Primary and secondary kinetic isotope effects in proton (H(+)/D(+)) and chloronium ion ((35)Cl(+)/(37)Cl(+)) affinities

Abstract

The Cooks' kinetic method and tandem-in-space pentaquadrupole QqQqQ mass spectrometry were used to measure primary and secondary kinetic isotope effects (KIEs) in H(+) and Cl(+) (X(+)) affinity for a series of A/A(') isotopomeric pairs. Gaseous, isotopomeric, and loosely bound dimers [A...X(+)...A(')] were formed in combinations in which X = H(+), D(+), (35)Cl(+) or (37)Cl(+) and A/A(') = acetonitrile/acetonitrile - d(3), acetonitrile/acetonitrile-(15)N, acetonitrile-d(3)/acetonitrile-(15)N, acetone/acetone-d(6), acetone/acetone-(18)O, acetone-d(6)/acetone-(18)O, pyridine/pyridine-d(5), pyridine/pyridine-(15)N, pyridine-d(5)/pyridine-(15)N, or 3-((35)Cl)chloropyridine/3-((37)Cl)chloropyridine. Under nearly the same experimental conditions, the dimers were mass-selected and then dissociated by low-energy collisions with argon, yielding AX(+) and A(')X(+) as the fragment ions. KIEs were measured from the changes in ion affinities of the neutrals (DeltaX(+)) as estimated by the AX(+)/A(')X(+) abundance ratios. Using [A...H(+)(D(+))...A(')] and [A...(35)Cl(+)((37)Cl(+))...A(')] dimers and by comparing their extent of dissociation under nearly identical collision-induced dissociation conditions, the kinetic method was also applied, for the first time, to measure primary KIEs of the central ion as well as their influence on secondary KIEs. Becke3LYP/6-311++G(2df,2p) calculations were found to provide Delta(DeltaZPE)s for the competitive dissociation reactions that accurately predict the nature (normal or inverse) of the measured KIEs.