Hydrocarbon probes, laser

Considerable interest in the subject of C-H bond activation at transition-metal centers has developed in the past several years (2), stimulated by the observation that even saturated hydrocarbons can react with little or no activation energy under appropriate conditions. Interestingly, gas phase studies of the reactions of saturated hydrocarbons at transition-metal centers were reported as early as 1973 (3). More recently, ion cyclotron resonance and ion beam experiments have provided many examples of the activation of both C-H and C-C bonds of alkanes by transition-metal ions in the gas phase (4). These gas phase studies have provided a plethora of highly speculative reaction mechanisms. Conventional mechanistic probes, such as isotopic labeling, have served mainly to indicate the complexity of "simple" processes such as the dehydrogenation of alkanes (5). More sophisticated techniques, such as multiphoton infrared laser activation (6) and the determination of kinetic energy release distributions (7), have revealed important features of the potential energy surfaces associated with the reactions of small molecules at transition metal centers. 

In our laboratory we have utilized multiphoton infrared laser activation of metal ion-hydrocarbon adducts to probe the lowest energy pathways of complex reaction systems (6). Freiser and co-workers have utilized dispersed visible and uv radiation from conventional light sources to examine photochemical processes involving organometallic fragments... 

Cyclohexane and cyclohexane-d12 have been used as the probe for crossover and, hence, the reactive multiplicity of the subject carbenes. By combining direct and triplet-sensitized generation of the carbene with kinetic analysis from laser spectroscopy and the results of the crossover experiments, a rather complete picture of the reaction of aromatic carbenes with hydrocarbons emerges. 

The measurements of temperature and species concentrations profiles in premixed, laminar flames play a key role in the development of detailed models of hydrocarbon combustion. Systematic comparisons are given here between a recent laminar methane-air flame model and laser measurements of temperature and species concentrations. These results are obtained by both laser Raman spectroscopy and laser fluorescence. These laser probes provide nonintrusive measurements of combustion species for combustion processes that require high spatial resolution. The measurements reported here demonstrate that the comparison between a model and the measured concentrations of CH, O2,... 

In these ultrafast UV laser studies, the pump energies varied between 4 eV and 5 eV, while the probe pulse energies varied between 0.55 eV and 3.1 eV. Upon biphotonic 5 eV or triphotonic 4 eV excitation, hydrocarbons (such as paraffins, isooctane, cylopentane, cyclohexane, and trans-decalin) yield both the solvent excited Si states and the electron-hole pairs [66]. Questions were raised... 

The world setting for chemistry has continued to change. The center of chemical activity has moved from Africa to India to Asia to Arabia to Europe to North America—if a center, that is, still exists. Chemistry has become increasingly delocalized and is now better described as a global enterprise. There have also been major technical advances affecting the course of chemistry—computers, transistors, lasers, space travel—and these have opened completely new areas of study. With such probes as Voyager we learned of planets with seas of hydrocarbons and bedrock of water and ammonia ice. Spectroscopists have found evidence of molecules in the empty space between stars, including carbon monoxide, ammonia, formaldehyde, cyanoacetylene, acetaldehyde, methyl mer-... 

Reactive molecular fragments or radicals, such as OH, H and O are very important in combustion. The combustion zone of a stoichiometric CH4/O2 flame contains about 10% OH, and 5% each of H and O. In the second of the two reactions given above the number of radicals is doubled. A fast increase in radical formation frequently leads to explosive combustion. Because of the high reactivity of radicals they cannot be measured by probe (extraction tube) techniques, since wall reactions immediately eliminate them. Thus, laser techniques are particularly valuable for radical monitoring. Pollution formation in flames should also be considered. Nitrogen and sulphur oxides, incompletely burnt hydrocarbons and soot particles form important pollutants. It is of the utmost importance to understand which elementary reactions form and eliminate pollutants. The formation of nitric oxide is reasonably well understood. At temperatures above 2000 K the nitrogen in the air is attacked ... 

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