ESR of the CN Radical in Inert Matrices

CN has been trapped as a randomly oriented molecule in argon, krypton, and neon matrices at 4°K. The ESR spectra of 12C14N and 13C14N in their ground Σ+2 states have been determined and yield the following magnetic parameters in solid argon: g‖ = 2.0015(5), g⊥ = 2.0003(5), A‖(14N) = + 18.2(4) MHz, A⊥ (14N) = − 28.0(3) MHz, A‖ (13C) = + 678(1) MHz, A⊥ (13C) = + 543.1(5) MHz. Weak forbidden ΔMI = ± 1 transitions are also observed. Upon slight warming, the sites are irreversibly changed so that CN thereafter exhibits hindered rotation, even at 4°K, and raising the temperature leads to the isotropic ESR spectrum of rotating CN. Variations in linewidths due to motional effects make it possible to confirm the relative signs of the A values. The g⊥ shift is shown to arise from small negative contributions from spin–orbit coupling with both the Πi2 and Πr2 excited states. Approximate spin densities in the molecule are qC(2s) = 0.19, qC(2p) = 0.48, qN(2p) = 0.32, qN(2s) = − 0.008. These values are compared with those obtained from a Hartree–Fock wavefunction provided by Cade.