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

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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 1107±7 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 S1 state it is slightly bent (3.4°±1.7°).