ESR Spectrum and Structure of HCN−in KCl at 4°K

A previous electron-spin resonance (ESR) study of HC${\mathrm{N}}^{-}$ in KCl at 77°K yielded an isotropic spectrum indicating rapid tumbling of the radical. At 4.2°K this tumbling is frozen out and ESR spectra recorded at this temperature are anisotropic, permitting a complete determination of the magnetic parameters of HC${\mathrm{N}}^{-}$. The experimental results show that the planar radicals lie in {110} crystal planes with the CN bonds parallel to $〈110〉$ crystal axes. The components of the electronic $g$-factor tensor, the proton hyperfine-splitting (hfs) tensor, and nitrogen hfs tensor are as follows: $g_1=2.0023\mathrm{}$, $g_2=2.0005\mathrm{}$, and $g_3=2.0035\mathrm{}$; ${A_1}^{\left(\mathrm{H}\right)}=136.5\mathrm{}$, ${A_2}^{\left(\mathrm{H}\right)}=141.3\mathrm{}$, and ${A_3}^{\left(\mathrm{H}\right)}=131.3\mathrm{}$ Oe; and ${A_1}^{\left(\mathrm{N}\right)}=20.9\mathrm{}$ and ${A_2}^{\left(\mathrm{N}\right)}={A_3}^{\left(\mathrm{N}\right)}=0.0\mathrm{}$ Oe. Axis 1 lies in the molecular plane and is perpendicular to the CN bond, axis 2 is parallel to the CN bond, and axis 3 is perpendicular to the molecular plane. The carbon-13 hfs tensor has the components: ${A_{1^{\prime }}}^{\left(\mathrm{C}\right)}=90.0\mathrm{}$, ${A_{2^{\prime }}}^{\left(\mathrm{C}\right)}=65.6\mathrm{}$, and ${A_3}^{\left(\mathrm{C}\right)}=68.6\mathrm{}$ Oe, where axes 1′ and 2′ lie in the molecular plane and make angles of 34° with axes 1 and 2, respectively. From these results we deduce an approximate value of 131° for the HCN bond angle and the following values for the unpaired electron density on the hydrogen, nitrogen, and carbon atoms: ${\rho }_{\mathrm{H}}=0.27\mathrm{}$, ${\rho }_{\mathrm{C}}=0.32\mathrm{}$ and ${\rho }_{\mathrm{N}}=0.41\mathrm{}$.