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of the
s
% contribution, which largely exceeds the increasingly negative
p
%
contribution. The opposite
% contributions trend for next neighbor C
and H atoms is clearly a consequence of the presence in the former and lack in the
latter of the
p
atomic orbitals and electrons.
6
s
% and
p
3.2.3 Second-Row Diatomic Hydrides
In Sect.
3.1
, application of the SF to the Li-
X
(
X
F, O, N, Cl, H) series was
reviewed to show how such a tool is able to assess whether an almost perfect or only
a compensatory chemical transferability of the Li atom - the fixed element through
the series - characterizes the given set of related compounds. In the following, the
SF description of the second-row diatomics H-
X
(
X
¼
¼
Li, Be, B, H, C, N, O, F)
is instead briefly reviewed [
9
] to inspect how this function is able to account for
the well-known change of nature of the H atom - the fixed element of this series -
with the change in the electronegativity of
X
. Figure
3
shows contour maps of
2
Fig. 3 H-
X
diatomics: contour maps of
L
(r)
¼r
r dashed red contours
,
L
(r)
<
0, indicate
regions of charge depletion and
solid blue contours
,
L
(r)
>
0, denote regions of charge concentra-
tion. The bond path and the intersection of the interatomic surface with the plane of the map are
shown for each diagram. The H-basin shape and the position of the bcp reflect the transition
through the series from closed-shell to shared atomic interactions and from a cationic to anionic
nature of the hydrogen. Changes in the H-basin shape and in its
L
(r) distribution significantly
affect both S(r
b
,H) and the percentage contribution of the H atom to the bcp electron density (see
Table
5
) (adapted from Fig. 1 with permission from [
9
], Copyright 2003, Wiley-VCH Verlag
GmbH & Co, KGaA)
6
Although H atoms do not possess p-electrons, H basins as defined by QTAIM, may nonetheless
contain
p
-electron MO contributions to their electron density.
