ESR studies of the formyl radical in a CO matrix: magnetophotoselective photolysis and thermally activated rotations

Photolysis of the formyl radical (HCO) in a CO matrix at 13 K by yellow light (λ>500 nm) polarized with its electric vector parallel to an external magnetic field (HDC) is orientationally selective, as shown by changes in the powder ESR spectrum of the radical. The depleted orientation in the cyclic photolysis [HCO→hν H+CO; H+ (a different CO)→(randomly oriented HCO)] is (HCO plane) ⊥ HDC, showing that the largest component of the optical transition moment is perpendicular to the HCO plane, as predicted by theory for a nonrotating HCO. Around 25 K part of the photoinduced orientational anisotropy is lost due to incipient rotation about an axis close to the CO bond, but the remaining orientational anisotropy persists to higher temperatures (∼40 K), where it decays only slowly. Around 35 K this rotation becomes fast enough to yield a pseudoaxially symmetric powder ESR spectrum with g∥=1.9973,g⊥=2.0021, and A∥=A⊥=383.0 MHz. The rotation axis is closest to but significantly different from the minimum inertial axis.