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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 0 and ± la, MG hydrogen bonds between ± 1s̀ and ± 2s̀, W hydrogen bonds beyond ± 2s̀ (where s̀ 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 θ (at the donor end) and ± and ± (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 conformational studies such as α‐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.

In bezug auf die Beteiligung der H‐Atome der ‐NH3+‐Gruppen von Aminosäuren an verschiedenen Zentren [neben dem Amino‐N das Carboxylat‐O, das Hydroxyl‐O, das Wasser‐O (in Hydraten), ein weiteres Amino‐N (in Dipeptiden), Cl‐ (in Hydrochloriden)] werden drei verschiedene Verbindungsklassen unterschieden: Aminosäuren mit drei Zweizentren‐, solche mit einer Drei‐ und zwei Zweizentren‐ und solche mit zwei Drei‐ und einer Zweizentren‐H‐Bindung (drei Dreizentren‐H‐Bindungen kommen nicht vor).