The donor OH stretching-libration dynamics of hydrogen-bonded methanol dimers in cryogenic matrices.

M Heger,M A Suhm,J Andersen,R Wugt Larsen

doi:10.1039/c5cp07387a

Abstract

FTIR spectra of the methanol dimer trapped in neon matrices are presented. The fundamental, overtone and combination bands involving the donor OH libration and stretching motions were observed in order to extract relevant anharmonicity constants. We find a stretching-libration coupling constant of +43(5) cm(-1) and a diagonal librational anharmonicity constant of -71(5) cm(-1). The spectra are compared to a number of VPT2 calculations and a torsionally localized monomer model in order to enhance previous explanations of the observable OH stretching red-shift upon dimerization.

Highlights

The easiest way to achieve this is to observe the according combination band at ns,l = ns + nl + xs,l
The spectra reveal some complex band patterns for the features assigned to the OH stretching fundamental of the methanol dimer.[7]
FTIR absorption spectra of the methanol dimer embedded in neon matrices have been analyzed with respect to the combination band of the donor OH stretching and libration motions, and the overtone band of the latter

Summary

Introduction

The easiest way to achieve this is to observe the according combination band at ns,l = ns + nl + xs,l (where s and l refer to the stretching and librational modes, respectively, and x denotes anharmonicity constants). As with any perturbational approach, one should be wary if the perturbation is of considerable magnitude; seeing that the best harmonic librational prediction of 660 cmÀ1 suggests an anharmonic perturbation of this mode on the order of À100 cmÀ1, the previously found agreement with experiment may be serendipitous or misleading.[8] It is indispensable to obtain direct experimental values for the coupling constant in order to test the predictions. We set out to explore the important xs,l anharmonicity constant by means of the less intense OH stretching–libration combination bands of the methanol dimer by this sensitive matrix isolation approach. Further data for the deuterated isotopologues are presented in the ESI.†

Experimental details

Computational details

Neon matrix spectral data

Quantum chemical predictions of anharmonic coupling constants

Analysis of the donor OH-stretching dimerization red-shift

Summary