Carbon-13 Impurity Effect on the a2u Infrared Exciton Spectrum of the Benzene Crystal

The C-13 shift for the a2u molecular benzene mode has been calculated to be −0.33 ± 0.04 cm−1. This confirms our previous assignment of the major pairwise molecular excitation exchange interactions for the a2u vibrational exciton and enables us to pick an exciton density-of-states for the pure crystal. We also calculate the natural-abundance isotopically mixed benzene crystal density-of-states and its spectrum. Predicted in-band spectral resonances are consistent with published experimental data and facilitate their interpretation in terms of Davydov components, density-of-states peaks, band center, site shift, and exciton interactions. The implications and applications of the C-13 presence in ``pure crystal'' spectra are discussed.