CHAPTER 17: SPECTROSCOPY

Question 1

How do you analyse a Carbon-13 NMR spectrum of an organic molecule?

Answer 1

• No. of Peaks = No. of carbon environments.
• Chemical Shift = Type of environment (use data sheet).
• C atoms bonded to different atoms/groups differ in environments & absorb at different chemical shifts.
• 2 C atoms positioned symmetrically are equivalent.
• This means the same: chemical environment, absorbtion at chemical shift & contribution to the peak.

Question 2

How do you analyse a proton NMR spectrum of an organic molecule?

Answer 2

• No. of Peaks = No. of Proton Environments.
• Chemical Shift = Type of environment (use data sheet).
• Relative Peak Areas = Relative no. of each type of H⁺.
• Splitting Pattern = No. of non-equivalent H⁺ on adjacent carbon atom + 1 (n + 1 rule), to give: singlets, doublets, triplets, quartets or multiplets (4+).

Question 3

What are uses of TMS, deuterated solvents & D₂O in NMR spectroscopy?

Answer 3

• Tetramethylsilane, TMS, is the standard for chemical shift measurements & is given a chemical shift of 0 ppm.
• All results of NMRs are calibrated by & relative to TMS.
• Deuterated solvents (e.g. CDCl₃) contain ²H (Deuterium) rather than ¹H.
• Deuterium gives no signal in frequency ranges used for ¹H and ¹³C NMR.
• Adding D₂O exchanges & replaces OH or NH protons with ²H atoms: eliminating the broad OH & NH peaks.