driven by the accumulation of non-covalent interactions in the transition state. This
is in agreement with the secondary interactions proposed by Hook. However, it
should be noted that the different substituants give rise to a different deformation,
leading to barriers as high as ca. 20 kcal/mol that can not be strickly attributed to
non-covalent interactions.
18.9 Summary and Conclusions
In conclusion, non-covalent interactions have a unique signature and their presence
can be revealed solely from the electron density. Non-covalent interactions are
highly nonlocal and manifest in real space thanks to the NCI analysis: in other
words, as low-gradient isosurfaces with low densities. The sign of the second
2
+
1
e
p
y
T
1
e
p
y
T
3
e
p
y
T
3
+
2
+
1
e
p
y
T
Fig. 18.18 Non-covalent interactions types in (in,in) (top left, top right and bottom left) and (out,
out) (bottom, right) transition states. Black, red and blue dashed lines represent type 1, type 2 and
type 3 interactions respectively
18 A Complete NCI Perspective: From New Bonds to Reactivity
521