Nuclear magnetic resonance (NMR) occurs when atomic nuclei that have spin are placed into a magnetic field; individual spins that are normally random in orientation line up parallel to the magnetic field. NMR is a property that can be used to determine the structure of molecules. Recall that the nucleus has a positive charge because it contains protons that are positively charged and neutrons that contain no charge. Just like electrons, nucleons also spin and are paired in the opposite direction. Therefore, if there is an odd number of either protons or neutrons resulting in an odd mass number, the nucleus has net spin, or nuclear spin. Nuclear spin is the magnetic moment resulting from the total angular momentum of certain nuclei, such as those with an odd number of either protons or neutrons. A nucleon is a proton or neutron in an atomic nucleus. The term is often used in nuclear chemistry when referring to the total number of protons and neutrons. Not all atoms have a nuclear spin. For example, carbon-12 contains six protons and six neutrons that have paired spins, yielding no net spin. Carbon-13, on the other hand, contains six protons and seven neutrons, yielding a net spin. Hydrogen-1 also has nuclear spin because it has one proton and no neutrons, also yielding a net spin.As suggested by the name, nuclear magnetic resonance requires nuclear spin. Spinning atoms with charges produce a magnetic moment. A nuclear magnetic resonance spectrometer (NMR spectrometer) is an instrument that uses the magnetic moment to gain information about the structure of a compound. Individual spins that are normally random in orientation line up parallel to the applied magnetic field (with the field) or antiparallel (against the field). The energy state that is parallel to the field and requires lower energy is called the alpha state (-state). The energy state that is antiparallel to the field and requires higher energy is called the beta state (-state). Protons may absorb energy to change their spin from the -state to the -state. The NMR spectrometer sends pulses of radio waves through the magnetic field to provide the protons with the energy necessary to change spins. When this occurs, the proton is in resonance with the nuclear field, thus it is in nuclear magnetic resonance. Then the instrument records the energy that is released by the protons when they relax back to the -state from the -state.
Alpha and Beta States
NMR Sample Tubes
|Original Solvent||Deuterated Solvent|
|Acetic acid||Tetradeuteroacetic acid|
Basics of 1H NMR Spectra
A 1H nuclear magnetic resonance spectrum (NMR spectrum) is a plot that has the radio frequency on the -axis and the signal or amount of energy on the -axis. The NMR spectrum identifies four pieces of information about an organic compound:
1. number of unique signals
2. shift of each signal
3. integration of each signal
4. splitting of each signal
First, the spectrum identifies the number of unique signals present. Each signal corresponds to a hydrogen atom that has that same frequency. Hydrogen atoms are often referred to as protons. This data provides information about the structure of the compound by indicating which protons are equivalent and which protons are not equivalent. Equivalent protons have identical chemical environments and absorb energy at the same frequency. Nonequivalent protons have different chemical environments and absorb energy at different frequencies.
|Name||Structure||Nonequivalent Protons and Descriptions||Number of Signals Predicted|
Symmetry and Protons
|Ethanol (carbon 2)||Homotopic|
|Ethanol (carbon 1)||Enantiotopic|
|(R)-2-chlorobutane (carbon 2)||Diastereotopic|
Second, the spectra identify the shift of each unique signal present. Nuclei that have strong electronegativities, such as F and O, have high-electron densities and the protons near them have low-electron densities. Deshielded is a characteristic of protons near highly electronegative nuclei in which the protons experience more of the magnetic field. Shielded is a characteristic of protons that do not experience the full effect of the magnetic field because of the electrons surrounding them. Downfield is the left part of the NMR spectra where a low field strength is applied. Deshielded protons appear in the downfield part of the spectra. Upfield is the right part of the NMR spectra where a high field strength is applied. Shielded protons appear in the upfield part of the spectra.
The effects of electronegative atoms are felt through the molecule but decrease the farther away the atoms get from the source. This results in a chemical shift, which is the difference in the resonant frequency of two protons because of their environment. A reference compound called tetramethylsilane is usually added to the NMR tube sample and is used as a reference point during NMR spectroscopy and has a frequency of zero parts per million (ppm).
|Name||Structure||Shielding/Deshielding||Predicted Chemical Shift|
|Ethanol||The three protons bonded to a carbon that is bonded to a carbon are shielded.||Upfield|
|The two protons bonded to the carbon that is bonded to the oxygen atom are deshielded.||Downfield|
|The one proton that is bonded to the oxygen atom bonded to a single carbon atom is deshielded.||Downfield|
Third, the spectra identify the integration of each unique signal present. Integration is the intensity of the signal on a proton nuclear magnetic resonance (1H NMR) spectrum, providing a ratio of the number of protons found in the signal. This ratio does not provide the exact number of protons but rather provides a ratio of this type of proton in this chemical environment to another type of proton in a different chemical environment.
Fourth, the spectra identify the number of neighboring protons for each signal. This information is derived from the splitting of each signal. A signal is split (or splitting) if it has multiple peaks of different areas and heights. Splitting is caused by spin-spin coupling, which is the result of the magnetic environment of one proton (or group of homotopic protons) being affected by the magnetic moment of neighboring, nonhomotopic protons.
Splitting occurs in an pattern, where N is the number of neighboring, nonhomotopic protons found on the same or adjacent carbon atom. For example, if there is only one neighboring, nonequivalent proton (), the signal is split into two equal peaks, called a doublet. Two neighboring, nonequivalent protons () produces a triplet, and three neighboring, nonequivalent protons () produces a quartet.Multiplets may appear in NMR spectra when one nonequivalent proton is coupled to two different nonequivalent protons. This can yield very complicated NMR spectra. For example, a doublet of doublet is four lines of equal intensities that is produced by the coupling of two protons. A triplet of doublets is a pattern of doublets in which the two center peaks have twice the intensity of the other two doublets.
Drawing 1H NMR Spectra
Consider the structure of ethyl acetate. It contains three different types of protons, which would result in three different signals. These signals will have splitting because of the nearby (neighboring) protons found in the structure.
To predict splitting, count the number of protons found on the adjacent carbon. In the case of ethyl acetate (), the acetate CH3 carbon contains three equivalent protons that are not adjacent to any protons. Splitting occurs in an pattern, and this would result in , a singlet. The carbonyl () carbon does not contain any protons; therefore, it would not produce a signal. The third CH2 carbon contains two protons that are equivalent and are adjacent to three protons. This would result in , a quartet. The ethyl CH3 carbon contains three protons that are equivalent and are adjacent to two protons. Splitting occurs, and this would result in , a triplet. The predicted outcome of this compound would be a singlet, a triplet, and a quartet.
The chemical shift for each of the three peaks can be predicted based on their environment. The quartet will be downfield because it is bonded to a carbon that is bonded to an oxygen atom. Oxygen atoms are electronegative, meaning that the protons on this carbon are deshielded relative to all of the other protons in the structure. The triplet will be found upfield because it is the farthest away from the electronegative atom and has a high electron density; the protons in the triplet are shielded. The singlet will fall between the triplet and the quartet because it is close to an electronegative atom but is not directly adjacent to it.The triplet represents three protons, and so does the singlet. The total area of these two peaks should be the same to show that there are three hydrogens in each. The quartet will have an area that is two-thirds that of the other two proton groups because there is one fewer proton bonded to this carbon.
Interpreting 1H NMR Spectra
Chemical Shift of Common Bonds
|Type of Bond||Example Structure||Approximate Chemical Shift (in ppm)|
|ROH||0.5 to 5|
An "Unknown" NMR Spectra
One triplet has a chemical shift of 1 ppm, which indicates that it is part of an alkyl (alkane) group, because upfield signals between 0.8 and 2 are usually typical of alkanes. The triplet appearing at 3.4 ppm indicates it is next to a halogen or oxygen. Putting this together, the structure should be CH3CH2CH2X, where X is a halogen or an oxygen. Equivalent protons in CH3 have two neighbors and are farthest away from the halogen. Next, CH2 has five neighbors and is nearer the halogen. Finally, CH2X appears last, with two neighbors. The X is a halogen, specifically Br, but to know that would require more information, such as mass spec data or infrared spectroscopy (IR) data.An NMR spectrum can differentiate between two different compounds that have the same chemical formula but different structures. For 1-bromopropane, the splitting and shift of signals in the NMR spectra can help determine to which carbon the halogen is attached. An NMR spectrum for 2-bromopropane would have two signals because there are only two types of protons because of the symmetry of the molecule. The signals are a doublet (a proton with one proton neighbor) and a septet (a proton with six proton neighbors). This makes for easy comparison between two very similar compounds, 1-bromopropane and 2-bromopropane.