Anharmonicity and hydrogen bonding: I. A near-infrared study of methanol trapped in nitrogen and argon matrices Original Research Article

The infrared spectra of six isotopic species of methanol trapped in solid nitrogen and argon were measured in the domain 7300–300 cm−1. Among the various combinations and overtones observed, the 2ν1 (OH or OD stretching), 2ν6 (OH or OD bending) and ν1+ν6 transitions were examined as a function of the concentration and compared to the ν1 and ν6 fundamentals. Most of the bands of monomer and dimer were identified. For aggregates larger than the dimer no absorption assignable to 2ν1 was detected, which, taking into account the accuracy of the intensity measurements, leads to a ν1/2ν1 intensity ratio greater than 1000 for these polymers, to be compared to ≈20 for the monomer and 400 for the proton donor molecule of the dimer. An opposite situation is encountered for the bending mode, the ν6/2ν6 intensity ratio decreasing from monomer to polymers, possibly because of a Fermi resonance between 2ν6 and ν1 whose frequencies get closer in case of aggregation. For the ν1+ν6 combination the effects of hydrogen bonding are not as dramatic as in the case of the overtones. From the frequencies and relative intensities some anharmonicity parameters were deduced. Noticeable differences between the spectra of the proton donor and of the proton acceptor molecules of the dimer are explained by changes in the dipole moment function upon H-bond formation.