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bonding, lengthening of the C-X bond, red shift, and intensity increase of
the C-X stretching vibration, they may equally be treated as a separate group
within the family of halogen-bonded complexes.
3.1
C-X···NComplexes
The interaction of a series of fluoroalkyl halides, CH
3
I, CH
2
FI, CHF
2
I, CF
3
I,
CF
3
Br, and CF
3
Cl, with the ammonia molecule was investigated theoretic-
ally with MP2 and DFT methods [48]. A linear C-X
Narrangementwas
assumed for all these complexes. In all cases, the calculated X
···
Ndistances
are shorter than the sum of van der Waals distances, and the interaction ener-
gies of the complexes are in a range of about 2-6 kcal
···
mol with comparatively
small charge-transfer contributions. With increasing fluorine substitution the
complexesbecomesignificantlymorestable.Thesametrendwasalsofound
for a broader range of C-X molecules interacting with ammonia [33]. Con-
trary to the interaction of XY dihalogens with methylated amines discussed
in Sect. 2.2, the interaction energy of molecules with C-X bonds and methy-
lated amines decreases upon successive methylation [33]. The same result was
also found in a study of CF
3
I interacting with trimethylamine and related
acceptors [49].
Among other halogen-bonded C-X
/
···
B complexes, theoretical vibrational
frequencies of the complexes CF
3
Cl
NH
3
were inves-
tigated too, with the interesting result that the calculated C-X stretching
frequencies turned out to be shifted to higher wavenumbers [50]. Because
of the analogy to the blue-shifting hydrogen bonds [51, 52], these particular
C-X
···
NH
3
and CF
3
Br
···
B complexes have been called
blue-shifting halogen bonds.
Comparing C-X bonds with different hybridization states of the carbon
atom revealed that
sp
-hybridized C-X bonds form the strongest halogen
bonds, followed by
sp
2
-and
sp
3
-hybridized C-X bonds [33], again a trend
similar to the relative hydrogen bonding ability of C-H bonds. Ab initio and
DFT studies on halogen bonding between appropriately substituted aromatic
molecules, such as halobenzenes and pyridines or Schiff bases, mimicking
the bonding situation in molecular crystals, are also available [53-56]. These
investigations confirm that halogen bonds are highly directional. Although
in most cases the interaction energies appear to be quite small, of the order
of 2 kcal
···
mol, they are sufficiently strong to have a prominent influence on
crystal packing.
/
3.2
C-X···H-C Complexes
Among the C-X
···
H-C complexes, the case of C-F
···
H-C is the best inves-
tigated. The C-F
···
H-C moiety is not linear. From the experimental side,
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