Pitfalls in the photoelectron spectroscopic investigations of benzyne. photoelectron spectrum of cyclopentadienylideneketene

Article abstract of DOI:10.1071/CH09641

The 9.24 eV ionization energy often quoted in photoelectron spectroscopic investigations of benzyne is not due to benzyne 1 but to benzene, C ₆H_6. The 8.9 eV ionization is not due to benzyne either but to cyclopentadienylideneketene 12 when a 10.2 eV band is also present, or to biphenylene 5 when a 7.6 eV band is simultaneously present. Cyclopentadienylideneketene 12 has been generated by flash vacuum thermolysis of four different precursors, which permit a linking of infrared, mass, and photoelectron spectroscopic observations.

Full text of DOI:10.1071/CH09641

RESEARCH FRONT
Full Paper
CSIRO PUBLISHING
Aust. J. Chem. 2010, 63, 1084–1090
www.publish.csiro.au/journals/ajc
Pitfalls in the Photoelectron Spectroscopic Investigations
of Benzyne. Photoelectron Spectrum
of Cyclopentadienylideneketene
Anna Chrostowska,A,C Genevieve Pfister-Guillouzo,A Françoise Gracian,A
and Curt WentrupB,C
AEquipe de Chimie Physique, Institut Pluridisciplinaire de Recherche sur l’Environnement et les
Matériaux, UMR CRNS 5254, Université de Pau et les Pays de l’Adour, 6400 Pau, France.
BSchool of Chemistry and Molecular Biosciences, The University of Queensland,
Brisbane, Qld 4072, Australia.
CCorresponding authors. Email: anna.chrostowska@univ-pau.fr; wentrup@uq.edu.au
The 9.24 eV ionization energy often quoted in photoelectron spectroscopic investigations of benzyne is not due to benzyne
1 but to benzene, C6H6. The 8.9 eV ionization is not due to benzyne either but to cyclopentadienylideneketene 12 when
a 10.2 eV band is also present, or to biphenylene 5 when a 7.6 eV band is simultaneously present. Cyclopentadienyli-
deneketene 12 has been generated by flash vacuum thermolysis of four different precursors, which permit a linking of
infrared, mass, and photoelectron spectroscopic observations.
Manuscript received: 9 December 2009.
Manuscript accepted: 20 April 2010.
O
Introduction
O
Dewar and Tien reported a photoelectron (PE) spectrum
attributed to o-benzyne 1 (in the following called benzyne), gen-
erated by flash vacuum thermolysis (FVT) of phthalic anhydride
2 and indanetrione 3 (the latter is also a precursor of benzocyclo-
butenedione 4 (Scheme 1)).[1] Three bands, at 9.24, 9.75, and
9.87 eV were assigned to the vertical ionization energies of ben-
zyne. Bands due to biphenylene 5 (7.6, 8.9, 9.7, 10.1, 11.0, and
11.5 eV) and possibly biphenyl (9.15 eV) were also present. The
interpretation of the benzyne PES was found to be consistent
with MNDO calculations.
Schulz and Schweig reported a PE spectrum of benzyne gen-
erated by FVT of indanetrione 3 and sulfobenzoic anhydride 6
as well as phthaloyl peroxide 7.[2] Three prominent bands were
observed at 9.23, 9.41, and 9.70 eV and assigned to benzyne
in full agreement with Dewar and Tien’s report.[1] The assign-
ment was supported by semiempirical calculations (LNDO/S
PERTICI).
O
O
O
2
1
O
5
3
N
O
N
N
8
O
4
O
O
O
O
O
O
S
O2
7
6
I
I
I
As we will show below, both of these assignments were
in error, in particular the assignment of the first band at 9.24
(9.23) eV to benzyne.
Hg
I
11
9
10
Zhang and Chen reported a time-of-flight mass and PE spec-
trum of benzyne, generated by supersonic jet flash thermolysis
of benzocyclobutenedione 4 (1300◦C, <1 hPa partial pressure
seeded in 1500 hPa of He; m/z 76; ionization energies (IE)
9.03 eV (weak, assigned to ionization of the out-of-plane π
molecular orbital) and 9.77 eV (strong, assigned to the in-plane
orbital of the CC triple bond).[3] At higher temperatures a peak
at m/z 50 appeared in the MS, accompanied by sharp peaks at
∼9.24 eV in the PES. The m/z 50 ion is ascribed to the butadiyne
molecular ion, C4H•2+; this is always a strong fragment peak in
Scheme 1.
mass spectra of benzyne (or rearranged C6H•4+ ions), and the
assignment of the butadiyne structure HC≡C-C≡CH is sup-
ported by collisional activation and neutralization-reionization
mass spectrometry.[4] C4H2 it is also a thermal decomposition
product of benzyne,[5] but it is not the origin of the PE ionization
at 9.24 eV; the IEs of butadiyne are 10.2 and 12.7 eV.[6]
© CSIRO 2010
10.1071/CH09641
0004-9425/10/071084

RESEARCH FRONT
Photoelectron Spectroscopic Investigations of Benzyne
1085
O
C
Werstiuk and coworkers reported the PES of benzyne gener-
ated from 1,2,3-benzotriazine 8 by loss of N2 and HCN induced
by vacuum pyrolysis with a CO2 laser.[7] With a path of only 1 cm
between the pyrolysis zone and the ionizing photon beam, this
apparatus allows the detection of very short-lived and/or reac-
tive species. A band at 9.7 eV was assigned to benzyne. Only
a minor amount of biphenylene 5 (7.6 eV) was observed. The
assignment of the benzyne PE spectrum is in agreement with
Zhang and Chen,[3] but not with Dewar[1] and Schweig.[2]
The ionization energy of benzyne has also been determined
by mass spectrometric methods. A mass spectrum of benzyne 1
(m/z 76) was obtained by Fisher and Lossing by pyrolysis of o-
diiodobenzene 9 at 960◦C in a reactor installed in the inlet system
of a mass spectrometer (residence time ∼10−3 s).[8] The ioniza-
tion energy of this species was measured as 9.75 eV referenced
to benzene = 9.50 eV. The vertical ionization energies measured
by mass spectrometry were asserted to be higher than those mea-
sured by photoelectron spectroscopy (IE(benzene) = 9.245 eV).
This would lead to a corrected IE(benzyne) = 9.5 eV. The same
value, IE(benzyne) = 9.50 eV, was measured by Grützmacher
and Lohmann by using mass spectrometry of benzyne generated
bypyrolysisofbis(o-iodophenyl)mercury 10at800◦C.[9] Genzel
and Osberghaus generated neutral benzyne by bombardment of
benzene, fluorobenzene, and toluene with 200 eV electrons; the
neutrals, including benzyne, were then investigated by electron-
ionization mass spectrometry. IE(benzyne) = 9.50 0.15 eV
was obtained. Interestingly, C6H2 (most probably hexatriyne)[4]
was also formed (IE(C6H2) = 9.85 0.15 eV).[10] However,
Rosenstock et al. concluded that the mass spectrometric
measurements were in error by 0.5 eV, thus giving IE(benzyne)
∼9 eV.[11] It must be noted that the C6H•+ ions formed
in the mass spectrometers most likely have4undergone ring
opening.[4] The most reliable ionization potential was deter-
mined by Guo and Grabowski, IE(benzyne) = 9.03 0.1 eV,
by using ion-molecule reactions of the benzyne radical anion
11 in a flowing afterglow apparatus.[12] This value is grati-
fyingly identical with the IEπ = 9.03 0.05 eV measured by
photoelectron spectroscopy of benzyne produced by supersonic
jet flash pyrolysis of benzocyclobutenedione 4 as described
above.[3]
O
C
or
O
C
O
C
O
O
12
2
1
Scheme 2.
O
O
O
O
O
CF3
O
O
FVT
FVT
FVT
13
14
C
C
O
12
O
O
O
O
O
FVT
FVT
O
O
O
15
O
O
1
O
O
16
Scheme 3.
O
O
O
CF3
O
C
C
Δ
ϪCF3CO2H
13
Δ
17
ϪC2H4
Δ
ϪCO2
O
O
ϪMe2CO
O
O
O
O
O
O
Δ
Δ
New research on the IR spectrum of benzyne led to a re-
interpretation of the PE and UV spectra as well. Early work by
Chapman and coworkers suggested that the benzyne CC triple
bond absorbed at 2085 cm−1 in an Ar matrix.[13] Photodetach-
ment of an electron from the benzyne radical anion 11 generated
a photoelectron spectrum featuring a vibrational progression,
from which a stretching vibration of 1860 15 cm−1 was deter-
mined for benzyne 1 in the gas phase.[14] A new investigation of
the matrix photolysis of 2 revealed that the benzyne CC stretch
is in fact at 1846 cm−1 (Ne matrix), and the 2085 cm−1 peak
does not belong to benzyne.[15]
C
C
ϪC2H4
ϪCO2
ϪMe2CO
O
12
Δ
14
18
Δ
−CΟ
ϪC5H5
Δ
O
O
O
O
O
1
5
O
O
O
15
A weak peak at 2085 5 cm−1 in the Ar matrix of the
product resulting from pyrolysis of phthalic anhydride 2 at 900–
920◦C (10−5 hPa) was assigned to cyclopentadienylideneketene
12 (Scheme 2).[16] The yield of the ketene 12 is very low
under these conditions, as most of the material is converted to
benzyne 1. Ketene 12 was also generated by FVT of the Mel-
drum’s acid derivatives 14 and 15 (Scheme 3), where a strong
peak at 2085 5 cm−1 was observed.[16] Further FVT-IR studies
with precursors 13–16 confirmed the assignment of the strong
2089 cm−1 band to ketene 12.[17] On-line FVT-MS studies of the
same precursors 13–15 allowed the assignment of the product
with m/z 104 to ketene 12.[4]
O
O
O
O
16
Scheme 4.
IR studies of the Ne and Ar matrix photolyses of benzo-
cyclobutenedione 4 and phthalic anhydride 2 demonstrated that
benzyne (1864 cm−1) in the presence of CO is in photoequilib-
rium with ketene 12 (2085 (2087) cm−1).[15,18]

RESEARCH FRONT
1086
A. Chrostowska et al.
(a)
C/S
(b)
C/S
IP [eV]
15
IP [eV]
10
5
15
10
5
(c)
(d)
C/S
C/S
IP [eV]
15
IP [eV]
15
10
5
10
5
(e)
(f)
C/S
C/S
IP [eV]
15
IP [eV]
15
10
5
10
5
Fig. 1. Photoelectron spectra of 13 at (a) 510 K, (b) 680 K, (c) 730 K, (d) 780 K, (e) 860 K, and (f) 1190 K.
The new insight into the IR spectra of benzyne 1
and cyclopentadienylideneketene[19] 12 caused Schweig and
coworkers to reinterpret both the UV[20] and the PE[21] spec-
tra previously assigned to benzyne. The 3.26 eV (380 nm) band
in the UV spectrum was tentatively assigned to ketene 12,[20]
but this role was later changed to the 4.23 eV (293 nm) band (12
has a strong UV absorption at 4.29 eV (289 nm)).[18] Ketene 12
(∼290 nm) may also have been formed in Kolc’s experiment on
photolysis of 4 at 77 K, but was not recognized as such.[22] A UV
absorption of gaseous benzyne with λmax ∼242 3 nm reported
in the early experiments by Berry et al.[23] was also observed in
Schweig’s matrix UV spectra (246 nm).[18,20]
due to another species, while the 9.75 and 9.87 eV bands were
still assigned to benzyne.[24] Thus, while the first (weak) ion-
ization energy of benzyne (9.03 eV) has now been identified
by Chen,[3] Grabowski,[12] and their coworkers, and the sec-
ond (strong) ionization energy (9.7 eV) by Chen,[3] Werstiuk,[7]
and their coworkers, the carrier of the 9.24 eV band observed
by many experimenters remains obscure in spite of extensive
investigation.
Results and Discussion
Ketene Precursors. Photoelectron Spectrum of 12
In the photoelectron spectrum, the 9.24 eV band previously
ascribed to benzyne[1,2] was now thought to be due to ketene
12, while ionizations at 9.65–9.9 eV and 10.0 eV were still
assigned to benzyne.[21] Already von Niessen et al. had carried
out Green function and CI calculations which indicated that the
9.24 eV band previously ascribed to benzyne[1,2] was possibly
The products of FVT of four precursors 13–16 of cyclopenta-
dienylidene ketene 12 were investigated by on-line PES
(Scheme 4). The spectra of the precursors are shown in Figs
1a, 2a, and 3a. The photoelectron spectrum of 13 (Fig. 1a)
exhibits two low energy bands at 10.1 and 10.65 eV; the band
at 9.5 eV is due to ionization of diethyl ether,[25] which was

RESEARCH FRONT
Photoelectron Spectroscopic Investigations of Benzyne
1087
(a)
(b)
C/S
C/S
IP [eV]
IP [eV]
(c)
(d)
C/S
C/S
IP [eV]
IP [eV]
(e)
C/S
IP [eV]
Fig. 2. Photoelectron spectra of 14 at (a) 600 K, (b) 750 K, (c) 800 K, (d) 830 K, and (e) 1030 K.
used as a solvent. The photoelectron spectrum of 14 (Fig. 2a)
to the fact that ethylene had not yet appeared. This assignment is
supported by calculations at the TDDFT (B3LYP/6–311G(d,p);
IE = 8.11 eV and MP2/6–311G(d,p); IE = 8.34 eV) levels (the
MOs are shown in theAccessory Publication). The 8.35 eV band
disappeared progressively with heating between 700 and 780 K,
while the band at 10.55 eV (C2H4) became more intense. Two
new bands at 8.9 and 10.2 eV are now ascribed to ketene 12
(Fig. 1c). With a thermolysis temperature of 780 K (Fig. 1d), the
first-formed band at 8.35 had disappeared, and the PE spectrum
now contained the ionizations at 8.9 and 10.2 eV ascribed to 12.
At higher temperatures (Fig. 1e), the presence of CO (14.0 eV) as
well as C2H4 (10.55 eV) and CO2 (13.8 and 18.05 eV) demon-
strates that extensive thermolysis was taking place. The peak
centred at 8.9 eV was still present. A new ionization appeared
at 8.15 eV. At 1190 K another new band at 9.25 (9.24) eV with
a shoulder at 9.65 (9.7) eV are clearly visible (Fig. 1f). The lat-
ter two bands are ascribed to benzene and benzyne, respectively
(see below).
displays the first ionisation energy at 9.2 eV, the second at
9.9 eV, and the third at 10.8 eV. The third precursor (15; Fig. 3a)
shows two first broad bands centred at 9.8 and 10.9 eV. The
precursors were subjected to FVT in the inlet system of the
PE spectrometer (Figs 1–3). Progress of the thermolyses was
evidenced by the appearance of the characteristic ionization
peaks due to CF3COOH (12.15 and 12.95 eV) and ethylene
(10.55 eV)[25] in the thermolysate of 13, ethylene, (CH3)2CO
(9.7 eV) and CO2 (13.8 and 18.05 eV)[25] for 14, (CH3)2CO and
CO2 for 15 and 16, and cyclopentadiene (8.6 and 10.65 eV)
for 15 (Figs 1–3). High temperatures result in elimination of
CO (14.0 eV)[25] from ketene 12 with formation of benzyne.
Whenever benzyne is formed, biphenylene 5 (7.6 and 8.9 eV)
appears too.
The first step of FVT of compound 13 is elimination of
CF3COOH, which starts at 680 K. We attribute the first ioniza-
tion energy at 8.35 eV (Fig. 1b) to the intermediate ketene 17 due

RESEARCH FRONT
1088
A. Chrostowska et al.
(a)
(b)
C/S
C/S
IP [eV]
(c)
IP [eV]
C/S
IP [eV]
Fig. 3. Photoelectron spectra of 15 at (a) 500 K, and (b) 810 K, and of 16 (c) 770 K.
In the case of the precursor 14, thermolysis at 750 K (Fig. 2b)
led to the appearance of new ionizations at 9.05 and 10.0 eV
possiblydueinparttotheintermediatefulvene18. Ketene17was
not observed clearly (but 17 is observed in analogous FVT-MS
experiments[4]). Starting at 800 K (Fig. 2c and 2d) bands due to
(CH3)2CO (9.7 eV) and CH2=CH2 (10.55 eV) were present, and
they were accompanied by two bands at 10.2 and 8.9 eV ascribed
to ketene 12. Compound 12 remains at higher FVT temperature
(Fig. 2d) when the first IP of biphenylene 5 at 7.6 eV starts to be
apparent. Finally, at 1030 K (Fig. 2e), the new ionization at 9.25
(9.24) eV appears.
Thermolysis of 15 above 700 K led to the formation of
cyclopentadiene (8.6 and 10.65 eV) but not yet acetone and
CO2. At 810 K acetone (9.7 eV), cyclopentadiene and CO2 were
present, as well as the two weak shoulders at 8.9 and 10.2 eV
matching the ionizations of ketene 12 described above (Fig. 3b).
The 8.9 eV band was also observed on FVT of 16 at 770 K
(Fig. 3c). The mystery band at 9.24 eV was absent in these cases.
While it is difficult to characterize all the generated products
by their ionization energies, and the PE bands are often super-
imposed on those due to byproducts (CH2=CH2, (CH3)2CO,
and cyclopentadiene), the coherence of the experimental data
obtained by IR,[16,17] mass,[4] and photoelectron spectroscopies
allows an interpretation in terms of the mechanism shown in
Scheme 4. Ionizations at 8.9 and 10.2 eV are present in the
spectra of all precursors in the temperature range 700–870 K.
They are associated with cyclopentadienylideneketene 12. The
first band at 8.9 eV contains two ionizations, namely that of
the π-system of the methyleneketene (as observed previously
for the substituted methyleneketenes at ∼9 eV)[26] and the πa2
MO of cis-butadiene.[27] The band at 10.2 eV is associated with
the removal of an electron from the MO representing the anti-
bonding combination of the π-system of the methyleneketene
(observed at 10.6 eV in substituted cases)[26] and the symmetric
combination πb1 of cis-butadiene.[27] These experimental val-
ues are in fair agreement with calculated ionization energies
at the TD-B3LYP/6–311G(d,p) and MP2/6–311G(d,p) levels
of theory (TD-B3LYP energies: [πC=C − nO] = 8.73 eV; [πC=C
(a2)] = 8.75 eV; [πC=C (b1) − πC=C + nO] = 9.98 eV). MP2
IEs: [πC=C − nO] = 8.72 eV; [πC=C (a2)] = 8.80 eV; [πC=C
(b1) − πC=C + nO] = 9.97 eV. The MOs are shown in the Acces-
sory Publication.
Benzyne Precursors
We now describe the thermolyses of benzocyclobutenedione
4 and phthalic anhydride 2. It has been established beyond
doubt that benzyne is formed by photolysis[15] as well as FVT,
where it has been identified in PES,[3,7] IR,[16,17] and MS[3–5]
studies. Additional evidence for the formation and structure of
benzyne has been provided by NMR[28] and microwave[29] spec-
troscopies. The FVT of both 2 and 4 at 1300 K results in PE
spectra featuring a large, well distinguished ionization at 9.24 eV,
together with two shoulders at 8.9 and 9.7 eV and a band at
10.2 eV. The two bands at 8.9 and 10.2 eV could be due to ketene
12 as described above, but because the 7.6 eV band belonging to
biphenylene is present, the 8.9 eV band is most likely due to the
second ionization of biphenylene 5. The 9.7 eV band is ascribed
to benzyne in accord with Chen, Schweig, and Werstiuk.[3,7]
The 9.24 eV band corresponds to Dewar’s[1] and Schweig’s[2]
‘mystery benzyne band’.
Most importantly, when the distance between the pyrolysis
nozzle and the ionization region of the PE spectrometer is
increased (50 cm distance at 10−3 hPa), the signal due to ben-
zyne at 9.7 eV disappears, but the one at 9.24 eV persists. Thus,
it is not due to a reactive intermediate. It is in fact assigned to
benzene,[30] C6H6 (IE 9.245, 11.5, 12.4, 14.0, 14.9, 16.8, and
16.9 eV[25,31]). Furthermore, FVT of benzocyclobutenedione 4
(Fig. 4) and phthalic anhydride 2 was carried out at 1000◦C with
isolation of the product on a liq. N2-cooled coldfinger. Analysis
oftheproductsbyGCMSdemonstratedtheformationofbenzene
in each case, in absolute yields of 3.5 1% from 4 and 4.5 1%

RESEARCH FRONT
Photoelectron Spectroscopic Investigations of Benzyne
1089
(a)
(d)
C/S
C/S
IP [eV]
IP [eV]
15
10
5
15
10
5
C/S
(b)
(e)
C/S
IP [eV]
IP [eV]
15
15
10
10
5
5
(c)
(f)
C/S
C/S
IP [eV]
IP [eV]
15
10
5
15
10
5
Fig. 4. Photoelectron spectra of benzocyclobutenedione 4 at (a) 500 K, (b) 1060 K, and (c) 1270 K, and of phthalic anhydride 2 at (d) 1030 K, (e) 1250 K,
and (f) 1300 K.
from 2. The main product was biphenylene 5. Triphenylene was
present in ∼160-fold smaller quantity.[4]
not observed. It should also be noted that estimates of ioniza-
tion energies based on measurements of heats of formation of
C6H•4+ radical cations formed by dissociative ionization must
be treated with caution: although evidence for the preserva-
tion of individual benzyne radical cation structures has been
published,[32] there is strong evidence for extensive isomeriza-
tion and ring opening before fragmentation of the molecular ions
of dihalobenzenes and benzonitrile.[4,33]
Conclusion
In conclusion, the ionization at 9.24 eV (observed in many
FVT-PE spectra of benzyne precursors with more or less high
intensity) is ascribed to the benzene molecule and not to benzyne
1 or cyclopentadienylideneketene 12. The ionization energies of
12 are at 8.9 and 10.2 eV. The band at 8.9 eV in the ‘benzyne’
photoelectron spectra also cannot be due to benzyne, but it cor-
responds to ketene 12 when the 10.2 eV band is also present, or
to biphenylene 5 when the 7.6 eV band is present. The strong
benzyne ionization is at ∼9.7 eV.[3,7] The weak benzyne ion-
ization at 9.03 eV[3] is not observable under the experimental
conditions employed here. The band at 9.87 (9.9) eV ascribed
to benzyne by Dewar,[1] Schweig,[21] and von Niessen[24] was
Experimental
The FVT-PE spectrometer has been described.[34] There is a 2 cm
flight distance at ∼10−3 hPa between the pyrolysis zone and the
ionization region. Photoelectron spectra were recorded with a
Helectros 0078 spectrometer fitted with a short-path coupled
FVT oven, as already described,[34] and equipped with a 127◦
cylindrical analyzer using 21.21 eV He I radiation as a photon
source and monitored by a microcomputer supplemented with a

RESEARCH FRONT
1090
A. Chrostowska et al.
digital-to-analogue converter. The helium ionization at 4.98 eV
and nitrogen ionizations at 15.59 and 16.98 eV were used for
calibration. The spectra result from single scans consisting of
2048 data points and are accurate to within 0.05 eV. Precursors
13–16werepreparedaccordingtoliteratureprocedures[17b,c] and
slowly vapourized at low pressure (∼10−4 hPa in the ionization
chamber) directly in the oven, with continuous analysis of the
gaseous thermolysates.
[18] J. G. G. Simon, N. Münzel, A. Schweig, Chem. Phys. Lett. 1990, 170,
187. doi:10.1016/0009-2614(90)87113-6
[19] (a) For calculations on cyclopentadienylideneketene 12 see:
J. G. Radziszewski, P. Kaszynski, A. Friedrichsen, J. Abildgaard,
Collect. Czech. Chem. Commun. 1998, 63, 1094. doi:10.1135/
CCCC19981094
(b) A. C. Scheiner, H. F. Schaefer, J. Am. Chem. Soc. 1992, 114, 4758.
doi:10.1021/JA00038A045
[20] A. Schweig, N. Münzel, H. Meyer, A. Heidenreich, Struct. Chem.
1990, 1, 89. doi:10.1007/BF00675788
[21] J. G. G. Simon, H. Specht, A. Schweig, Chem. Phys. Lett. 1992, 200,
459. doi:10.1016/0009-2614(92)80075-M
[22] J. Kolc, Tetrahedron Lett. 1972, 13, 5321. doi:10.1016/S0040-4039
(01)85240-0
[23] (a) R. S. Berry, G. N. Spokes, R. M. Stiles, J. Am. Chem. Soc. 1960,
82, 5240. doi:10.1021/JA01504A053
FVT of 2 and 4 at 1000◦C with isolation of the products
on a liq. N2-cooled coldfinger at 77 K and analysis by GCMS
revealed biphenylene 5 as the only major product, triphenylene
in ∼160-fold smaller quantity, and benzene (4.5 1% from 2,
and 3.5 1% from 4).[4]
Accessory Publication
(b) R. S. Berry, G. N. Spokes, R. M. Stiles, J. Am. Chem. Soc. 1962,
84, 3570. doi:10.1021/JA00877A031
Computational details and calculated molecular orbitals of
ketene 12 and 17 are available on the Journal’s website.
(c) M. E. Schafer, R. S. Berry, J. Am. Chem. Soc. 1965, 87, 4497.
doi:10.1021/JA00948A017
[24] I. H. Hillier, M. A. Vincent, M. F. Guest, W. von Niessen, Chem. Phys.
Lett. 1987, 134, 403. doi:10.1016/0009-2614(87)87162-2
[25] K. Kimura, S. Katsumata, Y. Achiba, T. Yamazaki, S. Iwata,
Handbook of HeI Photoelectron Spectra of Fundamental Organic
Molecules 1981 (Japan Scientific Societies Press: Tokyo, Halsted
Press: New York, NY).
Acknowledgement
This work was supported by the Australian Research Council and by the
CNRS.
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