Difference between revisions of "Micro view of NMR"
| Line 6: | Line 6: | ||
The formula for the number of states = 2I+1, thus a spin 1/2 nucleus such as a single hydrogen atom will have 2 states and 1 transition (when placed in a static magnetic field). For I greater than 1 there are more than two states and thus many transitions. The focus in NMR is on spin 1/2 nuclei since two states with one transition gives good, clean spectra that are easily interpretable. A lot of information about the environment of a nucleus can thus be obtained, making 1H NMR the most useful analytical technique in science. | The formula for the number of states = 2I+1, thus a spin 1/2 nucleus such as a single hydrogen atom will have 2 states and 1 transition (when placed in a static magnetic field). For I greater than 1 there are more than two states and thus many transitions. The focus in NMR is on spin 1/2 nuclei since two states with one transition gives good, clean spectra that are easily interpretable. A lot of information about the environment of a nucleus can thus be obtained, making 1H NMR the most useful analytical technique in science. | ||
| + | |||
| + | From here on, the focus will be on spin 1/2 nuclei, specifically 1H. This nucleus is referred to as "proton" in the NMR literature. Other [[Spins of nuclei|spin 1/2 nuclei]] will behave the same. Nuclei with spins other than 1/2 will have more than one transition and give much more complicated spectra. | ||
| + | |||
To obtain a spectrum, a spin 1/2 atom must first be placed into a static magnetic field (B<sub>0</sub>) to cause separation of the two states. The amount of separation depends on the [[gyromagnetic ratio]] for that atom (a constant) and the intensity of the static magnetic field according to the equation: | To obtain a spectrum, a spin 1/2 atom must first be placed into a static magnetic field (B<sub>0</sub>) to cause separation of the two states. The amount of separation depends on the [[gyromagnetic ratio]] for that atom (a constant) and the intensity of the static magnetic field according to the equation: | ||
| − | v=γ*B<sub>0</sub> | + | v<sub>0</sub>=γ*B<sub>0</sub> |
| − | The atom is then pulsed with a composite radio wave that includes the [[Relationship between frequency and energy|frequency]] v, called the Larmor Frequency. The atom absorbs just the v part of the radio wave (since it is a quantum transition), and a detector in the same axis as the radio wave (usually the same antenna) is then turned on to receive any emitted energy. | + | The atom is then pulsed with a composite radio wave that includes the [[Relationship between frequency and energy|frequency]] v<sub>0</sub>, called the Larmor Frequency. The atom absorbs just the v<sub>0</sub> part of the radio wave (since it is a quantum transition), and a detector in the same axis as the radio wave (usually the same antenna) is then turned on to receive any emitted energy. |
Making this happen in an instrument is thus mainly an [[NMR from an Instrument point of view|engineering problem]]. | Making this happen in an instrument is thus mainly an [[NMR from an Instrument point of view|engineering problem]]. | ||
==Complications== | ==Complications== | ||
| − | From the equation above, since γ is a constant and B<sub>0</sub> is a constant, there is just one frequency for that | + | From the equation above, since γ is a constant and B<sub>0</sub> is a constant, there is just one frequency for that proton, which makes sense since there is just one transition, thus there is just one peak expected in the spectrum. In reality though, an atom never exists in isolation, thus it never experiences the pure B<sub>0</sub>. The actual frequency v is thus slightly different for atoms of a particular γ based on the actual magnetic field around the atom. These slight differences in frequency are what gives a spectrum of peaks for a molecule rather than a single peak. There is still just one transition for that atom type (if that type is spin 1/2, such as 1H), but the transition energy is different for each 1H that is in a different magnetic environment. |
| + | |||
| + | If the 1H is part of a molecule then 1H atoms in the molecule in similar magnetic environments can be grouped and can be expected to give one peak per group. | ||
| + | |||
| + | ==Peak Intensity== | ||
| + | So far there has been no discussion of how big the peaks are, or how powerful the radio pulse needs to be. To look at these issues we have to increase the view beyond a single 1H atom and into a collection of 1H atoms as part of molecules, such as a solution of one compound in a solvent. | ||
Revision as of 14:33, 13 March 2020
NMR is an instrumental technique that uses photons of radio frequency energy to cause a transition, or change in state, in an atom. Radio is used because transitions at the atomic level are quantized, and the amount of energy needed to cause these transitions happens to fall in the radio region of the electromagnetic spectrum. Quantized energy means there has to be the right amount of energy to cause a change of state, too much or too little and no change occurs.
There are many transitions in atoms. The ones of interest in NMR are quantum spin transitions of protons and neutrons. Quantum spin in protons and neutrons has two states, which are normally equivalent, but when placed in a magnetic field they become non-equivalent, so adding the appropriate sized photon of energy can cause a transition from one to the other. This works fine for single protons and neutrons, but when these are combined into a nucleus the situation gets more complex and we have to rely on a net nuclear spin, called I.
Across the entire periodic table, net nuclear spin values ranging from I = 0 to I = 8 in ½-unit increments can be found. Protons and neutrons each have net spins of ½, but this derives from the elementary quarks and gluons of which they are composed. As a result of this complexity, no simple formula exists to predict I based on the number of protons and neutrons within an atom.
The formula for the number of states = 2I+1, thus a spin 1/2 nucleus such as a single hydrogen atom will have 2 states and 1 transition (when placed in a static magnetic field). For I greater than 1 there are more than two states and thus many transitions. The focus in NMR is on spin 1/2 nuclei since two states with one transition gives good, clean spectra that are easily interpretable. A lot of information about the environment of a nucleus can thus be obtained, making 1H NMR the most useful analytical technique in science.
From here on, the focus will be on spin 1/2 nuclei, specifically 1H. This nucleus is referred to as "proton" in the NMR literature. Other spin 1/2 nuclei will behave the same. Nuclei with spins other than 1/2 will have more than one transition and give much more complicated spectra.
To obtain a spectrum, a spin 1/2 atom must first be placed into a static magnetic field (B0) to cause separation of the two states. The amount of separation depends on the gyromagnetic ratio for that atom (a constant) and the intensity of the static magnetic field according to the equation:
v0=γ*B0
The atom is then pulsed with a composite radio wave that includes the frequency v0, called the Larmor Frequency. The atom absorbs just the v0 part of the radio wave (since it is a quantum transition), and a detector in the same axis as the radio wave (usually the same antenna) is then turned on to receive any emitted energy.
Making this happen in an instrument is thus mainly an engineering problem.
Complications
From the equation above, since γ is a constant and B0 is a constant, there is just one frequency for that proton, which makes sense since there is just one transition, thus there is just one peak expected in the spectrum. In reality though, an atom never exists in isolation, thus it never experiences the pure B0. The actual frequency v is thus slightly different for atoms of a particular γ based on the actual magnetic field around the atom. These slight differences in frequency are what gives a spectrum of peaks for a molecule rather than a single peak. There is still just one transition for that atom type (if that type is spin 1/2, such as 1H), but the transition energy is different for each 1H that is in a different magnetic environment.
If the 1H is part of a molecule then 1H atoms in the molecule in similar magnetic environments can be grouped and can be expected to give one peak per group.
Peak Intensity
So far there has been no discussion of how big the peaks are, or how powerful the radio pulse needs to be. To look at these issues we have to increase the view beyond a single 1H atom and into a collection of 1H atoms as part of molecules, such as a solution of one compound in a solvent.
