Spectroscopic and computational studies on the rearrangement of ionized [1.1.1]propellane and some of its valence isomers: The key role of vibronic coupling

Article abstract of DOI:10.1021/ja106024y

The [1.1.1]propellane radical cation 1^?+, generated by radiolytic oxidation of the parent compound in argon and Freon matrices at low temperatures, undergoes a spontaneous rearrangement to form the distonic 1,1-dimethyleneallene (or 2-vinylideneallyl) radical cation 3^?+ consisting of an allyl radical substituted at the 2-position by a vinyl cation. In similar matrix studies, it is found that the isomeric dimethylenecyclopropane radical cation 2^?+ also rearranges to 3^?+. The unusual molecular and electronic structure of 3^?+ has been established by the results of ESR, UV-vis, and IR spectroscopic measurements in conjunction with detailed theoretical calculations. Also of particular interest is an NIR photoinduced reaction by which 3^?+ is cleanly converted to the vinylidenecyclopropane radical cation 4^?+, a process that can be represented in terms of a single electron transfer from the allyl radical to the vinyl cation followed by allyl cation cyclization. The specificity of this photochemical reaction provides additional strong chemical evidence for the structure of 3^?+. Theoretical calculations reveal the decisive role of vibronic coupling in shaping the potential energy surfaces on which the observed ring-opening reactions take place. Thus vibronic interaction in 1^?+ mixes the ²A₁′ ground state, characterized by its "non-bonding" 3a₁′ SOMO, with the ²E″ first excited state resulting in the destabilization of a lateral C-C bond and the initial formation of the methylenebicyclobutyl radical cation 5^?+. The further rearrangement of 5^?+ to 3^?+ occurs via 2 ^?+ and proceeds through two additional lateral C-C bond cleavages characterized by transition states of extremely low energy, thereby explaining the absence of identifiable intermediates along the reaction pathway. In these consecutive ring-opening rearrangements, the "non-bonding" bridgehead C-C bond in 1^?+ is conserved and ultimately transformed into a normal bond characterized by a shorter C-C bond length. This work provides strong support for the Heilbronner-Wiberg interpretation of the vibrational structure in the photoelectron spectrum of 1 in terms of vibronic coupling.

Full text of DOI:10.1021/ja106024y

Published on Web 09/29/2010
Spectroscopic and Computational Studies on the
Rearrangement of Ionized [1.1.1]Propellane and Some of its
Valence Isomers: The Key Role of Vibronic Coupling
†
,†
‡
,‡
Beat M u¨ ller, Thomas Bally,* Robert Pappas, and Ffrancon Williams*
Department of Chemistry, UniVersity of Fribourg, Switzerland, and Department of Chemistry,
UniVersity of Tennessee, KnoxVille, Tennessee
Received July 12, 2010; E-mail: thomas.bally@unifr.ch; williams@ion.chem.utk.edu
•+
Abstract: The [1.1.1]propellane radical cation 1 , generated by radiolytic oxidation of the parent compound
in argon and Freon matrices at low temperatures, undergoes a spontaneous rearrangement to form the
•+
distonic 1,1-dimethyleneallene (or 2-vinylideneallyl) radical cation 3 consisting of an allyl radical substituted
at the 2-position by a vinyl cation. In similar matrix studies, it is found that the isomeric dimethylenecyclo-
propane radical cation 2• also rearranges to 3 . The unusual molecular and electronic structure of 3
+
•+
•+
has been established by the results of ESR, UV-vis, and IR spectroscopic measurements in conjunction
with detailed theoretical calculations. Also of particular interest is an NIR photoinduced reaction by which
•+
•+
3
is cleanly converted to the vinylidenecyclopropane radical cation 4 , a process that can be represented
in terms of a single electron transfer from the allyl radical to the vinyl cation followed by allyl cation cyclization.
The specificity of this photochemical reaction provides additional strong chemical evidence for the structure
•
+
of 3 . Theoretical calculations reveal the decisive role of vibronic coupling in shaping the potential energy
•
+
surfaces on which the observed ring-opening reactions take place. Thus vibronic interaction in 1 mixes
2
2
the A
1
′ ground state, characterized by its “non-bonding” 3a
1
′ SOMO, with the E′′ first excited state resulting
in the destabilization of a lateral C-C bond and the initial formation of the methylenebicyclobutyl radical
•
+
•+
•+
•+
cation 5 . The further rearrangement of 5 to 3 occurs via 2 and proceeds through two additional
lateral C-C bond cleavages characterized by transition states of extremely low energy, thereby explaining
the absence of identifiable intermediates along the reaction pathway. In these consecutive ring-opening
•
+
rearrangements, the “non-bonding” bridgehead C-C bond in 1 is conserved and ultimately transformed
into a normal bond characterized by a shorter C-C bond length. This work provides strong support for the
Heilbronner-Wiberg interpretation of the vibrational structure in the photoelectron spectrum of 1 in terms
of vibronic coupling.
5
Introduction
in energy, so that 1 is not substantially stabilized on lengthening
the C
1
3
-C bond.
1
,3
Although tricyclo[1.1.1.0 ]pentane (1), usually referred to
In some respects, this central bond in 1 corresponds to a
normal bond, and thus (a) it is only 6% longer than the C-C
as [1.1.1]propellane, possesses one of the highest strain energies
per C-C bond of all known cycloalkanes, the neutral molecule
1
10
bond in cyclopropane, (b) its thermochemical strength has been
is in fact remarkably stable to either thermal decomposition or
polymerization, and this lack of reactivity underlies its successful
1
,4,8
assessed to be ca. 70% of that of a conventional C-C bond,
1
0
2,3
and (c) its stretching force constant is in the expected range.
On the other hand, the analyses of overlap populations and
preparation. This high thermal stability of 1 has been attributed
5
to the clasping effect of the three methylene groups so that the
7
,8
other wave function-derived characteristics did not reveal an
unambigous “electronic basis” that might account for these
localized bonding properties, prompting the statement that “no
irrefutable case can be made either proving or disproving the
dissociation of the central C
strain relief. Moreover, due to a big HOMO-LUMO gap,
the open-shell singlet and triplet diradical states lie very high
1
3
-C bond brings about almost no
1,4
5-9
8
existence of a ‘bond’ between the bridgehead atoms in 1.”
†
University of Fribourg.
University of Tennessee.
‡
Very recently, some of the differing views on the nature of
the central bond in 1 have been reconciled by identifying it as
a “charge-shift” bond, such as that which prevails in molecular
(
(
(
1) Wiberg, K. B. Angew. Chem., Int. Ed. Engl. 1986, 25, 312.
2) Wiberg, K. B.; Walker, F. H. J. Am. Chem. Soc. 1982, 104, 5239.
3) Semmler, K.; Szeimies, G.; Belzner, J. J. Am. Chem. Soc. 1985, 107,
11
fluorine. However, leaving aside this enigmatic character of
6
410.
(
(
(
(
(
(
4) Wiberg, K. B. Chem. ReV. 1989, 89, 975.
1
the central bond, complete agreement exists that the 3a ′ HOMO
5) Newton, M. D.; Schulman, J. M. J. Am. Chem. Soc. 1972, 94, 773.
6) Stohrer, W.-D.; Hoffmann, R. J. Am. Chem. Soc. 1972, 94, 779.
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Chem. Soc. 1992, 114, 5820.
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(11) Wu, W.; Gu, J.; Song, J.; Shaik, S.; Hiberty, P. C. Angew. Chem., Int.
Ed. 2009, 48, 1407.
10.1021/ja106024y  2010 American Chemical Society
J. AM. CHEM. SOC. 2010, 132, 14649–14660 9 14649

A R T I C L E S
M u¨ ller et al.
Scheme 1
Scheme 2
The above expectation that 1 should survive one-electron
oxidation structurally intact, despite its considerable strain, was
2
0,21
tempered, however, by previous studies on oxiranes.
case, the seemingly contradictory results of photoionization from
In this
2
2
a nonbonding p-AO on the oxygen atom and C-C ring
opening by radiolytic oxidation at low temperatures were
2
0
observed (Scheme 1), these findings eventually being reconciled
2
1,23
theoretically.
Our studies of 1 now reveal an even more remarkable
oxidative rearrangement, one in which eventually three lateral
framework bonds of 1• break, and perhaps even more
unexpectedly, a “normal” C-C bond is reconstituted between
the carbons originally at the bridgehead positions, the final
product being the distonic 1,1-dimethyleneallene (or 2-vinylide-
neallyl) radical cation 3• possessing both substituted vinyl
cation and allyl radical moieties. The same product is obtained
on ionization of dimethylenecyclopropane 2, so the correspond-
ing radical cation is probably an intermediate in the decay of
+
Figure 1. Photoelectron spectrum of [1.1.1]propellane (1), reproduced from
ref 13, and pictorial representation of the molecular orbitals from which
ionization occurs.
+
5
-8,12
of 1 does not contribute appreciably to C
1
-C
3
bonding.
This can be ascribed to the “inverted-tetrahedron” geometry that
1
2
prevails at the bridgehead atoms, which means that most of
5
,7-9
the 3a
1
′ orbital’s amplitude lies outside the molecule
so
•+
1
(Scheme 2).
The key to understanding this unprecedented rearrangement
is provided by theoretical calculations that reveal the decisive
that the HOMO is essentially nonbonding (see MO drawing in
Figure 1). Experimental support for this HOMO description
comes from the observation of an intense sharp 0,0 vibrational
component in the first band of the photoelectron spectrum of
2
4,25
role of vibronic coupling
in shaping the potential energy
13
1,
implying that the rigid structure of the molecule is retained
landscape on which the observed reaction takes place. Es-
sentially, vibronic interaction mixes the 2A′ ground state,
on ionization with little or no change in geometry.
8
2
Similarly, theoretical calculations predict that the C
1
-C
3
characterized by its nonbonding 3a
1
′ SOMO, with the E′′ first
distance is largely unaffected by ionization, the small decrease
excited state, a process that results in a weakening of the lateral
C-C bonds. The possible importance of this vibronic coupling
was actually first suggested by Heilbronner, Wiberg, and their
•
+
from 1.6 Å for 1 to 1.55 Å for 1 formally suggesting, if
anything, a slightly antibonding character to the 3a ′ orbital. In
1
1
3
-1
view of the above, one would expect that ionization of 1 (i.e.,
removal of an electron from the HOMO) should lead only to
very minor structural changes. This is in contrast to the situation
for other strained hydrocarbons with HOMOs that are strongly
co-workers in discussing the low-frequency (360 ( 20 cm )
vibrational progression that they discerned in the first band in
the photoelectron spectrum of 1. They argued that this progres-
sion could not be assigned to any of the allowed totally
symmetric modes that may be stimulated on electron ejection
1
4-17
C-C σ-bonding, such as bicyclo[1.1.0]butane
and bicyclo-
1
4,18,19
[
2.1.0]pentane.
Indeed, the central bonds of these mol-
from the 3a ′ orbital of 1 because these would all be expected
1
ecules are found to undergo considerable weakening on ioniza-
tion, as clearly revealed by the above-cited ESR and theoretical
studies.
to have much higher frequencies. Instead, the authors proposed
that one explanation for the low-frequency mode could reside
in the aforementioned vibronic coupling of the ground state with
1
3
the proximate degenerate excited state. As a result of our
present studies which originate from a totally different direction,
(
(
12) Wiberg, K. B. Acc. Chem. Res. 1984, 17, 379.
13) Honegger, E.; Huber, H.; Heilbronner, E.; Dailey, W. P.; Wiberg, K. B.
J. Am. Chem. Soc. 1985, 107, 7172.
(
14) Gassman, P. G. In Photoinduced Electran Transfer Reactions: Organic
Substrates; Fox, M. A., Chanon, M., Eds.; Elsevier: Amsterdam, 1988;
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(20) Snow, L. D.; Wang, J. T.; Williams, F. Chem. Phys. Lett. 1983, 100,
193.
(
15) Newton, M. D.; Schulman, J. M. J. Am. Chem. Soc. 1972, 94, 767.
(21) Williams, F. Radiat. Phys. Chem. 2003, 67, 211.
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(23) Nobes, R. H.; Bouma, W. J.; McLeod, L.; Radom, L. Chem. Phys.
Lett. 1987, 135, 78.
(
16) Gerson, F.; Qin, X.-Z.; Ess, C.; Kloster-Jensen, E. J. Am. Chem. Soc.
1
989, 111, 6456.
(
(
17) Bally, T. J. Mol. Struct. 1991, 227, 249.
18) Williams, F.; Guo, Q.-X.; Kolb, T. M.; Nelsen, S. F. J. Chem. Soc.,
Chem. Commun. 1989, 1835.
(24) Bersuker, I. B. The Jahn-Teller Effect; Cambridge University Press:
Cambridge, U.K., 2006.
(
19) Adam, W.; Walter, H.; Chen, G.-F.; Williams, F. J. Am. Chem. Soc.
(25) K o¨ ppel, H.; Domcke, W.; Cederbaum, L. S.; Shaik, S. S. Angew.
Chem., Int. Ed. 1983, 22, 210.
1
992, 114, 3007.
1
4650 J. AM. CHEM. SOC. 9 VOL. 132, NO. 41, 2010

Rearrangement of Ionized [1.1.1]Propellane
A R T I C L E S
this earlier conjecture on the significance of vibronic coupling
in 1 is revealed to be well substantiated.
the above gas mixture was sprayed over the course of 1 h onto a
CsI window, held at 20 K in a closed-cycle cryostat, where the
gases condensed to form a transparent matrix. After cooling to 10
K, the matrix was exposed to X-irradiation (bremsstrahlung from
a tungsten target exposed to a 40 mA current of 40 kV). The
electron-hole pairs generated by that process in Ar are trapped by
•+
Experimental Section
Syntheses of Compounds 1, 2, and 4. [1.1.1]Propellane, 1, was
made by adding methyllithium to a solution of 1,1,-dibromo-2,2-
bis(chloromethyl)cyclopropane in cold diethyl ether according to
the added dopants (for the electron capture we use CH
2 2
Cl ).
2
6
Ionization of the hydrocarbon substrate then occurs by hole transfer
the procedure of Belzner et al. As 1 is difficult to separate from
+
33
from Ar , a processs that is exothermic by several eV. The excess
energy imparted onto the incipient hydrocarbon radical cations,
which cannot be easily dissipated in solid Ar, may induce
rearrangements that are not observed upon radiolytic oxidation in
Freon glasses. This is often the reason for differences observed in
the optical spectra observed in Freon and Ar matrices.
the solvent, it was reacted with I and thus converted to 1,3-
2
2
7
diiodobicyclopentane, a stable, crystalline compound that can be
reconverted as needed to 1 by reducing it with sodium cyanide in
DMSO, from which it can be separated by fractional condensation
27
under vacuum. Compound 1 is trapped as a white, powdery solid
at 77 K and can be stored at -78 °C for weeks in the absence of
1
Wavelength-selective photolyses were brought about by irradia-
tion with medium or high-pressure Hg/Xe or Ar resonance lamps
through appropriate cutoff or bandpass filters. For electronic
absorption spectroscopy, a Perkin-Elmer Lambda 900 instrument
was employed, and IR spectra were recorded on a Bomem DA3
interferometer using a liquid nitrogen cooled midrange MCT
detector.
air without appreciable decomposition. H NMR (CDCl
3
): δ 2.65
(
s, 6H).
ESR Experiments. Solutions containing 0.005-0.01 M con-
centrations of 1, 2, and 4 in Freon solvents (CFCl , CF CCl ,
3 3 3
CF ClCFCl ) were prepared on a vacuum line in Spectrosil ESR
2 2
sample tubes (3 mm i.d.) and γ-irradiated at 77 K for a typical
Above 350 °C in the gas phase, 1 decomposes to dimethyl-
enecyclopropane, 2, and vinylidenecyclopropane, 4, which are in
4
-1
radiation dose of 0.2 Mrad (1 Mrad ) 10 kGy ) 1 × 10 J kg ).
19,34
Additional details of sample preparation and reasons for the
2
8
thermal equilibrium under these conditions. Thus, the latter two
compounds can in principle be obtained by flash vacuum pyrolysis
of 1. However, we decided to use a higher yield “wet” synthesis
of 4 that is based on a procedure published by the group of Conia
recommended concentration range are described elsewhere.
After irradiation, the sample tube was quickly transferred from
liquid nitrogen into a variable-temperature Dewar insert mounted
inside the cavity of an ESR spectrometer (Bruker ER 200D SRC),
the initial temperature being ca. 80 K. The X-band microwave
frequency was recorded with a Systron-Donner 6054 B counter,
and the magnetic fields were determined by an NMR gaussmeter
2
9
for the preparation of bicyclopropylidene.
Thus, dibromocarbene was added to methylenecyclopropane
3
0
3
1
(
which was made from methallyl chloride and potassium amide )
to yield 2,2-dibromospiropentane, which upon dehalogenation with
methyllithium undergoes ring opening to give 4 in 25% isolated
yield (based on methylenecyclopropane). Samples of 4 used for
(
Bruker ER 035 M). Spectra were recorded at intervals of 5-10 K
on progressive annealing, the observed spectral changes being
monitored for reversibility by recycling to the lower temperature.
Photoelectron Spectra. The photoelectron (PE) spectrum of 1
matrix isolation were further purified by preparative GC on 35%
1
ODP/Chromosorb at 35 °C. H NMR (CDCl
3
): δ 1.52 (t, 4H), 4.81
13
was reproduced from the literature. That of 2 was measured on
(
q, 2H).
Dimethylenecyclopropane, 2. This compound was obtained by
a modified Perkin-Elmer PE 16 instrument operated in preretarda-
35
tion (and hence constant resolution) mode.
flash vacuum pyrolysis of 4 (0.1 mbar) through an empty 40 cm
quartz tube (10 mm i.d.) at 500 °C (where 2 predominates in the
Quantum Chemical Calculations. The geometries of all species
were optimized by the B3LYP/6-31G* method, which has proven
2
8
thermal equilibrium). The crude reaction mixture was separated
36
very reliable in predicting radical cation geometries. Force fields
1
3
by preparative GC (same conditions as for 4). H NMR (CDCl ):
calculated by the same method were used to model IR spectra (after
scaling of all frequencies by a suitable factor) and the thermo-
chemical corrections to the relative energies (see below). Fermi
contact terms to predict ESR hyperfine coupling constants were
also taken from B3LYP/6-31G* calculations. Derivative coupling
vectors between the ground and the first excited state of radical
cations (see Further Rearrangements in Results and Discussion)
were calculated by doing one cycle of a (4,4)CASSCF/6-31G*
conical intersection calculation. All of the above calculations were
δ 1.58 (m, 2H), 5.31 (m, 2H), 5.54 (m, 2H). Unreacted 4 was
recovered for further pyrolysis.
Matrix Isolation Spectroscopy. For the generation of radical
cations in Freon solutions, the hydrocarbons were dissolved to a
-
3
concentration of ca. 5 × 10 M in different Freons: CFCl
CF ClCCl (F-112a), and CF CCl (F-113a) for ESR spectroscopy,
and a 1:1 mixture of CFCl and CF BrCF Br, which forms a
3
(F-11),
2
3
3
3
3
2
2
3
2
transparent glass, for optical spectroscopy. After freezing to 77
K, the solutions were exposed to ca. 0.5 Mrad of γ-irradiation from
37
done using the Gaussian 98 and 03 suite of programs.
6
0
a
Co source. To obtain well resolved ESR spectra, the resulting
Absolute energies were calculated at the RCCSD(T)/cc-pVTZ
samples were warmed to 90-110 K.
For the purpose of Ar matrix isolation, the neutral precursor
molecules were mixed in a 2 L glass bulb in a ratio of 1:1000 with
38
level, using the Molpro program, to which zero-point vibrational
energies (ZPVE) from the above-described DFT calculations were
added. The relative energies shown and discussed in this paper are
ZPVE corrected RCCSD(T) energies. Cartesian coordinates, ab-
a 9:1 mixture of Ar and N
quality of matrices) to which an equimolar part of CH
added to act as an electron scavenger. Approximately 80 Torr of
2
(which serves to improve the optical
2
2
Cl was
(
33) Bally, T. In Radical Ionic Systems; Lund, A., Shiotani, M., Eds.;
Kluwer: Dordrecht, 1991; p 3.
(
26) Belzner, J.; Bunz, U.; Semmler, K.; Szeimies, G.; Opitz, K.; Schl u¨ ter,
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3366.
A.-D. Chem. Ber. 1989, 122, 397.
(
(
27) Alber, F.; Szeimies, G. Chem. Ber. 1992, 125, 757.
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Phenom. 1983, 31, 181.
9
, 798.
(36) Bally, T. ReV. Comput. Chem. 1999, 13, 1.
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Gaussin, Inc.: Wallingford, CT, 2003.
(
(
(
(
29) Le Perchec, P.; Conia, J. M. Tetrahedron Lett. 1970, 19, 1587.
30) Lukin, K. A.; Zefirov, N. S. Zh. Org. Khim. 1987, 23, 2548.
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32) Grimison, A.; Simpson, G. A. J. Phys. Chem. 1968, 72, 1776.
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College Cardiff Consultants Ltd.: Cardiff, 2004.
J. AM. CHEM. SOC. 9 VOL. 132, NO. 41, 2010 14651

A R T I C L E S
M u¨ ller et al.
Figure 2. (a) ESR spectrum obtained at 110 K after radiolytic oxidation
3 3
of 1 in CF CCl at 77 K. (b) Simulation of the spectrum based on the
assignment given at the bottom.
Figure 3. Difference spectrum for X-irradiation of 1 and for subsequent
NIR photolysis. The band labeled A belongs to the photobleachable species
that is created upon ionization of 1.
solute energies, and thermal corrections for all stationary points
evaluated in this work are listed in the Supporting Information.
Electronic absorption spectra were modeled by the CASSCF/
2 2
below 100 K and is broadened considerably in CF ClCFCl at
39
90 K. Confirmation that the primary species formed by radiolysis
of 1 in these Freon matrices are identical is provided by the
observation that the signal carrier, although unaffected by
exposure to visible light, is sensitive in all cases to photolysis
by near-IR light above 800 nm (vide infra).
CASPT2 method using the ANO-S basis set and the Molcas suite
40
of progams. The choice of active spaces is indicated in the tables
where results are listed. To avoid problems with intruder states in
the CASPT2 calculations, a series of calculations with level shifts
varying from 0.05 to 0.25 h were carried out, always ascertaining
that the relative energies of states that are not affected by intruders
do not change significantly.
3 2 2
Ionization of 1 in CFCl /CF BrCF Br, a Freon mixture that
forms a transparent glass, results in a rather nondescript EA
spectrum that increases gradually toward the UV. However,
ionization by X-irradiation in Ar gave a spectrum with three
To ascertain whether multideterminantal effects may prohibit the
CCSD(T) method from yielding reliable predictions of relative
energies, all stationary points were recalculated by CASSCF using
the ANO-S basis set and an active space of 11 electrons in 10
orbitals. However, the ground states are invariably described to
almost 90% by a single configuration, and no excited configuration
contributes more than 4%. Relative CASPT2 energies obtained on
this basis were found to be in reasonable accord with the results
from CCSD(T)(cc-pVTZ calculations (see Figure S7 in Supporting
Information).
distinct bands: a very weak structured one in the NIR (λmax
=
1
400 nm), a second weak one around 480 nm, and an intense
one at 310 nm, in addition to a few small peaks between the
second and the third band (see Figure 3). Irradiation through a
850 nm cutoff filter led to the complete bleaching of the NIR
band, whereas the other spectroscopic features remained unaf-
fected, except for a small increase around 350 nm and the onset
of a decrease below 270 nm.
4
1
Molecular orbitals were plotted using the MOPlot program.
Clearly the band at 1400 nm cannot belong to 1• because
+
Results and Discussion
2
2
(
a) the gap betweeen the first ( A
1
′) and the second band ( E′′)
Ionization of 1 and 2. Figure 2 shows the ESR spectrum
recorded at 110 K after the radiolytic oxidation of 1 in a CF CCl
in the PE spectrum in Figure 1 amounts to >1.5 eV and (b)
2
2
3
3
A ′ f E′′ transitions are electric dipole forbidden in D
1
3h
2
matrix at 77 K. The well-resolved hyperfine pattern is readily
symmetry. The transition to the E′ state, ca. 3 eV above the
2
1
analyzed as a 1:4:6:4:1 quintet (a(4H) ) 14.8 G) of 1:2:1 triplets
A ′ ground state, would be allowed, but that region of the
(
a(2H) ) 5.4 G), and this analysis is confirmed by the spectral
spectrum (around 400 nm) is devoid of absorptions. The bands
peaking at ca. 500 and at 310 nm seem to belong to the same
species, because they can be bleached by prolonged >475 nm
photolysis, whereby the characteristic bands of the radical cation
of cyclopentadiene are observed to arise. Such pairs of bands
simulation shown in Figure 2. Clearly, the form of the ESR
spectrum rules out the possibility that the carrier of this signal
•+
is the unrearranged radical cation 1 , which would be expected
to give rise to a septet pattern from six equivalent hydrogens.
The spectra obtained in other chlorofluorocarbon matrices show
the same distinct quintet pattern, although the triplet substructure
4
2
are typical for conjugated diene radical cations.
The same sequence of ionization and subsequent NIR
photolysis, this time monitored in the mid-IR range, is illustrated
in Figure 4 where three sections of the IR difference spectra
are shown. These spectra show the rise of a set of peaks on
ionization, at least 12 of which disappear again on subsequent
NIR photobleaching, as indicated by the vertical dashed lines
connecting the peaks labeled A in Figure 4. Group theory
3 2 3
is not as well resolved in CFCl and CF ClCCl at temperatures
(
(
39) Andersson, K.; Roos, B. O. In Modern Electronic Structure Theory;
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tb/moplot/moplot.html.
(
(42) Shida, T. Electronic Absorption Spectra of Organic Radical Ions;
Elsevier: Amsterdam, 1988.
1
4652 J. AM. CHEM. SOC. 9 VOL. 132, NO. 41, 2010

Rearrangement of Ionized [1.1.1]Propellane
A R T I C L E S
Scheme 3
Thus, it seems, in contrast to the expectations based on the
nature of the ground state of 1• (see Introduction), this cation
does not preserve its structural integrity but undergoes some
spontaneous relaxation to a species of lower symmetry. In the
following sections we will elucidate the nature of and the reasons
for this spontaneous decay of a cation that would at first sight
have no reason to depart significantly from the geometry of its
neutral precursor, 1.
+
Remarkably, similar experiments conducted on the oxidation
of 2 gave ESR and optical spectra that were nearly identical
with those derived from 1, when compared under the same
conditions of matrix and temperature (see Figure S1 of the
Supporting Information). The IR spectrum after ionization
differed more (see Figure S2 in the Supporting Information),
but all of the peaks of species A were clearly visible, and the
difference spectrum for bleaching of that species was again
almost identical to that shown in Figure 4b.
Thus, both 1 and 2 appear to yield a common radical cation
intermediate A despite the fact that the two parent molecules
have very different structures (Scheme 3). This is clearly
indicative of a rather extensive rearrangement of the parent
molecules upon ionization leading almost certainly to the
formation of a very stable intermediate corresponding to the
signal carrier A. Of course, it is not possible to conclude from
these results that the transformation of 1• proceeds directly
through the radical cation of 2 to this intermediate, although
this is a possible scenario given that 2 formally contains two
fewer cyclopropane rings than 1. This issue of the reaction
pathway will be addressed theoretically in detail later in this
paper.
+
Figure 4. IR difference spectra for (a) the ionization of 1 by X-irradation
in Ar and (b) subsequent bleaching by photolysis at >850 nm. The bands
labeled A belong to the photobleachable species that is formed on ionization
of 1.
Assignment of Photosensitive Radical Cation Obtained on
Ionization of 1 or 2. The first significant clue as to the unusual
structure of the signal carrier A came from thermal annealing
indicates that 1 has only 6 e′ and 2 a
2
′′ vibrations that are IR-
studies in CF
2 2
ClCFCl where ion-molecule reactions are well-
active, of which two are C-H stretches. Clearly more IR bands
are associated with the species that arises on ionization of 1 in
Ar and its subsequent bleaching, which indicates that this species
must have lower symmetry than D3h, in accord with the
conclusions based on the ESR spectrum. Interestingly, the most
known to occur above 100 K, leading to the formation of neutral
radicals as a result of proton or hydrogen-atom transfer.
4
3,44
As shown in Figure 5, the broadened five-line ESR pattern from
the primary species at 90 K (which corresponds to a poorly
resolved version of the spectrum in Figure 2) decays at 115 K
to give a quintet of very sharp lines with a similar splitting
constant of 14.6 G.
The spectrum at 115 K is unaffected by near-IR bleaching,
which shows that the spectral change observed on annealing
does not result from improved resolution, but that A had
irreversibly transformed into a secondary radical B. Most
revealing is the magnitude of the 4H-coupling (14.7 ( 0.1 G)
in both A and B. This is reminiscent of the 14.4 G splitting
observed for the spectra of neutral allyl radicals obtained from
-
1
intense peak in the IR spectrum of this species is at 1870 cm ,
a region where molecules containing cumulated double bonds
typically show strong absorptions.
However, it should be pointed out that more IR peaks arise
on ionization of 1 in Ar than are bleached by NIR photolysis,
some of which occur also in the region of 1900-2100 cm .
These bands may be associated with the bands at 500 and 310
nm in the UV-vis spectrum shown in Figure 3, a proposition
that will be examined later.
The conclusions from the above experiments are that ioniza-
tion of 1 in Freon or Ar matrices leads to a photosensitive
species with a set of four and a set of two equivalent hydrogens.
The sole electronic absorption that is discernible for this species
above 260 nm is a weak, structured band peaking at ca. 1400
nm that can be bleached by NIR irradiation. This photolysis
leads also to the disappearance of a set of at least 12 IR bands
that are formed (next to others) on ionization in Ar.
-
1
4
3,44
olefins in Freon matrices,
where the small difference of ca.
0
.8 G between the R-couplings from unsubstituted exo- and
45
endo-1,3-hydrogens is typically not resolved.
(43) Fujisawa, J.; Sato, S.; Shimokoshi, K.; Shida, T. J. Phys. Chem. 1985,
8
9, 5841.
(
(
44) Williams, F.; Qin, X.-Z. Radiat. Phys. Chem. 1988, 32, 299.
45) Linder, R. E.; Winters, D. L.; Ling, A. C. Can. J. Chem. 1976, 54,
1405.
J. AM. CHEM. SOC. 9 VOL. 132, NO. 41, 2010 14653

A R T I C L E S
M u¨ ller et al.
Figure 6. ESR spectrum of γ-irradiated 1 in CF
2
ClCCl
3
at 100 K before
(
a) and after (b) NIR photolysis for 1.5 h using a Corning 3962 filter.
Spectrum (a) is essentially the same as the quintet observed in Figure 1
except that the additional triplet splitting is only partially resolved in this
matrix. Spectrum (b) is identical in all respects to that recorded at the same
temperature after γ-irradiation of a CF ClCCl solution of 4.
Figure 5. ESR spectrum obtained after ionization by γ-irradation of 1 in
2 2
CF ClCFCl at 77 K and warming of the matrix to (a) 90 K and (b) 115 K.
Spectrum (c), recorded at 123 K where slow diffusion takes place in this
matrix, was obtained after reaction of the allenyl radical with allene, giving
rise to the species shown in the figure (cf. text).
2
3
a(1H) ) 12.2 G (see Figure 7). The latter coupling is
characteristic for the terminal H-atom in a propargyl radical (in
4
7
In Figure 5c, the spectrum of this product species B is
compared to that of the allylic radical product resulting from
the addition of the allenyl radical to the carbon-2 position of
•
MeHC -CtCH it is 11.8G ). Thus we conclude that the
species obtained after progressive annealing of the photoproduct
of A is the propargylic radical C obtained by deprotonation at
4
6
allene (both experiments were carried out in the CF
2
ClCFCl
2
•+
the terminal CH
2
group of 4 (in addition, DFT calculations
matrix at ca. 120 K). The similarity in the spectral profiles is
striking and strongly suggests that species B contains also an
allylic radical center. Furthermore, in view of the almost
identical quintet splittings in A and B, the structures of A and
B seem to be related through the loss of a proton from A in an
ion-molecule reaction, such that the spin distribution in the
allylic moiety is not significantly affected.
predict the coupling to the four protons attached to the ring to
be 22.0 G, in accord with that assignment). Furthermore, the
same species C was also obtained on ionization of 4 in the
2 2
CF ClCFCl matrix followed by warming to 120 K.
Thus, just as the neutral allylic radical B provided an
important clue to the structure of its parent radical cation A, so
the formation of the neutral propargyl radical C can be directly
attributed to a similar proton loss from the vinylidenecyclopro-
pane radical cation 4• in an ion-molecule reaction.
Further information about the structure of A was obtained
from its near-IR photolysis carried out in the CF ClCCl matrix
2 3
+
•+
at 83 K (where the triplet substructure of the ESR signal of A
is also not fully resolved, see Figure 6a). This led to the
replacement of the spectrum of A by a much narrower pattern
consisting of a well-resolved 1:2:1 triplet (a(2H) ) 14.6 G, see
Figure 6b). Identical spectra were obtained (under the same
conditions) after ionization and NIR photolysis of 2 and, more
importantly, after ionization of vinylidenecyclopropane, 4 (see
Figure S3 in the Supporting Information).
At this point the reader may wonder why 4 shows coupling
to only two of its six protons in its ESR spectrum. As a result
of its D2d symmetry, allene has a degenerate HOMO, and its
radical cation is therefore subjected to Jahn-Teller distortion
that leads to a reduction of the twisting of the allene moiety to
less than 90° (in allenes of lower symmetry, such as 4, the
HOMO is no longer degenerate, but distortion still occurs on
ionization, due to vibronic coupling of the ground state with a
4
8
Firm evidence that the spectrum resulting from the NIR
photolysis of A in the CF ClCCl matrix (Figure 6b) can indeed
2 3
low-lying excited state). As a consequence, the four protons
in the cyclopropylidene moiety become nonequivalent, resulting
in two identical pairs, as in the case of the similarly twisted
be assigned to the unrearranged vinylidenecyclopropane radical
cation 4• comes from additional NIR photolysis experiments
+
49
dicyclopropylidene radical cation.
in the CF
2
ClCFCl
2
matrix where, in contrast to the CF
2
ClCCl
3
According to DFT calculations, the coupling to two of these
matrix, ion-molecule reactions can occur as the temperature
is raised to >100 K (see above). The product observed by ESR
after NIR photolysis of species A in this matrix, followed by
annealing to 110 K shows couplings of a(4H) ) 21.2 G and
protons is 18.2 G, whereas that to the other two is only -2.8 G
(
47) Kochi, J. K.; Krusic, P. J. J. Am. Chem. Soc. 1970, 92, 4110.
(48) Bally, T.; M u¨ ller, B.; Gerson, F.; Qin, X.-Z.; von Seebach, M.;
Kozhushkov, S. I.; de Meijere, A.; Borovkov, V. I.; Potashov, P. A.
J. Phys. Chem. A 2006, 110, 1163.
(
46) Qin, X.-Z. Ph.D. Thesis, University of Tennessee, 1987, pp 142-
43.
(49) Gerson, F.; Schmidlin, R.; de Mejiere, A.; Sp a¨ th, T. J. Am. Chem.
Soc. 1995, 117, 8431.
1
1
4654 J. AM. CHEM. SOC. 9 VOL. 132, NO. 41, 2010

Rearrangement of Ionized [1.1.1]Propellane
A R T I C L E S
Scheme 4
it is driven by the additional formation of a (partial) π-bond in
•+
the allene moiety of 4 . A summary of the conclusions from
the above ESR experiments is given in Scheme 4.
•+
To corroborate the assignment of A to 3 , we calcluated the
spectroscopic properties of this species by quantum chemical
methods, to see if they match with our observations. First we
computed the proton ESR hyperfine coupling constants a
3
H
of
by the B3LYP functional with the 6-31G* basis set, a
method that has been shown to give reasonably reliable
predictions of a in hydrocarbons and in particular in the allyl
radical. For the allylic hydrogens the predicted a are -15.1
•+
H
5
3
H
(
endo) and -15.8 G (exo), in good agreement with experiment,
Figure 7. (a) ESR spectrum observed at 90 K after ionization of 1 in the
which showed couplings of ca. 14.8 G for these four protons.
CF
2
Cl-CFCl
2
matrix (this is a poorly resolved version of the spectrum of
shown also in Figures 2 and 5a); (b) same after NIR photolysis (cf.
The coupling to the allenic protons is predicted to be -6.4 G
•+
3
(
expt, 5.4 G). Both results point toward a slight overestimation
Figure 6b); (c) same after subsequent annealing to 110 K (which in this
matrix leads to ion-molecule reactions). The magnetic field markers are
at 3240.5 (left) and 3377.5 G (right), respectively.
of both π-σ and π-π spin polarization by the B3LYP
functional. A summary of the measured and calculated hyperfine
coupling constants for 3• and 4 as well as species B and C
and some reference values from the literature are given in Table
S1 of the Supporting Information.
+
•+
(
2
the coupling to the terminal CH group is negligibly small).
At the resolution that can be attained in Freon matrices, only
the larger of the two couplings (experimentally, 14.6 G) is
resolved. Actually, at higher temperatures, the twisted structure
of 4 begins to automerize through a perpendicular structure,
which expresses itself in an alternating line width effect in the
ESR spectrum. This process, as well as the structure of 4 and
its mode of interconversion with 3 , will be discussed in more
detail in a forthcoming publication.
For modeling the vibrational structure of 3• we used the same
method. On linear scaling of the calculated frequencies by the
recommended scaling factor of 0.9513, 13 observed vibrational
frequencies of A were reproduced with an rms deviation of 20.8
+
•+
•+
-1
cm , while the relative intensities of the corresponding IR bands
•
+
were in good accord with the observed ones (see Table S3 in
5
0
the Supporting Information for details). On scaling of the force
•+
54,55
On the basis of the previous assignment of species A to 3 ,
field by the SQM method of Pulay
(27 internal coordinates
•
+
its NIR photochemical conversion to 4 suggests a formal
intramolecular electron transfer to generate the corresponding
allyl cation, which then undergoes a disrotatory cyclization in
accordance with the low-energy pathway predicted by the
Woodward-Hoffmann rules for such electrocyclic reactions.
Although the reverse process is generally observed whereby
divided into 10 groups whose valence force constants are scaled
5
6
differently ) the rms error between calculated and predicted
-
1
frequencies dropped to 7.1 cm (see Supporting Information),
leaving little doubt that the IR assigment is indeed correct.
To model the very peculiar electronic absorption spectrum
51,52
•+
that we would like to attribute to 3 , we explored the electronic
structure of this species. As the ESR spectrum shows, the spin
is mainly located in the allylic moeity and so is therefore the
singly occupied MO. Figure 8 shows the π-MOs of 3• that
51
cyclopropyl cations readily open to allyl cations, the cycliza-
tion is likely to be the exothermic step in the present case since
+
(
(
(
50) Bally, T.; M u¨ ller, B.; Williams, F.; Pappas, R. Manuscript in
preparation.
(53) Batra, R.; Giese, B.; Spichty, M. J. Phys. Chem. 1996, 100, 18371.
(54) Pulay, P.; Fogarasi, G.; Pang, F.; Boggs, J. E. J. Am. Chem. Soc. 1979,
101, 2550.
51) Woodward, R. B.; Hoffmann, R. The ConserVation of Orbital
Symmetry; Verlag Chemie: Weinheim, 1970.
52) Longuet-Higgins, H. C.; Abrahamson, E. W. J. Am. Chem. Soc. 1965,
(55) Bally, T.; Tang, W.; Zhang, Y. J. Am. Chem. Soc. 1993, 97, 43645.
(56) M u¨ ller, B. Ph.D. disseration No. 1299, UniVersity of Fribourg, 2000.
8
7, 2045.
J. AM. CHEM. SOC. 9 VOL. 132, NO. 41, 2010 14655

A R T I C L E S
M u¨ ller et al.
near-UV range (barring a symmetry-forbidden transition at 365
nm that is not listed in Table 1), until a pair of very strong
transitions follows at 211 and 200 nm that we could, however,
not see because this region is masked by absorptions of neutral
. Thus, despite its very low-lying first excited state, 3• is
+
1
entirely colorless, but the predictions listed in Table 1 are in
good agreement with the observation of a single band of species
A at ca. 1400 nm (0.88 eV) and no other absorptions above
2
50 nm (cf. Figure 3, lower part).
Mechanism of the Spontaneous Decay of 1 to 3 . To
•
+
•+
elucidate the mechanism of this surprising reaction, we modeled
the potential surface that connects 1• to 3 by quantum
chemical calculations (in what follows, all relative energies are
from single-point CCSD(T)/cc-pVTZ calculations at the B3LYP/
+
•+
6
-31G* optimized geometries).
As expected, the D3h structure of 1• is a potential energy
+
minimum at the UHF, ROHF, UMP2, and QCISD levels, with
bond lengths that differ only little from those of 1. However, at
the B3LYP/6-31G* level, the D3h structure is a transition state
having a degenerate imaginary normal mode with a frequency
-1
of -202 cm depicted in Figure 9 (at the other levels of theory
-
1
the same mode has a positive frequency, e.g., +195 cm by
QCISD/6-31G*).
Following component (a) of that mode, which entails a
distortion to C symmetry, leads to a marginally more stable
s
potential energy minimum where one of the lateral bonds has
increased its length by 0.1 Å. Further stretching of that bond to
1
.86 Å leads to a transition state, 1.15 kcal/mol above the
Figure 8. Molecular orbitals of 3• that consistute the active space used
+
shallow minimum, at the far side of which the bond undergoes
complete cleavage to form a species 5 that may be described
in the CASSCF calculations shown in Table 1.
•+
Table 1. Excited States of 3•+ by CASPT2 Calculations
as a methylenebicyclobutyl radical cation (Figure 10, the SOMO
of 5• is shown in Figure 9).
+
a
CASPT2
nm
CASSCF
•+
Stretching the long central C-C bond in 5 leads to
b
c
states
eV
f
configurations
planarization of the carbon frame to give the radical cation of
12
12
12
A
90% (1a
)1
2
2
B
2
1
0.80
3.64
1543
0.007
81% 1a
2
f 3b
2
-6
B
341
239
2 × 10
60% [1a
6% 2b
2
f 4b
2
+ 2b
1
f 4b
2
]
1
1
f 1a
2
22
B
5.18
0.006
51% 1a
1% 2b
f 4b
2
f 1a
2
2
2
2
3
22
22
32
A
5.32
5.36
5.87
233
231
211
6 × 10
1 × 10
0.572
-4
-6
85% [1a
2 1
75% 1a f 6b
f 6b
+ 5b
+ 2b
f 1a
f 1a
]
]
2
2
1
1
2
2
B
1
A
2
66% 2b
5% [1a
2
f 4b
f 4b
2
1
2
2
2
32
B
2
6.20
200
0.226
61% [1a
1% [2b
f 4b
f 3b
+ 5b
+ 1a
f 3b
f 3b
]
]
2
2
1
2
2
1
2
1
1
a
The active space comprises the MOs shown in Figure 8. Averaging
was done over three 2
2
2
2 1 2
A , three B , and four B states, respectively.
Reference weights for the zero-order CASSCF wavefunctions were
b
>
80% for all CASPT2 states. Oscillator strength for electronic
c
transition. In terms of excitations between the MOs depicted in Figure
8
.
were all included in the active space of a CASSCF/CASPT2
calculation, the results of which are shown in Table 1.
This table shows that, in contrast to most radical cations, the
lowest energy transition (predicted at 0.803 eV or 1543 nm)
involves excitation of the unpaired electron (which occupies
Figure 9. (a) The two compontents of the e′′ imaginary normal mode of
1
(calculated in D3h symmetry at the B3LYP/6-31G* level; the symmetry
elements that remain on distortion along each component of the e′′ mode
are indicated). (b) The two components of the e′′ MO of 1. (c) The species
the purely allylic a
2
-MO) into a Virtual MO. Following this
•+
•+
5
that results from the distortion of 1 along component (a) of the e′′
•+
transition there is a gap that spans the entire visible and the
mode (the MO is the SOMO of 5 ).
1
4656 J. AM. CHEM. SOC. 9 VOL. 132, NO. 41, 2010

Rearrangement of Ionized [1.1.1]Propellane
A R T I C L E S
Figure 10. Potential energy surface for the decay of 1• to 3•+ via intermediates 5•+ and 2 . Relative energies are from CCSD(T)/cc-pVTZ//B3LYP/6-
+
•+
3
1G* calculations and include the contributions from zero-point vibrational energies. Numbers in parentheses indicate energies of transition states TS1-TS6
1
relative to the preceding minima. Italic numbers indicate C-C bond lengths in Å. If no symmetry is indicated, it is C . The same figure, complemented by
relative CASPT2 energies, is given as Figure S7 of the Supporting Information.
methylenecyclobutylidene, which represents actually the transi-
tion state TS2 for inversion of the bicyclobutyl moiety, 12.57
latter stabilization, and we believe that this is the reason why
the barrier for decay of 5• by cleavage of a lateral bond is so
low.
+
•+
kcal/mol above 5 . Only 1.54 kcal/mol above that structure
lies another transition state (TS3 in Figure 10) for a [1,2] shift
of one of the hydrogen atoms to the (carbenoid) apical carbon
atom, on the far side of which the species decays in a highly
exothermic process to the radical cation of methylenecy-
Although the minimum energy structure of 2• is slightly
+
distorted to C
2
symmetry (by twisting the two exocyclic CdC
bonds by ca. 3°), this structure appears as a stable potential
5
7
energy minimum. Nevertheless, stretching of one of the two
lateral bonds requires surprisingly little energy, and the transition
state for cleavage of this bond was found to lie only 2.09 kcal/
•+
clobutene, 6 . We will return to this species in a separate
5
0
paper that will deal with the assignment of the bands that
•+
•
+
mol above 2 , which therefore seems to decay to the observed
cannot be attributed to 1 in the spectra shown in Figures 3
•+
product, 3 , in a nearly barrierless process. Thus, to sum up
and 4. In the present context it suffices to note that a barrier of
•+
this section, the radical cation of [1.1.1]propellane, 1 , decays
14 kcal/mol is probably too high to be crossed by a thermalized
spontaneously, by consecutive cleavage of three lateral bonds,
species at 7 K, so another decay channel must be available to
•
+
•
+
via two fleeting intermediates, to the allylic species 3 . An
5
.
illustration of the potential surface that leads from 1• to 3 is
+
•+
Such a channel opens on stretching the second longest (1.57
given in Figure 10.
•
+
Å) lateral bond in 5 , which leads, at 1.93 Å, to a transition
We will now turn to the question of why the radical cation
of [1.1.1]propellane undergoes such a facile distortion and why
•
+
state TS4 that lies less than 3 kcal/mol above 5 . On the far
side of TS4 the species decays to the radical cation of
•+
the barriers for further decay to 3 are so surprisingly low.
•+
dimethylenecyclopropane, 2 . TS4 actually shows some struc-
tural similarity to TS5, the transition state for rotation of an
(57) The finding of 2• as a seemingly stable product of the decay of 1•+
led us to examine whether this species might be responsible for the
non-bleachable bands in the optical (Figure 3) and the IR spectrum
+
•
+
exocyclic π-bond of 2 . The barrier for this rotation is
surprisingly low (18.7 kcal/mol), which is due on the one hand
to the weakening of that π-bond by ionization and on the other
hand to the allylic resonance that TS5 enjoys. TS4, which lies
only 1.36 kcal/mol above TS5, appears to profit also from the
•+
(
Figure 4) we had observed after ionization of 1 . However, complete
lack of accord with the calculated electronic (CASPT2) and vibrational
B3LYP) spectra for 2 (see separate paper, ref 50) convinced us
that this is not the case.
•+
(
J. AM. CHEM. SOC. 9 VOL. 132, NO. 41, 2010 14657

A R T I C L E S
M u¨ ller et al.
Figure 12. Schematic representation of the transition density arising from
2
2
the interaction of the A
1
ground state with the two components of the E′′
•+
excited state of 1 , and the weakening of bonds it induces.
point on the B3LYP potential surface. The corresponding
imaginary mode resembles component (a) of the e′′ mode and
minimum of 1• to which
+
it points to the slightly lower-lying C
s
the species decays upon unconstrained geometry optimization.
2
•+
The spontaneous distortions that the A ′ ground state of 1
1
undergoes (at the B3LYP/6-31G* level) are therefore a conse-
quence of its mixing, through deformations along the two
•+
Figure 11. Distortions of 1 along the two components of the e′′ mode.
Dashed lines denote (qualitative) potential surfaces in the absence of vibronic
coupling, solid line with vibronic coupling. Numbers denote relative energies
from (TD)B3LYP/6-31G* calculations.
2
components of an e′′ mode, with the first E′′ excited state, which
1
3
according to photoelectron spectroscopy (see Figure 1) lies
only 1.56 eV above the ground state.
Figure 11 shows that the above deformations do not lead to
a pronounced splitting of the degeneracy nor to a stabilization
•
+
Role of Vibronic Coupling in Destabilizing 1 . The e′′ normal
mode along which 1• distorts so easily (even spontaneously
+
2
of the lower component of the E′′ excited state, in contrast to
-1
on the B3LYP potential surface) has a frequency of +1167 cm
in neutral 1, and to cleave a lateral bond in 1 (to form
-methylenecyclobutylidene) requires over 40 kcal/mol. What
is it that makes this process become so much more facile after
ionization? The answer to this question can be found by
considering the potential surfaces of the excited states and how
the e′ modes that lead to structures of C2V symmetry and to a
24
pronounced Jahn-Teller (JT) effect (see Figure S6 in the
3
Supporting Information). However, the pair of excited states
2
2
that result from this JT distortion ( A
2 1
and B ) both still have
2
a symmetry different from that of the ground sate ( A
1
), and
hence JT distortions do not entail a coupling of the ground state
2
5
these interact, upon suitable distortion, with the ground state,
2
to the E′′ excited state.
as illustrated in Figure 11.
An alternative way to look at this vibronic coupling effect is
to follow the development of the vibrationally induced transition
For 1• in D3h symmetry, the singly occupied MO (SOMO)
in the ground state (a ′) and that in the first excited state (e′′,
+
2
1,58-61
1
density
the two components of the e′′ MO of 1 , as illustrated in Figure
2. Considering first the a ′-e′′(a) interactions, it can be seen
1
resulting from the mixing of the a ′ orbital with
cf. Figure 9) belong to different irreducible representations, and
hence the corresponding states cannot mix. Distortion along
component (a) of the e′′ vibration reduces the symmetry from
•+
1
1
that the resulting increase (+) and decrease (-) of electron
density are symmetric with respect to the vertical plane shown
on the left-hand side of Figure 11, thus giving rise to a transition
2
2
D3h to C
s
, i.e., the E′′ state splits into a A′ state (which has
now the same symmetry as the ground state in C and can hence
s
2
mix with it) and a A′′ state (which cannot mix with the ground
s
density of C symmetry. The nuclei respond to this transition
state). Similarly, distortion along component (b) of the e′′
density mainly by shortening one and elongating the other of
the apical C-C bonds (at the same time the pair of C-C bonds
adjacent to the shortened bond lengthen and those adjacent to
the elongated bond shorten slightly, but this effect will be
weaker). This deformation, which corresponds to that along the
2
vibration reduces the symmetry from D3h to C
2
, i.e., the E′′
2
state splits into a A state (which has now the same symmetry
as the ground state) and a B state (which cannot mix with the
ground state). Thus, in both cases only one of the “branches”
s 2
of the surfaces for distortion of the excited state to C or C
2
symmetry, respectively, is effective in undergoing vibronic
coupling to the ground state, while the other is not. Note that
(
58) Longuet-Higgins, H. C. Proc. R. Soc. A 1956, 235, 537.
(59) Bader, R. F. W. Can. J. Chem. 1962, 40, 1164.
(
(
60) Salem, L. Chem. Phys. Lett. 1969, 3, 99.
2
the SOMO is not the same in the A branch (in C ) and in the
61) Vibronic coupling in radicals is suitably enhanced, compared to closed-
shell molecules, by the relatively small energy gap (≈ 0.5-1.5 eV)
that usually exists between their ground state and those excited states
that arise by promotion of an electron from the doubly occupied to
the close-lying singly occupied MO. On the other hand, excited states
of closed-shell molecules invariably arise by promotion of electrons
into virtual MOs, which involves typically much larger energy gaps
(≈ 4 eV).
A′ branch (in C ) that couple to the ground state.
s
•
+
We have already seen that, if one allows 1 to distort along
component (a) of the e′′ mode, this leads to a slight stabilization,
and to a potential energy minimum (by B3LYP/6-31G*).
Allowing distortion along component (b), i.e., descent to C
symmetry, leads to a similar stabilization but to a flat saddle
2
1
4658 J. AM. CHEM. SOC. 9 VOL. 132, NO. 41, 2010

Rearrangement of Ionized [1.1.1]Propellane
A R T I C L E S
e′′(a) normal mode in Figure 9, leads to the initial formation of
•
+
•+
5
in the stepwise process that ultimately yields 3 .
Turning to the a ′-e′′(b) interaction, one finds by analogy that
1
•+
a pair of unconnected C-C bonds in 1 undergo weakening
while the opposite diagonal pair of bonds is strengthened. If
this distortion (which corresponds to that along the e′′(b) normal
mode) is carried further, it leads to the simultaneous cleavage
•+
of two bonds, i.e., the direct formation of 2 . We have followed
this distortion computationally, but unsurprisingly the C
transition state that is reached along this coordinate lies much
2
•
+
higher than that leading to 5 by cleavage of only one bond.
•+
Further Rearrangements. The case of 1 illustrates well the
importance of vibronic coupling in weakening normally robust
bonds in radical cations. Thus we might ask whether (or to what
extent) this effect is also responsible for the facile further decay
•
+
on the way to the final observed product, 3 . What is required
for vibronic coupling to be effective is a low-lying excited state
6
2
(
which does not necessarily need to be the first one! ) that can
couple to the ground state by a normal mode of suitable
6
3
symmetry and has a moderate force constant, conditions that
•
+
are well fulfilled in 1 .
Figure 13. Photoelectron spectrum of 2, highest four occupied MOs, and
derivative coupling vectors for the coupling of the A and the B states.
2 1
2
2
If we turn to 5•+ we note that the ground state has A′
2
2
symmetry (cf. Figure 9c), so a A′′ state is needed to induce
Scheme 5
•
+
s
loss of C symmetry. TD-DFT calculations on 5 predict that
the lowest such state lies 4.35 eV above the ground state, which
makes effective coupling of the two states very unlikely. Indeed,
we have argued above that it actually is the gain in allylic
resonance energy, already at the transition state TS4, that drives
the decay of 5• to 2 .
+
•+
•
+
More intriguing is the case of 2 , where a normally quite
exocyclic bond lengthens, while the other one shortens (see
structure “dist-b ” in Scheme 5, which is attained by following
2
3
robust C(sp )-C(sp ) σ-bond breaks readily to yield the final
2
•+
product, 3 , in a process that is exothermic by less than 6 kcal/
mol. In an effort to understand the electronic structure of 2•
and its possible consequences on the surprising reactivity of
the derivative coupling vector shown in Figure 13). Similar
distortions occur on the way from 2• to TS6, which indicates
that these distortions are indeed facilitated by vibronic coupling
+
+
•
+
2
•+
2
2
, we measured its photoelectron spectrum (Figure 13) and
assigned it with the help of DFT calculations.
These calculations leave no doubt that the highest two
2 1
molecular orbitals are π-MOs of a and b symmetry, respec-
tively, and are followed by two σ-MOs of the Walsh type to
which the shoulder at 10.9 and the peak at 11.5 eV may be
2 1
between the A ground state of 2 and its B excited state.
-
1
However, with a (reduced) frequency of 1364 cm for this
mode, such distortions are still far from being spontaneous.
Hence this mode cannot represent the only perturbation leading
to the rearrangement of 2 , one possibility being that another
asymmetric mode of lower frequency takes over the cleavage
process at slightly distorted geometries.
•+
6
3
2
2
attributed. The B
1
first excited state is separated from A
) b
2
the ground state by only 1.4 eV, and the modes of a
2
× b
1
2
We have seen previously that higher excited states of a radical
cation may induce or facilitate distortions of the ground state
symmetry, which are apt to couple those two states, would in
principle be capable of inducing asymmetric distortions of a
kind that may eventually lead to cleavage of a lateral bond in
6
2
via vibronic coupling. At this point we may recall that the
equilibrium geometry of 2• does not have the C2V symmetry
+
•
+
2
. To see whether this is the case, we determined, by a (3 ×
of neutral 2 but was found to distort slightly to C , by following
2
3
) CASSCF calculation, the derivative coupling vector (i.e., the
an asymmetric twisting mode of the exocyclic CH groups. This
2
2
-1
nuclear displacements that lead to optimal coupling) of the A
2
a mode has a frequency of 780 cm in neutral 2, and its
lowering (to -66 cm ) in 2 points toward vibronic coupling.
Indeed, a modes are apt to couple the A ground state to the
2
2
-1
•+
and the B
to the b asymmetric CdC stretching mode (which has also
some contributions from CH scissoring). The frequency of this
mode is indeed lowered from 1711 cm in 2 to 1364 cm in
1
states. This vector (shown in Figure 13) corresponds
2
2
2
2
2
2
1
A excited state, and this particular twisting mode seems to be
-1
-1
very effective at that (unfortunately we are unable to calculate
derivative coupling vectors for higher excited states). Following
this very soft mode beyond the equilibrium structure will lead
to the σ-π mixing that is prerequisite for cleavage of a lateral
σ-bond, Thus, it could be that the combined effects of
•
+
2
, whereas the frequency of the symmetric CdC stretching
mode decreases only by 53 cm on ionization.
Apart from affecting the lengths of the exocyclic C-C bonds,
2
distortions along this b mode also change the lengths of the
-1
2
2
•+
lateral ring bonds, i.e., that which is geminal to the shortening
1 1
vibronically coupling the B and the A excited states of 2
2
2
to its A ground state are responsible for lowering the barrier
•+
for its rearrangement to 3 .
(
62) Bally, T.; Bernhard, S.; Matzinger, S.; Roulin, J.-L.; Sastry, G. N.;
Truttmann, L.; Zhu, Z.; Marcinek, A.; Adamus, J.; Kaminski, R.;
Gebicki, J.; Williams, F.; Chen, G.-F.; F u¨ lscher, M. P. Chem.sEur.
J. 2000, 5, 858.
Another possible reason for the occurrence of a low-lying
transition state would be an avoided state crossing (conical
intersection) that occurs near that transition state. We have
examined this possibility, by optimizing the geometries of the
(
63) Modes that couple two states must belong to the irreducible representa-
tion that results from the direct product of those of the two states.
J. AM. CHEM. SOC. 9 VOL. 132, NO. 41, 2010 14659

A R T I C L E S
M u¨ ller et al.
Figure 14. Energy profile and evolution of the SOMO for the 2• f 3•+ rearrangement (from B3LYP/6-31G* calculations).
+
•
+
lowest (excited) states of all four symmetries (see Figure S5 in
cation of dimethyleneallene, 3 . This extensive rearrangement
and the unusual structure of 3• have been established by matrix
ESR, IR, and UV-vis spectroscopy and is further confirmed
by its photochemical rearrangement to the radical cation of
+
the Supporting Information). However, none of those excited
2
states falls below the A
2
state or correlates with the ground
state of 3• (at TS6 the lowest excited state lies 2.36 eV above
the ground state which practically excludes a conical intersection
anywhere near TS6).
+
•+
vinylidenecyclopropane, 4 . A detailed quantum chemical study
finds that the pivotal cleavage of the first lateral bond in 1 is
•+
Finally, in the hope of obtaining a hint as to the precise nature
of this surprising rearrangement, we followed the evolution of
the SOMO in the course of the reaction (Figure 14). This shows
clearly the necessity of σ-π mixing in promoting the reaction
2
mediated by strong vibronic coupling between the A′ ground
state and the 2E′′ first excited state of 1 . The resulting
•+
•+
methylenebicyclobutyl radical cation 5 decays further to the
•+
radical cation of dimethylenecyclopropane, 2 , which in turn
cleaves spontaneously a lateral σ-bond to yield finally 3• (which
is also obtained by ionization of 2). Significantly, the present
study provides a clear and unambiguous example of the role of
vibronic coupling in determining the course of a chemical
reaction. It is particularly gratifying that the chemistry reported
here can be linked to the earlier observation of the unusual
(
see above), but there are no unusual or abrupt changes in the
+
nodal properties of the wave function, which could indicate a
switching of states. Thus, it is not immediately obvious why
the lateral bond in 2• breaks as easily as it does.
+
Returning to Figures 3 and 4 we note that, although the ESR
electronic absorption, and vibrational spectra unambiguously
•
+
testify to the presence of 3 , these spectra show additional
features that require further explanation. In particular, the
UV-vis spectrum shows an intense transition peaking at 310
nm and a weaker one with λmax ≈ 490 nm that cannot be
attributed to 3• and are indicative of a conjugated diene radical
cation system. In the IR spectra we note several peaks that
incerase on ionization but are not bleached with NIR light in
the region between the intense peak at 1870 cm- (which is
13
vibrational structure in the photoelectron spectrum of 1, both
effects being a direct consequence of vibronic coupling.
+
Acknowledgment. The workers in Fribourg would like to thank
the Swiss National Science Foundation for the support that they
have provided over the many years during which this project was
pursued. They are very indebted to the group of Prof. Michael Allan
for measuring the photoelectron spectrum of 2. The research at the
University of Tennessee was supported by the Division of Chemical
Sciences, Office of Basic Energy Sciences, U.S. Department of
Energy under Grant No. DE-FG02-88ER13852. The Tennessee
authors thank Dr. J. T. Wang and Mr. G.-F. Chen for their assistance
with the ESR experiments in the early stages of this work.
1
•+
characteristic of the cumulated double bond character of 3 )
-1
and ca. 2050 cm , i.e., a region that is indicative of allenes.
We will return to the assigment of these spectroscopic features,
which result from further rearrangements that appear to occur
5
0
only in Ar matrix experiment, in a forthcoming publication
that will also deal with the radical cation of vinylidenecyclo-
•
+
propane 4 and its photoconversion from 3 (cf. Figures 6 and
).
7
Supporting Information Available: UV-vis and IR spectra
of ionized 2, ESR spectrum of ionized 4, results of SQM
calcuations of the IR spectrum of 3 , state correlation diagram
for 2 , JT-surface for the E′′ excited state of 1 , Figure 10
with CASPT2 energies, and a text file contains the full ref 37
and the Cartesian coordinates, energies and thermal corrections
of all stationary points in Figure 10. This material is available
free of charge via the Internet at http://pubs.acs.org.
Conclusion
•+
•+
2
•+
Upon oxidation, [1.1.1]propellane, 1, undergoes spontaneous
cleavage of three lateral C-C bonds to yield the acyclic radical
JA106024Y
1
4660 J. AM. CHEM. SOC. 9 VOL. 132, NO. 41, 2010