Infrared spectra and quantum chemical characterization of weakly bound clusters of the benzoyl cation with Ar and H2O

Abstract

Weakly-bound clusters of the closed-shell benzoyl cation (C₆H₅CO⁺, PhCO⁺) with Ar and H₂O are investigated by infrared (IR) spectroscopy, mass spectrometry, and quantum chemical calculations in order to characterize the interaction of a closed shell aromatic cation with a nonpolar and a polar ligand. PhCO⁺–L dimers are produced by electron ionization of benzaldehyde in a supersonic plasma expansion. IR photodissociation (IRPD) spectra of PhCO⁺–L with L = Ar and H₂O are analyzed in the C–O, C–H, and O–H stretch ranges (2000–3900 cm⁻¹). The potential energy surface of the PhCO⁺–L dimers is characterized at the MP2/6-311++G(2df,2pd) level to locate the various minima and determine their energetic and vibrational properties. PhCO⁺–Ar prefers intermolecular π-bonding to the aromatic ring with a bond energy of D₀ = 6 kJ mol⁻¹. The weak interaction implies that the IRPD spectrum of PhCO⁺–Ar is very close to the IR spectrum of bare PhCO⁺. The detection of the C–H and C–O stretch fundamentals provides valuable information about the C–O and C–H bond strengths in this prototypical aromatic acylium ion. Moreover, a variety of weak combination and overtone bands are assigned. The global minimum on the PhCO⁺–H₂O potential has a planar charge–dipole configuration with D₀ = 41 kJ mol⁻¹ (with only the two H₂O protons being out-of-plane), in which the lone pairs of H₂O interact with the positive partial charges on the carbonyl carbon atom and the proton of the CH group in ortho position. The experimental IRPD spectra are in accord with the calculated global minima. The analysis of the charge distribution shows that the PhCO⁺ cation is best represented as an oxocarbenium ion (Ph–C⁺O) with smaller contributions of the ketene structure (Ph⁺CO). This view is supported by the geometrical and vibrational properties of PhCO⁺ as well as the shape of the intermolecular PhCO⁺–L dimer potentials.