NMR

Question 1

Why NMR works

Answer 1

• some atoms have nuclear spin, so generate a small magnetic field (e.g H-1 and C-13)
• placing them in a magnetic field aligns the atoms either against or with the main field
• the energy gap between the two states of alignment corresponds to the freq. of radiowaves roughly, so certain frequencies will be absorbed when passed through the sample

Question 2

Solvents

Answer 2

• the sample analysed is usually dissolved in a solvent and placed inside a testing machine
• the solvent cannot have any H-1 atoms as this would appear in the results so solvents use H-2 (deuterium)

Question 3

Calibration

Answer 3

• TMS is used to calibrate testing machines by adding it to samples as it has only 1 peak and its further right than most signals
• TMS is non-toxic, inert and can easily be removed from a sample

Question 4

Chemical shift

Answer 4

• the scale is derived from how far the signal is from TMS in ppm
• depends on what atoms/functional groups are near the Hydrogen’s in the molecule. The closer a Hydrogen is to an electronegative ion the greater it’s shift will be
• approximate values for different proton environments are known but they aren’t always accurate

Question 5

Splitting

Answer 5

• Number of lines on a peak = number of inequivalent H atoms on adjacent carbon atoms +1
• singlet = 1 peak, no H atoms on adjacent carbons
• doublet = 2 peaks, 1 H atom nearby
• triplet = 3 peaks, 2 H atoms nearby
• quartet = 4 peaks, three nearby H
• the relative height of the peak on the graph indicates the number of Hydrogens associated with that signal

Question 6

Carbon-13 NMR

Answer 6

• no link between integral of peaks and number of carbon atoms in environment
• no splitting of peaks
• peaks show different carbon environments rather than proton