ELECTRON SPIN RESONANCE SPECTRA OF ¹⁵N-CENTERED RADICALS AT LOW TEMPERATURES: I. THE RADICAL NH₃⁺ TRAPPED IN NH₄ClO₄

The electron spin resonance spectrum of NH 3 + trapped in ammonium perchlorate (95% 15 N) was examined at 300 °K, 77 °K, and 4 °K. The spectra are interpreted in terms of π-radical theory, and demonstrate the different stages in the freezing-out of the rotational motion of the ion.