Chemistry - 3.3.15: Nuclear Magnetic Resonance Spectroscopy

NMR spectroscopy

Nuclear Magnetic Resonance spectroscopy

How does NMR work? (4)

- Magnetic field applied to sample surrounded by source of radio waves and radio receiver
- Source has atoms with odd number of nucleons (e.g. C-13, H-2) as they have spin so have a magnetic field
- In magnetic field, most nuclei in lower energy state so are parallel to external magnetic field
- If electromagnetic energy equal to difference in energy between difference between parallel in anti-parallel states is supplied, some nuclei will flip - resonance

Are nuclei with more electrons around them better or worse shielded?

Better

Are nuclei with fewer electrons around them better or worse shielded?

Worse

Resonance frequency

The frequency of radio waves required to cause flipping for a particular magnetic field

What happens as the electron density around a C-13 / H-1 atom increases?

The nucleus feels a smaller magnetic field + the nucleus resonates at a lower frequency so has a smaller δ value

Chemical shift symbol and units

δ, ppm (parts per million)

What is chemical shift measured from?

From a defined zero related to TMS

TMS

Tetramethylsilane

Does the height of each bar matter in C-13 spectra?

No

How do you identify the same carbon/hydrogen environment?

1. Symmetry
2. Rotation
3. Rotation around a single carbon (e.g. 2 methyl groups on one carbon atom)

How do you deduce the structure of a compound from an H-1 NMR spectrum? (3)

1. State the integration values as the whole number ratio that is the same as the number of hydrogen atoms in the compound
2. Name the splitting pattern of each peak and draw the structure based on the number of hydrogens at each peak
3. Draw the final structure of the compound by joining each peak's structure together

What is CCl4 used in H-1 NMR and why? (4)

- It is used as a solvent
- It has no H atoms so won't create a peak
- It is non-polar so is a good solvent for non-polar molecules
- It is inert so is unlikely to react with sample

What is CDCl3 used in H-1 NMR and why? (4)

- It is used as a solvent
- It has no H atoms so won't create a peak
- It is polar so is a good solvent for polar molecules
- It is inert so is unlikely to react with sample

What is TMS used for in NMR and why? (5)

- It is used as a reference i.e. as a peak at 0 ppm
- It is inert so is unlikely to react with sample
- Lots of equivalent H to give one signal as a single environment
- Signal is in an area away from other H signals
- Volatile so easy to remove

What is the importance of the area under the peaks in H-1 NMR?

They are proportional to the number of hydrogen atoms of each type

What gives more complicated NMR spectra - C-13 or H-1?

H-1

Give the chemical formula of TMS and draw its structure.

Si(CH3)4

Spin-spin splitting

The patterns NMR peaks are split into due to the hydrogen atoms being affected by the magnetic field of the hydrogen atoms on the neighbouring carbon atoms

n + 1 rule

n hydrogens on an adjacent carbon atom will split a peak into n + 1 smaller peaks

5 types of splitting patterns

Singlet, doublet, triplet, quartet, multiplet

What is CCl4?

Tetrachloromethane

What is deuterium and why is it used in some solvents?

It is an isotope of hydrogen so has the same chemical properties. It is used as it does not produce an NMR signal.

Why might it be difficult to predict the chemical shift for a peak given by a H atom in a compound using the Data Booklet?

The H atom is attached to one bond and adjacent to another (not combined in Data Booklet), so doesn't fit with data in the Data Booklet

How do you explain why a hydrogen atom has a particular splitting pattern?

State the number of adjacent hydrogens/protons OR hydrogens on adjacent carbon

How do you explain the positioning of a peak?

State what it is attached to OR that it has shifted significantly downfield by electronegative atom