Secondary kinetic isotope effects in carbon-nitrogen rotation of amides

Abstract

A secondary deuterium kinetic isotope effect (KIE) κ H /κ D of 2.0 was recently reported for trans →cis of 1-phenylcyclohexene-2-d. The large value was attributed to loss of all the zero-point energy associated with the out-of-plane C-H bending mode. If this phenomenon is general, a large KIE might also be manifested in the C-N rotation of amides, whose rates can be determined by NMR methods. To demonstrate KIEs convincingly, it is necessary to measure the rates for H and D amides simultaneously in the same solution. Such measurements are possible because isotope shifts separate the signals