What factors determine whether a proton-bound homodimer has a symmetric or an asymmetric hydrogen bond?

Abstract

What factors determine whether a proton-bound homodimer with a central [X ··· H ··· X]⁺ moiety has a symmetric or an asymmetric hydrogen bond? Although hydrogen bonding may be considered the incipient step of protonation, quantum chemical computations indicate that the proton affinity of the base is not the governing influence as to whether the hydrogen bond is symmetric or asymmetric. Instead, the primary factor appears to be the electronegativity of the hydrogen-bonded heavy atom X, with a more electronegative X usually facilitating the formation of a symmetric hydrogen bond. Additional influences arise from the increased weakening of the X–H bond in the protonated monomer upon dimer formation for less electronegative X, as well as from stabilizing or destabilizing interactions involving more distant atoms that do not directly participate in the [X ··· H ··· X]⁺ hydrogen bond.

Keywords:

• hydrogen bonding
• quantum chemical calculations
• homodimers
• ab initio

Acknowledgements

We gratefully acknowledge generous allocations of computing time from the Australian Partnership for Advanced Computing (APAC) and the Australian Centre for Advanced Computing and Communications (ac3) (to L.R.), and from the Ohio Supercomputing Center (to J.E.D.B.). We also gratefully acknowledge the provision of funding by the U.S. National Science Foundation (Grant CHE-9873815 to J.E.D.B.), the award (to L.R.) of an Australian Professorial Fellowship, and funding from the ARC Centre of Excellence for Free Radical Chemistry and Biotechnology.