Entropy-driven proton-transfer reactions

Abstract

The relation between kinetics and thermochemistry in fast reactions is examined, including reactions with substantial entropy changes. Rate constants for such reactions, in the range of (0.02-3.0) {times} 10{sup {minus}9} cm{sup 2}s{sup {minus}1}, were measured by pulsed high-pressure mass spectrometry. The following relations were observed: (1) The reaction efficiency in either direction is controlled uniquely and completely by the overall reaction free energy change. Specifically, the efficiency r is determined by the equilibrium constant according to r = K/(1+K). (2) The sum of reaction efficiencies in the forward (exergonic) and reverse (endergonic) directions is near unity (r{sub f}+r{sub r}{approx equal} 1). These relations are observed in anionic and cationic systems, in reactions with {Delta}H{degree} up to 12 kcal/mol and with {Delta}S{degree} up to 15 cal/(mol K). Consistent with (1), reactions that are endothermic up to 7 kcal/mol can nevertheless proceed near the collision rate, when positive entropy changes make the reactions exergonic. The entropy changes are effective regardless of their structural origin. Relations analogous to (1) and (2) are also derived for reactions with multiple channels that proceed without significant barriers through a common intermediate.