Explicitly correlated coupled cluster calculations for the propargyl cation (H2C3H+) and related species

The vibrations of the propargyl cation (H3C3H+) have been studied by vibrational configuration interaction (VCI) calculations, using explicitly correlated coupled cluster theory at the CCSD(T*)-F12a level to determine the underlying 12-dimensional potential energy surface. The wavenumbers of the fundamental vibrations are predicted with an accuracy of ca. 5 cm(-1). Harmonic wavenumber shifts for three different energy minima of the complex H2C3H+ center dot Ar are combined with the corresponding VCI values in order to provide a comparison with recent infrared photodissociation (IRPD) spectra (A. M. Ricks et al., J. Chem. Phys., 2010, 132, 051101). An excellent agreement between experiment and theory is obtained for bands nu(2) (symm. CH stretch), nu(3) (pseudoantisymm. CC stretch), and nu(4) (CH2 scissoring). However, reassignments are suggested for the bands observed at 3238 cm(-1), the "doublets'' around 3093 and 1111 cm(-1), and the band at 3182 cm(-1). The assignment of the latter to the asymmetric CH stretching vibration of c-C3H3+center dot Ar is certainly wrong; the combination tone nu(3) + nu(5) of H2C3H+center dot Ar is a more likely candidate. Furthermore, accurate proton affinities are predicted for the carbenes H2Cn with n = 3-8, thereby providing data of interest for interstellar cloud chemistry.