April 9, 2008
by Arvin Moser, Team Manager, Application Scientists, ACD/Labs
Here is a lesson I learnt over time while working on small molecules—my Elucidation Evolution.
Thinking back to when I started doing elucidations of unknowns, my mindset was to collect loads of data (NMR, MS, IR, etc.) whether I needed it or not. Initially inexperienced, I was extracting bits and pieces of information from various datasets and building up a list of fragments that needed to be combined together to form a candidate structure. Although this offered a means to practice and learn how to interpret a wide array of data, it was not an efficient approach to an elucidation.
Please note that there is value in collecting additional data to confirm or verify a candidate structure. However, for elucidation purposes, one should maximize data extraction while minimizing data collection.
As my skills grew, I felt more comfortable working with less data. I began to maximize the information I could extract from a simple 1H NMR, thus avoiding the need to collect and analyze “duplicate data”. (An example of duplicate data is a 1H NMR and a 1H-1H COSY.) Although 1H-1H COSY data could be valuable in cases with complicated resonances and a high degree of signal overlap, a 1H NMR could suffice in extracting structural information.
For the purpose of an elucidation, one should extract the following bits of information from a 1H NMR spectrum:
presence of proton resonances,
spectrum purity (identifying impurities or mixtures),
identifying aromatic and/or aliphatic protons,
presence of exchangeable protons,
chemical shifts,
proton count from integrals,
presence of symmetry,
coupling patterns and constants,
second-order effects.
TIP: Every bit of information helps. A negative result is also telling you something.