AbstractThe proton and nitrogen hyperfine splittings of the short‐lived N‐hydropyridinyl radical have been measured over the temperature range ‐80 to 42°C in Propanol(‐2)/Acetone. The ESR‐data of the N‐deuteropyridinyl and the N‐hydropentadeuteropyridinyl radical at 22°C have been determined. The temperature coefficient of the NH proton splitting and the shift in the splitting caused by N‐deuterium substitution are unusually large. The effects of temperature and deuterium substitution have been analyzed using the model of localized out‐of‐plane vibrations of the protons. The estimated frequencies, in units of cm−1, are 290, 490, and 870 for the protons in position 1, 3(5), and 4, respectively. The result is discussed in connection with other ESR and MO studies on N‐hydropyridinyl and N, N′‐dihydrodiazinyl radicals. The previously observed non‐transferability of the spin polarization parameter QHNH is explained by the exceptionally low vibration frequency of the NH proton. A value of QHNH = −3.1 ± 0.4 mT is derived for the fixed planar structure of the radical. The vibrational analysis also confirms the negative sign of the π‐spin density on the carbon atoms in position 3 and 5.