Reconsideration of the Internal Tetramethylsilane Reference for Proton Magnetic Resonance Studies

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Abstract
The hypotheses implicit in the choice of tetramethylsilane (TMS) as an internal reference for liquid-phase proton magnetic resonance spectra are examined. Experiments are designed to test the postulated chemical inertness, and the equality of van der Waals shifts for TMS and the solute. It is found that TMS, as well as GeMe4, or PbMe4, gives rise to interactions with aromatic solvents such as pyridine, nitrobenzene, benzaldehyde, benzonitrile, or aniline. The apparent heat of formation for these complexes are in the 1-2-kcal/mole range, while the observed chemical shifts are of the order of 0.10-0.15 ppm. Caution is therefore urged in the use of TMS, and suitable molecules, not presenting these effects, are suggested for internal referencing.