NMR Spectrum of Aspirin

Aspirin (C₉H₈O₄), acetylsalicylic acid, is a salicylate drug used to treat a variety of conditions, including headaches, fever and inflammation. It is also used in low doses in preventive treatment against heart attacks, strokes and blood clots.

The ¹H NMR spectrum of a 4% (w/w; 330 mM) solution of aspirin in chloroform-D was measured at 82 MHz using the Thermo Scientific picoSpin 80 NMR spectrometer.

Aspirin contains aliphatic, aromatic and carboxylic acid protons that span a wide range of the 1H spectrum, and signal integration reveals a 3:4:1 intensity distribution, respectively. The carboxylic acid resonance (O=COH) is identified by a characteristic downfield chemical shift at 11.77 ppm. Carboxylic acids exist as stable hydrogen-bound dimers in non-polar solvents and these strongly deshielded protons typically appear between 11-12 ppm.

Signal broadening suggests a moderate rate of dynamic proton exchange of the labile proton. Aromatic protons appear as a series of multiplets. Dissimilar phenyl ring substituents cause ring proton chemical shifts to be dispersed between 7.0-8.2 ppm. The carboxylic acid substituent is moderately deactivating, drawing electron density away from the conjugated ð system at the ortho (C9) and para (C7) positions via the inductive effect, causing the C9 proton to shift downfield to 8.12 ppm while the C7 proton appears centered at 7.66 ppm. The acetoxy substituent (OAc) is moderately activating, donating electron density through the resonance effect causing an upfield shift of proton resonances ortho (C6; 7.16 ppm) and para (C8; 7.28) to this substituent. Multiplicity patterns result from vicinal 3J (7.62 Hz) and long-range 4J coupling (1.7-1.8 Hz) of protons around the ring. The methyl group (O=CCH3) appears as an uncoupled singlet at 2.36 ppm due to inductive deshielding on the oxygen side of the OAc functional group.

The picoSpin family of benchtop 1H NMR spectrometers can analyze a diverse range of organic compounds, allowing one to extract a wealth of chemical and structural information from high-resolution spectra. From this sample molecule, hydrogen bonding, resonance and inductive effects are identified by analyzing chemical shifts, signal integration confirms functional group assignment, and multiplet analysis reveals long and short-range coupling effects. picoSpin 1H NMR spectrometers offer a cost effective solution for funding-limited budgets, space-constrained labs and access-limited or over-burdened high-field resources, without compromising the need for quality spectroscopic results.

Chemical name: Aspirin (acetylsalicylic acid)

Concentration: 4% (w/w; 0.33 M) in Chloroform-D

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