Structure and vibrational dynamics of the benzene dimer

摘要

Point-wise evaluated coupled-cluster single double triple [CCSD(T)] stabilization energies are used to parameterize the nonempirical model (NEMO) empirical intermolecular potential of the benzene dimer in the ground electronic state. The potential is used for theoretical interpretation of the dimer structure and the dynamics of its intermolecular motions. Only one energy minimum, corresponding to the T-shaped structure, is found. A parallel displaced structure is the first-order transition structure separating the molecular symmetrically equivalent T-shaped structures. Due to a relatively high transition barrier (approx 170 cm~(-1)), the interconversion tunneling is unimportant in the energy region spanned by the available rotational spectra and is thus neglected (accordingly, the molecular symmetry group which is used for interpretation of the available experimental spectra is related to the T-shaped structure with two feasible internal rotations and nonequivalent monomers). The dimer undergoes a nearly free internal rotation along the axis connecting the benzene centers of mass in the T-shaped equilibrium geometry and a hindered internal rotation (the barrier being approx 46 cm~(-1)) along the axis that is perpendicular to the "nearly free" internal rotation axis. The tunneling splittings observed in the rotational spectrum are likely due to this hindered rotation. An analysis assuming the latter rotational as an independent motion and using purely vibrational tunneling splittings (obtained by extrapolating to zero values of the rotational quantum numbers) indicates that the genuine value of the hindered rotation barrier is nearly twice higher than its ab initio value. Similarly, the difference #DELTA#R = 0.25 A between the ab initio (equilibrium) and experimental (ground state) values for the distance of the mass centers of the benzene monomers is strong evidence that our theoretical potential is much shallower than the genuine one. The Raman bands observed at the 3-10 cm~(-1) region 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 polarizability tensor of the dimer and temperature at which the spectra were taken.