Chemistry Reference
In-Depth Information
3. A scale is given of the relative importance of such electrostatic
interactions in the order
ss > s
p
>
pp
, where
s
is a multipole
moment directed along the intermolecular axis and
a moment
p
perpendicular to it.
9
4. Highest stability (vs, very stable) is obtained for the angular con-
figuration of the dimer when all three contributions are favourable.
5. Secondary stability (s, stable) is obtained when terms beyond the
first are repulsive, opposing the largest attractive contribution of the
leading term.
6. Unstable (ns, not stable) angular configurations are obtained when
all three terms (or the leading term, at least) are repulsive.
For elongated molecules (such as CO
2
,C
2
H
2
and HCN) below
the sum of their VdW radii a further rule is introduced.
7. Of two angular configurations possible on the basis of the previous
rules, the configuration having smaller Pauli steric repulsions will be
more stable.
These rules are simple and immediately intuitive, once the electrical
characterization of a molecule in terms of its point-like multipoles is
accepted. The underlying physical assumption is that the electrostatic
interaction is the dominant attractive component of the intermolecular
potential determining the angular shape of the dimer, while short-range
forces are assumed to provide a repulsive uniform background balancing
attraction at the VdWminimum. Monomer size enters the model through
rule 7, which corrects for deviation from uniform repulsion when steric
interactions occur below the sum of the respective VdW radii.
10
Figures 5.7 and 5.8 sketch a picture of the first two permanent electric
moments (au) for a selection of noncentrosymmetric and centrosymmetric
molecules, respectively. The notation is the same as that given in Mag-
nasco et al. (1988). It is understood that the point-like multipoles are
placed at the centre of mass of the molecule, their sign in relation to the
molecular structure of the monomer being of fundamental importance in
determining the nature of the electrostatic interaction (attractive or
repulsive). The numbers shown in each figure are from SCF calculations
and so are little larger than those given in Table 5.2.
9
This
s
,
definition, strictly correct for dipoles, is here loosely extended to higher multipoles
p
with m
0.
10
Provided this situation is avoided, the exact value of the intermolecular distance R is not
particularly relevant for the qualitative prediction of the angular shapes of the dimers.
ΒΌ
