A b i n i t i o study of the infrared spectra of linear Cn clusters (n=6–9)

The harmonic frequencies, infrared intensities, and isotopic substitution bands of C6 have been evaluated at the MP2/6-31G* level. They confirm previous assignments of the 1952 and 1197 cm−1 matrix IR bands to C6. Isotopic substitution spectra at the MP2/6-31G* and HF/4-21G levels, as well as those with a newly developed ad hoc modified neglect of diatomic overlap (MNDO) method are shown to be in good agreement with experiment for C4, C5, and C6. On the basis of RHF/4-21G, RHF/6-31G*, MP2/4-21G, MP2/6-31G*, and ad hoc MNDO data, including theoretical frequencies, IR intensities, and isotopic substitution spectra, it is shown with virtual certainty that the 2128 and 1893 cm−1 matrix IR bands belong to C7. A recent assignment of the very intense 1997 cm−1 matrix IR feature to C8 is contradicted on the same grounds, and an alternative assignment to C9 put forward.