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By G. Desiraju, et. al.

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4 Å, the directionality rapidly becomes weaker. This could be due to the fact that some of these longer contacts are part of a bifurcated scheme, but these aspects have not been evaluated rigorously. The efficacy of a C H O/N interaction diminishes with falling and in practice, we recommend a lower limit of 110° when accepting a C H O geometry as a hydrogen bond. As for all cut-off criteria in this subject, this or any other angular delimiter should not be used indiscriminately. However, since the electrostatic interaction is zero at = 90°, this perpendicular geometry represents the formal end of hydrogen bonding.

1996). For the weak C H O hydrogen bonds, analogous effects should, in principle, be expected. 1. O interactions can be estimated using the valence bond model of the hydrogen bond, which is based on a strict relation between bond distances and valences (Brown 1992; Steiner 1998a). 7. 01 A, and this would be difficult to detect even with neutron diffraction.

These histograms represent 916 and 257 hits, respectively, and so are statistically significant samples. Here, the full range of C-O distances is considered rather than just the mean values, and it may be observed that the entire histogram for t-CO is offset towards longer C-O distances relative to the histogram for b-CO. 57 (2) C H O hydrogen bonds to b-CO ligands are shorter than those to t-CO ligands. In summary, one may conclude with some confidence that the basicity of CO ligands has a definite effect on C H O hydrogen bond lengths formed by them.

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