Nuclear magnetic resonance decoupling

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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 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. 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 compounds.

Explanation

NMR spectroscopy of a sample produces an NMR spectrum, which is essentially a graph 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 (rf) irradiated are the same isotope as the nuclei being observed (analyzed) in the spectrum. Heteronuclear decoupling is when the nuclei being rf irradiated are of a different isotope than the nuclei being observed in the spectrum. [1] For a given isotope, the entire range for all nuclei of that isotope can be irradiated in broad band decoupling, [2] 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 compounds. 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 hydrocarbons 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 rf 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. [1] 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). [2]

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. [3]

Related Research Articles

The nuclear Overhauser effect (NOE) is the transfer of nuclear spin polarization from one population of spin-active nuclei to another via cross-relaxation. A phenomenological definition of the NOE in nuclear magnetic resonance spectroscopy (NMR) is the change in the integrated intensity of one NMR resonance that occurs when another is saturated by irradiation with an RF field. The change in resonance intensity of a nucleus is a consequence of the nucleus being close in space to those directly affected by the RF perturbation.

In nuclear magnetic resonance (NMR) spectroscopy, the chemical shift is the resonant frequency of an atomic nucleus relative to a standard in a magnetic field. Often the position and number of chemical shifts are diagnostic of the structure of a molecule. Chemical shifts are also used to describe signals in other forms of spectroscopy such as photoemission spectroscopy.

<span class="mw-page-title-main">Nuclear magnetic resonance spectroscopy</span> Laboratory technique

Nuclear magnetic resonance spectroscopy, most commonly known as NMR spectroscopy or magnetic resonance spectroscopy (MRS), is a spectroscopic technique based on re-orientation of atomic nuclei with non-zero nuclear spins in an external magnetic field. This re-orientation occurs with absorption of electromagnetic radiation in the radio frequency region from roughly 4 to 900 MHz, which depends on the isotopic nature of the nucleus and increased proportionally to the strength of the external magnetic field. Notably, the resonance frequency of each NMR-active nucleus depends on its chemical environment. As a result, NMR spectra provide information about individual functional groups present in the sample, as well as about connections between nearby nuclei in the same molecule. As the NMR spectra are unique or highly characteristic to individual compounds and functional groups, NMR spectroscopy is one of the most important methods to identify molecular structures, particularly of organic compounds.

<span class="mw-page-title-main">Solid-state nuclear magnetic resonance</span>

Solid-state NMR (ssNMR) spectroscopy is a technique for characterizing atomic level structure in solid materials e.g. powders, single crystals and amorphous samples and tissues using nuclear magnetic resonance (NMR) spectroscopy. The anisotropic part of many spin interactions are present in solid-state NMR, unlike in solution-state NMR where rapid tumbling motion averages out many of the spin interactions. As a result, solid-state NMR spectra are characterised by larger linewidths than in solution state NMR, which can be utilized to give quantitative information on the molecular structure, conformation and dynamics of the material. Solid-state NMR is often combined with magic angle spinning to remove anisotropic interactions and improve the resolution as well as the sensitivity of the technique.

Carbon-13 (C13) nuclear magnetic resonance is the application of nuclear magnetic resonance (NMR) spectroscopy to carbon. It is analogous to proton NMR and allows the identification of carbon atoms in an organic molecule just as proton NMR identifies hydrogen atoms. 13C NMR detects only the 13
C
 isotope. The main carbon isotope, 12
C
 does not produce an NMR signal. Although ca. 1 mln. times less sensitive than 1H NMR spectroscopy, 13C NMR spectroscopy is widely used for characterizing organic and organometallic compounds, primarily because 1H-decoupled 13C-NMR spectra are more simple, have a greater sensitivity to differences in the chemical structure, and, thus, are better suited for identifying molecules in complex mixtures. At the same time, such spectra lack quantitative information about the atomic ratios of different types of carbon nuclei, because nuclear Overhauser effect used in 1H-decoupled 13C-NMR spectroscopy enhances the signals from carbon atoms with a larger number of hydrogen atoms attached to them more than from carbon atoms with a smaller number of H's, and because full relaxation of 13C nuclei is usually not attained, and the nuclei with shorter relaxation times produce more intense signals.

<span class="mw-page-title-main">Proton nuclear magnetic resonance</span> NMR via protons, hydrogen-1 nuclei

Proton nuclear magnetic resonance 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 (H) is used, practically all the hydrogen consists of the isotope 1H.

Nuclear magnetic resonance spectroscopy of proteins is a field of structural biology in which NMR spectroscopy is used to obtain information about the structure and dynamics of proteins, and also nucleic acids, and their complexes. The field was pioneered by Richard R. Ernst and Kurt Wüthrich at the ETH, and by Ad Bax, Marius Clore, Angela Gronenborn at the NIH, and Gerhard Wagner at Harvard University, among others. Structure determination by NMR spectroscopy usually consists of several phases, each using a separate set of highly specialized techniques. The sample is prepared, measurements are made, interpretive approaches are applied, and a structure is calculated and validated.

The heteronuclear single quantum coherence or heteronuclear single quantum correlation experiment, normally abbreviated as HSQC, is used frequently in NMR spectroscopy of organic molecules and is of particular significance in the field of protein NMR. The experiment was first described by Geoffrey Bodenhausen and D. J. Ruben in 1980. The resulting spectrum is two-dimensional (2D) with one axis for proton (1H) and the other for a heteronucleus, which is usually 13C or 15N. The spectrum contains a peak for each unique proton attached to the heteronucleus being considered. The 2D HSQC can also be combined with other experiments in higher-dimensional NMR experiments, such as NOESY-HSQC or TOCSY-HSQC.

Two-dimensional nuclear magnetic resonance spectroscopy is a set of nuclear magnetic resonance spectroscopy (NMR) methods which give data plotted in a space defined by two frequency axes rather than one. Types of 2D NMR include correlation spectroscopy (COSY), J-spectroscopy, exchange spectroscopy (EXSY), and nuclear Overhauser effect spectroscopy (NOESY). Two-dimensional NMR spectra provide more information about a molecule than one-dimensional NMR spectra and are especially useful in determining the structure of a molecule, particularly for molecules that are too complicated to work with using one-dimensional NMR.

In nuclear chemistry and nuclear physics, J-couplings are mediated through chemical bonds connecting two spins. It is an indirect interaction between two nuclear spins that arises from hyperfine interactions between the nuclei and local electrons. In NMR spectroscopy, J-coupling contains information about relative bond distances and angles. Most importantly, J-coupling provides information on the connectivity of chemical bonds. It is responsible for the often complex splitting of resonance lines in the NMR spectra of fairly simple molecules.

Nuclear magnetic resonance (NMR) in the geomagnetic field is conventionally referred to as Earth's field NMR (EFNMR). EFNMR is a special case of low field NMR.

In vivo magnetic resonance spectroscopy (MRS) is a specialized technique associated with magnetic resonance imaging (MRI).

Carbon satellites in physics and spectroscopy, are small peaks that can be seen shouldering the main peaks in the nuclear magnetic resonance (NMR) spectrum. These peaks can occur in the NMR spectrum of any NMR active atom where those atoms adjoin a carbon atom. However, Carbon satellites are most often encountered in proton NMR.

<span class="mw-page-title-main">Phosphorus-31 nuclear magnetic resonance</span> Spectroscopy technique for molecules containing phosphorus

Phosphorus-31 NMR spectroscopy is an analytical chemistry technique that uses nuclear magnetic resonance (NMR) to study chemical compounds that contain phosphorus. Phosphorus is commonly found in organic compounds and coordination complexes, making it useful to measure 31- NMR spectra routinely. Solution 31P-NMR is one of the more routine NMR techniques because 31P has an isotopic abundance of 100% and a relatively high gyromagnetic ratio. The 31P nucleus also has a spin of 1/2, making spectra relatively easy to interpret. The only other highly sensitive NMR-active nuclei spin 1/2 that are monoisotopic are 1H and 19F.

<span class="mw-page-title-main">Fluorine-19 nuclear magnetic resonance spectroscopy</span> Analytical technique

Fluorine-19 nuclear magnetic resonance spectroscopy is an analytical technique used to detect and identify fluorine-containing compounds. 19F is an important nucleus for NMR spectroscopy because of its receptivity and large chemical shift dispersion, which is greater than that for proton nuclear magnetic resonance spectroscopy.

Carbohydrate NMR spectroscopy is the application of nuclear magnetic resonance (NMR) spectroscopy to structural and conformational analysis of carbohydrates. This method allows the scientists to elucidate structure of monosaccharides, oligosaccharides, polysaccharides, glycoconjugates and other carbohydrate derivatives from synthetic and natural sources. Among structural properties that could be determined by NMR are primary structure, saccharide conformation, stoichiometry of substituents, and ratio of individual saccharides in a mixture. Modern high field NMR instruments used for carbohydrate samples, typically 500 MHz or higher, are able to run a suite of 1D, 2D, and 3D experiments to determine a structure of carbohydrate compounds.

<span class="mw-page-title-main">Nuclear magnetic resonance</span> Spectroscopic technique based on change of nuclear spin state

Nuclear magnetic resonance (NMR) is a physical phenomenon in which nuclei in a strong constant magnetic field are perturbed by a weak oscillating magnetic field and respond by producing an electromagnetic signal with a frequency characteristic of the magnetic field at the nucleus. This process occurs near resonance, when the oscillation frequency matches the intrinsic frequency of the nuclei, which depends on the strength of the static magnetic field, the chemical environment, and the magnetic properties of the isotope involved; in practical applications with static magnetic fields up to ca. 20 tesla, the frequency is similar to VHF and UHF television broadcasts (60–1000 MHz). NMR results from specific magnetic properties of certain atomic nuclei. Nuclear magnetic resonance spectroscopy is widely used to determine the structure of organic molecules in solution and study molecular physics and crystals as well as non-crystalline materials. NMR is also routinely used in advanced medical imaging techniques, such as in magnetic resonance imaging (MRI). The original application of NMR to condensed matter physics is nowadays mostly devoted to strongly correlated electron systems. It reveals large many-body couplings by fast broadband detection and it should not to be confused with solid state NMR, which aims at removing the effect of the same couplings by Magic Angle Spinning techniques.

Nucleic acid NMR is the use of nuclear magnetic resonance spectroscopy to obtain information about the structure and dynamics of nucleic acid molecules, such as DNA or RNA. It is useful for molecules of up to 100 nucleotides, and as of 2003, nearly half of all known RNA structures had been determined by NMR spectroscopy.

Triple resonance experiments are a set of multi-dimensional nuclear magnetic resonance spectroscopy (NMR) experiments that link three types of atomic nuclei, most typically consisting of 1H, 15N and 13C. These experiments are often used to assign specific resonance signals to specific atoms in an isotopically-enriched protein. The technique was first described in papers by Ad Bax, Mitsuhiko Ikura and Lewis Kay in 1990, and further experiments were then added to the suite of experiments. Many of these experiments have since become the standard set of experiments used for sequential assignment of NMR resonances in the determination of protein structure by NMR. They are now an integral part of solution NMR study of proteins, and they may also be used in solid-state NMR.

<span class="mw-page-title-main">Platinum-195 nuclear magnetic resonance</span>

Platinum-195 nuclear magnetic resonance spectroscopy is a spectroscopic technique which is used for the detection and characterisation of platinum compounds. The sensitivity of the technique and therefore its diagnostic utility have increased significantly starting from the 1970s, with 195Pt NMR nowadays considered the method of choice for structural elucidation of Pt species in solution.

References

  1. 1 2 Supplemental NMR Topics
  2. 1 2 Carbon NMR spectra
  3. ↑ Sherry AD; Woods M (2008). "Chemical exchange saturation transfer contrast agents for magnetic resonance imaging". Annu Rev Biomed Eng. 10: 391–411. doi:10.1146/annurev.bioeng.9.060906.151929. PMC   2709739 . PMID   18647117.
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