Gas-Phase Halide Affinity of Aliphatic Alcohols Estimated by the Kinetic Method

Abstract

The chloride affinity of ethanol was determined by the kinetic method by examining the dissociation of the [EtOH—Cl—HOR]− dimers (ROH=methanol, 1-propanol, 2-propanol and tert-butanol), generated in the ion source of a triple quadrupole mass spectrometer using chloroform as the chemical ionization reagent. Dimer mass selection and collision-induced dissociation using argon furnished, as products, the two individual alcohol–chloride anionic complexes, i.e. [EtOH+Cl]− and [ROH+Cl]−. The natural logarithm of the ratio of their abundances, i.e. ln ([ROH+Cl]–/[EtOH+Cl]−), correlated linearly with the experimental ROH chloride affinity. Using this linear relationship, the chloride affinity of ethanol is determined to be 43±3 kJ mol−1, which is consistent with the literature value of 43.5±8.4 kJ mol−1. From the slope of the kinetic method plot, an effective temperature of 643 ± 25 K was calculated, suggesting the weakly-bound nature of the cluster ions. The validity of the results was confirmed using the extended version of the kinetic method which showed that there is a negligible difference (–0.06 J mol K−1) in the entropy requirements of the two fragmentation channels. It is also demonstrated that this methodology can be applied to estimate the analogous bromide and fluoride affinities of aliphatic alcohols and to estimate halide affinities not previously known. Thus, these cases constitute additional examples of the advantageous application of the kinetic method in the determination of unknown thermochemical properties. Furthermore, in the dissociation of [EtOH—Cl—HO–CH2–CH2–Y]− cluster ions (Y=F, Cl, OMe and Me), a remarkable linear relationship was observed between the natural logarithm of abundance of product ratios, i.e. ln ([Y–CH2–CH2OH+Cl]−/[EtOH+Cl]−) and the electronegativity of the Y substituent. This is ascribed to the inductive effect in stabilizing the charge on chloride in the cluster ion.