Fulvenallenyl cation (C7H5(+)) and its complex with an argon atom: results of high-level quantum-chemical calculations

The fulvenallenyl cation (C(7)H(5)(+)) and its complex with an argon atom have been studied by explicitly correlated coupled cluster theory at the CCSD(T)-F12x(x = a, b) level and by the double-hybrid density functional B2PLYP-D. For the free cation, an accurate equilibrium structure has been established and ground-state rotational constants of A(0) = 8116.4 MHz, B(0) = 2004.3 MHz, and C(0) = 1606.9 MHz are predicted. The equilibrium dipole moment is calculated to be μ(e) = 1.305 D, with the positive end of the dipole at the acetylenic hydrogen site. Anharmonic wavenumbers of C(7)H(5)(+) were obtained by combination of harmonic CCSD(T*)-F12a values and B2PLYP-D anharmonic contributions. The most intense vibration is the pseudoantisymmetric CC stretching vibration at 2083 cm(-1). The potential energy surface of the complex C(7)H(5)(+)·Ar is characterized by two energy minima of C(s) symmetry which are separated by a very low energy barrier. The dissociation energy of the most stable structure is predicted to be D(0) = 530 ± 30 cm(-1).