New Perspective on Hydrogen Bonding.

A new model is proposed for hydrogen bonding in which an intermediate hydrogen atom acts as a bridge bond connecting two adjacent atoms, X and A, via quantum mechanical tunneling of the hydrogen electron. A strong hydrogen bond (X-H-A) is formed when the X-H and H-A interatomic distances are short and symmetric, thereby facilitating intense electron tunneling to and from both adjacent atoms. The hydrogen bond weakens (X-H···A) as the H···A interatomic distance lengthens compared to that of X-H since the H···A tunneling intensity degrades exponentially with increasing distance. Two modes of electron tunneling are distinguished. When an electron tunnels from H to either X or A (forward tunneling), the X-H···A bond is initially charge neutral but after tunneling is charged as either X-H···A or X-H···A. In contrast, electron tunneling from either X or A back to H (reverse tunneling) discharges the X-H···A bond, resetting it back into its neutral charge state. Reverse tunneling is central to understanding the nature of a hydrogen bond. When the H···A interatomic distance is sufficiently short, reverse tunneling occurs through a triangular energy barrier (Fowler-Nordheim tunneling) such that the reverse tunneling probability is almost 100%. Increasing the H···A interatomic distance leads to a decreasing H···A reverse tunneling probability, as tunneling occurs through an asymmetric trapezoidal energy barrier (direct tunneling) until finally the H···A interatomic distance is so large that the bond persists indefinitely in the X-H···A charge state such that it is incapable of acting as a bridge bond linking X and A.