Degenerate vibrations isotope effect

Recently, this view of secondary a-deuterium KIEs has had to be modified in the light of results obtained from several different theoretical calculations which showed that the Ca—H(D) stretching vibration contribution to the isotope effect was much more important than previously thought. The first indication that the original description of secondary a-deuterium KIEs was incorrect was published by Williams (1984), who used the degenerate displacement of methylammonium ion by ammonia (equation (4)) to model the compression effects in enzymatic methyl transfer (SN2) reactions. 

Recent works by Herman et al. and Field et al. have focused on molecules of the family of acetylene, in particular C2HD [112] and C2H2 (see Refs. 122 and 123 and Field et al., Intramolecular Dynamics in the Frequency Domain, this volume). These linear molecules have three stretching modes, 1, 2, and 3, and two doubly degenerate bending modes, trans 4 and cis 5. Isotopic effects appear particularly striking in the vibrational dynamics, as shown in the comparative study of the dynamics of the above isotopomers. 

Figure 6.12 Free energy correlation (shown schematically) for the H and D zero-point vibrations for a degenerate stepwise double hydrogen transfer reaction according to Eq. (6.31), where secondary kinetic isotope effects and isotopic fractionation between the initial and the intermediate state were neglected. Adapted from Ref [18c],...

Example 9-1. Given the wavenumbers and vibrational modes shown below for the BCI3 molecule in the two common isotopes of B, determine the symmetries of each vibrational mode determine whether the mode is IR-active, Raman-active, or both and rationalize the observed isotope effect. For the degenerate vibrational modes, only a single stretching frequency is observed. 

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