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, gauche- and 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.

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

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.

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.

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 syn- and gauche-

^aRef. [27] . ^bRef. [24] . ^cRef. [22] . ^dRef. [28] . ^eRef. [23] . ^fRef. [26] . ^gRef. [25] .

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-

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

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.