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.

Searching for Unknowns via the Internet

Searching for unknowns across an internal library or database can serve as a major time saver. A more accessible database is the Internet. Many chemists and elucidators search the World Wide Web using a mass, a molecular formula or a fragment.

Be aware that proprietary data sent over the Internet runs the risk of being intercepted.

Searching the Internet via Google™ for the molecular formula C17H11N5 produced 787 hits with Letrozole appearing in the first dozen hits or so. Using a variation of the molecular formula C17 H11 N5 with the elements spaced out produced over 34,000 hits without any clear reference to Letrozole.

Searching for Unknowns via the Internet

Searching for unknowns across an internal library or database can serve as a major time saver. A more accessible database is the Internet. Many chemists and elucidators search the World Wide Web using a mass, a molecular formula or a fragment.

Be aware that proprietary data sent over the Internet runs the risk of being intercepted.

Searching the Internet via Google™ for the molecular formula C17H11N5 produced 787 hits with Letrozole appearing in the first dozen hits or so. Using a variation of the molecular formula C17 H11 N5 with the elements spaced out produced over 34,000 hits without any clear reference to Letrozole.

Milestone for P2C2E

February 1, 2009 marked the 1 year anniversary of the weblog with the first post on ‘The Intricacies of solving for an unknown structure using solution NMR data’. The blog went live on February 15, 2008 with the second blog ‘Dealing with Mixture in a 13C NMR spectrum’.

 

I would like to thank all the blog readers; I truly appreciate the support and dedication.

  

As of January 17, 2009, the web counter marked 14,968 hits with an average of ~46 hits per day. January 18, 2009 had the most hits at 218.