Correlating 1D NMR data on an HMBC

There are many challenges present in deciphering correlations on an HMBC experiment. Barring any data collection issues, most of the challenges will arise from signals that are overlapping, thus, adding an element of ambiguity.   

The 1H -13C HMBC spectrum below shows a correlation between the 13C at 71.2 ppm and the overlapping 1H signals at 2.08 (d) and 2.12 (m) ppm. There is nothing usually here, right?

A nearby correlation on the same 1H -13C HMBC spectrum (see below) shows a correlation between the 13C at 75.01 ppm and the 1H at 2.28 ppm. The splitting pattern for this correlation suggests that the previous correlation might also be splitting. If this is the case, then the correlation is centered on 71.26 and 2.10 (m) ppm, and thus, there is no correlation to the doublet at 2.08 ppm. In the end, further analysis of the overlapping multiplets between 2.00-2.15 ppm is needed to resolve the ambiguity.

Correlating 1D NMR data on an HMBC

There are many challenges present in deciphering correlations on an HMBC experiment. Barring any data collection issues, most of the challenges will arise from signals that are overlapping, thus, adding an element of ambiguity.   

The 1H -13C HMBC spectrum below shows a correlation between the 13C at 71.2 ppm and the overlapping 1H signals at 2.08 (d) and 2.12 (m) ppm. There is nothing usually here, right?

A nearby correlation on the same 1H -13C HMBC spectrum (see below) shows a correlation between the 13C at 75.01 ppm and the 1H at 2.28 ppm. The splitting pattern for this correlation suggests that the previous correlation might also be splitting. If this is the case, then the correlation is centered on 71.26 and 2.10 (m) ppm, and thus, there is no correlation to the doublet at 2.08 ppm. In the end, further analysis of the overlapping multiplets between 2.00-2.15 ppm is needed to resolve the ambiguity.

Qualitative Elucidation via TLC Staining

TLC (Thin Layer Chromatography) offers a simple approach to identifying a solvent(s) for separating out a mixture, monitoring a reaction to completion, etc. Furthermore, the TLC plates can be stained to identify the presence or absence of various functional groups such as amines, ketones, etc. This qualitative experiment does offer various drawbacks.

The process of staining typically requires the items shown in the illustration below.

 The following websites offer more insight into the process of staining.

http://webpub.allegheny.edu/employee/s/smurphre/research/visualization.pdf

Org Prep Daily: http://orgprepdaily.wordpress.com/2006/09/27/tlc-staining-solutions/

Curly Arrow Blog: http://curlyarrow.blogspot.com/2006/11/lets-talk-about-tlcs-part-1-vanillin.html

Qualitative Elucidation via TLC Staining

TLC (Thin Layer Chromatography) offers a simple approach to identifying a solvent(s) for separating out a mixture, monitoring a reaction to completion, etc. Furthermore, the TLC plates can be stained to identify the presence or absence of various functional groups such as amines, ketones, etc. This qualitative experiment does offer various drawbacks.

The process of staining typically requires the items shown in the illustration below.

 The following websites offer more insight into the process of staining.

http://webpub.allegheny.edu/employee/s/smurphre/research/visualization.pdf

Org Prep Daily: http://orgprepdaily.wordpress.com/2006/09/27/tlc-staining-solutions/

Curly Arrow Blog: http://curlyarrow.blogspot.com/2006/11/lets-talk-about-tlcs-part-1-vanillin.html

Allene versus Carbonyl

Functional groups such as allene (C=C=C) and cumulene (C=C=C=C) present an interesting challenge in an elucidation project, especially when an elucidator is not expecting them. The 13C chemical shifts for allene carbons are typically expected at 80, 200 and 100 ppm +/- ~20 ppm per shift. The confusion, or better described as bias, arises at treating a 13C chemical shift of 200 ppm as a carbonyl (C=O) rather than considering the possibility of an allene group.

In the example below, there are an odd number of sp2 carbon atoms along with 1 oxygen atom and 6 hydrogen atoms.

The example leads into two possible candidates: an allene in 1-(ethenyloxy)propadiene ethenyl propadienyl ether or an alkene plus a carbonyl group in penta-1,4-dien-3-one.

Allene versus Carbonyl

Functional groups such as allene (C=C=C) and cumulene (C=C=C=C) present an interesting challenge in an elucidation project, especially when an elucidator is not expecting them. The 13C chemical shifts for allene carbons are typically expected at 80, 200 and 100 ppm +/- ~20 ppm per shift. The confusion, or better described as bias, arises at treating a 13C chemical shift of 200 ppm as a carbonyl (C=O) rather than considering the possibility of an allene group.

In the example below, there are an odd number of sp2 carbon atoms along with 1 oxygen atom and 6 hydrogen atoms.

The example leads into two possible candidates: an allene in 1-(ethenyloxy)propadiene ethenyl propadienyl ether or an alkene plus a carbonyl group in penta-1,4-dien-3-one.

Evaluating a Peak’s Line Shape on a Chromatogram

The line shape of a peak on a chromatogram can offer some insight into the presence of a functional group. However, issues with the instrument are also known to affect the line shape for a peak.

The GC/MS EI+ Total Ion Chromatogram/Current (TIC) below shows five peaks. All of the peaks with the exception of the peak at 1.8 minutes exhibit a symmetrical line shape. The asymmetry of the peak at 1.8 minutes is typical for a structure containing an acidic group (i.e. -COOH).

The peak at 1.8 minutes is 2-ethylhexanoic acid.

Evaluating a Peak’s Line Shape on a Chromatogram

The line shape of a peak on a chromatogram can offer some insight into the presence of a functional group. However, issues with the instrument are also known to affect the line shape for a peak.

The GC/MS EI+ Total Ion Chromatogram/Current (TIC) below shows five peaks. All of the peaks with the exception of the peak at 1.8 minutes exhibit a symmetrical line shape. The asymmetry of the peak at 1.8 minutes is typical for a structure containing an acidic group (i.e. -COOH).

The peak at 1.8 minutes is 2-ethylhexanoic acid.