Studies on hydrogen bonds. Part IV. Proposed working criteria for assessing qualitative strength of hydrogen bonds

The data obtained in the earlier parts of this series for the donor and acceptor end parameters of N-H...O and O-H...O hydrogen bonds have been utilised to obtain a qualitative working criterion to classify the hydrogen bonds into three categories: "very good" (VG), "moderately good" (MG) and weak (W). The general distribution curves for all the four parameters are found to be nearly of the Gaussian type. Assuming that the VG hydrogen bonds lie between O and +/- 1 sigma, MG hydrogen bonds between +/- 1 sigma and +/- 2 sigma, W hydrogen bonds beyond +/- 2 sigma (where sigma is the standard deviation), suitable cut-off limits for classifying the hydrogen bonds in the three categories have been derived. These limits are used to get VG and MG ranges for the four parameters 1 and theta (at the donor end) and zeta and xi (at the acceptor end). The qualitative strength of a hydrogen bond is decided by the cumulative application of the criteria to all the four parameters. The criterion has been further applied to some practical examples in conformation studies such as alpha-helix and can be used for obtaining suitable location of hydrogen atoms to form good hydrogen bonds. An empirical approach to the energy of hydrogen bonds in the three categories has also been presented.