM1 to yield correct predic- tions of conformational preferences in this molecule.

catechol is the planar C, conformer in Fig. 1, e1=e2=oo, in accordance with the available experimental data and with the previous ab initio calculations. The resulting geometry is compared in Table 2 with the x-ray experimental structure given in [ll] for the molecule in the crystal and also with the geometrical parameters found by

TABLE 2

Experimental and calculated structural parameters for catechol molecule

Bond lengths

&I

MNDO Exptl. a Expt1. b Bond angles ( a ) Expt1. B Exptl. b

cl-c2 1.435 1.390 1.400 c,-c,-c, 119.7 119.7 120.9 c2-c3 1.416 1.381 1.387 c,-c,-c, 120.2 120.1 119.4 c3-c4 1.403 1.384 1.401 c,-c,-c, 120.3 120.1 119.4 c4-cs 1.405 1.386 1.394 c,-c,-c, 120.6 119.8 120.3 cS-c6 1.403 1.385 1.400 H,-C,-C, 119.1 123.0 120.5 C3-H7 1.091 1.00 1.080 Hs-C,-C, 120.0 120.0 120.4 C4-Ha 1.090 O.% 1.078 H,-CS-C, 119.6 119.0 119.6 G-H9 1.091 0.98 1.078 H,o-C,-Cl 120.3 119.0 118.5 C,-ho 1.091 1.01 1.077 %-cl-c2 124.6 121.0 120.8 +011 1.357 1.364 1.367 013-c2-cl 117.5 123.3 114.1 c2-o13 1.363 1.373 1.383 H12-011-C, 114.3 111.0 106.4 011-H,, 0.947 0.80 0.964 H14-“13-C2 112.8 106.0 109.4 013-H,, 0.947 0.91 0.960 H,,... 013 2.453 2.31

’ Data from [ll]. b Data from [14].

F.J. Ram&x and J. T. L&Z Navarrete / Vii. Spectrosc. 4 (1993) 321-334 325

TABLE 3

Experimental a, MNDO-calculated and scaled frequencies (in cm-t) obtained for in-plane vibrations of catechol

Vibrations Calc. Scaled Obs. B Assignment P.E.D. ( > 10%)

Vl V2 3637

v3 3069

v4 3063

V5 3057

V6 3053

v7 1622

Vi3 1617

v9 1472

VlO 1456

Vll 1355

v12 1347

v13 1261

v14 1249

v15 1225

v16 1216

v17 1175

Vl8 1107

v19 1064

V20 834

v21 767

v22 584

V23 552

V24 466

V2!3 370

a Data from [S], except where indicated. b This work.

3997 3991 3418 3410 3404 3400 1770 1761 1707 1630 1523 1552 1318 1421 1417 1279 1202 1187 1147

932 865 592 562 477 372

3642 3663 3605 3069 3063 3060 3051 1616 1607 1504 1479 1365 1324 1275 1251 1195 1187 b 1151 1092 1035

859 768 581 b

/ 564s 449b 343b

v(013H14)

v(O11H12)

2 20a 24Ib

7b 8b 8a

19b 19a 14 S(OH)

3 7a

13 6(OH) 15 9b

18b 12

1 6a 6b

18a 9a

lOOv(OH) lOOv(OH) 1oovKH) 1oovKzH) 1oovKf-D 1oovKXi~

86v(CC), 126KC) 87&C), 116fCC) 45&C), 4oSU.X) 45&c), 33S(cH), 17NOH) 87&X), 456(OH) SSSKH), 366(OH) 416(CH), 27&X), 22v(CO), 1WOH) 34&c), 29v(CO), 176(CH), 156(OH), 14NX) 436(CH), 28&O), 19S(OH), 15&O) 536KH),17v(CO), 1WXOH) 54S(CH), 53&X), 2N(OH) U&C), 22S(CH), 22S(OH) SSv(CC), 21600 746(CC), 23vKO) 5Ov(CC), 25v(COI, 15sKCc) 69S(CC) 666(CO), 166KC) 54SKC), 22S(CO) 766fCO)

microwave spectroscopy of catechol in the gaseous state [14].

The calculated r(C-C) and r(C-H) values in general compare favourably with the observed values if one takes into account the condensed

situation in the solid. The r(C-0) parameters are also estimated accurately, but the r(O-H) values calculated with UNDO can be only successfully compared with the experimental data from mi- crowave spectroscopy. This is because the struc-

TABLE 4

Experimental a, WO-cakulated and scaled frequencies (in cm-‘) obtained for out-of-plane vibrations of catechol

Vibration Calc. Scaled obs. a

vl6 1055 964 %3b v27 1009 923 916 V28 937 841 850b

v29 836 741 741 v30 723 728 722 b v31 491 519 555 b v32 468 476 456 b v33 273 285 295 b v34 178 179 v35 169 171

v36 144 156 2O2b

B Data from [8X except where indicated. b This work.

Assignment P.E.D. ( > 10%)

5 98yKX-i~, 3ly(CC) 10a OoyKHI, 28Yec)

lob 82yKH), 21y(CO) 11 89yUJ-D 4 Sly(CO), 16yKC)

16a 54y(CC), 16y@.X), 15y(CO) 16b 65y(CO), 22YKC) 17b 49y(CC), 17y(CO), 16yKH) y(OH) 97y(OH), Uy@O) y(OH) 109y(OH), 24y(CO), 17y(CC) 17a 92yKC), 68y(OH), 27yKO)

326 FJ. Ramik and 3. i? l&a Navarrete / VI. Spectrasc. 4 (1993) 321-334

F.J. Ramhez and 1. T. L&z Nauamete / Vii. Spectrosc. 4 (I 993) 321-334 327

ture preferences for the OH groups in solid cate- chol is strongly affected by the environment, where both intra- and intermolecular hydrogen bonds are present. The calculated CCC and HCC angles in C, catechol also agree well with the experimental values. Evidently, the OCC and the HOC angles are more sensitive to intra- and intermolecular effects and the divergence be- tween the MNDO and the experimental values is more severe. In summary, the optimized equilib-

rium geometry by this semi-empirical method is very close to the experimental geometry (Table 2); this fact and the good results obtained with this method in vibrational property calculations on similar molecular led to the adoption of MNDO in the dynamic calculations on catechol.

Tables 3 and 4 summarize the observed, calcu- lated and scaled frequencies and the potential energy distribution corresponding to the scaled values obtained by the MNDO method. In order

-54

"g-23 -44Ry _36$-$4

-26 -37

"a'l:46 27+&r llj&-&

-.2 -'9

35%; 7$$ -16$+;

-43 -22 -21

16jk-&

-21

5j$

Fig. 5. Atomic displacements calculated for the out-of-plane vibrations of the catechol molecule.

328 F.J. Ramirez and .I. T. L&z Navamte / Vi. Spectrosc. 4 (1993) 321-334

TABLE 5 TABLE 6

Local symmetry coordinates for in-plane vibrations of cate- chol

Local symmetry coordinates for out-of-plane vibrations of catechol

No. Linear combination ’

1 r12

2 r23

3 r34

4 r45 5 ‘56 6 ‘61

7 rltl 8 r213

9 r37

10 r48

11 r59

l2 ‘610

13 Ball1 - I32111

l4 81213 - 83213

l5 8237 - 8437

16 8346 - 8548

Description

vom

VKQ

v(CC)

v(CC) v(CC) vex)

v(CO) v(CO)

l&HI

v0-n

VKm

vom

NCO)

6(CO)

6a-I)

mm

No. Linear combination ’ Description

26 c lout-of-plane 7KoI 27 C 2cmt-of-plane 7(Co) 28 c 3out-ofplane. 7&H) 29 C 4mf_of-plane r(f.W

30 Go”t-of-phe 7KH) 31 f%Nt+hne 700 32 712 - r23 + T34 - ?45 + 745 + r56 - r61 r(CC) 33 T12 - r34 + 745 - 761 r(CC) 34 - 712+27z3-734-r4~+2r56-r61 r(CO 35 7111 y(OH)

36 7213 r(OH)

a rij is the torsion of the bond between atoms i and j:

Ciout-of-plme is the out-of-plane bending centred in atom i.

l7 8459 - 8659 NCH)

l8 85610 - &610 SCI-U

l9 pal2 - 8123 + 8234 - 8345 + 8456 - 8561 SKC)

2o 2861Z-8123-8234 +28345-8456-8561 6(cc)

21 8123 - 8234 + p456 - 8561 See)

22 BllllZ g(OH) 23 821314 WH)

24 r1112 vW-0

25 r13 14 ~0-0

a rij is the distance between atoms i and j; Bijk is the angle between atoms i, j and k.

to give a better description of the normal modes, the atomic displacement for each fundamental was calculated; Figs. 4 and 5 display the results for in-plane and out-of-plane vibrations, respec- tively. Vibrations were approximately classified following the Wilson notation [25] for benzene modes. Because the calculations were performed with the isolated molecule, the frequencies re- ported by Wilson [8] for the vapour-phase in- frared spectrum were chosen; the bands mea- sured in the OH-stretching region indicate that there are no intermolecular associations by hy- drogen bonds in the way that occurs in the con- densed state, so MNDO will provide good results, as will be seen later. Tables 5 and 6 show the Pulay local symmetry coordinates for which asso- ciated force constants were corrected by applying the MNDO scaling factors listed in Table 7.

the frequencies obtained from the scaled force field are subject to smaller absolute errors than the directly calculated values, especially concern- ing normal modes which are not coupled with other vibrations, such as AOH) or v(CH). In addition, the scaled force field gives a better description of the vibrations, as can be seen from both the potential energy distribution and the atomic displacements. Thus, the v(CO) stretches, noted as 7a and 13 modes in the Wilson nomen- clature, are assigned to 1249 and 1225 cm-l without doubt in the scaled spectrum, whereas these vibrations could hardly be assigned in the

TABLE 7

Scaling factors applied to the MNDO force field of catechol

In-plane vibrations. As can be seen in Table 3,

Coordinates Description Factor ’

l- 6 VOX) 0.821 7- 8 v(CO) 0.520 b 9-12 v(CH) 0.807

13-14 6(CQ) 1.000 15-18 S(Cr-0 0.918 19-21 S(CC) 1.031 22-23 6(OH) 0.774 b 24-25 v(OH) 0.830 b 26-27 r(CO) 1.000 28-31 7m4) 0.778 32-34 7Km 1.230 35-36 y(OH) 1.000

’ Transferred from benzene molecule [19], except where indi- cated. b From [Zo].

F.J. Ram&z ad J. Z L.&z Navarrete/Vii. S)~~trosc. 4 (1993) 321-334 329

calculated spectrum because their corresponding coordinates are distributed over a large number of frequencies. The normal coordinates displayed in Fig. 4 permit the classification of the four 6(CH) modes of this molecule as 3, 15, 9b, and 18b vibrations, by means of comparison with the approximate vibrational forms given by Varsanyi [26]. However, it can be observed that there is considerable coupling between S(CH) and NOH) bending vibrations in the scaled spectrum, which gives a mixed character to the frequencies of 1347, 1261, 1216 and 1107 cm-‘. Mixing also occurs between 6(CC) and NCO) skeletal vibra- tions, as can be seen in both Table 3 and Fig. 4. In general, the description of the vibrations de- duced through the potential energy distribution matrix and the Cartesian atomic displacements is consistent with the assignments proposed by Wil- son [8] for the vapour-phase infrared spectrum of catechol. Table 3 also shows several frequencies that were measured here in the vibrational spec- tra of solid catechol because they were not re- ported by Wilson [8]. As can be seen, these fre- quencies are close to the scaled values because they have 6(CC) and NC01 skeletal bending vibrations that do not shift much with the phase transitions; however, there is a greater difference for the frequency of 1216 cm-’ as it has sufficient NOH) character and, as discussed above, the OH groups involve hydrogen bonds in the solid state, but they are free in the gas state.

Out-of-prcme vibrations. Only two frequencies were reported by Wilson [8] as out-of-plane vibra- tions of catechol, namely 916 and 741 cm-‘, which were assigned to y(CH) vibrations. The other observed bands listed in Table 4 corre- spond to solid catechol. The two y(OH) are not compared. The fitting between the observed and scaled frequencies is satisfactory, especially for the y(CH) out-of-plane bending vibrations be- cause their coordinates are’ slightly mixed with other modes, as Fig. 5 shows. The potential en- ergy distribution also shows that the y(CC) and r(CO) vibrations are coupled, so the correspond- ing frequencies could be properly named skeletal out-of-plane vibrations, although they have been classified by the Wilson notation on the basis of the corresponding atomic displacements. The lowest frequency, yS6, is calculated with a differ- ence of 46 cm-’ from the observed value because it has some y(OH) character.

Force fiefd. Table 8 lists the calculated and scaled diagonal force constants obtained. Off-di- agonal force constants have been excluded on account of their large number and small absolute values, but they are available on request. The MNDO force constants, which were in general overestimated, are corrected by the scaling and they compare well with values reported for other similar molecules. The averaged v(CC) force con- stant, 7.040 mdyn A-‘, is close to the values reported for 1,4-diethynylbenzene 1271, 7.400

TABLE 8

MNDO and scaled diagonal force constants calculated for catechol (all values in mdyn A-’ or mdyn A rad-‘1

Coordinate UNDO Scaled Coordinate MNDO Scaled Coordinate MNDO Scaled

1 8.306 6.819 13 1.242 1.242 25 8.949 7.428 2 8.553 7.022 14 1.190 1.190 26 0.528 0.528 3 8.740 7.175 15 0.582 0.534 27 0.500 0.500 4 8.679 7.125 16 0.549 0.504 28 0.394 0.337 5 8.6% 7.140 17 0.550 0.505 29 0.396 0.339 6 8.474 6.957 18 0.578 0.530 30 0.398 0.340 7 10.85 5.622 19 1.216 1.254 31 0.398 0.340 8 10.71 5.552 20 1.309 1.350 32 0.237 0.292 9 6.423 5.184 21 1.277 1.317 33 0.221 0.272

10 6.450 5.205 22 1.089 0.843 34 0.230 0.283 11 6.444 5.200 23 1.054 0.816 35 0.016 0.016 12 6.436 5.194 24 8.924 7.408 36 0.015 0.015

330 F.J. Ramik and 1. T. L&m Navarrete / Vi. Spectrosc. 4 (1993) 321-334

Fig. 6. Infrared spectrum of solid catechol.

mdyn A- ‘, and benzene [28], 7.006 mdyn A- ‘, and for v(CH) force constants the mean value is 5.196 mdyn A-r, near to 5.022 mdyn A-’ for the benzene molecule, and similarly for the other scaled force constants. The large correction ap- qlied to the Y(CO) force constant, 5,587 mdyn A-’ compared with 10.78 mdyn A-’ from MNDO, is emphasized. The scaled value is more consistent with the molecular structure of solid

I 1 *d&L: 36m SW0 l(oo lpoo ow 400

wannubu km-‘1

Fig. 7. Raman spectrum of solid catechol.

catechol [ll], where the C-C and C-O bond lengths are similar; in addition, for the optimized geometry of the isolated molecule the bond or- ders obtained from MNDO are 1.36 and 0.94, respectively, for the cited bonds, in good accord with these results.

The out-of-plane force constants are also con- sistent with the molecular structure. Thus, those corresponding to r(CO> are evaluated as being

F.J. Ramirez and J.T. tipez Navarrete/ VI. Spectrmc. 4 (1993) 321-334 331

larger than the y(CI-I) ones, 0.514 and 0.339 mdyn 8, radm2, respectively, and, as expected, the lower values are assigned to the torsional vibra- tions, especially in the case of the y(OH) coordi- nates.

vibrational spectra of solid catechol The infrared and Raman spectra of solid crys-

talline catechol were recorded in order to apply the normal coordinate analysis discussed above to aid in their interpretation. Figures 6 and 7 show the infrared and Raman spectra, respectively, of this molecule and Table 9 lists the observed fre- quencies and the proposed assignments. The de- polarization ratios from aqueous solution were also measured. The crystal structure of catechol has been studied by x-ray diffraction [ill and was found to be monoclinic and belonging to the space group-c;,, in the Schonflies notation. The unit cell has four molecular units (C,H,O,) and no local symmetry occurs. This structure is stabi- lized by a network of O-H.. .O intermolecular hydrogen bonds, in addition to the intramolecular type; therefore, the molecular symmetry is C,, as occurs in the gaseous state, and the 36 normal modes are distributed as 25 A’ in-plane vibra- tions and 11 A” out-of-plane vibrations. In addi- tion to this analysis, the depolarization ratio data indicated that, in aqueous solution, this molecule has C,*, symmetry because there are some depo- larized bands above 1000 cm-’ which must corre- spond to planar vibrations. Therefore, the polar- ized bands will be A’ vibrations in solid catechol, but the depolarized bands, which belong to B,, A, or B, species of the CT” point group, can be either in-plane or out-of-plane vibrations in the C, structure of the crystal. The normal modes were classified as benzene vibrations and sub- stituent vibrations, and the Wilson nomenclature [25] was followed as often as possible.

On the other hand, the four catechol molecules in a unit cell yield 144 internal optical modes, 21 external optical modes (9 translations and 12 rotations) and 3 acoustic modes. On the basis of the correlation between the molecular symmetry, C,, the site symmetry, C,, and the factor group, C2h, there are 72 Raman-active vibrations, which

TABLE 9

Infrared and Raman frequencies (in cm-‘) observed for solid catechol

Infrared ’ Raman ’ Assignments b

f z 5 Z PC

3450 S

3326 S

3080 VW

3052 m 3044 sh 1619 ms 1603 m 1597 sh 1528 sh 1514 S

1469 S

1364 S

1281 S

1256 S

1242 S

1193 sh 1187 S

1165 m

1096 S

1041 m

916 W

859 sh 850 m 769 S

756 S

742 S

722 W

637 m

564 m 555 m 500 S

456 m 449 m

3463 W

3330 W

3071 m

3062 m

3056 m

3044 W

1623 m

1611 m

1604 m

1517 W

1477 W

1344 W

1276 S

1255 W

1198 W

1180 W

1164 m

1104 W

1042 S

963 VW

910 VW

849 W

775 vs

752 m

719 W

598 sh 581 m 564 W

459 W

343 W

295 m

202 m

115 S

110 sh 74 S

54 vs

36 vs

P dp

dp

:P P P

dp

dp P

dp P

dp

P dp

dp

dp dp

40,&), A’ u(O13H1.J, A’ v(CH), A’, 2 &H), A’, 2Oa UKH), A’, 7b r&H), A’, 2Ob v(E), A’, 8b

r&C), A’, 8a

u(CC), A’, 19b v(E), A’, 19a l&x), A’, 14 6(OH), A’ WI-II, A’, 3 v(CO), A’, 7a &CO), A’, 13 NOH), A’ 6(CH), A’, 15 XT-I~, A’, 9b 6(CH), A’, 18b y(CI-0, A”, 5 y(CH), A”, 10a y(W), A’, 12 y(CH), A”, lob v(CC), A’, 1

yGJ-0, A”, 11 y(CC), A”, 4 y(OH), A’ 6(E), A’, 6a S(E), A’, 6b y(W), A”, 16a y(OH), A” y(CCI, A”, 16b S(CO), A’, 18a NCO), A’, 9a y(CO), A”, 17b y(CO), A”, 17a Lattice vibration Lattice vibration Lattice vibration Lattice vibration Lattice vibration

a G = Frequency; Z = intensity; p = depolarization ratio; w = weak, m = medium; s = strong; v = very; sh = shoulder; p = polarized; dp = depolarized. b Y = Stretching; 6 = in-plane bending; y = out-of-plane bending. ’ Data for catechol dis- solved in water.

332 F.J. Ram&z and J. T. L&z Navamte / Vii. Spectrosc. 4 (I 993) 321-334

belong to A, or B8 species of the C,, punctual group, and 72.infrared-active vibrations, A,, or B, species. In summary, every molecular normal mode gives rise to two crystalvibrations in both the Raman and infrared spectra. This effect can produce shifts between the Raman and infrated bands of the same molecular vibration and, in addition, one can observe the splitting of some bands, as a result of the correlation field effects.

Benzene vibrations. These are the stretching modes of C-C and C-H bonds as well as both in-plane and out-of-plane bendings, and they are all active in the infrared and Raman spectra. It can be expected that these vibrations will not shift greatly when the intermolecular hydrogen bonds are broken, so the scaled theoretical spec- trum will be a great aid in assigning them. The good fit between the experimental and the MNDO-optimized geometries for the benzene ring parameters of catechol is in agreement with this suggestion.

Between 3000 and 3100 cm-’ the four v(CH) stretching vibrations were assigned taking into account its usual order of appearance [26]. One of these was measured as a depolarized band from the Raman spectrum of the solution, be- cause it corresponds to a B, vibration according to the relevant symmetry of dissolved catechol, and for this reason it is assigned to the mode 20a, A’ in the Wilson notation. The depolarization ratio measurements also permit discrimination between the different v(E) modes; in addition, the 8a vibration can be observed as two peaks in both the infrared and Raman spectra, and the 19b vibration appears also as a doublet in the infrared spectrum. This result might be explained as a correlation field effect, because in this re- gion, 1700-1500 cm-‘, only the mentioned modes can be assigned. For the 8a normal mode, these splittings permit the observation of the four crys- tal vibrations: at 1611 and 1604 cm-’ in the Raman spectrum (A,, B, species), and at 1603 and 1593 cm-r in the infrared spectrum (A,, B, species). The other v(CC) vibration which ap- pears in this region, mode 14 in the Wilson nota- tion, is usually assigned near 1350 cm-’ in disub- stituted benzenes [26] and it is correlated with an A, mode in the C.,* group, so it was assigned to

the band at 1364 cm-‘. The v(CC) assignments proposed here are not in accord with Greaves and Griffith [lo], who reported mode 14 at 1515 cm-’ and the pair 19a-19b at 1606 and 1476 cm-‘, respectively, with the last one given as a polarized band. The v(CC), 14 normal mode has been assigned at 1346 cm-r in 1,2-dicyanoben- zene [29], at 1300 cm-’ in phthalic acid [30] and at 1376 cm-’ in phthalimide [311, and it is calcu- lated at 1355 cm-’ by the MNDO-scaled force field, all of them supporting the present assign- ment.

Three S(CC) bending vibrations are assigned between 1000 and 1200 cm-‘; they are denoted 15,9b and 18b modes. For vibration 3,6(CH), the band at 1242 cm-‘, which has high intensity in the infrared spectrum, was chosen. The other benzene vibrations appear below 1000 cm-‘. Data on deuterated catechol [lo] were useful in order to assign the y(CI-I) out-of-plane bending modes. One of them, mode 11 in the Wilson notation, is observed split in the infrared spectrum, as a consequence of the correlation field effect. How- ever, and in the same way as happened with the v(CC) vibrations, the shifts here are small, which confirms that in catechol the benzene vibrations are only slightly affected by the molecular envi- ronment. As will be seen later, the hydroxyl modes will shift significantly.

For the 6(CC) and S(CC) skeletal vibrations the range of their usual appearance [26] and the data from the normal coordinate analysis were taken into account. Vibrations 6a-6b were as- signed on the basis of the depolarization ratios. Finally, Table 9 lists several very intense bands below 200 cm- ’ that correspond to translational and rotational lattice vibrations, where the rela- tive atomic positions in a molecule are un- changed.

Substituent vibrations. The presence of hydro- gen bonds in solid catechol shifts the frequencies of substituent vibrations with respect to those from the isolated molecule, mainly stretching and bending modes of O-H bonds. In addition, it can be expected that these vibrations will be more sensitive to the correlation field effects. Because there are two different hydrogen bonds, intra and intermolecular, two frequencies were measured,

F.J. Ramikz and J.T. L&ez Navarrete / Vii. Spectrosc. 4 (1993) 32!-334 333

namely 3450 and 3326 cm-‘, corresponding to the v(OH) vibrations. Accordingly to the 0.. . H bond lengths of solid catechol [ll] ~(0,i-H&, 2.31 A for the Hi,. . . 0 intramolecular bond, is assigned at the former frequency, whereas v(O,~-H,,), 2.12 8, for the H,, . . .O intermolecu- lar bond, is assigned at 3326 cm-‘, because a greater H . . . 0 length involves a stronger O-H bond and therefore a higher frequency. Mukher- jee and Banerjee [32] assigned the bonded O-H stretching vibration of catechol in several solvents near 3550 cm-l; this is in accord with the present assignment at 3450 cm-’ because in solid cate- chol some non-linear hydrogen bonds involving the OH groups are present, in addition to the intramolecular type, which shift this vibration to a lower frequency. On the other hand, in the in- frared spectrum of catechol gas [8] there is about 60 cm-’ between the two v(OH) frequencies, which indicates that some intramolecular interac- tions remain, and the normal coordinate calcula- tion assigns in the same way as proposed here.

Between 1300 and 1500 cm-’ the u(C0) and 6(OH) vibrations are assigned to bands which shift only a few cm- ’ with respect to the gas, because they are coupled with other ring modes, mainly &CC), as can be observed from the po- tential energy distribution in Table 3. The differ- ences between the infrared and Raman frequen- cies are also a few cm-‘, in accord with the previous discussion, but they are greater than those observed for the benzene vibrations in this molecule.

On the basis of a Raman spectroscopic study of the OH and OD torsion in catechol and cate- chold,, Tylli and Konschin [9] reported the band at 459 cm-’ as a r(OH) mode, and frequencies measured by Greaves and Griffith [lo] for some deuterated derivatives of this molecule agree with this assignment. In the infrared spectrum the mentioned OH band is observed at 500 cm-‘, with a significant shift with respect to the Raman band, due to the correlation field effects, that can be explained if the major role of the hydroxyl moieties in the preservation of the crystalline structure of catechol [ll] is considered. As ex- pected, the assigned frequencies for the (OH) out-of-plane bending vibrations are different from

the corresponding modes in isolated catechol [81 and from the MNDO-scaled values. Finally, the S(C0) and y(C0) bending vibrations between 500 and 200 cm-’ were assigned following the criteria given by Varsanyi [26] for 1,2-disubsti- tuted benzenes.

Conclusions The infrared and Raman spectra of catechol

were recorded in the solid, crystalline state and the observed bands were reassigned on the basis of the experimental data, a normal coordinate calculation by using the semi-empirical MNDO method and factor group analysis. Previous data were taken into account, but some assignments were changed in this work. In order to obtain the structure of minimum energy the internal rota- tion of the two hydroxyl groups in the molecule was studied by using the semi-empirical AM1 method. The calculations suggest that there are two stable isomers, with C, and Cg symmetry, in catechol and the disrotatory motion is more prob- able than the conrotatory motion for the transi- tion from the more stable C, structure to the less stable C,*, ~MM isomer. The force field calcu- lated at the minimum energy point was scaled by using a set of MNDO scaling factors, and the force constants obtained compared favourably with values reported for some related molecules; calculated potential energy distribution is consis- tent with the experimental geometry.

The authors express their gratitude to the Junta de Andalucia (Spain), Grupo 6072, for financial support.

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