SPECTROSCOPIC, KINETIC, AND THERMODYNAMIC DEUTERIUM-ISOTOPE EFFECTS IN THE HEXAMETHYLBENZENE TETRACYANOETHYLENE CHARGE-TRANSFER COMPLEX/

The charge-transfer absorption and fluorescence spectra, resonance Ram an spectra, and equilibrium constants and molar absorptivities of the complexes between tetracyanoethylene and hexamethylbenzene (h(18)-HMB) or perdeuteriated hexamethylbenzene (d(18)-HMB) are compared. The ent halpies and entropies of complexation and the absorptivities in CCl4 s olution are the same for the two isotopes to within an experimental un certainty of about +/-10%. The vibrations that carry significant inten sity in the resonance Raman spectra are only slightly shifted by perde uteriation of the HMB, suggesting that hydrogen motions are only weakl y coupled to the charge-transfer transition. However, the fluorescence quantum yields in both CCl4 and cyclohexane solvents indicate that pe rdeuteriation decreases the rate of nonradiative return electron trans fer by a factor of about 1.6, implying more significant participation of modes involving hydrogen motion. Perdeuteriation shifts the absorpt ion spectra about 120 cm(-1) to the blue while having little effect on the fluorescence spectra. Complexes of HMB, durene, and p-xylene with tetracyanobenzene as acceptor similarly exhibit negligible isotope ef fects on the fluorescence band shapes but significant (factors of 1.3- 2.1) effects on the fluorescence yields. Calculations on HMB/TCNE with in the harmonic approximation are unable to reproduce the isotope effe cts on both the spectra and the kinetics with a common set of paramete rs. Anharmonicities of the CH (CD) stretches may play an important rol e as is thought to be the case in other radiationless transition proce sses.