NMR Spectroscopy

What is NMR spectroscopy?

An analytical technique that allows the structure of a molecule to be determined by analysing the energy of each bond environment

Why do different environments show different peaks on a spectra?

They absorb different amounts of energy

What is the standard molecule for NMR spec?

Tetramethyl silane (Si(CH₃)₄)

Why is TMS used as the standard in NMR?

It has 4 identical carbon and hydrogen environments

What is the chemical shift value of TMS?

0 ppm

Why is TMS used as the standard molecule for NMR?

It is inert, non-toxic and the signal is easily identifiable

What is C¹³ NMR?

A form of NMR spec that analyses different carbon environments in a molecule

Why does oxygen shift carbon environments to the left of the NMR spectrum?

It is very electronegative and changes how the bond environment absorbs energy 

How can the displayed structure be determined for molecules that have more than one equivalent carbon in each environment? 

Use the molecular formula of the compound 

What solvents are typically used for polar molecules?

CHCl₃

What solvents are typically used for non-polar molecules?

CCl₄

Why are CCl₄ and CHCl₃ used as solvents for NMR spec?

Their signal is easily recognisable so can be removed from the spectrum

How can the number of NMR peaks be predicted from the structure of a molecule?

Count the number of carbon environments in the molecule

What is H¹ NMR?

A form of NMR where hydrogen environments are analysed and displayed as peaks on the spectra

What is an essential property of the solvent in proton NMR? 

It must not contain any hydrogen atoms

What solvents are common for proton NMR?

CCl₄ or deuterated solvents

What does the height of the peaks show in proton NMR?

The relative intensity of the signals (how many H atoms)

How can the ratio of hydrogen atoms be calculated from the values given above the peaks?

Divide by the smallest value and then multiply up to give whole numbers

What do splitting patterns show in proton NMR?

Where each of the hydrogen environments are positioned

What rule is used to determine the number of hydrogens from the number of splits in a peak? 

The number of splits in the peak is the number of (non-equivalent) hydrogens adjacent +1 

What is represented by a singlet?

No H on the adjacent atom

What is represented by a doublet?

1 H on the adjacent atom

What is represented by a triplet?

2 Hs on the adjacent atom

What is represented by a quartet?

3 Hs on the adjacent atom

What is the signal called when there are more than 3 Hs on the adjacent atom? 

A multiplet 

What 2 types of atoms should NOT be coupled? 

Equivalent hydrogens

Hydrogens on Oxygen atoms 

What group is shown by a triplet and quartet of relative intensity 3:2?

CH₃CH₂

What group is shown by 2 triplets of relative intensity 2:2?

CH₂CH₂

What group is shown by a doublet and quartet of relative intensity 3:1?

CH₃CH

What group is shown by a singlet of relative intensity 3?

CH₃

What group is shown by a singlet of relative intensity 1?

-OH (alcohols)