Deuterium kinetic isotope effects in gas phase S(N)2 reactions

Citation
S. Kato et al., Deuterium kinetic isotope effects in gas phase S(N)2 reactions, INT J MASS, 210(1-3), 2001, pp. 223-229
Citations number
28
Language
INGLESE
art.tipo
Article
Categorie Soggetti
Spectroscopy /Instrumentation/Analytical Sciences
Journal title
INTERNATIONAL JOURNAL OF MASS SPECTROMETRY
ISSN journal
1387-3806 → ACNP
Volume
210
Issue
1-3
Year of publication
2001
Pages
223 - 229
Database
ISI
SICI code
1387-3806(20011001)210:1-3<223:DKIEIG>2.0.ZU;2-H
Abstract
Rate coefficients and secondary a-deuterium kinetic isotope effects (KIEs) for the nucleophilic substitution (S(N)2) reactions of Cl- + CH3Br --> CH3C l + Br-, Cl- + CH3I --> CH3Cl + I-, Br- + CH3I --> CH3Br + I-, and their pe rdeuterated analogs are remeasured in the gas phase at 300 K. The measureme nts confirm our previous results [Gronert et al., J. Am. Chem. Soc. 113 (19 91) 4009] and substantial inverse KIEs are measured with an improved accura cy: k(H)/k(D) = 0.77 (+/-0.03), 0.84 (+/-0.01), and 0.76 (+/-0.01), respect ively, for the above systems. Thus, the experimentally observed effect of d euterium substitution in these reactions is considerably more dramatic than predicted previously by transition state theory where KIEs are calculated to be closer to unity. The experimental KIE values for the series of S(N)2 reactions, F-, Cl- + CH3Br and F-, Cl-, Br- + CH3I are both found to decrea se (i.e. become more inverse) with an increase in the transition-state loos eness parameter (RTS) for larger halide anions, in contrast to those for li ghter methyl halides which show the expected positive correlations with RTs .