The proton-bound dimer of acetone

Abstract

Kinetic isotope effects (KIE) affect rates of ion–molecule reactions and provide access to important information on binding patterns of gaseous ions and neutral molecules and their reaction mechanisms.1 Mass spectrometric techniques have been widely used to measure KIE, and Cooks’ kinetic method has been invaluable to measure the intrinsic nature and extents of KIE, most particularly the relatively small values normally associated with secondary KIE.2 Recently, using electrospray ionization mass spectrometry (ESIMS), Schroder et al.3 used Cooks’ kinetic method to measure the secondary KIE associated with the 5–20 eV collision-induced dissociation (CID) of the proton-bound dimer of acetone-d0 and acetone-d6, presumably the loosely bonded ion 1′, and found an inverse secondary KIE of 0.82–0.79. This value is in marked contrast to the normal KIE of 1.17 reported by us4 for 15 eV CID with argon of the mass-selected proton-bound dimer of acetone-d0 and acetoned6 generated in the gas-phase by chemical ionization (CI). For the metastable ion, KIE of 1.012d and 0.933 have also been measured. Hence, for ions presumed to be 1′, different instruments and methods provide KIE ranging widely from 1.17 to 0.79. These values vary far beyond the experimental uncertainties, from normal to inverse KIE!