High resolution infrared spectroscopy of (HCl)₂ and (DCl)₂ isolated in solid parahydrogen: Interchange-tunneling in a...

Infrared spectroscopic studies of weakly bound clusters isolated in solid parahydrogen (pH₂) that exhibit large-amplitude tunneling motions are needed to probe how quantum solvation perturbs these types of coherent dynamics. We report high resolution Fourier transform infrared absorption spectra of (HCl)₂, HCl-DCl, and (DCl)₂ isolated in solid pH₂ in the 2.4-4.8 K temperature range. The (HCl)₂ spectra show a remarkable amount of fine structures that can be rigorously assigned to vibration-rotation-tunneling transitions of (HCl)₂ trapped in double substitution sites in the pH₂ matrix where end-over-end rotation of the cluster is quenched. The spectra are assigned using a combination of isotopically (H/D and ³⁵Cl/³⁷Cl) enriched samples, polarized IR absorption measurements, and four-line combination differences. The interchange-tunneling (IT) splitting in the ground vibrational state for in-plane and out-of-plane H³⁵Cl-H³⁷Cl dimers is 6.026(1) and 6.950(1) cm^-1, respectively, which are factors of 2.565 and 2.224 smaller than in the gas phase dimer. In contrast, the (DCl)₂ results show larger perturbations where the ground vibrational state IT splitting in D³⁵Cl-D³⁷Cl is 1.141(1) cm^-1, which is a factor of 5.223 smaller than in the gas phase, and the tunneling motion is quenched in excited intramolecular vibrational states. The results are compared to similar measurements on (HCl)₂ made in liquid helium nanodroplets to illustrate the similarities and differences in how both these quantum solvents interact with large amplitude tunneling motions of an embedded chromophore.