Chemistry Reference
In-Depth Information
Fig. 3 Contact distribution
functions for
halogen-halogen contacts.
Left
: aromatic (
taller
) and
aliphatic F
...
F;
right
:
aromatic (
taller
) and aliphatic
Cl
...
Cl
3
2.5
2
1.5
1
0.5
0
2
3 4
halogen-halogen distance
5
6
2
Fig. 4 Contact density
functions for oxygen-oxygen
contacts:
triangles
, nitro
oxygen,
squares
, carbonyl
oxygen. The
dotted line
is for
ether oxygen (no peak)
1.5
1
0.5
0
2.5
3.5
O...O distance
4.5
CDFs show distinct peaks (Fig.
3
); carbonyl C
¼
O
...
O
¼
C or nitro N
¼
O
...
O
¼
N
oxygen-oxygen CDFs show prominent peaks (Fig.
4
, while ether O
O CDFs do
not. Contact densities seem more dictated by an unknown admixture of steric and
electronic effects, and the identification of bonds on the basis of distance is unsafe
(see the example of oxygen; is there an oxygen-oxygen intermolecular bond?)
In general, geometry is but a poor substitute for physics; forthcoming sections in
this chapter will show that many short atom-atom contacts in crystals correspond
to neutral or even to slightly repulsive interactions.
...
