Jahn–Teller effect in van der Waals complexes; Ar–C6H6+ and Ar–C6D6+

Abstract

Please be advised that this information was generated on 2018-05-11 and may be subject to change. The two asymptotically degenerate potential energy surfaces of argon interacting with the X 2 E 1g ground state benzene ϩ cation were calculated ab initio from the interaction energy of the neutral Ar-benzene complex given by Koch et al. ͓J. Chem. Phys. 111, 198 ͑1999͔͒ and the difference of the geometry-dependent 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 ϩ ͑the z axis͒ at z e ϭ3.506 Å. The binding energy D e of 520 cm Ϫ1 is only 34% larger than the value for the neutral Ar-benzene complex. The higher one of the two surfaces is similar in shape to the neutral Ar-benzene potential, the lower potential is much flatter in the (x,y) bend direction. Nonadiabatic ͑Jahn-Teller͒ coupling was taken into account by transformation of the two adiabatic potentials to a two-by-two matrix of diabatic potentials. This transformation is based on the assumption that the adiabatic states of the Ar-benzene ϩ complex geometrically follow the Ar atom. Ab initio calculations of the nonadiabatic coupling matrix element between the adiabatic states with the two-state-averaged CAS-SCF͑5,6͒ method confirmed the validity of this assumption. The bound vibronic states of both Ar-C 6 H 6 ϩ and Ar-C 6 D 6 ϩ were computed with this two-state diabatic model in a basis of three-dimensional harmonic oscillator functions for the van der Waals modes. The binding energy D 0 ϭ480 cm Ϫ1 of the perdeuterated complex agrees well with the experimental upper bound of 485 cm Ϫ1 . The ground and excited vibronic levels and wave functions were used, with a simple model dipole function, to generate a theoretical far-infrared spectrum. Strong absorption lines were found at 10.1 cm Ϫ1 ͑bend͒ and 47.9 cm Ϫ1 ͑stretch͒ 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 Jahn-Teller 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 Ϫ1 . The level at 1.3 cm Ϫ1 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 Ϫ1 gives rise to weaker absorption. Several other weak lines in the frequency range of 10 to 60 cm Ϫ1 were found. Jahn-Teller effect in van der

Diabatic states, nonadiabatic coupling, and the counterpoise procedure for weakly interacting open-shell molecules

“…[15][16][17][18][19] In particular, some of the present authors have developed a multiple-property-based diabatization scheme specifically tailored towards weakly interacting open-shell molecules. 20 Strong nonadiabatic coupling is also encountered for complexes involving open-shell atoms [21][22][23][24][25][26][27][28] as well as for non-linear molecules such as the methane and benzene cations, [29][30][31] which also have spatially degenerate electronic states.…”