Comparison of ab initio MP2/6-311+G(d,p) predicted carbon-hydrogen bond distances with experimentally determined r0 (C-H) distances

Fifty different carbon-hydrogen distances have been predicted from ab initio MP2/6-311+G(d,p) calculations, which range from a short value of 1.0611 Angstrom A for HCNO to a long value of 1.1044 Angstrom for H2CO. The values include those predicted for a series of methyl (CH3) moieties where the two different C-H distances vary by as much as 0.005 Angstrom. These predicted values are compared to r(0)(C-H) distances obtained from the "isolated" carbon-hydrogen stretching frequencies, as well as to r(0) or r(s) parameters obtained from microwave data. Except for the very short C-H bonds, the ab initio values from the MP2/6 - 311CG(d, p) calculations can be used for the carbon-hydrogen distances with error limits of +/- 0.003 Angstrom. By utilizing the spectral data from CD3CClO, it is shown that combination bands in the C-H stretching region could cause problems in the identification of the "isolated" C-H stretching frequency from the CD2HCClO isotopomer. The value of the ab initio predicted C-H distances for checking unusually long or short r(s)(C-H) or r(0) values is demonstrated.