On the valence shell electronic spectroscopy of 2-vinyl furan

Article abstract of DOI:10.1063/1.1738642

The vacuum ultraviolet (VUV) absorption spectra of 2-vinyl furan was investigated. The spectroscopic properties of the 2-vinyl furan was analyzed using electron energy loss spectroscopy and synchrotron radiation source with absolute cross section determinations. The semiempirical molecular orbital calculations was used for the interpretation of the He_I photoelectron spectrum. A photoabsorption spectrum was observed in the first band in the UV absorption spectra in solution of furans in position 2 of polyenyl groups of increasing size.

Full text of DOI:10.1063/1.1738642

On the valence shell electronic spectroscopy of 2-vinyl furan
A. Giuliani, I. C. Walker, J. Delwiche, S. V. Hoffmann, C. Kech, P. Limão-Vieira, N. J. Mason, and M.-J. Hubin-
Franskin
Citation: The Journal of Chemical Physics 120, 10972 (2004); doi: 10.1063/1.1738642
View online: http://dx.doi.org/10.1063/1.1738642
View Table of Contents: http://scitation.aip.org/content/aip/journal/jcp/120/23?ver=pdfcov
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JOURNAL OF CHEMICAL PHYSICS
VOLUME 120, NUMBER 23
15 JUNE 2004
On the valence shell electronic spectroscopy of 2-vinyl furan
A. Giuliania)
´
´
`
ˆ
Laboratoire de Spectroscopie d’Electrons Diffuses, Universite de Liege, Institut de Chimie, Bat. B6c,
`
B-4000 Sart-Tilman Liege, Belgium
I. C. Walker
School of Engineering and Physical Sciences, Heriot-Watt University, Riccarton, Edinburgh EH144AS,
United Kingdom
J. Delwiche
´
`
ˆ
Thermodynamique et Spectroscopie, Universite de Liege, Institut de Chimie, Bat. B6c,
`
B-4000 Sart-Tilman Liege, Belgium
S. V. Hoffmann
˚
˚
Institute for Storage Ring Facilities, University of Arhus, Ny Munkegade, DK-8000 Arhus C, Denmark
C. Kech
´
`
ˆ
´
ˆ
Centre de Recherche du Cyclotron, Universite de Liege, Bat. B30, Allee du 6 aout,
`
B-4000 Sart-Tilman Liege, Belgium
b)
P. Limao-Vieira and N. J. Masonc)
˜
Department of Physics and Astronomy, University College London, Gower Street, London WC1 E6BT,
United Kingdom
M.-J. Hubin-Franskind)
´
´
`
ˆ
Laboratoire de Spectroscopie d’Electrons Diffuses, Universite de Liege, Institut de Chimie, Bat. B6c,
`
B-4000 Sart-Tilman Liege, Belgium
͑Received 14 July 2003; accepted 18 March 2004͒
The vacuum ultraviolet ͑VUV͒ absorption spectrum ͑3.50–10.33 eV, 350–120 nm͒ of gaseous
2-vinyl furan has been measured for the first time using both synchrotron radiation source and
electron energy loss spectroscopies with absolute cross section determinations. The He I
photoelectron spectrum obtained at higher resolution than previously has been interpreted with the
1
*
aid of semiempirical molecular orbital calculations. Three excited states of type ␲␲ are found
responsible for an intense and structured first band observed between 4.2 and 5.8 eV ͑295–214 nm͒.
Three triplet states were detected for the first time at about 2.46, 3.35, and 3.8 eV ͑477, 370, and 328
3
*
nm͒ which are, from the calculations, assigned as ␲␲ . Some partial Rydberg series, linked to IE1
and IE2 are identified. The VUV absorption spectrum bears little resemblance to that of the parent
compound, furan. The electronically excited molecule is found akin to a linear polyene. © 2004
American Institute of Physics. ͓DOI: 10.1063/1.1738642͔
I. INTRODUCTION
with furan, 2-vinyl furan is itself important as a synthetic
reagent in polymer chemistry being used to produce resins
and polymers.7 Moreover, the vinyl furan chemical structure
is known to have some antimicrobial, cytotoxic, and also
mutagenic properties that have been related to some elec-
tronic molecular parameters.8
Little work has been reported on the molecular proper-
ties of 2-vinyl furan. Its equilibrium structure and its vibra-
tional analysis have been revisited in a separated paper.9 It
appears that the planarity of the compound is well estab-
lished by both theory10,11 and experiment,12 which puts the
molecule into the Cs molecular symmetry point group. Evi-
dence exists that the trans form ͓depicted in inset Fig. 1͑a͔͒ is
the most stable conformer and predominates in the vapor
phase.9 To our knowledge, only one photoelectron
spectrum13 has been reported covering the 8–17 eV ioniza-
tion energy range. Assignments were made on a qualitative
basis by correlation with the ionization energies of furan and
ethylene. A photoabsorption spectrum has been reported only
The furan molecule is important in several fields of
chemistry. It is a pseudoaromatic heterocyclic ring that re-
tains properties related to those of the conjugated dienes and
this contributes to its versatility in organic synthesis.1 We
have already investigated the valence shell spectroscopy of
furan and some derivatives.2–6 We now present a comprehen-
sive spectroscopic study on 2-vinyl furan ͓Fig. 1͑a͒ inset͔ for
which there are only very few data in the literature.
As well as being of interest because of its relationship
a͒
Electronic mail: a.giuliani@ulg.ac.be
Also at: Departamento de Fisica, FCT-UNL, Quinta da Torre, P-2829-
b͒
516 Caparica, and Centro de Fisica Molecular, Complexo I, IST, Av. Ro-
visco Pais, P-1049-001 Liboa, Portugal.
Now at: Department of Physics and Astronomy, Centre for Molecular and
c͒
Optical Sciences, The Open University, Walton Hall, Milton
Keynes MK7 6AA, United Kingdom.
d͒
Directeur de Recherche F.N.R.S. Author to whom correspondence should
be addressed. Electronic mail: mjfranskin@ulg.ac.be
0021-9606/2004/120(23)/10972/11/$22.00
10972
© 2004 American Institute of Physics
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J. Chem. Phys., Vol. 120, No. 23, 15 June 2004
2-vinyl furan
10973
dc discharge in helium. The spectra were recorded by sweep-
ing a retarding voltage in steps of 1 meV between the ion-
ization chamber and the entrance slit of a hemispherical elec-
trostatic analyzer, operating in constant pass energy mode.
All recorded spectra were corrected for the electron transmis-
sion of the analyzing system. The overall resolution was
about 25 meV. The ionization energy scale was calibrated
2
using the xenon peaks (2P3/2 : 12.123 eV and P1/2 : 13.436
eV͒.18 The accuracy of the energy measurements is estimated
to be Ϯ2 meV.
B. The photoabsorption spectrometer
The photoabsorption spectrum was recorded using the
UV1 beamline19 of the ASTRID storage ring at Arhus Uni-
˚
versity, in Denmark. The normal incidence monochromator
was operated with the 2000 l/mm grating with an overall
resolution ͓full width at half maximum ͑FWMH͔͒ better than
0.1 nm. The radiation from the monochromator passed
through a LiF window into a gas cell of length 25 cm. A
Baratron capacitance manometer monitored the sample pres-
sure ͑less than 0.1 mbar͒, and the intensity of the radiation
exiting the cell through a CaF2 window was detected using
an UV enhanced photomultiplier tube ͑Electron Tube Lim-
ited, type 9406B͒. The spectral range extended from about
290 nm ͑4.275 eV͒ to the window cutoff at 113 nm ͑10.97
eV͒. This range was covered in sections of 11 nm using steps
of 0.1 nm; this step was found to be sufficiently small to
resolve any fine structure in the cross section. The radiation
transmitted through the empty cell (I0) was first recorded
over the limited ͑11 nm͒ range. The sample was then intro-
duced into the cell and two scans of transmitted radiation (It)
were recorded. The cell was subsequently evacuated and a
second (I0) recorded. The mean of each of the I0 and It
values was used in the Beer–Lambert law to evaluate the
cross section ␴:ItϭI0 exp(Ϫ␴ Nx), where N is the target gas
number density and x is the path length.
FIG. 1. ͑a͒ He I photoelectron spectrum of 2-vinyl furan in the 7.8–16.1 eV
region. ͑b͒ Enlargement of the 7.9–11.8 eV energy region. Inset ͑a͒: trans
2-vinyl furan molecule with atom numbering.
for the compound in solution over the limited 3.7–6.2 eV
energy range.14,15 The first band, centered around 4.8 eV, has
been interpreted as arising from transitions to two excited
states.
In the present study, we have measured for the first time
the electronic vacuum ultraviolet ͑VUV͒ excitation spectrum
of gaseous 2-vinyl furan using synchrotron radiation in the
3.5–10.3 eV energy range. A high-resolution electron-energy
loss ͑HREEL͒ spectrum has been obtained under electric di-
pole conditions, which allows direct comparison with the
optical spectrum and extends the excitation range up to 12
eV, not accessible with the synchrotron radiation. Electron
energy loss spectra have also been recorded using electrons
of near-threshold energy which favor the excitation of spin-
forbidden transitions. The photoelectron spectrum has been
remeasured at higher resolution than the previous measure-
ments. To help in the spectral assignments, semiempirical
neglect of diatomic differential overlaps ͑NDDO͒ calcula-
tions ͓PM3, AM1, and modified neglect of differential over-
lap ͑MNDO͔͒ with configuration interactions have been car-
ried out.
The averaging procedure compensated effectively for
decay of the radiation intensity as the storage ring current
decayed. Spectra are presented on an energy ͑eV͒ abscissa to
enable direct comparison with the electron energy loss data.
The accuracy of the cross section is estimated to be Ϯ5%.
C. The high resolution electron energy loss
spectrometer „HREELS…
The instrument used ͑VG-SEELS 400͒ has been de-
scribed in detail elsewhere.20 In this, an electrostatic electron
energy monochromator defines a narrow energy spread about
the mean incident electron energy and a three elements
lens focuses the electrons into the collision region. The elec-
tron beam intersects the gas beam, which flows through an
hypodermic needle, at 90°. The working pressure was
1ϫ10Ϫ5 mbar. The analyzer system is of the same type as
the monochromator. Both electron energy selectors work in
the constant pass-energy mode. The scattered electron signal
is detected by an electron multiplier of the continuous dyn-
ode type. Spectra were recorded for energy losses between
3.7 and 12 eV at step intervals of 8 meV. The resolution,
measured at the FWHM of the peak due to elastically scat-
II. EXPERIMENTAL METHODS
A. The photoelectron spectrometer
The photoelectron spectrometer has been described in
detail.16,17 Briefly, He I photons ͑58.4 nm͒ are produced by a
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10974
J. Chem. Phys., Vol. 120, No. 23, 15 June 2004
Giuliani et al.
tered electrons ͑‘‘elastic peak’’͒, was about 40 meV. The ap-
paratus was used with relatively high incident energy elec-
trons ͑100 eV͒ and a small scattering angle (␪ϳ0°), such
that electric dipole interaction conditions apply and the elec-
tron energy loss spectrum is comparable with the photoab-
sorption spectrum. The electron energy loss scale was cali-
brated to the ‘‘elastic peak.’’
The inelastic scattered intensity was converted to a rela-
tive differential oscillator strength distribution, using a
method developed previously.21 The latter was normalized
at 151.5 nm ͑8.184 eV͒ from the photoabsorption spec-
trum to obtain absolute cross section values, as explained
previously.4
Comparison of the HREELS cross section values with
those recorded using the synchrotron source provides a test
for any systematic errors in the optical values arising from
the line saturation effect and second order light from the light
source and beam line. These were found to be negligible in
this work.
level with configuration interaction (SDCIϭ10) and using
the PM3, AM1, and MNDO methods. The equilibrium struc-
tures were fully optimized using the eigenvector following
routine ͑EF keyword͒.27 The outer valence Green’s function
͑OVGF͒ method was used as described previously by
Danovich et al.28 for oxazole, up to the third order and using
18 occupied molecular orbital ͑MO͒ and 16 unoccupied MO.
Although those semiempirical methods provide helpful
results for the excited states, they are parameterized to best
reproduce some ground states properties. Therefore, their
outcomes should not be regarded as definitive but instead as
guides when excited state properties are computed. More-
over, MNDO, AM1, and PM3 rely on the same approxima-
tion ͑NDDO͒. Even though PM3 has been found to be usu-
ally in better agreement with experiment than the other
parameterizations,29 this is not uniform. Consequently, com-
parison of the three methods turns out to be useful and agree-
ments in the outcomes are believed to indicate more secure
results.
D. The trapped electron spectrometer
IV. RESULTS
Near-threshold electron energy loss spectra were re-
corded in a trapped electron spectrometer which has been
described elsewhere.22 Briefly, incident electrons emitted
from a filament are energy selected by a trochoidal mono-
chromator, which operates through the combined actions of
an axial magnetic field and a perpendicular electric field. The
energy-selected electrons enter the collision region where the
electrostatic potentials are such that those scattered electrons
whose residual energy ErрeW ͑where W is the so-called
trap well depth͒ are trapped and collected along with any
stable negative ions formed in dissociative attachment pro-
cesses. Negative ion currents can be identified because they
persist when Wϭ0, because the trajectories of the relatively
massive ions are not significantly influenced by the axial
magnetic field; ions therefore travel in straight lines from the
source and ͑unless emerging in a forward direction͒ are in-
tercepted by the collector. In the spectra presented here, any
negative ion signal has been removed leaving only signal due
to inelastically scattered electrons.
A. UV photoelectron spectrum
The He I photoelectron ͑PE͒ spectrum ͓Fig. 1͑a͔͒ was
recorded at room temperature and with a resolution of 24
meV, which is a little bit higher than in the previous study.13
It is quite similar to that published by Novak et al.13 At low
ionization energy three bands are observed ͓Fig. 1͑b͔͒, the
first two showing vibrational structure whose envelopes are
similar to those of the first two bands in furan and methyl
furan.4 The third band is diffuse and partly overlaps the sec-
ond band has no equivalent in furan.4 From about 12 to 16
eV, the spectrum shows a congested region of broad bands.
In this, it also exhibit similarities to the furan.
Based on the qualitative arguments of the composite
molecule approach, Novak et al.13 assigned the three lowest
energy bands to ionization from ␲ (a ) orbitals ␲ , ␲ , ␲
2
Љ
4
3
in order of increasing ionization energy ͑IE͒. The 12–16 eV
energy region was assigned to ionization from several ␴ or-
bitals together with ␲1 , the last having a vertical ionization
energy, IEVϳ15.4 eV. We have reviewed this interpretation
of the PE spectrum in light of our semiempirical calcula-
tions. The results are summarized at Table I. The PM3, AM1,
and MNDO methods give consistent results for the first ion-
The electron energy loss scale was calibrated with refer-
23
1
ence to the krypton P1 peak at 10.04 eV.
E. The sample
ization energies: 4a Ͼ3a Ͼ2a Ͼ14a Ͼ13a Ͼ12a
Ј
Љ
Љ
Љ
Ј
Ј
The synthesis was carried out according to Schmidt and
Werner.24 Briefly, thermal decarboxylation, catalyzed by
copper͑I͒ oxide, of furyl acrylic acid in quinoline under ar-
gon produced 2-vinyl furan. The crude material was dried
over magnesium sulfate and rectified on a 40 cm Vigreux
column. The purity, as determined by GC-MS, was better
than 99%. 1 ppm butylated hydroxytoluene was added as a
stabilizing agent. Boiling point 98–99 °C ͑Litt.
99–100 °C͒,24 1H–nuclear magnetic resonance.25
Ͼ11a . The average absolute energy difference between ex-
Ј
perimental and calculated SCF energies ranges from 0.53 for
AM1 to 0.70 eV for PM3. The use of the OVGF methods
gives better agreement with experiment ͑0.27 eV for PM3 to
0.36 for AM1͒, more secure assignations and, in addition,
solves apparent discrepancies at high ionization energies be-
tween the three parameterizations.
The previous assignments13 are confirmed for the three
lowest energy bands. Between 12 and 16 eV Novak et al.13
predicted seven ionic states but assigned only six of them
and suggested a ␴ orbital around 14 eV. The present calcu-
lations also find seven ionic states, ascribed to ionizations
from: 14a Ͼ13a Ͼ12a Ͼ11a Ͼ1a Ͼ10a Ͼ9a . As-
III. THEORETICAL METHODS
Semiempirical calculations were performed using the
MOPAC 2000 software package.26 All calculations were made
at the restricted Hartree–Fock self-consistent field ͑SCF͒
Ј
Ј
Ј
Ј
Љ
Ј
Ј
signments are straightforward for orbitals 13a –9a , the
Ј
Ј
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J. Chem. Phys., Vol. 120, No. 23, 15 June 2004
2-vinyl furan
10975
͓Fig. 1͑a͔͒ a vertical ionization energy of 12.25 eV for 14a .
Ј
We also reassign the first ␲ orbital (␲ ,1a ) placing its ion-
Љ
1
ization energy at 14.24 eV, below ␴ (10a ).
Ј
We turn now to consideration of the vibrational structure
2
˜
accompanying the two lowest ionization bands, X A and
Љ
2
˜
A A ͓Fig. 1͑b͔͒. As these states relate to removal of elec-
Љ
trons from ␲ orbitals, we expect the structure to involve
excitation of totally symmetric normal vibrational modes in
which the heavy atoms are displaced. This is a simplistic
picture, but it has been helpful in the interpretation of the
corresponding spectra in furan and its methyl derivative.4
The first adiabatic energy of vinyl furan is 8.057
Ϯ0.002 eV ͓Fig. 1͑b͒, Table I͔. There follows a vibrational
peak centered at 8.243 eV, 1498 cmϪ1 ͑186 meV͒ from the
0–0 transition. This peak is asymmetric on the low energy
side where a weak shoulder can be positioned at 1050 cmϪ1
͑130 meV͒ from the origin. The two weak broad bands at
higher energies may be mainly assigned to combination and
overtone bands of these two. However, the relatively poor
apparent resolution compared with, the corresponding
band in furan4 suggests that many vibrations are active,
adding to the line breadth. With this in mind, we propose
the following possible contributing vibrational modes
͑with ground state vibration wavenumbers͒; to the
main 1498 cmϪ1 peak-␯ (1419 cmϪ1), ␯ (1487 cmϪ1),
13
14
␯
(1573 cmϪ1), ␯ (1660 cmϪ1), ␯ (1753 cmϪ1), and
15
16 17
to the 1050 cmϪ1 feature-v4 (814 cmϪ1), ␯ (996 cmϪ1),
6
and ␯ (1175 cmϪ1). All the cited vibrations are totally
11
symmetric and involve displacement of the heavy atoms.9
FIG. 2. ͑a͒ Photoabsorption spectrum of 2-vinyl furan. ͑b͒ HREEL spectrum
of 2-vinyl furan recorded at 100 eV-0°, converted into differential oscillator
strength and normalized to the VUV spectrum at 8.184 eV. ͑c͒ Photoabsorp-
tion spectrum of furan ͑see Ref. 6͒. All spectra are in plotted in the 3.8–12
eV photon energy range ͑350–103 nm͒ on the same cross section scale. The
horizontal arrows indicate the cross section axis for each spectrum.
2
˜
Although diffuse, band A A resembles the correspond-
Љ
ing band in furan and methyl furan.4 The most intense peak
at 9.985Ϯ0.002 eV is assigned to the 0–0 transition. The
peak at 10.103 eV, is 952 cmϪ1 ͑118 meV͒ from this origin.
Possible contributing vibrational modes are ͑with ground
state wave numbers͒ v4 (814 cmϪ1), ␯ (996 cmϪ1), and
6
␯
(1175 cmϪ1). At least two weak shoulders are partly re-
spectral bands being sufficiently well resolved and calculated
energies being in reasonable agreement with the experiment.
11
solved on the low energy side of the peak around 10.05 and
10.08 eV, that is at 525 cmϪ1 ͑65.1 meV͒ and 766 cmϪ1 ͑95
For orbital 14a , calculated ionization energies lie between
Ј
meV͒ from the electronic origin. Modes ␯ (715 cmϪ1),
11.68 and 12.26 eV, that is on the low energy side of the
12.71 eV band where weak structure not well resolved is
visible as a shoulder around 12.25 eV. From this, we estimate
3
␯ (495 cmϪ1), and v4 (814 cmϪ1), may be implicated. The
2
2
˜
shoulders at higher energies, where band B A overlaps
Љ
TABLE I. Assignments of the ionization energies ͑eV͒ for trans 2-vinyl furan.
Experimental
PM3
AM1
MNDO
Orbital
a
b
SCF
OVGF
SCF
OVGF
SCF
OVGF
4a
3a
2a
14a
13a
12a
11a
1a
10a
9a
8a
8.271
10.264
10.74
12.25
12.72
13.04
13.68
14.24
14.97
15.3
8.14
10.07
10.76
¯
8.778
10.515
11.415
12.578
13.845
14.035
14.229
15.703
15.507
15.922
17.941
8.652
10.129
10.995
12.009
13.246
13.349
13.557
14.335
14.791
15.138
16.955
8.675
10.525
11.460
12.393
13.268
13.662
13.884
16.020
15.412
15.482
17.739
8.466
10.013
10.890
11.678
12.506
12.846
13.071
14.442
14.532
14.546
16.537
8.560
10.187
10.954
12.992
13.265
13.976
14.091
15.567
15.442
15.881
17.699
8.351
9.713
Љ
Љ
Љ
Ј
Ј
Ј
Ј
Љ
Ј
Ј
Ј
10.441
12.257
12.477
13.176
13.260
14.012
14.567
14.966
16.518
12.4
13.06
13.7
14.2
14.96
15.4
16.8
—
aThis work. Vertical IE are measured at the band center.
bReference 13.
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10976
J. Chem. Phys., Vol. 120, No. 23, 15 June 2004
Giuliani et al.
signed to
a
progression of 1450Ϯ150 cmϪ1 (180
Ϯ20 meV). Reijendam et al.14,15 have interpreted ͓from
Pariser-Parr-Pople ͑PPP͒ calculations͔ the 4.8 eV band as
arising from two transitions. The lowest transition, carrying
most of the intensity, was reported as nearly a pure N→V
transition. The other excited state was reported to be better
described as mixed. The authors did not label any of the
calculated states. A very weak, structured absorption is ob-
served as a tail to the low energy side of the 4.8 eV band
͓Fig. 3͑a͔͒. A corresponding feature may be discerned on the
low energy side of the solution spectrum of Reijendam
et al.,14,15 but it was not discussed by these workers.
Our computations find the three lowest singlet excited
*
valence states to be ␲␲ in nature and to be close in energy,
but they differ in the relative ordering of the states ͑Table II͒.
However, the computed oscillator strengths concur that the
*
␲ ␲ transition is the most intense and all three methods
4
5
*
predict the same lowest energy singlet ␲␲ state. Our as-
signments then follow: 2 1A (␲ ␲ ϩ␲ ␲ ) at ϳ4.50
*
*
Ј
4
3
6
5
1
1
eV, 3 1A ( ␲ ␲ ) at ϳ4.80 eV, and 4 A (␲ ␲
*
*
Ј
Ј
4
4
5
7
1
1
*
ϩ␲ ␲ ) at ϳ5.08 eV; the 2 A and 4 A states are found
Ј
Ј
2
5
to contain significant double excitations.
Both states 2 1A and 3 1A exhibit extensive, vibra-
Ј
Ј
tional structure. That accompanying the transition to state
2 1A is mainly due to two normal vibrational modes having
Ј
wave numbers of about 218 cmϪ1 ͑27 meV͒ and 459 cmϪ1
͑57 meV͒, respectively. We assign the lower of these to vi-
brational mode ␯ (1a ) whose ground state wave number is
Ј
1
about 214 cmϪ1; the higher is ␯ (2a ) whose ground state
Ј
2
wave number is 472 cmϪ1.9 The electronic origin is placed at
4.348 eV ͓Fig. 3͑a͔͒. Table III lists the band energies and
assignments.
FIG. 3. ͑a͒ Enlargement of the photoabsorption A band in the 4.2–5.7 eV
region. Adiabatic transitions to 2 1A , 3 1A , and 4 1A ͑thick lines͒ and
associated vibrational progressions are indicated. Hot bands are shown by
the vertical arrows. ͑b͒ Enlargement of the photoabsorption region contain-
ing the Rydberg states. The vertical lines marks the three lowest ionization
energies. The photoelectron spectrum of the first band is also shown.
Ј
Ј
Ј
The vibrational fine structure accompanying excitation
of state 3 1A is more complex ͓Fig. 3͑b͒ and Table IV͔. It is
Ј
dominated by a series of peaks regularly spaced by 206
cmϪ1, consistent with excitation of mode ␯1 ; it may be de-
scribed mainly as a symmetric C6 –C7 bending motion.9 This
allows us to place the electronic origin at the weak feature
located at 4.655 eV; excitation of a vibrational progression in
␯ ͑up to five quanta͒ follows. Repetition of the ␯ progres-
with the third band, are suggested to be ascribed to combi-
nation and overtone bands.
B. Spin allowed valence excited states
1
1
sion throughout the band may, in part, be rationalized and
The total photoabsorption spectrum is presented at Fig.
2͑a͒, with the 100 eV Ϫ0° normalized HREEL spectrum at
Fig. 2͑b͒. The two spectra are in excellent agreement, differ-
ences arising only from different energy resolution; the EEL
data extend measurements up to 12 eV. A single photoab-
sorption spectrum recorded on the condensed phase ͑solu-
tion͒ has been reported in a limited energy range ͑3.7–6.2
eV͒.14,15 Where overlapping, the shape and energy position
are in good agreement with the present data.
interpreted by combination of ␯ with other normal modes
1
having wave numbers of about 478, 833, and 1393 cmϪ1
,
respectively ͑Table V͒. The series of peaks indicated by ar-
rows in Fig. 5, also show the regular spacing of 206 cmϪ1
and lie about 130 cmϪ1 from the major ␯ progression. These
1
may be ‘‘hot’’ bands, arising from excitation of ␯ from a
1
ground state vibrationally excited of 1␯24 , as the wave num-
ber of the latter in the electronic ground state is 129 cmϪ1 9
.
We should also consider the possibility that they originate
from the 0–0 transition of the cis conformer of 2-vinyl furan.
Hexatriene provides a valuable matter for comparison since
the photoabsorption spectra of both rotamers have been re-
ported for the lowest energy transitions with identification of
the 0–0 transitions and the vibronic progressions.30 It ap-
pears in this system that the same progressions have been
observed in the cis and trans conformations and that the elec-
tronic origin transitions have been found separated by about
In the following discussion, we divide the spectra into
two spectral regions ͑A and B͒.
1. The A spectral region (4–6 eV)
The first optical band extends from about 4.1–5.8 eV
͓Fig. 3͑a͔͒ and clearly contains several transitions. The main
photoabsorption band, centered around 4.8 eV, is broad and
structured. Its position and intensity are in good agreement
with reported solution spectra.14,15 The condensed phase
measurements showed only weak vibrational structure, as-
130 cmϪ1 30
However, the relative intensity and the spacing
.
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J. Chem. Phys., Vol. 120, No. 23, 15 June 2004
2-vinyl furan
10977
TABLE II. Calculated sequential valence excited states for trans 2-vinyl furan and comparison with experiment. Energy values are given in eV.
PM3
AM1
MNDO
Leading opens
shellsa
Expt.c
EV
b
b
b
Eex
Eex
Eex
Label
fr
fr
fr
Singlet states
2
3
4
1
2
5
6
7
3
4
1A
1A
1A
1A
1A
1A
1A
1A
1A
1A
3.94
4.49
4.51
5.27
6.04
6.05
6.27
6.38
6.60
6.92
0.048
1.266
0.086
Ͻ10Ϫ4
0.002
0.023
1.184
0.357
4ϫ10Ϫ4
Ͻ10Ϫ4
3.82
4.46
4.37
5.01
6.37
5.94
6.27
6.36
6.73
6.88
0.053
1.393
0.004
2ϫ10Ϫ4
0.042
0.024
0.662
0.442
Ͻ10Ϫ4
6ϫ10Ϫ4
3.44
4.32
3.93
6.29
7.08
5.35
5.90
6.01
7.42
¯
0.025
1.455
0.002
4.50
4.80
5.08
—
*
*
␲ ␲ ϩ␲ ␲
Ј
Ј
Ј
Љ
Љ
Ј
Ј
Ј
Љ
Љ
4
3
6
5
*
␲ ␲
4
5
*
*
␲ ␲ ϩ␲ ␲
4
2
7
5
␲ 15a
Ͻ10Ϫ5
Ͻ10Ϫ5
0.043
0.796
0.656
Ͻ10Ϫ5
¯
*
*
Ј
Ј
4
␲ 16a
—
4
*
*
ϳ6.2
7.26
ϳ7.5
—
␲ ␲ ϩ␲ ␲
2
3
6
7
*
*
␲ ␲ ϩ␲ ␲
3
4
5
6
*
*
*
␲ ␲ ϩ␲ ␲ ϩ␲ ␲
4
2
3
7
5
6
*
␲ 15a
Ј
3
*
—
14a ␲
Ј
5
Triplet states
1
2
3
4
3A
3A
3A
3A
2.39
3.28
3.74
4.83
2.29
3.20
3.66
4.75
2.13
2.89
3.26
4.22
2.46
3.35
3.78
—
*
␲ ␲
Ј
Ј
Ј
Ј
4
5
*
*
␲ ␲ ϩ␲ ␲
4
3
6
5
*
*
*
␲ ␲ ϩ␲ ␲ ϩ␲ ␲
4
2
3
7
5
6
*
*
*
␲ ␲ ϩ␲ ␲ ϩ␲ ␲
3
3
3
6
7
5
aPM3 leading open shells.
bOscillator strengths.
cExperimental vertical excitation energies are measured at the center of the band.
of the vibrational progressions have not been reported ex-
actly the same in both hexatriene rotamers.30 This contrasts
with the present case in which the relative intensity and the
spacing of those four peaks are almost identical to those
A vibrational progression, spacing 196 cmϪ1, is visible
in the absorption assigned to excitation of state 4 1A . As for
Ј
the lower energy states, it is assigned to excitation of mode
␯ and the electronic origin is placed at 5.08 eV. The shift in
1
found in the main ␯ progression, which seems unlikely if
they relate to another electronic origin.
vibrational wave number from that observed in the adjacent
1
3 1A state supports assignment of this structure to a separate
Ј
Another mode, unlabeled in Fig. 3͑b͒, with a 2250 cmϪ1
wave number is also deduced from our analysis. In this en-
ergy region, no wave number has been found for the mol-
ecule in its electronic ground state,9 suggesting that this pro-
gression arises from multiple excitation or combination. As
the 2250 cmϪ1 value is unlikely to result from two quanta
excitation of the mode so far assigned, it could be due to a
excited state.
2. The B spectral region (6–7.75 eV)
A second absorption band extends from about 6–7.75 eV
with a maximum around 7.1 eV ͓Figs. 2͑a͒ and 3͑b͔͒. The
combination of ␯ and ␯13/14 . The possibility of a two quanta
TABLE IV. Band position, wave number and assignment of the vibrational
4
structure observed for the 3 1A excited state.
Љ
excitation of a nontotally symmetric a mode with a wave
Љ
number around 1125 cmϪ1 should also be considered.
⌬E ͑cmϪ1
͒
Eex ͑eV͒
Assignment
0–0
4.655
4.664
4.680
4.690
4.706
4.717
4.733
4.746
4.759
4.765
4.783
4.791
4.811
4.838
4.853
4.863
4.878
4.904
4.930
4.956
4.985
5.011
—
74
206
284
413
503
631
738
840
1␯1 –1␯
24
TABLE III. Band position, wave number, and assignment of the vibrational
1␯
structure observed for the 2 1A excited state.
1
Љ
2␯1 –1␯
24
2␯
⌬E ͑cmϪ1
͒
1
Eex ͑eV͒
Assignment
0–0
3␯1 –1␯
24
4.348
4.375
4.401
4.405
4.428
4.432
4.458
4.462
4.482
4.488
4.512
4.518
4.541
4.547
4.568
4.576
—
218
430
459
645
674
889
915
1081
1129
1323
1371
1557
1605
1783
1839
3␯
1
1␯
4␯1 –1␯
1
24
2␯
4␯
1
1
1␯
891
1␯2ϩ2␯
1␯4ϩ1␯1/5␯
1␯2ϩ3␯
1␯4ϩ2␯
1␯4ϩ3␯
1␯13/14ϩ1␯
1␯4ϩ4␯
1
1␯13/14ϩ2␯
1␯13/14ϩ3␯
1␯13/14ϩ4␯
1␯13/14ϩ5␯
1␯4ϩ1␯13/14ϩ2␯
1␯4ϩ1␯13/14ϩ3␯
2
1
3␯
1039
1099
1264
1476
1598
1683
1806
2016
2220
2435
2668
2868
1
1
1␯2ϩ1␯
1
1
1␯2ϩ2␯
1
1
2␯
2
1
1␯2ϩ3␯
1
1
2␯2ϩ1␯
1
2␯2ϩ2␯
1
1
3␯
2
1
1
1
2␯2ϩ3␯
1
3␯2ϩ␯
1
3␯2ϩ2␯
1
1
1
4␯
2
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10978
J. Chem. Phys., Vol. 120, No. 23, 15 June 2004
Giuliani et al.
TABLE V. Ground and excited state vibrational wave numbers observed for
2-vinyl furan expressed in cmϪ1
.
States
Normal mode
X
1A
2
1A
3
1A
4
1A
Ј
Ј
Ј
Ј
␯
␯
␯
214a
472a
218
459
—
206
478
833
196
—
—
1
2
885a
4
␯
1385a
1487a
—
1393
—
13
␯
14
aReference 9.
fine structure on this band is ascribed to excitation of Ryd-
berg states ͑see below͒ and the underlying continuum to
1
1
valence-excited states. Three ␲␲ (1A ), three ␲␴
*
*
Ј
( A ), and one ␴␲ (1A ) excited states are computed to
1
1
*
Љ
Љ
lie in this energy range ͑Table II͒. Oscillator strengths for the
1
*
*
A excited states (␲␴ and ␴␲ ) are predicted to be very
Љ
low and hence we suppose, absorption in the B region to be
1
mostly due to excitation of three ␲␲ (1A ) states. Of
*
Ј
these, the first, 5 1A (␲ ␲ ϩ␲ ␲ ) has the lowest com-
*
*
Ј
2
3
6
7
puted oscillator strength by all methods and is calculated to
be separated from 6 1A and 7 1A while the last two are
Ј
Ј
predicted to be close in energy. We therefore suggest that
excitation of state 5 1A gives the intensity around 6.2 eV
Ј
1
1
*
*
*
band. Then states 6 A (␲ ␲ ϩ␲ ␲ ) and 7 A (␲ ␲
4
7
Ј
Ј
3
4
5
6
*
*
ϩ␲ ␲ ϩ␲ ␲ ) are together responsible for the broad ab-
2
3
5
6
sorption between about 6.6 and 7.6 eV. The absolute calcu-
lated energies are adrift from the experimental values by 1–2
eV ͑Table II͒, and thus provide qualitative information only.
C. Spin forbidden valence excited states
Near-threshold electron energy loss spectra, recorded for
residual electron energies Er of about 1, 0.75, and 0.5 eV,
respectively, are displayed in Fig. 4. The HREEL 100 eV
Ϫ0° spectrum is included for comparison.
The threshold spectra of Fig. 4 show a transition at about
2.4 eV. A shoulder at about 3.7 eV gains intensity when the
residual electron energy is increased from 0.5 to 1 eV. Fur-
ther, the energy of maximum intensity shifts with increasing
residual electron energy, pointing to the presence of more
than one energy loss. That is, there are at least three energy
loss processes in vinyl furan between about 2 and 4 eV
which are absent from the 100 eV Ϫ0° HREEL spectrum;
they are assigned to optically forbidden, singlet–triplet tran-
sitions.
FIG. 4. Threshold electron energy loss spectra of 2-vinyl furan recorded at
different residual energies (Er). The dashed lines show the result from a
Gaussian fitting. The 100 eV–0° HREEL spectrum is added at the bottom.
The insets show subsequent fitting of the second lowest energy band by two
Gaussians.
The three lowest-energy triplet states are predicted to be
3
*
␲␲ in nature and to lie in the 2–4 eV region ͑Table II͒.
The first of these, 13A , is mainly highest occupied molecu-
Ј
lar orbital–lowest unoccupied molecular orbital ͑HOMO–
LUMO͒ in character; computed energies lie between 2.13 eV
modified neglect of differential overlap ͑MNDO͒ and 2.39
eV ͑PM3͒ ͑Table II͒. The 2.4 eV electron energy loss band is
attributed to excitation of this state. The two other triplet
states, which are mixed states, are predicted to be separated
by about 0.4 eV. As indicated above, the 3.7 eV band is
believed to contain more than one transition. In fact, the
band shape can be reproduced by fitting with two Gaussian
functions, each centered around a fixed energy loss, as illus-
trated in the insets to Fig. 4. From these fits, our experimen-
tal energies for states 2 3A and 3 3A are 3.35 and 3.78 eV,
Ј
Ј
values which are very close to those computed ͑Table II͒.
Allan et al.31 reported an electron impact observation in trans
octatetraene of 1 3Bu and 2 3Bu at, respectively, 2.1 and 3.55
eV ͑vertical͒ but could not localize 1 3Ag . They pointed out
that excitation of this state is complicated by the fact that in
addition of being spin forbidden it is also symmetry forbid-
den, which results in low cross section even in experimental
conditions that favor excitation of triplet states. In the
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J. Chem. Phys., Vol. 120, No. 23, 15 June 2004
TABLE VI. Energy value ͑eV͒, quantum defect and assignment for the Rydberg states.
2-vinyl furan
10979
Series
En
␦
Series
En
␦
␲4→ 3s
5.362
0.735
␲3→ 3s
6.981
8.72
9.225
0.872
0.72
0.769
4s
5s
␲4→ 3p
5.839
0.523
Avg.
0.787
␲4→ 3p
5.951
5.976
0.458
0.443
Std. dev.
Ϯ0.078
Ј
␲4→ 3p
␲3→ 3p
7.602
8.837
9.343
0.611
0.56
0.396
Љ
4p
5p
␲4→ 3d
6.129
7.14
7.465
0.343
0.148
0.206
Avg.
Std. dev.
0.522
Ϯ0.112
4d
5d
Avg.
0.232
Std. dev.
Ϯ0.100
␲3→ 3p
7.711
8.965
9.378
0.554
0.348
0.396
Ј
Ј
Ј
4p
5p
␲4→ 3d
4d
6.444
7.171
0.096
0.081
Avg.
Std. dev.
0.433
Ϯ0.108
Ј
Ј
Avg.
0.089
Std. dev
Ϯ0.011
␲4→ 3d
6.643
7.192
7.510
7.682
Ϫ0.102
0.035
0.014
Љ
Љ
Љ
Љ
4d
5d
6d
␲3→ 3d
8.428
9.11
9.42
0.044
0.057
0.093
Ј
Ј
Ј
4d
5d
Ϫ0.022
Avg.
Std. dev.
Ϫ0.019
Ϯ0.060
Avg.
Std. dev.
0.064
Ϯ0.025
␲3→ 3d
8.55
9.168
9.436
Ϫ0.079
Ϫ0.070
0.022
Љ
Љ
Љ
4d
5d
␲4→ 3d
4d
6.684
7.242
Ϫ0.148
Ϫ0.081
Avg.
Std. dev.
Ϫ0.042
Ϯ0.056
ٞ
ٞ
Avg.
Std. dev.
Ϫ0.115
Ϯ0.047
present case, although 2 3A seems to be of lower intensity
is the excitation energy of the Rydberg state, IE is the limit-
ing ionization energy, R is the Rydberg constant, n is the
principal quantum number of the upper orbital ͑here у3͒,
and ␦ is the quantum defect. Designation of a series as s,p, or
d depends on the quantum defect.4,6 Identification of Ryd-
berg states is further helped by the fact that the envelope of a
photoabsorption band resulting from excitation of a pure Ry-
dberg state resembles that of the corresponding ionization
band in the photoelectron spectrum.
Figure 3͑b͒ shows an enlargement of the photoabsorp-
tion spectrum in the 5.2–11 eV range, the Rydberg series
region. Table VI summarizes present assignments for partial
Rydberg series, giving peaks position and their quantum de-
fect. Also the mean ␦ value and the standard deviation is
reported for each series; a large standard deviation in ␦ indi-
cates less secure assignments.
Ј
than 3 3A , it is not symmetry forbidden. Its corresponding
Ј
singlet state (2 1A ) has finite oscillator strength and is de-
Ј
tected by photoabsorption. Therefore, fitting the 3.7 eV band
to determine the bands position makes sense.
The 5 eV optical band, assigned to mainly excitation of
2 1A , 3 1A , and 4 1A valence states, is also observed in
Ј
Ј
Ј
the threshold energy loss spectra. The position of maximum
energy loss shifts to higher energy as Er is increased, indica-
tive of several contributing processes whose cross sections
vary differently with increasing residual electron energy.
These may include the tripet state 4 3A .
Ј
D. Rydberg states
The three lowest ͑adiabatic͒ ionization energies have
been established at 8.057, 9.985, and 10.74 eV from photo-
electron spectroscopy. Rydberg states converging to these
three ionic states could contribute to the present photoab-
sorption spectrum. The Rydberg states are classified accord-
ing to the Rydberg formula: EnϭIEϪR/(nϪ␦)2, where En
The first members (nϭ3) for the ␲4→ns and np are
reported with consistent ␦ values ͑Table VI͒ but they are very
weak and higher members of these series could not be pos-
tulated. Some of the 3d members are relatively intense and
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10980
J. Chem. Phys., Vol. 120, No. 23, 15 June 2004
Giuliani et al.
TABLE VII. Experimental oscillator strength for 2-vinyl furan.
Optical oscillator strength
TABLE VIII. Adiabatic ionization energies ͑IE͒ for 2-vinyl furan, furan and
2-methyl furan ͑values in eV͒.
IE1
IE2
Energy region ͑eV͒
Photoabsorption
HREELS
d
e
d
e
Compound
Furana
Energy
8.883
⌬
⌬
Energy
10.308
⌬
⌬
2
1
2
1
4.24–6.04
6.04–7.708
7.708–8.806
8.806–9.620
9.620–10.772
10.772–12.000
0.383
0.407
0.325
0.230
0.519
—
0.413
0.403
0.322
0.224
0.443
0.582
—
—
—
—
—
—
2-methyl furanb 8.381 Ϫ0.502
10.024 Ϫ0.284
2-vinyl furanc
8.057 Ϫ0.826 Ϫ0.324 9.985 Ϫ0.323 Ϫ0.039
aReference 32.
bReference 4.
cThis work.
dEnergy difference from furan.
eEnergy difference from 2-methyl furan.
show the vibrational fine structure expected from the first
band in the photoelectron spectrum. ͓See Fig. 3͑b͒ where the
photoelectron band is included for comparison͔. The 4d se-
ries block lies in the 7.14–7.24 eV region ͓Fig. 3͑b͒, Table
VI͔, where several members have been identified. Some fea-
tures around 7.3–7.4 eV may be vibrational fine structure
accompanying these members.
lation. Further, in this simple picture, the third ionization in
vinyl furan, IEϳ10.7 eV, which encroaches on the furan-like
spectrum, is close to the ␲-ionization energy of ethene
͑10.51 eV͒. Novak and Klasinc13 also made this connection,
localizing ␲ on the ethene moiety. Indeed, most of the elec-
2
The intense peak at 6.981 eV, which dominates this spec-
tral region, is assigned to the 3s Rydberg state linked to the
second ionization limit ͑9.985 eV͒, ␦ϭ0.872; there is no
other plausible Rydberg assignment. The corresponding 4s
state should lie around 8.5 eV, but it cannot be identified in
the spectrum. In fact, all structures in this region are weak
and diffuse ͓Fig. 3͑b͔͒, suggesting interactions for 4s and
higher Rydberg states leading to autoionization and/or pre-
dissociation. This is in contrast to furan.
No Rydberg series converging to the third ionization
limit are reported. It is noted that the spectral region between
ionization limits two and three remains totally unstructured
͓Fig. 3͑b͔͒.
tron density in ␲ is found concentrated on the terminal C
2
atoms C –C , inset Fig. 1͑a͔͒.
͓
6
7
Table VIII compares the adiabatic ionization energy for
the three molecules and includes the energy shifts relatively
to furan32 and 2-methyl furan.4 Differences between the
␲-ionization energies of 2-methyl furan and those in furan
have been explained by ␴ –␲ hyperconjugation and induc-
␲
tive effects. These effects are more marked on the ␲3(a2)
orbital, in which electron density is concentrated along
C2 –C3 and C4 –C5 , than for ␲2(b1) which has a nodal plane
along C2 –C5 where the methyl is attached to the ring.
The effect of the vinyl group may be explained by con-
jugation with the rest of the molecule and the IE shift values,
⌬͑IE͒, are of the expected magnitude.33 Inspection of the
E. Oscillator strength
⌬͑IE͒ in Table VIII shows that the ␲ orbital of 2-vinyl furan
4
is more affected by conjugation than the ␲ orbital: the
former is shifted of Ϫ0.826 eV and the latter by Ϫ0.329 eV,
3
Photoabsorption and HREEL spectra are displayed in the
3.8–12 eV region at Figs. 2͑a͒ and 2͑b͒. Comparison of the
optical cross section values with those obtained by HREELS
is essential to be sure that the optical measurements were
free of line saturation effects and second order light. More-
over, the HREEL spectrum allows us to provide values of
oscillator strength up to 12 eV ͑103 nm͒, that is in a little
more extended energy range than available by photoabsorp-
tion due to the limited energy range of the monochromator.
Table VII gives the optical absorption oscillator strength
measured by HREELS and photoabsorption. There is an ex-
cellent agreement between the two sets of data at least up to
9.6 eV. Above this energy, the optical values become less
accurate possibly due to a reduction in the transparency of
the cell windows.
with respect to furan. Moreover, ␲ is lowered by Ϫ0.324
4
eV from methyl furan ␲2 , while ␲ is only slightly affected
3
͑Ϫ0.039 eV͒. This can be related to the respective position of
the molecular orbital’s nodal plane of furan along C2 –C5 in
the furan cycle.
We turn now to the electronically excited states. In ben-
zene and aromatics, substituent groups have been catego-
rized into two classes based on their effects on the electronic
spectrum of the parent compound.33 Briefly, the first class
͑which includes, for example, alkyl groups͒ causes energy
shifts and intensity changes in the benzenoid bands without
introducing new ones. Groups belonging to the second class
affect the spectrum in a similar way but introduce new ab-
sorption bands, reflecting stronger interactions with the aro-
matic ring. Styrene ͑‘‘vinyl benzene’’͒ is in the second class.
It presents essentially a benzene spectrum with additional
perturbing levels lying above the benzene set.33–35 In con-
trast, the photoabsorption spectrum of furan cannot be rec-
ognized in that of 2-vinyl furan as seen in Figs. 2͑a͒ and 2͑c͒
where the spectra of both molecules are compared on the
same scale. We therefore should consider whether the excited
states of 2-vinyl furan relates to another family of com-
pounds. Van Reijendam and Janssen14,15 have studied the
V. DISCUSSION
We first consider the information gleaned from the pho-
toelectron spectrum. As discussed above, the first two ioniza-
tion bands are ascribed to ionization out of ␲ and ␲ mo-
4
3
lecular orbitals which have been correlated by Novak and
Klasinc13 to the ␲3(a2) and ␲2(b1) orbitals of furan. The
similarity of the corresponding band shapes in furan,4
2-methyl,4 and 2-vinyl furan ͓Fig. 1͑b͔͒ supports this corre-
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J. Chem. Phys., Vol. 120, No. 23, 15 June 2004
2-vinyl furan
10981
theoretically predicted to occur between hexatriene and
octatetraene.42
The present results for 2-vinyl furan show a weakly al-
1
1
*
lowed ␲␲ excited state (2 A ) lying below ͑and overlap-
Ј
ping͒ another and intense ␲␲ transition (3 1A ). On in-
1
*
Ј
tensity grounds, we link this state 2 1A with A /A in furan
Ј
g
1
and polyenes. Further, in support of this, our results find the
1
*
*
2 A state to be a mixture of ␲ ␲ and ␲ ␲ containing
Ј
4
3
6
5
significant amounts of doubly excited configurations, similar
to Ag /A1 in the related molecules.36,42,43 Observation of the
2 1A state in 2-vinyl furan, although of low cross section, is
Ј
not surprising considering the low molecular symmetry
which does not forbid the transition. Also, the 3 1A state is
Ј
mainly HOMO–LUMO in character, as are the lowest en-
ergy Bu /B2 states.36,42,43 Those considerations plead in favor
of a polyenic description of the lowest excited states of
2-vinyl furan. This hypothesis may be further checked by the
comparison of the lowest vertical transition energies of the
molecule, for both triplet and singlet excited states, with
those of the linear polyenes. Previous works have shown
that, for the polyenes, plots of the transitions energies versus
the number of double bonds show a smooth and regular low-
ering of the energies as the number of double bonds
increase.40,42 In order to compare furan and 2-vinyl furan
with the polyene family, we have plotted in Fig. 5 the verti-
cal transition energies versus the number of pz atomic orbit-
als contributing to the conjugated system. The above-
mentioned regular lowering of the excitation energies of the
polyene is observed as the number of pz orbitals increases
from ethene (C2H4 ,pzϭ2) to octatetraene (C8H10 ,pzϭ8).
Obviously, furan (pzϭ5) does not fit this pattern; in particu-
lar, its triplet state energies are higher than those of butadiene
(pzϭ4). This is very likely because of the aromatic charac-
ter of the furan ring and its resulting stabilization energy.
Figure 5 confirms that 2-vinyl furan does not relate with
furan but instead matches the polyenes trends for both the
singlet and the triplets. The correlation with the polyene is
particularly smooth for the lowest triplet state, which is
thought to be, in all cases, a simple HOMO→LUMO transi-
tion.
FIG. 5. Excitation energy of lowest energy singlet ͑open symbols͒ and trip-
let ͑full symbols͒ excited states versus the number of pz orbitals for ethene
͑Refs. 40 and 43͒ (C2H4 ,pzϭ2), butadiene ͑Refs. 40 and 43͒ (C4H6 ,pz
ϭ4), furan ͑Refs. 2, 6, and 36͒ (pzϭ5), hexatriene ͑Refs. 40 and 43͒
(C6H8 ,pzϭ6), 2-vinyl furan ͑this work͒ (pzϭ7), and octatetraene ͑Refs.
31, 40, and 42͒ (C8H10 ,pzϭ8). The dashed lines join the linear polyenes
together, indicating the ‘‘pure’’ polyene trend. Octatetraene: the starred sym-
bols refer to calculations ͑Refs. 31 and 40͒.
first band in the UV absorption spectra in solution of several
furans substituted in position 2 by polyenyl groups of in-
creasing size. They have found that the ␭max of this family of
substituted furans follows that of the linear polyenes. We
consequently discuss in the following the possible polyenic
character of the excited states of 2-vinyl furan.
In cyclopentadiene and furan36 the
␲
molecular
orbitals
are,
in
order
*
of
*
increasing energy:
*
2b1(␲2) 1a2(␲3) 3b1(␲4 ) 2a2(␲5 ). The ␲␲ electronic
transitions lead to two excited states of symmetry A1 and two
of symmetry B2 . In furan a A1 /1B2 pair lies close together
1
within the band centered about 6 eV ͑Fig. 2͒, with most of
From these considerations, we relate the electronically
excited states of 2-vinyl furan (pzϭ7) to those of the linear
polyenes and propose that this molecule may be the shortest
polyene in which the lowest energy singlet excited state is
Ag /A1 in nature. This suggestion is not at variance with re-
cent theoretical works,42 which have predicted the Ag –Bu
crossing between hexatriene (pzϭ6) and octatetraene
(pzϭ8).
1
the intensity coming from B2 . It now seems likely, from
theoretical work,36–39 that this state, 1B2 , is the lowest-lying
1
valence-excited state with the A1 state at a slightly higher
energy. However, no secure experimental detection has been
reported for A1 and not all theoretical studies6,36–39 are in
1
accord with the energy ordering.
For linear polyenes, the nature of the lowest energy ex-
cited state has also been debated.40–43 It is now believed that,
for longer polyenes starting from octatetraene,40,42 the lowest
singlet transition leads to an excited state of the same sym-
metry as the ground states (Ag in C2h , A1 in C2v). As this
state is one photon forbidden for centro-symmetric mol-
ecules, the excitation energy is difficult to establish from
experiment. The next excited state is close in energy and, as
it is one photon allowed (Bu /B2), it gives rise to a strong
spectral band. For shorter polyenes, most recent
calculations42,43 suggest that in cis and trans butadiene and
hexatriene, the Ag /A1 state is not the lowest in energy but
that it lies slightly above Bu /B2 . The crossing has been
VI. CONCLUSIONS
2-vinyl furan spectroscopic properties have been inves-
tigated for the first time using a large number of techniques.
The experimental data have been interpreted guided by the
results of semiempirical calculations. The photoelectron
spectrum has been interpreted. Excited states, including low-
lying triplet states have been observed and characterized for
the first time. From these studies, we have concluded that the
electronically excited molecule is more akin to a linear poly-
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10982
J. Chem. Phys., Vol. 120, No. 23, 15 June 2004
Giuliani et al.
ene. This relationship with linear polyenes for the excited
states of 2-vinyl furan should lead to further interest in its
photophysical properties, as a great amount of research is
concerned with those properties in polyenes and related com-
pounds.
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19
`
The Patrimoine of the University of Liege, the Fonds
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25
3
͑250 MHz in CDCl3 , RT͒ ␦ϭ5.17 ppm ͓dd, 1H, 2J␤␥ϭ1.20 Hz, J␣␥
for the award of beam time and thank the staff there. We
acknowledge the support of the European Community-
Access to Research Infrastructure Action of the Improving
Human Potential Program ͑Contract No. HPRI-CT-2001-
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lowing synthesis in the Center de Recherche du Cyclotron,
ϭ11.34 Hz, H␥], ␦ϭ5.67 ppm ͓dd, 1H, 2J␤␥ϭ1.20 Hz, 3J␣␤
ϭ17.44 Hz, H␤], ␦ϭ6.27 ppm ͓d, 1H, 3J34ϭ3.24 Hz, H3], ␦ϭ6.38 pm
3
͓dd, 1H, 3J45ϭ1.78 Hz, 3J34ϭ3.16 Hz, H4], ␦ϭ6.51 ppm ͓dd, 1H, J␣␥
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ϭ11.28 Hz, J␣␤ϭ17.49 Hz, H ], ␦ϭ7.37 ppm s,1H,H
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