Chemistry Reference
In-Depth Information
tAbLe 4.7
thermodynamic Parameters of the Complexation of Alkali Cations in
Methanol at 25°C
Ligands
Calix[4]arene-diisopropoxy-crown-6
18-crown-6
Cations
K
Rb
Cs
Cs
log
K
(kJ mol
−1
)
4.5
5.93
6.1
4.8
25.6
33.8
35
27.4
-∆
G
c
(kJ mol
−1
)
18.1
40
50.2
47.2
-∆
H
c
(kJ mol
−1
)
7.5
T
∆
S
c
(kJ mol
−1
)
−6
−15
−19.8
25.3
∆
S
c
(J K
−1
)
−21
−52
Source:
A. Casnati, A. Pochini, R. Ungaro, F. Ugozzoli, F. Arnaud, S. Fanni, M.-J. Schwing,
R. J. M. Egberink, F. de Jong and D. N. Reinhoudt.,
J. Am. Chem. Soc.,
117
, 2767-
2777, 1995. With permission.
tAbLe 4.8
Dimethoxy-
p
-
tert-
butylcalix[4]arene-crown-6 in Water
na
+
→
k
+
k
+
→
Rb
+
Rb
+
→
Cs
+
na
+
→
Cs
+
17.6
5.1
7.7
30.4
∆
G
3
Cone
∆
G
4
21.1
6.6
8.9
36.6
∆
G
3
−∆
G
4
−3.5
−1.5
−1.2
−6.2
Partial cone
18.5
6.7
7.0
32.3
∆
G
4
∆
G
3
−∆
G
4
−0.9
−1.6
−0.7
−1.8
1,3 alternate
∆
G
4
15.0
3.2
5.9
24.1
∆
G
3
−∆
G
4
2.6
1.9
1.8
6.3
Note:
Differences in free energies and in free energies of binding (kcal mol
−1
) using M
+
Aqvist
parameters.
4.3.2.2.1 Gas Phase
For the three conformers, the binding sequence is Na > K > Rb > Cs. This is supported
by energy component analysis on the trajectories, as well as by Free energy perturba-
tion (FEP) calculations. Intrinsically, Cs
+
has the weakest interactions with both hosts.
The largest contribution of the cation/host interaction energy comes from the ether ring
rather than from the aromatic moieties. Each complex displays a clear conformational
preference. Sodium is most stable in the cone conformation, whereas cesium is most
stable in the 1,3-alternate conformation.
4.3.2.2.2 Water Solution
The binding selectivity ∆
G
c
depends on the conformation of the host and differs
from the selectivity calculated in the gas phase. The selectivity ∆
G
3
-∆
G
4
results
from the difference in the free energies for the complexed ions ∆
G
4
and for the free
