Optical and Resonance Raman Studies of the 1:1 and 2:1 Complexes of Hexamethylbenzene with Tetracyanoethylene

Article abstract of DOI:10.1021/j100265a015

The electron donor-acceptor (EDA) complexes of hexamethylbenzene (HMB) and tetracyanoethylene (TCNE) have been studied by optical and resonance Raman spectroscopy in a variety of solvents.The visible absorption spectra of the 1:1 and 2:1 complexes in cyclohexane have been resolved by multiwavelength linear regression of concentration-dependent absorbance data. the resonance Raman spectra of CCl4, CH2Cl2, and cyclohexane solutions of HMB and TCNE have been obtained with 5145-Angstroem excitation.A low-frequency mode at 163-169 cm^-1 is assigned to the first overtone of the donor-acceptor stretch, ν_DA.In cyclohexane, the second overtone of ν_DA is also observed, but is overlapped by an a_2u out-of-plane bending vibration of the methyl groups of HMB, which becomes allowed in the 2:1 complex due to vibronic coupling.A simple molecular orbital picture of the bonding in the 1:1 and 2:1 complexes is proposed which explains the resolved visible spectra as well as the low-frequency Raman data.It is concluded that the second donor molecule is not bound by "charge-transfer" forces, and that, although the ground state 2:1 complex has D_2h symmerty, the two D-A bonds are of unequal strngth.