Chemistry Reference
In-Depth Information
chemical shift of a target nucleus (d
target
) can be calculated using the following
equations:
90,91
δ
=−
σ
σ
(16.1)
target
ref
target
σ
=+
σ
DP
σ
(16.2)
target
σρ
D
∝
(16.3)
σ
P
∝∆
1
E
(16.4)
where s
target
= shielding constant of the target nucleus,
s
ref
= shielding constant of
the nucleus in the reference compound, r = electron density and
E
= average
excitation energy of electronic transitions. For details, please refer to references 90
and 91 .
In the case of
15
N nuclei, their shielding constants include both s
D
and s
P
terms,
which is a very different situation from those of
1
H. More importantly, the s
P
term
generally has a larger effect on s
target
than the s
D
term.
90,91
Therefore, as a very rough
approximation, the s
15
N
is proportional to the reciprocal of
∆
∆
E
:
σσ
15
≈∝∆
P
1
E
(16.5)
15
N
N
E
) is highly correlated with the HOMO-
LUMO transition or the allowed transitions near the HOMO-LUMO transition,
since such transitions are the most frequently occurring transitions. This is the
reason why the average excitation energy,
This average excitation energy (
∆
E
, is highly correlated with the energy
of the HOMO-LUMO or related excitations.
For T - Hg
II
-T base pair formation, the reaction goes though the pathway shown
in Scheme 16.5 .
From the
15
N chemical shifts of N3, Hg
II
-bound N3 nuclei were resonated at a
lower - fi eld than proton-bound N3 nuclei. This means that s
P
of Hg
II
- bound N3
nuclei [s
P
(N - Hg
II
)] makes a larger contribution than s
P
of proton-bound N3 nuclei
[s
P
(N - H)] (Figure 16.6 ).
∆
(
)
>
(
)
σ
P
NHg
−
II
σ
P
NH
−
(16.6)
Caution must be taken with the sign of s
P
, since its sign is opposite to the sign of
s
D
. According to Equations (16.5) and (16.6), this phenomenon means that the
excitation energy of the electron around the Hg
II
- bound N3 nuclei [
∆
E
(N - Hg
II
)]
would be smaller than the proton-bound N3 nuclei [
∆
E
(N - H)] (Figure 16.6 ).
O
O
O
H
+
Hg
II
Hg
II
N
NH
N
N
N
N
H
+
Hg
II
O
O
O
R
R
R
Scheme 16.5
