Comparison of the proton-transfer path in hydrogen bonds from theoretical potential-energy surfaces and the concept of conservation of bond order. II. (N-H...N)(+) hydrogen bonds.
The quantum-mechanically derived reaction coordinates (QMRC) for the proton transfer in (N-H-N)(+) hydrogen bonds have been derived from ab initio calculations of potential-energy surfaces. A comparison is made between the QMRC and the corresponding bond-order reaction coordinates (BORC) derived by applying the Pauling bond-order concept together with the principle of conservation of bond order. We find virtually perfect agreement between the QMRC and the BORC for intermolecular (N-H-N)(+) hydrogen bonds. In contrast, for intramolecular (N-H-N)(+) hydrogen bonds, the donor and acceptor parts of the molecule impose strong constraints on the N-N distance and the QMRC does not follow the BORC relation in the whole range. The X-ray determined hydrogen positions are not located exactly at the theoretically calculated potential-energy minima, but instead at the point where the QMRC and the BORC coincide with each other. On the other hand, the optimized hydrogen positions, with other atoms in the cation fixed as in the crystal structure, are closer to these energy minima. Inclusion of the closest neighbours in the theoretical calculations has a rather small effect on the optimized hydrogen positions. [Part I: Olovsson (2006). Z. Phys. Chem. 220, 797-810.].
Majerz
Irena
I
Faculty of Chemistry, University of Wroclaw, Joliot-Curie 14, 50-383 Wroclaw, Poland.
Olovsson
Ivar
I