THz spectroscopy of weakly bound cluster molecules in solid para-hydrogen: a sensitive probe of van der Waals interactions

The present work demonstrates that 99.9% enriched solid para-H₂ below 3 K provides an excellent inert and transparent medium for the exploration of large-amplitude intermolecular vibrational motion of weakly bound van der Waals cluster molecules in the THz spectral region. THz absorption spectra have been generated for CO₂/H₂O and CS₂/H₂O mixtures embedded in enriched solid para-H₂ and numerous observed transitions associated with large-amplitude librational motion of the weakly bound binary CO₂...H₂O and CS₂...H₂O van der Waals cluster molecules have been assigned together with tentative assignments for the ternary CS₂...(H₂O)₂ system. The interaction strength, directionality and anharmonicity of the weak van der Waals ‘‘bonds’’ between the molecules can be characterized via these THz spectral signatures and yield rigorous benchmarks for high-level ab initio methodologies. It is suggested that even a less stable linear conformation of the ternary CS₂(H₂O)₂ system, where one H₂O molecule is linked to each S atom of the CS₂ subunit, may be formed due to the kinetics associated with the mobility of free H₂O molecules in the soft para-H₂ medium. In addition, the spectroscopic observations confirm a linear and planar global intermolecular potential energy minimum for the binary CS₂...H₂O system with C_2v symmetry, where the O atom on the H₂O molecule is linked to one of the S atoms on the CS₂ subunit. A semi-experimental value for the vibrational zeropoint energy contribution of 1.93 ±  0.10 kJ mol^-1 from the class of large-amplitude intermolecular vibrational modes is proposed. The combination with CCSD(T)/CBS electronic energy predictions provides a semi-experimental estimate of 5.08 0.15 kJ mol^-1 for the binding energy D₀ of the CS₂...H₂O van der Waals system.