Journal title:

Journal of Chemical Physics

Abstract:

The two asymptotically degenerate potential energy surfaces of argon interacting with the X (2)E(1g) ground state benzene(+) cation were calculated ab initio from the interaction energy of the neutral Arbenzene complex given by Koch et al. [J. Chem. Phys. 111, 198 (1999)] and the difference of the geometrydependent ionization energies of the complex and the benzene monomer computed by the outer valence Green's function method. Coinciding minima in the two potential surfaces of the ionic complex occur for Ar on the C(6v) symmetry axis of benzene(+) (the z axis) at z(e)=3.506 A. The binding energy D(e) of 520 cm(1) is only 34% larger than the value for the neutral Arbenzene complex. The higher one of the two surfaces is similar in shape to the neutral Arbenzene potential, the lower potential is much flatter in the (x,y) bend direction. Nonadiabatic (JahnTeller) coupling was taken into account by transformation of the two adiabatic potentials to a twobytwo matrix of diabatic potentials. This transformation is based on the assumption that the adiabatic states of the Arbenzene(+) complex geometrically follow the Ar atom. Ab initio calculations of the nonadiabatic coupling matrix element between the adiabatic states with the twostateaveraged CASSCF(5,6) method confirmed the validity of this assumption. The bound vibronic states of both ArC(6)H(6) (+) and ArC(6)D(6) (+) were computed with this twostate diabatic model in a basis of threedimensional harmonic oscillator functions for the van der Waals modes. The binding energy D(0)=480 cm(1) of the perdeuterated complex agrees well with the experimental upper bound of 485 cm(1). The ground and excited vibronic levels and wave functions were used, with a simple model dipole function, to generate a theoretical farinfrared spectrum. Strong absorption lines were found at 10.1 cm(1) (bend) and 47.9 cm(1) (stretch) that agree well with measurements. The unusually low bend frequency is related to the flatness of the lower adiabatic potential in the (x,y) direction. The van der Waals bend mode of e(1) symmetry is quadratically JahnTeller active and shows a large splitting, with vibronic levels of A(1), E(2), and A(2) symmetry at 1.3, 10.1, and 50.2 cm(1). The level at 1.3 cm(1) leads to a strong absorption line as well, which could not be measured because it is too close to the monomer line. The level at 50.2 cm(1) gives rise to weaker absorption. Several other weak lines in the frequency range of 10 to 60 cm(1) were found.
