Techniques {nuclear magnetic resonance} (NMR) {magnetic resonance imaging} (MRI) can measure electron density around hydrogen nuclei and other nuclei that have magnetic fields.
purposes
NMR can detect proton transfers and isomer interconversions. Sample size is 0.1 ml to 0.4 ml. Accuracy is 1/10^8. MRI {functional MRI} (fMRI) can scan body or brain for regions where increased glucose containing oxygen-15 indicates high cellular activity. Blood-oxygen-level dependent (BOLD) method uses blood flow and volume increase with increased metabolism.
theory: spin
Atomic nuclei with odd atomic number or odd atomic mass number have an unpaired proton or neutron and so net spin. Hydrogen and nitrogen have atomic numbers 1 and 7, respectively, and have net spin. Radioactive elements carbon-11, nitrogen-13, and oxygen-15 have net spin.
theory: magnetic field
Charges with net spin make magnetic fields. Proton magnetic moment is small compared to electron magnetic moment, because proton mass is much larger. Applying strong 14000-gauss magnetic field aligns atomic-nucleus protons parallel or anti-parallel with field, because altering proton spin angle requires large torque. Spin and spin angle to applied magnetic field have quanta.
theory: microwave radiation
Parallel and anti-parallel alignments have energy difference at microwave energy levels. Applying microwave radiation can flip protons from parallel to anti-parallel, or vice versa. Microwave wavelength needed to flip indicates nucleus and neighboring-nuclei atomic mass.
theory: neighbor shielding
In molecules, neighboring atoms affect proton magnetic fields. More electrons near atomic nucleus shield it from outside magnetic fields, requiring more microwave energy to change field alignment. Different functional groups attached to hydrogen atom provide more or less shielding. Shielding from lowest to highest is methyl -CH3 [3 is subscript], methylene -CH2 [2 is subscript], methyne -CH, amine -NH2 [2 is subscript], and hydroxyl -OH. Polar molecules have greatest shielding, and non-polar molecules have smallest shielding. Trimethylsilane (TMS) is NMR standard compound, because it has silicon atom surrounded by three methyl groups and so minimum electron shielding.
theory: energy
Total energy of microwave frequency that changes alignment indicates number of hydrogens, or other magnetic nuclei, with same electron environment. For example, TMS has nine protons with same electron environment and has relative total energy nine. Frequency variations around microwave frequency can change alignment. Frequency-variation number indicates number of non-equivalent protons that affect shielding electrons. For example, TMS has zero variations, because all protons are equivalent.
theory: width
Peak width is greater for slow transitions, when inverse of time {relaxation time} in excited state is more, because transverse relaxation gives energy to neighboring nuclei. Transverse relaxation is rapid in solids, so proton NMR does not use solids. Longitudinal relaxation makes heat.
Physical Sciences>Chemistry>Analytical Chemistry>Resonance
5-Chemistry-Analytical Chemistry-Resonance
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Date Modified: 2022.0224