HOW STABLE IS CYCLOBUTYNE - THE ACTIVATION-ENERGY FOR THE UNIMOLECULAR REARRANGEMENT TO BUTATRIENE

The barrier height for the disappearance of the singlet ground state of cyclobutyne has been investigated using ab initio molecular quantum mechanical methods. Stationary-point geometries were determined using the two-configuration self-consistent-field (TCSCF) method in conjunction with double zeta plus polarization (DZP) basis sets. The transition state exhibits strong biradical behavior. At the DZ+P TCSCF level, the classical barrier for unimolecular rearrangement to butatriene is 55 kcal/mol. With configuration interaction including all single and double excitations with respect to both TCSCF reference configurations, the classical barrier is reduced to 51 kcal/mol. Significant further lowerings in the cyclobutyne barrier occur when coupled cluster (CC) methods are applied. Including all connected triple excitations with the CCSD(T) method, the classical barrier is reduced to 41 kcal/mol with the DZP basis. Further corrections for the inherent ''two reference configuration'' nature of the problem and for zero-point vibrational energy yield a final value of approximately 25 kcal/mol for the activation energy. Thus clyclobutyne is reasonably stable with respect to unimolecular isomerization to butatriene.