Distinguishing Impurities … Part 5

The past few blogs, Part 3 and Part 4, have examined impurity(ies) identification from short-range 2D NMR experiments without much success. If proton singlets, possibly attributed to impurities, are to be distinguished from the main unknown, then long-range 2D NMR experiments may help out by establishing long-range correlations to other atoms.

A 1H-1H TOCSY experiment with a mixing time of 30 ms is shown below. The correlations are colour-coded based on intensity, red for a high intensity and green for a low intensity. The black line spanning across the 2D NMR spectrum indicates the diagonal.

The alleged impurity singlets at 1.68 and 2.14 ppm do not show any long correlations to any of the remaining 1H signals as seen by a lack of any off-diagonal peaks. Although it is still possible that the singlets can pertain to the main unknown through a lack of any ‘neighbouring’ protons to couple to, more data is definitely needed to support/dismiss this claim.

Distinguishing Impurities … Part 5

The past few blogs, Part 3 and Part 4, have examined impurity(ies) identification from short-range 2D NMR experiments without much success. If proton singlets, possibly attributed to impurities, are to be distinguished from the main unknown, then long-range 2D NMR experiments may help out by establishing long-range correlations to other atoms.

A 1H-1H TOCSY experiment with a mixing time of 30 ms is shown below. The correlations are colour-coded based on intensity, red for a high intensity and green for a low intensity. The black line spanning across the 2D NMR spectrum indicates the diagonal.

The alleged impurity singlets at 1.68 and 2.14 ppm do not show any long correlations to any of the remaining 1H signals as seen by a lack of any off-diagonal peaks. Although it is still possible that the singlets can pertain to the main unknown through a lack of any ‘neighbouring’ protons to couple to, more data is definitely needed to support/dismiss this claim.

Distinguishing Impurities … Part 4

Certain NMR experiments offer clues to differentiate a signal from the main unknown and from the impurity(ies). Some clues are not as obvious as others and so it takes a little practice to understand what to look for in a dataset.

A 1H-1H double quantum filter (DQF) COSY experiment (shown below) is used to filter out uncoupled systems. The alleged impurity singlets at 1.68 and 2.14 ppm do not show any diagonal and off-diagonal correlations. As such, this 2D NMR experiment is a bad choice in cases where the impurities exhibit uncoupled proton signals.

Distinguishing Impurities … Part 4

Certain NMR experiments offer clues to differentiate a signal from the main unknown and from the impurity(ies). Some clues are not as obvious as others and so it takes a little practice to understand what to look for in a dataset.

A 1H-1H double quantum filter (DQF) COSY experiment (shown below) is used to filter out uncoupled systems. The alleged impurity singlets at 1.68 and 2.14 ppm do not show any diagonal and off-diagonal correlations. As such, this 2D NMR experiment is a bad choice in cases where the impurities exhibit uncoupled proton signals.

Distinguishing Impurities … Part 3

In the series Distinguishing Impurities, Part 1 pointed to certain signs in which an elucidator can differentiate a signal as pertaining to an impurity and not to the main unknown. Part 1 also made reference to using 2D NMR data as a practical approach to ascertain whether a signal from a 1H NMR was an impurity. With so many choices for 2D NMR experiments, the question is which one(s) will offer the best chance to assist with this method.

The diagram below is a 1H -13C HSQC spectrum related to the 1H NMR spectrum shown in Part 1. Originally flagged as possible impurities based on the 1H NMR data, the singlets at 1.68 and 2.14 ppm show a H-C correlation as do the remaining 1H signals. Based on this example, the HSQC data cannot differentiate any impurities from the main unknown.

Distinguishing Impurities … Part 3

In the series Distinguishing Impurities, Part 1 pointed to certain signs in which an elucidator can differentiate a signal as pertaining to an impurity and not to the main unknown. Part 1 also made reference to using 2D NMR data as a practical approach to ascertain whether a signal from a 1H NMR was an impurity. With so many choices for 2D NMR experiments, the question is which one(s) will offer the best chance to assist with this method.

The diagram below is a 1H -13C HSQC spectrum related to the 1H NMR spectrum shown in Part 1. Originally flagged as possible impurities based on the 1H NMR data, the singlets at 1.68 and 2.14 ppm show a H-C correlation as do the remaining 1H signals. Based on this example, the HSQC data cannot differentiate any impurities from the main unknown.

The 100th blog: a Milestone in Structure Elucidation

Structure elucidation is a skill learnt through years of practice. One of the best ways to learn is to learn by example. I feel that there is no better way to do this than mentoring through specific scenarios step-by-step. My intentions behind this blog are to offer a unique source for chemists who use a variety of tools such as NMR, MS, IR, etc. to determine the structural composition for an unknown.

Here are my top 10 picks for P2C2E that I feel have the most potential to impact a chemist who is learning to do elucidations of unknown compounds.

10. Molecular ion or adduct? - Identification of Adduct Ions

9. Missing a peak? - Hampering Data Interpretation

8. Exercise, exercise - The Coupling Exercise

7. NMR, MS, oh my! - Using complementary elucidation tools to solve for an unknown structure

6. Strong coupling? What’s that? - Interpreting strong coupling information from a 1H NMR spectrum

5. Problem Solver - Gearing up the Right Thought Process for Solving Problems

4. I can’t solve this - Stuck on a Structure Elucidation problem? You need Out of the Box thinking, right?

3.  1 + 1 = 1 - Assembling a set of Fragments to Complete a Candidate Structure

2. I need more data? - Why collect NMR or MS^n data? 

1. A 1H NMR is not enough? - Is it possible to elucidate an unknown with just a 1H NMR spectrum?