Tunneling splittings in the S0 and S1 states of the benzoic acid dimer determined by high-resolution UV spectroscopy.

Five different isotopologues of the benzoic acid dimer and a vibronic band located 57 cm(-1) above the electronic origin, which is assigned to the out-of-plane butterfly motion, are studied by rotationally resolved UV spectroscopy. From these measurements a ground-state structure with C(2h) symmetry is deduced, whereas the symmetry is lowered to C(s) in the S(1) state. The increase in the center-of-mass distance between the two monomers that is found on electronic excitation indicates a decrease in hydrogen-bond strength. The tunneling splittings in the S(0) and S(1) states are 1385.2+/-0.7 and 271.2+/-0.7 MHz, respectively, corresponding to an increase in barrier height by 7.2% on electronic excitation.