Carbon-13 (C13) nuclear magnetic resonance (most commonly known as carbon-13 NMR spectroscopy or
13C NMR spectroscopy or sometimes simply referred to as carbon NMR) is the application of
nuclear magnetic resonance (NMR) spectroscopy to
carbon
Carbon () is a chemical element; it has chemical symbol, symbol C and atomic number 6. It is nonmetallic and tetravalence, tetravalent—meaning that its atoms are able to form up to four covalent bonds due to its valence shell exhibiting 4 ...
. It is analogous to
proton NMR
Proton nuclear magnetic resonance (proton NMR, hydrogen-1 NMR, or 1H 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 stru ...
( NMR) and allows the identification of carbon
atom
Atoms are the basic particles of the chemical elements. An atom consists of a atomic nucleus, nucleus of protons and generally neutrons, surrounded by an electromagnetically bound swarm of electrons. The chemical elements are distinguished fr ...
s in an
organic molecule
Some chemical authorities define an organic compound as a chemical compound that contains a carbon–hydrogen or carbon–carbon bond; others consider an organic compound to be any chemical compound that contains carbon. For example, carbon-cont ...
just as proton NMR identifies
hydrogen
Hydrogen is a chemical element; it has chemical symbol, symbol H and atomic number 1. It is the lightest and abundance of the chemical elements, most abundant chemical element in the universe, constituting about 75% of all baryon, normal matter ...
atoms.
13C NMR detects only the
isotope
Isotopes are distinct nuclear species (or ''nuclides'') of the same chemical element. They have the same atomic number (number of protons in their Atomic nucleus, nuclei) and position in the periodic table (and hence belong to the same chemica ...
. The main carbon isotope, 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
Organic may refer to:
* Organic, of or relating to an organism, a living entity
* Organic, of or relating to an anatomical organ
Chemistry
* Organic matter, matter that has come from a once-living organism, is capable of decay or is the product ...
and
organometallic compound
Organometallic chemistry is the study of organometallic compounds, chemical compounds containing at least one chemical bond between a carbon atom of an organic molecule and a metal, including alkali, alkaline earth, and transition metals, and ...
s, 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
The nuclear Overhauser effect (NOE) is the transfer of spin polarization, nuclear spin polarization from one population of Spin (physics), spin-active nuclei (e.g. 1H, 13C, 15N etc.) to another via Relaxation (NMR), cross-relaxation. A phenomenolog ...
used in 1H-decoupled 13C-NMR spectroscopy enhances the signals from carbon atoms with a larger number of
hydrogen
Hydrogen is a chemical element; it has chemical symbol, symbol H and atomic number 1. It is the lightest and abundance of the chemical elements, most abundant chemical element in the universe, constituting about 75% of all baryon, normal matter ...
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 (for the sake of reducing the experiment time), and the nuclei with shorter relaxation times produce more intense signals.
The major isotope of carbon, the
12C isotope, has a
spin quantum number
In physics and chemistry, the spin quantum number is a quantum number (designated ) that describes the intrinsic angular momentum (or spin angular momentum, or simply ''spin'') of an electron or other particle. It has the same value for all ...
of zero and so is not magnetically active and therefore not detectable by NMR.
13C, with a spin quantum number of 1/2, is not abundant (1.1%), whereas other popular nuclei are 100% abundant, e.g.
1H,
19F,
31P.
Receptivity
13C NMR spectroscopy is much less sensitive (ca. by 4 orders of magnitude) to carbon than
1H NMR spectroscopy is to hydrogen, because of the lower
abundance
Abundance may refer to:
In science and technology
* Abundance (economics), the opposite of scarcities
* Abundance (ecology), the relative representation of a species in a community
* Abundance, the defining characteristic of abundant numbers
* ...
(1.1%) of
13C compared to
1H (>99%), and because of a lower(0.702 vs. 2.8)
nuclear magnetic moment
The nuclear magnetic moment is the magnetic moment of an atomic nucleus and arises from the spin of the protons and neutrons. It is mainly a magnetic dipole moment; the quadrupole moment does cause some small shifts in the hyperfine structure ...
. Stated equivalently, the
gyromagnetic ratio
In physics, the gyromagnetic ratio (also sometimes known as the magnetogyric ratio in other disciplines) of a particle or system is the ratio of its magnetic moment to its angular momentum, and it is often denoted by the symbol , gamma. Its SI u ...
(6.728284 10
7 rad T
−1 s
−1) is only 1/4th that of
1H.
On the other hand, the sensitivity of
13C NMR spectroscopy benefits to some extent from
nuclear Overhauser effect
The nuclear Overhauser effect (NOE) is the transfer of spin polarization, nuclear spin polarization from one population of Spin (physics), spin-active nuclei (e.g. 1H, 13C, 15N etc.) to another via Relaxation (NMR), cross-relaxation. A phenomenolog ...
, which enhances signal intensity for non-
quaternary
The Quaternary ( ) is the current and most recent of the three periods of the Cenozoic Era in the geologic time scale of the International Commission on Stratigraphy (ICS), as well as the current and most recent of the twelve periods of the ...
13C atoms.
Chemical shifts
The disadvantages in "receptivity" are compensated by the high sensitivity of
13C NMR signals to the chemical environment of the nucleus, i.e. the chemical shift "dispersion" is great, covering nearly 250 ppm. This dispersion reflects the fact that non-
1H nuclei are strongly influenced by
excited state
In quantum mechanics
Quantum mechanics is the fundamental physical Scientific theory, theory that describes the behavior of matter and of light; its unusual characteristics typically occur at and below the scale of atoms. Reprinted, Add ...
s ("paramagnetic" contribution to shielding tensor. This paramagnetic contribution is unrelated to
paramagnetism
Paramagnetism is a form of magnetism whereby some materials are weakly attracted by an externally applied magnetic field, and form internal, induced magnetic fields in the direction of the applied magnetic field. In contrast with this behavior, ...
). For example, most
1H NMR signals for most organic compounds are within 15 ppm.
The chemical shift reference standard for
13C is the carbons in
tetramethylsilane
Tetramethylsilane (abbreviated as TMS) is the organosilicon compound with the formula Si(CH3)4. It is the simplest tetraorganosilane. Like all silanes, the TMS framework is tetrahedral. TMS is a building block in organometallic chemistry but als ...
(TMS),
whose chemical shift is set as 0.0 ppm at every temperature.
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Typical chemical shifts in 13C-NMR
Coupling constants
Homonuclear
13C-
13C coupling is normally only observed in samples that are enriched with
13C. The range for one-bond
1J(
13C,
13C) is 50–130 Hz. Two-bond
2J(
13C,
13C) are near 10 Hz.
The ''trends'' in J(
1H,
13C) and J(
13C,
13C) are similar, except that J(
1H,
13C are smaller owing to the modest value of the
13C nuclear magnetic moment. Values for
1J(
1H,
13C) range from 125 to 250 Hz. Values for
2J(
1H,
13C) are near 5 Hz and often are negative.
Implementation
Sensitivity
As a consequence of low receptivity,
13C NMR spectroscopy suffers from complications not encountered in proton NMR spectroscopy. Many measures can be implemented to compensate for the low receptivity of this nucleus. For example, high field magnets with internal bores are capable of accepting larger sample tubes (typically 10 mm in diameter for
13C NMR versus 5 mm for
1H NMR).
Relaxation reagents allow more rapid pulsing. A typical relaxation agent is
chromium(III) acetylacetonate
Chromium(III) acetylacetonate is the coordination compound with the formula Cr(C5H7O2)3, sometimes designated as Cr(acac)3. This purplish coordination complex is used in NMR spectroscopy as a relaxation agent because of its solubility in nonpolar o ...
. For a typical sample, recording a
13C NMR spectrum may require several hours, compared to 15–30 minutes for
1H NMR. The
nuclear dipole
The nuclear magnetic moment is the magnetic moment of an atomic nucleus and arises from the spin of the protons and neutrons. It is mainly a magnetic dipole moment; the quadrupole moment does cause some small shifts in the hyperfine structure as ...
is weaker, the difference in energy between alpha and beta states is one-quarter that of proton NMR, and the
Boltzmann population
In statistical mechanics and mathematics, a Boltzmann distribution (also called Gibbs distribution Translated by J.B. Sykes and M.J. Kearsley. See section 28) is a probability distribution or probability measure that gives the probability tha ...
difference is correspondingly less. One final measure to compensate for low receptivity is isotopic enrichment.
Some NMR probes, called cryoprobes, offer 20x signal enhancement relative to ordinary NMR probes. In cryoprobes, the RF generating and receiving electronics are maintained at ~ 25K using helium gas, which greatly enhances their sensitivity. The trade-off is that cryoprobes are costly.
Coupling modes
Another potential complication results from the presence of large one bond
J-coupling
In nuclear chemistry and nuclear physics, ''J''-couplings (also called spin-spin coupling or indirect dipole–dipole coupling) are mediated through chemical bonds connecting two spins. It is an indirect interaction between two nuclear spins tha ...
constants between carbon and hydrogen (typically from 100 to 250 Hz). While potentially informative, these couplings can complicate the spectra and reduce sensitivity. For these reasons,
13C-NMR spectra are usually recorded with proton
NMR decoupling
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 part ...
. Couplings between carbons can be ignored due to the low natural abundance of
13C. Hence in contrast to typical proton NMR spectra, which show multiplets for each proton position, carbon NMR spectra show a single peak for each chemically non-equivalent carbon atom.
In further contrast to
1H NMR, the intensities of the signals are often not proportional to the number of equivalent
13C atoms. Instead, signal intensity is strongly influenced by (and proportional to) the number of surrounding
spins
The spins (as in having "the spins") is an adverse reaction of Substance intoxication, intoxication that causes a state of vertigo and nausea, causing one to feel as if "spinning out of control", especially when lying down. It is most commonly as ...
(typically
1H). Integrations are more quantitative if the delay times are long, i.e. if the delay times greatly exceed
relaxation times.
The most common modes of recording
13C spectra are proton-noise decoupling (also known as noise-, proton-, or broadband- decoupling), off-resonance decoupling, and gated decoupling. These modes are meant to address the large J values for
13C - H (110–320 Hz),
13C - C - H (5–60 Hz), and
13C - C - C - H (5–25 Hz) which otherwise make completely proton coupled
13C spectra difficult to interpret.
With proton-noise decoupling, in which most spectra are run, a noise decoupler strongly irradiates the sample with a broad (approximately 1000 Hz) range of radio frequencies covering the range (such as 100 MHz for a 23,486 gauss field) at which protons change their nuclear spin. The rapid changes in proton spin create an effective heteronuclear decoupling, increasing carbon signal strength on account of the
nuclear Overhauser effect
The nuclear Overhauser effect (NOE) is the transfer of spin polarization, nuclear spin polarization from one population of Spin (physics), spin-active nuclei (e.g. 1H, 13C, 15N etc.) to another via Relaxation (NMR), cross-relaxation. A phenomenolog ...
(NOE) and simplifying the spectrum so that each non-equivalent carbon produces a singlet peak.
Both the atoms, carbon and hydrogen exhibit spins and are NMR active. The nuclear Overhauser Effect is in general, showing up when one of two different types of atoms is irradiated while the NMR spectrum of the other type is determined. If the absorption intensities of the observed (i.e., non-irradiated) atom change, enhancement occurs. The effect can be either positive or negative, depending on which atom types are involved.
The relative intensities are unreliable because some carbons have a larger
spin-lattice relaxation time and others have weaker NOE enhancement.
In gated decoupling, the noise decoupler is gated on early in the free induction delay but gated off for the pulse delay. This largely prevents NOE enhancement, allowing the strength of individual
13C peaks to be meaningfully compared by integration, at a cost of half to two-thirds of the overall sensitivity.
With off-resonance decoupling, the noise decoupler irradiates the sample at 1000–2000 Hz upfield or 2000–3000 Hz downfield of the proton resonance frequency. This retains couplings between protons immediately adjacent to
13C atoms but most often removes the others, allowing narrow multiplets to be visualized with one extra peak per bound proton (unless bound methylene protons are non-equivalent, in which case a pair of doublets may be observed).
Distortionless enhancement by polarization transfer spectra
Distortionless enhancement by polarization transfer (DEPT) is an NMR method used for determining the presence of primary, secondary and
tertiary carbon
A tertiary carbon atom is a carbon atom bound to three other carbon atoms. For this reason, tertiary carbon atoms are found only in hydrocarbons containing at least four carbon atoms. They are called saturated hydrocarbons because they only con ...
atoms. The DEPT experiment differentiates between CH, CH
2 and CH
3 groups by variation of the selection angle parameter (the tip angle of the final
1H pulse): 135° angle gives all CH and CH
3 in a phase opposite to CH
2; 90° angle gives only CH groups, the others being suppressed; 45° angle gives all carbons with attached protons (regardless of number) in phase.
Signals from quaternary carbons and other carbons with no attached protons are always absent (due to the lack of attached protons).
The polarization transfer from
1H to
13C has the
secondary advantage of increasing the sensitivity over the normal
13C
spectrum (which has a modest enhancement from the
nuclear overhauser effect
The nuclear Overhauser effect (NOE) is the transfer of spin polarization, nuclear spin polarization from one population of Spin (physics), spin-active nuclei (e.g. 1H, 13C, 15N etc.) to another via Relaxation (NMR), cross-relaxation. A phenomenolog ...
(NOE) due to the
1H decoupling).
Attached proton test spectra
Another useful way of determining how many protons a carbon in a molecule is bonded to is to use an attached proton test (APT), which distinguishes between carbon atoms with even or odd number of attached
hydrogen
Hydrogen is a chemical element; it has chemical symbol, symbol H and atomic number 1. It is the lightest and abundance of the chemical elements, most abundant chemical element in the universe, constituting about 75% of all baryon, normal matter ...
s. A proper spin-echo sequence is able to distinguish between S, I
2S and I
1S, I
3S spin systems: the first will appear as positive peaks in the spectrum, while the latter as negative peaks (pointing downwards), while retaining relative simplicity in the spectrum since it is still broadband proton decoupled.
Even though this technique does not distinguish fully between CH
n groups, it is so easy and reliable that it is frequently employed as a first attempt to assign peaks in the spectrum and elucidate the structure.
Additionally, signals from quaternary carbons and other carbons with no attached protons are still detectable, so in many cases an additional conventional
13C spectrum is not required, which is an advantage over DEPT. It is, however, sometimes possible that a CH and CH
2 signal have coincidentally equivalent chemical shifts resulting in annulment in the APT spectrum due to the opposite phases. For this reason the conventional
13C spectrum or HSQC are occasionally also acquired.
See also
*
Nuclear magnetic resonance
Nuclear magnetic resonance (NMR) is a physical phenomenon in which nuclei in a strong constant magnetic field are disturbed by a weak oscillating magnetic field (in the near field) and respond by producing an electromagnetic signal with a ...
*
Hyperpolarized carbon-13 MRI
Hyperpolarized carbon-13 MRI is a functional medical imaging technique for probing perfusion and metabolism using injected substrates.
It is enabled by techniques for hyperpolarization of carbon-13-containing molecules using dynamic nuclear ...
*
Triple-resonance nuclear magnetic resonance 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 s ...
References
External links
Carbon NMR spectra where there are three spectra of ethyl phthalate,
ethyl ester
In chemistry, an ester is a compound derived from an acid (either organic or inorganic) in which the hydrogen atom (H) of at least one acidic hydroxyl group () of that acid is replaced by an organyl group (R). These compounds contain a distin ...
of
orthophthalic acid: completely coupled, completely decoupled and off-resonance decoupled (in this order).
* For an extended tabulation o
13C shiftsan
{{DEFAULTSORT:Carbon-13 Nmr
Nuclear magnetic resonance
Carbon