Carbon vibrational relaxation

The dynamics of fast processes such as electron and energy transfers and vibrational and electronic deexcitations can be probed by using short-pulsed lasers. The experimental developments that have made possible the direct probing of molecular dissociation steps and other ultrafast processes in real time (in the femtosecond time range) have, in a few cases, been extended to the study of surface phenomena. For instance, two-photon photoemission has been used to study the dynamics of electrons at interfaces [ ]. Vibrational relaxation times have also been measured for a number of modes such as the 0-Fl stretching m silica and the C-0 stretching in carbon monoxide adsorbed on transition metals [ ]. Pump-probe laser experiments such as these are difficult, but the field is still in its infancy, and much is expected in this direction m the near fiitiire. 

The lowest excited triplet states of a-dicarbonyl compounds are considerably less energetic than those of simple carbonyls. For instance the energy of the vibrationally relaxed triplet of glyoxal is 55 kcal,366 as compared to 72 kcal for formaldehyde. Irradiation of glyoxal at 4358 A populates the lowest vibrational levels of the first excited singlet, 30% of which fluoresce and 70% of which cross over to the triplet manifold.388 Almost all of the triplet molecules then decompose to formaldehyde and carbon monoxide, the phosphorescence yield being only 0.1%. 

Whittenburg SL, Wang CH. Light scattering studies of rotational and vibrational relaxations of acetonitrile in carbon tetrachloride. J Chem Phys 1977 66 4255-4262. 

Very few experiments have been performed on vibrational dynamics in supercritical fluids (47). A few spectral line experiments, both Raman and infrared, have been conducted (48-58). While some studies show nothing unique occurring near the critical point (48,51,53), other work finds anomalous behavior, such as significant line broadening in the vicinity of the critical point (52,54-60). Troe and coworkers examined the excited electronic state vibrational relaxation of azulene in supercritical ethane and propane (61-64). Relaxation rates of azulene in propane along a near-critical isotherm show the three-region dependence on density, as does the shift in the electronic absorption frequency. Their relaxation experiments in supercritical carbon dioxide, xenon, and ethane were done farther from the critical point, and the three-region behavior was not observed. The measured density dependence of vibrational relaxation in these fluids was... 

Figure 15 shows the lifetime as a function of temperature at the critical density of carbon dioxide. With CO2 as the solvent there is no inverted region in which the lifetime becomes longer as the temperature is increased. Instead, the lifetime decreases approximately linearly. Thus the inverted behavior is not universal but is specific to the properties of the particular solvent. The fact that the nature of the temperature dependence changes fundamentally when the solvent is changed from ethane to C02 demonstrates the sensitivity of the vibrational relaxation to the details of the solvent properties. The solid line is the theoretically calculated curve. The calculation of the temperature dependence is done with no adjustable... 

We have presented experimental and theoretical results for vibrational relaxation of a solute, W(CO)6, in several different polyatomic supercritical solvents (ethane, carbon dioxide, and fluoroform), in argon, and in the collisionless gas phase. The gas phase dynamics reveal an intramolecular vibrational relaxation/redistribution lifetime of 1.28 0.1 ns, as well as the presence of faster (140 ps) and slower (>100 ns) components. The slower component is attributed to a heating-induced spectral shift of the CO stretch. The fast component results from the time evolution of the superposition state created by thermally populated low-frequency vibrational modes. The slow and fast components are strictly gas phase phenomena, and both disappear upon addition of sufficiently high pressures of argon. The vibrational... 

Dr. Rohlfing s research interests include the experimental characterization of transient molecules relevant to combustion processes, linear and nonlinear laser spectroscopies, trace detection of pollutants, molecular beam and mass spectrometric studies of carbon and metal clusters, and vibrational relaxation dynamics. He is the author of approximately 50 peer-reviewed articles, holds membership in the American Chemical Society and the American Physical Society, and is a fellow of the American Association for the Advancement of Science. 

From this state, ring strain facilitated predissociation to a "biradical-like" transition state [135] or vibrational relaxation (k ) to S may occur. It is also conceivable that transition state [135] could be produced directly from S °. Alternatively, molecules in the S ° state could intersystem cross (kST) to the triplet manifold (T ). For 2-alkylidenecyclobutanones, reactivity is manifested in isomerization about the exocyclic carbon-carbon double bond, while for the saturated cyclobutanone derivatives studied, definitive evidence for solution-phase reactivity is not available. If analogy is again made to the vapor-phase photochemistry of cyclobutanone [21], reactivity could conceivably result in decarbonylated products. Indeed, preliminary evidence has been obtained from sensitization experiments employing m-xylene as triplet sensitizer that decarbonylation of a saturated cyclobutanone is enhanced by selective population of its state (35). ... 

Mechanistic details for the attack of olefins on CS-substituted cyclopentenones are discussed at length by Scharf et al. [114]. All steps of the photocycloaddition are reversible except for the final cyclization, and diastereoselectivity is dominated by only one mechanistic step, that being the approach of the olefin to the vibrationally relaxed furanone (56, see Fig. 7). The P carbon structure... 

Hill J R ef a/1995 Vibrational relaxation of carbon monoxide in model heme compounds 6-coordinate metalloporphyrins (M = Fe, Ru, Os) Chem. Phys. Lett. 224 218-23... 

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. 

Vibrational relaxation in 0332 and enhancement of reaction rates of Oa by vibrational excitation 222 have been discussed. Reactions of ozone with saturated hydrocarbons,327 olefins,328 H2S,329 metal alkyls,330 acetals,331 carbon systems,332 and CH3, CH30, and CH3Oa radicals 383 have been reported. These last studies are, of course, of great importance in polluted urban atmospheres, but lack of space forbids detailed discussion of these papers. 

Stretching motion (amide I mode) was found to occur on a 1 picosecond or subpicosecond timescale [24-28], whereas vibrational relaxation of photolyzed carbon monoxide in the heme pocket can last from tens to hundreds of picoseconds in different Mb mutants [7,29,30], Starting from a basis of harmonic normal modes and including the contribution of cubic anharmonicity in the potential energy, providing nonlinear coupling, Leitner and coworkers estimated the thermal diffusivity of Mb at 300K to be 14 ps", approximately the value for water. The thermal conductivity... 

Sagnella, D. E. Straub, J. E., A study of vibrational relaxation of B-state carbon monoxide in the heme pocket of photolyzed carhoxymyoglobin. Biophys. J. 1999, 77, 70-84. 

Margottin-Maclou M, Doyennette L and Henry L 1971 Relaxation of vibrational energy in carbon monoxide, hydrogen chloride, carbon dioxide and nitrous oxide App/. Opt. 10 1768-80... 

That the carbon—metal or carbon—metalloid bonds are preserved at all in these reactions is quite surprising. With tetramethylgermanes, for example, this free radical reaction must be a 24 step process. The success in preserving carbon-germanium bonds must arise from very rapid molecular vibrational, rotational, and translational relaxation processes occurring on the cryogenically cooled surfaces such that the energy from the extremely exothermic reaction is smoothly dissipated. 

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