Chemistry Reference
In-Depth Information
Table 2 Source function (SF) and compensatory transferability. Source contributions to the C-C
bcp in the H-CH
2
-CH
2
B(B
¼
CH
3
,NH
2
, OH, F) series
a,b
B
D
S(H-CH
2
)
c
D
S(CH
2
-B)
c
D
S(C)
d
D
S(H
2
)
d
r
b
(
r
b
)
av
(
Dr
b
)
H
0.2514
-
-
-
-
-
0.0006
0.0007 (
0.0008)
0.0002 (0.0001)
CH
3
0.2532
0.2532 (0.0000)
0.0006
NH
2
0.2592
0.2581 (0.0011)
0.0044
0.0033
0.0040 (
0.0032) 0.0002 (0.0000)
OH 0.2651
0.2632 (0.0019)
0.0078
0.0059
0.0067 (
0.0047) 0.0004 (
0.0005)
F
0.2688
0.2652 (0.0036)
0.0108
0.0072
0.0092 (
0.0060) 0.0007 (
0.0005)
a
Data from [
1
], all quantities in au
b
(
r
b
)
av
is the mean of the
r
b
value for C-C bond in HCH
2
-CH
2
H and in BCH
2
-CH
2
B, while
Dr
b
represents the deviation from the mean value,
Dr
b
¼ r
b
(
r
b
)
av
c
D
S(H-CH
2
) and
D
S(CH
2
-B) are the source differences for these groups in H-CH
2
-CH
2
B and in
the corresponding homomolecules, HCH
2
-CH
2
H and BCH
2
-CH
2
B
d
D
S(C) and
D
S(H
2
) are the changes in the source contributions from the C and H
2
in the H-CH
2
group with respect to the values for the same group in HCH
2
-CH
2
H. In parenthesis, the
corresponding changes in the CH
2
B group with respect to the values for the same group in
BCH
2
-CH
2
B
from the arithmetic mean of the homosubstituted values. The
Dr
b
are very small
relative to
r
b
and increase with the electronegativity of B. The SF analysis reveals
that the small
Dr
b
values are the result of much larger perturbations which almost
compensate each other to yield very small deviations from the arithmetic
mean additivity rule.
S(CH
2
-B) in Table
2
represent the
deviations of source contributions to C-C
r
b
from the HCH
2
j
D
S(H-CH
2
)and
D
groups
in H-CH
2
-CH
2
B with respect to the values of these sources in the corresponding
homomolecules. Their values are almost equal in magnitude but have a different
sign and are from three to five times larger in magnitude than
and BCH
2
j
Dr
b
.Table
2
also
shows that both
S(CH
2
-B)aredominatedbythechanges
experienced by the carbon atoms involved in the investigated C-C bond. The
sources from the HCH
2
j
D
S(H-CH
2
)and
D
groups, and the corresponding sources from
the C atoms of the C-C bond, are found to, respectively, increase, relative to
ethane, and decrease, relative to B-CH
2
-CH
2
B, by similar amounts so as to yield
the final small
and BCH
2
j
Dr
b
values.
Although the discussed results seem quite promising, application of the SF
analysis to the study of transferability is certainly still in its infancy. The SF
capability to reveal and detail external, possibly long-distance contributions in
determining the electron density and its changes at relevant points, upon chemical
substitution or perturbation by the environment, might become of some interest in
pharmacological and biological studies. For instance, as an interesting tool for drug
design or in the analysis of how close to the actual drug/receptor interactions are the
models taken from their simplified and perturbed interactions in crystal assemblies
[
31
]. A preliminary work along these directions, aimed at exploring whether the SF
descriptor is able to finely disentangle the effect of the environment on the charge
distribution of inhibitor species, is included in the chapter by Bernd Engels et al.
[
32
] of the present topic.
