6.9.5 Proton NMR

Features of a ¹H NMR spectrum

• An NMR spectrum shows the intensity of each peak against its chemical shift
• The area under each peak gives information about the number of protons in a particular environment
• The area under each peak shows the intensity / absorption from protons
• A single sharp peak is seen to the far right of the spectrum
  • This is the reference peak from TMS
  • By definition the chemical shift is at 0 ppm

A low resolution ¹H NMR for ethanol showing the key features of a spectrum

Molecular environments

• ¹H nuclei that have different neighbouring atoms (said to have different chemical environments) absorb at slightly different field strengths
• The difference environments are said to cause a chemical shift away from TMS
  • Ethanol has the structural formula CH₃CH₂OH
  • There are 3 chemical environments: -CH₃, -CH₂ and -OH

• The hydrogen atoms in these environments will appear at 3 different chemical shifts
• Different types of protons are given their own range of chemical shifts

Answer:

Two different ¹H chemical environments occur in 2-methylpropane

• • The three methyl groups are in the same ¹H environment
  • The lone hydrogen is in its own ¹H environment

Chemical shift values for ¹H molecular environments table

Environment of proton	Chemical shift range, δ / ppm
R–CH	0.5 - 2.0
N–CH	2.0 - 3.0
O–CH Cl–CH Br–CH	3.0 - 4.2
	4.5 - 6.0
	6.2 - 8.0
	9.0 - 10.0
	10.0 - 12.0
R–OH R–NH	0.5 - 12.0

• Protons in the same chemical environment are chemically equivalent
  • 1,2-dichloroethane, Cl-CH₂-CH₂-Cl has one chemical environment as these four hydrogens are all exactly equivalent

• Each individual peak on a ¹H NMR spectrum relates to protons in the same environment
  • Therefore, 1,2-dichloroethane would produce one single peak on the NMR spectrum as the protons are in the same environment

Low resolution ¹H NMR

• Peaks on a low resolution NMR spectrum refer to molecular environments of an organic compound
  • Ethanol has the molecular formula CH₃CH₂OH
  • This molecule as 3 separate proton environments: -CH₃, -CH₂, -OH
  • So 3 peaks would be seen on its spectrum at 1.2 ppm (-CH₃), 3.7 ppm (-CH₂) and 5.4 ppm (-OH)
  • The strengths of the absorptions are proportional to the number of equivalent ¹H atoms and are proportional to the area underneath each absorption peak
  • Hence, the areas of absorptions of -CH₃, -CH₂, -OH are in the ratio of 3:2:1 respectively

A low resolution NMR spectrum of ethanol showing 3 peaks for the 3 molecular environments

High resolution ¹H NMR

• More structural details can be deduced using high resolution NMR
• The peaks observed on a high resolution NMR may sometimes have smaller peaks clustered together
• The splitting pattern of each peak is determined by the number of protons in neighbouring environments

Spin-Spin Splitting

• A high resolution ¹H NMR spectrum can show you the structure of the molecule but also the peaks can be split into sub-peaks or splitting patterns
• These are caused by a proton's spin interacting with the spin states of nearby protons that are in different environments
  • This can provide information about the number of protons bonded to adjacent carbon atoms
  • The splitting of a main peak into sub-peaks is called spin-spin splitting or spin-spin coupling

  

High resolution ¹H NMR spectrum of ethanol showing the splitting patterns of each of the 3 peaks. Using the n+1, it is possible to interpret the splitting pattern

The n+1 rule

• The number of sub-peaks is one greater than the number of adjacent protons causing the splitting
  • For a proton with n protons attached to an adjacent carbon atom, the number of sub-peaks in a splitting pattern = n+1

• When analysing spin-spin splitting, it shows you the number of hydrogen atoms on the adjacent carbon atom
• These are the splitting patterns that you need to be able to recognise from a ¹H spectra:

¹H NMR peak splitting patterns table

• Splitting patterns must occur in pairs, because each protons splits the signal of the other
• There are some common splitting pairs you will see in a spectrum however you don't need to learn these but can be worked out using the n+1 rule
  • You will quickly come to recognise the triplet / quartet combination for a CH₃CH₂  because it is so common

Common pair of splitting patterns

• A quartet and a triplet in the same spectrum usually indicate an ethyl group, CH₃CH₂-
• The signal from the CH₃ protons is split as a triplet by having two neighbours
• The signal from the CH₂ protons is split as a quartet by having three neighbours
• Here are some more common pairs of splitting patterns

Common pairs of splitting patterns

¹H NMR spectrum of propane

• The CH₂ signal in propane (blue) is observed as a heptet because it has six neighbouring equivalent H atoms (n+1 rule), three either side in two equivalent CH3 groups
• The CH₃ groups (red) produce identical triplets by coupling with the CH₂ group

Answers:

i) 3 peaks

ii) (CH₃)₂CHOH at 0.7 - 1.2 ppm, (CH₃)₂CHOH at 3.1 - 3.9 ppm, (CH₃)₂CHOH at 0.5 - 5.5 ppm

iii) Ratio 6 : 1 : 1

iv) (CH₃)₂CHOH split into a doublet (1+1=2), (CH₃)₂CHOH split into a heptet (6+1=7)

Tetramethylsilane

• The horizontal scale on an NMR spectrum represents chemical shift (δ)
• Chemical shift is measured in parts per million (ppm) of the radio frequency needed for resonance compared to a reference chemical called tetramethylsilane, abbreviated to TMS

The displayed formula of tetramethylsilane

• TMS is used universally as the reference compound for NMR as its methyl groups are particularly well shielded and so it produces a strong, single peak at the far right of an NMR spectrum
• The signal from the hydrogen atoms in TMS is defined as having a chemical shift of 0 ppm

The NMR reference peak for TMS

Hydroxyl and amino protons

• Organic compounds often contain protons from part of a functional group rather than being bonded to a carbon, examples include:
  • Alcohols, ROH
  • Carboxylic acids, RCOOH
  • Phenols, ArOH
  • Amines, RNH₂ 
  • Amides, RCONH₂ 
  • Amino acids, H₂NCHRCOOH
• These hydroxyl, OH, protons and amino, NH, protons can be involved in hydrogen bonding which causes their peaks to be broader than normal and they can appear at almost any chemical shift
  • The broadening of the peak means that they are not usually involved in spin-spin coupling and therefore appear as singlets

Deuterated solvents

• Deuterated solvents such as D₂O (heavy water) and CDCl₃ can be used to identify OH and NH protons
• This is achieved by:
  • Performing a standard proton NMR on the sample compound
  • A small amount of the deuterated solvent is added to the sample compound and mixed
  • A second proton NMR is then performed
• Deuterium atoms from the solvent replace the OH and NH protons in the sample, e.g for ethanol

C₂H₅OH + CDCl₃ ⇌ C₂H₅OD + CHCl₃ 

• This means that the second proton NMR will still have the peaks for the CH₃ and CH₂ protons of ethanol but will not have the OH peak as that proton has been exchanged