Spectroscopy and Structure of Dichlorocarbene Ylides
A R T I C L E S
predict that the lowest-energy transition of dichlorodioxirane
On the basis of the experimental results and the computational
studies, we suggest the following (speculative) scenario for the
experimental result presented in Figure 5. LFP of 4 under air
generates CCl2, which is captured by oxygen, affording triplet
CCl2-carbonyl oxide 22T. The latter undergoes ISC to singlet
22S. Both of these processes are extremely rapid, with rate
2
5 should appear at 377 nm (f ≈ 0.0008).
The calculations predict that singlet 22S should encounter a
significant barrier to ring closure and could absorb in the desired
UV region, but how efficiently can it be formed from triplet
2
(
2T? To answer this question, we have carried out CASSCF-
6,6)/6-311+G(d) calculations on 22S and 22T, and have located
the minimum-energy crossing point (conical intersection)
9
constants in the 10 regime, and both are substantially exother-
mic. CCl2-carbonyl oxide 22S, which faces a significant barrier
to cyclization to dioxirane 25, is relatively long-lived (τ ≈ 4-5
µs), and is currently the best candidate for the observed UV
3
9
between the triplet and singlet potential energy surfaces.
According to these CASSCF calculations, the 22S-22T elec-
tronic energy separation is 16.2 kcal/mol, and a crossing point,
i.e., a molecular geometry where 22T and 22S are isoenergetic,
is located only 1.8 kcal/mol above the 22T minimum. The
crossing point shows a change in the ClCOO dihedral angle of
approximately 23° away from the perpendicular orientation in
4
4
band at 465 nm.
Summary
Dichlorodiazirine (4) can be prepared by the reaction of 2,4-
dinitrophenoxychlorodiazirine (13) and chloride. Photolysis of
dichlorodiazirine generates dichlorocarbene which, in LFP
experiments, forms chromophoric ylides or oxides with pyridine,
2-picoline, thioanisole, and oxygen. These species are readily
observable by UV absorption spectroscopy. However, with the
assignment of the 465 nm absorption from the LFP-UV of 4
under air to carbonyl oxide 22, we can find no absorption
attributable to CCl2 itself, certainly not the σ f p absorption
computed near 500 nm. Perhaps the CCl2 absorption is simply
too weak to be detected under our current conditions, in which
case it is not surprising that CCl2 absorbance was not found in
LFP experiments with precursor 1, especially in light of the
2
2T toward planarity (22S). The magnitude of the spin-orbit
-
1
coupling between the two electronic states is 10.4 cm at the
crossing point.
Thus, three factors combine to indicate that the triplet-to-
singlet intersystem crossing (ISC) of 22T to 22S should be
facile: (1) the activation energy is very small, (1.8 kcal/mol ≈
3
RT, T ) 298 K); (2) the geometry of the crossing point is
very similar to the equilibrium geometry for 22T, hence, easily
accessible at ambient temperature; and (3) the magnitude of
-
1
the spin-orbit coupling (10.4 cm ) is not insignificant,
reflecting the presence of two “heavy” Cl atoms in the molecule.
A numerical estimate of the rate constant for ISC in 22 can
5
interference from triplet phenanthrene. It would certainly be
be obtained by application of “Fermi’s Golden Rule“ to
In the high-temperature limit, the
rate constant expression takes the form:
of interest to study the low-temperature photolysis of matrix-
isolated 4, where higher concentrations of CCl2 could perhaps
be obtained.
radiationless transitions.4
0,41
Experimental Section
2
2
kISC ) (4π /h)|H | FCWD
(4)
SO
2
,4-Dinitrophenoxychlorodiazirine (13). A solution of 0.85 g (5
15
mmol) of phenoxychlorodiazirine (7) in 10 mL of nitromethane was
added dropwise over 10 min into a magnetically stirred suspension of
Here |HSO| is the magnitude of the spin-orbit coupling vector,
and FCWD is the Franck-Condon weighted density of vibra-
tional states (h ) Planck’s constant). FCWD may be expressed
in terms of the exothermicity of the reaction (∆E) and the
reorganization energy (λ)
2
.0 g (15 mmol) of nitronium tetrafluoroborate in 10 mL of ni-
tromethane at 0 °C. After the addition, the reaction mixture was stirred
at 0 °C for 2 h until the reaction was complete (as monitored by TLC).
Then, 200 mL of water was added, and the resulting solution was
extracted with 5 × 50 mL of pentane. The pentane extract was dried
2
4
over MgSO , filtered, concentrated on the rotary evaporator, and
(∆E + λ)
-1/2
FCWD ) (4πλRT)
exp -
(5)
chromatographed over a silica column using 1:5 ether/pentane as eluent.
We obtained 0.95 g (75%) of 13 as pale-yellow oil, which solidified at
(
[
]
)
(4λRT)
-20 °C in the freezer. CAUTION: All diazirines should be regarded
If we ignore solvation energy differences, we may approximate
λ by the difference in triplet energies for 22 evaluated at the
triplet and singlet equilibrium geometries, respectively. Using
CASSCF derived energies (∆E ) -16.2 kcal/mol, HSO ) 10.4
as explosive. Work should be carried out behind safety shields, and in
general, the diazirines should not be handled neat. It is best to store 13
4
2
as a solution in ether or pentane in the freezer.
1
H NMR (400 MHz, δ, CDCl ): 8.09-8.12 (d, 1H, J ) 9.2 Hz),
3
-
1
9
-1
13
cm , λ ) 27.7 kcal/mol), we obtain kISC ≈ 3.5 × 10 s at T
8.62-8.64 (d, 1H, J ) 9.2 Hz), 8.84 (s, 1H). C NMR (100 MHz, δ,
CDCl ): 68.82, 118.78, 121.48, 122.56, 125.97, 129.12, 149.29.
)
298 K, thus confirming the qualitative analysis presented
3
4
3
1
3
Dichlorodiazirine (4). A 25-mL flask with a tall neck bearing a
side exit tube and containing a magnetic stirring bar was charged with
above. The formation of 22S from reaction of CCl2 and O2
should be very rapid at ambient conditions.
0.87 g (5.0 mmol) of anhydrous 1-butyl-3-methylimidazolium chloride,
(
38) Additional excitations are computed at 425.2 nm (f ≈ 0.0003) and at 356.2
nm (f ≈ 0.005). A near-infrared transition with no intensity is also predicted
at λ ) 1217 nm (f ) 0.0000)
0.56 g (2.0 mmol) of TBACl, and 0.34 g (2.0 mmol) of CsCl. The exit
tube was connected to a series of two traps; the first was cooled to
-
20 °C, and the second, containing 1 mL of pentane, benzene, or other
(
(
(
(
(
39) Bearpark, M. J.; Robb, M. A.; Schlegel, H. B. Chem. Phys. Lett. 1994,
2
23, 269.
solvent, was cooled to 77 K. The second trap was fitted with stopcocks
40) Loudon, R. The Quantum Theory of Light; Oxford University Press: Oxford,
983.
41) Newton, M. Chem. ReV. 1987, 87, 113. Marcus, R. A. ReV. Mod. Phys.
993, 65, 599.
42) Beljonne, D.; Shuai, Z.; Pourtois, G.; Bredas, J. L. J. Phys. Chem. A 2001,
03, 3899.
at both its inlet and outlet.
1
1
(44) This assignment should still be regarded as provisional. Although the 465
2
nm absorption clearly results from the reaction of CCl with oxygen, and
1
computational studies provide for an extremely rapid formation of 22, we
are unable to quench the absorbing species with TME, acetaldehyde, or
tris(trimethylsilyl)silane, nor do we observe the formation of other UV active
species (over 10 µs) as the 465 nm species decays.
43) Using PBEPBE/6-311+g(d) derived energies (∆E ) -28.4 kcal/mol, λ )
1
9
-1
2
)
3.0 kcal/mol) and HSO ) 10.4 cm- , we obtain kISC ≈ 3.1×10 s at T
298 K.
J. AM. CHEM. SOC.
9
VOL. 129, NO. 16, 2007 5173