Vibrational analysis of the inelastic neutron scattering spectrum of tetracyanoethylene-hexamethylbenzene by electronic structure calculations

The vibrational spectrum of the electron donor-acceptor complex formed from tetracyanoethylene and hexamethylbenzene was measured at 15 K by inelastic neutron scattering (INS). Direct analysis of the measured spectra was accomplished using both an isolated complex approximation at the B3LYP/6-31G** theory level and a full periodic representation at the BLYP/dnd theory level. Six intermolecular modes were identified below 100 cm^-1. The 165 cm^-1 vibrational mode was identified to arise from the TCNE symmetric bend but a small component from the first overtone of v₈ was also found. Comparison of the observed and calculated INS spectra revealed that the BLYP/dnd calculations for the crystalline solid provide a good description of the experimental INS spectrum and clearly demonstrates the power of the combined experimental/theoretical approach.