X-ray-induced transformation of o-vinylbenzaldehyde and 2-methylbenzocyclobutenone to an o-quinoid ketene and its radical cation

Article abstract of DOI:10.1021/ja964243w

Upon ionization in argon matrices by X-irradiation, o-vinylbenzaldehyde (VBA) partially undergoes transfer of the formyl hydrogen atom to the vinylic CH₂ group, thereby forming a quinoketene radical cation of the type previously described (Bally, T.; Michalak, J. J. Photochem. Photobiol. A: Chem. 1992, 69, 185). This process does not manifest itself clearly in the optical spectra because the transitions of the quinoketene cation coincide with the much more intense ones of VBA^.+ and are mostly obscured by those. However, the IR spectra show unambiguously that the rearrangement occurs rather efficiently. Also they show that in the course of the X-irradiation, the tautomerized cations are reneutralized to form the quinoketene tautomer of VBA in good yield. The quinoketene and its radical cation were independently obtained from 2-methylbenzocyclobutenone which undergoes spontaneous ring-opening on photolysis and/or ionization. The optical spectra of the radical cations are assigned on the basis of CASPT2 calculations and the different species involved in this study are characterized at the B3LYP/6-31G* level.

Full text of DOI:10.1021/ja964243w

J. Am. Chem. Soc. 1997, 119, 2825-2831
2825
X-ray-Induced Transformation of o-Vinylbenzaldehyde and
2-Methylbenzocyclobutenone to an o-Quinoid Ketene and Its
Radical Cation
Krzysztof Huben,†,‡ Zhendong Zhu,‡ Thomas Bally,*,‡ and Jerzy Gebicki*,§
Contribution from the Institute of Physical Chemistry, UniVersity of Fribourg, Pe´rolles,
CH-1700 Fribourg, Switzerland, and Institute of Applied Radiation Chemistry,
Technical UniVersity of Lodz, 90-924 Lodz, Poland
ReceiVed December 9, 1996X
Abstract: Upon ionization in argon matrices by X-irradiation, o-vinylbenzaldehyde (VBA) partially undergoes transfer
of the formyl hydrogen atom to the vinylic CH2 group, thereby forming a quinoketene radical cation of the type
previously described (Bally, T.; Michalak, J. J. Photochem. Photobiol. A: Chem. 1992, 69, 185). This process does
not manifest itself clearly in the optical spectra because the transitions of the quinoketene cation coincide with the
much more intense ones of VBA•+ and are mostly obscured by those. However, the IR spectra show unambiguously
that the rearrangement occurs rather efficiently. Also they show that in the course of the X-irradiation, the tautomerized
cations are reneutralized to from the quinoketene tautomer of VBA in good yield. The quinoketene and its radical
cation were independently obtained from 2-methylbenzocyclobutenone which undergoes spontaneous ring-opening
on photolysis and/or ionization. The optical spectra of the radical cations are assigned on the basis of CASPT2
calculations and the different species involved in this study are characterized at the B3LYP/6-31G* level.
1. Introduction
2-methylbenzocyclobuten-1-one (MBC)7,8 led us to investigate
the fate of VBA and MBC on ionization by X-irradiation in
solid Ar.
Phototautomerizations (such as for example enolizations of
ketones)1 or photoinduced valence isomerizations (such as
pericyclic reactions)2 represent widely accepted and well-
documented phenomena in neutral chemistry. Conversely, the
corresponding reactions induced by electron transfer are just
beginning to be explored systematically and it is found that they
often take a different course than in the neutrals.3,4 Frequently
the order of stabilities of two isomeric forms is reversed by
oxidation or reduction such that processes which require
activation by light may occur spontaneously in the radical ions.
This was found to be the case in several classes of tautomers
where enol forms are generally more stable than keto forms. In
the case of cycloreversions or ring-opening reactions the order
of stabilities is usually not reversed from the neutrals, but the
activation barriers are lowered to the extent that the rearrange-
ments occur spontaneously on ionization.5
Scheme 1. Photochemistry of Neutral o-vinylbenzaldehyde
(VBA)6
Recently, Kessar et al. found that o-vinylbenzaldehyde (VBA)
undergoes a novel kind of phototautomerization in that it does
not form the expected hydroxyvinylallene (HVA), but instead
reacts from a less stable rotamer to transfer the formyl hydro-
gen atom and generate an o-quinoid ketene, 6-ethylidene-
2,4-cyclohexadiene-1-ylidenemethanone (ECM). This and
the fact that ECM could presumably also be generated from
2. Methods
2.1. Syntheses. o-Vinylbenzaldehyde (VBA) was prepared from
3,4-dihydroisoquinoline as reported by Dale et al.9 and purified by
distillation (113-115 °C/18 mmHg). 2-Methylbenzocyclobuten-1(2H)-
one (MBC) was obtained by flash vacuum pyrolysis of 2-(1′-hydroxy-
1′,2′-dihydro-2′-methylbenzocyclobuten-1′-yl)-2,4-dimethylpen-
tanone,10 which was in turn prepared by photolysis of 1-(o-ethylphenyl)-
2,2,4-trimethylpentane-1,3-dione (100 h of irradiation by a 200-W
medium-pressure Xe-Hg arc through a Pyrex filter).10 The precursor
† On leave from the Technical University of Lodz, Poland.
‡ University of Fribourg.
§ Technical University of Lodz.
X Abstract published in AdVance ACS Abstracts, March 1, 1997.
(1) Rappoport, Z., Ed. The Chemistry of Enols; Wiley: Chichester, 1990.
(2) See e.g.: Kopecky, J. Organic Photochemistry: A Visual Approach;
VCH Publishers: New York, 1991; p 77 ff.
(3) (a) Bally, T. In Radical Ionic Systems; Lund, A., Shiotani, M., Eds.;
Kluwer: Dordrecht, 1991. (b) Gebicki, J. Pure Appl. Chem. 1995, 67, 55.
(4) Gebicki, J.; Bally, T. Acc. Chem. Res. Submitted for publication.
(5) See e.g.: Dunkin, I. R.; Andrews, L. Tetrahedron 1985, 41, 145.
Aebischer, N.; Bally, T.; Roth, K.; Haselbach, E.; Gerson, F.; Qin, X.-Z. J.
Am. Chem. Soc. 1989, 111, 7909.
(7) Hacker, N. P.; Turro, N. J. J. Photochem. 1983, 22, 131.
(8) Bally, T.; Michalak, J. J. Photochem. Photobiol. A: Chem. 1992,
69, 185.
(9) Dale, W. J.; Starr, L.; Strobel, C. W. J. Org. Chem. 1961, 26, 2225.
(10) Yoshioka, M.; Momose, S.; Nishizawa, K.; Hesagawa, T. J. Chem.
Soc., Perkin Trans. 1 1992, 499. The pyrolysis in the inlet of the GC column
did not work in our experience.
(6) Kessar, S. V.; Mankotia, A. K. S.; Scaiano, J. C.; Barra, M.; Gebicki,
J.; Huben K. J. Am. Chem. Soc. 1996, 118, 4361.
S0002-7863(96)04243-6 CCC: $14.00 © 1997 American Chemical Society

2826 J. Am. Chem. Soc., Vol. 119, No. 12, 1997
Huben et al.
dione was obtained by condensation of 2,4-dimethylpentan-3-one with
o-ethylbenzaldehyde followed by Jones oxidation.11
2.2. Matrix Isolation and Spectroscopy. A 1:1 mixture of VBA
or MBC and CH2Cl2 (which acts as an electron scavenger) was diluted
with a 1000-fold excess of argon and deposited on a CsI plate held at
20 K. After the sample was cooled to the lowest temperature attainable
by the closed-cycle cryostat (12 K), the sample was exposed to 90
min of X-irradiation as described previously.3,12 Photolyses were
effected with a 1 kW Ar plasma arc with use of interference filters.
Electronic absorption (EA) spectra were taken between 190 and 1500
nm with a Perkin Elmer Lambda 19 instrument whereas IR spectra
were obtained on a Bomem DA3 interferometer (1 cm-1 resolution)
equipped with an MCT detector (500-4000 cm-1).
2.3. Quantum Chemical Calculations. The geometries of all
species were optimized by the B3LYP density functional method13 as
implemented in the Gaussian 94 suite of programs,14,15 using the 6-31G*
basis set. All potential energy minima for the most stable rotamers
(and, in the case of neutral VBA, also those of the others) were
identified by Hessian calculations which yielded also some valuable
information on the IR spectra.
Excited state calculations on the radical cations were carried out at
the B3LYP/6-31G* geometries by the CASSCF/CASPT2 procedure16
with the MOLCAS program.17 The π excited states were calculated
with a (9π,10π) active space.18 In order to ensure orthogonality, the
CASSCF wave function was averaged over the five lowest π excited
states which resulted in a satisfactory description of these. In all cases
the final CASPT2 states were described to >65% by the first-order
CASSCF wave function and no single state outside the active space
contributed by more than 1%. Any attempt to calculate higher excited
states resulted in severe problems with intruder states in the CASPT2
part whose remediation would have required an extension of the active
space beyond the limits imposed by the program and hardware. Due
to the expected small oscillator strengths of σ f π transitions, σ states
were not considered. Satisfactory agreement with experiment was
obtained with the simple [C]3s2p1d/[H]2s ANO DZ basis set,19
therefore we saw no necessity to add higher angular momentum and/
or diffuse functions. Transition moments were calculated on the basis
of the CASSCF wave functions, using CASPT2 energy differences in
the denominator.
Figure 1. (a) Difference spectrum after 90 min of X-irradiation of
MBC (the dashed line is the difference spectrum for 15 min of
photolysis of MBC or VBA through a Pyrex filter); (b) difference
spectrum for photolysis of the above sample at 365 nm and (c) for
subsequent 6 h of photolysis at >420 nm; (d) difference spectrum after
90 min of X-irradiation of VBA and (e) after subsequent 45 min of
photolysis at 600 nm.
3. Results
Our experimental findings are summarized in Figure 1 (elec-
tronic absorption, EA) and Figure 2 (IR spectra). There, traces
(11) Nielsen, A. T.; Gibbons, C.; Zimmerman, C. A. J. Am. Chem. Soc.
1951, 73, 4696.
(12) Bally, T. Chimia 1994, 48, 378.
(13) (a) Becke, A. D. Phys. ReV. A 1988, 39, 3098. (b) Becke, A. D. J.
Chem. Phys. 1993, 98, 5648. (c) Lee, C.; Yang, W.; Parr, R. G. Phys. ReV.
B 1988, 37, 785.
(a) show the spectral changes on X-irradiation of MBC while
the dashed lines indicate the changes on UV photolysis of MBC.
In both experiments MBC with its intense pair of IR peaks at
1776 and 1811 cm-1 undergoes ring-opening to the o-quinoid
ketene, ECM,8 which distinguishes itself by the characteristic
group of peaks near 2100 cm-1 (strongly split by site effects)
and by a broad EA band with λmax ) 380 nm (dashed lines).7,8
In addition to these features, X-irradiation gives rise to new IR
peaks around 2170 cm-1 and indicates also the loss of some
CO (2143 cm-1), whereas the EA spectrum shows a shoulder
at 430 nm and a weak, broad band peaking at 610 nm.
(14) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Gill, P. M. W.;
Johnson, B. G.; Robb, M. A.; Cheeseman, J. R.; Keith, T.; Petersson, G.
A.; Montgomery, J. A.; Raghavachari, K.; Al-Laham, M. A.; Zakrzewski,
V. G.; Ortiz, J. V.; Foresman, J. B.; Cioslowski, J.; Stefanov, B. B.;
Nanayakkara, A.; Challacombe, M.; Peng, C. Y.; Ayala, P. Y.; Chen, W.;
Wong, M. W.; Andres, J. L.; Replogle, E. S.; Gomperts, R.; Martin, R. L.;
Fox, D. J.; Binkley, J. S.; DeFrees, D. J.; Baker, J.; Stewart, J. P.; Head-
Gordon, M.; Gonzalez, M.; C.; Pople, J. A. Gaussian, Inc.; Pittsburgh: PA,
1995.
(15) For a description of the density functionals as implemented in the
Gaussian series of programs, see: Johnson, B. G.; Gill, P. M. W. L.; Pople,
J. A. J. Chem. Phys. 1993, 98. 5612.
(16) (a) Andersson, K.; Malmqvist, P.-A° .; Roos, B. O.; Sadlej A. J.;
Wolinski, K. J. Phys. Chem. 1990, 94, 5483. (b) Andersson, K.; Malmqvist,
P.-A° .; Roos, B. O. J. Chem. Phys. 1992, 96, 1218. (c) Andersson, K.; Roos,
B. O. In Modern Electronic Structure Theory; World Scientific Publish-
ing: Singapore, 1995; Part 1, Vol. 2, p 55.
Subsequent photolysis of the X-irradiated sample at 365 nm
leads to the disappearance of the 380-nm band and the associated
ketene stretching bands in the IR while the 1776/1811-cm-1
peaks of MBC reappear concomitantly, thus confirming the
photoreversibility of the (neutral) ring-opening reaction. In
addition, a new IR peak grows in at 1706 cm-1, which is due
to one of the rotamers of o-vinylbenzaldehyde (VBA),6 as will
be shown below (note that the same peak had already appeared
weakly after X-irradiation of MBC). Conversely, the IR bands
at 2170 cm-1 as well as the EA bands at 420 and 610 nm remain
virtually unaffected by this photolysis. Only on prolonged
(17) Molcas, Version 3: Andersson, K.; Blomberg, M. R. A.; Fu¨lscher,
M. P.; Kello¨, V.; Lindh, R.; Malmqvist, P.-A° .; Noga, J.; Olsen, J.; Roos,
B. O.; Sadlej, A.; Siegbahn, P. E. M.; Urban M.; Widmark, P.-O. University
of Lund: Sweden, 1994.
(18) In the case of VBA•+ we found it necessary to include the next
higher virtual π-MO to arrive at a satisfactory description of all π excited
states of interest, resulting in a (9,11) active space.
(19) Widmark, P.-O.; Malmqvist, P.-A° .; Roos, B. O. Theor. Chim. Acta
1990, 77, 291.

Formation of an o-Quinoid Ketene and Its Radical Cation
J. Am. Chem. Soc., Vol. 119, No. 12, 1997 2827
4. Discussion
4.1. Mechanistic Aspects. There is a considerable body of
experience which shows that X-irradiation of organic mole-
cules M embedded in argon matrices together with some elec-
tron scavenger S (CH2Cl2 in our case) leads primarily to
formation of the corresponding radical cations M•+ through hole
transfer from ionized argon while the electrons are trapped by
forming S•+.3a,12 In addition we found recently that, once
formed, the radical cations appear to compete efficiently with
S for the electrons which are liberated in the process of
X-irradiation and are thus reneutralized.8 This process limits
the yield of charged species that can be obtained in such
experiments, but it usually goes unnoticed because it re-forms
the starting M. However, if M•+ spontaneously undergoes some
rearrangement to isomeric N•+ (or loses a fragment such as
molecular nitrogen), then re-neutralization leads to formation
of neutral N which may be a photoproduct of M (cf. Scheme
2). In principle, this mechanism allows the net M f N
rearrangement to be carried to completion which is not always
possible otherwise.4
Scheme 2. Mechanism for the Formation of Neutral
Rearranged Products N by Way of Ionization/
Reneutralization on X-irradiation of Argon Matrices
We believe that all of the events observed in the present work
and described above can be explained on the basis of these three
processes, i.e. ionization, rearrangement, and re-neutralization.
Thus, upon ionization in argon, both MBC and VBA appear to
undergo spontaneous rearrangement to ECM•+, the former by
ring-opening, the latter by transfer of the formyl hydrogen atom.
Whereas the rearrangement appears to be complete in the case
of MBC (as in the parent molecule,8 we could find no trace of
MBC•+), it is only partial in VBA•+, which manifests itself
clearly in the EA spectrum (see below) and less obviously but
still distinctly in the IR (1534-cm-1 peak). In both cases we
find substantial quantities of neutral ECM which is presumably
formed by re-neutralization of its radical cation. On X-
irradiation of MBC we also notice some neutral VBA (IR band
Figure 2. IR spectra for steps (a)-(e) as described in the caption to
Figure 1.
irradiation at >420 nm do we note a slight decrease in these
features, as illustrated in traces (c).
Building on the recent results of Kessar et al., who found
that ECM is also formed from VBA on photolysis,6 and on
our own earlier work on ionization-induced tautomerizations,4
we endeavored to investigate the fate of VBA on ionization.
The corresponding results are shown in the last two traces of
Figures 1 and 2. Spectra (d) show that X-irradiation of VBA
(see strong IR peaks around 1710 cm-1) leads also to formation
of ECM as well as the other group of IR bands around 2170
cm-1. In the UV/vis range we note the appearance of a strong
pair of peaks at 355/370 nm in addition to a weaker band at
610 nm and some wiggles around 470 nm.
Scheme 3. Formation of ECM and Its Radical Cation from
VBA or MBC
In contrast to the above experiments starting from MBC,
bleaching of the visible EA bands was quite efficient in this
case: 45 min of irradiation through a 600 nm interference filter
sufficed to remove the blue band and the 355/370 nm pair of
peaks (trace (d)) which left essentially the broad ECM band at
380 nm and the wiggles around 470 nm. The latter proved to
be photostable whereas neutral ECM showed the same bleach-
ing behavior as illustrated in spectra (b) above. In searching
for features in the IR spectrum which rise and fall in concert
with the new optical absorptions detected after X-irradiation,
we only found a relatively weak peak at 1534 cm-1 which
decays on bleaching while the group of IR peaks at 2170 cm-1
rises.

2828 J. Am. Chem. Soc., Vol. 119, No. 12, 1997
Huben et al.
Table 1. Energies of Different C9H8O Isomers and Their Radical
Cations from B3LYP/6-31G* Calculationsa
neutral
radical cation
s,a-VBA
a,a-VBA
a,s-VBA
s,s-VBA
MBC
s-ECM
s-HVA
-422.96906b
+0.81 [+0.95]c
+1.54 [+1.58]c
+3.74
-422.67018b
+3.97
+4.85
(-22.16)d
+16.61e
-13.86
+6.14
+3.86
+24.11
+43.94
a Except where noted all energies are in kcal/mol relative to s,a-
VBA or its radical cation and correspond to fully optimized structures
that are available in the Supporting Information. b Total energies in
hartrees. c Difference in ∆G(298K). d s,s-VBA collapses to an o-quinoid
pyran on ionization (cf. scheme above). e At the geometry of netural
MCB (cation collapses to ECM•+ on optimization).
Figure 3. Schematic representation of the energies of four neutral (open
bars) and cationic (solid bars) C9H8O isomers relative to neutral VBA
as obtained by B3LYP/6-31G* calculations. Only the energies of the
most stable rotamers (drawn at the bottom) are plotted; those of the
less stable rotamers are listed in Table 1. All energies are at equilibrium
geometries except that for MBC•+ (which collapses spontaneusly to
ECM•+), which is given at the neutral equilibrium geometry.
Scheme 4. B3LYP/6-31G* Geometries of s,s- and a,s-VBA
compared to those of benzaldehyde and styrene
at 1706 cm-1) which may arise by way of ionization and
subsequent rearrangement and reneutralization from the second-
ary product, ECM. However, the virtual absence of the strong
EA bands of VBA•+ in these experiments would seem to suggest
that VBA may also be formed by another mechanism in this
case.
4.2. Energetics and Vibrational Structure. In order to
substantiate our propositions and assignments, and to gain more
detailed insight into the behavior of different VBA conformers,
we carried out quantum chemical calculations. Table 1 and
Figure 3 show the results of the B3LYP/6-31G* calculations
on VBA, ECM, MBC, and their radical cations, including also
the other possible product of H-transfer from VBA, i.e. the
o-quinoid hydroxyvinylallene, HVA. Analogous to earlier cases
of spontaneous hydrogen transfer reactions of radical cations,4
we note the marked changes in relative stabilities of VBA and
the two H-shifted products on ionization. The same applies to
the pair of valence isomers, MBC and ECM.
Firstly, we would like to comment on the different conformers
of neutral VBA from which ionization may occur. B3LYP/
6-31G*, which gives rotational barriers for styrene and benzal-
dehyde in good agreement with experiment,20 predicts the most
stable of these to be the s,a-conformer followed by the a,a-
conformer. This order of stabilities may appear surprising at
first sight, but we think that it can be explained on the basis of
the geometric features of the formyl group (cf. Scheme 4).
Due to the small exocyclic C-C-H angles in benzaldehyde
and styrene the two hydrogens come in very close contact in
the planar a,a-conformer, thus necessitating a significant twisting
of the vinyl group (and to a smaller degree even of the formyl
group) to avoid steric strain. Conversely, the large C-CdO
angle (which is even widened by 3° in s,a-VBA) allows for
more planarity in the s,a-conformer which may furthermore
profit from a weak stablizing interaction between the vinylic
hydrogen and the carbonyl oxygen.
The “reactive” a,s-conformer is even higher in energy than
the former two, whereas the s,s-conformer need not be
considered in the calculation of the equilibrium composition.
After transformation of the energy differences into ∆G298K
(based on the B3LYP structures and vibrations), these translate
into an equilibrium composition of the s,a-, a,a-, and a,s-
conformers of 78.8%:15.8%:5.4%. Hence most of the neutral
VBA is predisposed for formation of s-HVA whereas only a
(20) (a) Styrene: calculated 4.42 kcal/mol, experimental 4 ( 0.4 kcal/
mol (Facchine, K. L.; Staley, S. W.; van Zijl, P. C. M.; Mishra, P. K.;
Bother-By, A. A. J. Am. Chem. Soc. 1988, 110, 4900. (b) Benzaldehyde:
calculated 9.92 kcal/mol, experimental (NMR, ∆H‡) 8.3 ( 0.2 kcal/mol
(Lunazzi, L.; Macciantelli, D.; Baicelli, A. C. Tetrahedron Lett. 1975, 1205).
Note that there is a discrepancy between the rotational barrier of benzal-
dehyde from NMR and from IR measurements, the latter giving much lower
values which are, however, in disaccord with all calculations (cf.: Head-
Gordon, M.; Pople, J. A. J. Phys. Chem. 1993, 97, 1147).

Formation of an o-Quinoid Ketene and Its Radical Cation
J. Am. Chem. Soc., Vol. 119, No. 12, 1997 2829
Figure 5. MOs of VBA and ECM involved in the electronic transitions
of their radical cations listed in Tables 2 and 3.
Figure 4. EA spectra of VBA•+ () difference spectrum (e) in Figure
1 reversed) and ECM•+ () spectrum (a) in Figure 1 minus that of
neutral ECM) on a wavenumber scale and CASPT2 predictions (cf.
Tables 2 and 3).
In contrast to parent benzocyclobutenone,8 we found the ring-
opening of MBC•+ to be entirely activationless. This is
presumably due to the stabilization of the lowest σ state (which
is dissociative with regard to ring-opening)8 by the inductive
effect of the methyl group, so that it falls below the π ground
state of the parent cation. Hence, vertical ionization of MBC
leads directly to a dissociative surface. But even if a π state
would be attained vertically, breaking of the Cs symmetry by
the methyl group entails a mixing of π and σ states, which may
lead to the disappearance of the symmetry-imposed barrier for
the ring-opening of the parent cation.8 In fact, no potential
energy minimum could be located for MBC•+ (the black bar in
Figure 3 indicates the cation’s energy at the neutral geometry).
Finally we note that we could not find a local minimum
corresponding to the ionized form of the least stable VBA
rotamer (s,s) which undergoes spontaneous ring closure to a
much more stable quinoid pyran cation (cf. Table 1).
small percentage can directly yield s-ECM (or the respective
cations after ionization).
However, B3LYP/6-31G* predicts that the VBA f HVA
rearrangement is strongly endothermic, which is perhaps the
reason why no allenic or OH stretching vibrations were observed
after photolysis of VBA.21 Although strongly attenuated, the
endothermicity of this process persists in the radical cation which
may explain why we found no sign of HVA•+ after ionization
of VBA,22 in contrast to the exothermic rearrangement to
ECM•+. If the rearrangement to HVA•+ does not occur, then
most of the VBA will retain its structure after ionization, i.e.
the yield of VBA•+ is expected to be much larger than that of
ECM•+ in those experiments. Nevertheless, the latter may show
up more prominently in the IR spectra due to the exceptionally
high intensity of the ketene stretching vibration. Neutral ECM
appears most strongly because it continues to be formed (by
re-neutralization of ECM•+, cf. Scheme 2) in the course of
X-irradiation.
The calculated IR spectra of the different cations show also
features which are in general agreement with our observations:
on ionization of ECM, the calculated ketene stretching fre-
quency increases by 58 cm-1 (experimentally 50-70 cm-1
)
because the HOMO of ECM is slightly CdO antibonding (cf.
Figure 5). Conversely, the CdO stretching frequency of VBA
is predicted to decrease by a similar amount which puts it
unfortunately in the region where water molecules associated
with the very polar VBA show strong, sharp peaks (not shown
in Figure 2). However, the most intense IR band of VBA•+ is
predicted at 1583 cm-1, in good accord with our observation
(21) The energy of the photons used in the VBA photolysis6 would have
sufficed to make up for the endothermicity, but perhaps the “funnels” on
the excited state surface are ill disposed for relaxation to HVA. Also, our
calculations show that the vinylic proton remains coordinated to the incipient
allenic π-bond after its transfer to the carbonyl group (i.e. formation of
HVA). This may cause the barrier for re-ketonization to be too small for
HVA to persist after photolytic formation. A calculation of the corresponding
reaction path could elucidate this possibility.
(22) We believe that the small IR bands which rise around 1900 cm-1
on exhaustive photolysis of MBC•+ are not due to HVA•+ because this
would necessitate previous formation of VBA•+, which is not observed in
this experiment. They are probably due to an allenic product of ring-opening
that may be formed in a single step.
of the only band which we attribute to this cation at 1534 cm-1
.
4.3. Electronic Structure of Radical Cations. To gain
some insight into the electronic structure of the radical cations
whose EA spectra we have observed in this study we embarked

2830 J. Am. Chem. Soc., Vol. 119, No. 12, 1997
Huben et al.
Table 2. Excited States of VBA•+ a by CASSCF/CASPT2
CASSCFb
CASPT2
state
Erel/eV
compositionc
Erel/eV
fd
% refe
exp Erel/eVf
12A′′ g
22A′′
32A′′
(0)
1.25
2.26
80% ... (π3)2(π4)2(π5)1
(0)
1.16
2.00
71
71
70
(0)
76% ... (π3)2(π4)1(π5)2
0.001
0.040
54% ... (π3)1(π4)2(π5)2
2.03
(≈3.5)
3.33
19% ... (π3)2(π4)2(π5)0(π6)1
45% ... (π2)1(π3)2(π4)2(π5)2
17% ... (π3)2(π4)2(π5)0(π7)1
45% ... (π3)2(π4)2(π5)0(π6)1
18% ... (π3)1(π4)2(π5)2
42A′′
52A′′
3.76
4.31
3.46
3.30
0.031
0.431
69
64
5% ... (π2)1(π3)2(π4)2(π5)2
a Calculations were carried out at the B3LYP/6-31G* optimized geometry of the most stable rotamer, i.e. the s,a-rotamer (cf. Table 1). b For
active space, see Section 2.3. c Percent contribution of the most important configurations to the CASSCF wave function (cf. MO plots in Figure 5).
The σ-MO is doubly occupied except where indicated. d Oscillator strenghts. e Percent of the CASPT2 wave function described by the reference
CASSCF wave function. f From the spectra in Figure 4. g Ground state.
Table 3. Excited States of ECM•+ a by CASSCF/CASPT2
CASSCFb
compositionc
CASPT2
/eVf
state
Erel/eV
Erel/eV
fd
% refe
exp Erel
12A′′g
22A′′
(0)
2.38
82% ... (π3)2(π4)2(π5)1
62% ... (π3)2(π4)2(π5)0(π6)1
8% ... (π3)1(π4)2(π5)2
(0)
2.07
69
69
(0)
2.06
0.015
0.049
0.105
0.090
6% ... (π3)2(π4)1(π5)2
32A′′
42A′′
52A′′
3.13
3.18
4.31
51% ... (π3)1(π4)2(π5)2
10% ... (π3)2(π4)2(π5)0(π6)1
9% ... (π3)2(π4)1(π5)2
2.86
2.95
3.70
69
68
68
≈2.8
54% ... (π3)2(π4)1(π5)2
8% ... (π3)2(π4)2(π5)0(π6)1
6% ... (π3)2(π4)2(π5)0(π7)1
37% ... (π3)2(π4)2(π5)0(π7)1
11% ... (π3)1(π4)2(π5)2
11% ... (π3)1(π4)2(π5)1(π6)1
5% ... (π3)2(π4)1(π5)2
≈2.8
>3.6
a Calculations were carried out at the B3LYP/6-31G* optimized geometry of the rotamer where thje methyl group is syn to the ketene group.
b-gSee footnotes for Table 2.
on excited state calculations by the CASSCF/CASPT2 protocol.
The results of these are displayed in Figure 4 which juxtaposes
the EA features which we attribute to VBA•+ and ECM•+,
respectively, to the CASPT2 predictions on an energy scale.
Tables 2 and 3 list the corresponding numbers and assignments,
including also those from the CASSCF calculations, whereas
Figure 5 shows the MOs which are involved in the observed
transitions. Firstly we note the very satisfactory quantitative
agreement between experiment and theory23 which gives us a
degree of confidence that our assignments are correct.
In the Vinylbenzaldehyde cation the first excited state is
predicted to arise by ionization from the π4 MO of VBA which
corresponds essentially to a benzene HOMO (or by π4 f π5
excitation in VBA•+). We could not observe any bands
associated with VBA•+ in the NIR range, presumably because
of the very small predicted oscillator strength of the corre-
sponding EA transition, but the 1.16 eV excited state energy is
in good agreement with Iv,2-Ia,1 in the photoelectron (PE)
spectrum of VBA.24 The EA band at 610 nm corresponds
predominantly to a state arising by ionization from π3 of VBA
(or π3 f π5 excitation in VBA•+), with a small admixture of
the HOMO f LUMO excited configuration. It also shows up
as the third band in the PE spectrum of VBA.24
associated with the 370-nm peak (in pefect quantitative agree-
ment) whereas the higher-lying state must be buried in the UV
tail of that band.
In the quinoketene cation, ECM•+, the analysis shows that
the first excited state which gives rise to the sharp visible band
is dominated by the HOMO f LUMO excited configuration,
as is typical for the o-quinodimethane cation chromophore.25
Again, the second EA band appears to comprise transitions to
two excited states, again in excellent quantitative agreement with
experiment. Both of these are dominated by Koopmans’
configurations (ionization from π3 and π4, respectively) but carry
substantial Non-Koopmans’ contributions, i.e. excitations into
virtual MOs. Finally, CASPT2 predicts another rather intense
transition at 335 nm which is difficult to detect unambiguously
because strong absorptions of the neutral precursor, MBC, begin
to rise in this region. Nevertheless, the difference spectrum in
Figure 4 indicates a band peaking at ≈325 nm whose position
is in reasonable agreement with this prediction.
5. Conclusions
X-irradiation of the C9H8O isomers, o-vinylbenzaldehyde
(VBA) and 2-methylbenzocyclobuten-1-one (MBC), in solid
argon in the presence of an electron scavenger leads to creation
of a common o-quinoid ketene radical cation, ECM•+. Concur-
rently, the corresponding neutral ECM arises by re-neutraliza-
tion of the cation which competes effectively with the added
electron scavenger. B3LYP calculations predict that the
predominant conformation of neutral VBA (80%) persists after
According to CASPT2, the second, intense near-UV band of
VBA•+ comprises two transitions of strongly mixed character.
According to the calculations, the lower-lying state is to be
(23) It was impossible to achieve reasonable agreement with semiem-
pirical methods. We attempted INDO/S calculations of VBA•+ and ECM•+
,
but as the SCF procedure invariably converged to a σ-state we could not
evaluate the π-electronic structure by this method.
(24) Asmis, K.; Zhu, Z.; Bally, T. Unpublished. We thank Dr. Asmis
(University of Fribourg) for recording this spectrum for us.
(25) Marcinek, A.; Michalak, J.; Rogowski, J.; Tang. W.; Bally, T.;
Gebicki, J. J. Chem. Soc., Perkin Trans. 2 1992, 1353.

Formation of an o-Quinoid Ketene and Its Radical Cation
J. Am. Chem. Soc., Vol. 119, No. 12, 1997 2831
ionization, which explains why the electronic absorption
spectrum of VBA•+ could be very clearly detected after
ionization of VBA. Conversely, the radical cation of MBC
proved to be elusive, presumably because it is formed in a state
which is dissociative with respect to ring-opening to ECM•+.
The ketene stretching frequency of ECM is shifted to higher
energies on ionization because the HOMO is CdO antibonding.
The EA spectra of VBA•+ and ECM•+ can be modeled very
satisfactorily by CASSCF/CASPT2 calculations which givesapart
from good quantitative agreementsa picture of the electronic
structure in accord with qualitative expectations. This proves
once again the validity and usefulness of this new method for
modeling EA spectra of radical cations.26
Acknowledgment. This work has been funded through
Grants No. 2028-047212.96 of the Swiss National Science
Foundation and Grant No. 3.T09A.097.11 of the Polish State
Committe for Scientific Research.
JA964243W
(26) See also: Fu¨lscher, M. P.; Matzinger, S.; Bally, T. Chem. Phys.
Lett. 1995, 236, 167. Truttmann, L.; Asmis, K. R.; Bally, T. J. Phys. Chem.
1995, 99, 17844.