Solvent-Induced Frequency Shifts as a Probe for Studying Very Weak Molecular Complexes: Evidence for van der Waals Complex Formation between COF2and N2in Cryogenic Solutions

Abstract

FTIR (4000−400 cm-1) spectra of COF2 dissolved in liquefied argon (LAr), krypton (LKr), xenon (LXe), nitrogen (LN2), and in LAr/LN2 mixtures have been investigated. In addition, spectra of COF2 and COF2/N2 mixtures isolated in solid argon matrices have been studied. Vibrational frequencies of COF2 dissolved in LN2 exhibit unusual temperature behavior as compared to the solutions in the other solvents. This, and the effects on the vibrational frequencies and IR band shapes of COF2 in mixed nitrogen/argon solvents, is attributed to the formation of a complex COF2·N2 in the solutions. A phenomenological model is proposed for the calculation of the temperature and concentration effects on the vibrational frequencies. By fitting the experimental data, the complexation enthalpy, ΔH° = −1.4(3) kJ mol-1, was determined from this model. The existence of a COF2·N2 complex was confirmed by the matrix isolation spectra. Geometry, energy, and vibrational spectrum of COF2·N2 have been calculated by ab initio at the MP2/6-311++G(2d,2p) level. The ab initio predictions for ΔH° and for the complexation shifts of the vibrational bands agree well with the values determined from the phenomenological model.