How to reference 1D and 2D NMR spectra? … Part 2

When working with multiple NMR datasets, it is key to set a reference point(s) in order to align all the datasets. The elucidation purpose -- as opposed to standard NMR purposes of referencing to a standard such as TMS -- behind referencing the spectral data to a common point(s) is to facilitate the interpretation and thus minimize any incorrect grouping of data. For heteronuclear 2D NMR experiments, such as 1H-13C HMQC and HMBC experiments, two reference points are needed. Typically, an elucidator selects a peak from a 1H and a 13C NMR spectrum as the reference points and the 2D NMR data is adjusted accordingly. If a 13C NMR spectrum is not available, as is the case sometimes, then a correlation on an HSQC or HMQC experiment can serve as the stand-in reference point.

From the 1H NMR spectrum below, the resonance at 5.90 ppm is selected as the reference point for the 1H dimension. The resonance at 5.90 ppm is a good choice for a reference point, as it is isolated from other signals and thus is clearly visible. In the case where there are multiple choices for a reference point, I typically lean towards an intense singlet as the 2D counterpart tend to be easy to locate on a 2D NMR spectrum.

Checking the COSY data, the diagonal correlation at 5.9, 5.9 ppm is properly aligned to the 1D resonance at 5.9 ppm. Therefore, no further data manipulation is needed. The HMQC spectrum also shows good alignment with the 1H resonance. Since there is no 1D 13C NMR spectrum available, the correlation at 122.2 ppm in the HMQC is selected as the 13C reference point for the HMBC. The F1 dimension of the HMBC spectrum needs to be referenced as the carbon at 123.3 ppm is off by 1.1 ppm.

How to reference 1D and 2D NMR spectra? … Part 2

When working with multiple NMR datasets, it is key to set a reference point(s) in order to align all the datasets. The elucidation purpose -- as opposed to standard NMR purposes of referencing to a standard such as TMS -- behind referencing the spectral data to a common point(s) is to facilitate the interpretation and thus minimize any incorrect grouping of data. For heteronuclear 2D NMR experiments, such as 1H-13C HMQC and HMBC experiments, two reference points are needed. Typically, an elucidator selects a peak from a 1H and a 13C NMR spectrum as the reference points and the 2D NMR data is adjusted accordingly. If a 13C NMR spectrum is not available, as is the case sometimes, then a correlation on an HSQC or HMQC experiment can serve as the stand-in reference point.

From the 1H NMR spectrum below, the resonance at 5.90 ppm is selected as the reference point for the 1H dimension. The resonance at 5.90 ppm is a good choice for a reference point, as it is isolated from other signals and thus is clearly visible. In the case where there are multiple choices for a reference point, I typically lean towards an intense singlet as the 2D counterpart tend to be easy to locate on a 2D NMR spectrum.

Checking the COSY data, the diagonal correlation at 5.9, 5.9 ppm is properly aligned to the 1D resonance at 5.9 ppm. Therefore, no further data manipulation is needed. The HMQC spectrum also shows good alignment with the 1H resonance. Since there is no 1D 13C NMR spectrum available, the correlation at 122.2 ppm in the HMQC is selected as the 13C reference point for the HMBC. The F1 dimension of the HMBC spectrum needs to be referenced as the carbon at 123.3 ppm is off by 1.1 ppm.

How to reference 1D and 2D NMR data? … Part 1

Like driving a car, the driver navigates the vehicle within the designated lanes on a roadway, thus using the lane markers as a reference. The same can be said when analyzing spectral data. Correctly referencing multiple NMR data is critical in a structure elucidation process. For the case where 1D and 2D NMR data is present, the typical approach is to reference a correlation in the 2D NMR data to a resonance from a 1D NMR spectrum.

The spectral data below shows two 1H-13C HMBC experiments (optimized for 8 Hz) differing in the reference point. From the correctly referenced HMBC, there are 4 correlations indicating H to C bond separation of 2, 3 or 4 bonds away: 1H resonance 3.71 ppm to carbons 127.1 and 140.5 ppm and 1H resonance 3.86 ppm to carbons 132.7 and 142.1 ppm (illustrated by the green arrows on the structure).

The incorrectly referenced HMBC shows the same 4 correlations, however, the H-C bond separation is unreasonable for data. Although the 2 and 3-bond separation is coincidently workable for the 1H resonance at 4.14 ppm to carbons 127.1 and 140.5 ppm, it is the 1H resonance at 4.29 ppm to the carbon resonances at 132.7 and 142.7 ppm that shows unreasonable 5 and 6-bond separations. Ultimately, if the 2D NMR data is not correctly aligned to the 1D NMR data, then the corresponding fragments most likely cannot be pieced together to complete a structure.

How to reference 1D and 2D NMR data? … Part 1

Like driving a car, the driver navigates the vehicle within the designated lanes on a roadway, thus using the lane markers as a reference. The same can be said when analyzing spectral data. Correctly referencing multiple NMR data is critical in a structure elucidation process. For the case where 1D and 2D NMR data is present, the typical approach is to reference a correlation in the 2D NMR data to a resonance from a 1D NMR spectrum.

The spectral data below shows two 1H-13C HMBC experiments (optimized for 8 Hz) differing in the reference point. From the correctly referenced HMBC, there are 4 correlations indicating H to C bond separation of 2, 3 or 4 bonds away: 1H resonance 3.71 ppm to carbons 127.1 and 140.5 ppm and 1H resonance 3.86 ppm to carbons 132.7 and 142.1 ppm (illustrated by the green arrows on the structure).

The incorrectly referenced HMBC shows the same 4 correlations, however, the H-C bond separation is unreasonable for data. Although the 2 and 3-bond separation is coincidently workable for the 1H resonance at 4.14 ppm to carbons 127.1 and 140.5 ppm, it is the 1H resonance at 4.29 ppm to the carbon resonances at 132.7 and 142.7 ppm that shows unreasonable 5 and 6-bond separations. Ultimately, if the 2D NMR data is not correctly aligned to the 1D NMR data, then the corresponding fragments most likely cannot be pieced together to complete a structure.

Blog on hiatus for 2 weeks

With the birth of my baby girl, I will not be posting any blogs -- except this one -- for at least 2 weeks. In the meantime, I included a little snap shot of the bundle of joy, δ, held by my overjoyed wife. I'm standing in the background appreciating the moment.

Please note that viewer comments will not appear on the blog until I return.

Blog on hiatus for 2 weeks

With the birth of my baby girl, I will not be posting any blogs -- except this one -- for at least 2 weeks. In the meantime, I included a little snap shot of the bundle of joy, δ, held by my overjoyed wife. I'm standing in the background appreciating the moment.

Please note that viewer comments will not appear on the blog until I return.

Hunting for Weak 1H resonances

Among intense NMR resonances, weak resonances can be easily overlooked. This is commonly seen with 1H NMR spectra whereby 1H resonances with complicated couplings are spread out over a larger area in comparison to uncoupled 1H resonances that appear as sharp signals.

The 1H NMR spectrum below shows 3 intense resonances at 2.39, 2.66 and 3.11 ppm. The weak resonances at 2.33 and 3.26 ppm are barely visible. Upon integration, the weak resonances approximate to whole integral numbers relative to the intense resonances.

The region between 3.0 to 3.4 ppm clearly shows a multiplet at 3.26 ppm. The 1H -13C HSQC spectrum shows a correlation between the 1H at 3.26 ppm and the 13C at 31 ppm. The data from a 2D NMR spectrum serves as an additional flag to the elucidator that there is a 1H resonance in this particular region.

Hunting for Weak 1H resonances

Among intense NMR resonances, weak resonances can be easily overlooked. This is commonly seen with 1H NMR spectra whereby 1H resonances with complicated couplings are spread out over a larger area in comparison to uncoupled 1H resonances that appear as sharp signals.

The 1H NMR spectrum below shows 3 intense resonances at 2.39, 2.66 and 3.11 ppm. The weak resonances at 2.33 and 3.26 ppm are barely visible. Upon integration, the weak resonances approximate to whole integral numbers relative to the intense resonances.

The region between 3.0 to 3.4 ppm clearly shows a multiplet at 3.26 ppm. The 1H -13C HSQC spectrum shows a correlation between the 1H at 3.26 ppm and the 13C at 31 ppm. The data from a 2D NMR spectrum serves as an additional flag to the elucidator that there is a 1H resonance in this particular region.

Differentiating Tautomers using 15N chemical shift information

As with many nitrogen-containing compounds, 15N chemical shift information can be critical in elucidating or confirming a candidate structure. Many elucidators extract 15N chemical shifts from such experiments as 15N NMR, 1H-15N HSQC or 1H-15N HMBC.

Below are two tautomers, from a previous blog, that differ in the hybridization state of the nitrogen. The 15N chemical shift for the structure on the left is expected somewhere between 260-300 ppm (referenced to NH3 (liq.)) whereas the 15N chemical shift for the structure on the right is expected around 160-190 ppm.

Differentiating Tautomers using 15N chemical shift information

As with many nitrogen-containing compounds, 15N chemical shift information can be critical in elucidating or confirming a candidate structure. Many elucidators extract 15N chemical shifts from such experiments as 15N NMR, 1H-15N HSQC or 1H-15N HMBC.

Below are two tautomers, from a previous blog, that differ in the hybridization state of the nitrogen. The 15N chemical shift for the structure on the left is expected somewhere between 260-300 ppm (referenced to NH3 (liq.)) whereas the 15N chemical shift for the structure on the right is expected around 160-190 ppm.