Chemistry Reference
In-Depth Information
O
1.263
1.43
O
122º
HC
HC
1.43
1.481
N
147º
1.17
N
1
3c
26
FIGURE 12.5.
Selected bond lengths (A
) and angles of singlet nitrene and oxazirene
predicted at the HF/STO-3G level.
58
or spectroscopic experiments. Among alkanoyl azides, only pivaloyl azide was
studied in some details, and most likely that both singlet and triplet nitrenes (
1
3b
and
3
) participate in the reaction with
trans
-4-methyl-2-pentene.
68
The elucidation of
the unusual properties of carbonyl nitrenes was reported only recently.
16,17
Most of the calculations available in the early 1990s were concerned with the
simplest acyl azide and acyl nitrene, formyl azide (
3b
).
58,89-93
However, these calculationswere performed at relatively low level of theory, at least by
current standards, and their results do not agreewith the modern view on the electronic
structure of singlet acyl nitrenes.
13-19,36,37
For instance, Rauk and Alewood
58
employed the RHF/STO-3G method and optimized structures of both the acyclic
1
A
0
state of
1c
), and formyl nitrene (
3c
3c
and the oxazirene
26
(Fig. 12.5), which were calculated to be very close
in energy (within 1 kcal/mol).
Mavridis and Harrison
93
recalculated the electronic structures of the triplet and
two low-lying singlet states (
1
A
0
and
1
A
00
) of formyl nitrene
3c
using the SCF and
GVB procedures with double
P
) basis set.
However, the geometries of the singlet states were not optimized and the open-
shell singlet
1
A
00
was assumed to have a geometry identical to its companion,
3
A
00
state. In constructing the wave function for closed-shell singlet,
1
A
0
, all bond angles
were taken to be 120
. They found the triplet
3
A
00
state to be the ground state while
the first two excited states (
1
A
0
and
1
A
00
) were calculated to be 36.8 and 39 kcal/mol
above the triplet state.
93
Thus, calculations available in the early 1990s did not
support experimental findings, which were most consistent with a singlet ground
state of aroyl nitrenes.
To understand the reason for the singlet multiplicity of the ground state of
carbonyl nitrenes, the singlet-triplet splitting (
z
plus polarization functions (
DZ
þ
D
E
ST
¼
E
S
E
T
) for benzoyl-(
3a
)
) was calculated at the B3LYP/6-31G
level of theory.
16
The triplet states were still computed to be lower in energy by about 5.0 and
4.5 kcal/mol for
3
3a
and 2-naphthoyl nitrenes (
13
and
3
13
, respectively. However, these values are much lower
E
ST
values for NH (36 kcal/mol),
82-84
CH
3
N (31.2
than the well-known
D
0.3
kcal/mol),
85
and PhN (18
0.7 kcal/mol).
86,87
The significant stabili-
zation of the singlet state relative to the triplet state in aroyl nitrenes is attributed to
the bonding interaction between the nitrogen and the oxygen atoms (the Mulliken
bond order is 0.87 for
1
3a
2 and 18.3
).
16
Due to this bonding, the structure of the singlet species
(Fig. 12.5) resembles that of a cyclic oxazirene, although the calculated N
O
O single bond (about 1.5 A
in
strained rings).
88
Thus, structures of acyclic singlet nitrene and oxazirene shown in
Figure 12.5 do not correspond to the minima on the PES obtained with higher level of
1.76 A
distance (
) is much longer than a normal N
