Knowledge Bank

The nuclear magnetic double resonance spectra of $N14$ and $K15$ ammonium ion have been obtained by observing the proton magnetic resonance while applying a strong radio frequency field near the nitrogen resonance frequency. The main features of the spectra are in agreement with spectra calculated using a Hamiltonian that is made stationary by transformation to a coordinate system rotating at the frequency of the Strong r.f. field. In the rotating frame. [ H_{R}=\Sigma_{i} \left(\frac{-\gamma_{i}H_{0}}{2\pi}+\frac{\omega_{2}}{2\pi}\right)+ \Sigma_{i < j}\quad J_{ij}; I_{i};I_{j} - \Sigma_{i} \frac{\gamma_{1}H_{2}}{2\pi}\quad I_{xi} ] where $\omega 2$ and $H2$ refer to the frequency and amplitude of the strong radio frequency field. Transitions in addition to those predicted from Equation (1) are observed in both the $N14$ and $N15$ spectra. The dependence of these transition frequencies and intensities on $\omega 2$ and $H2$ is discussed in term of possible double quantum transitions, changes in symmetry of the ammonium ion, and relaxation effects. The complicating effects of sweeping magnetic field rather than frequency are shown to be useful in obtaining accurate values of nitrogen chemical shifts.