ANHARMONIC-FORCE FIELD AND EQUILIBRIUM STRUCTURE OF METHANE

The anharmonic force field of methane has been refined to fit spectroscopic data from the isotopic species12CH4,13CH4,12CD4,12CH3D,12CHD3 and12CH2D2. Six of the thirteen cubic force constants have been determined experimentally, the remaining cubic constants being fixed at values derived from ab initio calculations. The quartic force field is very crude, in that only frrrr has been refined. It is concluded however that the cubic and quartic force fields, even though they are subject to limitations, provide a considerable improvement in the experimental determination of the re structure and the quadratic force field. The equilibrium bond length is found to be re(CH) = l.0858± 0-001 Å. © 1979 Taylor & Francis Group, LLC.