The infrared spectrum of the benzene–Ar cation

Abstract

The infrared (IR) absorption spectra of the jet-cooled C6H6 and C6D6 cations, complexed with Ar, are measured throughout the 450–1500 cm−1 region via IR-laser-induced vibrational dissociation spectroscopy. The IR spectrum of the C6H6–Ar cation is dominated by a Fermi resonance between the IR active ν11 mode and two components of the combination mode of the lowest frequency modes ν6 and ν16. A stringent upper limit of 316 cm−1 is found for the value of the dissociation limit D0 of the neutral C6D6–Ar complex.

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

“…This value of D 0 for Ar-C 6 D 6 ϩ agrees well with the experimental upper bound of 485 cm Ϫ1 . 11 Hence, we may conclude that the well depth of our ͑adia-batic͒ potentials for Ar-benzene ϩ is reliable. The difference in D 0 with the neutral complex, 150 cm Ϫ1 according to our calculations, is somewhat smaller than the experimental difference of 170 cm Ϫ1 obtained from the redshift of the ionization energy of benzene upon complexation with Ar.…”

“…Further dipole contributions are due to the polarization of Ar by the benzene cation and to other interaction-induced effects. The spectra measured are infrared spectra 11,22 due to combination bands of the van der Waals modes with some of the intramolecular modes of the benzene cation, or they were obtained by selectively exciting the van der Waals excited states of the cationic complex 20 or the corresponding Rydberg series of the neutral complex 21 by a resonance-enhanced two-photon process. The actual dependence of the dipole function on the Ar position vector R is more complicated, but we simply assumed a dipole linear in (x,y,z) with a proportionality constant of one.…”

“…11,[20][21][22] The vibronic levels from our calculations are quite dense, and in order to help with the assignment of the experimental spectra we constructed a model dipole function, calculated transition strengths, and simulated the far-infrared spectrum. An obvious contribution to the dipole moment function of the complex is due to the motion of the charged benzene ϩ monomer relative to the center of mass of the complex.…”

“…9,10 Experiments revealed, however, that the binding of the ionic complex in this case is not much stronger. The binding energies D e and D 0 are 387 and 328 cm Ϫ1 in neutral argonbenzene according to the ab initio calculations 6 and slightly less according to experiment, 11 while it is known from ionization energies 12 that D 0 is only 170 cm Ϫ1 larger in the ionic complex. We will show further on in this paper that in both the ionic and neutral systems the equilibrium position of the Ar atom is located on the sixfold symmetry axis of benzene and that the equilibrium distance R e is only slightly smaller in the ionic complex.…”

“…The Ar-benzene ϩ complex has been studied spectroscopically by Dopfer et al, 19 by Neusser and coworkers, 20,21 and by Meijer and co-workers. 11,22 The first paper concerns the intramolecular C-H stretch modes; the latter four studies involve also the intermolecular or van der Waals modes of the complex. In the present paper we describe a theoretical study of the Ar-benzene ϩ complex that considers these van der Waals modes and, in particular, the effect of the nonadiabatic Jahn-Teller coupling on these modes.…”