Benzene intramolecular dynamics

Reddy, K. V., Heller, D. F., and Berry, M. J. (1982), Highly Vibrationally Excited Benzene Overtone Spectroscopy and Intramolecular Dynamics of CsH6, C6D6, and Partially Deuterated or Substituted Benzenes, J. Chem. Phys. 76, 2814. 

II. Intramolecular Dynamics in Electronically Excited S State of Benzene... 

Even in a molecule the size of benzene the resolution achieved in this way is sufficient to investigate the dynamic behavior of individual rotational states. For this it is necessary to eliminate the Doppler broadening of the rovibronic transitions. Two methods have been applied (i) the elimination of Doppler broadening in a Doppler-free two-photon-transition and (ii) the reduction of Doppler broadening in a molecular beam. Measurements of the dynamic behavior have been performed in the frequency [3] and time domain [4]. We will briefly summarize the results from high-resolution measurements and discuss the conclusions on the intramolecular decay mechanism. Then it will be discussed how the intramolecular dynamics is influenced by the attachment of an Ar or Kr atom to the benzene molecule, leading to a weakly bound van der Waals complex. 

In this section we present experimental results for the lifetime of individual rovibronic states of benzene at different excess energies in the Si electronic state. In this way the dependence of the lifetime of the states on their excess energy and their rotational quantum number is studied. A general model for the underlying coupling mechanism is presented, and the influence of a van der Waals bound noble-gas atom on the intramolecular dynamics is investigated. 

To discover smaller specific effects on the intramolecular dynamics after attachment of an Ar atom to the benzene molecule, we performed lifetime measurements of single rovibronic states in the 6q band of the benzene-Ar and the benzene-84 Kr complex. No dependence of the lifetime on the J K> quantum number within one vibronic band was found [38]. This is in line with the results in the bare molecule and points to a nonradiative process in the statistical limit produced by a coupling to a quasi-continuum, for example, the triplet manifold. 

In conclusion, we have found that the intramolecular dynamics in the benzene molecule at low excess energy is not strongly influenced by the additional three vibrational degrees of freedom of the benzene-Ar complex. The coupling of the excited intramolecular modes to the low-frequency inter-molecular modes is weak. The observed 40% decrease of the lifetime of the 61 state does not depend on the individual excited rotation and points to an external heavy-atom effect as the source of the lifetime shortening observed for the same selectively excited rovibronic state. 

Classical trajectory calculations have given some support to the local mode model. Trajectories for the H-C-C H + C=C system are quasiperiodic in an internal coordinate representation. A classical trajectory study of the intramolecular dynamics of highly excited CH bonds in benzene shows that energy remains localized in CH bonds on a picosecond time scale.Deuteration destroys this result, presumably as a result of increased momenta coupling (larger g terms) for CD bonds as compared to CH bonds. Also, the decomposition of model H-Si-Si is found to be more intrinsically non-RRKM than is H-C-C decomposition. This result is consistent with decreased local mode momenta coupling for H-Si-Si. 

Deuterium NMR has recently been used to study molecular motion of organic adsorbates on alumina (1.) and in framework aluminosilicates (2). The advantage of NMR is that the quadrupole interaction dominates the spectrum. This intramolecular interaction depends on the average ordering and dynamics of the individual molecules. In the present work we describe NMR measurements of deuterated benzene in (Na)X and (Cs,Na)X zeolite. 

Compound 220 in solution gives rise to a dynamic equilibrium between the enaminoketone (i )-220 and iV-acyl forms (Z)-221 in the ground state (Scheme 14). In nonpolar solvents, such as hexane, benzene, and toluene, the equilibrium is displaced toward isomer ( )-220, which is stabilized by intramolecular hydrogen bond it absorbs in the region of 470 nm. In polar solvents like DMSO, the equilibrium shifts almost completely toward the N-acylated form (Z)-221. 

A significant reason for determining accurate anharmonic force fields for benzene is that they serve as input into dynamical studies of overtone spectra and intramolecular energy transfer. In previous studies, we have used the force fields developed by Pulay et... 

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. 

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