Chirped-pulse Fourier transform microwave spectrum of the HCl–DCl heterodimer

Hydrohalic acid dimers provide a fundamental opportunity to study hydrogen bond rearrangement dynamics at a high level of detail. The (HCl)2 and (DCl)2 homodimers do not have a pure rotational spectrum due to rapid geared tunneling motions that interchange the role of the hydrogen bond donor and acceptor. In this work, we report the pure rotational spectrum of HCl-DCl, which has a preference for deuterium in the hydrogen bond donor position. However, the quadrupole coupling constants indicate a significant amount of geared tunneling, consistent with significant zero-point wavefunction amplitude in the less stable DCl-HCl configuration. A comparison of experimental results with previously published wavefunction calculations based on model and ab initio potentials is consistent with a picture in which about 14% of the probability density distribution is located in the less stable well.