Timothy A. Wildman

Temperature dependence of hydrogen tunnelling rate constants

Abstract A theoretical model based on tunnelling through a two-dimensional barrier is developed and used to interpret hydrogen transfer rate constants that deviate stronly from the standard Arrhenius equation, such as those of hydrogen abstraction by methyl and phenyl radicals.

Conformational preferences and internal rotation in toluene, o-xylene and hexamethylbenzene☆

Abstract Barriers to methyl internal rotation have been investigated through ab initio MO calculations with geometry optimization on toluene (STO-3G and 4-31G), o -xylene (STO-3G), and hexamethylbenzene (STO-3G), and compared with experimental results. Toluene is found to prefer a conformation in which one methyl CH bond makes a dihedral angle of 90° with the ring plane. However, the sixfold barrier to internal rotation is only 17 J mol −1 at the 4-31G level. The potential energy surface for coupled methyl group rotation in o -xylene is displayed and the preferred conformer is that known from microwave spectroscopy. Neglect of coupling and the sixfold component are partially responsible for over-estimates of the barrier from experiments. The most stable conformer of hexamethylbenzene has D 3d symmetry with slight puckering of the ring and tilting of the methyl groups to reduce steric crowding. The barrier to geared internal rotation is 2.42 kJ mol −1 .

Structural interpretation of hole burning in inhomogeneously broadened absorption bands

Abstract Spectroscopic hole-burning data are reported for four dyes in two organic glasses. Hole depths, monitored via fluorescence, vary neither exponentially nor logarithmically with time, but can be interpreted in terms of two-level systems as due to Gaussian distributions of barrier widths.

An ab initio quantum chemical study of hydrogen abstraction from methane by methyl

Abstract Ab initio MO calculations with a double-zeta basis plus polarization functions and truncated Cl (all singles and doubles) have been used to study hydrogen abstraction from methane by methyl. The optimum transition state geometry and an adiabatic barrier which is in agreement with the experimental activation energy have been determined.

Structural interpretation of low-temperature heme-ligand recombination rates in myoglobin

The nonexponential recombination of photodissociated heme‐CO and heme‐O2 in myoglobin, which is geminate at T < 180 K, is interpreted as being due to a narrow, random distribution of ligand transfer distances in the heme pocket. This permits evaluation of the most probable recombination rate which is shown to be consistent with ligand tunneling.

Electron spin resonance (ESR) investigation of the structure of methyl radical trapping sites in methanol glass

Measurements are reported of ESR spectra of methyl radicals trapped in methanol glasses. In these spectra, forbidden lines appear as satellites of the lines of the methyl quartet as a result of dipolar coupling of the unpaired electron with protons of neighboring methanol molecules. The relative intensity of the satellites is used to study the structure of the sites where the radicals are trapped. Comparison of intensities observed in CH3OH, CH3OD, CHD2OD, CD3OH, and CD3OD indicates a structure that is locally similar to the (disordered) β‐phase crystal structure of methanol, with the methyl radical replacing a methanol molecule and occupying a position close to its methyl position. The resulting methyl–methyl distances are compared with those deduced from the observed rate constants of the hydrogen abstraction reaction taking place at the trapping sites. If volume changes due to cooling and phase transitions are taken into account, the distances obtained in the two experiments are found to be compatible....

Hydrogen abstraction by methyl radicals in glasses

An experimental and theoretical study is reported of the abstraction of hydrogen atoms by methyl radicals from organic glasses, in particular methanol and several of its deuterated analogues. Rate constants are obtained for hydrogen and deuterium transfer as a function of temperature for a distribution of trapping sites of the radical in the glass. The site dependence is investigated by an analysis of the non-exponential decay of the methyl radical concentration based on the use of Laplace transforms. With the help of a recently developed theoretical model for hydrogen tunnelling, a relationship between tunnelling rate and tunnelling distance is established on the basis of these observations. The results yield a detailed picture of the structure of the trapping sites: roughly spherical cages in which the tumbling radical is surrounded on average by eleven rotating methyl groups, with the closest of which it reacts. The model yields a quantitative description of the rate of this reaction, expressed in terms of spectroscopic, thermodynamic and quantum-chemical input parameters.

Relaxation in disordered systems

Abstract Non-exponential relaxation in disordered systems may be interpreted in terms of a distribution of exponential rate constants that arises naturally from a narrow, random distribution in the value of a structural parameter (distance or angle) which characterizes the inequivalent sites within the system. This analysis is applied to the decay of methyl radicals in glassy methanol and to other systems. In the former case, the underlying assumptions have been tested experimentally. Such an approach indicates in general terms which kind of disorder is responsible for the observed decay. It also offers the prospect of obtaining a detailed description of the microscopic structure of a reactive site.

Hydrogen Tunneling in Chemical Reactions. Comparison of the Golden-Rule Approach with Transition-State Theory

A recent treatment of hydrogen-tunneling reactions based on the Golden Rule of time-dependent perturbation theory is compared with the conventional treatment based on transition-state theory. As a specific example, hydrogen abstraction by methyl radicals from methyl groups is studied in detail. Empirical barriers derived by the Golden Rule and by transition-state theory are compared with a theoretical barrier calculated by ab initio methods for the symmetric reaction of ·CH3 with CH4. It is concluded that the methods yield barriers of essentially the same height and width. This agreement, however, does not extend to the dynamics derived from the barrier shape. The reason for the discrepancy is discussed as a first step towards unification of the two methods.