Isotope Effects on the Unimolecular Dissociation of Ionized 3-Methyl-2-butanol:Reactions via a Long-Lived C-H-C Hydrogen-Bridged Ion-Neutral Complex

摘要

A pronounced isotope effect causes metastable CD_3CHOHCH(CH_3)_2~(+centre dot)ions to expell C_3H_6D_2 in preference to C3H7D in a ratio of ~33:1;a number of related compounds show similar effects.High-level ab initio calculations suggest that the reactant alcohol molecular ion possesses an extraordinarily long a-carbon-carbon bond and that the reaction proceeds via the formation of an intermediate hydrogen-bridged complex of propane and ionized vinyl alcohol,in which the bridging hydrogen atom is almost midway between the two carbon termini.The isotopic preference reflects the difference between the zero-point vibrational energies of the isotopically different product pairs rather than kinetic isotope effects on the hydrogen atom transfer reactions that precede dissociation.