Chemistry Reference
In-Depth Information
2
L
(r)
X
surface with the plan of
the map for all investigated diatomics. Changes in the H-basin shape and in its
L(
r)
distribution are anticipated to have a significant effect on both the SF value and its
percentage contribution to the bcp electron density. The shape of the H-basin and
the position of the bcp reflect the transition through the series from closed-shell to
shared atomic interactions and from a cationic to an ionic nature of the H atom. On
going from H-Li to H-F, the bcp moves from a region closer to the
X
nucleus of
negative
L
, hence positive Laplacian, to a region closer to the H nucleus and with
increasingly smaller Laplacian values (this is detailed in Table
5
, where the symbol
R
H
denotes the distance from the H nucleus to the bcp). As a consequence, the
S(r
b
,H) value is found to increase from 0.02 au in HLi up to about 0.140 au in HO
and HF. However, the
r
b
value increases along this series by a quantity about three
times as large, namely from 0.033 au to 0.354 au, leading to a monotonic decrease
in the S%(r
b
, H) values along the series from about 60% in HLi to 38.8% in HF.
Therefore, the decreasing electron population of H, from 1.874 e
for the hydridic
HinHLitotheonly0.399e
for the almost cationic H in HF and the corresponding
increase of
X
's electronegativity, translates into a significantly decreased H's source
function percentage contribution to the bcp density through the series. Attention has
been called [
9
] to the fact that the decreasing percentage share from H atom with
increasing electronegativity of
X
complies with the previously described parallel
decrease of the local Shannon entropy in the H-basin, taken as an indication of an
increasingly structured and localized H-atom's density along the series [
39
].
The LS, LG, and LV profiles along the bond path have been analyzed in detail
for the H-
X
series [
13
], with the bcp being taken as rp. Only the main conclusions of
that study by Gatti and Bertini are reviewed here. The LS profiles were found to
show an increasing asymmetry, with respect to the bcp location, as the
X
's electro-
negativity increases through the series. The observed behavior was proposed to set
up a clear and sensitive indicator of the increased (decreased) asymmetry and polar
character of a bond along a series or of the changes of these features when the bond
is placed in different chemical environments. From HC to HF, the LS asymmetry is
the result of a corresponding asymmetry of LV, the term related to the potential
¼r
r
and the intersections of the interatomic Li
j
Table 5 Bond critical point properties and source contributions to its density
r
b
in the second-row
diatomic hydrides
a
H-
X
R
H
%
b
q
(H)
b
2
R
e
r
b
r
r
b
S%(r
b
,H)
H-Li (
1
+
S
0.874
)
3.016
54,1
0.033
0.155
60.1
H-Be (
2
+
S
)
2.538
56,4
0.798
0.083
0.188
60.3
H-B (
1
+
S
)
2.328
53,1
0.574
0.163
0.488
52.2
H-H (
1
g
+
)
S
1.400
50,0
0
0.243
1.012
50.0
H-C (
2
P
)
2.116
37,6
0.052
0.245
0.664
44.8
H-N (
3
)
S
1.958
30,2
0.219
0.305
1.161
43.2
H-O (
2
P
)
1.833
23,7
0.450
0.340
1.548
41.0
H-F (
1
+
)
S
1.733
20,2
0.601
0.354
1.686
38.8
a
Data from [
9
], all quantities in au
b
R
H
% is the distance from the H nucleus to the bcp, expressed as bond length percentage; q(H) is
the QTAIM net charge of H atom
