Nuclear Magnetic Resonance (NMR) spectroscopy exploits the magnetic properties of nuclei with non-zero spin (e.g., ¹H, ¹³C, ¹⁹F, ³¹P) placed in a strong external magnetic field. Under these conditions, nuclei occupy distinct energy levels, and radiofrequency pulses induce transitions between them. The resonance frequency depends on the local magnetic field at each nucleus, which is modified by the surrounding electron cloud — this phenomenon produces chemical shift, measured in parts per million (ppm) relative to a reference standard.
The universal reference standard for NMR is tetramethylsilane (TMS), whose 12 equivalent protons produce a single sharp peak defined as 0 ppm. Most organic protons resonate between 0 and 12 ppm, while ¹³C signals span 0-220 ppm. The chemical shift is influenced by electronegativity, hybridization, and anisotropic effects from nearby π systems. For example, aldehydic protons appear at 9-10 ppm, aromatic protons at 7-8 ppm, and methyl protons adjacent to oxygen at 3-4 ppm.
The integration of a ¹H NMR signal (the area under the peak) is directly proportional to the number of protons producing it, enabling quantitative determination of proton ratios. Spin-spin coupling (J-coupling) splits signals into multiplets according to the n+1 rule: a proton with n equivalent neighboring protons appears as a multiplet of n+1 lines. The coupling constant J (in Hz) provides information about bond connectivity and dihedral angles through the Karplus equation. Common splitting patterns include singlets, doublets, triplets, quartets, and more complex patterns such as doublets of doublets when couplings to inequivalent neighbors occur.
¹³C NMR is routinely acquired with broadband proton decoupling, producing singlet signals for each unique carbon environment. DEPT (Distortionless Enhancement by Polarization Transfer) experiments distinguish between CH₃, CH₂, CH, and quaternary carbons by varying the decoupler pulse angle (45°, 90°, and 135°). Two-dimensional techniques including COSY (COrrelation SpectroscopY) and HSQC (Heteronuclear Single Quantum Coherence) map through-bond and through-scaling interactions, respectively, enabling complete assignment of complex molecular structures.
Practical interpretation requires awareness of common artifacts. Residual solvent peaks (e.g., CDCl₃ at 7.26 ppm in ¹H, 77.16 ppm in ¹³C) must be recognized. Dynamic effects such as rotamer equilibria or proton exchange can broaden or coalesce signals. Temperature-dependent measurements can resolve exchange-broadened peaks and provide insight into molecular motion and conformational flexibility.
