Isotopic and anharmonic perturbations to the dipole active vibrational modes of buckminsterfullerene

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Abstract
Force constants of C60 are obtained by a least squares fit to experimental infrared spectra. Vibrational normal modes are computed and used as a zero order basis to calculate the isotopic perturbation as one of the 60 carbon atoms is continuously varied from 12 to 15 amu. A perturbing mass of 13 amu splits the four, threefold degenerate, dipole active modes by amounts ranging from ∼0.5 to over 2 cm−1. Stereoscopic figures of the perturbed normal modes are displayed. The perturbed modes are used as initial conditions for numerically integrating Hamilton’s equations of motion. Using a realistic anharmonic potential, power spectra are generated for one quanta of excitation, and display anharmonic redshifts that range from 0.06 to 0.67 cm−1.