Stepwise microsolvation of HCl revisited: Infrared investigation of selectively deuterated (HCl)m(H2O)n (m + n ≤ 4) cluster molecules.

In a recent theoretical investigation of DCl-H2O, HCl-D2O, and DCl-D2O [Felker et al., J. Phys. Chem. A, 125(29), 6437 (2021)] employing an accurate 9D permutation invariant polynomial-neural network potential energy surface and a highly efficient bound-state methodology, all the intramolecular vibrational eigenstates and dimerization spectral shifts of the three isotopic binary 1:1 complexes have been predicted. By means of dedicated annealing procedures, relative concentration dependencies, and a specialized dual inlet deposition procedure enabling complexation between specific isotopically substituted subunits, the present work identifies the intramolecular vibrational transitions experimentally for these three isotopologues of the binary complex and the most stable cyclic conformations of selectively deuterated mixed (HCl)m(H2O)n (m + n ≤ 4) cluster molecules embedded in inert neon "quantum matrices" at 4 K. The vibrational assignments up to mixed ternary cluster molecules are supported by harmonic CCSD(T)-F12b/cc-pVTZ-F12 frequency predictions in conjunction with anharmonic corrections employing second-order vibrational perturbation theory (VPT2) at the MP2/aug-cc-pVTZ level of theory. While the assigned O-H and O-D stretching transitions in neon are systematically spectrally redshifted by 0.2%-0.5% relative to previously reported observations in supersonic jets, the assigned H-Cl and D-Cl stretching transitions all reveal anomalous excessive spectral redshifts in neon increasing with the size of the cluster molecules. These cluster-size dependent excessive H-Cl/D-Cl spectral redshifts in neon indicate that the extent of charge transfer is enhanced strongly with the complexation of an increasing number of H2O molecules as predicted by quantum chemical models for more than a decade.