Chemistry Reference
In-Depth Information
measure of the electron sharing between basins A and B. They convey information
about the special electronic connection between two basins, whereas the S(r,
)
values simply relate the Laplacian electron distribution in one basin with its
contribution to the electron density value at a single, given point. However, when
this reference point is the bcp, taken as the most representative point for the
interaction between two linked atoms, one may envisage that delocalization indices
and source function contributions might empirically be related in some way and
under special circumstances. For instance, when bond covalency is known to increase
along a series of related compounds, one anticipates that both the delocalization index
between the covalently bonded atoms and their source function contributions to the
electron density at the associated bcp will correspondingly increase along the series.
In the hydrocarbon series discussed earlier, we indeed found [
14
] that the increased
SF percentage contributions from the C atoms is paralleled by an increased delocali-
zation index, from 1.03 in ethane, to 1.92 in ethane and 2.76 in ethyne. Conversely,
the very small source function percentage contribution from the C atom to the bcp
density of the C
0
-H
0
bond (1.7%, 4.0%, and 4.7% for the three members of the series)
is paralleled by a corresponding
d
(C, H
0
) value as low as 0.042, 0.062, and 0.084,
in ethane, ethene, and ethyne, respectively. Coming back to the diborane case, the
d
(B,B
0
) value is 0.084, while the electronic share between the B and the bridged
hydrogen is about four times as large,
d
(B,H
bridge
)
O
0.332. Moreover, both the
electron share between the two bridged hydrogens,
d
(H
bridge
,H
0
bridge
)
¼
¼
0.214 and
that between a terminal and a bridged H,
d
(H
bridge
,H)
0.109, are (much) greater
than the electron sharing between the two B atoms. All these results are in line with
the delocalized nature of SF contributions to the density at the B-H
bridge
bcp and the
small SF contribution to such density from the other B atom. Conversely, the
localized description of source contributions to the bcp density of the terminal B-H
bond properly complies with a delocalization index value for this pair of bonded
atoms,
d
(B,H)
¼
0.59, which is almost twice as big as that for the B and the bridged
hydrogen atom. Figure
2
also shows the large delocalization of source contributions
to the electron density at the diborane ring critical point.
A spread of sources is also observed in the case of the Lewis adduct, suggesting
that its formation involves the two molecules in their entirety, rather than the
electron-rich and the electron-deficient atoms only. Indeed, the BH
3
and PH
3
moieties are found to determine to similar extent the density at the bcp of the
adduct, with even larger contribution from the acidic moiety, S(BH
3
)
¼
58% [
33
].
However, despite being much closer to the bcp, the electron-deficient B atom
contributes only about one half the source from the P atom and are the much larger
sources due to its three linked hydridic H, as compared to those due to the acidic H
linked to P, that eventually lead to the dominance of the acid moiety contributions.
¼
3.2.2 Cyclic Conjugated and Aromatic Hydrocarbons
Table
4
lists source contributions to the C-C bonds in three cyclic conjugated and
aromatic hydrocarbons (see Scheme
2
for atomic numbering). As anticipated earlier
