Advances in use of p-H₂ as a novel host for matrix IR spectroscopy

A review of recent advances in IR spectroscopy using p-H₂ as a matrix host in our laboratory is presented. The softness of this novel p-H ₂ matrix host not only makes high-resolution spectroscopy feasible but also provides new applications. Molecular motion, such as the rotation of C₂H₄, internal rotation (torsion) of CH₃OH, and rotation of CH₃F about a single axis are characterized in this p-H₂matrix. In contrast, no evidence for rotation of CH₃I and CH₃ was found. The diminution of the cage effect leads to the possibility of production of free radicals via either photolysis in situ or bimolecular reactions of molecules with atoms that are produced in situ from their molecular precursors. Taking advantage of these unique features, we identified the IR spectrum of CH₃ produced from photolysis of CH ₃I and CH₃S produced from photolysis of CH ₃SCH₃ and CH₃SSCH₃. The reaction of Cl with CS₂ in solid p-H₂ yields ClSCS instead of ClCS₂. The reaction of Cl with HCOOH in solid p-H₂ yields only a complex, Cl·HCOOH, not HCl and HOCO. The presence of o-H ₂ in trace proportions turned out to be an important factor in the interpretation of IR spectra of species embedded in solid p-H₂. Experimental techniques to minimize the amount of o-H₂ and to characterize its relative concentration with spectra of H₂O·o- H₂ are discussed.