Chemistry Reference
In-Depth Information
O
N
NH
O
Enzymatic
activati
o
n
N
N
NH
2
H
N
N
Ar
R'
NH
10
N
N
Ar
R
+
Ar
R
R
N
N
NH
2
8
9
R'
11
N-C-8 adduct
FIGURE 6.4.
Formation of N-C-8 adducts
11
of guanosine
10
is attributed to metabolic
generation of nitrenium ions
9
from aromatic amines
8
.
intermediates
9
which added to the C-8 position of guanosine
10
, resulting in genetic
mutations and, eventually tumor formation (Fig. 6.4).
6.2 NH
2
þ
AND OTHER TRIATOMIC NITRENIUM IONS
Due to its simplicity, the parent nitrenium ion, NH
2
þ
, sometimes called the
“imidonium ion” in the early literature, has been subjected to the earliest and
most thorough theoretical examinations. The earliest HF calculations identified the
3
B
1
(corresponding to the
p
triplet in Fig. 6.1) as the lowest energy state, with a
substantial energy separation (30-40 kcal/mol) to the next highest state, the closed-
shell (
s
2
) singlet state.
11
More sophisticated treatments using correlation corrections
and multireference approaches converged on a gap of
s
29.9 kcal/mol,
12,13
þ
in
excellent agreement with the experimental value of
þ
30.1 kcal/mol derived from
photoionization of the corresponding neutral radical.
14
The geometries and the relative energies of four states of NH
2
þ
are shown in
Figure 6.5.
13
The triplet ground state is quasi-linear. Which is to say that its
equilibrium geometry is bent with a 151
bond angle with a barrier to inversion
that is below its zero-point energy. As would be expected on the basis of valence shell
electron pair repulsion (VSEPR), the
1
A
1
state, having a doubly occupied in-plane
s
orbital, shows a smaller HNH angle of 108
. The
p
2
or
1
B
1
singlet state resembles the
triplet, having a bond angle of 156
. The open-shell singlet is also nearly linear with a
bond angle of 175
.
107º
156º
152º
H
H
175º
H
H
H
H
H
H
N
N
N
1.04 Å
N
1.04 Å
1.06 Å
1.04 Å
+
+
+
3
B
1
1
A
1
1
B
1
1
A
1
0
30.06 (30.1 ± 0.2)
77.94
46.14 (51.2)
FIGURE 6.5.
Calculated geometries of the parent nitrenium ion and relative energies in
kilocalories per mole for its four lowest electronic states. Values in parentheses are exper-
imentally determined values.
13
