Previous posts have considered correlations between different physical properties of hydrogen bonded complexes (D-H...A). A key correlation is that as the donor-acceptor (D-A) separation R decreases the D-H stretch vibrational frequency (corresponding to varying r in the figure below) decreases.
There is a second vibrational mode associated with angle phi in the figure above. This can also be viewed as a torsional or bending mode. Hydrogen bonding leads to a stiffening of this mode.
The figure below shows how the frequency of this mode (horizontal axis) is correlated with the D-H stretch frequency (vertical axis).
The figure is taken from a classic 1974 review by Novak.
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