Proton tunneling in the benzoic acid dimer studied by high resolution ultraviolet spectroscopy

Abstract

High resolution ultraviolet spectroscopy has been used to investigate the rotationally resolved excitation spectrum of the first singlet-singlet transition in the benzoic acid dimer. The measured spectrum consists of two overlapping components. The corresponding lines in the two components are shown to originate in different levels of the ground state potential separated by a tunneling splitting produced by concerted proton exchange between the two subunits forming the dimer. The frequency separation between the two components is equal to the difference between the tunneling splittings in the ground and the excited electronic state. This frequency separation is found to be 1107Ϯ7 MHz. From the analysis, it is estimated that the barrier for proton tunneling changes by about 20% upon electronic excitation. The structure of the dimer in the ground state is determined to be linear, while in the excited S 1 state it is slightly bent (3.4°Ϯ1.7°).

The structure of 4-methylphenol and its water cluster revealed by rotationally resolved UV spectroscopy using a genetic algorithm approach

“…This method has been shown by Helm et al to be helpful in the analysis of the phenol-water cluster spectrum 31 and later by Remmers et al in the analysis of the tunneling spectrum of the benzoic acid dimer. 32 Nevertheless, if one component of the complete spectrum amounts to more than 50% of the total intensity, the GA is capable of fitting this part of the spectrum, although it might be concealed in the dense spectrum of other component͑s͒ and would therefore be inaccessible to a classical analysis using line-positionassigned fits. From the difference of the partly fitted spectrum to the experimental one, the missing components can easily be recognized and subsequently fitted.…”

The infrared absorption spectrum of the gas phase neutral benzoic acid monomer and dimer

“…Its high symmetry makes the BA dimer a very interesting system that can serve as a prototype for doubly hydrogen bonding, and it has been studied quite extensively using various techniques. [20][21][22][23] From a direct measurement of the IR spectra, one can draw conclusions on possible changes in the vibrational structure upon dimerization of individual BA monomer units. In an earlier study, Stepanian et al recorded the matrix isolation spectrum of BA and found substantial differences between the IR spectra of the monomer and dimer.…”

Theoretical Studies of Dynamic Interactions in Excited States of Hydrogen-Bonded Systems

“…This effect is a vibrational analogue of the vibronic coupling, such as the pseudo-Jahn-Teller effect, occurring in the electronic spectra of symmetric dimers [64]. From the experimental data it has been concluded that in the first excited electronic state benzoic acid dimer is in-plane bent as an effect of localised electronic excitation on one moiety of the dimer [14,18,19,21].…”

“…Tomioka et al [20] studied the correlation between the frequencies of intermolecular hydrogen bond vibrations between the fluorescence excitation and dispersed fluorescence spectra and concluded that potentials for such vibrations are affected very little upon electronic excitation. Significant discovery was made by Remmers and et al [21]. On the basis of their high resolution ultraviolet rotationally resolved excitation spectrum of benzoic acid dimer, they have demonstrated convincingly that the linear and planar (C 2h symmetry) ground state geometry of the dimer is slightly in-plane bent (C s symmetry) upon electronic excitation.…”

“…In the first excited singlet state benzoic acid dimer is in-plane bent [14,19,21]. Such symmetry lowering (from C 2h to C s ) causes that degeneracy is removed and Davydov coupling significantly decreases.…”