Chemistry Reference
In-Depth Information
p
electron-donating capability of the substituent, the
D
E
ST
of the nitrene likewise
shows a reasonably direct correlation with the
-donating ability of the substituent.
This correlation can be seen in a qualitative way from Scheme 2.1—increasing the
strength of the
p
E
ST
).
However, substituents must be in powerful, direct electron communication with
the nitrene center for a significant substituent effect on the electronic states to be
observed. For example, when the donor group is incorporated as a
para
substituent
on phenylnitrene, the donating ability of the substituent does not perturb the
singlet-triplet gap significantly. The
p
D
donor decreases the singlet-triplet gap (
E
ST
of
para
-substituted phenylnitrenes is
computed to remain between 16 and 20 kcal/mol over a wide range of substitu-
ents
17
(Table 2.1). Presumably this is because in phenylnitrene the frontier orbitals
are nearly degenerate, and so the
para
D
-donor orbital (typically associated with a
lone pair) and the
para
electron-withdrawing group orbitals are not so close in
energy to the nitrene's frontier orbitals as to mix in a significant way. This behavior
stands in sharp contrast to phenylcarbenes and phenylnitrenium ions that show
strong manipulation of the
p
p
substituents.
18-22
These species have a much larger energy separation between
their frontier orbitals and so at least one of the orbitals are close in energy to mix
with donors or acceptors.
D
E
ST
by substitution of the
para
position with
TABLE 2.1. CASPT2 and BPW91 Singlet Energies (kcal/mol) Relative to T
0
for para-
Substituted Phenylnitrenes
S
1
S
2
S
3
Para
group
s
CASPT2
BPW91
CASPT2
BPW91
CASPT2
BPW91
p
-NHCH
3
0.46
16.7
12.2
30.4
23.3
63.7
53.9
p
-OH
0.38
18.3
13.4
34.5
27.6
61.2
49.9
p
-N(CH
3
)
2
0.32
16.3
12.2
30.5
24.4
63.2
52.4
p
-NH
2
0.30
17.1
12.5
31.2
23.9
63.5
53.4
p
-CH
3
0.14
18.8
13.8
36.9
32.2
58.6
44.5
p
-OCH
3
0.12
18.3
13.3
34.5
27.7
65.2
49.4
H
0.00
19.3
14.3
37.4
33.9
57.8
43.0
p
-F
0.15
19.1
14.1
36.3
30.8
59.5
47.1
p
-Cl
0.24
18.4
13.7
37.3
32.6
58.2
45.1
p
-Br
0.26
18.4
13.8
37.5
33.0
57.9
44.5
p
-I
0.28
18.2
13.7
37.7
33.1
57.4
43.7
p
-CO
2
H
0.44
18.0
15.4
38.7
38.1
55.9
38.3
p
-CO
2
CH
3
0.44
18.0
13.4
38.6
36.7
56.0
37.7
p
-COCH
3
0.47
17.5
13.0
38.7
37.1
56.1
33.9
p
-CHO
0.47
17.2
12.9
38.9
37.9
56.0
36.1
p
-CF
3
0.53
19.1
14.2
38.3
40.7
62.8
36.1
p
-CN
0.70
17.5
13.2
39.5
37.0
56.7
40.6
p
-NO
2
0.81
18.1
13.5
38.8
38.3
55.5
34.1
Table values are excerpted from Ref. 17.
