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

Abstract

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⁺·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 ν₂ (symm. CH stretch), ν₃ (pseudoantisymm. CC stretch), and ν₄ (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-C₃H⁺₃·Ar is certainly wrong; the combination tone ν₃ + ν₅ of H₂C₃H⁺·Ar is a more likely candidate. Furthermore, accurate proton affinities are predicted for the carbenes H₂C_n with n = 3–8, thereby providing data of interest for interstellar cloud chemistry.