Ibuprofen

Ibuprofen

courtesy.....http://www.magritek.com/

1H spectrum


The simplified 2-dimensional molecular structure of the common NSAID (non-steroidal anti-inflammatory drug) Ibuprofen is shown below, along with the 1-Dimensional proton (1H) NMR spectrum of this compound. They hydrogen atoms are colored, to correspond with the peaks in the NMR spectrum.


The protons that are different (chemically speaking) have different NMR frequencies because the chemical environment causes the local magnetic field for that nucleus to be unique. The "PPM" axis is used to report frequency differences that are scaled by the magnetic field strength. This allows the peak positions to be compared for spectra acquired at various magnetic field strengths, without having to do any conversions. Zero, on this PPM axis, is the frequency position (orchemical shift) of a compound called Tetramethyl Silane or TMS.
The NMR signals from the various chemically unique protons appear not as single peaks, but as multiplets of peaks. This is because the spins of the hydrogen nuclei bonded to neighboring carbon atoms perturb the energy of the NMR signals. This effect is often called "J-coupling," "spin-spin splitting," or "scalar coupling." For example, the CH hydrogen (shown in green) is affected by six equivalent CH3 (red) hydrogen nuclei, and two equivalent (dark blue) CH2 hydrogens. Because there are eight hydrogen atoms on the neighboring carbon atoms, the NMR signal from the (green) CH is split into 9 different individual peaks (or a nonet). This follows the so-called "n+1" rule, which is over-simplified, but often taught to beginning students learning NMR spectroscopy. The (purple) CH that is adjacent to the (cyan) CH3 group appears as 4 peaks (or a quartet) because of the 3 hydrogens on the neighboring carbon. Click on each peak above to see an expanded view of the NMR signal.

Above is an NMR Spectrum of Ibuprofen measured by Spinsolve. Below is a 2D COSY of the same sample taken in just 8 minutes. No special NMR knowledge was required, a single click was required by the operator. 

13 C NMR

125 MHz 13C NMR spectrum with broadband 1H decoupling and molecular structure of the non-steroidal anti-inflammatory drug ibuprofen dissolved in DMSO-d6.

MASS

IR

See how the 2D COSY enables the small coupling peaks of the proton at position 10 to become apparent, even though they are hidden in the 1D proton spectrum. This shows how the power of 2D COSY NMR is particularly important for benchtop NMR spectrometers. 

Two-dimensional 600 MHz 
1H–1H COSY NMR spectrum and molecular structure of the non-steroidal anti-inflammatory drug ibuprofen dissolved in DMSO-d6.

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1H COSY

1H homonuclear j-resolved 2D spectrum

T1 analysis

T2 analysis

13C spectrum

13C-1H HETCOR

1H-13C HSQC

1H-13C HMQC

1H-13C HMBC

COSY, HETCOR, etc spectrum of Ethyl-2-butenoate.

Ethyl-2-butenoate

1H-NMR proton decoupled spectrum of Ethyl-2-butenoate in CDCl3.

1H-NMR proton coupled spectrum of Ethyl-2-butenoate in CDCl3.

13C-NMR proton decoupled spectrum of Ethyl-2-butenoate in CDCl3.

DEPT spectrum of Ethyl-2-butenoate

COSY spectra

• The information on the H that are coupling with each other is obtained by looking at the peaks inside the grid.  These peaks are usually shown in a contour type format, like height intervals on a map.
• In order to see where this information comes from, let’s consider an example shown below, the COSY of ethyl 2-butenoate 
• First look at the peak marked A in the top left corner.  This peak indicates a coupling interaction between the H at 6.9 ppm and the H at 1.8 ppm.  This corresponds to the coupling of the CH3 group and the adjacent H on the alkene.
• Similarly, the peak marked B indicates a coupling interaction between the H at 4.15 ppm and the H at 1.25 ppm.  This corresponds to the coupling of the CH2 and the CH3 in the ethyl group.
• Notice that there are a second set of equivalent peaks, also marked A and Bon the other side of the diagonal.

(COSY spectra recorded by D. Fox, Dept of Chemistry, University of Calgary on a Bruker Advance DRX-400 spectrometer)

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HETCOR spectra

• The information on how the H are C are matched is obtained by looking at the peaks inside the grid.  Again, these peaks are usually shown in a contour type format, like height intervals on a map.
• In order to see where this information comes from, let’s consider an example shown below, the HETCOR of ethyl 2-butenoate.
• First look at the peak marked A near the middle of the grid.  This peak indicates that the H at 4.1 ppm is attached to the C at 60 ppm.  This corresponds to the -OCH2- group.
• Similarly, the peak marked B towards the top right in the grid indicates that the H at 1.85 ppm is attached to the C at17 ppm.  Since the H is a singlet, we know that this corresponds to the CH3- group attached to the carbonyl in the acid part of the ester and not the CH3- group attached to the -CH2- in the alcohol part of the ester.
• Notice that the carbonyl group from the ester has no “match” since it has no H attached in this example.

(HETCOR spectra recorded by D. Fox, Dept of Chemistry, University of Calgary on a Bruker Advance DRX-400 spectrometer)

Ethyl crotonate

1H spectrum

1H COSY

1H homonuclear j-resolved 2D spectrum

T1 analysis

T2 analysis

13C spectrum

13C-1H HETCOR

1H-13C HSQC

1H-13C HMQC

1H-13C HMBC

COSY------------ETHYL ACETATE

HMBC (multiple-bond CH correlation) of codeine

HMBC   (multiple-bond CH correlation) of codeine

This is a 2D experiment used to correlate, or connect, ¹H and ¹³C peaks for atoms separated by multiple bonds (usually 2 or 3).  The coordinates of each peak seen in the contour plot are the ¹H and ¹³C chemical shifts.  This is extremely useful for making assignments and mapping out covalent structure.

The information obtained is an extension of that obtained from an HMQC spectrum, but is more complicated to analyze.  Like HMQC, this is an "inverse detection" experiment, and is possible only on newer model spectrometers.  Acorn NMR's new JEOL Eclipse⁺ 400 is equipped to perform inverse experiments, and uses Z-gradients for improved spectral quality.

The time required for an HMBC depends on the amount of material, but is much greater than for HMQC, and can take from an hour to overnight.

The contour plot shown below is of 3.3 mg codeine in ~ .65 ml CDCl_3.  See also comparison to the HMBC spectrum of an 18 mg sample.

Normal 1D ¹H and ¹³C spectra are shown along the edges.  Peaks occur at coordinates in the 2 dimensions corresponding to the chemical shifts of a carbon and protons separated by (usually) 2 or 3 bonds.  The experiment is optimized for couplings of ~8 Hz.  Smaller couplings are observed, but their intensities are reduced.  Compare to the spectrum obtained when the experiment is optimized for 4 Hz.

The experiment is designed to suppress 1-bond correlations, but a few are observed in most spectra.  In concentrated samples of conjugated systems, 4-bond correlations can be observed.  There is no way to know how many bonds separate an H and C when a peak is observed, so analysis is a process of attempting to assign all observed peaks, testing for consistency and checking to be sure none of the assignments would require implausible or impossible couplings.

Because of the large number of peaks observed, analysis requires several expanded plots.  In this case, the spectrum has been divided into 4 sections, each of which is discussed below.

The discussion below uses the numbering system shown at right.  The numbers were assigned to peaks in the 1D 13C spectrum, starting downfield, moving upfield, and numbering each sequentially.   This generates a unique identifier for each Carbon, even before knowing any assignments.

In aromatic rings, the most common correlations seen in HMBC spectra are 3-bond correlations because they are typically 7-8 Hz, which is the value for which the experiment is optimized.  The coupling constant is affected by substituents, so 2-bond correlations are also sometimes observed.  

The red lines in the plot above show correlations from aromatic proton H-8 to aromatic carbons C-1 and C-6 (both are 3-bond couplings) and a weak correlation to C-2, a 2-bond coupling.

The other aromatic proton, H-7, has correlations to C-2 and C-4, both of which are 3-bond couplings.

The green lines in the plot above show correlations from proton H-9 to carbons C-1, C-3 and C-4 (all are 3-bound couplings).  With the poor digital resolution of the spectrum in the carbon dimension (512 data points spread over 17kHz), the peaks for C-3 and C-4 run together because they are barely resolved.

The peaks indicated in red above are due to 1-bond coupling in CHCl₃ solvent.  Note that the pair of peaks don't line up with any H peaks, but are symmetrically located about the CHCl₃ peak, with a separation equal to the 1-bond C-H coupling constant.

The other 2 peaks in this plot are H-7 to C-18 and H-9 to C-17.

The region in the above plot shows correlations between aliphatic Hs and aromatic Carbons.  In the lower left corner is a peak showing the 3-bond coupling between the methoxy Hs and C-2, the aromatic carbon bearing the methoxy.

Two protons show correlations to the same 3 carbons.  These are the geminal H-18 protons, showing coupling to C-7 and C-4 (both 3-bond couplings) and C-6 (2-bond coupling).  The remaining peak is H-17 to C-4.  Note that the corresponding coupling from H-17' is not observed.  As with H-H couplings, the value of the 3-bond coupling constant is dependent on the dihedral angle.

The last quadrant is shown above, showing correlations between aliphatic Hs and Cs.  The peaks are identified below, for each H, starting from the left.  Carbons 14 and 15 are only 0.1 ppm apart, and are not resolved in the 2D spectrum.

H-11 to C-14 and/or C-15
H-18 to C-16, C-11
H-13 to C-17, C-14 and/or C-15, C-11
H-14 to C-13 and C-11
H-18' to C-11
H-17 to C14 and/or C-15, C-13

During the process of assigning HMBC peaks, it can be useful to indicate on the plot the positions of the 1-bond correlations.  NUTS has the ability to do this, using the compare command.  In the HMBC plot below, 1-bond HMQC peaks are indicated by X.

Acquisition Parameters:

512 complex points in direct dimension
128 t₁ increments
8 scans
2 sec. relaxation delay
Total acquisition time: 35 min

Processing:

sine squared window function in both dimensions with 0 degree phase shift in t₂ and 90 degree phase shift in t₁
2x zero-fill in the indirect dimension
magnitude calculation (no phasing is required)
final data size 512 x 512

HMBC  of codeine optimized for small couplings

See explanation of the HMBC experiment.

Long range C-H couplings cover a range of values, typically less than 10 Hz.  The experiment is optimized for a specific coupling, and selection of that value is a trade-off.  Correlations due to couplings of other values are reduced in intensity.  To optimize for a specific J value, a delay in the pulse sequence is set to 1/(2J).  For very small couplings, this delay becomes so long that much of the magnetization is lost to relaxation.  Typically, the delay is optimized for J of 7-8 Hz, the expected value for aromatic 3-bond couplings.  However, longer-range couplings can be observed if the delay is optimized for smaller couplings.

The contour plots shown below are of 8 mg codeine in ~ .65 ml CDCl₃, in which the experiment was optimized for couplings of 4 Hz.

Compare to the HMBC spectrum optimized for 8 Hz.

Because of the large number of peaks observed, analysis requires several expanded plots.  In this case, the spectrum has been divided into 4 sections, each of which is discussed below.

The discussion below uses the numbering system shown at right.  The numbers were assigned to peaks in the 1D 13C spectrum, starting downfield, moving upfield, and numbering each sequentially.   This generates a unique identifier for each Carbon, even before knowing any assignments.

Compare to the same quadrant in the HMBC optimized for 8 Hz.  Several additional 2-bond couplings are observed, as are 1-bond couplings for 7 and 8.  The peaks indicated by red circles are 4-bond couplings: H-8 to C-4 and H-7 to C-1.  

Compared to the 8 Hz-optimized spectrum, one additional peak is observed in this quadrant, indicated by the red circle.  This is a 4-bond coupling from H-7 to C-15.  

In this quadrant, three 4-bond peaks are indicated by red circles (H-18 to C-8 and C-1; H-18' to C-8).  In addition, two 5-bond couplings are indicated by blue circles.  These are H-18 and H-18' to C-2.

The only additional correlations in the quadrant not seen in the 8 Hz-optimized spectrum are from H-13' (just upfield of the intense N-Me singlet) to carbons 17, 14/15 and 11.

Acquisition Parameters:

512 complex points in direct dimension
128 t₁ increments
64 scans
2 sec. relaxation delay
Total acquisition time: 4.5 hrs

Processing:

sine squared window function in both dimensions with 0 degree phase shift in t₂ and 90 degree phase shift in t₁
2x zero-fill in the indirect dimension
magnitude calculation (no phasing is required)
final data size 512 x 512