Interaction frames in solid-state NMR: A case study for chemical-shift-selective irradiation schemes

TL;DR

This paper explores how different interaction frames in solid-state NMR influence the understanding and efficiency of chemical-shift-selective irradiation schemes, highlighting the advantages of various frame choices for better spin dynamics analysis.

Contribution

It provides a detailed case study comparing different interaction frames in solid-state NMR, emphasizing their impact on analyzing frequency-selective dipolar recoupling.

Findings

Different interaction frames offer unique insights into spin dynamics.

Including complete RF Hamiltonian improves analysis accuracy.

Choice of interaction frame affects the convergence of perturbation theories.

Abstract

Interaction frames play an important role in describing and understanding experimental schemes in magnetic resonance. They are often used to eliminate dominating parts of the spin Hamiltonian, e.g., the Zeeman Hamiltonian in the usual (Zeeman) rotating frame, or the radio-frequency-field (rf) Hamiltonian to describe the efficiency of decoupling or recoupling sequences. Going into an interaction frame can also make parts of a time-dependent Hamiltonian time independent like the rf-field Hamiltonian in the usual (Zeeman) rotating frame. Eliminating the dominant term often allows a better understanding of the details of the spin dynamics. Going into an interaction frame can also reduce the energy-level splitting in the Hamiltonian leading to a faster convergence of perturbation expansions, average Hamiltonian, or Floquet theory. Often, there is no obvious choice of the interaction frame to use but some can be more convenient than others. Using the example of frequency-selective dipolar recoupling, we discuss the differences, advantages, and disadvantages of different choices of interaction frames. They always include the complete radio-frequency Hamiltonian but can also contain the chemical shifts of the spins and may or may not contain the effective fields over one cycle of the pulse sequence.