Atom Group Transfer Processes

Abstract

Atom group transfer (AT) processes represent a special class of vibrational and electronic relaxation phenomena in which large reorganization occurs in certain intramolecular modes, i.e. those associated with the motion of the transferring atom group. This kind of process is widely represented in chemical reactions such as nucleophilic substitution (32,237,238), inner sphere electron transfer processes (13,24,32), and in proton and hydrogen atom transfer reactions (11,19). However, AT between different molecular fragments also occurs in several other — largely solid-state — processes, among which we notice in particular: (a) In amorphous solids atom groups may be located in two different neighbouring positions separated by an energy barrier. At low temperatures this gives rise to the formation of ‘tunnelling-states’, i.e. a splitting of the vibrational energy levels in the neighbouring potential wells, associated with the finite probability of atom group tunnelling through the barrier (6–8). Since the barrier widths and heights in the amorphous solids are almost continuously distributed this gives rise at low temperature to an additional specific heat (and thermal conductivity) from the phonon energy absorption associated with the transition between the tunnelling levels. Both experimental data and a closer theoretical analysis show that this ‘anomalous’ specific heat is proportional to the absolute temperature (as opposed to the T3 dependence expected for crystalline materials at low temperatures). (b) Rotational and translational motion of lattice impurities in crystalline solids, e.g. rotation of CN− and OH− or hindered translation of Li+ in alkali halide lattices (239). These effects are revealed by fine-structure in the infrared absorption spectra and by the temperature dependence of the thermal conductivity at low temperatures (a few K). At low temperatures the ‘impurities’ are localized in individual librational wells but interaction between neighbouring wells leads to a splitting, most frequently of the range up to a few cm−1. The appearance of these new ‘tunnelling levels’ are the origin of the effects listed. (c) The dynamics of hydrogen-bonded systems (e.g. the vibrational properties of water, ice, carboxylic acids etc, proton mobility) is physically closely related to proton transfer reactions in chemical systems (240). These systems are for example subject to both single and correlated proton tunnelling giving rise to a fine-structure in the infrared absorption peaks. Moreover, the theory of local defect dynamics is commonly handled within a modified Born-Oppenheimer approximation — in which the proton stretching modes constitute the fast subsystem and are strongly coupled to slower local modes or to the lattice phonon modes constituting the slow subsystem — to give optical line shape functions analogous to the ones derived in chapter 3. (d) Diffusion of ‘interstitials’ or lattice defects through crystal lattices also involves essentially atom group transfer between neighbouring local equilibrium positions. Examples are the diffusion of excess cations or anions in alkali and earthalkaline halides (5,128,241) and diffusion of light atoms (in particular hydrogen) in metals (242). (e) We should finally notice that intramolecular conversions in ‘fluxional’ molecules bears a resemblance to the kind of AT processes to be discussed in the following. The classical example is the inversion of the ammonia molecule which exists in two equilibrium configurations corresponding to inversion of the pyramidal structure. The molecules can thus be represented by a symmetric double-well potential energy surface in which the ‘zero order’ levels in each well are split due to finite probability of localization of the proton in either well. Direct evidence for the proton tunnelling between the two configurations was first obtained by the microwave spectrum of gaseous ammonia which shows a characteristic peak at about 1 cm−1 corresponding to the magnitude of the level splitting (243).