Paramagnetic Resonance of Free Radicals at Millimeter Wave Frequencies

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 gII=3.0035, g_⊥=2.0043 for the first and gII=2.0095, g_⊥=2.0035 for the second. The third radical has a lower symmetry with gₓ=2.0042, gy=2.0064, and gz=2.0083 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 N¹⁴ nuclei of the N¹⁴ 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 N¹⁴ 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.