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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 A for HCNO to a long value of 1.1044 A 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 A. 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 MP2/6−311+G(d,p) calculations can be used for the carbon–hydrogen distances with error limits of ± 0.003 A. 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. |
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