Topological aspects of protonation and hydrogen bonding: the dihydrogen bond case

The topological theory of the chemical bond suggests that the favored protonation site of a molecule satisfies a least topological change principle. This generally means that the numbers of basins in the neutral and protonated species are equal. If the molecule has one or more lone pair the protonation takes place in a mono-synaptic basin yielding the formation of a covalent bond B-H. This process is named covalent protonation accordingly. In the case of hydrides, MHn the conservation of the number of basins implies the formation of a complex involving MHn-1+ and H-2 It is proposed to name this process as predissociative protonation, The conventional hydrogen bond A-H . . .B can be considered as the initial step of a proton transfer reaction leading to the covalent protonation BH+, whereas the dihydrogen bond, A-H . . .H-M, corresponds to the predissociative protonation. This hypothesis has been investigated on systems involving LiH, BeH2, BH3, CH4 and BH3NH3 as proton acceptor. (C) 2000 Elsevier Science B.V. All rights reserved.