Hydrogen–hydrogen bonding in biphenyl revisited |
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Authors: | Jesús Hernández-Trujillo Chérif F Matta |
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Institution: | (1) Departamento de Física y Química Teórica, Facultad de Química, Universidad Nacional Autónoma de México, Mexico, DF, 04510, Mexico;(2) Department of Chemistry and Physics, Mount Saint Vincent University, Halifax, NS, Canada, B3M 2J6;(3) Department of Chemistry, Dalhousie University, Halifax, NS, Canada, B3H 4J3 |
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Abstract: | The evidence for the stabilizing nature of the H–H bonding in planar biphenyl is succinctly reviewed. The stabilizing nature
of the H–H bonding is revealed through a comparison of the atomic energy of every atom in planar biphenyl with the same atom
in the twisted equilibrium structure. It is shown that the barrier to rotation via the planar transition state is the net
resultant of a stabilisation of the four ortho-hydrogen atoms (by 8 kcal/mol each), a stabilisation of the two para-carbon atoms (by 3 kcal/mol each) and by the dominant destabilisation of the two carbon atoms joining the two rings—the two
junction carbon atoms—(by 22 kcal/mol each). The energetic stabilisation of the four ortho-hydrogen atoms is further shown to be in large proportion due to the formation of the hydrogen–hydrogen interatomic surface.
Furthermore, neither the “bond order” between the two junction carbon atoms nor the total electron delocalisation between
the two rings exhibit a significant change in going from the planar to the twisted equilibrium geometry. These findings are
in contrast with the classical view of a balance between “steric non-bonded repulsion” and better electron delocalisation
as a function of the twist dihedral angle. Similar conclusions have been recently reached by Pacios and Gómez through a study
of the electrostatic potential at the position of the hydrogen nuclei.
We dedicate this article to Professor TM Krygowski on the occasion of his 70th birthday wishing him a long and productive
life. |
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Keywords: | Hydrogen– hydrogen bonding Dihydrogen bonding Electron density Quantum theory of atoms in molecules (QTAIM) Bond path |
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