Chemistry Reference
In-Depth Information
Fig. 1 Profile of source con-
tributions from the Li atom,
S(r, Li), to the electron density
along the internuclear Li-
X
(
X ¼
F, O, N, Cl, H) axis.
The Li nucleus is at
r ¼
0.0.
Source function contributions
in the region of the bcps are
enlarged in the inset where
the vertical bar denotes the
position of the LiF bcp,
which is the closest to the Li
nucleus. The observed
changes in S(r, Li) with
X
,
despite the almost constant
Li electron population, are
due to the bcp shift toward
X
through the series (repro-
duced from Fig. 3 with per-
mission from [
9
], Copyright
2003, Wiley-VCH Verlag
GmbH & Co, KGaA)
distance. It is the shift in the bcp position along the series and not the small
difference in the S(r,Li) profiles which lead to the observed trend in the S(r
b
,Li)
values and which ensures a constancy in S%(r
b
,Li) and in N(Li). The decrease in S
(r,Li) with increasing distance from the Li nucleus is clearly evident from Fig.
1
.It
results from the increasing weight of the positive Laplacian regions when the rp
moves toward
X
[
9
].
The third example on transferability reviewed in this section concerns the series
of heteromolecules HCH
2
-CH
2
B with B
H, CH
3
,NH
2
, OH, and F [
1
]. Energies
of this series of molecules are found to equal the arithmetic mean of the energies of
the corresponding homomolecules HCH
2
-CH
2
H and BCH
2
-CH
2
B to within a few
Kcal/mol or less, computationally and experimentally [
1
,
30
]. These additivity rules
suggest that the groups HCH
2
j
¼
(with vertical bars indicating the zero-
flux surface of the C-C bond) undergo small changes in forming the heteromole-
cule. The largest density perturbations, when HCH
2
-CH
2
B is formed, should occur
in the vicinity of the new C-C interatomic surface and in particular at its highest
density value
r
b
. Table
2
reports
r
b
values at the C-C bcp and their departures,
and BCH
2
j
Dr
b
,
