Matrix Isolation and Computational Study on the Photolysis of CHCl2COCl ()
1. Introduction
Dichloroacetyl chloride (CHCl2COCl) is known to be produced in the oxidation of chlorinated ethenes [1] -[4] . In the chlorine atom initiated oxidation of chlorinated ethenes, relatively high product yields of chlorinated acetyl chloride were reported by Hasson and Smith [5] . Conformations of CHCl2COCl were studied by vibrational spectroscopy [6] -[9] , electron diffraction [10] , and theoretical method [11] . Two conformers exist in the CHCl2 internal rotation potential: syn conformer having an H-C-C=O dihedral angle of 0˚ and gauche conformer having a non-zero value of the dihedral angle. As for the photolysis of chlorinated acetyl chloride in rare gas matrix, one chlorine atom substitution to methyl group of acetyl chloride opened the additional reaction paths in the T1 state [12] [13] . Without chlorination the ketene∙∙∙HCl complex was exclusively produced in the S0 state after the internal conversion from the S1 state [14] [15] . In the CCl3COCl photolysis in an Ar matrix, the C-C bond cleavage was found to be the major reaction path [16] .
In the present study, the UV light photolysis of CHCl2COCl was investigated in cryogenic Ar, Kr, Xe, and O2 matrices with the aid of the calculation using the B3LYP and MP2 methods to clarify how the two chlorine atom substitutions affect the reaction mechanism.
2. Experimental
Light irradiation was performed using a low pressure mercury arc lamp (HAMAMATSU L937-04, λ > 253.7 nm). IR spectra were measured in the range 4000 - 700 cm−1 with 1.0 cm−1 resolution by a SHIMADZU 8300A Fourier transform IR spectrometer with a liquid-nitrogen-cooled MCT detector. Each spectrum was obtained by scanning over 128 times. A closed-cycle helium cryostat (Iwatani M310/CW303) was used to control the temperature of the matrix.
Argon (Nippon Sanso, 99.9999%), krypton (Taiyo Sanso), xenon (Nippon Sanso), and O2 (Okaya Sanso) were used without further purification. Dichloroacetyl chloride (Wako Pure Chemicals) was used after freezepump-thaw cycling at 77 K. Chloroform (Wako Pure Chemicals) was used as an authentic sample for product identification. Samples were deposited on a CsI window at 6 K.
For product identification and energetic consideration, molecular orbital calculation was utilized. Geometry optimizations were performed using the second-order Møller-Plesset theory (MP2) and density functional theory (B3LYP [17] [18] , CAM-B3LYP [19] , and M06-2X [20] ) with the 6-311++G(3df,3pd) and aug-cc-pV(T+d)Z basis sets. Harmonic vibrational frequency calculation was performed to confirm the predicted structures as local minima and to elucidate zero-point vibrational energy corrections (ZPE). The vertical transition energy was calculated at the SAC-CI/D95+(d,p) level based on the structures optimized at the CCSD/D95+(d,p) level. All calculations were performed using Gaussian 09 [21] .
3. Results and Discussion
3.1. CHCl2COCl/Ar
A mixture of CHCl2COCl/Ar was deposited on a CsI window with a ratio of CHCl2COCl/Ar = 1/1000. In the infrared spectrum obtained after deposition, two conformers, gaucheand syn-CHCl2COCl were distinguished by the C=O stretching vibration bands at 1816 and 1784 cm−1, respectively [8] [9] . Figure 1(a) shows the infrared difference spectrum obtained upon λ > 253.7 nm irradiation of a matrix CHCl2COCl/Ar for 60 min. The positive and negative bands indicate the growth and depletion, respectively, during the irradiation period. Table 1 lists the observed wavenumbers of the growth bands. In the CO stretching region, a strong band observed at 2138 cm−1 assignable to the CO stretching continued to grow during the prolonged irradiation period. A band at 2155 cm−1 showed growth and decay behavior accompanied with the bands at 1293 and 934 cm−1, whose frequencies are consistent with those of CCl2=C=O observed in the CCl3COCl photolysis in Ar [16] . The bands at 2844 and 2836 cm−1 were assigned to the stretching vibration of HCl complexed with the CCl2=C=O. With the different growth rate from those of CO and CCl2=C=O, three bands at 2150, 1297 and 1113 cm−1 showed continuous growth which are assignable to the C=O stretching, C=C stretching, and C-H in-plane bending vibrations of CHCl=C=O, respectively [12] . The C-Cl stretching band observed in the photolysis of CH2ClCOCl in Ar was difficult to be discerned due to the overlapping with the strong depletion band of syn-CHCl2COCl. A band at 1878 cm−1 was assigned to the CO stretching vibration of COCl [22] . Photolysis counterpart of COCl, CHCl2, showed the C-H bending and CCl2 antisymmetric stretching vibrations at 1219 and 898 cm−1, respectively [23] . Prolonged irradiation caused the depletion in intensities of the bands due to CCl2=C=O as shown in Figure 1(b). A band at 1969 cm−1 showing an induction period was assigned to the CO stretching vibration of CCO [24] . A band at 766 cm−1 grew continuously to be the strongest in the spectrum after 360 min irradiation, which was assigned to the C-Cl stretching vibration of CHCl3. The C-H bending vibration of CHCl3 was observed at 1223 cm−1.
3.2. CHCl2COCl/Kr, CHCl2COCl/Xe
Figure 2 shows the infrared difference spectra obtained upon λ > 253.7 nm irradiation of the matrix CHCl2COCl/Xe. In Kr, similar results were obtained. In addition to the photolysis products in Ar, the products of Kr2H+ and Xe2H+ were observed in Kr and Xe, respectively [25] . The growth bands at 1814, 1262, 987, and 740 cm−1 in Kr and 1809, 1259, 984, and 736 cm−1 in Xe were assigned to the C=O stretching, CH bending, C-C stretching, and CCl2 symmetric stretching vibrations of gauche-CHCl2COCl, respectively [9] . It is controversial
Figure 1. Infrared difference spectra upon λ > 253.7 nm irradiation of the matrix CHCl2COCl/Ar = 1/1000. (a) 60 - 0 min and (b) 360 - 60 min.
which of the two conformers is more stable [11] . Table 2 compares the relative electronic energies calculated at the several calculation levels. The barrier height for the conversion from the syn to gauche rotamer is calculated to be approximately 1200 cm−1 in the S0 ground state indicating that the conversion between the syn and gauche rotamers is not expected to occur at 7 K in the absence of UV irradiation. UV irradiation yielded an increase of the population of the less stable rotamer.
3.3. CHCl2COCl/O2
In order to clarify the route of the ketenes and CHCl3 formation i.e. the radical or concerted mechanism, the reactive O2 matrix was used. Figure 3 shows the infrared difference spectrum obtained after 480 min irradiation of CHCl2COCl. The product bands were assigned by comparison with the spectrum observed in the photolysis of the matrix CCl3COCl/O2. Due to the photolysis in O2 at 253.7 nm, ozone formation is prominent at 1038 cm−1 (ν3) [26] . Other O3 absorption bands were observed at 2107 (ν1 + ν3) and 1101 cm−1 (ν1) [26] [27] . The 2342 and 2276 cm−1 bands are assigned to ν3 vibrations of 12CO2 and 13CO2, respectively. The 2037 cm−1 band is attributed to CO3 complexed with Cl [16] . A broad band at 1436 cm−1 was assigned to ClOO ν1 [28] . In O2, compared with the ratio of CHCl3 or CO absorbance with CHCl2COCl absorbance in Figure 1, the CO and CHCl3 formation was depressed. Formation of CHCl2 and ketenes was negligible. Instead major product was found to be CO2 which would be produced via reactions of COCl and CHCl2 with O2. These indicate the reaction predominantly proceed by radical mechanism in the photolysis of CHCl2COCl similar to that of CCl3COCl.
3.4. Reaction Mechanism
Figure 4 shows the integrated absorbance changes of syn-CHCl2COCl (1784 cm−1), gauche-CHCl2COCl (1816 cm−1), CHCl3 (766 cm−1), CHCl=C=O (2150 cm−1), CO (2138 cm−1), CCl2=C=O (2155 cm−1), and CHCl2 (898 cm−1) observed in Ar, where the IR intensities of these absorption bands were calculated to be 283, 242, 320, 618, 80, 621, and 163 km mol−1, respectively, at the B3LYP/aug-cc-pV(T+d)Z level. The synand gauche
Table 1. FTIR spectra of the CHCl2COCl photolysis products in the Ar, Kr, Xe, and O2 matrices.
aRef. [27] . bRef. [24] . cRef. [22] . dRef. [28] . eRef. [23] . fRef. [26] . gRef. [25] .
Figure 2. Infrared difference spectra upon λ > 253.7 nm irradiation of the matrix CHCl2COCl/Xe = 1/1000. (a) 30 - 0 min and (b) 420 - 30 min.
Table 2. Calculated relative electronic energies in cm−1 including zero-point vibrational energy corrections.
CHCl2COCl possess the different decay rates. The CCl2=C=O and CHCl2 showed the growth and decay profiles. The relative yield of CHCl3:CHCl=C=O:CCl2=C=O at the irradiation time of 360 min was found to be 1:0.09:0.008. There is an obvious contrast as compared with the relative yield obtained in the photolysis of the matrix CH2COCl/Ar where the ratio of CH2Cl2:CHCl=C=O was found to be 1:7.5 [12] .
Even in O2, the ketene species were found to be produced, though the yields decreased greatly. It indicates the majority of the ketene species were formed in the triplet state by the radical mechanism. It seems plausible to explain the dominant radical mechanism in the triplet state by the enhanced intersystem crossing from S1 caused by substitution of the chlorine atoms with methyl hydrogen atoms of acetyl chloride. Therefore, we focus on the triplet surface reaction after intersystem crossing and the ground state reaction after internal conversion. Figure 5 shows the energy diagram for the CHCl2COCl photolysis initiated by 253.7 nm irradiation. The photon energy at a wavelength of 253.7 nm corresponded to 113 kcal∙mol−1. The reaction enthalpies of three elementary reac-
Figure 3. Infrared difference spectrum upon λ > 253.7 nm irradiation of the matrix CHCl2COCl/O2 = 1/1000 for 480 min.
Figure 4. Integrated absorbance changes of (○) syn-CHCl2COCl, (□) gaucheCHCl2COCl, (●) CHCl3, (+) CHCl=C=O, (Δ) CO, (▲) CCl2=C=O, and (×) CHCl2 upon λ > 253.7 nm irradiation of the matrix CHCl2COCl/Ar = 1/1000.
tions, C(O)-Cl, C-C, and CHCl-Cl bond cleavages from the T1 equilibrium states are calculated to be −1.7, −14.8, and −19.5 kcal∙mol−1 for syn-CHCl2COCl and −2.8, −15.1, and −20.9 kcal∙mol−1 for gauche-CHCl2COCl, respectively, where the reaction barriers are calculated to be 4.4, 5.8, and 2.6 kcal∙mol−1 for syn-CHCl2COCl and3.7, 5.6, and 2.2 kcal∙mol−1 for gauche-CHCl2COCl, respectively. The C-C dissociation on the T1 surface possesses the highest barrier, while CHCl-Cl dissociation the lowest barrier. Radical species CHCl2 and COCl
Figure 5. Energy diagram for the CHCl2COCl photolysis.
can be also produced from the dissociation of CHCl2CO into CHCl2 and CO, followed by the recombination of CO with Cl. The CHClCOCl would be further photodissociated. The reaction barrier for the formation of CHCl=C=O + Cl2 in the S0 state was calculated to be higher compared with that for the formation of CCl2=C=O + HCl. The SAC-CI calculation showed the S1 and T1 states of CCl2=C=O possess the mixing characters of 
and πRydberg, ‒0.87 (HOMO → LUMO) + 0.30 (HOMO → LUMO+3). Upon UV irradiation the C-Cl bond dissociation would occur to form CCO.
For the CHCl2COCl photolysis in the rare gas matrices, the C-C bond cleaved CHCl3, CO, CHCl2, and COCl were dominantly produced similar to the CCl3COCl photolysis and contrary to the CH2ClCOCl photolysis, where ketene formation was a major process. For the CHCl2COCl photolysis in O2, both ketene and CHCl3 formations were greatly depressed, while for CH2ClCOCl, the formation of ketene was slightly depressed. On the basis of these results it will be reasonable to consider that the reaction mechanism drastically changed between CH2ClCOCl and CHCl2COCl from the concerted mechanism in the S0 state to the radical mechanism in the T1 state.
4. Conclusion
UV light photolysis of CHCl2COCl was investigated in cryogenic Ar, Kr, Xe, and O2 matrices. In Ar, Kr, and Xe, the formation of CHCl3 and CO became the dominant process over the ketene formation. The C-C bond cleaved products CHCl2 and COCl were also observed. In Kr and Xe, photoisomerization from synto gaucheCHCl2COCl was observed at the early stage of the irradiation. As the number of the chlorine atom substitution to methyl group of acetyl chloride increased, the C-C bond cleaved product yield in the triplet state increased, which can be attributed to an internal heavy-atom effect where the intersystem crossing rate was enhanced.
Acknowledgements
The author thanks Prof. Tsuneo Fujii and Prof. Hiromasa Nishikiori (Shinshu University) for their helpful discussions.