Transient Nutations in Nuclear Magnetic Double Resonance. Assignment of Transitions to an Energy-Level Diagram

Abstract

A nuclear magnetic double resonance technique is described which permits the assignment of transitions of complex high-resolution spectra to the appropriate energy-level diagram. A monitoring radio frequency (H1 at ω1) is maintained at exact resonance for a chosen line in the spectrum with the level H1 set well above saturation, while a second radio frequency (H2 at ω2) of about the same strength is swept through the entire spectrum. A transient nutation of the type described by Torrey is detected whenever ω2 passes through a line that shares an energy level with the monitored transition. This is attributed to the sudden transfer of spin population to the common energy level as a result of population inversion across the connected transition caused by adiabatic passage through resonance. The new magnetization vector then precesses around H1 in the rotating frame of reference, giving the characteristic decaying oscillation first observed by Torrey. This transient oscillation is positive going when the two connected transitions span an interval Δm = 2 (the ``progressive'' configuration), and negative going when they span an interval Δm = 0 (the ``regressive'' configuration), thus making a clear distinction between these two possibilities. The technique is illustrated by transient double resonance experiments on 2-chlorothiophene.