Infrared and Raman measurements of halogen bonding in cryogenic solutions.

Because they create a weakly interacting environment which, combined with the low temperatures used, leads to small bandwidths and thus facilitates the detection of complex bands only slightly shifted from the monomer modes, solutions in liquefied inert gases have proven to be an ideal medium to study molecular complexes held together by weak and medium-strong C-X…Y (with X=I, Br, Cl and Y=O, N, S, F, Cl, π,…) halogen bonds. In this chapter, experimental setups for infrared and Raman study of cryosolutions are described, and general methodologies used to examine weakly bound molecular complexes are discussed. The methods are illustrated using data obtained for a variety of halogen-bonded complexes involving, amongst others, the trifluorohalomethanes CF3Cl, CF3Br, and CF3I, and a variety of Lewis bases. The results are compared with theoretical data obtained from ab initio calculations, and with experimental and theoretical data obtained for complexes involving weak C-H proton donors such as CHF3. Preliminary data for mixed proton donor/halogen donors such as CHClF2, CHBrF2 are also discussed.