NMR Facility

Department of Chemistry University of Oxford
A collection of common NMR acronyms are explained below along with a very brief description of the techniques to which they relate. This is designed to help chemists quickly become familiar with terms they may come across in publications but may not recognise.
 

APT: Attached Proton Test
An experiment derived from the JMOD experiment and also used for the multiplicity editing of carbon-13 spectra. The modification in APT aims to improve the intensity of the slowly relaxing non-protonated centres by using a smaller initial excitation pulse to reduce saturation.

See also: JMOD DEPT DEPTQ

COLOC: Correlation through Long-Range Coupling
A 2D experiment for determining long-range (2 or 3-bond) correlations between proton and carbon nuclei and thus piecing together organic structures. This carbon-detected experiments has largely been superseded by the more sensitive proton-detected HMBC experiment.

See also: HMBC HETCOR

COSY: Correlation Spectroscopy
The original 2D experiment. Used to identify nuclei that share a scalar (J) coupling. The presence of off-diagonal peaks (cross-peaks) in the spectrum directly correlates the coupled partners. Most often used to analyse coupling relationships between protons, but may be used to correlate any high-abundance homonuclear spins eg 31P, 19F and 11B.

See also: DQF-COSY TOCSY

DEPT: Distortionless Enhancement by Polarisation Transfer
A 1D experiment used for enhancing the sensitivity of carbon observation and for editing of 13C spectra. The sensitivity gain comes from starting the experiment with proton excitation and subsequently transferring the magnetisation onto carbon (the process known as polarisation transfer). This gain stems from the larger population differences associated with protons, which are four times those of carbon.
The editing feature alters the amplitude and sign of the carbon resonances according to the number of directly attached protons, allowing the identification of carbon multiplicities. The experiment is typically run using different final proton pulse angles (the numbers below), resulting in differing signs (+ve or -ve) for the various carbon resonances:

  DEPT-45 DEPT-90 DEPT-135 DEPTQ
C 0 0 0 -
CH + + + +
CH2 + 0 - -
CH3 + 0 + +
 

Note that because quaternary carbons do not possess a directly bonded proton, they do not produce responses in standard DEPT experiments, although these can be seen (often more weakly) in the DEPTQ variant.

See also: INEPT JMOD APT DEPTQ

DEPTQ: DEPT with retention of Quaternaries
A variant of the above DEPT experiment in which the signals of non-protonated carbons (such as quaternary centres, hence the "Q") are also retained (albeit with reduced intensities) and are displayed with signs similar to that of CH2 groups.

See also: DEPT

DIPSI: Decoupling or mixing sequence
The DIPSI element (decoupling in the presence of scalar interactions) is sequence of pulses clustered together that serve to transfer magnetisation between protons that share scalar couplings. It is most often used in the TOCSY experiment as a so-called "isotropic mixing" sequence, for which the DIPSI-2 element is most commonly employed. The multi-pulse element is applied as a repeated sequence to define a total "mixing time" during which magnetisation is allowed to flow between coupled spins.

See also: TOCSY HSQC-TOCSY HMQC-TOCSY DPFGSE-TOCSY

DOSY: Diffusion Ordered Spectroscopy
A pseudo-2D NMR experiment which presents chemical shifts on one axis versus the self-diffusion coefficients of the solutes on the other. The diffusion coefficients are determined from the NMR signal intensity decays in a sequence of 1D spectra recorded with increasing amplitudes of pulsed field gradients (PFGs) which are used to map the translational behaviour of the solutes. The method can, for example, be employed to investigate molecular size, complexation phenomena, binding and aggregation.

See also: PFG

DPFGSE excitation: Double Pulsed Field Gradient Spin-Echo excitation
An excitation method based on pulsed field gradients (PFGs) to selectively excite a resonance or group of resonances in a spectrum. The method is quick and clean, typically providing complete suppression of all other signals. The selected target may then represent the starting point for magnetisation transfer through scalar coupling (TOCSY) or the NOE (NOESY), thus providing specific information on the NMR interactions of the target spins.

See also: PFG

DPFGSE-NOESY:
A 1D NOE technique based on the use of the DPFGSE for selective excitation of a target proton. The NOEs observed originate only from the selected proton(s) and are generated during a mixing period in a similar manner to the 2D NOESY experiment. Thus, so-called transient NOEs are sampled with this method and percentage NOE enhancements are not recorded directly (nor do thay have the same significance) as in the 1D NOE difference experiment. The gradient selection generally provides superior quality data in considerably less time than the traditional NOE difference method.

See also: DPFGSE NOESY NOE-Difference NOE

DPFGSE-TOCSY
A 1D technique based on the use of the DPFGSE for selective excitation of a target proton followed by magnetisation transfer with a TOCSY mixing period. This aims to propagate magnetisation from the selected proton to others within the same scalar-ccoupled spin-system and is therefore very useful for the analysis of the proton spectra of isolated systems such as amino-acids and saccharides.

See also: TOCSY DPFGSE

DQF-COSY: Double-Quantum Filtered Correlation Spectroscopy
A variation of the standard 2D COSY experiment, used for identifying scalar (J) couplings between protons. The double-quantum filter serves to alter the phase properties of the spectrum, enabling a phase-sensitive presentation and thus higher resolution. In practice this aids the analysis of crowded spectra, reducing cross-peak overlap, and allows the potentially informative fine-structure within cross-peaks to be studied in detail. The filter also suppresses singlets in the spectrum (along the diagonal).

See also: COSY TOCSY

Excitation sculpting:
A method for the selective exciation (or removal) of a resonance or resonances in a spectrum based on the use of a sequence of pulsed field gradient spin-echoes, the best known example being the "double pulsed field gradient spin-echo" sequence. The method provides very clean excitation (or removal) of the target resonance(s) and may be used in selective 1D experiments or in solvent suppression methods, for example.

See also: DPFGSE WATERGATE

EXSY: Exchange Spectroscopy
A homonuclear 2D method used to identify equilibrium chemical exchange pathways and, in favourable cases, quantify the kinetic processes. Its appearance is similar to the basic COSY experiment, but crosspeaks now indicate an exchange process between the correlated spins. Its use is limited to those systems in which the exchange kinetics are faster than, or comparable to, spin relaxation rates. Most often used in the study of fluxional inorganic or organometallic systems, but can be applied to conformational exchange in organic systems, for example.

GARP: Broadband Decoupling Sequence
The GARP (globally-optimised, alternating-phase, rectangular pulses) element is a cluster of pulses applied repeatedly to a heteroatom to achieve spin-decoupling, typically between protons and the irradiated heteroatom (so called "heteronuclear decoupling"). Commonly it is used for carbon-13 decoupling in the HSQC experiment whilst proton magnetisation is detected, leading to decoupling of the observed 13C satellites. The sequence is effective over a very wide spectral width, as required for carbon decoupling.

See also: WALTZ

HETCOR: Heteronuclear Shift Correlation (or HETERO-COSY)
The traditional 2D experiment used to identify couplings between heteronuclear spins. Most often employed to correlate carbons with their directly bonded protons. This rather old experiment makes use of carbon detection, and has nowadays largely been superseded by more sensitive proton detected heteronuclear shift correlation experiments such as HMQC and HSQC. HETCOR may, however, still find use when very high carbon resolution is required, since this is easier to achieve in the directly observed dimension of a 2D experiment.

See also: HMQC HSQC COLOC HMBC

HMBC: Heteronuclear Multiple-Bond Correlation
A 2D experiment (closely related to HMQC, its 1-bond analogue), used to identify long-range couplings between protons and carbons. "Long-range" generally refers to 2- or 3-bonds since couplings over more bonds are usually vanishingly small (exceptions include those across unsaturation). The experiment utilises proton detection so has good sensitivity, and benefits considerably from the use of pulsed field gradients. It is an extremely powerful tool for piecing together organic structures, and is now a routine technique in organic chemistry.

See also: HMQC COLOC H2BC

HMQC: Heteronuclear Multiple-Quantum Correlation
A 2D experiment used to correlate directly bonded carbon-proton nuclei. Utilises proton detection and has very high sensitivity (and can be quicker to acquire than a 1D carbon spectrum). The correlations can be used to map known proton assignments onto their directly attached carbons. The 2D spectrum can also prove useful in the assignment of the proton spectrum itself by dispersing the proton resonances along the 13C dimension and so reducing proton multiplet overlap. It also provides a convenient way of identifying diastereotopic geminal protons (which are sometimes difficult to distinguish unambiguously, even in COSY) since only these will produce two correlations to the same carbon. Alongside COSY, this represents the front-line 2D technique of organic chemistry.

See also: HMBC HSQC HMQC-TOCSY HETCOR

HMQC-TOCSY: Heteronuclear Multiple-Quantum Correlation with additional TOCSY transfer
An extension of the 2D HMQC experiment in which a TOCSY transfer between protons is added prior to data acquisition. This relays the original proton-carbon correlation peak onto neighbouring protons within the same spin-system, thus producing a 13C-dispersed TOCSY spectrum. This proves useful when analysing complex proton spectra for which the 2D TOCSY becomes too crowded for unambiguous interpretation.

See also: HMQC TOCSY

HOESY: Heteronuclear Overhauser Effect Spectroscopy
Experiments that detect the nOe phenomenon between dissimilar nuclides (as opposed to the classical homonuclear proton-proton NOESY). Both 1D and 2D methods exist and most often one nuclide is the proton. The often poor sensitivity of the method can limit the application of HOESY, such as for 1H-13C experiments, whereas 1H-19F HOESY can have sensitivity comparable to that of 1H-1H NOESY and so finds use in fluorine chemistry.

See also: NOE NOESY

HOHAHA: Homonuclear Hartmann-Hahn Spectroscopy
An alternative and essentially identical experiment to TOCSY. Historically, these names were often used interchangeably in the literature, but TOCSY is now the favoured term.

See also: TOCSY

HSQC: Heteronuclear Single Quantum Correlation
A 2D proton-detected heteronuclear shift correlation experiment which provides the same information as the closely related HMQC, that is, one-bond H-X correlations. The principle advantage of HSQC is the slightly better resolution that can be obtained in the X-dimension where the resonances are broadened by homonuclear proton couplings in HMQC but not in HSQC. For most routine applications this difference is barely noticeable, but where crowding occurs in the X dimension, the HSQC should provide better results (provided sufficiently high digital resolution is used).

See also: HMQC HMBC HETCOR COLOC

HSQC-TOCSY: Heteronuclear Single Quantum Correlation with additional TOCSY transfer
An extension of the 2D HSQC experiment in which a TOCSY transfer between protons is added prior to data acquisition. This relays the original proton-carbon correlation peak onto neighbouring protons within the same spin-system, thus producing a 13C-dispersed TOCSY spectrum. This proves useful when analysing complex proton spectra for which the 2D TOCSY becomes too crowded for unambiguous interpretation.

See also: HSQC TOCSY

HSQMBC: Heteronuclear Single Quantum Multiple-Bond Correlation
A variant of the HSQC experiment that has been optimised for the detection of long-range couplings (as observed in HMBC) but is more specifically tailored for the measurement of the magnitudes of the coupling constants themselves. A number of variants of HSQMBC exist, including Clean in-phase (CLIP) and Pure in-phase (PIP) experiments. Most useful for measuring nJCH and nJNH coupling constants. 

See also: HSQC HMBC

H2BC: Heteronuclear Multiple-Bond Correlation over two bonds
A conceptually similar experiment to HMBC for identifying correlations between 1H and 13C nuclei but which shows only two-bond correlations (that is, H-X-C correlations). The experiment does, however, use quite a different transfer pathway than HMBC and rather than exploiting long-range H-C couplings (2/3JCH) it uses vicinal (three-bond, 3JHH) proton and one-bond C-H couplings (1JCH) to generate cross peaks (the actual transfer pathway is therefore effectively H->H->C). The consequence of this is that ONLY two-bond correlations to adjacent protonated carbons can be observed (because of the need for 3JHH). Further, the peaks observed may not always match the equivalent two-bond correlations in HMBC because different couplings pathways are used in the two experiments.

See also: HMBC

INEPT: Insensitive Nuclei Enhanced by Polarization transfer
A 1D experiment used to enhance the sensitivity of nuclei with low magnetogyric ratios, g, (eg 15N or 13C) by transferring the greater population differences of a high-g spin (eg 1H, 19F or 31P) onto the heteronucleus via the process of polarization transfer. The transfer is most often from protons onto a directly bound heteronucleus. Can also be used for multiplicity editing, although the DEPT sequence is more widely used, especially for the editing of carbon-13 spectra.

See also: DEPT

J-MOD: J-modulated spin-echo
A 1D method used for the multiplicity editing of, typically, carbon-13 spectra. By judicious choice of delay periods in the spin-echo, the experiment can be tuned to produce spectra in which different multiplicities produce differing responses. A typical result would provide spectra in which the quaternary and methylene signals have opposite phase to those of methine and methyl resonances. Used as an alternative to the DEPT experiment, but does not gain from polarisation transfer although it does retain quaternary resonances (although these are often rather weak).

See also: DEPT APT

J-RES: J-Resolved Spectroscopy
A family of 2D methods which separate chemical shifts and scalar (J) couplings into different dimensions. Can operate in homonuclear or heteronuclear modes and thus provides a means of measuring homonuclear or heteronuclear couplings in the J-dimension. The homonuclear experiment in particular is plagued by problems arising from strong-couplings, so is best performed at the highest available field strength.

NOE: Nuclear Overhauser Effect
A through-space phenomenon used in the study of 3D structure and conformation. It gives rise to changes in the intensities of NMR resonances of spins I when the spin population differences of neighbouring spins S are altered from their equilibrium values (by saturation or population inversion). Proton-proton NOEs are by far the mostly widely used in structure elucidation. Since the effect has a (non-linear) distance dependence, only protons "close" in space (within 4-5 Å) give rise to such changes and the NOE is thus an extremely useful probe of spatial proximity. The NOE is a spin relaxation phenomenon and has very different behaviour depending on molecular motion and, in particular molecular tumbling rates. Small molecules (<1000 Da) under typical solution conditions tumble rapidly and produce weak, positive proton NOEs that grow rather slowly whereas, in contrast, large molecules (> 3000 Da) tumble slowly in solution and so produce large, negative NOEs that grow quickly. Mid-size molecules (ca 1000-3000 Da) tumble at "intermediate " rates where the NOE crosses from the positive to the negative regime and thus can have vanishingly small NOEs. Under such circumstances conventional NOEs may not be observed and it is necessary to either alter solution conditions (eg temperature, solvent viscosity) to change the motional properties or use so-called rotating-frame NOE (ROE) measurements. ROEs are generated under rather different physical conditions but from the chemist's perspective the key feature is that they remain positive for any tumbling rate.

See also: NOE DIFF NOESY ROESY DPFGSE-NOESY HOESY

NOE DIFF: NOE Difference Spectroscopy
A 1D method for measuring proton NOEs in small (rapidly tumbling) molecules. Involves the collection of "NOE spectra" in which a suitable target spin is subject to saturation and thus generates NOEs at its near neighbours, and a "Control spectrum" where the radio-frequency is placed far from all resonances and thus no NOEs are generated. In processing, the Control is subtracted from each NOE spectrum to produce the "Difference spectra". These contain responses only from the saturated resonance and from any NOE enhancements that exist (plus artifacts!), so making it easier to visualise and quantify the enhancements, which are often rather small ( < 10 %). Widely used in structural organic chemistry, but not well suited to the observation of NOEs in mid-sized or very large (slowly tumbling) molecules.

See also: NOE NOESY ROESY DPFGSE-NOESY

NOESY: Nuclear Overhauser Effect Spectroscopy
A 2D method used to map NOE correlations between protons within a molecule. Most popular with, and best suited to, the study of very large molecules such as bio-polymers, although it still has a place in small molecule work. The observed NOEs are termed "transient NOES" and should not be confused with the "steady-state NOEs" that are observed with the NOE difference experiment. The spectra have a layout similar to COSY but crosspeaks now indicate NOEs between the correlated protons. Positive NOEs (rapidly tumbling molecules) have opposite phase to the diagonal peaks whereas negative NOEs (slowly tumbling molecules) have the same phase as the diagonal (saturation transfer from chemical or conformational exchange also has the same phase as the diagonal and may be confused with negative NOEs).

See also: NOE NOE DIFF ROESY DPFGSE-NOESY HOESY

PFG: Pulsed Field Gradient
This is the application of a short (pulsed) magnetic field gradient across the NMR sample which momentarily destroys the magnetic field homogeneity within the sample. The effect is such that chemically similar spins that exist in different locations within the NMR sample precess with different frequencies during the pulse (in contrast to the usual requirement for high-resolution NMR spectra where, in a well shimmed magnet, these should all precess with identical frequencies). The net result of the pulse is that these spins are dispersed in the transverse plane (defocussed) and produced zero net magnetisation. This is the basis on which pulsed field gradients may be used to suppress unwanted resonances in a spectrum. Furthermore, appropriate combinations of these pulses can be employed to selectively refocus signals that we do wish to see in the final spectrum whilst leaving the unwanted resonances defocussed and thus unobservable. Thus, pulsed field gradients provide a means for signal selection in NMR experiments that provide clean, high-quality data sets often very quickly when sample concentrations are not limiting. Many modern NMR methods are thus referred to as "gradient-selected", gradient-enhanced" or simply "gradient" experiments.

ROESY: Rotating-Frame NOE Spectroscopy
A 2D experiment that measures NOEs in the "rotating-frame" and is used to map NOE correlations between protons, particularly for mid-sized molecules (1000-3000 Da) that have close-to-zero conventional NOEs. Again has a similar overall appearance to COSY, but cross-peaks (which have opposite phase to the diagonal regardless of molecular tumbling rates) now indicate ROEs between the correlated spins. The experiment is also prone to interference from TOCSY transfers (between J-coupled spins) and requires careful analysis.

See also: NOE NOESY

STD: Saturation Transfer Difference
An experiment used to detect the binding of small molecule ligands to macromolecules such as proteins. The technique relies on the transfer of magnetisation between the protein (whose proton resonances are saturated by direct irradiation) and the bound ligand via the proton-proton NOE, followed by the release of the ligand back into free solution where its proton spectrum is observed. The free ligand carries the negative NOE from when it was bound, so has reduced proton signal intensities. Subtraction of a control spectrum (recorded without protein saturation) reveals resonances of the binding ligands. Any molecule that does not bind should not appear in the resulting STD spectrum. The technique is often complimentary to waterLOGSY.

See also: WATERLOGSY NOE

TOCSY: Total Correlation Spectroscopy
A 2D homonuclear correlation experiment used to analyse scalar (J) coupling networks between protons. It has a similar appearance to the 2D COSY spectrum. However, COSY crosspeaks are limited to identifying directly coupled spins, that is, those spins that share a mutual J-coupling, A-B. TOCSY is able to “relay” magnetisation between spins, A-B-C-D.., and can therefore show correlations amongst spins that are not directly coupled (eg A-C and A-D) but exist within the same spin system. This proves useful in the analysis of crowded spectra where correlations from a single resolved proton may be used to trace the coupling network. Popular for the analysis of peptides and oligosaccharides where molecules are typically composed of discrete subunits (spin systems) ie. amino-acids or saccharide units.

See also: COSY HMQC-TOCSY HSQC-TOCSY DPFGSE-TOCSY DIPSI

WALTZ: Broadband Decoupling Sequence
The WALTZ (wideband, alternating-phase, low-power technique for residual splitting (!)) element is a cluster of pulses applied repeatedly to achieve spin-decoupling, typically of protons during observation of a heteroatom (an example of so called "heteronuclear decoupling"). Most commonly it is used for proton decoupling during the acquisition of carbon-13 spectra. The sequence produces very effective removal of couplings with little residual broadening of peaks, as required for high-resolution heteronuclear NMR measurements.

See also: GARP

WaterLOGSY: Water-Ligand Observed through Gradient Spectroscopy
An experiment used to detect the binding of small molecule ligands to macromolecules such as proteins. The technique relies on the transfer of magnetisation from irradiated water onto the protein and then onto the bound ligand via the proton-proton NOE, followed by the release of the ligand back into free solution where its proton spectrum is observed. The free ligand carries the negative NOE from when it was bound. The ligand in the free state will also receive a positive NOE directly from water, so it is often necessary to record the waterLOGSY spectrum in the absence of the protein and look for differences in peak intensity as indicative of binding. Any molecule that does not bind should not show such intensity changes in the presence of the protein. The technique is often complimentary to STD, and often finds use in fragment-based ligand screening.

See also: STD NOE

WATERGATE: Water suppression through gradient tailored excitation
A method for solvent suppression that employs pulsed field gradient spin-echoes to destroy the unwanted solvent resonance but retain all others. Most commonly employed in the study of biological molecules in 90%H2O/10%D2O where the water signal dominates all others. Other variations, including those based on excitation sculpting, are also widely employed.

See also: PFG

Acronyms

Some common NMR acronyms are listed below and their definitions may be found in the neighbouring text.

APT

COLOC

COSY

DEPT

DEPTQ

DIPSI

DOSY

DPFGSE excitation

DPFGSE-NOESY (1D)

DPFGSE-TOCSY (1D)

DQF-COSY

Excitation sculpting

EXSY

GARP

HETCOR

HMBC

HMQC

HMQC-TOCSY

HOESY

HOHAHA

HSQC

HSQC-TOCSY

HSQMBC

H2BC

INEPT

J-MOD

J-RES

NOE

NOE-DIFF

NOESY

PFG

ROESY

STD

TOCSY

WALTZ

WaterLOGSY

WATERGATE