Infrared spectrum of CCH+ in solid argon and neon

Laser-ablation of over ten different transition, lanthanide, and actinide metals with concurrent codeposition of acetylene/argon samples at 7 K produced metal independent absorptions for CCH, CCH−, C4H, and C4H2, in agreement with previous matrix isolation work, and a sharp new 1820.4 cm−1 band. Isotopic substitution showed this band to be due to a largely C–C stretching mode of a species with one H and two inequivalent carbon atoms. The same species were observed in solid neon samples at 4 K, and the neon matrix counterpart of the new band was found at 1832.2 cm−1. When CO2 was added to serve as an electron trap, the yield of CCH− at 1772.8 cm−1 decreased and the 1832.2 cm−1 band increased relative to CCH at 1837.9, 1835.0 cm−1. Quantum chemical calculations at the coupled-cluster and density functional levels predict the C–C stretching mode of CCH+ between this mode for CCH and CCH− and support assignment of this new infrared absorption to the CCH+ cation in solid argon and neon.