3-Methylglutaric acid as a 13C solid-state NMR standard

Abstract

The calibration of a solid-state NMR spectrometer requires setting the magic angle, setting the reference and decoupler frequencies, ensuring that the magnetic field is homogeneous across the sample volume, optimizing the signal-to-noise ratio, determining the π/2 pulse durations, and optimizing the Hartman–Hahn matching condition. Each task has one or more widely accepted standards, such as potassium bromide for setting the magic angle, adamantane for optimizing magnet homogeneity, and hexamethylbenzene or glycine for measuring the signal-to-noise ratio. We show that all of these tasks can be performed using 3-methylglutaric acid (MGA). In the case of high-powered decoupling, the CH 2 and CH carbon peaks of MGA provide an opportunity to evaluate the decoupling in a manner that is superior to any of the commonly used standard compounds. Thus, MGA can be used as a single solid-state NMR standard compound to perform all calibration steps except for magnet shimming.