Halogen bond distance as a function of temperature

The halogen bond, that is, the attractive interaction between a halogen atom and an electron lone pair, was studied as a temperature function in the range 90-292 K. Three model structures containing the bonds Ar-I...N, Ar-I...O, and Ar-Br...N, where Ar is a perfluorinated aromatic residue, were considered: (E)-1,2-bis- (4-pyridyl)ethylene 1,4-diiodo-2,3,5,6-tetrafluorobenzene (bpe.F4dIb), (E)-1,2-bis-(4-pyridyl)ethylene 1,4-dibromo- 2,3,5,6-tetrafluorobenzene (bpe.F4dBrb), and 4,4'-dipyridyl-N,N'-dioxide 1,4-diiodo-2,3,5,6-tetrafluorobenzene (bpNO. F4dIb). All three Ar-X...B systems (B ) lone pair donor) are nearly linear, and the change of X...B bond lengths spans from 0.030 to 0.059 Å in the studied temperature range; the least change is found in bpNO...F4dIb where the interaction between the two molecules is enforced by the presence of an H...O hydrogen bond. Other weak packing features, present in all the three structures, such as H...F, show larger variations in the same temperature range.