Benzene vibrational relaxation

Fluorescent and triplet state yields for benzene add to between 0.9 and 1.0 for exciting wavelengths greater than 2530 A. There is, however, some effect of wavelength and as the pressure is lowered it appears that vibrational relaxation is more rapid for some modes of vibration than for others. These data are based on the progression for which in the excited state there is one unit of the frequency 521 cm"1 (an asymmetric mode of vibration) and 0, 1, 2, etc., units of the frequency 923 cm-1 (symmetrical stretching). 

It is clear that a number of questions need to be answered. Why, in the condensed phase, is the intersystem crossing between two nn states so efficient What is the explanation of the conflict between the linewidth studies of Dym and Hochstrasser and the lifetime studies of Rentzepis and Busch, with respect to the vibrationally excited levels It was in an attempt to provide some answers to these questions that Hochstrasser, Lutz and Scott 43 carried out picosecond experiments on the dynamics of triplet state formation. In benzene solution the build up of the triplet state had a lifetime of 30 5 psec, but this could only be considered as a lower limit of the intersystem crossing rate since vibrational relaxation also contributed to the radiationless transition to the triplet state. The rate of triplet state build-up was found to be solvent-dependent. 

The Parmen ter-Tang rules combine in a very sensible manner the common analytic and numerical features of the SSH-type theories currently used to describe vibrational relaxation. They give a very good description of the pattern of energy flow as a result of collisions of benzene and... 

Smalley and co-workers have probed intramolecular vibrational relaxation by viewing the yields and the time-dependence of the fluorescence from Sj in alkylated benzenes. They focus attention on those ring modes whose vibrational frequencies are unshifted by alkylation these are vibrations with nodes at the alkylated ring carbon atom. The absorption lines are sharp, but as the alkyl chain is lengthened, the emission spectrum develops a broad relaxed component, while the intensity of the sharp unrelaxed resonance fluorescence diminishes in intensity as the intensity of the relaxed spectrum increases. The time-dependence of the relaxed and unrelaxed emission is found to be a single exponential decay, so unfortunately, the rapid intramolecular dephasing decay has not yet been followed. 

Sibert EL, Reinhardt WP, Hynes JT. 1984. Intramolecular vibrational relaxation and spectra of CH and CD overtones in benzene and perdeuterobenzene. J. Chem. Phys. 81 1115-1134. 

See, for example, the following and references contained therein E. L. Sibert 111, W. P. Reinhardt, and J. T. Hynes, /. Chem. Phys., 81, 1115 (1984). Intramolecular Vibrational Relaxation and Spectra of CH and CD Overtones in Benzene and Perdeuterobenzene. S. P. Neshyba and N. De Leon,. Chem. Phys., 86, 6295 (1987). Qassical Resonances, Fermi Resonances, and Canonical Transformations for Three Nonlinearly Coupled Oscillators. S. P. Neshyba and N. De Leon,. Chem. Phys., 91, 7772 (1989). Projection Operator Formalism for the Characterization of Molecular Eigenstates Application to a 3 4 R nant System. G. S. Ezra, ]. Chem. Phys., 104, 26 (1996). Periodic Orbit Analysis of Molecular Vibrational Spectra Spectral Patterns and Dynamical Bifurcations in Fermi Resonant Systems. Also see Ref. 6. 

Benzene is a non polar molecule and as such cannot exhibit dielectrically active reorientational relaxation. Investigation of the microwave and far infrared dielectric spectrum indicates that pure benzene exhibits a distinct loss feature. It is well known from ultrasonic studies that molecules with a high degree of symmetry can exhibit translational-vibrational relaxation "24 jp molecules collide inelasti-cally part of their momentum can be used to excite an internal vibrational mode to a higher state. In the case of benzene it is assumed that this is one of the low frequency ring vibrational modes . Deactivation of this excited mode is not readily... 

ABSTRACT. The mechanisms for energy flow from overtone excited HC and HO local modes have been elucidated in two mode model Hamiltonians of benzene and trihalomethanes and in a six mode model of HOOH molecule. Intramolecular vibrational relaxation (IVR) from the excited 2 1 Fermi resonance is shown to be very sensitive to the stretch-bend potential energy coupling in connection with the stability of the HC stretch periodic orbit. The overtone induced dissociation of HOOH, which is a slow process in comparison with the initial HO overtone relaxation, is explained in terms of the details of the potential energy surface. 

Iachello, F., and Oss, S. (1993b), Vibrational spectroscopy and intramolecular relaxation of benzene, 7. Chem. Phys. 99, 7337. 

---