Coherence Transfer by Radiation Damping Combined with J‐Couplings in 2D Correlation Spectroscopy at High‐Field Solution NMR

Abstract

Radiation damping, one of the oldest known macroscopic interactions in solution nuclear magnetic resonance (NMR), 1 has been recently receiving much interest since it can generate unexpected cross-peaks in two-dimensional (2D) NMR experiments. [2][3][4][5] This effect is quite dramatic for concentrated protons (e.g., in protonated solvents) and in most modern high-field NMR spectrometers. For example, the COSY type spectrum of water has many harmonic peaks along the indirectly detected dimension. 6,7 The theoretical framework describing this effect has tremendously progressed over the last couple of decades. An analytical solution, 2 numerical simulations, 4 and other studies have proved that radiation damping during the detection period in a 2D NMR experiment (t 2 period) is responsible for it, as also confirmed experimentally via Q-switching methods. 6,8 Radiation damping is also an ongoing problem in biological NMR applications, where the solvent peak is usually much larger than others. It generates many artifacts in 2D experiments and, hence, its suppression has been intensively investigated. 9-11 However, it may be constructively used to suppress the water signal in some cases. 1 Radiation damping has also a soft-pulse effect that turns the corresponding spin into its equilibrium state, affecting the spin relaxation. Therefore, the suppression of the radiation damping effect is crucial to achieving high precision in quantitative NMR (qNMR) spectroscopy, a technique that has recently received much attention. 12,13 In the past, this effect has been considered only when dealing with the signal of concentrated spins, such as for solvents. However, since the use of high-sensitivity highfield NMR spectrometers has recently increased, conditions suitable for observing the radiation damping effect even in dilute spins have been established. Radiation damping can also be combined with J-coupling, producing various types of cross-peaks along the indirectly detected dimension. 5 Therefore, the radiation damping associated coherence transfer pathways in 2D NMR spectra showing cross-peaks induced by spin couplings should be understood.In this study, we show how the effects of radiation damping associated with J-coupling appear in 2D COSY spectra. To understand them, we investigated the coherence transfer pathways in a J-coupled spin system while applying various COSY pulse sequences. In addition, we demonstrate that the radiation damping effect can be observed even in dilute samples.The time evolution of magnetization for equivalent spins in an external magnetic field, including the relaxation processes (T 1 and T 2 ), is described by the Bloch equation:As seen in Eq. (1), the conventional Bloch equation is linear since the external magnetic field B is independent of the magnetization itself, without collective effects such as the dipolar field and the radiation damping field. 2,14 Radiation damping occurs when the current induced by the transverse magnetization (i.e., the free induction decay signal) of the spins in rec...