Mosey has a nice follow-up study on the origin of Woodward-Hoffman forbidden ring opening of cyclobutene under mechanical stress.1 (See this blog post discussing the earlier work of Martinez.2) Pulling on cis substituents of a cyclobutene causes the ring to open in a disrotatory fashion. Normally, the WH forbidden pathway is accessed by photolysis which creates a new electronic state. Mosey asks if this same mechanism is occurring during mechanical stress.
On the face of things, this seems unlikely; how can a mechanical force lead to a new electronic state? CASSCF computations with either no applied external force or with varying sized external forces and IRC computations help answer this question. Without an external force, a diradical (or at least a species with high diradical character – and this could be the transition state) is found along the disrotatory pathway. This same diradical is found regardless of the size of the externally applied mechanical force. What does change is the position of the TS along the pathway: as the force increases, the TS becomes earlier, and the reaction barrier diminishes. No change in the electronic state is affected by the applied mechanical stress.
References
(1) Kochhar, G. S.; Bailey, A.; Mosey, N. J., "Competition between Orbitals and Stress in Mechanochemistry," Angew. Chem. Int. Ed., 2010, 49, 7452-7455, DOI: 10.1002/anie.201003978
(2) Ong, M. T.; Leiding, J.; Tao, H.; Virshup, A. M.; Martinez, T. J., "First Principles Dynamics and Minimum Energy Pathways for Mechanochemical Ring Opening of Cyclobutene," J. Am. Chem. Soc., 2009, 131, 6377-6379, DOI: 10.1021/ja8095834
Henry Rzepa responded on 30 Dec 2010 at 2:56 am #
Entropy, or disorder, one might think would be affected by mechanical stress. Thus application of stress might constrain the vibrational motions, reducing entropy. This is an important feature of reactions occurring in cavities, which one can think of as application of stress (a molecular anvil) to a reacting system. Although the study Steve quotes was done using molecular dynamics, there is no discussion of entropy. Does the transition state become earlier because of a change in the free energy, which itself is changed by the entropy? Is there a relationship between mechanically stressed reactions and those happening inside a cavity?
Of course, internal stress is often used to change reaction characteristics, and molecular stability. I am thinking eg of the mechanical stress which can be applied to a cyclo-octatetraene to induce its planarity.