Molecular spectroscopy III: Nuclear Magnetic Resonance (NMR)

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1 Molecular spectroscopy III: Nuclear Magnetic Resonance (NMR) Nuclear magnetic resonance (NMR) is a physical phenomenon in which magnetic nuclei in a magnetic field absorb electromagnetic radiation at a specific resonance frequency which depends on the strength of the magnetic field and the magnetic properties of the isotope of the atoms. NMR is also routinely used in magnetic resonance imaging (MRI) Nobel prizes: 1. Nuclear magnetic resonance was first described and measured in molecular beams by Isidor Rabi in 1938, and in 1944, he was awarded the Nobel Prize in Physics for this work. 2. In 1946, Felix Bloch and Edward Mills Purcell expanded the technique for use on liquids and solids, for which they shared the Nobel Prize in Physics in Richard R. Ernst received Nobel Prize in Chemistry in 1991 for his contributions towards the development of Fourier Transform nuclear magnetic resonance spectroscopy. 4. In 2002 Kurt Wüthrich shared the Nobel Prize in Chemistry for his work with protein Fourier Transform nuclear magnetic resonance in solution.

2 neutron Hydrogen (contains 1 proton) Deuterium (contains 1 proton and 1neutron) Note that two very common nuclei, 12 C and 16 O, have zero spin, hence zero magnetic moment: they are invisible in NMR The range of typical chemical shifts for 1 H resonances The 1 H NMR spectrum of ethanol. The bold letters denote the protons giving rise to the resonance peak, and the step like curve is the integrated signal.

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11 Origin of the shielding constant Electron density around the nucleus studied Neighboring chemical groups in the molecule studied ( local) ( neighbour) ( solvent) ( local) d p (local) > 0, if d > p (local) < 0, if d < p Diamagnetic contribution Paramagnetic contribution Neighbouring group contributions The increasing value of δ (that is, the decrease in shielding) is consistent with the electron withdrawing power of the O atom: it reduces the electron density of the OH proton most, and that proton is strongly deshielded. It reduces the electron density of the distant methyl protons least, and those nuclei are least deshielded. The determination of the area under an absorption line is referred to as the integration of the signal (just as any area under a curve may be determined by mathematical integration). Spectrometers can integrate the absorption automatically. In ethanol the group intensities are in the ratio 3:2:1 because there are three CH 3 protons, two CH 2 protons, and one OH proton in each molecule.

12 The shielding and deshielding effects of the ring current induced in the benzene ring by the applied field. Protons attached to the ring are deshielded but a proton attached to a substituent that projects above the ring is shielded. The solvent contribution An aromatic solvent (benzene here) can give rise to local currents that shield or deshield a proton in a solvent molecule. In this relative orientation of the solvent and solute, the proton on the solute molecule is shielded. The fine structure The splitting of resonances into individual lines is called the fine structure of the spectrum. It arises because of the coupling between spins of different nuclei. This spin spin coupling arises because each magnetic nucleus may contribute to the local field experienced by the other nuclei and so modify their resonance frequencies. The origin of spin spin coupling The electron and nucleus interact: Fermi contact interaction: The 1s electron 1s can penetrate the field resulting from the rotating charge of nucleous and the spherical average of the field it experiences is not zero. Hence the spin of the electron 1s prefer to be antiparallel to the magnetic field of the proton.

13 Two magnetic nuclei interact though the electrons: According to Pauli s principle, two electrons must have antiparallel spins in a bond, hence the two nuclear spins for X and Y are linked to each other via the bonding electrons: that is the polarization mechanism for spin spin coupling. The two arrangements have slightly different energies. Lower energy corresponds to the case when when the nuclear spins are antiparallel. Example: H H, 13C 1H, F H, P H... Two protons interact via an intermediate nucleus (non magnetic): The spin information is transmitted from one bond to the next by a version of the mechanism that accounts for the lower energy of electrons with parallel spins in different atomic orbitals: Hund's rule of maximum multiplicity. In this case, lower energy corresponds to the case when the nuclear spins are parallel. Example: 1H 12C 1H IMPACT ON MEDICINE: Magnetic resonance imaging If an object containing hydrogen nuclei (a tube of water or a human body) is placed in an NMR spectrometer and exposed to a homogeneous magnetic field B 0, then a single resonance signal will be detected with a frequency Consider a magnetic field that varies linearly in the z direction, B = B 0 + Gz. Then the resonant frequencies are All the protons within a given slice (that is, at a given field value) come into resonance and give a signal of the corresponding intensity. B = B 0 + Gz Changing the orientation of the field shows the shape along the corresponding direction, and computer manipulation can be used to build up the three dimensional shape of the sample.

14 Magnetic resonance imaging X rays are known to be dangerous on account of the ionization they cause; The MRI technique is considered safe. The special advantage of MRI is that it can image soft tissues, whereas X rays are largely used for imaging hard, bony structures and abnormally dense regions, such as tumours.