Knowledge Bank

A peculiar distortion of the hydrogen fluoride fundamental absorption band at 2.47 microns has been observed in mixtures of hydrogen fluoride and chlorine trifluoride. This distortion is caused by an infrared band coincident with the P branch of hydrogen fluoride. The absorbence of this band depends upon the product of the partial pressures of hydrogen fluoride and chlorine trifluoride and is hence attributable to a molecular complex formed by one molecule of hydrogen fluoride combining with one molecule of chlorine trifluoride. The complex is present only in low concentration. The equilibrium constant was estimated to be about $1\times 10-4$ per mm at $21\circ C$. The temperature variation per unit product of the pressures indicates a heat of reaction of 3.9 kcal per mole for the reaction $HF+ClF3\leftrightarrow HF\cdot ClF3$. The small displacement of this band from the hydrogen fluoride fundamental suggests that the complex involves a bonding of the fluorine of the hydrogen fluoride to the chlorine trifluoride rather than a hydrogen bond. The 3.51 micron band of $DF\cdot CIF3$ has been observed as coincident with the P branch of deuterium fluoride.