The infrared spectrum of matrix isolated guaiacol: self-association and complexation with nitrogen

Journal of Molecular Structure, 176 (1988) 245-251

245

Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands

THE I N F R A R E D S P E C T R U M OF M A T R I X I S O L A T E D GUAIACOL: S E L F - A S S O C I A T I O N A N D C O M P L E X A T I O N WITH NITROGEN

HENRIK TYLLI and HENRIK KONSCHIN

Department of Chemistry, University of Helsinki, E. Hesperiankatu 4, SF-O0100 Helsinki (Finland) (Received 2 October 1987)

ABSTRACT The self-association of guaiacol and guaiacol-d3 under cryogenic matrix conditions, and the complexation with the low-temperature nitrogen matrix medium has been studied using IR spectroscopy. In the OH stretching region matrices with high M/A ratios display spectra of almost purely monomeric guaiacol. Lowering the M/A ratio results in self-association, the dominant aggregates being the dimer and the trimer. Annealing the matrix lowers the intensity of the dimer band and increases the intensity of the trimer band. Further annealing increases the relative amount of trimer and larger aggregates. The self-association tendency for guaiacol is found to be much less pronounced than for phenol. Thus the hydrogen bond in guaiacol is sufficiently strong to ensure the trapping of the molecules solely in the intramolecularly bonded form. The complexation of guaiacol with nitrogen under matrix conditions is seen as a splitting of the OH torsional transition into a multiplet, whereas the same transition in a matrix of pure argon gives rise to a single sharp band. The intensity distribution between the different bands in the torsional multiplet shows temperature dependence.

INTRODUCTION

In a previous contribution from this laboratory [ 1 ] we studied the self-association of phenol and its complexation with nitrogen under cryogenic matrix conditions. It was found that the tendency for phenol to form structured aggregates was fairly high and that complexation with nitrogen took place under the matrix conditions. The complexation was seen most clearly in the OH torsion region where a multiplet was found instead of a single band. Substituted phenols with hydrogen bond accepting substituents in the ortho position could be expected to have a lower tendency for self-association, and perhaps also a lower tendency to form complexes with nitrogen. In the present contribution we therefore extend our study to guaiacol (ortho-methoxyphenol). The literature concerning the vibrational spectroscopy of guaiacol has been reviewed previously [2 ]. The intramolecular hydrogen bonds in ortho 0022-2860/88/$03.50

© 1988 Elsevier Science Publishers B.V.

246

substituted phenols and the formation of complexes between guaiacol and various H-bond acceptor solvents has been extensively studied in the past. Most of these studies deal with the behaviour of the OH stretching mode as a function of solvent polarity, state of aggregation and temperature [3-16]. Only a few studies have been devoted to the low-frequency part of the vibrational spectrum [ 2,12,17-20 ], although the most direct information concerning hydrogen bonding, self-association and complexation could be obtained from this part of the spectrum. Gebicki and Krantz have previously studied the complexation of phenol [21] and several substituted phenols, including guaiacol [22 ], with carbon monoxide using matrix isolation IR spectroscopy. This technique is well suited for the study of association and complexation phenomena due to the high resolution and the controlled conditions which can be employed. EXPERIMENTAL

The sample of guaiacol (Rh6ne-Poulenc) was purified by repeated sublimation at 0.15 Torr and 30°C. Guaiacol-d3 was prepared by methylation of catechol according to ref. 2, and purified by column chromatography and sublimation. Due to the hygroscopic properties of this compound special care was taken to eliminate water vapour. The purified samples were stored under dry nitrogen and all sample manipulations were carried out in a dry box. Matrix samples were prepared by two different methods. Matrices with medium and high M / A ratios were obtained by depositing matrix gas which previously had been saturated with sample vapour in a storage bulb on the vacuum line. Matrices with lower M / A ratios were prepared by allowing matrix gas to pass through a sample tube containing solid guaiacol onto a cooled CsI window. The M / A ratio was varied by changing the temperature of the sample; matrices with the lowest M / A ratios examined were prepared with the sample tube at a temperature just below the melting point of the sample. The temperature for deposition, normally 14 K, was maintained with a Displex CS-202 cryocooler and the spectra were recorded at a sample temperature of 11-13 K with the same equipment as reported in ref. 1. The resolution and wavenumber accuracy were of the order of 1 cm-1. RESULTS AND DISCUSSION

The spectra measured under different matrix conditions are collected in Figs. 1-7 and the assignments are given in Table 1. For guaiacol in an argon matrix the monomer OH stretching band is found at 3572 c m - 1, whereas Gebicki and Krantz [ 22 ] in their study concerning the complexation of substituted phenols with carbon monoxide reported the value 3576.5 cm-1, with a shoulder at 3572 cm-1. The matrix value is close to the results obtained in dilution studies for guaiacol in inert solvents [9,11,12]. For phenol in an argon matrix the mono-

247

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mer OH stretching transition has been observed at 3631 c m - 1 [ 1 ] , and as a doublet at 3633.5 c m - 1 and 3638.5 c m - ] by Gebicki and Krantz [21 ]. T h e shift in the v ( OH ) wavenumber for guaiacol compared to that for phenol ( 59 c m - 1 ) is largely due to the intramolecular hydrogen bond, which stabilizes the molecule in the m i n i m u m energy cis conformation. T h e spectra show no indications of the presence of the trans conformer in the matrix samples. Evidently, the hydrogen bond is strong enough to ensure the trapping of the molecules solely in the intramolecularly hydrogen bonded form. This is also in accordance with the observations of Gebicki and Krantz [22]. T h e intramolecular

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hydrogen bond considerably lowers the tendency for self-association of guaiacol as compared to phenol. In nitrogen matrices with high M / A ratios, the selfassociation was found to be negligible (compare Figs. 1 (a) and 2 (a)). In spectra of matrices with medium M / A ratios the dimer band is found at 3470 c m - 1, and the trimer band at 3390 cm -1 (Fig. 2(b)). Annealing the matrices with medium M / A ratios to 27 K lowers the intensity of the dimer band and favours the formation of the trimer (Fig. 2 (c)). Further warming of the matrix to 34 K for 15 min gives rise to increased amounts of trimer and larger aggregates (compare Figs. 2(d) and 3(b) ). The structure of the OH stretching band depends on the method used for preparation of the matrix samples. This is seen in Fig. 4 which shows the moItlilllllllltl

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Fig. 6. The OH torsional transition for guaiacol-d3 in Ar matrix. Fig. 7. Temperature dependence of the OH torsional multiplet of guaiacol in N2 matrix.

nomer v (OH) band for guaiacol in nitrogen matrix expanded. If the sample is prepared by depositing matrix gas presaturated with sample vapour on the vacuum line the monomer band can be resolved into three sub-bands. On the other hand, if the sample is prepared by depositing matrix gas which has been passed through a sample tube containing solid guaiacol, a single unresolved band with a shoulder on the low-wavenumber side is obtained (Fig. 4 (b) ). The intensities of these subbands display a reversible temperature dependence (Fig. 5). The OH torsional transition for guaiacol was observed as a very strong and sharp band at 426 cm -1 (and at 423 cm -1 for guaiacol-d3) in an argon matrix (Fig. 6). The value obtained for this transition is in close agreement with the solution state results of Nyquist [ 12 ], Carlson and Fateley [ 17 ] and Fateley et al. [ 18 ]. No indication of a second torsional transition, originating from the TABLE 1 Assigned bands (cm- ' ) in the IR spectra of guaiacol and gnaiacol-d3 in Ar and N2 matrices Assignment

Monomer ~ (OH)a Monomer v (OH)b (Different trapping sites) Dimer v(OH) Trimer p (OH) ~(OH) T(OH) in open dimer ~(OH) (complex with N2)

Ar matrix

N2 matrix

Guaiacol

Guaiacol-da

Guaiacol

Guaiacol-d3

3572

3571

3567 3565 3570 3578 3470 3390

3570 3564 3570 3579

426 411

423 408 425-474

422-473

aSample prepared by direct sublimation, bSample prepared by depositing matrix gas saturated with sample vapour.

250 "free" OH group in a t r a n s conformer was found in the matrix spectra. The fact that two torsional bands were observed in refs. 17 and 18 indicates that a partial rupture of the intramolecular hydrogen bond occurs in cyclohexane solution. The value obtained for the OH torsion can be compared with the value 307 c m - 1 for the "free" OH group in phenol, also in an argon matrix [ 1 ]. The shift of 119 c m - ' may be taken as measure of the strength of the intramolecular hydrogen bond in guaiacol. The OH torsional transition for guaiacol and guaiacol-d3 in an N2 matrix is split into a complicated multiplet extending over a range of about 50 c m - 1 (see Table 1 ). This clearly indicates that complexation with nitrogen occurs. Broad OH torsion bands exhibiting fine structure have been observed previously by Murto et al. for methanol in nitrogen and carbon monoxide matrices [23 ], and by us for phenol in a nitrogen matrix [ 1 ]. The origin of the torsional multiplet is not clear, but a mechanism which could produce the observed fine structure pattern would be a coupling between the torsion and the low-frequencyOH..' N2 modes. A formalism similar to that of the Witkowski-Mardchal model [24-28] used for the evaluation of band shapes for hydrogen bonded solids would then have to be considered. The intensity distribution between the different subbands in the torsional multiplet shows both reversible and irreversible temperature dependence (Fig. 7). In a nitrogen matrix doped with 4% argon we observed a spectrum of the torsion region which essentially was a sum of those observed in pure nitrogen and argon matrices. Upon warming the sample, the band originating from uncomplexed guaiacol decreased in intensity. The structure of the guaiacol dimer is interesting. In principle both an open dimer and a cyclic dimer may be formed. In the matrix spectra of the torsional region we find support for predicting an open structure. On the low-wavenumber side of the monomer torsion band (Fig. 6) we observe transitions (at 411 c m - 1 for guaiacol and at 408 c m - 1 for guaiacol-d3) which gain intensity upon warming the matrix. We assign these bands to the torsion of the "free" OH group in an open dimer structure of these compounds. In contrast to the situation in phenol [ 1 ] the dimer torsion band is shifted to lower frequency compared to the monomer band. This is probably due to the weakening of the intramolecular hydrogen bond when the intermolecular H-bond is formed, resulting in a net lowering of the torsional potential for the OH group. ACKNOWLEDGEMENTS We are indebted to Associate Prof. J. Murto for kindly permitting us to perform the matrix isolation measurements in his laboratory.

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