NMR (Nuclear Magnetic Resonance)

NMR (Nuclear Magnetic Resonance) Some (but not all) nuclei, such as 1H, 13C, 19F, 31P have nuclear spin. A spinning charge creates a magnetic moment, so these nuclei can be thought of as tiny magnets. When placed in a magnetic field, these nuclei can line up with or against the field. N S spin state Nuclei aligned with magnetic field

Spin states  state is lower in energy than the  state. We probe the energy difference by irradiating with 300 MHz (radio waves). The absorbance of this energy is measured. N S spin state Nuclei aligned with magnetic field N S b spin state Nuclei aligned against magnetic field

Which nuclei won’t absorb? Nuclei that don’t experience spin (12C). Do not behave like magnets. Therefore it isn’t possible to perform NMR spectroscopy on 12C or 2D.

Using the absorbances Magnets of different strengths may be used. Absorbances reported using the “chemical shift” scale. Expressed in d values (ppm) relative to tetramethylsilane (d = 0 ppm).

1H NMR Most commonly probed nuclei in NMR. Used to distinguish between 1H environments. Different 1H environments appear at different chemical shifts (d).

NMR Data Chemical shifts – indicate the type of 1H nuclei present. Solve the puzzle and propose a structure. Chemical shifts – indicate the type of 1H nuclei present. Integrals – indicate the ratio of each type of 1H nuclei present. 1H-1H coupling – indicate the position of the 1H nuclei present.

1H NMR Spectrum of CH3COOCH2CH3 Integral 1H-1H coupling Chemical shift δ/ppm

Chemical Shift Data Scale showing where common kinds of 1H typically appear. Most appear between 0 (the reference, tetramethylsilane, TMS) and 10 ppm.

Integrals Lines, the heights of which give the ratio of each type of 1H. E.g. ethyl ethanoate CH3COOCH2CH3

Assigning 1H-NMR spectra The ratio of 1H signals is 3 : 2 : 3 3H 3H 2H

1H - 1H coupling Most signals are not single lines. 1H nuclei feel the presence of adjacent nuclei with different chemical shifts.

Effect of more than one neighbour We get more lines.

1H- 1H Coupling If a 1H has n neighbouring 1H that are equivalent, that 1H will be split into n+1 lines. Four equivalent neighbours -> five lines. Six equivalent neighbours -> seven lines. The intensity of the lines is given by Pascal’s triangle.

Assigning the spectrum of CH3COOCH2CH3

Assigning the spectrum of CH3CH2Br

Assign the spectrum below