Decoupling of Magnetic Resonance Signals in Time and Frequency Domain

Nuclear magnetic resonance (NMR) was discovered in the first half of the 20th century. Today, neither analytical chemistry without NMR spectroscopy nor medical diagnostics without magnetic resonance imaging (MRI) could be imagined. A magnetic resonance signal decays with a time constant T2, the transverse relaxation time. This parameter contains a great deal of information about structure and dynamics of the molecule, where the observed nuclear spin is located. A method to measure T2 is the so-called spin echo, discovered by Erwin Hahn in 1950. Usually, the spin echo can generate an exponential decay of the signal and T2 can be extracted. But if the indirect spin-spin interaction, the J -coupling, connects nuclear spins of the same isotope, the echo signal is modulated. The present work discusses novel methods to quench these echo modulations. This enables one, on the one hand, to measure the transverse relaxation in homonuclear spin systems. On the other hand, this decoupling technique can simplify NMR spectra by making splittings due to J-couplings disappear. These two processes can be seen to be equivalent, since the Fourier transform of a oscillating time signal corresponds to a line splitting of the frequency signal. To measure T2’s of protons (the nuclei of the most abundant hydrogen isotope 1H) or molecules isotopically enriched in carbon-13, we use a “train of echo pulses” with a moderate amplitude of the irradiated radio-frequency. To decouple various nuclear spins in a 1H NMR spectrum, we use a selective radio-frequency irradiation. These experiments are performed as Fourier transform spectroscopy, introduced by Ernst and Anderson in 1966. Based on the same pulse sequence, we measure so-called “spin tickling” experiments, described by Freeman and Anderson in 1962. All lines in the spectrum split that share a common energy-level with the irradiated resonance. “Tickling” and decoupling are complementary and belong both to the category of double resonance NMR experiments.