THE INFRARED-SPECTRUM OF THE H-2-HCO+ COMPLEX

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 H 2-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. © 1995 American Institute of Physics.