| Comments |
"Mean distances are listed in Table 3 for fairly linear X-H···O[W] hydrogen bonds (W=water molecule) from 47 X-H
donor types (X=O, N, S, and C). If one uses the ranking of
distances to define a "donor strength" of X-H, one finds a
general ranking O-H>N-H>S-H>C-H. These are only
rough categories, however, with strong internal variations.
Hydrogen bonds of the strongest C-H types (Cl3CH, C=CH)
are clearly shorter on average than those with the weakest
N-H donors (C[sp2]-NH2, N-NH2). The ranking within the
X-H groups follows a simple rule: basically, the donor
strength is increased by neighboring electron-withdrawing
groups and reduced by electron-donating groups. In consequence,
the ranking of O-H donor strengths is H3O+>O=C-OH>Ph-OH>C[sp3]-OH>H2O>OH-. The difference
in mean bond lengths D within this sequence amounts
to over 0.7Å! Remember, however, that each of the lines in
Table 3 corresponds to a broad histrogram (such as in
Figure 11a). The corresponding 95% ranges are normally
over 0.3Å broad (last two columns in Table 3), which implies
there are large overlapping regions of hydrogen bond geometries
between most donor types." |