Cyclobutyne: Minimum or Transition State?

A cornucopia of very high-level theoretical methods has been used to study cyclobutyne, a molecule that has been the center of much speculation. We conclude that cyclobutyne is a transition state in its singlet ground state, based on new coupled cluster and multireference computations presented in this research. This is substantially different from other theoretical studies proposing the existence of singlet cyclobutyne as a minimum. The singlet cyclobutyne transition state ( C2 v) exhibits a ring puckering imaginary vibrational mode, leading to two equivalent minima, cyclopropylidenemethylenes in C s symmetry, with a barrier height of ∼23 kcal/mol. In contrast with previous studies, singlet cyclopropylidenemethylene in C2 v symmetry was predicted to be a transition state, not a minimum. Triplet cyclobutyne is a genuine minimum and higher-lying than the lowest singlet state by ∼15 kcal/mol. New computations give the total ring strain of the singlet cyclobutyne to be 101 kcal/mol, with an in-plane π-bond strain of 71 kcal/mol.