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

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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+, 35Cl+ or 37Cl+ and A/A′ = acetonitrile/acetonitrile − d3, acetonitrile/acetonitrile‐15N, acetonitrile‐d3/acetonitrile‐15N, acetone/acetone‐d6, acetone/acetone‐18O, acetone‐d6/acetone‐18O, pyridine/pyridine‐d5, pyridine/pyridine‐15N, pyridine‐d5/pyridine‐15N, or 3‐(35Cl)chloropyridine/3‐(37Cl)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 (ΔX+) as estimated by the AX+/A′X+ abundance ratios. Using [A···H+(D+)···A′] and [A···35Cl+(37Cl+)···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 Δ(ΔZPE)s for the competitive dissociation reactions that accurately predict the nature (normal or inverse) of the measured KIEs. Copyright © 2001 John Wiley & Sons, Ltd.