C2H radical: 13C hyperfine interaction and optical spectrum

C2H was prepared by photolysis of acetylene, 90% 13C-substituted acetylene, or iodoacetylene and trapped in solid argon at 4°K. A complete analysis of all hyperfine-interaction and g tensor components was made for the randomly oriented 2Σ molecules yielding the following parameters (in solid argon): g∥=g⊥=2.0025(5), A⊥(H)=41(1), A∥(H)=51(1), A⊥(13Cα)=863(1), A∥(13Cα) =980(1), A⊥(13Cβ)=139(1), A∥(13Cβ) =191(1) MHz. Approximate spin densities are compared with an extended basis INDO calculation of Morikawa and Kikuchi. Weak optical absorption spectra were observed at about 10 000 and 3300 Å. The former is assigned to an A2∏i←X2Σ transition with (0,0,0) at 10 080 Å (in solid argon) where the upper state C–C stretching frequency ν′3 =1560 cm−1. The assignment of the 3300 Å system (s) is less definite, but it may involve a B2A′←X2Σ, bent ← linear transition since the observed bands cannot definitely be assigned to an expected B2Σ←X2Σ transition.