Nuclear Magnetic Resonance (NMR) Spectroscopy

Nuclei with nonzero spin can have magnetic moments and NMR transitions. Hydrogen-1 and carbon-13 are familiar spin-half examples; carbon-12 has spin zero and is not directly observed by ordinary NMR. Nuclear spin cannot be assigned simply from the total nucleon count alone.

A magnetic field separates nuclear energy levels. For an appropriate nucleus, the Larmor relationship is ν = |γ|B0/(2π), with consistent units. NMR instrument frequency labels commonly refer to the proton frequency, so a nominal 500 MHz system measures carbon-13 at a different frequency in the same field.

A radio-frequency pulse changes the ensemble magnetisation. The transverse magnetisation precesses and induces a receiver signal, the free induction decay. Fourier transformation converts the time-domain signal to frequency information after appropriate acquisition and processing.

Longitudinal relaxation T1 concerns return of magnetisation along the field. Transverse relaxation T2 and field inhomogeneity contribute to signal decay, often described through an effective T2*. They affect experiment timing, linewidth and quantitative response in different ways.

Proton and carbon-13 differ in abundance and magnetic properties, so their practical sensitivities differ. Probe, field, concentration, relaxation and pulse sequence also matter. A fixed sample mass, scan count or overnight acquisition is not required for every carbon experiment.

Signals can overlap or be broadened beyond useful recognition. NMR does not guarantee one distinct peak for each atom: equivalent nuclei share environments, different environments can coincide, and dynamic exchange can alter what is observed.

Chemical shift compares resonance frequency with a defined reference relative to operating frequency and is expressed in ppm. Its normalisation helps compare data across fields. Solvent, temperature, concentration, chemical form and reference procedure still influence the measured position.

Shielding reflects the local electronic response to the magnetic field. Electronegativity, anisotropic fields, hydrogen bonding and other effects influence it. A simple rule that more electron density always gives a predictable shift is insufficient for every molecular environment.

Broad proton regions can guide interpretation: many saturated alkyl signals occur below about 3 ppm, heteroatom-adjacent protons often occur farther downfield, alkenyl and aromatic signals commonly occupy intermediate-to-downfield regions, and aldehydic signals often occur around 9–10 ppm. These are approximate patterns with exceptions.

Carbon spectra span a wider range, with many saturated carbons upfield, unsaturated carbons farther downfield and carbonyl carbons extending into still higher shifts. A shift near 200 ppm can suggest an aldehyde or ketone carbonyl; it does not uniquely assign one structure.

Exchangeable O–H and N–H signals can be broad, variable or absent under particular conditions. Their absence is not proof that the functional group is absent. Exchange experiments and other data may help when compatible with specimen preservation.

Use the full set of assignments rather than a rule that one aromatic block determines a drug. A few plausible chemical shifts do not uniquely distinguish related NPS structures, and a structure cannot be guaranteed to emerge within a fixed number of minutes.

Scalar coupling links nuclear spin behaviour through bonding interactions. Coupling can occur over different numbers of bonds and is not limited to protons on adjacent carbons. The observed pattern depends on equivalence, coupling constants and chemical-shift separation.

For a suitable first-order proton system with n equivalent spin-half neighbours sharing a coupling, the n+1 rule predicts a multiplet. An ethyl group can show a methyl triplet and methylene quartet with matching relevant J values. Nonequivalent neighbours or second-order behaviour can produce more complex patterns.

J is commonly reported in hertz and is largely field-independent for the same chemical conditions. Chemical-shift separation in hertz increases with field. Higher field can therefore reduce some second-order effects without changing the molecule's underlying coupling constants.

Coupling magnitude can support geometric or structural assessment, but a single value does not uniquely establish a ring substitution or conformation. Karplus-type relationships involve assumptions and can allow alternatives. Overlap can also make an apparent spacing misleading.

Under suitable acquisition conditions, proton integral areas are proportional to contributing nuclei. Integrate the full relevant multiplet and evaluate baseline, overlap, relaxation and exchange effects. A 3:2 integral ratio is not proof that the signals must be a CH3 and CH2 in one molecule; mixtures or other groupings can give the same ratio.

Combine shift, multiplicity, J and integration with molecular constraints and additional experiments. A first-order ethanol example illustrates a pattern rather than universal OH behaviour. Exchange can remove or alter OH coupling, and solvent conditions shift the signals.

High-field systems commonly use superconducting magnets, while benchtop systems can use permanent magnets. Cryogen requirements depend on the magnet design. Price, service cost and field strength should come from actual specifications rather than an unsourced fixed national inventory.

The probe delivers radio-frequency pulses and detects the response. Shim adjustments improve field homogeneity over the specimen. A lock system commonly monitors deuterium to help stabilise the field, but not every experiment requires the analyte to be dissolved directly in a deuterated solvent.

Deuterated solvents reduce large proton-solvent signals but are not perfectly free of residual protons. Water and impurities also contribute. Residual chloroform in CDCl3 commonly appears near 7.26 ppm and residual DMSO-d6 near 2.50 ppm under familiar conditions; use an appropriate solvent reference chart rather than assume every unexplained peak is analyte.

Select solvent for solubility, stability and compatibility with the information needed. Protic or exchange-promoting conditions can affect labile protons. Filtration, purification or solvent exchange can alter evidence and should be justified rather than prescribed automatically for every specimen.

COSY reveals suitable scalar-coupling correlations between protons. HSQC commonly associates protons with directly attached carbons, while HMBC targets longer-range correlations with experiment-dependent behaviour. A missing cross-peak is not automatically proof that no connection exists.

NOE-based experiments provide through-space information under appropriate conditions. They should not be read as ordinary bond-connectivity maps. Selecting complementary experiments and testing alternative assignments is more reliable than treating one 2D spectrum as an unrestricted structural map.

Seized-drug characterisation can use NMR to evaluate ring substitution, side chains and other features that remain ambiguous in mass-spectral data. Adequate purity, signal and assignment are required. NMR is not legally mandatory for every NPS, and MS can provide more than molecular weight alone.

Pharmaceutical and adulteration examinations can compare structures or assess component amounts using a suitable method. An acetyl singlet and two aromatic features are not unique proof of paracetamol in every mixture. Complementary evidence and realistic interference assessment remain important.

Cannabinoid or natural-product profiles can support chemical comparisons, but a terpene pattern does not independently determine geographic origin. Storage, processing, variety and mixture effects require suitable reference populations and interpretation.

Quantitative proton NMR can compare assigned analyte and reference integrals after accounting for proton counts, molar masses, reference purity and weighed amounts. It can avoid needing a separate response-calibration standard for every analyte under suitable conditions, but it is not reference-free measurement.

Adequate relaxation, excitation, baseline treatment and separation of the measured resonances are essential. A peak can look well resolved while still containing an unnoticed contribution. Suitable uncertainty assessment includes reference, weighing and spectral factors.

US, UK, Indian and other laboratories may provide NMR through specialist or shared facilities. Confirm their forensic scope, specimen requirements and chain-of-custody arrangements. Instrument access alone does not establish the validity or legal status of the resulting assignment.

In a conventional DEPT-135 phase convention, CH and CH3 signals share one phase, CH2 signals have the opposite phase, and non-protonated carbons are absent. Phase can be inverted in processing, so relative phase and experiment setup matter more than an absolute “up” direction.

DEPT-90 commonly selects CH signals. Compare edited data with an appropriate conventional carbon spectrum to assess non-protonated carbons and assignments. DEPT-135 alone cannot identify the location of every quaternary carbon that it does not display.

Sensitivity, relaxation and pulse-sequence conditions can make some carbon signals weak or missing. HMBC can help connect selected non-protonated carbons when useful long-range correlations are observable, but it is not a guaranteed workaround for every absent signal.

Mixtures, stereochemistry, conformational exchange and insufficient signal can leave alternatives unresolved. Enantiomers ordinarily have identical spectra in an achiral environment. A claimed unique stereochemical assignment needs appropriate evidence rather than ordinary one-dimensional data alone.

Report acquisition conditions, references, assignments and the supported level of structural or quantitative conclusion. Preserve original data and processing choices. A synthetic teaching spectrum or short feature list should not be presented as an actual laboratory case or complete structure proof.

Related reading: Mass Spectrometry: Principles, Ionisation and Mass Analysers.

Related reading: Raman Spectroscopy and Comparison with IR.

Related reading: Method Validation, Measurement Uncertainty and Proficiency Testing.