Alkanes, excited, decomposition

Because fluorescence plays a very small role in the depopulation of alkane excited molecules the sum of the quantum yields of the chemical decompositions in the thermally activated and nonactivated channels is practically unity 4>(S Sx) + 4>(S T ) is 1. Using Eq. (3), the temperature dependencies of the product yields formed in the activated and nonactivated channels have the following forms ... 

The excitation-decomposition reaction can also occur in alkanes. Lee et al. (1960b) showed that the yield of labeled propylene from cyclopropane increased with decreasing cyclopropane pressure. 

In the group of nonfluorescing alkanes the lifetimes are very short, x < 0.3 nsec an upper limit of i < 3 x 10 sec is estimated. The absence of fluorescence for these compounds may have two causes the low R, i.e., low rate coefficient of the Si Sq radiative transition and the short lifetime, i.e., the very fast chemical decomposition. In the C5 and C7-C10 cycloalkanes the ring strain, which is mainly caused by the repulsive interaction of their unfavorably displaced H atoms, may enhance the rate coefficient of the chemical decay by C-H decompositions. In the excited sates of the geminally branched... 

The ko. A, and Fa parameters obtained for a few alkanes are collected in Table 3. kg is around 10 sec A 10 to 10 sec and Fa 10 to 20 kJ mol h In principle, the decay of excited states may involve Si- Sx-type internal conversion transitions [IC, where Sx is some singlet state that gives the product(s) of chemical decomposition] and Si T -type intersystem crossing processes (ISC). The temperature-independent decay was attributed, on the basis of the size of the rate parameter (ko 10 sec ), to Si T -type intersystem crossing. At the same time the temperature-activated decay with a frequency factor of 10 to 10 sec was attributed to an internal conversion process that takes place by overcoming a barrier of Fa 10-20 kJ mol and leads finally to some... 

The photodecomposition of -alkanes at excitation energies slightly above the absorption onset involves both C-H and C-C bond decompositions [18]. The dominant process is the C-H scission, (H2) 0.8-0.9, and the contribution of C-C decomposition is small. In the photolysis of cyclohexane, cycloheptane, cyclooctane, and cyclodecane, however, only hydrogen evolution was observed [[Pg.375]

Similarly to the fluorescence quantum yields, the yields of individual primary decomposition steps generally show considerable excitation energy dependence the yields of the unimolecular H2 and alkane eliminations and also those of the radical-type decompositions show a continuous variation with photon energy [27,39,42,107,115]. In cyclohexane photolysis the sum of the quantum yields of the two primary decompositions described by Reactions (5) and (6) is practically unity between photon energies 7.6 and 11.6 eV yield decreases with the energy, [Pg.382]

Studies on a range of saturated hydrocarbons indicate that the main decomposition reactions of electronically excited alkane molecules formed by electron impact are118,122-124... 

Recently, the primary processes were investigated using pulse radiolysis with two extractant-alkane systems (182, 292). Transient optical absorption spectra proved that in the presence of ligands like TODGA, the excited species of -dodecane (singlet excited state and radical cation) disappeared immediately. Results showed that an energy transfer occurred from the excited alkane to the extractant molecule (TBP, TOPO, or amide), which constituted an additional decomposition route, as described in the following set of reactions ... 

In recent years it has been shown that the primary event in the Hg-sensltized decomposition of many hydrogen-bearing compounds, in particular paraffins and H2, can be considered the process of H abstraction by the excited Hg. There is evidence that the excited Hg atom forms complexes with alcohols (248), amines (249), and alkanes (250). This has been concluded from fluorescence observation by Phillips and co-workers (248,249), and Gunning and co-workers (250). The transient molecule HgH has been detected spectroscopically in such a system by Vikis and LeRoy (251). The sequence 71 could be of importance in the... 

Bill Hase received his Ph.D. in chemistry in 1970, working in the research area of experimental physical chemistry under the direction of John W. Simons at New Mexico State University. His research included studies of the methylene singlet-triplet energy gap and of the unim-olecular decomposition of vibrationally excited alkane and alkylsilane molecules prepared by chemical activation. His career as a computational chemist began during his postdoctoral work with Don Bunker at the University of California, Irvine. In 1973 he joined the Chemistry Department at Wayne State University, where he remained until 2004, when he assumed the Robert A. Welch Chair in Chemistry at Texas Tech University. He remembers that his hrst computational chemistry classical trajectory computer program was written in assembly language and run on a PDP-10. 

While product analysis is an important source of information on the early stages of radiolysis, this approach has a serious problem For all of the alkanes studied, the yield of primary decomposition estimated from the total product yields significantly exceed the ionization yield 6 to 6.5 per 100 eV vs. 4.5 to 5 per 100 eV [1]. This suggests that some of the products were counted twice i.e., that fragmentation of the excited states is more extensive than was assumed. For example, instead of H2 elimination, the exited state may eliminate... 

The same types of products are obtained from alkanes and alkenes in the liquid phase as are obtained in the gas phase. As might be expected from the possibility of more rapid collisional de-excitation, there is a marked decrease in those products which result from decomposition of an excited species (Lee and Rowland, 1962) such as the hot radical produced by tritium addition to a double bond. 

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