CH32 - Structure determination

How does NMR work?

A magnetic field is applied to a sample, surrounded by a source of radio waves and a radio receiver, which generates an energy change in the nuclei of atoms in the sample that can be detected. Electromagnetic energy is emitted, which can be interpreted by a computer.

The importance of C13 in NMR:

• Carbon 12, does not have an electron spin, as there are an equal number of protons and neutrons the spins cancel out.

• While Carbon 13 has a spin, however the chance of carbon 13 in the environment is very rare, so instruments are very sensitive.

How can shielding of electrons, effect how it absorbs energy?

• Carbon with different functional groups, feel the magnetic field differently, this is because the nuclei are shielded by the electrons surrounding them.

• Nuclei with more electrons around them are better shielded, so smaller the magnetic field felt by the nucleus, lower the frequency at which it resonates.

About Chemical shift:

• Chemical shift is measured in parts from million

• TMS compound is used, as a standardised 0, so it can be used universally (as a means of calibration)

• The key idea, different environments, will give chemical shift values

• If carbon next to a lot of electronegative atoms, these will draw electrons away from the carbon being bonded, which deshields the carbon, feeling a greater magentic field

Why is proton NMR easier to use than carbon NMR?

• Nearly all hydrogen atoms are H¹ so they will have a electron spin, without there being any cancellations.

  • While for carbon NMR, C13 is very rarely found in environment

Key ideas behind proton NMR:

• The greater the electron density around the proton, the smaller the chemical shift

• The area under each peak represents the hydrogen atoms producing it.

About spin-spin splitting in proton NMR:

The hydrogens are effected by the neigbouring hydrogens, which are on the neighbouring carbon.

If there are no neighbouring hydrogens, this will form a singlet.

If there is one neighbouring hydrogen, this will form a Doublet.

• This is because there are two outcomes either the hydrogen will have a spin up (towards magnetic field) or a spin down (against the magnetic field)

If there is two neighbouring hydrogens, this will form a Triplet.

n = hydrogens on the adjacent carbon atom

n + 1 = The number of peaks it will split up into

The type of solvent used for proton NMR:

• The solution must not contain any protons, or else the signal from hydrogen atoms in the solution will swap out from the signals of sample

• D = deuterium is used an isotope of H which does not form a signal

• CCl4 => to dissolve non-polar sunstances

• CDCl3 => to dissolve polar substances