Matrix trapping of two structural arrangements of weak complexes

FTIR spectra of Ar/CO/HF mixture rapidly frozen at 12 and 5 K contained a weak new absorption at 3907 cm^-1 in addition to the strong OC- -HF complex band at 3789 cm^-1. The DF counterpart absorbance at 2866 cm^-1 was a larger fraction of the absorbance for OC- -DF at 2781 cm^-1. The former new bands were generally favored by slower spray-on rates and were destroyed on annealing that increased the latter bands. These weaker new bands are assigned to the reverse complexes CO- -HF and CO- -DF that calculations predict to be less stable than the carbon-bonded structures observed in the gas phase and and larger yield in matrices. An analogous situation was found for Ar/HCN/HF mixtures which revealed both HF- -HCN and HCN- -HF. Apparently matrix formation relaxes these molecules fast enough to trap the less stable complex structures that are not observed in supersonic nozzle expansion experiments.