Infrared spectroscopy of matrix-isolated carbon clusters, with emphasis on C8 and C9 Original Research Article

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 C8 and C9 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 Hütter, Lüthi and Diederich (HLD) for the linear C8 cluster. A moderately intense band has also been located at 2078 cm−1 (Ar matrix) and its shown to correlate well with the two known C9 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 v5 mode of linear C9. Combination bands involving symmetric and asymmetric stretching modes have been observed for C5, C6, C7 and C9 and are used to estimate the totally symmetric frequencies for these species. Good agreement with theory ios found. Finally, anharmonicity constants for the prominent asymmetric stretching frequencies are determined from the overtone bands of C3, C5, C6, C7 and C9.