The EFT-90 Spectra of Ibuprofen

The name of Ibuprofen (C₁₃H₁₈O₂) comes from the molecular fragments, iso-butyl, propionic, phenyl. The formal name of this non-steroidal anti-inflammatory drug is (RS)-2-(4-(2-methylpropyl)phenyl)propanoic acid. It is used to treat pain and swelling.

For permanent magnet based NMR instruments, the demonstration sample of choice is the 2M sample of ibuprofen in CDCl₃ due to the insignificant overlapping of the signals and the exhibition of interesting spectral phenomenon. This article discusses ¹H and ¹³C 1D and 2D spectra and their interesting features.

EFT-90 Spectrometers

The resolution capability of the EFT-90 spectrometers is demonstrated by the proton spectrum shown in Figure 1. In all of the peaks, the ¹H-¹H proton coupling is resolved even in the CH at position '8' split by the methylene (H₇) and the two methyls (H₉, H₁₀). At 12.25ppm, the integral for the carboxyl resonance was normalized to 1.00.

Figure 1. ¹H NMR spectrum of 2M Ibuprofen taken with a single scan

¹³C Spectrum of Ibuprofen

Figure 2 shows the ¹³C spectrum collected with 12 scans in one minute, containing nine resonance signals with varying intensity. The carbons without a directly bonded proton (C2, C4, and C7) are represented by weak signals (181.5, 140.7, and 137.0ppm). Carbons with a bonded proton are generally much larger than those without a directly bonded proton.

Figure 2. ¹³C Spectrum of 2M Ibuprofen recorded with 12 scans with a total acquisition time of 1 minute

The chemical structure analysis reveals that carbons 12 and 13 are magnetically and chemically equivalent like the aromatic protonated carbons of the phenyl ring, i.e., carbons 6 and 8 and carbons 5 and 9 are equivalent, due to a line of symmetry between carbon 4 and 7. This line of symmetry describes the approximately 2:1 intensity difference for the resonances at 20, 127 and 130ppm. Further analysis is required for resonance at 45ppm.

Distortion-less Enhancement via Polarization Transfer (DEPT)

Each carbon signal (CH, CH₂, or CH₃) can be differentiated using the ¹³C DEPT experiment, which is called ‘spectral editing’ owing to the manipulation of the signals by the pulse sequence to acquire more data about the chemical structure. Quaternary, methine, methylene and methyl groups can be differentiated using a set of three DEPT experiments.

EFT spectrometers used in the ¹³C DEPT experiment collect DEPT45, DEPT90 and DEPT135 spectra as a single data array. The NUTS processing software can automatically process the acquired spectra. Single experiments are also available such as the DEPT135 and APT (Attached Proton Test) .

As shown in Figure 3, CH peaks have the greatest polarization transfer in the DEPT90 experiment. As a result, the CH peaks in the DEPT45 and DEPT135 spectra must be smaller than the CH peaks in the DEPT90 spectrum. The peak at 45ppm in the DEPT135 spectrum is pointing down, thus representing CH₂.

Figure 3. DEPT 45, 90 and 135 (plotted from bottom to top) spectra of ibuprofen.

Heteronuclear Correlation (HETCOR)

As a two-dimensional experiment, the Heteronuclear Correlation (HETCOR) experiment reveals coupled C-H pairs. The horizontal axis of the contour plot represents the ¹³C spectrum and the vertical axis is the ¹H spectrum.

It is also possible to run this experiment for other X-nuclei. In fact, EFT spectrometers can be used to obtain a ¹⁹F-¹³C correlation spectrum. Long range coupling information greater than 3 bonds CH couplings can be obtained by further modifying the delays in HETCOR.

As shown in Figure 4, the ¹³C peak at 45ppm is two different ¹³C peaks assigned to C11 which is attached to the H falling at 3.7ppm, and C7 which is bonded to the doublet resonance at 2.5ppm. High field spectrometers (600 MHz) can also reveal this accidental degeneracy.

The dissolution of ibuprofen in DMSO eliminates the degeneracy as indicated by a slight < 1ppm chemical shift difference by the resonances. This shows the exotic results of the DEPT experiment as well as the ¹³C one-dimensional spectrum without 10 peaks.

Figure 4. {¹H}-¹³C HETCOR spectrum for 2M ibuprofen at 90 MHz.

About Anasazi Instruments

Anasazi Instruments has been providing high quality, rugged, easy-to-use 60 and 90 MHz NMR spectrometers and upgrades to the educational and industrial markets. These instruments have been successfully implemented at hundreds or institutions ranging from large companies and top-tier universities to community colleges throughout North and South America.

In research environments, the Eft is a cost-effective workhorse for synthetic and analytical laboratories. These permanent magnet based FT-NMR spectrometers have applications in industrial labs for quality testing or as a "walk-up" NMR resource. Crucial to the success of the Eft is that, over the lifetime of the instrument, the total annual cost is fixed, whereas for a supercon-based NMR, annual costs increase.

In education, the Eft gives thousands of undergraduates the hands-on opportunity to learn to acquire and analyze FT-NMR data. Additionally, the wide appeal of the Eft spectrometer is due to the ease of obtaining high quality NMR spectra on an instrument that does not required cryogens and has minimal maintenance requirements.

This information has been sourced, reviewed and adapted from materials provided by Anasazi Instruments.

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