Wednesday, July 29, 2009

Distinguishing Impurities … Part 6

Although a 1H-13C HMBC experiment may take a long time to collect with adequate signal-to-noise, it can offer an idea as to which signals belong 'together'. Generally, signals belonging to the same structure leave a trail of connectivity information via long range correlations. If the atoms belong to the same structure, then a typical HMBC pattern is as follows: proton A is correlated to carbon B, carbon B is correlated to proton C, proton C is correlated to carbon D, etc.


The 1H-13C HMBC experiment below shows a lack of long range connectivity information for the singlets at 1.68 and 2.14 ppm to any of the other signals. This leaves two possibilities: either the singlets do not pertain to the main unknown structure or the protons are separated far enough from any carbon atoms to show any correlations.


ImpuritiesOnHMBC_6_Jul282009



Distinguishing Impurities … Part 6

Although a 1H-13C HMBC experiment may take a long time to collect with adequate signal-to-noise, it can offer an idea as to which signals belong 'together'. Generally, signals belonging to the same structure leave a trail of connectivity information via long range correlations. If the atoms belong to the same structure, then a typical HMBC pattern is as follows: proton A is correlated to carbon B, carbon B is correlated to proton C, proton C is correlated to carbon D, etc.


The 1H-13C HMBC experiment below shows a lack of long range connectivity information for the singlets at 1.68 and 2.14 ppm to any of the other signals. This leaves two possibilities: either the singlets do not pertain to the main unknown structure or the protons are separated far enough from any carbon atoms to show any correlations.


ImpuritiesOnHMBC_6_Jul282009



Tuesday, July 21, 2009

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.


ImpuritiesOnTOCSY_5_Jul1212009


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.


ImpuritiesOnTOCSY_5_Jul1212009


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.



Wednesday, July 15, 2009

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.


ImpuritiesOnCOSYTOCSY_4_Jul142009




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.


ImpuritiesOnCOSYTOCSY_4_Jul142009




Tuesday, July 7, 2009

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.


ImpuritiesOnHSQC_3_Jul72009



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.


ImpuritiesOnHSQC_3_Jul72009