David J. Malcolme-Lawes

Dynamics of some hydrogen isotopic exchange reactions at high energies

Classical trajectory studies have been performed using an empirical potential energy surface for the interactions H + H2, D + D2, T + H2, T + D2, H + T2 and T + HD. Reaction and dissociation cross sections in each system are examined and a simple rule of thumb for estimating the low energy reaction efficiency is described. The integrated reaction cross section ratios for the tritium atom reactions are found to be in good agreement with observed experimental isotope effects. The vibrational excitation of the product molecule and the centre of mass scattering distributions are also discussed.

Reactions of energetic hydrogen atoms with bromine and hydrogen bromide

The reactions of high energy hydrogen atoms with HBr and Br2 have been studied using the classical trajectory technique. Comparison of the excitation functions obtained for H, D and T atoms reacting with Br2 shows a high energy isotope effect which favours the heavier isotope. The functions are considered in the light of the available experimental evidence for scavenger competition in hot tritium chemistry.Excitation functions for reaction of H and D atoms with HBr and DBr suggest that at collision energies below ≈ 500 kJ mol–1 the hydrogen atomic exchange reaction is considerably more efficient than the H-abstraction reaction. Values of the average H-abstraction cross section and hot reaction rate constant ratios predicted from the calculated excitation functions are compared with the available experimental data; good agreement is obtained.

Tritium recoil ranges and their influence on the helium moderator anomaly

The original data of Seewald and Wolfgang showed an anomaly in the kinetic analysis of the recoil tritium + methane system when helium was used as a moderator. This anomaly was interpreted by postulating the presence of tritium ions in systems containing large amounts of helium. New calculations indicate that the tritium recoil ranges used in the original work may have been too large, resulting in a significant overestimate of product yields. A re-examination of the original data using new recoil ranges shows that the anomaly has disappeared.A list of tritium recoil ranges in a variety of gases is included.

High energy reaction between tritium atoms and molecular oxygen

An excitation function for the T + O2 reaction has been derived using the quasi-classical trajectory technique and is compared with the available experimental evidence from hot atom chemistry.

Energy loss from hot tritium atoms in HF

The moderating efficiency of HF for energetic tritium atoms, αHF, is evaluated from the results of quasi-classical trajectory calculations for non-reactive encounters. αHF is shown to vary with energy, particularly in the energy region where the reaction cross section is large. The implications of this variation on kinetic analyses of hot atom experiments are considered.

Excitation functions for the reactions of tritium atoms with HF and DF

Reaction and dissociation cross sections for the interaction of tritium atoms with HF and DF in the energy range 20–3000 kJ mol–1 are evaluated by classical trajectory methods. Integrated cross sections for the primary reaction products and approximate energy loss parameters are discussed.

Reactions of energetic hydrogen atoms with simple hydrocarbons. Part 1.—The effect of moderators

The reactions of recoil tritium atoms with ethane have been studied in the presence of N2 and CF4 moderators. The results suggest that collisional dissociation of excited HT, initially produced by direct reaction, may be more important in determining the ratio of final products than the commonly described effect of energy shadowing. It is suggested that hydrocarbons behave essentially as carbon atoms as far as their ability to collisionally dissociate HT is concerned, and that moderators which consist of groups which are similar in mass to carbon atoms, exert very little effect on observed product ratios.

Reactions of energetic hydrogen atoms with simple hydrocarbons. Part 2.—Substitution reactions in ethane

The products of reaction of recoil tritium atoms with ethane are studied in the presence of Kr and Xe moderators. It is found that the relative yields of substitution products remain essentially unaltered over a wide range of moderation by these moderators or Ne, N2, CF4 and He. The relative moderating efficiencies of Ne, Kr, Xe, N2 and CF4 are found to be 0.066, 0.042, 0.036, 0.208 and 0.536 respectively. It is proposed that all products, other than HT, may be regarded as resulting from collision of the hot T atom with a carbon atom followed by dissociation of one of the C-ligand bonds. It is suggested that the rapidity of dissociation of a C—H bond may leave the labelled product in a vibrationally excited state which may decay by further C—C or C—H bond rupture.

Isotope effect on the lifetime of excited ethane

The mean lifetimes of excited C2H5T and C2D5T are estimated from pressure dependence studies of the unimolecular decomposition reactions of these molecules following the substitution reaction with recoil tritium.

Variation in the abstraction isotope effect for energetic tritium atoms

The HT/DT yield ratio from the reaction of energetic tritium atoms with 1:1 mixtures of protonated and deuteriated alkanes, is found to fall on addition of neon.