Fourier transform infrared and theoretical isotopic study of the ν4(σu) and ν5(σu) modes of linear C7

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Abstract
A Fourier transform infrared, 13C isotopic study has been made of two previously identified fundamentals, ν4(σ u ) and ν5(σ u ), of the linear C7carbon cluster which, in the present investigation, was formed by trapping the products of the evaporation of graphite in solid Ar at ∼10 K. Measuredisotopic shifts were compared with theoretical predictions in order to confirm the assignments as well as to investigate the general quality of such predictions. The shifts for the ν4(σ u ) mode were found to be highly sensitive to the level of calculation, whereas consistently good agreement between theory and experiment was found for the ν5(σ u ) mode. This difference in behavior between the two modes is predicted by the isotopic sensitivity index which is shown to be a useful guide for future identifications of vibrational modes based on comparisons between measured and theoretically calculated isotopic shifts.