# Pseudo rotary resonance relaxation dispersion effects in isotropic samples

## TL;DR

The paper reports a surprising transverse signal decay in rotating liquids under rotary resonance conditions, which could impact the design of MAS experiments.

## Contribution

The study identifies and explains pseudo-RRD effects in isotropic samples, a novel observation in MAS experiments.

## Key findings

- A drastic reduction in spin-lock signal intensities occurs when the spin-lock frequency matches the MAS rate.
- Oscillations in the signal suggest a coherent origin of the pseudo-RRD effect.
- Simulations show pseudo-RRD arises from sample rotation and inhomogeneous fields due to instrumental imperfections.

## Abstract

Enhanced transverse relaxation near rotary resonance conditions is a well-documented effect for anisotropic solid samples undergoing magic-angle spinning (MAS). We report transverse signal decay associated with rotary resonance conditions for rotating liquids, a surprising observation, since first-order anisotropic interactions are averaged at a much faster timescale compared with the spinning frequency. We report measurements of
13C
and
1H
signal intensities under spin lock for spinning samples of polybutadiene rubber, polyethylene glycol solution, and 99.96 %
D2O
. A drastic reduction in spin-lock signal intensities is observed when the spin-lock frequency matches 1 or 2 times the MAS rate. In addition, oscillations of the signal are observed, consistent with a coherent origin of the effect, a pseudo rotary resonance relaxation dispersion (pseudo-RRD). Through simulations, we qualitatively describe the appearance of pseudo-RRD, which can be explained by time dependence caused by sample rotation and an inhomogeneous field, the origin of which is an instrumental imperfection. Consideration of this effect is important for MAS experiments based on rotary resonance conditions and motivates the design of new MAS coils with improved radio frequency (RF)-field homogeneity.