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 ${}^2{\Sigma }^{+}$ states have been determined and yield the following magnetic parameters in solid argon: $g_{\Vert }\text{\hspace{0.17em}=\hspace{0.17em}2.0015(5)}$ , $\text{g⊥\hspace{0.17em}=\hspace{0.17em}2.0003(5)}$ , $A_{\Vert }{(}^{14}\mathrm{N}\text{)\hspace{0.17em}=\hspace{0.17em}+\hspace{0.17em}18.2(4)}\mathrm{MHz}$ , ${\mathrm{A\perp \; (}}^{14}\mathrm{N}\text{)\hspace{0.17em}=\hspace{0.17em}−\hspace{0.17em}28.0(3)}\mathrm{MHz}$ , $A_{\Vert }{(}^{13}\mathrm{C}\text{)\hspace{0.17em}=\hspace{0.17em}+\hspace{0.17em}678(1)}\mathrm{MHz}$ , ${\mathrm{A\perp \; (}}^{13}\mathrm{C}\text{)\hspace{0.17em}=\hspace{0.17em}+\hspace{0.17em}543.1(5)}\mathrm{MHz}$ . Weak forbidden ${\mathrm{\Delta M}}_I\text{\hspace{0.17em}=\hspace{0.17em}±\hspace{0.17em}1}$ 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 $\mathrm{g\perp }$ shift is shown to arise from small negative contributions from spin–orbit coupling with both the ${}^2{\Pi }_i$ and ${}^2{\Pi }_r$ excited states. Approximate spin densities in the molecule are $q_C\text{(2s)\hspace{0.17em}=\hspace{0.17em}0.19}$ , $q_C\text{(2p)\hspace{0.17em}=\hspace{0.17em}0.48}$ , $q_N\text{(2p)\hspace{0.17em}=\hspace{0.17em}0.32}$ , $q_N\text{(2s)\hspace{0.17em}=\hspace{0.17em}−\hspace{0.17em}0.008}$ . These values are compared with those obtained from a Hartree–Fock wavefunction provided by Cade.