Nuclear magnetic resonance spectroscopy of stereoisomers most commonly known as NMR spectroscopy of stereoisomers is a

chemical analysis Analytical chemistry studies and uses instruments and methods to separate, identify, and quantify matter. In practice, separation, identification or quantification may constitute the entire analysis or be combined with another method. Separati ...

method that uses

NMR spectroscopy Nuclear magnetic resonance spectroscopy, most commonly known as NMR spectroscopy or magnetic resonance spectroscopy (MRS), is a spectroscopic technique to observe local magnetic fields around atomic nuclei. The sample is placed in a magnetic fiel ...

to determine the

absolute configuration Absolute configuration refers to the spatial arrangement of atoms within a chiral molecular entity (or group) and its resultant stereochemical description. Absolute configuration is typically relevant in organic molecules, where carbon is bonded ...

stereoisomers In stereochemistry, stereoisomerism, or spatial isomerism, is a form of isomerism in which molecules have the same molecular formula and sequence of bonded atoms (constitution), but differ in the three-dimensional orientations of their atoms in ...

. For example, the ''cis'' or ''trans''

alkene In organic chemistry, an alkene is a hydrocarbon containing a carbon–carbon double bond. Alkene is often used as synonym of olefin, that is, any hydrocarbon containing one or more double bonds.H. Stephen Stoker (2015): General, Organic, an ...

s, ''R'' or ''S''

enantiomer In chemistry, an enantiomer ( /ɪˈnænti.əmər, ɛ-, -oʊ-/ ''ih-NAN-tee-ə-mər''; from Ancient Greek ἐνάντιος ''(enántios)'' 'opposite', and μέρος ''(méros)'' 'part') – also called optical isomer, antipode, or optical ant ...

s, and ''R,R'' or ''R,S''

diastereomer In stereochemistry, diastereomers (sometimes called diastereoisomers) are a type of stereoisomer. Diastereomers are defined as non-mirror image, non-identical stereoisomers. Hence, they occur when two or more stereoisomers of a compound have dif ...

s.Frank J. Hollis. "NMR Through the Looking Glass: Uses of NMR Spectroscopy in the Analysis and Synthesis of Chiral Pharmaceuticals." 1994

/ref> In a mixture of enantiomers, these methods can help quantify the

optical purity In stereochemistry, enantiomeric excess (ee) is a measurement of purity used for chiral substances. It reflects the degree to which a sample contains one enantiomer in greater amounts than the other. A racemic mixture has an ee of 0%, while a sing ...

by integrating the area under the NMR peak corresponding to each stereoisomer. Accuracy of integration can be improved by inserting a

chiral derivatizing agent A chiral derivatizing agent (CDA) also known as a chiral resolving reagent, is a chiral auxiliary used to convert a mixture of enantiomers into diastereomers in order to analyze the quantities of each enantiomer present within the mix. Analysis can ...

with a nucleus other than hydrogen or carbon, then reading the heteronuclear NMR spectrum: for example

fluorine-19 NMR Fluorine-19 nuclear magnetic resonance spectroscopy (fluorine NMR or 19F NMR) 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 ...

phosphorus-31 NMR 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 (as ...

Mosher's acid Mosher's acid, or α-methoxy-α-trifluoromethylphenylacetic acid (MTPA) is a carboxylic acid which was first used by Harry Stone Mosher as a chiral derivatizing agent. It is a chiral molecule, consisting of ''R'' and ''S'' enantiomers. Applicati ...

contains a -CF₃ group, so if the adduct has no other fluorine atoms, the ¹⁹F NMR of a racemic mixture shows just two peaks, one for each stereoisomer. As with NMR spectroscopy in general, good resolution requires a high

signal-to-noise ratio Signal-to-noise ratio (SNR or S/N) is a measure used in science and engineering that compares the level of a desired signal to the level of background noise. SNR is defined as the ratio of signal power to the noise power, often expressed in deci ...

, clear separation between peaks for each stereoisomer, and narrow line width for each peak. Chiral lanthanide shift reagents cause a clear separation of chemical shift, but they must be used in low concentrations to avoid

line broadening A spectral line is a dark or bright line in an otherwise uniform and continuous spectrum, resulting from emission or absorption of light in a narrow frequency range, compared with the nearby frequencies. Spectral lines are often used to ident ...

Methods

Karplus equation The Karplus equation, named after Martin Karplus, describes the correlation between 3J-coupling constants and dihedral torsion angles in nuclear magnetic resonance spectroscopy: :J(\phi) = A \cos^2 \phi + B \cos\,\phi + C where ''J'' is the 3'' ...

Chiral derivatizing agent A chiral derivatizing agent (CDA) also known as a chiral resolving reagent, is a chiral auxiliary used to convert a mixture of enantiomers into diastereomers in order to analyze the quantities of each enantiomer present within the mix. Analysis can ...

**Chiral solvating agent **Chiral lanthanide shift reagent (e.g. Eufod) * NMR database method

Methods

References

See also