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.