MECHANISMS OF ELIMINATION REACTIONS .11. THEORETICAL CALCULATIONS OF ISOTOPE EFFECTS IN ELIMINATION REACTIONS

Deuterium and sulfur isotope effects have been calculated for E2 reactions of hydroxide ion with ethyl-dimethylsulfonium ion, using the Wolfsberg-Schachtschneider FG matrix program. The dependence of the isotope effects on transition-state force constants (including off-diagonal F matrix elements used to achieve a zero or imaginary reaction coordinate frequency) was explored. The deuterium isotope effect goes through a maximum when the proton is half-transferred, and is not strongly affected by the extent of weakening of the carbon-sulfur bond. Similarly, the sulfur isotope effect increases with increasing extent of carbon-sulfur bond weakening, and does not depend much on the extent of proton transfer. Less reassuring are two other observations. The deuterium isotope effect can be changed drastically by changes in the degree of coupling (off-diagonal F matrix elements) of the proton-transfer motion with other atomic motions, and the sulfur isotope effect can remain very small until the stretching force constant of the carbon-sulfur bond has decreased to less than half of its original value. The implications of these results for the use of isotope effects in determining transition-state structures are discussed. © 1969, American Chemical Society. All rights reserved.