From all these previous considerations, it seems judicious to define the following
descriptor that involves the main energetic components linked to the primary atoms:
DE
tot
XN ¼ E
IQA
XN þ E
disp
XN þ DE
intra
IQA ðXÞ þ DE
intra
IQA ðNÞ;
ð16:21Þ
where ΔE
intra is calculated with respect to the intraatomic energy in the relevant free
isolated fragment. The evolutions of this quantity are depicted in Graph 16.9.
A clear dichotomy is then revealed: DE
tot
FN is always positive and is increasing
when the distance F…N is decreased, in concordance with the absence of an
equilibrium geometry, whereas DE
tot
BrN and DE
tot
ClN are negative and decreasing when
the two moieties get closer, the effect being enhanced for bromine. Obviously,
Graph 16.8 Variations of intraatomic (“self”) energies with respect to the X…N distance (the
origin corresponding to the self energies inside the isolated optimized fragment) for the Cl 3 COX…
NH 3 complexes
Graph 16.9 DE
tot
XN (as
defined by Eq. 16.21)
variations with respect to the
X…N distance for the
Cl 3 COX…NH 3 complexes
16 Following Halogen Bonds Formation …
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