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

The electron spin resonance spectrum of NH3+ trapped in ammonium perchlorate (95% 15N) 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.