Halogen Bonding in Crystal Engineering - Halogen Bonding: Fundamentals and Applications - page 110

withdrawing power of the groups neighbouring the halogen on the carbon
skeleton [51]. Chlorine in CH 3 Cl does not present any positive electrostatic
potential, but it appears in CH 2 FCl, and becomes even greater in CF 3 Cl; the
most positive chlorine potential in HOCH 2 CH 2 Cl is - 1.1 but it becomes 7.8
in O 2 NCH 2 CH 2 Cl. Similarly, the atomic charge increases with the fluorina-
tion degree in iodomethane derivatives, being 0.009 in CH 3 I and 0.165 in
CF 3 I [52].
The possibility to carefully tune the XB strength of a given halocarbon
by modifying the substituents on the carbon skeleton is also confirmed ex-
perimentally. As expected, the stronger an XB, the shorter the interaction
length (see onwards). The N I-C interaction length in the infinite chain

given by 1,4-BPY with 1,4-diiodobenzene (1,4-DIB) is 3.032 A and the N

···

··· I

Table 2 Interaction of 1,4-DIB and 1,4-DITFB with XB acceptors

Co-crystals I ··· Ndistance I ··· Ninteraction

X-ray DFT calculations energy (DFT)

[A] [A] [kcal mol -1 ]

1,4-BPY · 1,4-DIB 3.032 3.151 3.29

1,4-BPY · 1,4-DITFB 2.851 2.936 5.81

BPE · 1,4-DIB 2.996 3.119 3.44

BPE · 1,4-DITFB 2.810 2.891 6.02

·

HMTA 1,4-DIB 3.011 3.124 3.52

HMTA · 1,4-DITFB 2.844 2.919 6.07

On interaction with a variety of XB acceptors, 1,4-DITFB forms shorter and stronger con-
tacts than 1,4-DIB. Clearly, the electron withdrawing ability of fluorine boosts the donor
ability of the iodine atoms

Fig. 3 Due to the anisotropic distribution of the electron density, halogen atoms show
a negative electrostatic potential and a larger radius ( r max ) in the equatorial region and
a positive electrostatic potential and a smaller radius ( r min ) in the polar region. As a con-
sequence of this, halogens behave as nucleophiles at the equator and as electrophiles at
the pole