Chemistry Reference
In-Depth Information
TABLE 12.4. The Singlet-Triplet Splitting (DE
ST
) for Formyl Nitrene Calculated
Without ZPE Correction Using Different Methods, Basis Sets, and Geometries
17
D
E
ST
(kcal/mol)
Method
B3LYP/6-31G(
d
)
8.1
B3LYP/cc-pVDZ
a
8.5
B3LYP/cc-pVTZ
a
6.8
B3LYP/cc-pVQZ
a
6.7
B3LYP: complete basis set extrapolation
b
6.6
CCSD/cc-pVDZ
c
14.1
CCSD/cc-pVTZ
c
11.0
CCSD/cc-pVQZ
c
9.9
CCSD: complete basis set extrapolation
b
9.4
CCSD(T)/6-31G(
d
)
c
3.5
CCSD(T)/cc-pVDZ
c
5.2
CCSD(T)/cc-pVTZ
c
1.2
CCSD(T)/cc-pVQZ
c
0.1
CCSD(T): complete basis set extrapolation
b
0.7
a
At the B3LYP/6-31G(
d
) geometry.
b
Complete basis set extrapolation was performed using the Dunning-Feller formula.
96-98
c
At the CCSD(T)/cc-pVTZ geometry.
CH
2
NO
94
and CHNO
95
isomers. The cyclic structure
1
3c
was found to be lower in
energy than triplet formyl nitrene
3
3c
at G2 level (by 2.9 kcal/mol), but it was
predicted to be slightly higher than
3
3c
at the CASPT2/cc-pVTZ//CASSSF/cc-pVZD
(by 3.2 kcal/mol) and QCISD(T)/cc-pVTZ//CASSSF/cc-pVZD (by 0.4 kcal/mol)
levels of theory.
94
Note, that acyclic singlet formyl nitrene at the
1
A
00
state was
also optimized and predicted to be much higher in energy than the
1
A
0
state (by
11.9 kcal/mol at G2 level).
94
However, Shapley and Bacskay
94,95
did not correlate
their findings to the specific properties of singlet carbonyl nitrenes.
Later,
17
the geometry of
3c
in the lowest singlet and triplet states was reoptimized
at the CCSD(T)/cc-pVTZ level of theory. Figure 12.6 shows the resulting geometries
of
1
3c
and
3
3c
that fit well with those obtained by DFT calculations. Thus, B3LYP/6-
31G(
d
) provides adequate geometries of the lowest singlet and triplet states of
carbonyl nitrenes.
The singlet-triplet splitting for
calculated at different levels of theory are given
in Table 12.4. The B3LYP and CCSD calculations provide strongly positive
3c
E
ST
(i.e., they predict triplet ground states), whereby the gap is slightly reduced on going
to larger basis sets. The value of
D
E
ST
decreases dramatically on perturbative
inclusion of triple excitations in the coupled cluster scheme, and the further reduction
on increasing the basis set leads to an extrapolated value of
D
0.7 kcal/mol. It
was concluded that the two spin states must lie very close in energy, and that the
B3LYP/6-31G(
d
) calculations overestimate
D
E
ST
ΒΌ
D
E
ST
in
3c
by about 9 kcal/mol.
E
ST
values calculated by the B3LYP/6-31G(
d
) method
for benzoyl and naphthoyl nitrenes are positive (
As aforementioned, the
D
5 kcal/mol) but about 3 kcal/mol
.
16
Taking the overestimation of the B3LYP calculations
smaller than that for
1c
