Chemistry Reference
In-Depth Information
TABLE 6.2. Alkyl Nitrenium Ions Singlet-Triplet Energy Gaps (
D
E
st
) Rearrangement
Barriers in Approximate Structures of Isomers Calculated at the B3LYP/6-31G(d,p)
Level
Barrier
Singlet-
Triplet
Isomer
Energy
Singlet-Triplet
Nitrenium Ion
E
st
Isomer
D
þ
8.3
þ
3.1/
þ
44.1
94.8/
6.1
H
H
N
CH
3
H
18
CN
H
H
þ
3.8
þ
3.8/
þ
13.4
69.7/
þ
3.0
H
H
N
CN
CH
3
CH
3
19
H
CH
3
11.3]
a
Nsm
b
/
þ
37.5
[
þ
85.7/
10.7
H
H
CH
3
N
CN
H
CH
2
20
H
3
C
H
6.15]
a
Nsm
b
/(nd)
CH
3
[
þ
93.1/
15.8
CH
3
H
N
H
C
CN
CH
3
H
3
C
H
21
H
[þ8.03
a
]
Nsm
b
/(nd)
63.1/
þ
16.1
CH
3
CH
3
CH
3
N
H
C
CN
CH
3
CH
3
H
3
C
22
H
[þ9.7]
a
Nsm
b
/(nd)
68.3/
2.07
H
CH
3
CH
3
N
CN
CH
2
CH
3
23
H
CH
3
þ
4.6/
c
43.7/
c
31.1
N
H
H
N
24
Energies are in kilocalories per mole, a negative value implies a ground state singlet (D. D. Falvey,
unpublished results).
a
Values in italics are vertical energy gaps at the triplet equilibrium geometry.
b
Nsm
¼
no singlet minimum at a nitrenium ion geometry.
c
Triplet state of isomer appears to spontaneously revert to the triplet nitrenium ion.
relatively high-energy C
orbitals stabilizes the singlet nitrenium ion relative to
the corresponding triplet by with a gap of
C
s
31 kcal/mol in favor of the singlet. Here,
the relevant rearrangement process appears to be a ring expansion that would create a
strained unsaturated azetidinium cation
. Nonetheless, the barrier to this process is
still only 4.6 kcal/mol. Also located was a transition state to ring opening, but the
barrier to this reaction is higher.
These species were also investigated using multireference CASSSCF and
CASPT2 methods. However, both species appear to be well described by a single
determinant. Indeed, the main source of ambiguity in the singlet-triplet gaps appears
to be uncertainties in the geometry of the singlet state. At best, the singlets appear to
25
