Thesaurus of NMR Spectroscopy

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Nuclear magnetic resonance (NMR) is spectroscopy method that is even more

important to the organic chemist than infrared spectroscopy in which magnetic nuclei in a

magnetic field absorb and re-emit electromagnetic radiation.

Spin is quantum-mechanical property of a nucleus. In NMR, only nuclei with spin I>0

are observable. In conventional NMR jargon “spin” often means “NMR active nucleus”.

Gyromagnetic ratio (γ) is the ratio of its magnetic dipole moment to its angular

momentum

Proton NMR (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 structure of its molecules.

Carbon-13 NMR (13C NMR) is the application of nuclear magnetic resonance (NMR)

spectroscopy with respect to carbon-13 nuclei within the molecules of a substance for

determining the structure of its molecules.

Resonance is the absorption of energy by a precessing nucleus and the resulting “flip” of

its nuclear spin from a lower energy state to a higher energy state.

Resonance assignment is the process by which the signals in a spectrum are correlated to

the NMR-active nuclei that give rise to them.

Signal is a recording in an NMR spectrum of a nuclear magnetic resonance.

Resonance frequency is the resonance frequency of a NMR-active nucleus at which its

signal appears in the spectrum.

Resonance line is NMR signal of a 1D (or a 1D trace in a 2D or 3D). A resonance line is

characterised by its position in the spectrum (resonance frequency), its height (signal

intensity) and its width (linewidth).

Diamagnetic current is The circulation of electrons around a nucleus in an applied field.

Diamagnetic shielding is the nuclear shielding resulting from an applied field.

Chemical shift (δ) is the resonance frequency (the characteristic frequency of a nucleus)

of NMR signals is most commonly given as chemical shift in units of ppm.

ppm (parts per million) is units of chemical shift.

Tetramethylsilane (TMS) is standard reference substance that is used universally

(CH3)4Si.

Upfield is the shift of an NMR signal to the right on the chart because of shielding effect.

Downfield is the shift of an NMR signal to the left on the chart because of deshielding

effect.

Chemically equivalent is all of the protons found in chemically identical environments

within a molecule.

Integration is a ratio for the number of hydrogens that give rise to the signal, thereby

helping calculate the total number of hydrogens present in a sample.

Peak is the units into which an NMR signal is split; doublet, triplet, quartet, multiplet,

etc.

Signal splitting is splitting of an NMR signal into a set of peaks by the influence of

neighboring nonequivalent hydrogens.

(n + 1) rule describes “If a hydrogen has n hydrogens nonequivalent to it but equivalent

among themselves on the same or adjacent atom(s), its 1H-NMR signal is split into (n + 1)

peaks”.

Singlet peak is high resolution NMR spectrum of compound which has one peak.

Doublet peak is high resolution NMR spectrum of compound which has two peaks.

Triplet peak is high resolution NMR spectrum of compound which has three peaks.

Signal coupling is an interaction in which the nuclear spins of adjacent atoms influence

each other and lead to the splitting of NMR signals.

Coupling constant (J) is the separation on an NMR spectrum (in hertz) between adjacent

peaks in a multiplet.

Spin-spin Coupling (scalar coupling) is the result of Fermi contact interaction between

electrons in the s orbital of one nucleus and the nuclear spin of a bonded nucleus.

Geminal coupling (2J) is the coupling for nonequivalent H on the same carbon.

Vicinal coupling (3J) is the coupling for H atoms three bonds apart.

Allylic coupling (4J) is the coupling for H atoms four bonds apart.

Anisotropy effect is responsible for the largest part of shift caused by the pi electrons in

the aromatic compouds.

Ring current is π electrons in the aromatic ring system induced to circulate around the

ring when it is placed.

Exchangeable proton is 1H atom of a macromolecule replaced by a H atom from the

solvent (water) after a certain time.

Correlation time (τc) is a time constant that is a measure of how fast a molecule tumbles

in solution. It depends on the size and the shape of a molecule.

Fourier Transform (FT) is a mathematical operation that transforms time-domain data

(intensities measured at discrete time intervals) into a frequency-domain spectrum and

vice versa.

Free-Induction Decay (FID) is a superimposed combination of all the frequencies

emitted and can be quite complex.

DEPT (Distortionless Enhancement by Polarization Transfer) is a very useful method

for determining the presence of primary, secondary and tertiary carbon atoms.

Two-dimensional nuclear magnetic resonance spectroscopy (2D NMR) 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.

Homonuclear through-bond correlation methods is a methods in which magnetization

transfer occurs between nuclei of the same type, through J-coupling of nuclei connected

by up to a few bonds.

Heteronuclear through-bond correlation methods is a methods in which magnetization

transfer occurs between different nuclei.

COSY (Correlation Spectroscopy) is an experiment correlates two spins that are

connected via a single J-coupling.

ECOSY (Exclusive Correlation Spectroscopy) is an NMR correlation experiment for

the accurate measurement of small J-couplings.

TOCSY (Total Correlation Spectroscopy) is similar to the COSY experiment, in that

cross peaks of coupled protons are observed. However, cross peaks are observed not only

for nuclei which are directly coupled, but also between nuclei which are connected by a

chain of couplings.

INADEQUATE (Incredible Natural-Abundance Double-Quantum Transfer

Experiment) is a method often used to find 13C couplings between adjacent carbon

atoms.

NOECY (Nuclear Overhauser Effect Spectroscopy) is one of the most useful

techniques as it allows to correlate nuclei through space (distance smaller than 5Å). By

measuring cross peak intensity, distance information can be extracted.

ROESY (Rotating frame nuclear Overhauser Effect Spectroscopy) is similar to

NOESY, except that the initial state is different. Instead of observing cross relaxation

from an initial state of z-magnetization, the equilibrium magnetization is rotated onto the

x axis and then spin-locked by an external magnetic field so that it cannot precess.

HSQC (Heteronuclear Single-Quantum Correlation) is type of experiment correlates 1H to a heteronucleus, for example to 15N. In fact, the 2D 1H,15N-HSQC is the most

popular spectrum in biomolecular NMR because it yields one peak per 1H/15N group.

HMQC (Heteronuclear Multiple-Quantum Correlation) is very similar to the HSQC.

It also correlates 1H to a heteronucleus. It is mostly used for 1H,13C correlations.

HMBC (Heteronuclear Multiple-Bond Correlation) is heteronuclear correlations over

longer ranges of about 2–4 bonds.

Larmor frequency is a nucleus which begins to process about it own or spin with

angular frequency when the magnetic field is applied.

Linebroadening is the linewidth of a NMR signal (resonance line), the transverse

relaxation rate.

Linewidth is width of a NMR resonance line, measured at half-height.

Lorentzian is a function that is obtained by Fourier transformation of an decaying

exponential function.

Paramagnetic relaxation enhancement (PRE) is the effect of a paramagnetic group

(e.g. spin-label) on nuclear spins.

Residual dipolar couplings (RDCs) is dipolar couplings are a through-space interaction

between NMR active nuclei. In solution NMR, these couplings are averaged to zero due

to the fast rotation of the molecule. It is however possible to re-introduce dipolar

couplings by putting the molecule into a liquid medium that has some degree of order

(alignment medium).

Restraint (structural R.) is structural information of NMR spectra is stored in a format

called restraint. A restraint defines a geometric value (a distance between two atoms, an

angle between two bonds) with appropriate upper and lower bounds. In a structure

calculation, the molecule is forced to fulfil the restraint, i.e. the value of the distance or

the angle is restrained to stay within the upper and lower bounds of the restraint.

Spin-label is a paramagnetic group that is attached to a macromolecule.

Spin-system is all nuclei of the same isotope type that are connected via J-couplings. For

example, all protons in an aromatic ring belong to a spin system. More importantly, the

amino acid residue constitutes a spin-system for both carbon and proton, because there

are no homonuclear J-couplings across the peptide bond.

Triple resonance experiments is 3D experiments that connect the resonances of three

different isotopes (1H, 15N, 13C). They are primarily used for protein backbone

assignment.

Solid-state NMR (SSNMR) spectroscopy is a kind of nuclear magnetic resonance

(NMR) spectroscopy, characterized by the presence of anisotropic (directionally

dependent) interactions.

Quadrupolar nucleus is the Nucleus with a spin greater than one-half which have a non

spherical charge distribution.

Magic Angle Spinning (MAS) is a technique often used to perform experiments in solid-

state NMR spectroscopy.