On the violation of Legon–Millenʹs rules for predicting angular geometries of hydrogen-bonded dimers: an ab initio study

The potential energy surfaces of furan⋯HCl, furan⋯ClF, thiophene⋯HCl, and thiophene⋯ClF complexes were extensively explored at the MP2/aug-cc-pVDZ level. QCISD(T)/aug-cc-pVDZ//MP2/aug-cc-pVDZ calculations were carried out to improve the energy estimates. Ab initio calculations confirm that, as demonstrated by recent experimental work, there are exceptions to the third Legon–Millenʹs rule for predicting angular geometries of hydrogen-bonded dimers. Besides electrostatic considerations, charge transfer contributions may play an important role in determining the experimentally observed geometry of the furan⋯ClF complex.