Structure and vibrations of small carbon clusters from coupled-cluster calculations

Abstract

The vibrational spectrum of the linear carbon clusters C-2-C-9 and of cyclic C-4, C-6, C-8, and C-10 has been investigated using the augmented coupled-cluster, CCSD(T), method and correlation-consistent basis sets, Particular emphasis is placed on assignment of bands in matrix infrared spectra of trapped graphite vapor, The relative energetics of linear and cyclic isomers of C-6, C-8, and C-10 were investigated at the CCSD(T) level using basis sets of [4s3p2d1f] quality. Our best calculations predict that the ground state structure of C-6 is a cumulenic D-3h ring and that of C-8 a polyacetylenic C-4h ring. Linear (3) Sigma(g)(-) states are close in energy (13 +/- 2 and 8 +/- 2 kcal/mol, respectively), and both forms should be observable. The ground state structure of cyclic C-10 clearly has D-5h rather than D-10h symmetry, at least for the r(e) geometry. A linear regression of computed CCSD(T)/[3s2p1d] harmonic frequencies vs observed fundamentals for the previously assigned stretching frequencies of linear C-n clusters has a very high correlation coefficient (up to r = 0.99975 at 10 data points), From the regression line and the calculated frequencies, band origins for linear C-8 are predicted at 2063 +/- 18 and 1711 +/- 14 cm(-1) (99% confidence intervals) and ancillary bands for linear C-9 at 2093 +/- 18 and 1604 +/- 16 cm(-1). From the calculated infrared-active CCSD(T) frequencies for the cyclic clusters and anharmonicity considerations, we suggest that bands near 1695, 1818, and 1915 or 1921 cm(-1) belong to cyclic C-6, C-8, and C-10, respectively, while bands near 1590 and 1715 cm(-1) are probably due to linear C-9 and C-8, respectively. Finally, a broad feature at 2070-2090 cm(-1), which upon deconvolution appears to consist of three bands, is suggested to contain bands of both linear C-8 and C-9.