Excited alkane molecule

We confine our discussion to alkanes and cyclic alkanes. Some of the features found in them are common to other hydrocarbons as well. An H atom is produced by the dissociation of highly excited alkane molecules ... 

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... 

Excited alkane molecules may dissociate into alkyl radicals and hydrogen atoms. 

In other alkanes the reaction of excited molecules with nitrous oxide is less important, and in some cases it may not occur at all. In n-hexane the N2 yields are about one-half what they are in cyclohexane. In other alkanes such as 2,2,4-trimethylpentane the yields of N2 were quite low. A small yield could be attributed to one of the other effects discussed, but if it is attributed to excited alkane molecules, then energy transfer to nitrous oxide is much less important in 2,2,4-trimethylpentane than in cyclohexane. 

It would be elegant to finish the part on photophysics and photochemistry of liquid alkanes by giving a picture that unifies the temperature- and energy-dependence results obtained in fluorescence and photodecomposition studies. However, the spectroscopic information available for alkane molecules is not sufficient to identify the exact excited states involved in the radiative and nonradiative processes [55]. Because of the lack of information, there are different views on the positions and identities of excited states involved [52,55,83,121,122]. 

Energy transfer from an electronic excited state of an alkane molecule to a solute molecule was first suggested 50 years ago [123]. Since then over 100 papers discussed the various... 

Alkane elimination is a basic photodecomposition mode of highly branched alkanes, e.g., most of the excited neopentane molecules split directly to methane and isobutene. 

The radical-forming reactions are suggested to take place mostly after an Si T type ISC the reactions have nonactivated character. The homolytic split to H atom and alkyl radical has a considerable yield in the photolysis of n-alkanes and cycloalkanes, while the scission to two radicals is characteristic of the decay of excited branched alkane molecules. 

The initial interaction of the excited mercury atom with the alkane molecule presumably takes the form of a slight overlap of the p-orbital of the 3Pi atom with the C-H sigma bond of the alkane molecule. In the formation of the cyclic intermediate (TS-2), the two hybrid sp-orbitals of mercury are very likely simultaneously involved through... 

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 ... 

Recent synchrotron radiation experiments showed that the probability of the energy deposition on the alkane molecules was the highest at about 16 18 eV [95]. With the energies above 16 eV, excited states of alkane radical cations can be produced efficiently. In irradiated cyclohexane, for example, the following reactions were considered to be the formation reactions of alkyl radical. 

Radiolysis of cryogenic trichlorofluoromethane containing a suitable n-alkane as solute has proven very suitable for the study of symmetric proton transfer from alkane radical cations to alkane molecules. At low concentration of the alkane solute (RH) in the binary CCljF/alkane system, absorption of ionizing radiation mainly occurs by trichlorofluoromethane resulting in its excitation and ionization. 

Time-resolved anisotropy measurements of excitation hopping between two anthryl moieties attached to both ends of alkane molecules have been interpreted by a model based on conformational... 

In the 1470-A. photolysis of cyclohexane-nitrous oxide solutions, nitrous oxide reacts with excited cyclohexane molecules to form nitrogen and oxygen atoms. The reaction of N20 with photoexcited 2,2,4-trimethylpentane molecules is much less efficient than with cyclohexane. In the radiolysis of these solutions, G(N2) is the same for different alkanes at low 5 mM) N20 concentrations. At higher concentrations, G(N2) from the radiolysis of cyclohexane is greater than G(N2) from the radiolysis of 2,2,4-trimethylpentane solutions. The N2 yields from 2,2,4-trimethylpentane are in excellent agreement with the theoretical yields of electrons expected to be scavenged by N20. The yield of N2 in the radiolysis of cyclohexane which is in excess of that formed from electrons is attributed to energy transfer from excited cyclohexane molecules to nitrous oxide. 

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. 

When alkanes are dissolved in superacids, they are transformed into various products [52]. In protic media, the first step of the reaction is the protonation to form in the case of methane, the methonium ion, CHs. For the theoretical description of such a structure see refs. [53]. The ground state of this ion has symmetry which may be regarded as a complex of CHi cation with Hi molecule (H-H distance has been found to be 0.936 A which suggests formation of a bond). The ground state is only insignificantly different ca. 1 kcal moP ) from the excited state of C4v symmetry. Even closer are two transition states of other symmetries [53c], This finding corresponds to extremely fast hydrogen atom movements in CH5 and other similar carbonium ions therefore, fast isomerization, e.g., epimerization of alkane molecules, is expected to proceed in... 

The application of both criteria to gas-phase reactions is complicated further by the formation of vibrationally excited products. Both the insertion and addition reactions of methylene are exothermic by approximately 93 kcal. mole (based on recent estimates of AH (CH2) = 94 kcal.mole" ). Vibrationally excited alkanes and alkenes may dissociate into free radicals, and excited cyclopropanes may undergo structural and geometrical isomerizations unless collisionally stabilized . The occurrence of hot molecule reactions excludes any reasonable estimation of singlet and triplet methylene fractions. The data presented in the following paragraphs have been taken from experiments at high-pressures", which are thought to ensure complete collisional deactivation of excited reaction products. 

The excited singlet states of alkanes show a weak fluorescence with wavelength of maximum around 200-230 nm and a quantum yield of (f) = 0.001-0.02 (Rothman et al. 1973). The lifetimes of the excited states are generally around 1 ns (Hermann et al. 1985). In gas phase, the lifetime decreases with the decreasing pressure probably due to decreased collisional deactivation of vibrationally excited Si molecules. In liquid phase, a characteristic temperature dependence of the fluorescence lifetime was detected (Flamigni et al. 1982 Dellonte et al. 1984 Wickramaaratchi et al. 1985). 

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