1 Nuclear Magnetic Resonance Nuclear Magnetic Resonance (NMR) Applying Atomic Structure Knowledge to Chemical Analysis

2 Nuclear Magnetic Resonance Nuclear Magnetic Resonance Spectroscopy (NMR) Spectroscopy is the study of the interaction of electromagnetic radiation with matter. Nuclear Magnetic Resonance Spectroscopy uses the NMR phenomena to study the biological, chemical, and physical properties of matter.

3 Nuclear Magnetic Resonance Nuclear Magnetic Resonance Spectroscopy (NMR) 1-D NMR is used to determine the structure of simple molecules. 2-D NMR is used to determine the structure of more complicated molecules. Solid state NMR is used to determine the molecular structure of solids. Time domain NMR is used to study molecular dynamics in solutions.

4 Nuclear Magnetic Resonance Nuclear Magnetic Resonance Spectroscopy Atoms are put into a static magnetic field and then exposed to a second, oscillating magnetic field. Once in these fields, the nuclei of atoms that do have “spin” interact with the oscillating magnetic field (note: not all nuclei have spin though). The interaction is detected and recorded. Spin in the nucleus can have the following values: # Neutrons AND # Protons are even NO SPIN # Neutrons + # ProtonsODD1/2, 3/2, 5/2, 7/2, etc. (half integer spin) # Neutrons + # ProtonsEVEN1, 2, 3, 4, 5, etc. (integer spin)

5 Nuclear Magnetic Resonance Nuclear Magnetic Resonance Spectroscopy A spinning nucleus has more energy when its magnetic field opposes the applied magnetic field (B) than when its magnetic field is aligned with the applied field. The gap between these 2 energies is a radio frequency. This energy can be adsorbed and re-emitted (relaxed). The amount of relaxation energy is characteristic of a particular element in a specific chemical environment. Energy level of a spinning nucleus not in a magnetic field B (Applied Magnetic Field) Energy gap

6 Nuclear Magnetic Resonance Nuclear Magnetic Resonance Spectroscopy The difference of the energy levels varies depending on the electrical environment of nucleus (i.e. how much the nuclear charge is shielded by surrounding electrons). Because the energy gap varies for the same nucleus in different chemical environments, we can use this property for chemical analysis.

7 Nuclear Magnetic Resonance Nuclear Magnetic Resonance Spectroscopy This property is called chemical shift. The more electropositive the nucleus is, the bigger the shift. A chemical shift is measured in parts per million (ppm) and is the ratio of the shift energy to the energy of the applied magnetic field.

8 Nuclear Magnetic Resonance Nuclear Magnetic Resonance Spectroscopy For a Hydrogen-NMR (H-NMR), we investigate the hydrogen atoms in a sample. Look at the following sample spectra of ethanol. Ethanol has the formula C 2 H 6 O and has the arrangement of atoms shown below. CH 3 – CH 2 – OH H H H-C - C - OH H H or Three hydrogen atoms are bonded to the first carbon atom. The second carbon has two hydrogen atoms bonded to it. The last hydrogen is bonded to the single oxygen atom.

9 Nuclear Magnetic Resonance Chemical shift as a ratio to applied magnetic field Nuclear Magnetic Resonance Spectra: Ethanol Ethanol: CH 3 –CH 2 –OH H H H-C-C-OH H H or This grouping of 3 peaks reflects the presence of 2 hydrogens on the neighboring carbon This single peak reflects the hydrogen on the oxygen (hydroxyl hydrogen) This grouping of 4 peaks reflects the presence of 3 hydrogens on a neighboring carbon H H-C - H - C - H –OH

10 Nuclear Magnetic Resonance The hydrogen bonded to the oxygen has the largest chemical shift (~ 5 on the x-axis). Oxygen is very electronegative and “hoards” electrons, leaving the hydrogen bonded to it more positive. The more exposed the hydrogen nucleus is, the larger the effective positive charge. The strongest positive charge interacts the strongest with the applied magnetic field and has the largest chemical shift. CH 3 – CH 2 - OH Methyl methylene oxygen Hydrogens hydrogens hydrogen Chemical shift Nuclear Magnetic Resonance Spectra: Ethanol H H-C - H - C - H –OH

11 Nuclear Magnetic Resonance Life Sciences: clinical Magnetic Resonance Imaging (MRI) Agriculture and Food Sciences: identifying food additives Biotechnology: using NMR probes during formula processing Spectroscopy: analysis of unknowns Material Science: identify nanotechnology composites of silicon and different polymers NMR Applications

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