Toward the laboratory identification of cyclopropenylidene

Nonempirical molecular electronic structure theory has been used to predict the geometries and energetics of the lowest singlet and triplet states of C-HC=CH. The closed-shell singlet ground state is predicted to lie about 70 kcal/mol below the lowest triplet state. Actually there are two low-lying triplet states, /sup 3/B/sub 1/ and /sup 3/A/sub 2/, which lie very close energetically. Ground-state cyclopropenylidene is predicted to have a dipole moment ..mu.. approx. 3.4 D, making it a very nucleophilic carbene. Vibrational frequencies are predicted for all three low-lying electronic states at the double-zeta plus polarization (DZ+P) self-consistent-field (SCF) or two-configuration (TC) SCF levels of theory. A comparison of the predicted harmonic frequencies of singlet cyclopropenylidene and cyclopropene at the SCF level of theory is made. These predictions should assist in the identification of cyclopropenylidene from matrix-isolation ir spectroscopy. 37 references, 3 figures, 8 tables.