Singlet cyclobutyne: a relative minimum on the C/sub 4/H/sub 4/ potential energy hypersurface
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 level 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.
- Research Organization:
- Univ. of California, Berkeley
- DOE Contract Number:
- AC03-76SF00098
- OSTI ID:
- 6000600
- Journal Information:
- J. Am. Chem. Soc.; (United States), Journal Name: J. Am. Chem. Soc.; (United States) Vol. 105:4; ISSN JACSA
- Country of Publication:
- United States
- Language:
- English
Similar Records
Toward the laboratory identification of cyclopropenylidene
Nitromethylene (H-C-NO[sub 2]). A comparison of the lowest lying triplet and singlet states of a highly unconventional carbene