NMR Spectroscopy

What is chemical shift?

Chemical shift is a measure of the energy difference

Delta means chemical shift in ppm

What does NMR need?

A reference / standard substance

Energy absorbed by sample is compared to the energy absorbed by the standard

Tetramethylsilane Si(CH3)4

• all carbons and hydrogens in same environment

• Therefore only produce one characteristic peak

• Long way away from any other peaks from the sample

• Delta = 0ppm for this peak

• It is an inert chemical - therefore won’t react w/ sample

• Non-toxic

• Highly volatile - therefore can remove from sample very easily once you’ve run the NMR spectrum

What determines a chemical shift value?

• what atoms are nearby?

• Highly electronegative atoms deshield a nucleus, which gives a larger delta value

• Common examples: (1H NMR)

Delta/ppm 2.1-2.6ppm - hydrogen attached to a carbon adjacent to a carbonyl group

Shown as a range - because precise values will vary from one molecule to another

Delta/ppm 3.7-4.1ppm - hydrogen attached to a carbon that is connected to the oxygen of an ester group

(13C NMR)

Delta/ppm 20-50ppm - carbon adjacent to carbonyl group

Delta/ppm 110-160ppm - carbon atom in a benzene ring

Delta/ppm 160-185ppm - carbonyl group in an ester or a carboxylic acid will have a carbon which has this peak

What is the integration trace?

(1H NMR only)

Area of the peak is proportional to the number of hydrogen atoms causing the peak

Splitting pattern - high resolution NMR