Paramagnetic Resonance of Free Radicals at Millimeter Wave Frequencies

Abstract

Paramagnetic resonance experiments have been made at 36 kMc/sec and at 75 kMc/sec on single crystals and on solid and liquid solutions of the free radicals: diphenyl picryl hydrazyl (DPPH), $p$-anisyl nitrogen oxide, and 2-(phenyl nitrogen oxide)-2-methyl pentane-4-one-oxime-$N$-phenyl ether. The measurements indicate fields axially symmetric about the paramagnetic element in the first two, with $g_{\mathrm{II}}=3.0035\mathrm{}$, $g_{\perp }=2.0043\mathrm{}$ for the first and $g_{\mathrm{II}}=2.0095\mathrm{}$, $g_{\perp }=2.0035\mathrm{}$ for the second. The third radical has a lower symmetry with $g_x=2.0042\mathrm{}$, $g_y=2.0064\mathrm{}$, and $g_z=2.0083\mathrm{}$ along the principal axes of susceptibility. The hyperfine structure of DPPH in dilute solution (earlier observed by others) consists of five components, arising from interactions with the two ${\mathrm{N}}^{14}$ nuclei of the ${\mathrm{N}}^{14}$ group. The other two radicals of the present study were found to have in dilute liquid solution a triplet hyperfine structure arising from interaction with a single ${\mathrm{N}}^{14}$ nucleus. This structure is interpreted as indicating that the odd electron is localized mainly on an —N=O group in each radical. The anisotropies in the $g$ factors are believed to arise mainly from a residual spin-orbit coupling. A sensitive millimeter-wave magnetic-resonance spectrometer employing superheterodyne detection with a crystal multiplier as the beat frequency oscillator is described.