The infrared spectrum of the H2–HCO+ complex

Abstract

A combined experimental and theoretical study of the structural properties of the H₂–HCO⁺ ion‐neutral complex has been undertaken. Infrared vibrational predissociation spectra of mass selected H₂–HCO⁺ complexes in the 2500–4200 cm⁻¹ range display several vibrational bands, the most intense arising from excitation of the C–H and H₂ stretch vibrations. The latter exhibits resolved rotational structure, being composed of Σ–Σ and Π–Π subbands as expected for a parallel transition of complex with a T‐shaped minimum energy geometry. The determined ground state molecular constants are in good agreement with ones obtained by ab initio calculations conducted at the QCISD(T)/6–311G(2df,2pd) level. The complex is composed of largely undistorted H₂ and HCO⁺ subunits, has a T‐shaped minimum energy geometry with an H₂...HCO⁺ intermolecular bondlength of approximately 1.75 Å. Broadening of the higher J lines in the P and R branches of the Π–Π subband is proposed to be due to asymmetry type doubling, the magnitude of which is consistent with the calculated barrier to H₂ internal rotation. The lower J lines in the Σ–Σ and Π–Π subbands have widths of 0.06 cm⁻¹, around three times larger than the laser bandwidth, corresponding to a decay time of ≊90 ps for the upper level. The absence of discernible rotational structure in the ν₂ band suggests that it has predissociation lifetime of less than 1 ps.