Photoinduced reactions of chloroacetone in solid Ar: Identification of CH2=COClCH3

Article abstract of DOI:10.1016/j.cplett.2014.09.053

The UV light-induced reactions of chloroacetone in a cryogenic Ar matrix were investigated using infrared spectroscopy. The photoinduced isomerisations of gauche-chloroacetone to syn-chloroacetone and hypochlorous acid 1-methylethenyl ester were confirmed by comparing the experimental and calculated spectra. In addition, the photolysis products were found to be CH₂CO and a cyclopropanone-HCl complex. The cyclopropanone-HCl complex was further decomposed into CH₂CH₂, CO and HCl. The hypochlorous acid 1-methylethenyl ester was further isomerized to 2-chloro-2-methyloxirane.

Full text of DOI:10.1016/j.cplett.2014.09.053

Chemical Physics Letters 614 (2014) 258–262
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Chemical Physics Letters
journal homepage: www.elsevier.com/locate/cplett
Photoinduced reactions of chloroacetone in solid Ar: Identification of
CH2 COClCH3
Nobuaki Tanaka∗, Yoshitaka Urashima, Hiromasa Nishikiori
Department of Environmental Science and Technology, Faculty of Engineering, Shinshu University, 4-17-1 Wakasato, Nagano 380-8553, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
The UV light-induced reactions of chloroacetone in a cryogenic Ar matrix were investigated using
infrared spectroscopy. The photoinduced isomerisations of gauche-chloroacetone to syn-chloroacetone
and hypochlorous acid 1-methylethenyl ester were confirmed by comparing the experimental and calcu-
Received 25 August 2014
In final form 20 September 2014
Available online 28 September 2014
lated spectra. In addition, the photolysis products were found to be CH2
C
O and a cyclopropanone· · ·HCl
complex. The cyclopropanone· · ·HCl complex was further decomposed into CH2 CH2, CO and HCl. The
hypochlorous acid 1-methylethenyl ester was further isomerized to 2-chloro-2-methyloxirane.
© 2014 Elsevier B.V. All rights reserved.
1. Introduction
polarity [10,13]. However, there is little information available on
the photochemistry of chloroacetone trapped in a rare gas matrix.
In the past four decades, halogenated volatile organic com-
pounds (VOCs) have captured the attention of environmental
chemists and photochemists because of their role in the depletion
of the ozone layer [1]. The VOCs react in the atmosphere leading to
the formation of secondary organic aerosols [2], which affect the
Earth’s radiative balance [3].
The present study investigates photoinduced reactions of
chloroacetone trapped in solid Ar. Owing to the narrower band-
width and lower complexity of the spectra obtained in the
matrix-isolated species compared with that of spectra of the vapor
or liquid state, the absorption bands of the different conformers
can be separated. Because of the matrix cage effect, the fragment
species formed by photodissociation cause the addition or abstrac-
tion reaction to occur within the available energy. As a result, the
coexistence of the gauche and syn conformers in chloroacetone was
confirmed. The infrared bands of the less stable conformer were
separated and assigned. Moreover, the photolysis products caused
by the initial C Cl bond fission were identified.
The reactions of chloroacetone with OH and Cl have been inves-
tigated from an experimental [4] and a theoretical [5] point of view.
Hydrogen atom abstraction from a CH2Cl group was found to be a
dominant initial process. Waschewsky et al. found the evidence of
competing C Cl and C C bond fission reactions during the photoly-
sis of chloroacetone vapor at 308 nm [6]. Kitchen et al. determined
the absolute branching ratio between C Cl and C C bond fission
at 308 nm to be 4.63:1.0 at 180 ◦C [7]. Alligood et al. determined
a branching ratio of 11:1 upon excitation at 193 nm [8]. The con-
formations of chloroacetone have been studied using vibrational
spectroscopy [9–11], electron diffraction [12], NMR spectroscopy
[13] and theoretical methods [11–13]. In all of these studies, two
conformers have been proven to exist from calculations of the
CH2Cl internal rotation potential: the gauche and syn conform-
2. Experimental
Chloroacetone (Wako Pure Chemicals Industries, Ltd.) was puri-
fied by a freeze–pump–thaw cycle at 77 K and was diluted with
Ar gas (Nippon Sanso, Japan, 99.999% purity) to approximately
1/500 (0.4 Torr chloroacetone and 200 Torr Ar). It was then slowly
sprayed onto a CsI plate cooled by a closed-cycle helium refriger-
ator (Iwatani CryoMini M310/CW303) to approximately 7 K. The
infrared absorption spectra were measured in the 3500–700 cm−1
range with a resolution of 1.0 cm−1 using a SHIMADZU FTIR 8300
spectrophotometer equipped with a liquid-nitrogen-cooled MCT
detector. Each spectrum was obtained by acquiring 128 scans.
Under the sample conditions mentioned above, the populations of
conformers just after deposition were assumed identical to those
at the temperature of the gaseous sample just before deposition
(298 K). The photoexcitation effect was examined by irradiating
ers have the dihedral angles, ϕ(Cl
C C
O), of 138(7)–155◦ and
0.0–5.5◦, respectively. The gauche conformer was more stable in
the vapor state, while the syn conformer was predominant in the
liquid state. In solution, the syn/gauche ratio increased with solvent
∗
Corresponding author.
E-mail address: ntanaka@shinshu-u.ac.jp (N. Tanaka).
http://dx.doi.org/10.1016/j.cplett.2014.09.053
0009-2614/© 2014 Elsevier B.V. All rights reserved.

N. Tanaka et al. / Chemical Physics Letters 614 (2014) 258–262
259
Table 1
Observed and calculated wavenumbers of photoproducts obtained upon UV irradi-
ation of a matrix chloroacetone/Ar.
Wavenumber
cm−1
Species
Obs
Calca
2950
2934
2863
2805
2745
2542
2146
2139
1831
1761
1681
1448
1440
1436
1407
1381
1306
1298
1237
1163
1158
1058
976
syn-CH2 COClCH3
HCl
2698
2490
2166
2186
1827
1771
1680
1455
1451
1435
1376
1390
1347
1280
1204
1138
1150
1052
944
HCl· · ·CH2 CH2
HCl· · ·cyclopropanone
CH2
CO
C O
Cyclopropanone· · ·HCl
syn-CH2ClCOCH3
syn-CH2 COClCH3
syn-CH2 COClCH3
CH2 CH2· · ·HCl
syn-CH2 COClCH3
syn-CH2 COClCH3
2-Chloro-2-methyloxirane
2-Chloro-2-methyloxirane
syn-CH2ClCOCH3
syn-CH2 COClCH3
2-Chloro-2-methyloxirane
syn-CH2ClCOCH3
Cyclopropanone· · ·HCl
Cyclopropanone· · ·HCl
CH2 CH2· · ·HCl
966/960/954/949
935
879
867
838
987
919
870
856
syn-CH2 COClCH3
2-Chloro-2-methyloxirane
syn-CH2 COClCH3
syn-CH2ClCOCH3
801
820
767
828
739
syn-CH2 COClCH3
syn-CH2ClCOCH3
Figure 1. IR difference spectra upon UV irradiation of the matrix chloroace-
tone/Ar = 1/500 obtained by the spectral subtraction of (a) 70 − 0 min and (b)
280 − 200 min. Calculated infrared spectra of (c) gauche- (downward) and syn-
chloroacetone (upward) and (d) syn- (upper) and gauche-CH2 COClCH3 (lower) at
the B3LYP/6-311++G(2d,2p) level with anharmonic correction.
a
Anharmonic wavenumbers calculated at the B3LYP/6-311++G(2d,2p) level.
subtracting the spectrum measured before UV irradiation from the
one measured after 70 min of irradiation without the UV-29 filter
and (c) the calculated infrared difference spectra of chloroacetone,
where the upward and downward bands of the calculated spectra
correspond to those of the syn and gauche conformers, respectively.
The UV irradiation resulted in a decrease or an increase in inten-
sities of the bands due to a reactant and the appearance of new
bands. The observed and calculated wavenumbers of the product
bands are given in Table 1. The bands that increased at 1761 and
1298 cm−1 were also present in the as-deposited spectrum. The cal-
culated infrared spectra at the B3LYP/6-311++G(2d,2p) level with
anharmonic treatment reproduced well the observed spectrum.
Comparison of the observed and calculated spectra indicates that
the bands that decreased and increased upon UV irradiation cor-
respond to those of the gauche and syn conformers, respectively.
The increased bands at 1761, 1298, 1158, 838 and 767 cm−1 were
assigned to C O stretching, CH2 wagging, CH3 rocking, CH2 rocking
and C Cl stretching vibrations of syn-CH2ClCOCH3, respectively.
This conformation change was also observed when the irradiation
was conducted with the UV-29 filter.
the deposited samples with UV light. A Xe short arc lamp (HAMA-
MATSU, C2577) with a sharp-cut filter (HOYA UV-29) was used as
the UV light source combined with a water filter to avoid thermal
radiation.
For product identification and energetic considerations, molec-
ular orbital calculations were performed using the gaussian 09
program [14]. Geometry optimizations were performed using
Becke’s three-parameter hybrid density functional in combina-
tion with the Lee–Yang–Parr correlation functional (B3LYP) [15,16]
and MP2 with the aug-cc-pV(T+d)Z or 6-311++G(2d,2p) basis
set. Harmonic vibrational frequency calculations were performed
in order to confirm the predicted structures as local minima
or transition states and to elucidate the zero-point vibrational
energy corrections. An anharmonic vibrational frequency calcu-
lation was performed at the B3LYP/6-311++G(2d,2p) level. The
vertical transition energies of the parent and intermediate species
were calculated at the TD B3LYP/aug-cc-pV(T+d)Z level and at the
SAC-CI/D95+(d,p)//CCSD/D95+(d,p) level. Vibrational energy distri-
bution analysis was performed using VEDA 4 [17].
Taking into consideration the solvent dependence of the 13C
NMR spectra, Doi et al. determined that the energy difference
between the two conformers was 1.7 kcal mol−1 in the vapor
phase [13]. The calculation at the B3LYP/aug-cc-pV(T+d)Z level also
shows that the gauche conformer is more stable than the syn con-
former by 1.2 kcal mol−1 and the barrier height is 3.5 kcal mol−1
in the ground state. This means that the syn/gauche population
ratio before UV irradiation is 0.069/1 at 298 K and that the con-
version from the gauche to syn conformer is not expected to
occur at 7 K in the absence of UV irradiation. However, UV irra-
diation led to an increase in the population of the less stable
conformer.
3. Results and discussion
3.1. Separation and assignment of the infrared bands of the less
stable conformer
A chloroacetone/Ar mixture was deposited on a CsI window
(chloroacetone/Ar = 1/500). In the infrared spectrum obtained after
deposition, strong bands were observed at 1738 and 1360 cm−1
,
which were attributed to the C O stretching and CH3 symmetric
deformation vibrations of chloroacetone, respectively [11]. Figure 1
shows (a) the observed infrared difference spectrum obtained by

260
N. Tanaka et al. / Chemical Physics Letters 614 (2014) 258–262
Figure 2. Absorbance changes of the bands at 977 (ꢀ), 960 (•), 934 (×) and 838 (ꢀ) cm−1 upon UV irradiation of the chloroacetone/Ar = 1/500 (a) without and (b) with a
UV-29 filter.
radical [8]. Thus, the band at 2146 cm−1 was assigned to C
antisymmetric stretching vibration of CH2 O.
C O
3.2. Identification of additional products
C
The three strong bands at 1681, 935 and 820 cm−1 from group
B were prominent in an early stage of the irradiation, indicat-
ing that the bands from group B are due to one of the primary
products. The absorption bands due to the conjugated C O stretch-
ing vibration and C C stretching vibration are known to appear
in the ∼1680 cm−1 region. The band at 935 cm−1 would indi-
cate the presence of a ꢁC CH2 group. Therefore, we assumed
the formation of a recombination product of hypochlorous acid
1-methylethenyl ester, CH2 COClCH3, from the photolysis prod-
ucts of CH2COCH3 and Cl. Two conformers, the syn and gauche
forms, were found to exist with the equilibrium dihedral angles
The UV irradiation led to the increase of other bands in the differ-
ence spectra besides the vibration bands of the syn-chloroacetone.
The bands that increased in intensity were classified into three
groups (A, B and C) according to their different behavior. Figure 2
shows the changes in the absorbance of the bands at 977, 935, 960
and 838 cm−1 for groups A, B and C, and syn-chloroacetone, respec-
tively, upon UV irradiation with and without the UV-29 filter. The
absorption bands from group A reached a peak after 300 min irra-
diation time. The bands from group B showed an increase followed
by a decrease during the irradiation period, as shown in Figure 2a.
The absorption bands from group C exhibited an induction period.
The band due to syn-chloroacetone continued to grow.
of C(H2)
C O
Cl calculated to be 0.0 and 127.5◦, respectively, at
The wavenumbers of the bands from group A are similar to those
of cyclopropanone measured in Ar [18] except for the bands at 2542
and 2146 cm−1. In the 2800–2500 cm−1 region, an absorption band
appeared because of the formation of a hydrogen-bond complex of
HCl [19,20]. Two stationary points were previously predicted for
the cyclopropanone· · ·HCl complex from the DFT and MP2 calcu-
lations, due to the interactionof the hydrogen atom from HCl with
an oxygen atom or a C C bond in cyclopropanone [21]. The former
complex was calculated to be more stable and have a larger shift in
wavenumber for the HCl stretching vibration compared to the free
HCl. Therefore, the observed wavenumbers were compared with
those for the ꢁC O· · ·HCl complex. The calculated wavenumbers
were in good agreement with those observed, as listed in Table 1.
The bands at 2542, 1831, 1058 and 976 cm−1 were assigned, respec-
tively, to H Cl stretching, C O stretching, C H in-plane bending
and C H in-plane bending vibrations of the cyclopropanone· · ·HCl
complex.
the B3LYP/aug-cc-pV(T+d)Z level, as shown in Figure 3. The syn
conformer was determined to be more stable by 1.5 kcal mol−1
than the gauche conformer. The barrier heights to internal rotation
from the syn to gauche conformer and gauche to gauche conformer
were calculated to be 3.6 and 3.8 kcal mol−1, respectively, at the
B3LYP/aug-cc-pV(T+d)Z level. Figure 1d compares the calculated
spectra of the syn- and gauche-CH2 COClCH3 conformers. The cal-
culated spectrum of the syn conformer was in good agreement with
the observed one. Table 2 lists the wavenumber and the assign-
ment of the infrared spectrum for syn-CH2 COClCH3. The bands
at 1681, 935, 867 and 820 cm−1 were assigned to C C stretching,
CH2 in-plane bending, C C stretching and CH2 out-of-plane bend-
ing vibrations of syn-CH2 COClCH3, respectively. The optimized
structural parameters of syn-CH2 COClCH3 are listed in Table S1.
Supplementary Table S1 related to this article can be found, in
the online version, at doi:10.1016/j.cplett.2014.09.053.
The initial growth rate of the bands from group C was smaller
than those of the bands from groups A and B, indicating that
the bands from group C belong to one of the secondary prod-
ucts. After prolonged irradiation the bands from group C were
clearly discernible, as shown in Figure 1b. The photodissociation
of cyclopropanone was studied experimentally [25] and theoret-
ically [26–28]. Thomas and Rodriguez found that CH2 CH2 and
In the ∼2140 cm−1 region, two bands were observed at 2146
and 2139 cm−1. The former band behaved as a group A band, and
the latter as a group C band. Ketene and CO absorption bands are
known to emerge in this region [22–24]. In fact, during the photo-
lysis of chloroacetone in the vapor phase, upon 193 nm excitation,
CH2
C O was produced via the vibrationally excited CH2COCH3

N. Tanaka et al. / Chemical Physics Letters 614 (2014) 258–262
261
Figure 3. Optimized structures of (a) syn- and (b) gauche-CH2 COClCH3 at the B3LYP/aug-cc-pV(T+d)Z level.
CO were the only volatile products resulted upon excitation at a
selected wavelength between 292 and 365 nm [25]. The MCSCF
calculation showed that photodecarbonylation was initiated pre-
dominantly from the lowest excited state, taking the bent-in-plane
path, and the ground state CH2 CH2 and CO were produced via
the biradical intermediate [26]. Cui et al. found the two conical
intersections between the S1 and S0 states of cyclopropanone using
the state-averaged CASSCF method; one leads to an ␣-bond fission
and the other to two ␣-bond fissions [27,28]. In the present exper-
iments, a cyclopropanone· · ·HCl complex was formed. Therefore,
during the photolysis, CH2 CH2, CO and HCl would be formed.
The characteristic bands belonging to group C were assigned by
comparing the observed wavenumbers with those of the three pos-
sible monomers. The band at 2139 cm−1 was attributed to CO. The
band at 2745 cm−1 was assigned to the stretching vibration of H Cl
in CH2 CH2· · ·HCl. The bands at 1440 and 960 cm−1 from group
C were assigned, respectively, to the CH2 scissor and CH2 wag-
ging vibrations of CH2 CH2· · ·HCl [29]. The normalized absorbance
changes of the bands at 2745, 2139 and 960 cm−1 also support the
concomitant formation of these three species, as shown in Figure
S1. As for the photolysis of syn-CH2 COClCH3, the vertical tran-
sition energies were calculated at the SAC-CI/D95+(d,p) level. The
S1 (3.54 eV), S2 (4.93 eV) and S3 (5.16 eV) states were character-
ized as the ␲␴OCl* (HOMO → LUMO), n␴OCl* (HOMO − 2 → LUMO)
Table 2
Observed and calculated wavenumbers of syn-hypochlorous acid 1-methylethenyl ester.
Obs
Calca
PED ≥ 10%
Assignmentb
ꢀ
Relative intensity
Harmonic ꢀ
Anharmonic ꢀ
Intensity
km mol−1
cm−1
cm−1
cm−1
3270.6
3183.8
3136.1
3098.7
3046.7
1714.2
1495.4
1474.2
1435.6
1408.7
1239.1
1074.9
1017.2
939.1
3129.1
3033.1
3001.3
2957.4
2950.3
1680.3
1455.0
1434.9
1393.7
1376.3
1204.1
1051.9
1003.8
918.9
3.0
0.2
5.2
99␯CH
99␯CH
92␯CH
99␯CH
92␯CH
81␯CC
68␦CH + 18␶HCCC
78␦CH + 12␶HCCC
79␦CH
␯aCH2
␯sCH2
␯aCH3
␯aCH3
␯sCH3
␯C C
␦aCH3
␦aCH3
␦sCH3, ␦sCH2 scissor
␦sCH3, ␦sCH2 scissor
␯CO
␥CH3 rock
␦CH3 rock
␯CO
␯CC
␥CH2 wagging
␯OCl
9.8
2934
1681
1448
1436
13
100
32
14.2
99.8
10.2
6.4
1.1
5.6
49.9
0.2
2.9
33.6
14.7
55.6
5.3
9
1407
1237
13
29
93␦CH
34␯CO + 29␦CH
63␶HCCC + 20␦CH + 11␥OCCC
58␶HCCC + 24␦CH
55␯CO + 37␦CH
80␯CC
92␥CH
58␯OCl + 27␦CCO
86␶HCCO
935
867
820
96
25
89
873.2
851.0
723.9
718.8
855.8
828.0
714.9
711.5
0.0
␶HCCO
501.5
493.3
365.6
261.6
497.0
494.6
368.7
262.8
3.3
3.8
0.8
0.5
71␥OCCC + 11tHCCC
45␦CCO + 16␦CCC + 15␯OCl + 10␦CH
65␦CCC + 17␯OCl
87␦COCl
␥OCCC
␦CCO
␦CCC
␦COCl
168.2
172.1
0.0
85␶HCCC
␶HCCC
103.6
111.0
3.0
97␶CCOC
␶CCOC
a
Calculated at the B3LYP/6-311++G(2d,2p) level.
␯, stretching; ␦, in-plane bending; ␥, out-of-plane bending; ␶, torsion.
b

262
N. Tanaka et al. / Chemical Physics Letters 614 (2014) 258–262
Scheme 1.
and ␲␴OCl* (HOMO − 1 → LUMO) states with oscillator strengths
of 0.0, 0.0039 and 0.0, respectively. Once the syn-CH2 COClCH3 is
excited upon irradiation, O Cl bond dissociation occurs to form the
CH2COCH3 radical and Cl. If the Cl atom recombination occurs at the
C2 carbon of CH2COCH3, 2-chloro-2-methyloxirane is formed. The
observed and calculated wavenumbers are compared in Table 2.
The weak bands at 1381, 1306, 1163 and 879 cm−1 are tentatively
assigned to the vibrations of 2-chloro-2-methyloxirane. No bands
attributed to the ethylenic hydrogen atom abstracted products
photoinduced isomerisations of gauche-chloroacetone to syn-
chloroacetone and hypochlorous acid 1-methylethenyl ester
were confirmed by comparison with the calculated spec-
tra. In addition, the photolysis products were found to be
CH2
C
O, a cyclopropanone· · ·HCl complex, HCl and CO. The
cyclopropanone· · ·HCl complex was further decomposed into
CH2 CH2, CO and HCl. The hypochlorous acid 1-methylethenyl
ester was further isomesized to 2-chloro-2-methyloxirane.
¨
¨
(HCCOCH3, methyloxirene and CH(= O)CCH3) were observed.
Supplementary Figure S1 related to this article can be found, in
the online version, at doi:10.1016/j.cplett.2014.09.053.
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As shown in Scheme 1, the UV irradiation of the matrix
chloroacetone/Ar leads to; (1) rotational isomerization from
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4. Conclusions
UV light-induced reactions of chloroacetone in a cryogenic
Ar matrix were investigated using infrared spectroscopy. The