nmr spectroscopy l.o.: state that nmr spectroscopy involves interaction of materials with...

NMR spectroscopy

L.O.:State that NMR spectroscopy involves interaction of materials with low-energy radio frequency radiation.

Describe the use of tetramethylsilane, TMS, as the standard for chemical shift measurement.

State the need for deuterated solvents, e.g. CDCl3, when running an NMR spectrum.

Some nuclei have a magnetic spin: 1H, 13C, 19F

In a strong magnetic field they can line up either with the field or opposed the field.

Different nuclear spins in an external magnetic field

Excitation: a nucleus in its low –energy spin state can be promoted to an upper energy level.

This energy is provided by a radio-frequency radiation.

Relaxation: The excited nucleus will drop back to the lower energy level.

Resonance: the energy provided by the radiation is the same as the energy gap between the tow spin states.

The magnetic field felt by each nucleus depends on:

o the applied magnetic fieldo Chemical environment. Electrons

surrounding the nucleus generate a magnetic field.

Nuclear shielding – electrons modify the magnetic field experienced by the nucleus

Chemical shift, , is a scale that compares the frequency of an NMR absorption with the frequency of the reference peak of TMS at =0.

Tetramethylsilane (TMS), (CH3)4Si

Carbon-13 NMR spectroscopy

L.O.:• Analyse carbon-13 NMR spectra to make

predictions about the different types of carbon atoms present.

• Predict the chemical shifts of carbons in a given molecule.

Carbon-13 chemical shifts

Electron withdrawing groups (O, CO, F, N) are less shielding.

The less shielding, the higher the chemical shift.

How many carbon environments?

Carbon-13 NMR spectra of propan-1-ol and propan-2-ol

Carbon-13 NMR spectra of propan-1-ol and propan-2-ol showing peak assignments

Carbon-13 NMR spectra of (a) CH3CHBrCH3 and (b) CH3COOCH3

13C NMR TASK 1For each of these compounds indicate the number of signals in the 13C NMR spectrum predict the approximate chemical shift of each signal using the data sheet a)methylpropeneb)propenec)2-chloropropaned)Propanonee)methylaminef)ethyl propanoateg) 1,2-dibromopropaneh)dimethylethyl propanoate i)but-2-ene

L.O.:• Analyse carbon-13 NMR spectra to make

predictions about possible structures for an unknown compound.

Carbon-13 NMR spectra of isomer of C3H6O

Structural isomers of the carbonyl isomers of C3H6O showing different carbon environments

Carbon-13 NMR spectra of an aromatic compound with molecular formula C8 H8

Structural isomers of the aromatic isomers of C8H8

13-C NMR of butane