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. CDCl 3, when running an NMR spectrum.

Some nuclei have a magnetic spin: 1 H, 13 C, 19 F 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 field o 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), (CH 3 ) 4 Si

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) CH 3 CHBrCH 3 and (b) CH 3 COOCH 3

13 C NMR TASK 1 For each of these compounds indicate the number of signals in the 13 C NMR spectrum predict the approximate chemical shift of each signal using the data sheet a)methylpropene b)propene c)2-chloropropane d)Propanone e)methylamine f)ethyl propanoate g) 1,2-dibromopropane h)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 C 3 H 6 O

Structural isomers of the carbonyl isomers of C 3 H 6 O showing different carbon environments

Carbon-13 NMR spectra of an aromatic compound with molecular formula C 8 H 8

Structural isomers of the aromatic isomers of C 8 H 8

13-C NMR of butane