Atom Tunneling and Molecular Structure

Abstract

Proton transfer is one of the most elementary chemical reactions and plays important roles in polar solvents, organic reactions, and biological systems (Bountis 1991). Proton transfer dynamics have been extensively studied, and it has been shown that proton transfer cannot be well described by a one-dimensional potential along the reaction coordinate in many polyatomic molecules. When a potential barrier exists along the reaction coordinate the mechanism of proton transfer below the barrier is quantum mechanical tunneling. Since the whole atoms that constitute the molecule participate in the process, tunneling in polyatomic molecules exhibits a multi-dimensional nature. Due to the multi-dimensionality of tunneling, our understanding of proton tunneling in large molecules and complexes is still incomplete. However, various laser spectroscopic techniques combined with the molecular beam technique enable us to obtain fruitful data for the determination of the tunneling PESs (potential energy surfaces) in the electronic excited state as well as in the electronic ground state of the isolated molecules. These data are available to construct new models of tunneling. In this chapter, various molecules and complexes that show multi-dimensional tunneling are described. In particular, we focus on vibrational mode-selective tunneling in tropolone, 9-hydroxyphenalenone, and their derivatives. In Sect. 10.2 basic spectroscopic techniques for investigation of proton transfer reactions in the isolate state are described. The multi-dimensional nature of tunneling in several molecules is discussed in Sect. 10.3.1 on the basis of various experimental data together with theoretical calculations on the molecular structure. The effects of the intermolecular interaction on proton tunneling will be discussed in Sect. 10.3.2 with calculated two-dimensional PESs for the 9-hydroxyphenalenone-CO₂/H₂O complexes. The doubly hydrogen-bonded 7-azaindole dimer has been of considerable interest in double proton transfer as a model base pair. Recent experimental results of doubly hydrogen-bonded dimers are described in Sect. 10.3.2. The conclusion and future developments are described in Sect. 10.4.