In proton (1H^1\text{H}1H) NMR spectroscopy, tetramethylsilane (TMS), Si(CH3)4\text{Si(CH}_3)_4Si(CH3)4, is used as the reference standard with its chemical shift defined as δ=0 ppm\delta = 0\text{ ppm}δ=0 ppm. Which statement correctly explains the electronic basis for the highly shielded nature of the protons in TMS and the position of its NMR signal?
Silicon is less electronegative than carbon, which decreases electron density (deshielding) around the protons, causing them to resonate at a higher frequency (downfield) than most organic protons.
Silicon is less electronegative than carbon, which increases electron density (shielding) around the protons, causing them to resonate at a lower frequency (upfield) than most organic protons.
Silicon is more electronegative than carbon, which increases electron density (shielding) around the protons, causing them to resonate at a higher frequency (downfield) than most organic protons.
Silicon is more electronegative than carbon, which decreases electron density (deshielding) around the protons, causing them to resonate at a lower frequency (upfield) than most organic protons.
28 exam-style questions on OCR A A Level Chemistry Spectroscopy. Each one has a worked solution and a mark scheme showing where the marks go.