Distinguishing Impurities … Part 2

A previous blog described how integrals and coupling information can assist in discerning an unknown structure signal from an impurity(ies).  Before proceeding to acquire 2D NMR data to confirm the information, there is the option to purify/rewash the sample (assuming there is enough sample to do this) followed by a spectral comparison of the before and after the wash; any differences in the integrals may assist in identifying signals from an impurity.

Illustrated below are 2 1H NMR spectra differing in sample preparation. The top spectrum, washed once before acquisition, shows 2 impurity signals at 1.68 and 2.14 ppm. The bottom spectrum is for the same sample but washed a second time. When comparing the 2 spectra, only the 2 singlets have changed significantly in area, thus, indicating a separate entity in relation to the remaining signals.  

One advantage of knowing which signals belong to an unknown structure  is that these signals, and only these signals, are used in searching across a library or database. This will help in reducing false positives and allow more flexibility to narrow down the number of hits.

Distinguishing Impurities … Part 2

A previous blog described how integrals and coupling information can assist in discerning an unknown structure signal from an impurity(ies).  Before proceeding to acquire 2D NMR data to confirm the information, there is the option to purify/rewash the sample (assuming there is enough sample to do this) followed by a spectral comparison of the before and after the wash; any differences in the integrals may assist in identifying signals from an impurity.

Illustrated below are 2 1H NMR spectra differing in sample preparation. The top spectrum, washed once before acquisition, shows 2 impurity signals at 1.68 and 2.14 ppm. The bottom spectrum is for the same sample but washed a second time. When comparing the 2 spectra, only the 2 singlets have changed significantly in area, thus, indicating a separate entity in relation to the remaining signals.  

One advantage of knowing which signals belong to an unknown structure  is that these signals, and only these signals, are used in searching across a library or database. This will help in reducing false positives and allow more flexibility to narrow down the number of hits.

Distinguishing Impurities … Part 1

One of the trickiest parts of interpreting NMR data is identifying what signal belongs to the unknown compound and what can be classified as an impurity. In this case, impurities are described as extra signals coming from a solvent(s), a side product(s), extraction/purification process, etc. 

The 1H NMR spectrum below is for an unknown compound. The spectrum shows a highly coupled system with the exception of two singlets at 2.14 and 1.68 ppm. In addition, the integrals of the 2 singlets show a relative integral of 1 proton. This indicates the possibility of 2 CH groups adjacent to 3 quaternary carbons. Although is not necessarily incorrect, it does raise 2 yellow flags that either singlet (or both) does not belong to the unknown. The next step would be to check this possibility against 2D NMR data.

Distinguishing Impurities … Part 1

One of the trickiest parts of interpreting NMR data is identifying what signal belongs to the unknown compound and what can be classified as an impurity. In this case, impurities are described as extra signals coming from a solvent(s), a side product(s), extraction/purification process, etc. 

The 1H NMR spectrum below is for an unknown compound. The spectrum shows a highly coupled system with the exception of two singlets at 2.14 and 1.68 ppm. In addition, the integrals of the 2 singlets show a relative integral of 1 proton. This indicates the possibility of 2 CH groups adjacent to 3 quaternary carbons. Although is not necessarily incorrect, it does raise 2 yellow flags that either singlet (or both) does not belong to the unknown. The next step would be to check this possibility against 2D NMR data.

Exchangeables Protons acquired in different Deuterated Solvents

A deuterated solvent can impact whether exchangeable protons, such as OH or NH, are visible on a 1H NMR spectrum. The advantage of eliminating any contribution from an exchangeable proton(s) is to simplify spectral interpretation. One disadvantage of not seeing exchangeable protons is the deficiency to the total proton count in establishing the molecular formula.

The diagram below compares two 1H NMR spectra for the same functional group in two different deuterated solvents. The 1H NMR spectrum in deuterated-DMSO shows two doublets whereas the spectrum acquired in deuterated-methanol shows only the singlet for the CH group.

Exchangeables Protons acquired in different Deuterated Solvents

A deuterated solvent can impact whether exchangeable protons, such as OH or NH, are visible on a 1H NMR spectrum. The advantage of eliminating any contribution from an exchangeable proton(s) is to simplify spectral interpretation. One disadvantage of not seeing exchangeable protons is the deficiency to the total proton count in establishing the molecular formula.

The diagram below compares two 1H NMR spectra for the same functional group in two different deuterated solvents. The 1H NMR spectrum in deuterated-DMSO shows two doublets whereas the spectrum acquired in deuterated-methanol shows only the singlet for the CH group.

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.