Structure and vibrational dynamics of the benzene dimer

Point-wise evaluated coupled-cluster single double triple [CCSD(T)] stabili zation energies are used to parameterize the nonempirical model (NEMO) empi rical intermolecular potential of the benzene dimer in the ground electroni c state. The potential is used for theoretical interpretation of the dimer structure and the dynamics of its intermolecular motions. Only one energy m inimum, corresponding to the T-shaped structure, is found. A parallel displ aced structure is the first-order transition structure separating the molec ular symmetrically equivalent T-shaped structures. Due to a relatively high transition barrier (similar to 170 cm(-1)), the interconversion tunneling is unimportant in the energy region spanned by the available rotational spe ctra and is thus neglected (accordingly, the molecular symmetry group which is used for interpretation of the available experimental spectra is relate d to the T-shaped structure with two feasible internal rotations and nonequ ivalent monomers). The dimer undergoes a nearly free internal rotation alon g the axis connecting the benzene centers of mass in the T-shaped equilibri um geometry and a hindered internal rotation (the barrier being similar to 46 cm(-1)) along the axis that is perpendicular to the "nearly free" intern al rotation axis. The tunneling splittings observed in the rotational spect rum are likely due to this hindered rotation. An analysis assuming the latt er rotation as an independent motion and using purely vibrational tunneling splittings (obtained by extrapolating to zero values of the rotational qua ntum numbers) indicates that the genuine value of the hindered rotation bar rier is nearly twice higher than its ab initio value. Similarly, the differ ence Delta R=0.25 Angstrom between the ab initio (equilibrium) and experime ntal (ground state) values for the distance of the mass centers of the benz ene monomers is strong evidence that our theoretical potential is much shal lower than the genuine one. The Raman bands observed at the 3-10 cm(-1) reg ion seem to involve states associated with the nearly free rotation and the "energy minimum path" bending motion. Unambiguous assigning of the weaker Raman features is infeasible, partly due to limitations in the accuracy of the theoretical potential, and partly due to the lack of knowledge of the p olarizability tensor of the dimer and temperature at which the spectra were taken. (C) 1999 American Institute of Physics. [S0021-9606(99)00526-7].