3.3.15-16 NMR + Chromatography

Question 1

what is the formula for TMS and why is it a suitable solvent?

Answer 1

tetramethylsilane (Si(CH3)4)
- non-toxic
- does not react with the sample
- easily separated from the sample molecule
- volatile / low boiling point
- produces one strong, sharp absorption peak on the spectrum

Question 2

what is the ‘n+1’ rule in proton NMR?

Answer 2

• shows the number of protons on the adjacent carbon atom

Question 3

why Si(CH3)4, CCl4 and CDCl3 are used in 1H NMR spectroscopy; explain how their properties make them suitable for use in 1H NMR
spectroscopy.

Answer 3

• used as solvents
• both CCL4 and CDCL3 have no H atoms so won’t give any signals
• CCl4 = non-polar; good solvent for non-polar organic molecules
• CDCl3 = polar covalent - same reason as above
• TMS = non-toxic, easily separated from the sample molecule due to low boiling point, does not react with the sample, etc.

Question 4

what is the splitting pattern for a singlet in proton nmr

Answer 4

• no splitting
• only one single peak
• means there’s no proton on the adjacent carbon

Question 5

what’s the splitting pattern for a doublet in proton nmr

Answer 5

• 2 peaks - 1:1 ratio

Question 6

what’s the splitting pattern for a triplet in proton nmr

Answer 6

• 3 peaks - 1:2:1 ratio