Photochemistry of cyclopentadiene isolated in low-temperature argon matrices

Article abstract of DOI:10.1016/j.molstruc.2004.01.023

The photochemistry of cyclopentadiene isolated in low-temperature argon matrices was studied by means of IR and UV/VIS spectroscopy. Bicyclo[2.1.0]pent-2-ene was formed by the irradiation of matrix-isolated cyclopentadiene using a super-high-pressure mercury lamp. When the matrix-isolated cyclopentadiene was irradiated with shorter wavelength using a low-pressure mercury lamp, further reactions of bicyclo[2.1.0]pent-2-ene were found to produce allylacetylene and vinylallene. While the photochemistry of cyclopentadiene to form bicyclo[2.1.0]pent-2-ene is known in a solution system, the production of allylacetylene and vinylallene in a matrix-isolated system has never been previously reported. The assignments of the species and the determination of the reaction mechanisms were performed using molecular orbital calculations.

Full text of DOI:10.1016/j.molstruc.2004.01.023

Journal of Molecular Structure 692 (2004) 145–153
www.elsevier.com/locate/molstruc
Photochemistry of cyclopentadiene isolated
in low-temperature argon matrices
*
Jun Miyazaki, Yasuhiro Yamada
Department of Chemistry, Faculty of Science, Science University of Tokyo, 1-3 Kagurazaka, Shinjuku-ku, Tokyo, 162-8601, Japan
Received 14 October 2003; revised 20 January 2004; accepted 20 January 2004
Abstract
The photochemistry of cyclopentadiene isolated in low-temperature argon matrices was studied by means of IR and UV/VIS spectroscopy.
Bicyclo[2.1.0]pent-2-ene was formed by the irradiation of matrix-isolated cyclopentadiene using a super-high-pressure mercury lamp. When
the matrix-isolated cyclopentadiene was irradiated with shorter wavelength using a low-pressure mercury lamp, further reactions of
bicyclo[2.1.0]pent-2-ene were found to produce allylacetylene and vinylallene. While the photochemistry of cyclopentadiene to form
bicyclo[2.1.0]pent-2-ene is known in a solution system, the production of allylacetylene and vinylallene in a matrix-isolated system has never
been previously reported. The assignments of the species and the determination of the reaction mechanisms were performed using molecular
orbital calculations.
q 2004 Elsevier B.V. All rights reserved.
Keywords: Cyclopentadiene; Photochemistry; Matrix-isolation; Infrared spectroscopy; Molecular orbital calculation
1
. Introduction
Cpd, with sensitizers such as acetophenone, was irradiated
with a UV lamp to produce endo- and exo-dicyclopenta-
diene and trans-(3,0,3,0)-tricyclo-2,8-decadiene in approxi-
mately a 1:1:1 ratio [1].
Cyclopentadiene (Cpd) is commonly used as a diene-
donor in the Diels–Alder reaction [1]. It is also known as a
ligand which coordinates to metal atoms in the form of
cyclopentadienyl anion (Cp). It is well known that
ferrocene, which has two Cp ligands, coordinates to iron
atoms; the delocalized electrons in the five-membered ring
Matrix isolation is a technique for trapping a highly
reactive atom or unstable molecule, radical, or ion in inert
solid such as low-temperature Ar. Using this technique, it is
possible to study photochemical reactions of the sample
without the intervening effects of solvents. The reaction of
iron atoms with Cpd in an argon matrix [5] and hydrogen-
bonded complexes of Cpd with the strong bases [6] have
been reported using the matrix isolation technique. More-
over, unstable species have been reported; cyclopentadienyl
radical and cyclopentadienyl anion were produced by
pyrolysis of bis(cyclopentadienyl)nickel in argon matrix
5
of Cp bonds to the center iron metal in h . However, in the
1
case of HgCp , h bonding takes place with diene character,
2
wherein an electron is localized in one of the carbon atoms
in the five-membered ring of Cp. The differences in such
bonding character are observed as differences in the infrared
spectra [2]. The structure and electronic state of Cp correlate
to the bonding nature of Cp with the center metal. It is
therefore important to investigate electronic structure and
photochemistry of Cpd in order to understand the photo-
chemistry of metal complexes with Cp, which could be used
to synthesize novel compounds.
[7], and a radical cation of Cpd was produced by X-ray
irradiation of Cpd in a matrix [8]. However, to the best of
our knowledge, the photoisomerization of matrix-isolated
Cpd is yet to be reported.
In the present study, the photochemistry of Cpd isolated
in a low-temperature argon matrix was studied using a
super-high-pressure mercury lamp (SHPML) and a low-
pressure mercury lamp (LPML), and the reactivity and
electronic properties of photochemical products of Cpd
were investigated.
The photochemistry of Cpd in solution systems has been
described previously. Bicyclo[2.1.0]pent-2-ene (BCP) is
formed by UV irradiation of Cpd in ethanol at 0 8C [3,4].
*
Corresponding author. Tel.: þ81352288276; fax: þ81332352214.
E-mail address: yyasu@rs.kagu.tus.ac.jp (Y. Yamada).
0022-2860/$ - see front matter q 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2004.01.023

146
J. Miyazaki, Y. Yamada / Journal of Molecular Structure 692 (2004) 145–153
2
. Experimental
monitoring the pressure change in the gas-line before and
after introduction of the gas.
Cpd was prepared by cracking the dimer, dicyclopenta-
Infrared spectra were recorded with an FTIR spec-
trometer (Perkin Elmer, System 1720x). UV/VIS spectra
were recorded by a Shimadzu MPS-2000 spectrometer. For
infrared spectra measurements, CsI and KBr were employed
as the cold substrate and optical window, respectively. For
diene, (Kanto Chemical Co., Inc, more than 97% purity) and
further purified by freeze-thaw cycles at 77 K. The
preparation of the pure Cpd was confirmed by Fourier
transform infrared spectroscopy (FTIR). Argon (Nippon
Sanso, more than 99.9995%) was used without further
purification as the matrix gas.
measurement of UV/VIS spectra, CaF and quartz were
2
employed as the cold substrate and external windows,
respectively.
Matrix samples were condensed on a cold substrate (CsI
or CaF ) that had been cooled down to 18 K by a closed-
The excitation sources were a super-high-pressure
mercury lamp (SHPML) (USHIO, USH-500D) and a low-
pressure mercury lamp (LPML). For the irradiation using
SHPML (200 nm , l , 800 nm, 17.5 mW), the infrared
region in the light was filtered out by a water filter to prevent
heating of the sample. The wavelength was controlled by an
optical glass filter. The LPML with strong bright lines at
184.9 and 253.7 nm was located in the vacuum chamber to
irradiate the sample directly without loss of UV-light.
2
cycle helium refrigerator (Iwatani Cryo Techno, Cryo Mini
D510). The temperature of Cpd in the glass tube was kept at
2
73 K by a crushed-ice bath, and Cpd vapor was introduced
onto the cold substrate through a needle valve. The amount
of deposited Cpd was estimated by weighing the glass tube
before and after the vaporization. The matrix gas, Ar, was
introduced in a controlled manner with a fine needle valve.
The amount of argon matrix gas was estimated by
Fig. 1. Infrared Spectra of Cpd isolated in solid Ar matrices (18 K). (a) Cpd/Ar ¼ 1/25, (b) Irradiated with SHPML for 10 min, (c) Irradiated with SHPML for
80 min.
1

J. Miyazaki, Y. Yamada / Journal of Molecular Structure 692 (2004) 145–153
147
3
. Results
Table 1
2
Observed frequencies (cm ) for Cpd and reaction products
1
3
.1. Infrared spectra
Cyclopentadiene Assignment
Cpd)
SHPML LPML Assignment
(
(BCP)
Infrared spectra of Cpd isolated in the solid argon matrix
ꢀ
(
molar ratio: Cpd/Ar ¼ 1/25) are shown in Fig. 1. Similar
experiments with other mixture ratios (Cpd/Ar ¼ 1/1000–
/25) were also performed, and the same IR spectra were
3122
3333
CH stretching
CH stretching
3113
3101
3087
3083
(allylacetylene)
ꢀ
ꢀ
ꢀ
9
1
3087
3070
3046
CH stretching
CH stretching
obtained. In this wave-number region, vibrations of CH and
ring deformation of Cpd are expected. The ring stretching
3058
050
2910
2
and CH bending are observed at 959 cm , and CH rocking
1
3
2
2
is observed at 894 cm . The absorptions at 666 and
1
CH
CH
2
2
stretching
stretching
2906
3022
2985
2
1
6
63 cm
are assigned to CH in-plane bending. Other
>
2
899 =
observed peaks and their assignments are summarized in
Table 1 [5].
2894
>
;
2891
1953
1648
CyCyC stretching
(vinylallene)
On irradiation using SHPML (200 nm , l , 280 nm),
absorption bands of Cpd decreased and new bands appeared
1
1
1
573
383
370
CyC stretching
CH scissoring
CyC stretching
2
1
2
1286
1284
(allylacetylene)
at 776 and 720 cm (Fig. 1(b)). The species represented by
the new bands is denoted as photochemical species
A. Though dicyclopentadiene may be formed by photo-
aggregation in a matrix, the corresponding absorptions at
C–H bending
C–H bending
þ ring stretching
C–H bending
þ ring stretching
1294
1
240
091
1175
1087
21
7
55, 727, 720 cm were not observed in this experiment.
1
2
CH wagging
Therefore it was concluded that species A was not
dicyclopentadiene. Further irradiation using SHPML
1
060
1013
9
91
CH
2
allylrocking
(
(
180 min) increased the intensity of species A bands
Fig. 1(c)), and no other absorption was observed. In order
(allylacetylene)
ꢀ
ꢀ
9
9
61
59
ring stretching
946
to see the effects of annealing, the temperature of the sample
was increased to 30 K and maintained at this temperature for
þ C–H bending
Ring
918
914
ꢀ
ꢀ
6
species A was stable at 30 K.
0 min; No spectral change was observed indicating that
902
899
yCH
2
894
CH
2
rocking
(vinylallene)
869
When the Cpd in the matrix was irradiated with longer-
wavelength radiation (l . 280 nm) using a SHPML light
filtered by an optical short-cut filter (UV-28), no photo-
chemical reaction was observed. Therefore, this indicates
that only light in the region of 200 nm , l , 280 nm can
promote the photoreaction of Cpd isolated in a matrix to
produce species A.
849
CyCyCH
vinylallene)
2
(
806
ring bending
C–H bending
776
7
7
7
74
73
20
ꢀ
666
663
641
629
CxCH
(allylacetylene)
The IR spectrum of Cpd isolated in the solid argon
matrix, Cpd/Ar ¼ 1/50, is shown in Fig. 2(a). When the
matrix sample was irradiated by LPML light, the absorption
of Cpd decreased and the bands of species A at 776 and
2
1
allylacetylene, and vinylallene, respectively. The assign-
ments are confirmed by their chemical behavior and
molecular orbital calculations as described later.
7
20 cm appeared, as in the case of irradiation by SHPML
Fig. 2(b)). But further irradiation (180 min) resulted in the
(
formation of other bands at 641, 849, 1953, and 3333 cm
at the expense of the Cpd and species A (Fig. 2(c)). The
1
absorption at 849 cm is assigned to C–H out-of-plane
2
1
The intensities of absorbance in infrared spectra were
checked by varying the irradiation time. SHPML irradiation
increased the production of species A (BCP) at the cost of
Cpd. LPML irradiation initially increased species A (BCP)
at the cost of Cpd. However, prolonged LPML irradiation
(30 to 60 min) increased species B (allylacetylene) and C
(vinylallene) at the cost of Cpd and species A (BCP). Since
the production of species B (allylacetylene) and C
(vinylallene) was observed only after the formation of
species A (BCP) and the increase of species B (allylacety-
lene) and C (vinylallene) accompanies the consumption of
species A (BCP), it was demonstrated that the new species B
2
2
1
deformation of an allene group, the absorption at 1953 cm
is assigned to C ¼ C ¼ C stretching of an allene group, the
2
absorption at 641 cm is assigned to C–H out-of-plane
1
21
deformation, and 3333 cm is assigned to C–H stretching
of CxCH ethynyl group. Therefore the new species
produced by irradiation by LPML contains ethynyl (species
B) or allene (species C) groups (Fig. 2(c)). No spectral
change was observed on annealing at 30 K, indicating that
the new species B and C were stable. The species A, B,
and C are assigned to bicyclo[2.1.0]pent-2-ene (BCP),

148
J. Miyazaki, Y. Yamada / Journal of Molecular Structure 692 (2004) 145–153
Fig. 2. Infrared spectra of Cpd isolated in solid Ar matrices (18 K). (a) Cpd/Ar ¼ 1/50, (b) Irradiated with LPML for 10 min, (c) Irradiated with LPML for
80 min.
1
(allylacetylene) and C (vinylallene) arose only from species
A (BCP) on irradiation by short wavelengths.
of Cpd, species C (vinylallene) are not produced directly
from Cpd. It was shown that species C (vinylallene) are
produced via intermediates, which may be species A
(BCP).
3
.2. UV/VIS spectra
3
.3. Molecular orbital calculations
Cpd isolated in a solid argon matrix (18 K) was
irradiated using SHPML (Fig. 3(a)) and LPML (Fig.
(b)), and UV/VIS spectra were observed by varying the
irradiation time. On irradiation using SHPML, the bands
3
Molecular orbital calculations were performed in order to
confirm the assignments. The Gaussian98 [9] software
program with B3LYP/6-311 þ G p basis sets was
employed to estimate the structures of photochemical
species A, B, and C. We performed the geometry
optimization and frequency analysis for Cpd and bicy-
clo[2.1.0]pent-2-ene (BCP), as well as the various species
which are expected to be produced on irradiation:
dicyclopentadiene, its isomers, and species containing
allene and ethynyl groups.
p
at 237 nm (p–p transition of Cpd) decreased: Cpd may
decrease to form species A (BCP) that is not observed in
this spectral region. On irradiation using LPML, the
absorption at 237 nm decreased up to 30 min, and then
new bands at 226 nm appeared by further irradiation up
to 180 min. The new bands at 226 nm is assigned to the
p
p–p transition of the allene group. As the increase of
species C (vinylallene) was observed after the decrease

J. Miyazaki, Y. Yamada / Journal of Molecular Structure 692 (2004) 145–153
149
Fig. 3. UV/VIS spectra of Cpd isolated in solid Ar matrices (18 K). (a) is irradiated using SHPML, (b) is irradiated using LPML. Irradiation times (min) are
indicated in the figures.
Frequency analysis of BCP is in agreement with
observed IR absorption peaks of species A. Species B,
containing the ethynyl group, is assigned to allylacetylene
agreement with experimentally observed infrared spectra
(Fig. 5). The assignments are summarized in Table 1.
(
allene group is assigned to vinylallene (penta-1,2,4-triene:
CHxCCH CH ¼ CH ), and the species C containing the
2
2
4. Discussion
CH ¼ C ¼ CHCH ¼ CH ) from comparison of observed
2
2
IR spectra with calculated frequencies. The optimized
geometries and energy differences based on the ground
states are summarized in Fig. 4. All the photochemical
products have higher potential energies than that of Cpd.
Allylacetylene and vinylallene have cis and trans isomers
for each species. When the ground state energies were
compared, the trans form isomer of vinylallene is
For the photochemical products with ethynyl group
species B) and allene group (species C) produced by
(
irradiation using LPML, methylacetylene (CH CxCH) and
3
allene (CH ¼ C ¼ CH ) are also candidates. Infrared
2
2
spectra of these compounds in low-temperature matrices
have already been reported including hydrogen discharge
photolysis of methylacetylene or allene [10], and vacuum-
ultraviolet photolysis of propylene to produce methylacety-
lene and allene [11]. It was reported that methylacetylene has
2
.836 kcal/mol more stable than the cis isomer, and the
cis isomer of allylacetylene is 0.145 kcal/mol more stable
than the trans isomer. Calculated frequencies for the
optimized species using B3LYP/6-311 þ G p are listed
in Table 2, and the scaling factor was estimated to be 0.98.
The calculated frequencies and their intensities are in
2
1
absorptions at 3330 and 628 cm , and allene has absorp-
2
1
tions at 1954 and 838 cm , which are very similar to our
experimental results. However the reaction of Cpd to
produce methylacetylene or allene should be accompanied

150
J. Miyazaki, Y. Yamada / Journal of Molecular Structure 692 (2004) 145–153
Fig. 4. Optimized geometries and energy differences based on the ground state using B3LYP/6-311 þ G p .
2
1
by the formation of acetylene (3300 and 735 cm ), the
bands for which were not observed in our experiments.
Therefore it was concluded that the species B and C observed
in this experiments cannot be methylacetylene nor allene.
Species B and C are produced simultaneously and
therefore one could suspect the presence of ethynyl group
and allene group in one molecule. Ethynylallene (Penta-3,4-
in agreement with experimental data. Infrared spectra of
vinylallene in the vapor phase, in solution, as well as in
the solid state [14,15] have been reported to have bands
n
21
of H C ¼ C ¼ CH stretching (1940 cm ), ¼ CH
2
2
2
2
2
1
out-of-plane deformation (895 cm ), and C ¼ C ¼ CH
2
1
out-of-plane deformation (845 cm ). These bands are
compatible with the absorption bands in Fig. 2(c). The
n
absorptions of allylacetylene were observed at CxCH
dien-1-yne: CHxCCH ¼ C ¼ CH ) has ethynyl group and
2
2
stretching (3330 cm ),
1
n
allene group in the same molecule, and infrared spectra have
been measured in CS [12], or in gas-phase [13], which have
-HC ¼ CH
stretching
2
2
1
n
(1640 cm ), -HC ¼ CH out-of-plane deformation
2
2
2
1
21
(990 cm ), ¼ CH out-of-plane deformation (920 cm
21
absorptions at 3333, 1950, 853 cm . These bands are very
close to those in our experimental results. But the band at
)
2
[15]. In the UV/VIS spectrum (Fig. 3), increase of the bands
at 226 nm was observed after LPML irradiation. As
2
99 cm was observed in the infrared spectrum of matrix
1
8
isolated Cpd irradiated with LPML for 180 min (Fig. 2(c)),
p
vinylallene in ethanol is reported to have bands of p–p
which was assigned to ¼ CH out-of-plane deformation.
transition at 228 nm [15], the production of vinylallene in
our experiment is also supported by UV/VIS spectra.
Thermal decomposition of Cpd has been studied earlier
using the shock tube technique [16], and the tube flow
reactor method [17]. The main products of the decompo-
sition were acetylene, ethylene, benzene, allene, propylene,
methane, and 1-3butadiene. Two pathways of thermal
decomposition of Cpd have been proposed:
2
Therefore, it was concluded that ethynylallene was not a
product in our experiment.
Infrared spectra of allylacetylene or vinylallene isolated
in a low-temperature matrix have never been reported. We
therefore performed molecular orbital calculations using
Gaussian98 software in order to estimate their IR spectra in
an isolated system. While infrared spectra of the species
isolated in low-temperature matrices have never been
reported, the spectra in conventional conditions have indeed
been previously reported. The results of the calculations are
(i) Cpd forms cyclopentadienyl radical by fission of the
CH bond, and then decomposition of cyclopentadienyl

J. Miyazaki, Y. Yamada / Journal of Molecular Structure 692 (2004) 145–153
151
Table 2
Calculated frequencies (cm ) and IR intensities (kM/mol) using B3LYP/6-311 þ G p
2
1
Cpd
BCP
Vinylallene (trans)
Allylacetylene (cis)
a
a
a
a
Frequencies
Intensity
Frequencies
Intensity
Frequencies
Intensity
Frequencies
Intensity
3
3
3
3
2
2
1
1
1
1
1
1
1
1
1
9
9
9
9
9
9
8
8
6
6
5
3
154.92
148.42
130.60
120.54
973.68
953.98
602.76
518.56
400.61
375.32
294.04
249.60
115.69
109.73
092.52
93.62
2.82
26.43
17.96
3.65
3134.20
3125.76
3117.81
3111.63
3098.98
3027.08
1564.70
1454.98
1287.12
1267.04
1200.16
1179.02
1087.78
1062.41
1023.15
1016.25
1013.61
943.10
5.16
59.20
24.20
3.80
8.43
39.78
9.20
2.91
23.28
1.07
0.10
0.98
1.88
4.46
0.24
8.49
1.87
4.02
4.55
7.72
2.66
1.10
0.13
36.89
77.59
1.02
8.05
3156.3
16.4465
2.9974
7.5522
10.9297
3.0263
11.4507
76.9024
19.8359
4.391
3392.18
3157.68
3081.05
3069.03
2960.05
2946.02
2175.36
1677.16
1448.57
1421.74
1335.68
1305.53
1227.99
1077.27
1003.65
950.127
944.47
52.795
13.5206
11.3308
18.4524
7.5186
17.9337
4.3668
16.5331
13.7051
3.8407
3.9938
0.2905
1.1322
4.8987
21.3287
1.4886
4.8481
45.9302
0.1352
56.586
55.4518
1.9361
16.1693
3.0547
9.1108
0.7538
0.0479
3110.26
3088.91
3070.5
9.82
3052.34
3043.45
1988.34
1646.39
1462.38
1422.83
1315.11
1291.37
1168.29
1093.22
1007.96
1001.73
904.776
904.069
885.787
856.85
11.97
0.01
1.03
13.37
13.02
1.03
0.4498
3.8339
3.9074
4.0994
1.5926
19.0634
8.3367
61.0571
2.8697
0.1922
59.6454
6.2726
10.1225
7.5142
6.4463
5.6485
0.8107
0.8565
2.35
0.00
0.32
2.67
0.20
52.63
16.47
0.57
27.18
924.897
849.442
635.872
611.802
603.655
554.619
371.977
322.322
158.399
146.018
26.04
0.00
924.51
06.94
11.51
33.23
6.02
866.14
01.59
833.30
715.454
557.763
497.667
394.71
03.42
814.80
01.14
0.01
784.87
92.38
0.00
765.86
64.68
93.98
0.00
718.24
345.761
163.483
126.368
10.38
443.51
41.20
7.76
413.15
a
The scaling factor 0.98 applied.
radical occurs [18,19]. Cyclopentadienyl radical iso-
lated in a low-temperature matrix has been previously
reported [7], and its molecular orbital calculations have
been performed [18]. However, no corresponding
absorption bands were observed in our experiments.
and vinylallene are proposed as the intermediate
species [19]. However, in this study, it was shown
that BCP exists as an intermediate product to form
allylacetylene and vinylallene.
(
ii) Cpd forms carbene intermediate by proton shift, and
then acetylene plus propylene or allene are produced.
In the process of forming acetylene, allylacetylen
There have been no previous reports indicating that
allylacetylene and vinylallene are produced from Cpd via
BCP. In order to examine this photochemical reaction
Fig. 5. Calculated frequencies using B3LYP/6-311 þ G p and the scaling factor 0.98 applied.

152
J. Miyazaki, Y. Yamada / Journal of Molecular Structure 692 (2004) 145–153
Table 3
Calculated of electronic spectra (nm) and oscillator strength (f) using ZINDO
Cpd
BCP
Vinylallene
Allylacetylene
State
l/nm
f
l/nm
f
l/nm
f
l/nm
f
1
2
3
4
5
238.27
198.61
193.94
192.11
157.59
0.2229
0.0392
0.0006
0.0000
0.9126
233.64
203.67
186.84
175.96
163.47
0.1087
0.0139
0.0364
0.1132
0.0087
287.61
270.64
232.79
204.33
188.08
0.0000
0.0000
1.2631
0.0039
0.0178
276.50
248.26
248.17
180.54
178.36
0.0000
0.0002
0.0002
0.0001
0.0162
mechanism, we performed electronic states calculations
using ZINDO in Gaussian98 and INDO/S in MOS-F ver.
l ¼ 187 nm) of BCP using INDO/S calculations is shown
in Fig. 6. By the transition from the ground to the excited
state, the electron is localized at the three-membered ring,
4
.2C [20]. The structure employed in the calculation is the
optimized structure using B3LYP/6-311 þ G p . The calcu-
lated electronic spectra are summarized in Table 3. The
bands at 237 nm observed in UV/VIS spectra that
especially on the carbon at CH . Consequently, BCP
2
could decompose from the three-membered ring by UV
irradiation.
p
corresponds to p–p transition of Cpd are in agreement
with the results of these calculations. Moreover, the
calculations show that allylacetylene has no strong UV/
VIS absorption, which correlates with our experimental
results.
5
. Conclusions
Matrix isolated cyclopentadiene (Cpd) forms bicy-
clo[2.1.0]pent-2-ene (BCP) on irradiation with a super-
high-pressure mercury lamp (SHPML) (200 nm ,
l , 280 nm). Irradiation with a low-pressure mercury
lamp (LPML) promotes subsequent reaction of BCP to
produce allylacetylene and vinylallene. The assignments
were made with the aid of molecular orbital calculations,
and the calculated results are in good agreement with
observed IR spectra and UV/VIS spectra. The reaction of
BCP to form allylacetylene and vinylallene is promoted by
the three-membered ring opening of BCP which corre-
sponds to absorption at 187 nm. This is the first reported
photochemical reaction of BCP in low-temperature
matrices.
BCP is calculated to have three bands at 234 nm ðf ¼
0
:1087Þ; 204 nm ðf ¼ 0:0139Þ and 187 nm ðf ¼ 0:0364Þ
above l . 180 nm region. The band at 234 nm is
compatible with the 232 nm ðemax ¼ 1000Þ band in
UV/VIS absorption spectra of BCP in ethanol [4]. The
photoreaction of BCP to form allylacetylene and viny-
lallene was only observed by irradiation using LPML,
which has a shorter wavelength, and absorption of BCP at
187 nm may promote the reaction. The difference atomic
charge between the ground and the excited states (state 3;
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