Gas-phase halide affinity of aliphatic alcohols estimated by the kinetic method

Eur. J. Mass Spectrom. 10, 847–856 (2004)
DOI: 10.1255/ejms.695

Gas-phase halide affinity of aliphatic alcohols estimated by the kinetic method

Rodinei Augusti,^a Daniella V. Augusti,^a Hao Chen,^b and R. Graham Cooks^b,*
^aDepartamento de Química, Universidade Federal de Minas Gerais, Belo Horizonte/ MG, Brazil 31270-901
^bDepartment of Chemistry, Purdue University, West Lafayette, IN 47907, USA

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, 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 was 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 was 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–CH₂–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.

Keywords: kinetic method; halide affinity; aliphatic acohols; tandem mass spectrometry; kinetic isotope effect

You can buy this paper on-line in PDF format; it costs only £11.75. Just click on the BUY on-line button. You can pay on-line through a secure server and get access immediately.