Tetramethylsilane

Tetramethylsilane is the chemical compound with the formula Si(CH₃)₄ or SiMe₄ (Me = CH₃). Commonly abbreviated TMS, it is the simplest tetraorganosilane. Like all silanes, the SiC₄ framework is tetrahedral and consequently it is a non-polar. TMS is a building block in organometallic chemistry but also finds use in diverse applications.

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Synthesis and reactions

TMS is a by-product of the production of methyl chlorosilanes, SiCl_x(CH₃)_4-x, via the "direct reaction" of methyl chloride with silicon. The more useful products of this reaction are those for x = 1, 2,and 3.

TMS undergoes deprotonation upon treatment with butyl lithium to give Si(CH₃)₃CH₂Li. The latter, tetramethylsilyl lithium, is a relatively common alkylating agent.

In chemical vapor deposition, TMS is the precursor to silicon dioxide or silicon carbide, depending on the deposition conditions.

Uses in NMR spectroscopy

Tetramethylsilane is an internal standard for calibrating chemical shift in ¹H, ¹³C and ²⁹Si NMR spectroscopy. Because all twelve hydrogen atoms in a tetramethylsilane molecule are equivalent, the ¹H NMR spectrum of TMS consists of a singlet. The chemical shift of this singlet is assigned as δ0.0 in the spectrum, and all other chemical shifts are determined relative to it. The silicon in tetramethylsilane shifts its ¹H NMR signal upfield. The majority of compounds studied by ¹H NMR have hydrogen peaks downfield of the tetramethylsilane peak, so there is usually no interference between the standard and the sample, and the tetramethylsilane singlet can usually easily be identified for the chemical shift calibration.

Similarly, all four carbon atoms in a tetramethylsilane molecule are equivalent. In a fully decoupled ¹³C NMR spectrum, the carbon in the tetramethylsilane appears as a singlet, allowing for easy identification. The chemical shift of this singlet is also set to be δ0.0 in the ¹³C spectrum, and all other chemical shifts are determined relative to it.

Because of its high volatility, TMS can easily be evaporated, which is convenient for recovery of samples analyzed by NMR spectroscopy.