in ammonia and OH 
in water
By far, the most striking similarity between NH in ammonia and
OH in water is the unusually high coordination number around the
heavy atom. Based on the number of lone pairs around the heavy
atom, a naive estimate of the number of hydrogen bonds would
be 3 for OH in water and 2 for NH in ammonia. Indeed,
gas phase quantum chemical calculations of OH with three
water molecules confirm that threefold coordination of the oxygen
atom is the lowest energy structure [73]. However, ab initio
MD simulations of OH in water at 300 K [8, 9]
showed that, in the liquid, structures in which four or five hydrogen
bonds surround the oxygen are possible, and, in addition, the
OH hydrogen can also form a weak hydrogen bond with
nearby water molecules. The overall coordination number predicted
from these simulations is approximately 5.3, which is in excellent
agreement with the recently measured value from dieletric relaxation
experiments of 5.5 
0.5 [74].
Recent quantum chemical calculations of OH with
four water molecules [73] show that threefold coordination
of the OH oxygen with the fourth water molecule in the second solvation
shell is the lowest energy structure but that fourfold coordination
of the oxygen is only about 1.2 kcal/mol higher in energy and represents
a local minimum on the potential energy surface.
In the case of NH in ammonia, the coordination is also considerably
higher than expected based on the number of lone pairs. In
Sec. 5.1, it was seen that the coordination number of
NH in ammonia is between 7 and 8. However, unlike the OH case,
not all of this is due to hydrogen bonding (cf. Fig. 9)
and the discussion of Fig. 5.1).
The ELF used in Sec. 4.3 and 5.4 can be used to
provide an explanation of why the coordination of the anionic defects
is so large. Figure 5.4 shows the ELF for NH in the gas phase
and in some typical complexes. For comparison, the ELF for OH
in the gas phase and in some typical complexes is shown in Fig. 6.2.
in the gas phase (a), with a threefold coordinated
first solvation shell (H 
O 
)(b),
and with a fourfold coordinated first solvation shell (H 
O 
)(c).
Because 
corresponds to perfect localization,
the contour cutoff level is chosen to be 0.93 in the figure.
Plots such as these were also introduced in Ref. [76]
in the context of fully quantum mechanical studies of OH
solvation in water. The figure shows the dramatic delocalization
of the three lone pairs of OH leading to the formation of
a torus of nearly equal probability. This negatively
charged ring allows hydrogens to form hydrogen bonds to the
hydroxyl oxygen without needing to adhere to specific
``basins'' of negative attraction, thereby allowing for a higher
coordination than would be expected based on the actual number of lone pairs.
Similarly, the ELF
for NH shows that half of such a ring is formed by the
two lone pairs. This half ring also allows hydrogen bonding
to the NH nitrogen to occur with no adherence to basins
of attraction and leads to the structures observed in Fig. 5.1.
Several key differences exist between these ions as well. One clear
difference is that the NH hydrogens do not form hydrogen
bonds to neighboring ammonia molecules, while the OH hydrogen
is capable of forming weak hydrogen bonds to solvent
molecules [8, 9, 76].
The formation of such hydrogen bonds has been implicated as a
crucial element in the structural diffusion of the OH defect
in water [76]. The inability of NH hydrogens to
form such weak hydrogen bonds, combined with the coordination number
mismatch alluded to in the previous section, is the most likely
explanation for the lack of proton transfer mediated structural
diffusion of this anionic defect in ammonia.
