STUDIES ON HYDROGEN BONDS: Part IV. Proposed Working Criteria for Assessing Qualitative Strength of Hydrogen Bonds*

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Abstract
The data obtained in the earlier parts of this series for the donor and acceptor end parameters of N-H.cntdot..cntdot..cntdot.O and O-H.cntdot..cntdot..cntdot.O H bonds were utilized to obtain a qualitative working criterion to classify the H-bonds into 3 categories: very good (VG), moderately good (MG) and weak (W). The general distribution curves for all the 4 parameters are nearly of the Gaussian type. Assuming that the VG H-bonds lie between 0 and .+-. 1 .sigma., MG H-bonds between .+-. 1 .sigma. and .+-. 2 .sigma., W H-bonds beyond .+-. 2 .sigma. (where .sigma. is the SD), suitable cut-off limits for classifying the H-bonds in the 3 categories were derived. These limits are used to get VG and MG ranges for the 4 parameters 1 and .theta. (at the donor end) and .zeta. and .xi. (at the acceptor end). The qualitative strength of a H-bond is decided by the cumulative application of the criteria to all the 4 parameters. The criterion was further applied to some practical examples in conformational studies such as .alpha.-helix and can be used for obtaining suitable location of H atoms of form good H-bonds. An empirical approach to the energy of H-bonds in the 3 categories was also presented.

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