Hindered rotation in ion-neutral molecular complexes: The ν1 vibration of H2–HCO+ and D2–DCO+

Infrared spectra of the mass selected ionic complexes H2–HCO+ and D2–DCO+ have been recorded in the vicinity of their ν1 vibrations (H2/D2 stretch) by means of photofragmentation spectroscopy. The anomalous rotational constants obtained by fitting the observed line positions to a semirigid Watson A-type Hamiltonian reflect the appreciable zero-point excursions of the H2/D2 molecule. Barriers for this internal motion are estimated utilizing a simple atom–diatom hindered rotor Hamiltonian. According to this one-dimensional model, the barrier increases by about 15% upon vibrational excitation which is mainly attributed to electrostatic effects.