Small-ring cyclic alkynes: ab initio molecular orbital study of cyclohexyne (original) (raw)

1987, The Journal of Organic Chemistry

The structures of the lowest energy singlet and triplet electronic states of cyclohexyne have been determined by ab initio MO theory at the G V B and UHF levels, respectively, with the split-valence 3-21G basis set. Both electronic states prefer a Cz nonplanar structure. Harmonic force constant calculations on the optimized structures demonstrate that both singlet ('A) and triplet (3B) cyclohexyne are relative minima on the corresponding C6H8 potential-energy hypersurface. The triplet state is predicted to lie about 42 kcal/mol above the ground-state singlet. Harmonic vibrational frequencies are predicted for both electronic states. The triple-bond harmonic stretching frequency for singlet cyclohexyne is predicted to be 2003 cm-', whereas the analogous stretching frequency for the triplet state is 1663 cm-'. In terms of C=C bond distance, diradical character, singlet-triplet energy separation, and C=C stretching frequency, cyclohexyne should be considered as a normal, yet strained, alkyne. (1) (aPericls, M. A.; Riera, A.; Solb, A. J. Chem. Res. (s) 1985,328. (b) Olivella, S.; Pericls, M. A,; Riera, A.; Solb, A. J. Am. Chem. SOC. 1986. 108. 6884. (8) Olivella,'S.; Pericls, M. A,; Riera, A.; Solb, A. J. Chem. Soc., Perkin (9) Krebs, A.; Cholcha, W.; Muller, M.; Eicher, T.; Pielartzik, H.;