NMR Spectroscopy

Question 1

When is the M+ peak an even number in mass spectroscopy?

Answer 1

• alkanes
• alkenes
• alcohols
• aldehydes
• ketones
• carboxylic acids

Question 2

What is NMR spectroscopy?

Answer 2

A type of spectroscopy that tells us about the structure of a compound

Question 3

What are chemical environments?

Answer 3

Looking at the carbon atoms and the hydrogen atoms (protons) and deciding on their chemical environment

Looking at the atoms and/or functionality on carbons adjacent to a particular environment

Question 4

What are equivalent environments?

Answer 4

When two atoms are in an identical environment within the molecule

Question 5

What are non-equivalent environments?

Answer 5

When two atoms are not in an identical environment within the molecule

Question 6

What is chemical shift?

Answer 6

Depending on the environment of a particular carbon or hydrogen, the associated peak will appear at a certain position in the spectrum

Question 7

What is integration (1H NMR only)?

Answer 7

The area under the peak is given as a trace line on the spectrum and gives the relative number of protons in that given environment

Question 8

What is spin-spin splitting pattern (1H NMR only)?

Answer 8

When two proton environments interact (couple) with each other (up to 3 bonds), they split each others peak from a single line into multiple lines depending on the number of protons from each environment

Question 9

What are the key points of 13C NMR?

Answer 9

• number of peaks on the spectrum is equal to the number of different carbon environments
• identifying the type of carbon environments present in the molecule can be done by using the chemical shift
• all 13C NMR spectra used will be decoupled and all peaks will appear as singlets
• the closer a carbon is to an electronegative atom (e.g. oxygen atom in OH group) the further to the left its peak will appear on the spectrum

Question 10

What are the key recognisable points of 1H NMR?

Answer 10

• recognise the different proton environments from the number of non-equivalent protons and hence the number of peaks
• relative ratios of each type of proton (from integration traces - ratio numbers from relative peak area)
• types of proton environment present from the chemical shifts
• number of non-equivalent protons adjacent to the proton from the spin-spin splitting pattern

Question 11

How to recognise the non-equivalent protons?

Answer 11

• determined by the number of non-equivalent proton environments in the molecule

Question 12

How is the relative ratio of numbers of protons in each environment determined?

Answer 12

• ratios of area under the peaks will help determine
• relative ratios can sometimes be the same as actual ratio
• may have to measure integrator traces and create a ratio
• ratio numbers may be given in relative terms of 0.x:0.y

Question 13

How is the number of protons attached to the carbon atom worked out?

Answer 13

The spin-spin splitting pattern

• number of sub-peaks in a cluster is one more than the number of protons attached to the adjacent carbon
• n-1 rule

Question 14

What are the terms for the n-1 rule?

Answer 14

Singlet = no H on adjacent groups
Doublet = adjacent to a CH group
Triplet = adjacent to a CH2 group
Quartet = adjacent to a CH3 group
Multiplet = adjacent to carbons with more than 3 protons

Phenyl group peak will always be one or more multiples, OH and NH group will always appear as singlets