Experimental Procedures in NMR Spectroscopy

Abstract

In order to record the proton NMR spectrum of a substance, it must first be dissolved in a suitable solvent in which it is sufficiently soluble (preferably 10- 50 mg per ml). The majority of solvents used are deuterated analogues of the common organic solvents (e.g., CDC1₃, CD₃COCD₃, C₆D₆, CD₃CN, CD₃OD, C₅D₅N, DMSO-d ₆, etc.) to avoid the overlap of large peaks of the protons present in the solvents with the spectrum of the material. Since deuterated solvents are expensive, it is essential that solubility be first checked in the corresponding undeuterated solvents before attempting dissolution in the deuterated materials. Spectroscopic grade CC1₄ is relatively inexpensive and may be used if the solubility is sufficient. Compounds bearing carboxyl groups have a high solubility in pentadeuteropyridine (C₅D₅N) while basic substances can be dissolved in CF₃COOD. A list of some useful NMR solvents is given in Table 3.1. A small amount of a reference compound, usually tetramethylsilane, (CH₃)₄Si, is added as an internal standard. For a routine continuous-wave type NMR spectrometer, about 10–20 mg of the compound is required for a good proton NMR spectrum, but in the FT instruments 1–10-mg quantities of sample are usually used. It is possible to record proton NMR spectra using as little as 50 µg of sample but this requires accumulation of several thousand scans, the use of special microcells and high-purity solvents and materials, as even trace impurities in solvents can be significantly magnified and can interfere with the spectrum of the substance being recorded.