A theoretical study of the thermal racemization of trans-cyclooctene

Quantum mechanical calculations of trans-cyclooctene find a chiral structur e for the rate-determining transition state for racemization. This structur e has a distorted double bond geometry (C-C=C angle = 157 deg) and a compre ssed ( 1.05 Angstrom) vinylic C-H bond, due to severe transannular interact ions. Interconversion of the lowest-energy ("crown" conformation) ground st ate enantiomers can occur via either of two mirror-image pathways, each con taining two intermediates and three transition states. Semiempirical (AM1) and ab initio (HF/6-31G*) methods overestimate the enthalpy barrier for rac emization by 18 kcal/mol and 6 kcal/mol, respectively, but the B3PW91 densi ty functional theory method with the DZVP basis set reproduces the experime ntal value determined by Cope and coworkers in the 1960's (Delta H = 34.9 k cal/mol (calculated) versus 34.7 +/- 0.9 kcal/mol (experimental).