Electron Spin Resonance of Free Radicals Formed from Group-IV and Group-V Hydrides in Inert Matrices at Low Temperature

The electron-spin-resonance spectra of molecular free radicals formed from the group-IV and group-V hydrides at 4.2°K in the xenon matrix have been observed and compared with those observed in krypton and other matrices. From these spectra, certain properties of the free radicals and certain matrix effects have been derived. Molecular free radicals formed by $\gamma$ irradiation of a matrix containing P${\mathrm{H}}_3$ or As${\mathrm{H}}_3$ are P${\mathrm{H}}_2$ and As${\mathrm{H}}_2$, respectively. From analysis of the hyperfine structure, the P-bonding orbitals of P${\mathrm{H}}_2$ were found to have 20.6% $3s$ character in the Xe matrix, compared with 19% in the Kr matrix. Because of anisotropies in the coupling and in the $g$ tensor the hyperfine structure of As${\mathrm{H}}_2$ could not be measured. In the Xe matrix the average $g$ is 2.0050 for P${\mathrm{H}}_2$, and 2.034 for As${\mathrm{H}}_2$. No spectra could be observed for Sb${\mathrm{H}}_2$, although evidence for dissociation of Sb${\mathrm{H}}_2$ was indicated by the strong H-atom lines observed for a $\gamma$-irradiated sample of Sb${\mathrm{H}}_3$ in the Xe matrix. Molecular free radicals formed by $\gamma$ irradiation of the group-IV hydrides are C${\mathrm{H}}_3$, Si${\mathrm{H}}_3$, Ge${\mathrm{H}}_3$, and Sn${\mathrm{H}}_3$. The observed hyperfine structure caused by the isotopes ${}_{}{}^{29}{}_{}{}^{}\mathrm{Si}$, ${}_{}{}^{73}{}_{}{}^{}\mathrm{Ge}$, and ${}_{}{}^{117,119}{}_{}{}^{}\mathrm{Sn}$ indicate that the radicals Si${\mathrm{H}}_3$, Ge${\mathrm{H}}_3$, and Sn${\mathrm{H}}_3$ are not planar like C${\mathrm{H}}_3$, but are pyramidal in structure. Noticeable difference in the isotropic coupling of ${}_{}{}^{29}{}_{}{}^{}\mathrm{Si}$ for the Kr matrix, 240 G, and for the Xe matrix, 190 G, indicates strong interaction of the matrix with the Si${\mathrm{H}}_3$ radicals.