Abstract: The curvature correction factor Ais introduced into the tunneling probability expression derived by W.H.Miller.Then the modification for the microcanonical unimolecular rate constant formula is obtained.The application of the formula to the unimolecular hydrogen transfer reaction of F─C≡C─H indicates that the rate constants have asignificant enhancement in the tunneling region when the factor Ais involved,but the selection of rotation states has little effect on the reaction rate.

Key words: Microcanonical ensemble, Unimolecular rate constant, Curvature, Selection of rotation state