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-CH 2-CH 2-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-CH 2-CH 2-OH + 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.

Pyridine N‐oxide and pyridine‐d5N‐oxide: an electrospray/tandem mass spectrometric study carried out at high mass resolution

“…When a value of T eff = 500 K is assumed and with R = 8.3143 J mol −1 K −1 , the difference in PA values is found to range from 2.1–2.7 kJ mol −1 ; this value is only in fair agreement with the CBS‐QB3 computed value of 0.6 kJ mol −1 . When the CBS‐QB3 computed value for the excess in PA for pyridine‐ d 5 N ‐oxide is used together with the experimentally determined ratio of ion signal intensities, T eff is found to range from 111–147 K. This result is somewhat low given that a recent report37 on the gas‐phase halide affinity of aliphatic alcohols estimated by the kinetic method found T eff = 643 ± 25 K. When a value of T eff = 500 K is assumed together with the CBS‐QB3 computed value for the excess in PA for pyridine‐ d 5 N ‐oxide, the expected ratio of ion signal intensities is 100:87. Thus the experimental ratio of 100:56 ± 4 corresponds to a greater contribution due to protonated pyridine‐ d 5 N ‐oxide by some 50% than expected on the basis of the calculated difference in PAs and a reasonable value for T eff .…”

Tetrafluoroterephthalonitrile as an anion‐π donor: theoretical evaluation and application to anion recognition

“…In the case that weak interactions are involved within the dimer, this ion dissociates to generate two monomers [AH] + and [BH] + , the most intense ion corresponding to the molecule having the highest proton affinity. One could therefore transpose this approach to measure relative halide affinities [26][27][28] provided heterodimers could be presently generated. To this end, we studied equimolar 1 : 1 : 1 mixtures of 2/3/NBu 4 X by electrospray to generate these heterodimers.…”

Final Report

“…24 Although extensively applied to evaluate gas phase proton and cation affinities, the kinetic method has seen little application to the determination of anion affinities. Therefore, a collaboration with the Department of Chemistry at Universidade Federal de Minas Gerais (Belo Horizonte, Brazil) was established to evaluate the potential of the kinetic method in estimating halide (chloride, bromide and fluoride) affinities of simple aliphatic alcohols (i.e.…”