Interpreting an HSQC or HMQC or HETCOR experiment

The 1H-13C HMQC, HSQC, DEPT-HSQC, HSQC-TOCSY and HETCOR experiments offer the elucidator information on the proton-carbon connectivity. The interpretation process comes down to 3 basic assignments: the correlation belongs to a methyl, methylene or methine carbon. A methyl or methine carbon exhibits at most a single correlation between the 1H and 13C axes. A methylene group can exhibit 1 or 2 correlations depending on whether the protons are anisochronous.

Three regions of a 1H -13C HSQC spectrum for quinine are shown below.

A single 1H-13C correlation is observed for a CH and CH3.

For anisochronous methylene protons, 2 correlations are observed.

Interpreting an HSQC or HMQC or HETCOR experiment

The 1H-13C HMQC, HSQC, DEPT-HSQC, HSQC-TOCSY and HETCOR experiments offer the elucidator information on the proton-carbon connectivity. The interpretation process comes down to 3 basic assignments: the correlation belongs to a methyl, methylene or methine carbon. A methyl or methine carbon exhibits at most a single correlation between the 1H and 13C axes. A methylene group can exhibit 1 or 2 correlations depending on whether the protons are anisochronous.

Three regions of a 1H -13C HSQC spectrum for quinine are shown below.

A single 1H-13C correlation is observed for a CH and CH3.

For anisochronous methylene protons, 2 correlations are observed.

Anisochronous Protons on a 1H NMR Spectrum

In NMR, nuclei can be classified as isochronous or anisochronous. “Where diastereotopic protons show the same chemical shift, they are said to be accidentally equivalent or isochronous, and where they have different chemical shifts the protons are described as anisochronous.” Stereochemistry by David G. Morris, Royal Society of Chemistry (Great Britain) Published by Royal Society of Chemistry, 2001.

The 1H NMR spectrum is shown for geminal protons, coloured in red, within a ring system. The spectrum illustrates protons from a methylene group (CH2) with different chemical shifts that are coupled to each other. The protons are termed as diastereotopic and anisochronous.

Other examples of this property are seen at the following links: DEPT-HSQC, HSQC/HMBC and 1H/DEPT-HSQC.

Anisochronous Protons on a 1H NMR Spectrum

In NMR, nuclei can be classified as isochronous or anisochronous. “Where diastereotopic protons show the same chemical shift, they are said to be accidentally equivalent or isochronous, and where they have different chemical shifts the protons are described as anisochronous.” Stereochemistry by David G. Morris, Royal Society of Chemistry (Great Britain) Published by Royal Society of Chemistry, 2001.

The 1H NMR spectrum is shown for geminal protons, coloured in red, within a ring system. The spectrum illustrates protons from a methylene group (CH2) with different chemical shifts that are coupled to each other. The protons are termed as diastereotopic and anisochronous.

Other examples of this property are seen at the following links: DEPT-HSQC, HSQC/HMBC and 1H/DEPT-HSQC.

Assembling a set of Fragments to Complete a Candidate Structure

The progression of a structure elucidation process is to examine the experimental data, compare the results to literature if possible, build a set of fragments based on the available data and finally assemble the fragments until a candidate structure(s) is reached. The assembly part is very much like working on a jigsaw puzzle. When all the pieces are present, it is as easy as matching up the right pieces by examining the overlap and/or trying out different combinations.

The following fragments #1-5 represent all the available information extracted from a set of experimental data. The letter A represents an open point of attachment.

The goal of the elucidator is to sort out what pieces belong together and which do not. Although more than one answer is possible, a candidate structure, 6-{[5-(trifluoromethyl)cyclopenta-1,4-dien-1-yl]methyl}-2,3-dihydropyridin-4(1H)-one, is colour-coded to illustrate how the fragments fit together to complete the structure.

Assembling a set of Fragments to Complete a Candidate Structure

The progression of a structure elucidation process is to examine the experimental data, compare the results to literature if possible, build a set of fragments based on the available data and finally assemble the fragments until a candidate structure(s) is reached. The assembly part is very much like working on a jigsaw puzzle. When all the pieces are present, it is as easy as matching up the right pieces by examining the overlap and/or trying out different combinations.

The following fragments #1-5 represent all the available information extracted from a set of experimental data. The letter A represents an open point of attachment.

The goal of the elucidator is to sort out what pieces belong together and which do not. Although more than one answer is possible, a candidate structure, 6-{[5-(trifluoromethyl)cyclopenta-1,4-dien-1-yl]methyl}-2,3-dihydropyridin-4(1H)-one, is colour-coded to illustrate how the fragments fit together to complete the structure.

Differentiating M+ and M+H+ ions in an ESI+ MS experiment

In electrospray ionization MS (ESI-MS), ions are produced by the addition of a proton ([M+H]+). However, in cases where the analyte molecule is already charged, e.g. quaternary amine salts, the resulting ion may be an M+ ion.

Two ESI+ mass spectra are shown below. To test whether the molecular ion is M+ or M+H+, deuterium is added to the sample and the data is recollected. If the ion peak does not change in position relative to the original MS, the ion peak is considered an M+ and not M+H+.

Note: the deuterium exchange test may also occur with OH/NH/SH groups too. It is best to have a fair idea of the structure prior to performing this test.