The Interaction of Dihalogens and Hydrogen Halides with Lewis Bases in the Gas Phase: An Experimental Comparison of the Halogen Bond and the Hydrogen Bond - Halogen Bonding: Fundamentals and Applications - page 37

rotational spectroscopy. Each complex has the planar, T-shaped geometry of
C 2 v symmetry that is predicted by applying rule 2 using the π -bonding model

of ethyne.

3.2.2
B Carries Pseudo-π-Pairs

Cyclopropane resembles an alkene in its chemical behaviour. This fact led
Coulson and Moffitt [136] to propose a model for cyclopropane in which
apseudo- carbon-carbon bond is formed by overlap of a pair of sp 3 hybrid

orbitals on adjacent carbon atoms. A schematic diagram showing the elec-
tron density distribution between a pair of C atoms in cyclopropane resulting
from such a model is shown in Fig. 12. The symmetry axis of the pseudo-

π

or-
bital coincides with a median of the cyclopropane equilateral triangle. Hence,
according to rule 2, the angular geometry of cyclopropane

π

··· ClF [73], or of

cyclopropane HCl [137], is predicted to have C 2 v symmetry, with ClF, or

HCl, lying along the extension of the median. The electrophilic end

···

δ + Cl of

ClF, or δ + H of HCl, is expected to interact with the pseudo-

π electron dens-

Fig. 12 ClF

(drawn to scale) and the Coulson-Moffitt pseudo- π -electron model of cyclopropane. See
Fig. 1 for key to the colour coding of atoms

The experimental geometries of cyclopropane ··· HCl and cyclopropane ···