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

The proton‐bound dimer of acetone

“…Hence, a normal KIE (>1) for the dissociation of the proton-bound dimer of acetone-d 6 /acetone-d 0 1 is observed, as ZPE calculations predict. 4 For the ion of m/z 123 pre-formed in solution, a possible rationale for its contrasting behavior could consider that solvent or counter-ion effects, or both, may alter the stability order as compared with that prevailing for the gaseous ions ( Fig. 1).…”

Dissociation of ionized benzophenones investigated by the kinetic method: effective temperature, steric effects and gas‐phase CO+• affinities of phenyl radicals

“…We re‐examined this system using the kinetic method,4, 5 an approach for the estimation of thermochemical properties based on the rates of competitive dissociations of mass‐selected cluster ions, usually but not always loosely bound complexes: This method has been widely applied to obtain ion affinities,6–10 acidities,11–13 electron affinities14–17 and ionization potentials18–20 and also to evaluate steric effects,21, 22 agostic bonds,23, 24 entropy changes,25–27 isotopic effects28 and, more recently, the quantitative composition of enantiomeric mixtures 29–35. The advantages include its versatility, simplicity and the ability to measure very small energy differences (<1 kcal mol −1 (1 kcal = 4.184 kJ)).…”

Deuterium isotope effects observed during competitive binding chiral recognition electrospray ionization—mass spectrometry of cinchona alkaloid‐based systems

“…A simple, yet elegant, technique that can also be used to measure IEs is Cook's kinetic method (KM) 20. Eberlin et al have employed the KM to measure relative kinetic IEs through collision induced dissociation (CID) of bound dimer ions in the form A·H·A′; where A′ is the isotopically labeled counterpart of analyte A 21. Multiple small molecules studied in this way gave rise to both normal and inverse kinetic IEs.…”