Infrared spectroscopy of matrix-isolated carbon clusters, with emphasis on C-8 and C-9

Infrared spectra of neutral linear all-carbon clusters, produced by laser ablation of graphite, have been investigated in Ne, Ar and Kr matrices. The effects of thermal annealing on the infrared intensities have been used together with results from recent high level ab initio and density functional theory (DFT) calculations to locate several heretofore unknown infrared bands of linear C-8 and C-9 clusters. Specifically, two infrared bands located at 2071.5 cm(-1) and 1710.5 cm(-1) (Ar matrix), are shown to be well correlated and to match closely the density functional theory (DFT) predictions of 2109 cm(-1) (1366.3 km/mol) and 1703 cm(-1) (984.8 km/mol) by Hutter, Luthi and Diederich (HLD) for the linear C-8 cluster. A moderately intense band has also been located at 2078 cm(-1) (Ar matrix) and is shown to correlate well with the two known C-9 bands at 1601 cm(-1) and 1998 cm(-1). This band is also shown to match well with the DFT predictions of HLD and is assigned to the nu(5) mode of linear C-9. Combination bands involving symmetric and asymmetric stretching modes have been observed for C-5, C-6, C-7 and C-9 and are used to estimate the totally symmetric frequencies for thesez species. Good agreement with theory is found. Finally, anharmonicity constants for the prominent asymmetric stretching frequencies are determined from the overtone bands of C-3, C-5, C-6, C-7 and C-9.