The notion that tensile force along a bond accelerates its scission is so intuitive that it is rarely if ever questioned. It is often argued that the obligatory elongation of the scissile bond in the transition state allows (partial) relaxation of the strain that generates the force thus stabilizing the transition state. However, kinetics of chemical reactions in multiatomic reactants is governed by reaction paths that rarely if ever depend on a single internuclear distance. If the formation of the transition state requires contraction of some distances, they may completely mask the elongation of the scissile bond. We have recently shown that electrocyclic homolysis of the C-C bond in cyclobutene is accelerated 10-fold per 120 pN of tensile force along the scissile bond. In contrast, the kinetics of thiol/disulfide exchange is force-independent even though the S-S bond, which undergoes nucleophilically-assisted heterolysis, elongates by 0.5 Å in the transition state. This is an example when intuitive ideas from the macroscopic word cannot be mapped onto the molecular dynamics. Another example is the acceleration of bond scission by force that is orthogonal to the scissile bond.
Both examples are consistent with the known geometry of a pentacoordinate transition state through which these reactions most likely proceed. In nucleophilic scission of the S-S bond, the movement of the leaving group into an axial position decreases the separation between the two atoms bound by the scissile bond; while the elongation of the scissile bond increases it. The net result is little change in the dimensions of the reacting group along the molecular axis containing the scissile bond but a significant elongation of the reactive group along orthogonal directions. We suggest that even simple homolytic scissions of covalent bond would follow similar patterns.
- Nucleophilic scission of the S-S bond is insensitive to force even though the scissile bond elongates substantially in the transition state and the force is along the bond. Because the reaction proceeds through a pseudo-trigonal bipyramidal transition state, the elongation of the S-S bond is compensated by the contraction of the C…C distance when one of the CH2 groups moves into the equatorial position.
- Nucleophilic scission of a P-O bond is accelerated by force orthogonal to the scissile bond because the spectator ligands on which the force is applied are farther apart in the transition state than in the reactant as required when they move from a tetrahedral arrangement to the trigonal bipyramidal one.