The reaction path is a key concept in the theoretical description of a chemical reaction. The intrinsic reaction coordinate is defined as the steepest descent path in mass-weighted Cartesian coordinates that connects the transition state to reactants and products on the potential energy surface. Recently, a new Hessian based predictor-corrector reaction path following algorithm was presented that is comparable to a fourth-order algorithm developed earlier. Although the method is very accurate, it is costly because second derivatives of the energy are required at each step. In this work, the efficiency of the method is greatly enhanced by employing Hessian updating. Three different updating schemes have been tested:  Murtagh and Sargent, Powell-symmetric Broyden, and Bofill. Bofill's update performs the best and yields excellent speed-up.

Using Hessian Updating To Increase the Efficiency of a Hessian Based Predictor-Corrector Reaction Path Following Method.

Publisher Summary This chapter discusses the reduction potentials involving inorganic free radicals in aqueous solution. Because free radicals are usually transients, knowing their thermodynamic properties is primarily useful in mechanistic studies. Thus, the useful redox couples associated with a given free radical correspond to plausible elementary steps in reaction mechanisms. All potentials are expressed against the normal hydrogen electrode (NHE). Apart from the NHE, the standard state for all solutes is the unit molar solution at 25 °C. This violates the usual convention for species, such as O 2 that occur as gases, but because the rates of bimolecular reactions in solution are significant, the unit molar standard state is most convenient. The emphasis is on electron transfer reactions in which no bonds are formed or broken, electron transfer reactions in which concerted electron transfer and bond cleavage could occur, and certain atom transfer reactions. The chapter also presents a tabulation of ∆ f G ° values for all the radicals. A common approach in estimating the thermochemistry of aqueous free radicals is to use gas-phase data with approximations of solvation energies.

Reduction potentials involving inorganic free radicals in aqueous solution

Photophysics of photosynthesis. Structure and spectroscopy of reaction centers of purple bacteria

Ion‐neutral complexes in unimolecular decompositions

Gas phase analogues of solvolysis reactions

An analysis of thermochemical and kinetic data on the bromination of the halomethanes CH4–nXn (X = F, Cl, Br; n = 1–3), the two chlorofluoromethanes, CH2FCl and CHFCl2, and CH4, shows that the recently reported heats of formation of the radicals CH2Cl, CHCl2, CHBr2, and CFCl2, and the CH bond dissociation energies in the matching halomethanes are not compatible with the activation energies for the corresponding reverse reactions. From the observed trends in CH4 and the other halomethanes, the following revised ΔH°f,298 (R) values have been derived: ΔH°f(CH2Cl) = 29.1 ± 1.0, ΔH°f(CHCl2) = 23.5 ± 1.2, ΔHf(CH2Br) = 40.4 ± 1.0, ΔH°f(CHBr2) = 45.0 ± 2.2, and ΔH°f(CFCl2) = −21.3 ± 2.4 kcal mol−1. The previously unavailable radical heat of formation, ΔH°f(CHFCl) = −14.5 ± 2.4 kcal mol−1 has also been deduced. These values are used with the heats of formation of the parent compounds from the literature to evaluate CH and CX bond dissociation energies in CH3Cl, CH2Cl2, CH3Br, CH2Br2, CH2FCl, and CHFCl2.

Bond dissociation energies and radical heats of formation in CH3Cl, CH2Cl2, CH3Br, CH2Br2, CH2FCl, and CHFCl2

The reactions of photochemically generated Cl(2PJ) atoms with a number of fluorohydrocarbons have been investigated in the temperature range 8–95 °C by the competitive photochlorination technique using CH4 as a primary standard. Relative and absolute rate parameters are reported for CH3F, CH2F2, CH3CH2F, CH2FCH2F, CH3CHF2, CH3CF3, CH2FCHF2, CHF2CHF2, and several auxiliary reactions including CH3Cl, C2H6, and C3H8. The internal competition for hydrogen abstraction in asymmetric fluorethanes is examined in detail. The reactivity trends are discussed and it is found that the activation energies in the fluoromethane series correlate with the known C–H bond dissociation energies. The hydrogen reactivity in the fluoroethane series for which a sufficient data base of DH°(C–H) values is not available is best rationalized in terms of inductive effects and resonance interactions.

Reactions of ground state chlorine atoms with fluorinated methanes and ethanes

The reactions of photochemically generated Cl(/sup 2/P/sub J/) atoms with CH/sub 3/Br, CH/sub 2/Cl/sub 2/, CH/sub 2/Br/sub 2/, CH/sub 2/ClF, and CH/sub 2/BrCl have been studied over the temperature range 0-95/sup 0/C by using the competitive method with methane as a primary standard. Relative and absolute rate constants are reported. The preexponential factors for this group of compounds fall within the range (3.1-9.5) x 10/sup -11/ cm/sup 3/s/sup -1/ and activation energies vary from 2.4 to 3.1 kcal mol/sup -1/. The results are compared with the results for other members of this series including CH/sub 3/F, CH/sub 2/F/sub 2/, CHF/sub 3/, CH/sub 3/Cl, CH/sub 2/Cl/sub 2/, and CHCl/sub 3/ from this and other laboratories and discussed in terms of the thermochemistry involved. A sample calculation of the A factor using the thermochemical kinetics version of transition-state theory (TST) shows reasonable agreement with experiment. The sensitivity of the input parameters in the bond energy-bond order (BEBO) method prediction of activation energy and tunneling is discussed in some detail with CH/sub 2/Cl/sub 2/ as an example

Kinetics of photochlorination of halogen (F, Cl, Br) substituted methanes

The abstraction of hydrogen and deuterium from ethane, 1,1-dichloroethane, 1,1,1-trichloroethane, and some of their deuterated analogs by photochemically generated ground state chlorine atoms has b...

Competitive photochlorination and kinetic isotope effects for hydrogen/deuterium abstraction from the methyl group in C2H6, C2D6, CH3CHCl2, CD3CHCl2, CH3CCl3, and CD3CCl3

This investigation presents a detailed examination of the relative reliability of the comparative and absolute rate methods as they are applied to kinetic studies in the single-pulse shock tube. For this purpose two previously studied reactions, the thermal elimination of HCl from ethyl chloride and n-propyl chloride, were selected and mixtures of these compounds were shock heated to temperatures in the range of 960°-1100°K. The experimental results were analyzed by both methods and the rate constants obtained from these analyses are compared with those of previous studies. The advantages and shortcomings of both methods are noted and it is concluded that reliable kinetic data can be obtained by the absolute rate (isolation technique) method with careful examination of the gas-dynamic flow conditions and taking cognizance of the incident shock deceleration. The limitations of the comparative rate technique encountered in the present study were similar to those detailed in previous investigations.

E. Tschuikow-Roux

Papers

Bond dissociation energies and radical heats of formation in CH3Cl, CH2Cl2, CH3Br, CH2Br2, CH2FCl, and CHFCl2

Reactions of ground state chlorine atoms with fluorinated methanes and ethanes

Kinetics of photochlorination of halogen (F, Cl, Br) substituted methanes

Competitive photochlorination and kinetic isotope effects for hydrogen/deuterium abstraction from the methyl group in C2H6, C2D6, CH3CHCl2, CD3CHCl2, CH3CCl3, and CD3CCl3

A comparison of two single-pulse shock-tube techniques: The thermal decomposition of ethyl chloride and n-propyl chloride