6.3.2 Spectroscopy

What are the two types of NMR spectroscopy?

• Carbon-13

• ¹H (proton)

Give the 3 requirements for NMR spectroscopy

• Strong magnetic field provided by electromagnet

• Sample dissolved in suitable solvent and a bit of TMS

• Low frequency radiowaves

What is nuclear spin? What is the relevance of this to NMR?

Just like electrons, the nucleus will have a nuclear spin which is especially significant with an odd number of nucleons (neutrons and electrons)

• Therefore ¹H and ¹³C are used as they contain odd number of nucleons

What is resonance? What is the relevance of this to NMR?

Like electrons, nuclei have 2 different flip states with different energies

• When the nucleus absorbs the right combo of strong magnetic field and radiowaves, the nucleus can absorb enough energy to flip between flip states (resonance)

• NMR measures the frequency of radiowaves absorbed for resonance to occur

What does the frequency required for the NMR spectrometer depend on?

The electromagnetic field strength

• The small quantity of energy can only be detected in strong and uniform magnetic fields

What is chemical shift?

The scale used to compare frequency of NMR absorption with the frequency of the reference peak of TMS at delta = 0ppm

• I.e. place in NMR spectrum at which a nucleus absorbs energy for resonance to occur

What is NMR calibrated by and what are the units?

By the delta scale

• Units are ppm

What is TMS and what is its use in NMR?

Tetramethylsilane, (CH₃)₄Si - the standard reference chemical against which all chemical shifts are measured

What is the chemical shift of TMS and why?

0 ppm as it contains 12 equivalent protons (H⁺), giving it a single shapr NMR signal at 0

What are the properties of TMS?

• Volatile so can evaporate from sample after running NMR

• Chemically unreactive

Give the process of running the NMR:

1. Sample is dissolved in solvent and a bit of TMS, then placed in narrow NMR sample tube

2. Placed in NMR spectrometer and spun to even out imperfections in magnetic field in the sample

3. Spectrometer zeroed against TMS standard and sample is given pulses of radiation

4. Absorption of energy from resonance are detected and displayed

5. Solvent and TMS is evaporated from sample

What is the issue with using normal solvents for NMR?

They contain carbon and hydrogen atoms which would produce a signal

Which 2 solvents are used for NMR?

Deuterated compounds:

1. Deuterium (²H) - stable isotope of hydrogen with an even number of nucleons

   • 1 proton and 1 neutron

2. Deuterated trichloromethane (CDCl₃)

   • Will produce a peak in the carbon-13 NMR but the peak is identified and filtered out by the computer