The infrared spectrum of matrix-isolated methane - rotation or dimerization?

The infrared spectra of methane trapped in argon and nitrogen matrices at 17 K have been obtained. Significant changes have been observed in the spectra in the triply degenerate stretching and bending regions as a result of annealing to 35 K. Early interpretations of these temperature effects included the proposal that raising the temperature populated the rotational levels of the trapped monomer molecules, while reducing the temperature again caused the reversal of the population process. Ab initio calculations have identified a weakly bound hydrogen bonded dimer, in which one of the CH bonds of one molecule interacts with the triangular face of the second molecule in an axially symmetric species. The computed infrared spectrum of this dimer is characterized by a number of bands in the CH stretching and CH3 bending regions whose positions, after allowing for the overestimation of the wavenumbers typically associated with ab initio calculations, are remarkably close to those observed in nitrogen matrices. An alternative interpretation of the observed spectra is proposed on the basis of the predicted dimer spectrum. (C) 2000 Elsevier Science B.V. All rights reserved.