Directionality is a key property of hydrogen bonds

Today I re-read a nice paper The hydrogen bond: a molecular beam microwave spectroscopist’s view with a universal appeal, by  Mausumi Goswami and E. Arunan

I found the following sentences very helpful

one aspect about hydrogen bonding that is widely accepted is the directionality, i.e. X–HY is found to be linear in most cases. Although secondary interactions in a system could force X–HY away from linearity, it is the directionality in hydrogen bonding resulting in an anisotropic intermolecular potential that separates it from the more general ‘van der Waals forces’, which are expected to be isotropic.

The main point of the paper is

For a ‘hydrogen bonded complex’, the zero point energy along any large amplitude vibrational coordinate that destroys the orientational preference for the hydrogen bond should be significantly below the barrier along that coordinate so that there is at least one bound level. These are vibrational modes that do not lead to the breakdown of the complex as a whole. If the zero point level is higher than the barrier, the ‘hydrogen bond’ would not be able to stabilize the orientation which favors it and it is no longer sensible to characterize a complex as hydrogen bonded.

Next week I will be visiting the Indian Institute for Science in Bangalore and look forward to meeting the authors then.

This directionality of is incorporated in my effective Hamiltonian for hydrogen bonding via the directional dependence of the matrix element which couples the two diabatic states. It is also responsible for the hardening of vibrational modes associated with rotation of the D-H unit relative to the acceptor atom A (discussed in my last H-bond post).