# NMR Recap¶

## NMR Active Nuclei¶

Fo an atom to be NMR active, it needs to have a quantum number $$I\neq0$$.

Nuclei with an odd number of protons (atomic number), neutrons (atomic mass) or both with have a nuclear spin.

# Protons # Neutrons $$I$$ Examples
Even Even 0 $$\ce{^{12}C, ^{16}O, ^{32}S}$$
Odd Even $$1/2$$ $$\ce{^{1}H, ^{19}F, ^{31}P}$$
Odd Even $$3/2$$ $$\ce{^{11}B, ^{35}Cl, ^{79}Br}$$
Even Odd $$1/2$$ $$\ce{^{13}C}$$
Even Odd $$3/2$$ $$\ce{^{127}I}$$
Even Odd $$5/2$$ $$\ce{^{17}O}$$
Odd Odd 1 $$\ce{^{2}H, ^{14}N}$$

NMR needs these isotopes to be relatively high in concentration, or it needs to sample for an incredibly long period of time.

## Chemical Shift¶

Occurs as the different atoms will have a different electron density surrounding them. Each additional electron will have its own associate magnetic field and so as electrons withdrawn or donated, the atom will have a slightly larger or smaller response to the magnetic field. This can happen for many reasons, including intramolecular h-bonding and hybridisation state.

The chemical shift is measured as a difference from the $$0\:ppm$$ standard reference (TMS for $$\hnmr$$ and $$\cnmr$$ )

$\delta=\frac{\text{Shift in }\nu\text{ from TMS (Hz)}}{\nu\text{ of the spectrometer (Hz)}}$

The values themselves are scaled up by $$\e{6}$$, as the shifts are realistically incredibly small.

$1\:ppm=\frac{1\:Hz}{10^6\:Hz}$

## $$\pi$$ Induced Magnetic Fields¶

The $$\pi$$ bond’s natural ability to conjugate and have free moving electrons, allows the to create an equivalent magnetic field that will reinforce the magnetic field of the NMR spectrometer. This causes a downfield shift for any aromatic or pi bonded carbon atoms.

## Splitting¶

Covered in Spectroscopy and Instrumentation, though it’s worth noting that most $$\cnmr$$ spectra are decoupled from protons, so you likely won’t see splitting in a $$\cnmr$$ spectra.

## Integration¶

Covered in Spectroscopy and Instrumentation, though again, $$\cnmr$$ spectra are not so easily integrated. This is because the $$\ce{^13C}$$ nuclei has a much more varied relaxation period