CH3 Hsc-si-CHs Tetramethylsilane CH3 TMS is added to the sample. Since silicon is less electronegative than carbon,TMS protons are highly shielded.Signal defined as zero. Organic protons absorb downfield (to the left)of the TMS signal. => Chapter 13 13 Chapter 13 13 Tetramethylsilane • TMS is added to the sample. • Since silicon is less electronegative than carbon, TMS protons are highly shielded. Signal defined as zero. • Organic protons absorb downfield (to the left) of the TMS signal. => Si CH3 CH3 CH3 H3C