Direct detection of chlorine-35 multiple-quantum NMR transitions in a single crystal of sodium chlorate†

Abstract

35Cl (spin 3/2) in sodium chlorate has a strong quadrupole coupling, similar in size to its interaction with a magnetic field of 11.7 T (500 MHz for protons). This means that it obeys neither the standard nuclear quadrupole resonance (NQR) nor the nuclear magnetic resonance (NMR) rules. Recently, we have published a theoretical approach that allows us to calculate the transition frequencies of such a system without approximations. This exact solution is applicable to any spin, with any relative ratio of Zeeman to quadrupole coupling, so we can map out how NQR becomes NMR as the magnetic field is increased. In the general situation, the selection rules restricting direct observation to only single quantum transitions break down and nominal multiple-quantum transitions become directly observable. The transition frequencies depend on the Zeeman interaction, the quadrupole coupling and the orientation of the crystal in the magnetic field. Sodium chlorate is an ideal sample, since we can use the 23Na NMR spectrum to determine the orientation of the crystal. 23Na (spin-3/2) has a weak quadrupole coupling and there are four molecules per unit cell. The positions of the quadrupole satellites from the four molecules are sufficient to determine the orientation, so the sodium nucleus acts as an internal goniometer. Since we already know the magnetic parameters for 35Cl, we can predict where to look for its resonances, which can range from less than 10 MHz to greater than 150 MHz. A single spin-3/2 has 6 possible transitions, so the four molecules in the unit cell should show 24 transitions. We have observed 17 of them.