A Fourier transform infrared, C-13 isotopic study has been made of two previously identified fundamentals, nu(4)(sigma(u)) and nu(5)(sigma(u)), of the linear C-7 carbon cluster which, in the present investigation, was formed by trapping the products of the evaporation of graphite in solid Ar at similar to 10 K. Measured isotopic 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 nu(4)(sigma(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 nu(5)(sigma(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.