Deuterium Kinetic Isotope Effects in the Gas-Phase S_N2 Reactions of Solvated Fluoride Ions with Methyl Halides

Abstract

Rate constants and deuterium kinetic isotope effects (KIEs) are measured for gas-phase nucleophilic substitution (SN2) reactions of solvated fluoride ions of F-(methanol) + CH3X (X = Br, I), F-(isopropyl alcohol) + CH3I, and F-(hydrogen fluoride) + CH3I at 300 K. The isotope effects are determined as the rate constant ratio k H/k D for specifically deuterated reactants, that is, methanol (CH3OH, CD3OH, CH3OD, and CD3OD), isopropyl alcohol (i-C3H7OH and i-C3H7OD), hydrogen fluoride (HF and DF), and methyl halides (CH3X and CD3X). The data reveal identical trends to those previously observed for F-(water) + CH3X (O'Hair, R. A. J.; Davico, G. E.; Hacaloglu, J.; Dang, T. T.; DePuy, C. H.; Bierbaum, V. M. J. Am. Chem. Soc. 1994, 116, 3609). The SN2 reactivities decrease as reaction exothermicity decreases (CH3I > CH3Br > CH3Cl) and as the nucleophile is solvated. Moderate inverse kinetic isotope effects (k H/k D < 1) are observed for the deuteration of the methyl halide, whereas substantial inverse KIEs are measured for the deuteration of the hydroxyl group of the solvent (or the deuteration of hydrogen fluoride). Moderate inverse KIEs are also measured for the deuteration of the methyl group in methanol. The observed trends and magnitudes of the isotope effects are rationalized qualitatively in terms of the SN2 transition-state structure and bonding interactions analogous to those in the F-(H2O) + CH3X system.

Deuterium kinetic isotope effects in microsolvated gas-phase E2 reactions

“…The solvent KIEs, as summarized in Table 1, are moderately inverse and consistent with our previous results for solvated S n 2 reactions [17,18]. This effect is attributed to a decrease in bond length within the solvent molecule in the transition state, relative to those bonds in the reactant ion.…”

“…These effects are multiplicative [17,18]; the KIE for the overall perdeutero reaction is the product of the KIE for the perdeutero alkyl halide reaction and the perdeutero solvent reaction. Errors in branching fractions are estimated to be Ϯ50% due to correction for mass discrimination and the presence of ϳ15% F Ϫ reactant anion from collisional dissociation of the solvated anion upon injection.…”

“…Hu and Truhlar [10] computationally studied the reaction of F Ϫ (H 2 O) with CH 3 Cl, and their results are in excellent agreement with experiment. Kato et al [18] expanded the experimental work by examining the reactions of F Ϫ (HF) and F Ϫ (CH 3 OH) with methyl halides. Small to moderate inverse KIEs were reported for deuteration of the methyl halide.…”

Direct ab initio molecular dynamics study on a microsolvated SN2 reaction of OH−(H2O) with CH3Cl

“…[1][2][3][4][5][6][7][8][9][10][11][12][13][14][15] Nevertheless, fewer studies have addressed the role of solvation in these reactions. 16,17 From an experimental point of view, O'Hair et al investigated solvent effects on the S N 2 reaction of fluoride ion with methyl chloride and measured the product ions formed by the reaction F − ͑H 2 O͒ +CH 3 Cl. 18 They detected two ions, Cl − and Cl − ͑H 2 O͒, as products.…”

Fundamental Aspects of Gas Phase Ion Chemistry Studied Using the Selected Ion Flow Tube Technique

“…71) We speculate that a similar mechanism is operative for the KIE upon the deuteration of H(3) in methanol. 74) Isotope scrambling occurs via interconversion between the non-classical 1 and classical 2 structures. The reaction is all the way downhill relative to the reactants, since the relative energies of those intermediates as well as of the connecting transition states are significantly lower than the reactant energy by at least 20 kcal mol ῍1 .…”