The most stable isomers in HC4N and HC6N

The relative stability of the triplet-linear A to the singlet B with a C-3-ring in an HC4N molecule was investigated at the density functional level of theory with Becke's three parameters (B3LYP) and the coupled-cluster singles and doubles with triple contributions (CCSD(T)). The various qualities of the correlation-consistent basis sets were used. After the zero-point vibrational energy corrections, the singlet B is by 4.2 kcal/mol lower in energy than the tripler A at the present highest level of theory (CCSD(T)/cc-pCVTZ with core electron correlations (CCSD(T)(full)/cc-pCVTZ)). The B3LYP calculations failed to predict the relative stability, however, the optimized geometries even with the cc-pVDZ basis set are in good agreement with the accurate CCSD(T)/cc-pVTZ ones. For the larger HC6N molecule, the CCSD(T)/cc-pVTZ and cc-pCVDZ energy calculations (with and without core electron correlations) on the basis of the B3LYP optimized parameters were performed to predict the relative stabilities of the tripler-linear C to the two singlet-rings (D and E). The triplet C is the most stable isomer, which are by 2.4 and 4.0 kcal/mol lower in energy than the singlets D and E, respectively, after the corrections due to the zero-point vibrational energies. (C) 2000 Elsevier Science B.V. All rights reserved.