Chemistry Reference
In-Depth Information
Table 7
SAPT2/aug-cc-pVDZ Results for Contributions to
the Interaction Energy (kcal mol
1
) at CCSD(T)/aug-cc-pVQZ
Intermolecular Distances for Two Configurations of H
2
S-
Benzene
a
Energy
A
B
E
elst
2.37
0.01
E
exch
4.19
1.03
E
ind
0.81
0.17
E
disp
4.16
2.14
E
int
(SAPT2)
3.15
1.27
E
int
(MP2)
3.06
1.21
a
Data from Ref. 162.
3.15 kcal mol
1
,SAPT2/
aug-cc-pVDZ) than the hydrogens-up configuration, B (
configuration, A, binds significantly more favorably (
1.27 kcal mol
1
).
SAPT analysis can help explain this difference.
As shown in Table 7, the individual components of the interaction
energy change significantly between the hydrogens-down and hydrogens-up
configurations. The exchange-repulsion and dispersion terms are the most sen-
sitive to geometry. In fact, in our studies of substituent effects in
p
-
p
interac-
tions, we have usually kept the distance between the
rings constant when
comparing different substituted complexes, even when they might have
slightly different optimal vertical separations, simply to make the comparison
of the SAPT components more straightforward.
157
In the case of configura-
tions A and B for H
2
S-benzene, there is an unavoidable significant difference
in the geometries. The hydrogens-down configuration has a much larger
exchange energy (4.19 kcal mol
1
) because the hydrogens are much closer to
the
p
cloud of the benzene, leading to greater steric repulsion. When the hydro-
gens are rotated away from the benzene, the steric repulsion is much less
(1.03 kcal mol
1
). On the other hand, differences in exchange-repulsion ener-
gies usually lead to roughly opposite changes in dispersion interactions. Here,
as one goes from A to B, the exchange energy becomes more favorable (less
repulsive), but the dispersion energy becomes less favorable (less attractive).
In such situations, it is often helpful to consider the sum of the exchange
and dispersion terms. In this case, the change in the repulsion term is even lar-
ger than that in the dispersion term, so the sum of exchange and dispersion is
attractive for configuration B (
p
1.11 kcal mol
1
) while it is almost zero for
configuration A. This is consistent with the expectation that the (net) disper-
sion interaction should be larger for configuration B because the sulfur lone
pairs will have a more favorable dispersion interaction with the benzene
p
cloud than will the hydrogens.
However, the change in the electrostatic term is even larger than this;
configuration A is favored by 2.38 kcal mol
1
electrostatically compared to
configuration B. This is consistent with the expectation that the partial positive
charge on the hydrogens will interact much more favorably with a partial
negative charge on the
p
cloud of benzene than would the sulfur lone pairs.
