Photoionization reversible reactions

A number of studies of H-atom transfer from hydrogen halides to free radicals, R + HX - RH + X, have been done by FPTRMS in which R was detected by photoionization, and its decay was monitored as a function of [HX] under pseudo-first-order conditions. When the rate coefficient is combined with determinations of the rate coefficient of the reverse reaction to obtain the equilibrium constant, the enthalpy of formation of the radical can be deduced. If the kinetics are accurately measured in isolation, this is a direct kinetic method which can be used to confirm (or otherwise) thermodynamic data obtained by classical, indirect kinetic methods which depend on correct mechanistic interpretation. In a number of instances free radical enthalpies of formation by these two different approaches have not been in good agreement. It is not the purpose of this short survey to discuss the differences, but rather to briefly indicate the extent to which the FPTRMS method has contributed to the kinetics of these reactions and to free radical thermochemistry. 

CF from CF4 are generally too high, and even photoelectron-photoion coincidence experiments provide only upper limits (Brehm et al., 1974 Powis, 1980). We therefore performed a series of experiments designed to determine this heat of formation (Fisher and Armentrout, 1990d). In contrast to a literature report (Babcock and Streit, 1981), we demonstrated that there can be no fluoride transfer equilibrixrm between CFj and SF at thermal energies by examining both the forward and reverse reactions (21) ... 

Later, a very fine investigation was carried out on this reaction by Chupka et al. [204]. Producing their reactant H2 ions in selected vibrational states, or even in selected rotational states, by the use of their high-resolution photoionization mass spectrometer, they studied the cross-section of reaction (123) as a function of vibrational and kinetic energy of the reactant ions. The found that the cross-section decreases as vibrational excitation increases when ion kinetic energies are low, but the reverse is true when ion kinetic energies are high. 

Radical cations are produced photochemically by electron transfer transitions (p. 419) but only as ion pairs with the radical anion formed simultaneously. In most cases, the ion pair reverts to reactant by reverse electron transfer before any reaction can take place. Radical cations can, however, be prepared as stable entities by photolysis of compounds with low ionization potentials at low temperatures in glasses containing electron acceptors. Electrons expelled by photoionization wander through the glass until they are trapped by an acceptor. Since the acceptor is not in contact with the cation radical and since energy is required to extract the electron... 

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