Nuclear Magnetic Resonance Spectroscopy

Nuclear Magnetic Resonance (NMR) Spectroscopy

Nuclear magnetic resonance (NMR) is a property of some molecules that allows scientists to determine the structures of hydrocarbon compounds.

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 (α\alpha-state). The energy state that is antiparallel to the field and requires higher energy is called the beta state (β\beta-state). Protons may absorb energy to change their spin from the α\alpha-state to the β\beta-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 α\alpha-state from the β\beta-state.

Alpha and Beta States

A nuclear magnetic resonance (NMR) spectrophotometer utilizes the nuclear spin of a proton to measure the frequency of the energy required to reach nuclear magnetic resonance.
Most NMR spectrometers range from moderately sized to large sized because they need to be large enough to create the magnetic field necessary for resonance. However, there have been advances in making the instruments smaller. Although the machine itself is quite large, it only uses 2–10 milligrams (mg) of sample in 0.6–1 milliliter (mL) of solvent. The dissolved sample is housed in a 5-millimeter (mm) NMR tube and needs to be clear in order to provide a good spectrum. A deuterated solvent is often used to dissolve the sample.

NMR Spectrometer

A nuclear magnetic resonance (NMR) spectrometer produces a large magnetic field and sends a pulse of radio waves through the samples to provide enough energy to achieve nuclear magnetic resonance.

NMR Sample Tubes

The sample of interest is dissolved in a deuterated solvent or a compound that does not contain hydrogen atoms. The sample needs to be clear-not necessarily colorless-so that the radio waves can pass through the sample.
A compound is deuterated when a hydrogen atom is replaced with a deuterium atom that has one proton and one neutron as compared to hydrogen that does not have any neutrons. A deuterated solvent is a liquid that is used to dissolve other substances (a solvent) that has at least one of its hydrogen atoms replaced with a deuterium atom that has one proton and one neutron as compared to hydrogen that does not have any neutrons. Deuterium (2H) is an isotope of hydrogen that contains one proton and one neutron. Using a deuterated solvent reduces the background noise of the solvent so that the peaks in the spectrum represent the protons of interest.

Deuterated Solvents

Original Solvent Deuterated Solvent
Name Structure Name Structure
Acetic acid
Tetradeuteroacetic acid
Benzene
Hexadeuterobenzene
Water
Deuterated water

The structure for a deuterated solvent is identical in every way to its original solvent except that each hydrogen-1 is replaced with a hydrogen-2. They behave in the same way and dissolve the same compounds.

Basics of 1H NMR Spectra

An NMR spectrum contains four pieces of information that scientists use to determine structure: the number of signals, the chemical shift for each signal, integration, and splitting.

A 1H nuclear magnetic resonance spectrum (NMR spectrum) is a plot that has the radio frequency on the xx-axis and the signal or amount of energy on the yy-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.

Nonequivalent Protons

Name Structure Nonequivalent Protons and Descriptions Number of Signals Predicted
Ethanol
  • Three protons bonded to a carbon that is bonded to another carbon
  • Two protons bonded to a carbon that is bonded to another carbon and an oxygen atom
  • One proton bonded to an oxygen atom that is bonded to a carbon atom
3
3-chloropropanoic acid
  • Two protons bonded to a carbon that is bonded to another carbon atom and a chlorine atom
  • Two protons bonded to a carbon atom that is bonded to two carbon atoms, one of which is bonded to a chlorine and the other is bonded to two oxygen atoms
  • One proton bonded to an oxygen atom that is bonded to a carbon atom that is double-bonded to an oxygen atom
3

Protons are equivalent if they are found in the same chemical environment. The structures are color coded such that equivalent protons are the same color and new colors are used for nonequivalent protons.

Protons can look the same because they are bonded to the same carbon, but they might not be the same. The NMR spectra can help show the symmetry of a molecule and help identify which hydrogen atoms are the same and which are different. Homotopic is a characteristic of hydrogen atoms that are in the same chemical environment and produce the same signal in NMR spectroscopy. Enantiotopic is a characteristic of hydrogen atoms that are not in the same chemical environment and produce different signals in a chiral environment, such as in chiral solvents in NMR spectroscopy. Diastereotopic is a characteristic of hydrogen atoms that are not in the same chemical environment and produce different signals in NMR spectroscopy. To determine if protons do indeed have the same environment, imagine replacing each one in turn with another element or group in the compound. For example, a proton could be replaced by a deuterium atom or a chlorine atom or a methyl group. If the resulting compounds are the same, the protons are homotopic; if they are enantiomers or diastereomers, the protons are enantiotopic or diastereotopic, respectively. A single compound can contain homotopic protons on one carbon and enantiotopic or diastereotopic protons on another carbon.

Symmetry and Protons

Name Replacement Type
Ethanol (carbon 2)
Homotopic
Ethanol (carbon 1)
Enantiotopic
(R)-2-chlorobutane (carbon 2)
Diastereotopic

On carbon 2 in ethanol, three indistinguishable compounds are produced by switching one proton with a deuterium atom. On carbon 1 in ethanol, two chiral compounds are produced that are only distinguishable in a chiral environment. The proton/deuterium switch on carbon 2 in (R)-2-chlorobutane results in two diastereomers that are not mirror images.

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).

Chemical Shift

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

Equivalent protons that are bonded to an electronegative element or near an electronegative element will be deshielded and have a downfield chemical shift. The reverse is also true.

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 N+1 {\rm {N}}+1 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 (N=1{\rm {N}}=1), the signal is split into two equal peaks, called a doublet. Two neighboring, nonequivalent protons (N=2{\rm {N}}=2) produces a triplet, and three neighboring, nonequivalent protons (N=3{\rm {N}}=3) 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.

Complex Multiplets

Multiplets appear in nuclear magnetic resonance (NMR) spectra when one nonequivalent proton is coupled to two different nonequivalent protons. This can yield very complicated NMR spectra.

Drawing 1H NMR Spectra

To draw the spectra of a compound, first the number of unique signals is determined by examining the symmetry of the molecule and determining if there are any diastereotopic CH2 in the molecule.

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 (CH3COOCH2CH3{\rm{CH_3{-}COO{-}CH_2{-}CH_3}}), the acetate CH3 carbon contains three equivalent protons that are not adjacent to any protons. Splitting occurs in an N+1 {\rm {N}}+1 pattern, and this would result in 0+1=10+1=1, a singlet. The carbonyl (C=O{\rm{C{=}O}}) 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 3+1=43+1=4, 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 2+1=32+1=3, 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.

Acetate Coupling

The signals of a compound are split because of the magnetic moments of the protons on adjacent carbons. Signal C indicates that there are three adjacent protons to protons found in the signal. Signals A, B, and C would show integration ratio of 3:3:2, indicating three hydrogens each in signals A and B, and two hydrogens in signal C.

Interpreting 1H NMR Spectra

The identity of a compound can be determined by examining each signal and determining the integration, the split, and the shift. Each signal is then stitched together with other signals until the compound is determined.
If a structure is known, such as ethyl acetate, NMR spectroscopy can be used as a confirmatory test. NMR spectroscopy can also be used to determine the structure of an unknown compound by interpreting the shift, split, integration, and number of signals.

Chemical Shift of Common CH{\rm{C{-}H}} Bonds

Type of Bond Example Structure Approximate Chemical Shift (in ppm)
RCH3{\rm{R{-}{CH}}_3} CH3CH3{\rm{CH_3{-}C}}{\color{#c42126}\rm {H}}_3 0.9
RCH2R\rm{R{-}{\rm{CH}}_2{-}\rm{R}} CH3CH2CH3{\rm{CH}_3{-}C}{\color{#c42126}\rm{H}}_2{-}{\rm{CH}}_3 1.3
ROH CH3OH{\rm{CH}_3{-}O}{\color{#c42126}\rm{H}} 0.5 to 5
ROCH3 CH3OCH3{\rm{CH}_3{-}O{-}C}{\color{#c42126}\rm{H}}_3 3.8

A partial list of common hydrogen chemical shifts used in determining structure from a spectrum

An "Unknown" NMR Spectra

The nuclear magnetic resonance (NMR) spectrum of 1-bromopropane shows three different types of protons, two different triplets, and a sextet. The integration shows a relative ratio of 1.5:1:1, indicating that there are more protons in the first triplet than in the other proton groups.
The NMR spectra of 1-bromopropane has three different types of protons, or three unique signals. The first peak at 0 ppm is the reference peak and is not a signal. One signal close to 1 ppm is a triplet, the next signal close to 2 ppm is a sextet downfield from the triplet, and the final signal at 3.4 ppm is another triplet found farther downfield. From the splitting of the signals, the triplets (N+1=3;N=2{\rm {N}}+1=3;\;{\rm {N}}=2) are both adjacent to two protons, and the sextet (N+1=6;N=5{\rm {N}}+1=6;\;{\rm {N}}=5) is adjacent to five protons.

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.

NMR Spectra for 2-bromopropane

The nuclear magnetic resonance (NMR) spectrum of 2-bromopropane shows two different types of protons: one doublet and one septet. The integration shows a relative ratio of 5.93:1, indicating that there are six times as many protons in the chemical environment than in the other proton group.