Excitation of Delocalized Long-Lived States in Aliphatic Protons at Low and High Magnetic Fields

Abstract. Long-lived states (LLS) can be excited in geminal protons of aliphatic chains by mono- or poly-chromatic spin-lock induced crossings (SLIC), i.e., by application of one or more selective radio-frequency (RF) fields to create delocalised population imbalances between states belonging to different symmetries under spin permutations. At low fields (in this work at 1.4 T, or 60 MHz for proton NMR), these experiments are challenging due to the proximity of the chemical shifts and the need to consider the full untruncated J-coupling Hamiltonian. Five molecules were studied in this work: ethanolamine, lysine, vitamin B1, metronidazole, and phenoxyethylamine (POEA). For POEA and metronidazole, the LLS are reported for the first time. Measurements were carried out at low and high magnetic fields (1.4 T and 11.7 T, or 60 and 500 MHz for protons) using 60 MHz Magritek and 500 MHz Bruker NEO spectrometers. The rates 𝑅_LLS = 1/T_LLS and R₁ = 1/T₁ were determined using monochromatic SLIC excitation at both fields. We describe strategies for optimising SLIC conditions in cases where the signals of neighbouring CH₂ groups are relatively close to each other.