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ACD/Labs Blog

Arvin Moser
A 1H-1H COSY dataset provides information on coupled spin systems. A coupled spin system does not necessarily imply that the protons are bonded to only carbon atoms . An example of an exchangeable proton (OH) coupled to a proton from a CH group is shown below. Three spectral datasets, 1H NMR, HMQC and a COSY-45...

Arvin Moser
Although peak crowding can be a nuisance, a monosubstituted benzene fragment can be identified by a 1H NMR. A good marker for a monosubstituted benzene ring, and thus how an elucidator can clue in to its presence for an unknown, is whether the sum of the relative integrals for the aromatic resonances add up to...

Arvin Moser
Methoxy groups have a distinct NMR signature that make them easy to spot in virtually every case. The basic pattern is a singlet integrating to 3 and ranging between 2.4 to 4.4 ppm on a 1H NMR spectrum (assuming no overlap with other resonances). On the 13C end, a 13C resonance is seen between 46...

Arvin Moser
An integral part of an elucidation is to recognize and associate an NMR spectral pattern to a structure or fragment. A challenging elucidation can be simplified by being aware of as many as possible NMR patterns. One example is morpholine; it typically shows a distinct NMR pattern. A portion of the 1H-13C DEPT-HSQC spectrum for...

Arvin Moser
Ideally, identifying whether a fluorine atom(s) is present is as simple as acquiring a 19F NMR. However, running ‘uncommon’ experiments is not usually my first step in an elucidation (MDE). Imagine a scenario where the elucidator is unsure whether fluorine is present. He/she acquires a 19F NMR and sees no 19F resonances. Therefore the extra...

Arvin Moser
When faced with a crowded region in a 1H NMR spectrum, a 2D NMR experiment can assist in removing the ambiguity and in narrowing down the proton count. For the 1H NMR spectrum below, Multiplet F (the region between 1.4 to 1.8 ppm) displays an integral of 6.71. Although several factors may contrbute to the...

Arvin Moser
A very useful bit of information on the proton count comes from integrating a 1H NMR spectrum. However, overlapping resonances can make integration less than straightforward and in some cases ambiguous. Integration involves the following decisions to be made by the elucidator: which resonances to integrate, where the integral begins and ends, and what the...

Arvin Moser
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...

Arvin Moser
The second-order effects (also known as tilting or roofing) exhibited by multiplets can be used to identify multiplets coupled to each other. The 1H NMR spectrum below illustrates an example of strong coupling among three multiplets. The peak intensities across Multiplets A and B are different, that is, the peak on the right side of...

Arvin Moser
A window function (also known as apodization function or convolution) is applied to an FID to emphasize a region of the FID over another. The general goal is to improve the SNR (Signal-to-noise) ratio. The early stages of processing such as applying the “correct” window function can affect the time spent on an elucidation especially...