Matrix Isolation and Computational Study on the Photolysis of CHCl 2 COCl

UV light photolysis of dichloroacetyl chloride (CHCl2COCl) has been investigated by infrared spectroscopy in cryogenic Ar, Kr, Xe, and O2 matrices. The formation of CHCl3 and CO was found to be the dominant process over the ketene formation. The C-C bond cleaved products CHCl2 and COCl were also observed. 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.


Introduction
Dichloroacetyl chloride (CHCl 2 COCl) 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 CHCl 2 COCl were studied by vibrational spectroscopy [6]- [9], electron diffraction [10], and theoretical method [11].Two conformers exist in the CHCl 2 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 T 1 state [12] [13].Without chlorination the ketene•••HCl complex was exclusively produced in the S 0 state after the internal conversion from the S 1 state [14] [15].In the CCl 3 COCl 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 CHCl 2 COCl was investigated in cryogenic Ar, Kr, Xe, and O 2 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.

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 O 2 (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.

CHCl2COCl/Ar
A mixture of CHCl 2 COCl/Ar was deposited on a CsI window with a ratio of CHCl 2 COCl/Ar = 1/1000.In the infrared spectrum obtained after deposition, two conformers, gauche-and syn-CHCl 2 COCl 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 CHCl 2 COCl/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 CCl 2 =C=O observed in the CCl 3 COCl photolysis in Ar [16].The bands at 2844 and 2836 cm −1 were assigned to the stretching vibration of HCl complexed with the CCl 2 =C=O.With the different growth rate from those of CO and CCl 2 =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 CH 2 ClCOCl in Ar was difficult to be discerned due to the overlapping with the strong depletion band of syn-CHCl 2 COCl.A band at 1878 cm −1 was assigned to the CO stretching vibration of COCl [22].Photolysis counterpart of COCl, CHCl 2 , showed the C-H bending and CCl 2 antisymmetric stretching vibrations at 1219 and 898 cm −1 , respectively [23].Prolonged irradiation caused the depletion in intensities of the bands due to CCl 2 =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 CHCl 3 .The C-H bending vibration of CHCl 3 was observed at 1223 cm −1 .

CHCl2COCl/Kr, CHCl2COCl/Xe
Figure 2 shows the infrared difference spectra obtained upon λ > 253.7 nm irradiation of the matrix CHCl 2 COCl/Xe.In Kr, similar results were obtained.In addition to the photolysis products in Ar, the products of Kr 2 H + and Xe 2 H + 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 CCl 2 symmetric stretching vibrations of gauche-CHCl 2 COCl, respectively [9].It is controversial 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 S 0 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.

CHCl2COCl/O2
In order to clarify the route of the ketenes and CHCl 3 formation i.e. the radical or concerted mechanism, the reactive O 2 matrix was used.Figure 3 shows the infrared difference spectrum obtained after 480 min irradiation of CHCl 2 COCl.The product bands were assigned by comparison with the spectrum observed in the photolysis of the matrix CCl 3 COCl/O 2 .Due to the photolysis in O 2 at 253.7 nm, ozone formation is prominent at 1038 cm −1 (ν 3 ) [26].Other O 3 absorption bands were observed at 2107 (ν 1 + ν 3 ) and 1101 cm −1 (ν 1 ) [26] [27].The 2342 and 2276 cm −1 bands are assigned to ν 3 vibrations of 12 CO 2 and 13 CO 2 , respectively.The 2037 cm −1 band is attributed to CO 3 complexed with Cl [16].A broad band at 1436 cm −1 was assigned to ClOO ν 1 [28].In O 2 , compared with the ratio of CHCl 3 or CO absorbance with CHCl 2 COCl absorbance in Figure 1, the CO and CHCl 3 formation was depressed.Formation of CHCl 2 and ketenes was negligible.Instead major product was found to be CO 2 which would be produced via reactions of COCl and CHCl 2 with O 2 .These indicate the reaction predominantly proceed by radical mechanism in the photolysis of CHCl 2 COCl similar to that of CCl 3 COCl.
The relative yield of CHCl 3 :CHCl=C=O:CCl 2 =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 CH 2 COCl/Ar where the ratio of CH 2 Cl 2 :CHCl=C=O was found to be 1:7.5 [12].Even in O 2 , 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 S 1 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 CHCl 2 COCl 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-   For the CHCl 2 COCl photolysis in the rare gas matrices, the C-C bond cleaved CHCl 3 , CO, CHCl 2 , and COCl were dominantly produced similar to the CCl 3 COCl photolysis and contrary to the CH 2 ClCOCl photolysis, where ketene formation was a major process.For the CHCl 2 COCl photolysis in O 2 , both ketene and CHCl 3 formations were greatly depressed, while for CH 2 ClCOCl, 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 CH 2 ClCOCl and CHCl 2 COCl from the concerted mechanism in the S 0 state to the radical mechanism in the T 1 state.

Conclusion
UV light photolysis of CHCl 2 COCl was investigated in cryogenic Ar, Kr, Xe, and O 2 matrices.In Ar, Kr, and Xe, the formation of CHCl 3 and CO became the dominant process over the ketene formation.The C-C bond cleaved products CHCl 2 and COCl were also observed.In Kr and Xe, photoisomerization from syn-to gauche-CHCl 2 COCl 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.

Figure 5 .
Figure 5. Energy diagram for the CHCl 2 COCl photolysis.canbe also produced from the dissociation of CHCl 2 CO into CHCl 2 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 + Cl 2 in the S 0 state was calculated to be higher compared with that for the formation of CCl 2 =C=O + HCl.The SAC-CI calculation showed the S 1 and T 1 states of CCl 2 =C=O possess the mixing characters of * C Cl πσ − 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 CHCl 2 COCl photolysis in the rare gas matrices, the C-C bond cleaved CHCl 3 , CO, CHCl 2 , and COCl were dominantly produced similar to the CCl 3 COCl photolysis and contrary to the CH 2 ClCOCl photolysis, where ketene formation was a major process.For the CHCl 2 COCl photolysis in O 2 , both ketene and CHCl 3 formations were greatly depressed, while for CH 2 ClCOCl, 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 CH 2 ClCOCl and CHCl 2 COCl from the concerted mechanism in the S 0 state to the radical mechanism in the T 1 state.

Table 2 .
Calculated relative electronic energies in cm −1 including zero-point vibrational energy corrections.