Isotope Effects on the Unimolecular Dissociation of Ionized
3-Methyl-2-butanol: Reactions via a Long-Lived C-H-C Hydrogen-Bridged
Steen Hammerum, Lars Bo Jensen and Malene Mohr
J. Phys. Chem. A, 109, 3159-3165 (2005).
A pronounced isotope effect causes metastable
CD3CHOHCH(CH3)2+ ions to expell
C3H6D2 in preference to
C3H7D in a ratio of approx. 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 alpha-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.