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The lowest singlet and triplet electronic states of cyclobutyne have been investigated by using the nonempirical molecular electronic structure theory. Initially, singlet cyclobutyne was studied with use of two-configuration self-consistent-field (TCSCF) methods and triplet cyclobutyne with single-configuration SCF theory. Three standard basis sets, of double-xi (DZ) through double-xi plus polarization (DZ + P) caliber, were employed, the largest being designated C(9s5p1d/4s2p1d), H(4s1p/2s1p). Geometrical structures and harmonic vibrational frequencies were determined with use of analytic gradient techniques. From purely geometrical considerations, the short carbon-carbon bond in singlet cyclobutyne may be labeled a weak (or long) triple bond, with r/sub e/(C double bond C) = 1.258 A. For triplet cyclobutyne a much longer carbon-carbon distance (1.323 A) is predicted, consistent with the sort of C double bond C double bond expected for this diradical species. The triple-bond CC harmonic stretching frequency for singlet cyclobutyne is predicted to be 1911 cm/sup -1/, while the analoguous stretching frequency for the triplet diradical species is 1779 cm/sup -1/. Vibrational analyses demonstrate that (at the levels of theory considered) both singlet and triplet cyclobutyne are relative minima on the C/sub 4/H/sub 4/ potential-energy hypersurface. For the former, the lowest frequency predicted at the DZ+d TCSCF levelmore » of theory is 276 cm/sup -1/, corresponding to the ring-puckering mode. These remarks notwithstanding, it is to be emphasized that singlet cyclobutyne is a highly energetic species, predicted to lie approx. 78 kcal above vinyl acetylene, HC double bond C - CH double bond CH/sub 2/, the absolute minimum among C/sub 4/H/sub 4/ molecules. Finally, singlet cyclobutyne is predicted to lie 13 kcal below its triplet diradical.« less

Singlet cyclobutyne: a relative minimum on the C4H4 potential-energy hypersurface?