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The anharmonic force field of methane has been refined to fit spectroscopic data from the isotopic species 12CH4, 13CH4, 12CH4, 12CH3D, 12CHD3 and 12CH2D2. 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 r e structure and the quadratic force field. The equilibrium bond length is found to be r e(CH) = 1·0858 ± 0·001 Å.
Gray et al. (1979) studied this question.