Study of the HCl-DCl Van der Waals Complex and its Isotopologues via Rotational Spectroscopy and Quantum Chemistry

Hydrohalic acid dimers are the simplest prototypes for understanding intermolecular hydrogen bonding interactions. 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. The HCl-DCl dimer, on the other hand, 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.The experimental 𝜃_eff values agreed poorly with previously reported four-dimensional eigenstate calculations [Elrod, Saykally J. Chem. Phys. 103, 921 (1995)] on the 6D analytical ab initio potential of Bunker and co-workers [J. Mol. Spectrosc. 146, 200 (1991)]. However, the one-dimensional model of Schuder and Nesbitt, based on a reduced dimension electrostatic potential [J. Chem. Phys. 100, 7250 (1994)] was in fairly good agreement with experimental results.