Infrared absorption of CH₃SO₂ observed upon irradiation of a p-H₂ matrix containing CH₃I and SO ₂

Irradiation with a mercury lamp at 254 nm of a p-H₂ matrix containing CH₃I and SO₂ at 3.3 K, followed by annealing of the matrix, produced prominent features at 633.8, 917.5, 1071.1 (1072.2), 1272.5 (1273.0, 1273.6), and 1416.0 cm^-1, attributable to ₁₁ (C-S stretching), ₁₀ (CH₃ wagging), ₈ (SO₂ symmetric stretching), ₇ (SO₂ antisymmetric stretching), and ₄ (CH₂ scissoring) modes of methylsulfonyl radical (CH₃SO₂), respectively; lines listed in parentheses are weaker lines likely associated with species in a different matrix environment. Further irradiation at 365 nm diminishes these features and produced SO₂ and CH₃. Additional features at 1150.1 and 1353.1 (1352.7) cm^-1 are tentatively assigned to the SO₂ symmetric and antisymmetric stretching modes of ISO₂. These assignments are based on comparison of observed vibrational wavenumbers and ¹⁸O- and ³⁴S-isotopic shifts with those predicted with the B3P86 method. Our results agree with the previous report of transient IR absorption bands of gaseous CH₃SO₂ at 1280 and 1076 cm^-1. These results demonstrate that the cage effect of solid p-H₂ is diminished so that CH₃ radicals, produced via UV photodissociation of CH₃I in situ, might react with SO₂ to form CH₃SO₂ during irradiation and upon annealing. Observation of CH₃SO₂ but not CH₃OSO is consistent with the theoretical predictions that only the former reactions proceed via a barrierless path.