Localized Saturation Effects in Nuclear Magnetic Double Resonance

In a high-resolution nuclear magnetic resonance spectrum where the dominant line-broadening influence is the spatial inhomogeneity of the magnetic field, it is possible to impose a sufficiently weak rf field (H2) on a given line ωpq such that saturation is localized in a restricted region of the sample volume. This has been called “burning a hole” in the line. When a second line ωqr that is regressively connected to ωpq is examined in a double resonance experiment, a similar hole appears in an exactly corresponding region of the line profile, due to the disturbance of the spin population on the common level q. This correlation, which arises from the intramolecular nature of the coupling, forms the basis of a technique for measuring frequency separations with a precision that is not primarily limited by the inhomogeneous linewidth. Experimental evidence suggests a reliability of ±1 mHz. The persistence of this effect down to rf levels as low as γH2 / 2π = 0.005 Hz distinguishes it from the well-known coherence effect of the irradiation field (“spin tickling”). The technique has been employed to detect the small displacements of the lines of a complex spectrum attributable to the presence of the two radiofrequency fields H1 and H2—the indirect Bloch–Siegert shifts. The transition frequencies of a complex three-spin system (2-chlorothiophene) have been measured by an extension of this double resonance method in order to provide a test case for the iterative analysis programs marip and laocoon. Experimental and calculated transition frequencies agree within a mean deviation of 0.0008 Hz.