Identifying the Molecular Ion using Dimer Information

A previous blog described how a sodiated ion peak can be used to locate or calculate the molecular ion for an unknown compound. In a similar fashion, the dimer ion peak can be used to identify the mass of the unknown even if the molecular ion is no visible.

The ESI+ MS data below shows 4 ion peaks at m/z 401.0, 422.9, 801.0 and 823.0. Assuming the ion peak at m/z 801.0 is 2M+H+, the weight of the unknown is (801.0-1.007) / 2 = 400.0 Da. The same holds true if the sodiated dimer ion (2M+Na+) is used: (823.0-22.989) / 2 = 400.0 Da.

Identifying the Molecular Ion using Dimer Information

A previous blog described how a sodiated ion peak can be used to locate or calculate the molecular ion for an unknown compound. In a similar fashion, the dimer ion peak can be used to identify the mass of the unknown even if the molecular ion is no visible.

The ESI+ MS data below shows 4 ion peaks at m/z 401.0, 422.9, 801.0 and 823.0. Assuming the ion peak at m/z 801.0 is 2M+H+, the weight of the unknown is (801.0-1.007) / 2 = 400.0 Da. The same holds true if the sodiated dimer ion (2M+Na+) is used: (823.0-22.989) / 2 = 400.0 Da.

Extracting Proton Information from 13C NMR data

In addition to using chemical shift information to ascertain a carbon’s proton count (i.e. C, CH, CH2 or CH3), 13C NMR experiments can be set up in a variety of ways to assist with this process.

The following simulated spectra compare a variety of 13C NMR experiments for aspartame. Please note that there are variations to the list below that are not being shown. These include such experiments as DEPTQ-135 and DEPT-HSQCSE (sensitivity enhanced).

 

   

 

   

 

 

Extracting Proton Information from 13C NMR data

In addition to using chemical shift information to ascertain a carbon’s proton count (i.e. C, CH, CH2 or CH3), 13C NMR experiments can be set up in a variety of ways to assist with this process.

The following simulated spectra compare a variety of 13C NMR experiments for aspartame. Please note that there are variations to the list below that are not being shown. These include such experiments as DEPTQ-135 and DEPT-HSQCSE (sensitivity enhanced).

 

   

 

   

 

 

The Coupling Exercise

When presented with an NMR dataset, a highly-experienced elucidator can interpret the dataset without any direct need to do peak picking, integration and multiplet analysis through software. (This may not be the case every time as severe peak overlap can complicate matters.) In essence, this NMR interpretation skill equates to speed reading and can be learned with some practice.

The 1H NMR spectrum below displays 5 multiplets with the chemical shift and integration information removed. In this exercise, can you recognize which multiplets are coupled to each other?

The first step is to roughly estimate the integral values for each multiplet and thus eliminate any notion of overlapping peaks. From the 1H NMR spectrum, there are 5 multiplets roughly integrating to an equal area, i.e. the integral ratio is approximately 1:1:1:1:1.

The next step is to judge the coupling constants for each multiplet, i.e. the spacing between the peaks. Without getting too technical here, a good approach is to eyeball the coupling constants as small, medium and/or large. From this point, it becomes a simple task of matching up the same couplings: A to B (small coupling), A to E (large coupling), and C to D (medium coupling).

 

The Coupling Exercise

When presented with an NMR dataset, a highly-experienced elucidator can interpret the dataset without any direct need to do peak picking, integration and multiplet analysis through software. (This may not be the case every time as severe peak overlap can complicate matters.) In essence, this NMR interpretation skill equates to speed reading and can be learned with some practice.

The 1H NMR spectrum below displays 5 multiplets with the chemical shift and integration information removed. In this exercise, can you recognize which multiplets are coupled to each other?

The first step is to roughly estimate the integral values for each multiplet and thus eliminate any notion of overlapping peaks. From the 1H NMR spectrum, there are 5 multiplets roughly integrating to an equal area, i.e. the integral ratio is approximately 1:1:1:1:1.

The next step is to judge the coupling constants for each multiplet, i.e. the spacing between the peaks. Without getting too technical here, a good approach is to eyeball the coupling constants as small, medium and/or large. From this point, it becomes a simple task of matching up the same couplings: A to B (small coupling), A to E (large coupling), and C to D (medium coupling).

 

Searching for Unknowns via the Internet … Part 2

A previous blog described the Internet as a valuable tool in searching for previous work based on a molecular formula. In addition to searching the Internet “directly”, certain online sites can assist and facilitate the search process. Some of these sites are free while others have a fee.

Be aware that the Internet is full of misleading and often times incorrect data. It is one thing to make a mistake in the elucidation process; it is another to follow someone else’s mistake. As always, the next step is to verify the data and publish once.

Searching for Unknowns via the Internet … Part 2

A previous blog described the Internet as a valuable tool in searching for previous work based on a molecular formula. In addition to searching the Internet “directly”, certain online sites can assist and facilitate the search process. Some of these sites are free while others have a fee.

Be aware that the Internet is full of misleading and often times incorrect data. It is one thing to make a mistake in the elucidation process; it is another to follow someone else’s mistake. As always, the next step is to verify the data and publish once.