How to determine whether intramolecular H•••H interactions can be classified as dihydrogen bonds

Different types of intramolecular H center dot center dot center dot H interactions have been analyzed using the MP2/6-311++G(d,p) level of approximation. These are C-H center dot center dot center dot H-B, C-H center dot center dot center dot H-Al, C-H-H-C, C-H center dot center dot center dot H-O, O-H center dot center dot center dot H-Al and O-H center dot center dot center dot H-B contacts. Quantum theory of atoms in molecules and natural bond orbitals methods were applied to better understand the nature of these interactions. It was found that some of the species analyzed possess the characteristics of typical hydrogen bonds, such as the O-H center dot center dot center dot O ones. The electron charge transfer from the Lewis base to the antibonding X-H (for example O-H) orbital of the Lewis acid is one such characteristic. The NBO method may be considered decisive to classify any system as dihydrogen bonded.