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The noncovalent interactions between pairs of halogen (X) atoms on separate molecules are commonly categorized as Type I or II (T1 or T2). T2 contains a standard halogen bond where the two molecules are roughly perpendicular to one another, while the two units in T1 are nearly antiparallel. Quantum calculations reveal that the T2 geometry is more stable than T1 for FX pairs (X = Cl, Br, I) but only by a small amount. In mixed heterodimers FX 1 ···X 2 F, there is a preference for the heavier X atom to serve as an electron acceptor. The T1 conformation is bound by a pair of bent halogen bonds, along with some positive cooperativity between them. The internal FX bond is stretched within the acidic subunit and contracted in the base, accompanied respectively by red and blue shifts of the FX stretching frequency. The NMR chemical shielding of the X atoms is increased by the formation of each complex, with a smaller deshielding observed on the F atoms. These changes rise rapidly in the order Cl < Br < I.
Steve Scheiner (Wed,) studied this question.