Primary isotope effects and reacting bond orders

2 PRIMARY ISOTOPE EFFECTS AND REACTING BOND ORDERS 

Experimental data are available for the primary hydrogen isotope effect, kH/kDy in some proton transfer reactions from a-CH bonds of aliphatic carbonyl and nitro compounds to various oxygen bases. Bell and 

Reacting bond orders, calculated and experimental isotope effects in proton transfer reactions 

Reaction rtmax kH/kD (25 °C) Calculated stretching vibrational contribution feH/feD Experimental3 

These findings are in agreement with theoretical predictions [93—96] (see Vol. 2, pp. 360—361). A maximum isotope effect is expected if the hydrogen ion is approximately one-half transferred in the transition state, and lower isotope effects are expected if the transition state is either more reactant-like (n2 0.5, n 0.5) or more product-like (n2 0.5, n 0.5). In Table 8, experimental isotope effects are compared with calculated nmax values and with calculated stretching vibrational contributions to the isotope effects. The calculations have been done on the basis of the semi-empirical model, as outlined in Sect. 4.1. As expected, calculated n values close to 0.5 correspond to high experimental isotope effects and n values different from 0.5 correspond to low experimental isotope effects. However, the calculated isotope effects exhibit very little dependence on nmax i they are close to the maximum value of the stretching vibrational contribution even for the examples with n = 0.18 and n = 0.12. This is a consequence of the relatively high curvature of the barrier [96], as computed from the semi-empirical model. 

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