Cyclopropyne and Silacyclopropyne:  A World of Difference

The lowest singlet and triplet states of cyclopropyne and silacyclopropyne have been investigated using ab initio electronic structure methods. Employing DZP and TZ(2df,2pd) basis sets, optimum geometries and harmonic vibrational frequencies have been obtained with the following methods: restricted Hartree−Fock or self-consistent-field (SCF), two-configuration self-consistent-field (TCSCF), single and double excitation configuration interaction (CISD) and coupled cluster (CCSD), and CCSD incorporating perturbative estimates of connected triple excitations [CCSD(T)]. Although silacyclopropyne has been observed via matrix isolation, cyclopropyne remains a high-lying saddle point on the C3H2 potential energy surface. Structural and electronic differences between these two molecules are explored. The triplet states of cyclopropyne and silacyclopropyne are minima on their potential energy surfaces and lie higher in energy than the corresponding singlet states, by 10 and 40 kcal mol-1, respectively.