Solvent-induced frequency shifts as a probe for studying very weak molecular complexes: Evidence for van der Waals complex formation between COF2 andN-2 in cryogenic solutions

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 be en investigated In addition, spectra of COF2 and COF2/N-2 mixtures isolated in solid argon matrices have been studied. Vibrational frequencies of COF2 dissolved in LN2 exhibit unusual temperature behavior as compared to the s olutions in the other solvents. This, and the effects on the vibrational fr equencies and IR band shapes of COF2 in mixed nitrogen/argon solvents, is a ttributed to the formation of a complex COF2. N-2 in the solutions. A pheno menological model is proposed for the calculation of the temperature and co ncentration effects on the vibrational frequencies. By fitting the experime ntal data, the complexation enthalpy, Delta H degrees = -1.4(3) kJ mol(-1), was determined from this model. The existence of a COF2. N-2 complex was c onfirmed by the matrix isolation spectra. Geometry, energy, and vibrational spectrum of COF2. N-2 have been calculated by ab initio at the MP2/6-311+G(2d,2p) level. The ab initio predictions for Delta H degrees and for the c omplexation shifts of the vibrational bands agree well with the values dete rmined from the phenomenological model.