Reactions between chlorine atom and acetylene in solid para-hydrogen: Infrared spectrum of the 1-chloroethyl radical

摘要

We applied infrared matrix isolation spectroscopy to investigate the reactions between Cl atom and acetylene (C_2H_2) in a para-hydrogen (p-H_2) matrix at 3.2 K; Cl was produced via photodissociation at 365 nm of matrix-isolated Cl_2 in situ. The 1-chloroethyl radical (CHClCH_3) and chloroethene (C_2H _3Cl) are identified as the main products of the reaction Cl C _2H_2 in solid p-H_2. IR absorption lines at 738.2, 1027.6, 1283.4, 1377.1, 1426.6, 1442.6, and 2861.2 cm~(-1) are assigned to the 1-chloroethyl radical. For the reaction of Cl C _2D_2, lines due to the CDClCH_2D radical and trans-CHDCDCl are observed; the former likely has a syn-conformation. These assignments are based on comparison of observed vibrational wavenumbers and ~(13)C- and D-isotopic shifts with those predicted with the B3LYPaug-cc-pVDZ and MP2aug-cc-pVDZ methods. Our observation indicates that the primary addition product of Cl C_2H_2, 2-chlorovinyl (CHCHCl) reacts readily with a neighboring p-H_2 molecule to form CHClCH_3 and C_2H_3Cl. Observation of CDClCH _2D and trans-CHDCDCl from Cl C_2D_2 further supports this conclusion. Although the reactivity of p-H_2 appears to be a disadvantage for making highly reactive free radicals in solid p-H _2, the formation of 1-chloroethyl radical indicates that this secondary reaction might be advantageous in producing radicals that are difficult to prepare from simple photolysis or bimolecular reactions in situ.