Nitrogen and oxygen nuclear quadrupole and nuclear magnetic resonance spectroscopic study of NO bonding in pyridine 1‐oxides

¹⁴N nuclear quadrupole resonance (NQR) spectra of the NO functional group in a series of 4‐substituted pyridine 1‐oxides (R = H, CH₃, C₆H₅, OCH₃, OCH₂C₆H₅, Cl, CN, NO₂) and the natural abundance ¹⁷O NQR and nuclear magnetic resonance (NMR) solution spectra of a number of selected systems were obtained. The orientations of the principal axes of the ¹⁴N electric field gradient tensor (efg) in the NO group were determined using a Townes‐Dailey treatment of the NQR results, combined with a ¹³C cross‐polarization magic‐angle sample‐spinning NMR line shape analysis of carbons bonded directly to nitrogen. The major principal axis (Z) is coincident with the NO bond. The orientation of the principal (Z) axis of the ¹⁷O efg tensor also lies along the N‐O bond, as determined by Townes‐Dailey analysis. The NO π‐bond orders show considerable sensitivity to the nature of the para‐substituent, ranging from 0.17 to 0.31. ¹⁷O and ¹⁵N NMR chemical shifts parallel these changes in π‐bond order.