Nuclear magnetic resonance decoupling
Encyclopedia
Nuclear magnetic resonance decoupling (NMR decoupling for short) is a special method used in nuclear magnetic resonance (NMR) spectroscopy
where a sample to be analyzed is irradiated at a certain frequency
or frequency range to eliminate fully or partially the effect of coupling
between certain nuclei
. NMR coupling refers to the effect of nuclei on each other in atoms within a couple of bonds distance of each other in molecules. This effect causes NMR signals in a spectrum to be split into multiple peaks which are up to several hertz
frequency from each other. Decoupling fully or partially eliminates splitting of the signal between the nuclei irradiated and other nuclei such as the nuclei being analyzed in a certain spectrum. NMR spectroscopy and sometimes decoupling can help determine structures
of chemical compound
s.
of signal intensity on the vertical axis vs. chemical shift
for a certain isotope on the horizontal axis. The signal intensity is dependent on the number of exactly equivalent nuclei in the sample at that chemical shift. NMR spectra are taken to analyze one isotope
of nuclei at a time. Only certain types of isotopes of certain elements
show up in NMR spectra. Only these isotopes cause NMR coupling. Nuclei of atoms having the same equivalent positions within a molecule also do not couple with each other. 1H (proton) NMR spectroscopy
and 13C NMR spectroscopy analyze 1H and 13C nuclei, respectively, and are the most common types (most common analyte isotopes which show signals) of NMR spectroscopy.
Homonuclear decoupling is when the nuclei being radio frequency
(r f) irradiated are the same isotope as the nuclei being observed (analyzed) in the spectrum.
Heteronuclear decoupling is when the nuclei being r f irradiated are of a different isotope than the nuclei being observed in the spectrum.
For a given isotope, the entire range for all nuclei of that isotope can be irradiated in broad band decoupling,
or only a select range for certain nuclei of that isotope can be irradiated.
Practically all naturally occurring hydrogen
( H ) atoms have 1H nuclei, which show up in 1H NMR spectra. These 1H nuclei are often coupled with nearby non-equivalent 1H atomic nuclei within the same molecule. H atoms are most commonly bonded to carbon
( C ) atoms in organic compound
s. About 99% of naturally occurring C atoms have 12C nuclei, which neither show up in NMR spectroscopy nor couple with other nuclei which do show signals. About 1% of naturally occurring C atoms have 13C nuclei, which do show signals in 13C NMR spectroscopy and do couple with other active nuclei such as 1H. Since the percentage of 13C is so low in natural isotopic abundance samples, the 13C coupling effects on other carbons and on 1H are usually negligible, and for all practical purposes splitting of 1H signals due to coupling with natural isotopic abundance carbon does not show up in 1H NMR spectra. In real life, however, the 13C coupling effect does show up on non-13C decoupled spectra of other magnetic nuclei, causing satellite signals
.
Similarly for all practical purposes, 13C signal splitting due to coupling with nearby natural isotopic abundance carbons is negligible in 13C NMR spectra. However, practically all hydrogen bonded to carbon atoms is 1H in natural isotopic abundance samples, including any 13C nuclei bonded to H atoms. In a 13C spectrum with no decoupling at all, each of the 13C signals is split according to how many H atoms that C atom is next to. In order to simplify the spectrum, 13C NMR spectroscopy is most often run fully proton decoupled, meaning 1H nuclei in the sample are broadly irradiated to fully decouple them from the 13C nuclei being analyzed. This full proton decoupling eliminates all coupling with H atoms and thus splitting due to H atoms in natural isotopic abundance compounds. Since coupling between other carbons in natural isotopic abundance samples is negligible, signals in fully proton decoupled 13C spectra in hydrocarbon
s and most signals from other organic compounds are single peaks. This way, the number of equivalent sets of carbon atoms in a chemical structure
can be counted by counting singlet peaks, which in 13C spectra tend to be very narrow (thin). Other information about the carbon atoms can usually be determined from the chemical shift
, such as whether the atom is part of a carbonyl group or an aromatic ring, etc. Such full proton decoupling can also help increase the intensity of 13C signals.
There can also be off-resonance decoupling of 1H from 13C nuclei in 13C NMR spectroscopy, where weaker r f irradiation results in what can be thought of as partial decoupling. In such an off-resonance decoupled spectrum, only 1H atoms bonded to a carbon atom will split its 13C signal. The coupling constant, indicating a small frequency difference between split signal peaks, would be smaller than in an undecoupled spectrum.
Looking at a compound's off-resonance proton-decoupled 13C spectrum can show how many hydrogens are bonded to the carbon atoms to further help elucidate the chemical structure. For most organic compounds, carbons bonded to 3 hydrogens (methyls) would appear as quartets (4-peak signals), carbons bonded to 2 equivalent hydrogens would appear as triplets (3-peak signals), carbons bonded to 1 hydrogen would be doublets (2-peak signals), and carbons not bonded directly to any hydrogens would be singlets (1-peak signals).
Another decoupling method is specific proton decoupling (also called band-selective or narrowband). Here the selected "narrow" 1H frequency band of the (soft) decoupling RF pulse covers only a certain part of all 1H signals present in the spectrum. This can serve two purposes: (1) decreasing the deposited energy through additionally adjusting the RF pulse shapes/using composite pulses, (2) elucidating connectivities of NMR nuclei (applicable with both heteronuclear and homonuclear decoupling). Point 2 can be accomplished via decoupling e.g. of a single 1H signal which then leads to the collapse of the J coupling pattern of only those observed heteronuclear or non-decoupled 1H signals which are J coupled to the irradiated 1H signal. Other parts of the spectrum remain unaffected. In other words this specific decoupling method is useful for signal assignments which is a crucial step for further analyses e.g. with the aim of solving a molecular structure.
Note that more complex phenomena might be observed when for example the decoupled 1H nuclei are exchanging with non-decoupled 1H nuclei in the sample with the exchange process taking place on the NMR time scale. This is exploited e.g. with chemical exchange saturation transfer (CEST) contrast agents in in vivo magnetic resonance spectroscopy
.
NMR spectroscopy
Nuclear magnetic resonance spectroscopy, most commonly known as NMR spectroscopy, is a research technique that exploits the magnetic properties of certain atomic nuclei to determine physical and chemical properties of atoms or the molecules in which they are contained...
where a sample to be analyzed is irradiated at a certain frequency
Frequency
Frequency is the number of occurrences of a repeating event per unit time. It is also referred to as temporal frequency.The period is the duration of one cycle in a repeating event, so the period is the reciprocal of the frequency...
or frequency range to eliminate fully or partially the effect of coupling
Angular momentum coupling
In quantum mechanics, the procedure of constructing eigenstates of total angular momentum out of eigenstates of separate angular momenta is called angular momentum coupling. For instance, the orbit and spin of a single particle can interact through spin-orbit interaction, in which case the...
between certain nuclei
Atomic nucleus
The nucleus is the very dense region consisting of protons and neutrons at the center of an atom. It was discovered in 1911, as a result of Ernest Rutherford's interpretation of the famous 1909 Rutherford experiment performed by Hans Geiger and Ernest Marsden, under the direction of Rutherford. The...
. NMR coupling refers to the effect of nuclei on each other in atoms within a couple of bonds distance of each other in molecules. This effect causes NMR signals in a spectrum to be split into multiple peaks which are up to several hertz
Hertz
The hertz is the SI unit of frequency defined as the number of cycles per second of a periodic phenomenon. One of its most common uses is the description of the sine wave, particularly those used in radio and audio applications....
frequency from each other. Decoupling fully or partially eliminates splitting of the signal between the nuclei irradiated and other nuclei such as the nuclei being analyzed in a certain spectrum. NMR spectroscopy and sometimes decoupling can help determine structures
Chemical structure
A chemical structure includes molecular geometry, electronic structure and crystal structure of molecules. Molecular geometry refers to the spatial arrangement of atoms in a molecule and the chemical bonds that hold the atoms together. Molecular geometry can range from the very simple, such as...
of chemical compound
Chemical compound
A chemical compound is a pure chemical substance consisting of two or more different chemical elements that can be separated into simpler substances by chemical reactions. Chemical compounds have a unique and defined chemical structure; they consist of a fixed ratio of atoms that are held together...
s.
Explanation
NMR spectroscopy of a sample produces an NMR spectrum, which is essentially a graphGraphics
Graphics are visual presentations on some surface, such as a wall, canvas, computer screen, paper, or stone to brand, inform, illustrate, or entertain. Examples are photographs, drawings, Line Art, graphs, diagrams, typography, numbers, symbols, geometric designs, maps, engineering drawings,or...
of signal intensity on the vertical axis vs. chemical shift
Chemical shift
In nuclear magnetic resonance spectroscopy, the chemical shift is the resonant frequency of a nucleus relative to a standard. Often the position and number of chemical shifts are diagnostic of the structure of a molecule...
for a certain isotope on the horizontal axis. The signal intensity is dependent on the number of exactly equivalent nuclei in the sample at that chemical shift. NMR spectra are taken to analyze one isotope
Isotope
Isotopes are variants of atoms of a particular chemical element, which have differing numbers of neutrons. Atoms of a particular element by definition must contain the same number of protons but may have a distinct number of neutrons which differs from atom to atom, without changing the designation...
of nuclei at a time. Only certain types of isotopes of certain elements
Chemical element
A chemical element is a pure chemical substance consisting of one type of atom distinguished by its atomic number, which is the number of protons in its nucleus. Familiar examples of elements include carbon, oxygen, aluminum, iron, copper, gold, mercury, and lead.As of November 2011, 118 elements...
show up in NMR spectra. Only these isotopes cause NMR coupling. Nuclei of atoms having the same equivalent positions within a molecule also do not couple with each other. 1H (proton) NMR spectroscopy
Proton NMR
Proton NMR is the application of nuclear magnetic resonance in NMR spectroscopy with respect to hydrogen-1 nuclei within the molecules of a substance, in order to determine the structure of its molecules. In samples where natural hydrogen is used, practically all of the hydrogen consists of the...
and 13C NMR spectroscopy analyze 1H and 13C nuclei, respectively, and are the most common types (most common analyte isotopes which show signals) of NMR spectroscopy.
Homonuclear decoupling is when the nuclei being radio frequency
Radio frequency
Radio frequency is a rate of oscillation in the range of about 3 kHz to 300 GHz, which corresponds to the frequency of radio waves, and the alternating currents which carry radio signals...
(r f) irradiated are the same isotope as the nuclei being observed (analyzed) in the spectrum.
Heteronuclear decoupling is when the nuclei being r f irradiated are of a different isotope than the nuclei being observed in the spectrum.
For a given isotope, the entire range for all nuclei of that isotope can be irradiated in broad band decoupling,
or only a select range for certain nuclei of that isotope can be irradiated.
Practically all naturally occurring hydrogen
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...
( H ) atoms have 1H nuclei, which show up in 1H NMR spectra. These 1H nuclei are often coupled with nearby non-equivalent 1H atomic nuclei within the same molecule. H atoms are most commonly bonded to carbon
Carbon
Carbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...
( C ) atoms in organic compound
Organic compound
An organic compound is any member of a large class of gaseous, liquid, or solid chemical compounds whose molecules contain carbon. For historical reasons discussed below, a few types of carbon-containing compounds such as carbides, carbonates, simple oxides of carbon, and cyanides, as well as the...
s. About 99% of naturally occurring C atoms have 12C nuclei, which neither show up in NMR spectroscopy nor couple with other nuclei which do show signals. About 1% of naturally occurring C atoms have 13C nuclei, which do show signals in 13C NMR spectroscopy and do couple with other active nuclei such as 1H. Since the percentage of 13C is so low in natural isotopic abundance samples, the 13C coupling effects on other carbons and on 1H are usually negligible, and for all practical purposes splitting of 1H signals due to coupling with natural isotopic abundance carbon does not show up in 1H NMR spectra. In real life, however, the 13C coupling effect does show up on non-13C decoupled spectra of other magnetic nuclei, causing satellite signals
Carbon-13 NMR satellite
Carbon satellites are small peaks that can be seen shouldering the main peaks in an NMR spectrum. These peaks can occur in the NMR spectrum of any NMR active atom where those atoms adjoin a carbon atom...
.
Similarly for all practical purposes, 13C signal splitting due to coupling with nearby natural isotopic abundance carbons is negligible in 13C NMR spectra. However, practically all hydrogen bonded to carbon atoms is 1H in natural isotopic abundance samples, including any 13C nuclei bonded to H atoms. In a 13C spectrum with no decoupling at all, each of the 13C signals is split according to how many H atoms that C atom is next to. In order to simplify the spectrum, 13C NMR spectroscopy is most often run fully proton decoupled, meaning 1H nuclei in the sample are broadly irradiated to fully decouple them from the 13C nuclei being analyzed. This full proton decoupling eliminates all coupling with H atoms and thus splitting due to H atoms in natural isotopic abundance compounds. Since coupling between other carbons in natural isotopic abundance samples is negligible, signals in fully proton decoupled 13C spectra in hydrocarbon
Hydrocarbon
In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons from which one hydrogen atom has been removed are functional groups, called hydrocarbyls....
s and most signals from other organic compounds are single peaks. This way, the number of equivalent sets of carbon atoms in a chemical structure
Chemical structure
A chemical structure includes molecular geometry, electronic structure and crystal structure of molecules. Molecular geometry refers to the spatial arrangement of atoms in a molecule and the chemical bonds that hold the atoms together. Molecular geometry can range from the very simple, such as...
can be counted by counting singlet peaks, which in 13C spectra tend to be very narrow (thin). Other information about the carbon atoms can usually be determined from the chemical shift
Chemical shift
In nuclear magnetic resonance spectroscopy, the chemical shift is the resonant frequency of a nucleus relative to a standard. Often the position and number of chemical shifts are diagnostic of the structure of a molecule...
, such as whether the atom is part of a carbonyl group or an aromatic ring, etc. Such full proton decoupling can also help increase the intensity of 13C signals.
There can also be off-resonance decoupling of 1H from 13C nuclei in 13C NMR spectroscopy, where weaker r f irradiation results in what can be thought of as partial decoupling. In such an off-resonance decoupled spectrum, only 1H atoms bonded to a carbon atom will split its 13C signal. The coupling constant, indicating a small frequency difference between split signal peaks, would be smaller than in an undecoupled spectrum.
Looking at a compound's off-resonance proton-decoupled 13C spectrum can show how many hydrogens are bonded to the carbon atoms to further help elucidate the chemical structure. For most organic compounds, carbons bonded to 3 hydrogens (methyls) would appear as quartets (4-peak signals), carbons bonded to 2 equivalent hydrogens would appear as triplets (3-peak signals), carbons bonded to 1 hydrogen would be doublets (2-peak signals), and carbons not bonded directly to any hydrogens would be singlets (1-peak signals).
Another decoupling method is specific proton decoupling (also called band-selective or narrowband). Here the selected "narrow" 1H frequency band of the (soft) decoupling RF pulse covers only a certain part of all 1H signals present in the spectrum. This can serve two purposes: (1) decreasing the deposited energy through additionally adjusting the RF pulse shapes/using composite pulses, (2) elucidating connectivities of NMR nuclei (applicable with both heteronuclear and homonuclear decoupling). Point 2 can be accomplished via decoupling e.g. of a single 1H signal which then leads to the collapse of the J coupling pattern of only those observed heteronuclear or non-decoupled 1H signals which are J coupled to the irradiated 1H signal. Other parts of the spectrum remain unaffected. In other words this specific decoupling method is useful for signal assignments which is a crucial step for further analyses e.g. with the aim of solving a molecular structure.
Note that more complex phenomena might be observed when for example the decoupled 1H nuclei are exchanging with non-decoupled 1H nuclei in the sample with the exchange process taking place on the NMR time scale. This is exploited e.g. with chemical exchange saturation transfer (CEST) contrast agents in in vivo magnetic resonance spectroscopy
In vivo magnetic resonance spectroscopy
In vivo magnetic resonance spectroscopy is a specialised technique associated to magnetic resonance imaging ....
.