Is it possible to elucidate an unknown with just a 1H NMR spectrum? … Part 3

Misconstrued coupling patterns on a 1H NMR spectrum can halt an elucidation process in its track. As such, structural connectivity is best validated with additional data such as a 1H-1H COSY.

A good use of coupling patterns and coupling constants is at the end of the elucidation process. Candidate structures can be verified or eliminated by the use of coupling information.

The following coupling patterns are some examples where the observed coupling pattern is not as simple as tallying the number of observed lines.

 

 

Is it possible to elucidate an unknown with just a 1H NMR spectrum? … Part 3

Misconstrued coupling patterns on a 1H NMR spectrum can halt an elucidation process in its track. As such, structural connectivity is best validated with additional data such as a 1H-1H COSY.

A good use of coupling patterns and coupling constants is at the end of the elucidation process. Candidate structures can be verified or eliminated by the use of coupling information.

The following coupling patterns are some examples where the observed coupling pattern is not as simple as tallying the number of observed lines.

 

 

Is it possible to elucidate an unknown with just a 1H NMR spectrum? … Part 2

Peak shape, and subsequently the observed coupling pattern, is highly dependent on a range of factors such as temperature, concentration, pH, FID processing, etc. Relying solely on a 1H NMR spectrum for structure determination can lead into the risky realm of misinterpreting a coupling pattern. One of the simplest solutions to most elucidations is to acquire a 1H-1H COSY in addition to a 1H NMR spectrum and thus validate the coupled systems.

The 1H NMR spectrum below shows 3 multiplets at 3.1 (CH3), 3.5 (CH2) and 4.0 ppm (CH) with identical J-coupling constants. Does the coupling pattern for the CH2 group at 3.5 ppm indicate a coupling to the methine at 4.0 ppm or the methyl group at 3.1 ppm? To rephrase the question: does the 1H NMR spectrum indicate the presence of a CH-CH2 group or a CH2-CH3 group?

The peak shape for the multiplet at 3.5 ppm (CH2) appears to be a doublet and so can be considered to be coupled to the CH triplet at 4.0 ppm. However, upon closer examination of the doublet, it appears to be a poorly-resolved quartet, and thus, be considered coupled to the CH3 triplet at 3.1 ppm. In conclusion, further spectral data is needed to confirm the coupling system.

Is it possible to elucidate an unknown with just a 1H NMR spectrum? … Part 2

Peak shape, and subsequently the observed coupling pattern, is highly dependent on a range of factors such as temperature, concentration, pH, FID processing, etc. Relying solely on a 1H NMR spectrum for structure determination can lead into the risky realm of misinterpreting a coupling pattern. One of the simplest solutions to most elucidations is to acquire a 1H-1H COSY in addition to a 1H NMR spectrum and thus validate the coupled systems.

The 1H NMR spectrum below shows 3 multiplets at 3.1 (CH3), 3.5 (CH2) and 4.0 ppm (CH) with identical J-coupling constants. Does the coupling pattern for the CH2 group at 3.5 ppm indicate a coupling to the methine at 4.0 ppm or the methyl group at 3.1 ppm? To rephrase the question: does the 1H NMR spectrum indicate the presence of a CH-CH2 group or a CH2-CH3 group?

The peak shape for the multiplet at 3.5 ppm (CH2) appears to be a doublet and so can be considered to be coupled to the CH triplet at 4.0 ppm. However, upon closer examination of the doublet, it appears to be a poorly-resolved quartet, and thus, be considered coupled to the CH3 triplet at 3.1 ppm. In conclusion, further spectral data is needed to confirm the coupling system.

Is it possible to elucidate an unknown with just a 1H NMR spectrum? … Part 1

Yes, it is possible to elucidate an unknown with just a 1H NMR spectrum but only for specific cases*. Case 1: an unknown with a low molecular weight and a “well-resolved” 1H NMR spectrum. Case 2: the unknown has been identified previously, or at least part of it, in a database/library/literature and a spectrum search produced a hit(s).

*Please note that as I write this blog, only two cases come to mind perhaps there are more. If anyone can think of more cases, please feel free to comment so I may expand this blog.

Elucidations based solely on a 1H NMR spectrum can fail when:

-peaks are poorly resolved or crowding each other,

-the 1H NMR spectrum exhibits low signal-to-noise,

-the peaks for the unknown are not clearly distinguished from impurities or artifacts,

 -the 1H NMR spectrum is too complicated,

-the unknown contains few characteristic proton signals and/or numerous heteroatoms.

 

Is it possible to elucidate an unknown with just a 1H NMR spectrum? … Part 1

Yes, it is possible to elucidate an unknown with just a 1H NMR spectrum but only for specific cases*. Case 1: an unknown with a low molecular weight and a “well-resolved” 1H NMR spectrum. Case 2: the unknown has been identified previously, or at least part of it, in a database/library/literature and a spectrum search produced a hit(s).

*Please note that as I write this blog, only two cases come to mind perhaps there are more. If anyone can think of more cases, please feel free to comment so I may expand this blog.

Elucidations based solely on a 1H NMR spectrum can fail when:

-peaks are poorly resolved or crowding each other,

-the 1H NMR spectrum exhibits low signal-to-noise,

-the peaks for the unknown are not clearly distinguished from impurities or artifacts,

 -the 1H NMR spectrum is too complicated,

-the unknown contains few characteristic proton signals and/or numerous heteroatoms.

 

Identifying Peak Overlap on an HMBC Spectrum … Part 2

One of the trickiest parts of interpreting a 1H-13C HMBC is deciding whether a 13C resonance is coinciding with another 13C resonance, i.e. overlapping 13C peaks. A past blog, Part 1, describes a specific case where the possibility of two coinciding 13C resonances can be deciphered based on a high carbon correlation count. Herein, we present a specific case for a 1H-13C HMBC with paired 1J responses to distinguish peak overlap.

The 1H -13C HMBC spectrum below illustrates 2 protons multiplets at 1.89 and 2.37 ppm and 2 carbon resonances at 26.8 and 43.0 ppm. The paired 1J responses at 43.0 ppm indicate a CH2 group with the proton multiplet at 1.89 ppm. Similarly, the carbon at 26.8 ppm shows a paired 1J response with the proton multiplet at 2.37 ppm (CH group). In addition, a single correlation at exactly 2.37, 26.8 ppm indicates a quaternary carbon coinciding with the carbon resonance for the CH group.

The quaternary carbon and CH group are in the vicinity of each other.

Identifying Peak Overlap on an HMBC Spectrum … Part 2

One of the trickiest parts of interpreting a 1H-13C HMBC is deciding whether a 13C resonance is coinciding with another 13C resonance, i.e. overlapping 13C peaks. A past blog, Part 1, describes a specific case where the possibility of two coinciding 13C resonances can be deciphered based on a high carbon correlation count. Herein, we present a specific case for a 1H-13C HMBC with paired 1J responses to distinguish peak overlap.

The 1H -13C HMBC spectrum below illustrates 2 protons multiplets at 1.89 and 2.37 ppm and 2 carbon resonances at 26.8 and 43.0 ppm. The paired 1J responses at 43.0 ppm indicate a CH2 group with the proton multiplet at 1.89 ppm. Similarly, the carbon at 26.8 ppm shows a paired 1J response with the proton multiplet at 2.37 ppm (CH group). In addition, a single correlation at exactly 2.37, 26.8 ppm indicates a quaternary carbon coinciding with the carbon resonance for the CH group.

The quaternary carbon and CH group are in the vicinity of each other.