Chemistry Reference
In-Depth Information
104º
32.8
+
1.50
27.2
130º
110
32.2
1.80
140º
104º
1.59
27.1
TS3
25.9
96º
2.56
1.10
1.1
TS2
1.81
27.3
2.18
TS4
10.8
10.8
TS1
12.0
4.8
5.6
0.0
4.5
+
0.0
1
3d
123
115º
118º
1.50
111º
1.52
117º
1.12
1.24
118º
1.24
1.13
2.42
2.53
syn-ld
anti-ld
-61.1
-61.1
+
2d
+
2d
-61.9
-61.9
FIGURE 12.1.
G
(298 K), kcal/mol) for stepwise and concerted
Curtius rearrangement of acetyl azide (
1d
,R
¼
CH
3
) at the B3LYP/6-311
þ
G(
d
,
p
)//B3LYP/6-
31G(
d
) (red, top) and CBS-QB3 (black, bottom) levels of theory.
18
Distances are shown in
A
and angles are in degrees.
Source
: Reprinted with permission from Ref. 18. Copyright 2004 ACS
Publications.
The free energy surface (
D
Recently,
18
the stationary points on the PES for the thermal decomposition of
acetyl azide (
CH
3
) have been investigated using high-level CBS-QB3
approach as well as density functional theory at the B3LYP level. Two stable
18
conformations of
1d
,R
¼
in a manner reminiscent of diazo carbonyl compounds,
59
were
located. Calculations predict that for
1d,
, in the less stable conformer, the azide group
is positioned
anti
to carbonyl group (Fig. 12.1). In the gas phase, the difference in the
free energy of two conformers is predicted to be
1d
4.5 kcal/mol at the CBS-QB3
level. The free energy difference is slightly reduced in cyclohexane (4.1 kcal/mol)
and acetonitrile (3.2 kcal/mol). The activation free energy for the conversion of the
syn
conformer to the
anti
one was found to be 10.8 kcal/mol (Fig. 12.1).
Note that similar results were obtained for the conformers of fluorocarbonyl
azide.
60
According to the results of the gas-phase electron diffraction and vibrational
spectroscopy the
syn
conformer of fluorocarbonyl azide has lower energy by about
1.3 kcal/mol.
60
Calculations of Liu and coworkers
18
predict that less stable conformer of acetyl
azide (
anti
-
D
G
¼
1d
) undergoes dissociation with formation of molecular nitrogen and
