Atomic hydrogen as high-precision field standard for high-field EPR.

We introduce atomic hydrogen trapped in an octaisobutylsilsesquioxane nanocage (H@iBuT₈) as a new molecular high-precision magnetic field standard for high-field EPR spectroscopy of organic radicals and other systems with signals around g=2. Its solid-state EPR spectrum consists of two 0.2 mT wide lines separated by about 51 mT and centered at g≈2. The isotropic g factor is 2.00294(3) and essentially temperature independent. The isotropic ¹H hyperfine coupling constant is 1416.8(2) MHz below 70 K and decreases slightly with increasing temperature to 1413.7(1) MHz at room temperature. The spectrum of the standard does not overlap with those of most organic radicals, and it can be easily prepared and is stable at room temperature.