Heavy stretching frequencies

Table 11.4 Separative effect / and isotopic fractionation constant K for heavy isotopes, computed through equation 11.47. is angular totally symmetric stretching frequency derived from Raman spectra (see Bigeleisen and Mayer, 1947 for references).

In terms of both mean absolute error (in symmetric stretching frequencies) and of individual frequencies, density functional models perform significantly better than Hartree-Fock models. As with diatomic molecules, local density models appear to provide the best overall account, but the performance of the other models (except for B3LYP models) is not much different. B3LYP models and MP2 models do not appear to fare as well in their descriptions of frequencies in one-heavy-atom hydrides, and the performance of each appears to worsen in moving from the 6-3IG to the 6-311+G basis set. 

Table 7-2 Mean Absolute Errors in Symmetric Stretching Frequencies for One-Heavy-Atom Hydrides... 

CH3X molecules provide an excellent opportunity to assess the ability of the calculations both to reproduce gross trends in measured vibrational frequencies, for example, trends in CX stretching frequencies, as well as to account for what are presumed to be subtle differences associated with the methyl rotor with change in X. Data are provided in Appendix A7 (Tables A7-9 to A7-16) for the usual collection of theoretical models. The reader can easily verify that the same comments made for diatomic molecules and for one-heavy-atom, main-group hydrides generally apply here as well. 

Weigl studied the exchange reaction between ascorbic acid and heavy water. An examination of the 0—H and C—H stretching frequency region of the normal and the deuterated compound led him to the conclusion that ascorbic acid contains labile hydrogen atoms attached to both carbon and oxygen. 

Ketenyl radical (27) is a species involved both in ketene formation and in ketene reactions, and may be formed by hydrogen atom abstraction from ketene. Ethoxy-acetylene is a thermal source of ketene and forms ketenyl radical by 193 nm photolysis in helium, argon, or xenon, with time-resolved IR emission spectroscopy used to examine the lowest quartet state of the radical. The radical with 1785 36 cm for the fundamental transition of the V2 asym-CCO stretch was observed in helium while CO did not form in this medium, but in Ar or Xe vibrationally excited CO was observed, suggesting rapid coUision-induced intersystem crossing occurred due to a heavy-atom effect (Eqn (4.17)). The presumed product CH formed with CO was not however observed and the expected stretching frequencies would be quite weak. 

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