Question

What compound is used as a standard for location of NMR signals?

Select one:

a. acetone, CH₃COCH₃

b. silicon grease

c. water

d. deuterated water, D₂O

e. deuterated chloroform, CDCl₃

f. trimethylsilane, (CH₃)₄Si

g. chloroform, CHCl₃

Answer 1

Ans: trimethylsilane, (CH₃)₄Si

Tetramethylsilane is the accepted internal standard for calibrating chemical shift for ¹H, ¹³C and ²⁹Si NMR spectroscopy. Because of its high volatility, TMS can easily be evaporated, which is convenient for recovery of samples analyzed by NMR spectroscopy. Because all twelve hydrogen atoms in a tetramethylsilane molecule are equivalent, its ¹H NMR spectrum consists of a singlet. The chemical shift of this singlet is assigned as δ 0, and all other chemical shifts are determined relative to it. The majority of compounds studied by ¹H NMR spectroscopy absorb downfield of the TMS signal, thus there is usually no interference between the standard and the sample. 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 δ 0 in the ¹³C spectrum, and all other chemical shifts are determined relative to it.