SECONDARY H/D ISOTOPE EFFECTS IN METHYL-TRANSFER REACTIONS DECREASE WITH INCREASING LOOSENESS OF THE TRANSITION STRUCTURE

The barriers of identity gas-phase methyl-transfer reactions X- + CH3X --> XCH3 + X- are directly related to the looseness of their transition structures, and inversely related to the secondary kinetic isotope effects in the methyl group (k(H)/k(D) < 1). This means that the isotope effects decrease with increasing looseness of the transition structure, in contrast to current belief. Similar trends are observed at the 4-31G, 6-31 +G*, and MP2/6-31 +G* computational levels, and at each level, the C-H bond lengths of the transition structures do not depend upon X. The isotope effects are inverse primarily as a result of the increase in C-H (C-D) stretching frequencies (C-H (C-D) bond shortening) that accompanies the tetrahedral to trigonal change in geometry. Because of the constant C-H bond lengths in the transition structures, the largest isotope effect (smallest k(H)/K(D)) is seen with the substrate having the longest C-H bond. The contributions to the isotope effects from the complementary changes in the C-H (C-D) bending vibrations along the reaction coordinates are normal.