Dissecting enthalpic and entropic barriers to ultrafast equilibrium isomerization of a flexible molecule using 2DIR chemical exchange spectroscopy.

Ultrafast 2DIR chemical exchange spectroscopy was used to study the dynamic equilibrium between different isomers of dicobalt octacarbonyl. Exchange of population between bridged and unbridged isomers takes place on the time scale of a few picoseconds, corresponding to activation barriers of several kcal/mol. Despite overlapping spectral features in the 2DIR spectrum, the exchange component of the waiting time dependence was isolated by exploiting the well-characterized coherent modulation of nonexchange crosspeaks. The temperature dependence of the forward and reverse rate constants enabled extraction of isomerization energy barriers, where analysis using the Eyring equation indicated a substantial entropic contribution to the free energy barrier (DeltaS(double dagger)(exp) > 0). Comparison to quantum chemical calculations showed reasonable enthalpy agreement, but qualitative disagreement for the entropy of the transition state relative to the isomers (DeltaS(double dagger)(comp) < 0).