Hydrogen-bonded systems proton tunneling

High resolution gas phase spectroscopy has now been able to nail down the size of the tunneling splitting for proton transfer in double hydrogen bonded systems. 

The previous studies of concerted hydrogen atom and proton transfer in hydrogen-bonded systems have been limited to studies of reaction pathways for simple model systems [4-8] with simple, reduced dimensionality methods for including quantum tunneling [13,14,34,35]. The applicability of modem computational methods to such systems is exemplified by more recent studies of the energetics of intramolecular hydrogen atom transfer in molecules such as malonaldehyde [36-39] and models of... 

THEORETICAL MODELING OF VIBRATIONAL SPECTRA AND PROTON TUNNELING IN HYDROGEN-BONDED SYSTEMS... 

The instanton theory of tunneling splittings in hydrogen-bonded systems and decay of metastable states in polyatomic molecules was studied by Nakamura et al. [182, 192, 195, 201-204, 216] They formulated a rigorous solution of the multidimensional Hamiltonian-Jacobi and transport equations, developed numerical methods to construct a multidimensional tunneling instanton path, and applied this method to HO [201], malonaldehyde [192, 195], vinyl radical [203], and formic acid dimer [202]. Coupled electron and proton transfer reactions were recently reviewed by Hammes-Schiffer and Stuchebrukhov [209]. 

In this chapter we have described the state of research in theoretical vibrational spectroscopy of hydrogen-bonded systems, interpretation of spectra of water in ices and in aqueous ionic solutions, and theoretical modeling of mnltidimensional proton tunneling. 

Theoretical Modeling of Vibrational Spectra and Proton Tunneling in Hydrogen-Bonded Systems... 

Thus, the spectroscopic results eliminate the model with symmetric hydrogen bonds, but they do not tell us much about the tunnelling of the protons. Actually, this should cause a splitting of the vibrational levels, but a rough estimation [14] of the splitting of the levels for a linear (which is probably not the case) O H 9 O system with an assumed 0—O distance of 2 7 A, yields a value of 15 cm- 1, This is too small a splitting to be observable with such broad bands, having moreover the contour complicated by the overlap with other bands. 

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