Spectroscopy in Organic Chemistry: IR, NMR, and Mass Spectrometry

Infrared (IR) Spectroscopy

Infrared spectroscopy measures the absorption of infrared light by molecular vibrations. When the frequency of infrared radiation matches the natural vibration frequency of a bond, the molecule absorbs that radiation, and a detector records decreased transmittance at that frequency. The resulting IR spectrum plots transmittance (or absorbance) against wavenumber (cm-1), producing a unique pattern of absorption bands that serves as a molecular fingerprint.

Different functional groups absorb at characteristic wavenumber ranges, making IR spectroscopy the fastest way to identify which functional groups are present. Broad O-H stretches appear between 2500-3300 cm-1 for carboxylic acids and 3200-3600 cm-1 for alcohols. Sharp N-H stretches appear near 3300-3500 cm-1 (primary amines show two peaks, secondary amines show one). The strong, sharp C=O stretch near 1700-1750 cm-1 is one of the most distinctive absorptions in organic spectroscopy and shifts position depending on whether the carbonyl is an aldehyde, ketone, ester, amide, or acid.

C-H stretches appear between 2850-3000 cm-1 for sp3 C-H, near 3020-3100 cm-1 for sp2 C-H (alkene and aromatic), and near 3300 cm-1 for sp C-H (terminal alkyne). C=C stretches appear near 1620-1680 cm-1 for alkenes and near 1450-1600 cm-1 for aromatic rings (often as two bands). The triple bond region (2100-2260 cm-1) shows C triple bond C and C triple bond N absorptions that are easy to identify because few other bonds absorb in this region.

The fingerprint region (below 1500 cm-1) contains complex overlapping absorptions that are difficult to assign individually but create a pattern unique to each compound. Matching the fingerprint region of an unknown sample to a reference spectrum provides definitive identification. Modern IR spectroscopy uses Fourier transform instruments (FTIR) that collect an entire spectrum in seconds and can analyze gases, liquids, solids, and thin films.

Nuclear Magnetic Resonance (NMR) Spectroscopy

NMR spectroscopy exploits the magnetic properties of certain atomic nuclei (most importantly 1H and 13C) to reveal detailed structural information. When placed in a strong external magnetic field, these nuclei absorb radiofrequency radiation at frequencies that depend on their electronic environment. The result is a spectrum where each chemically distinct hydrogen or carbon produces a signal at a characteristic position (chemical shift), providing a map of the molecular framework.

In proton NMR (1H NMR), the chemical shift (measured in parts per million, ppm, relative to tetramethylsilane) reveals the electronic environment of each hydrogen. Alkyl hydrogens appear between 0.5-2.5 ppm, hydrogens on carbons adjacent to electronegative atoms are shifted downfield (2.5-4.5 ppm), vinyl hydrogens appear at 4.5-6.5 ppm, aromatic hydrogens at 6.5-8.5 ppm, aldehyde hydrogens near 9-10 ppm, and carboxylic acid hydrogens between 10-12 ppm.

The integration (area under each peak) is proportional to the number of hydrogens producing that signal. A signal integrating for three hydrogens often indicates a methyl group (CH3), while a signal integrating for two hydrogens suggests a methylene (CH2). Splitting patterns arise from spin-spin coupling between neighboring hydrogens. The n+1 rule states that a hydrogen with n equivalent neighboring hydrogens splits into n+1 peaks: a singlet (no neighbors), doublet (one neighbor), triplet (two neighbors), quartet (three neighbors), and so on.

Carbon-13 NMR (13C NMR) provides complementary information about the carbon framework. Because the 13C isotope is only 1.1% abundant, 13C NMR requires more sample and longer acquisition times than 1H NMR. Each chemically distinct carbon produces one signal, and chemical shifts span a wider range (0-220 ppm). Carbonyl carbons appear farthest downfield (170-220 ppm), aromatic carbons at 110-160 ppm, and alkyl carbons at 0-50 ppm. 13C spectra are typically proton-decoupled, showing each carbon as a single line.

Two-dimensional NMR techniques (COSY, HSQC, HMBC, NOESY) establish correlations between nuclei that are connected through bonds or close in space, enabling the complete structural elucidation of complex molecules. COSY identifies which hydrogens are coupled to each other (adjacent hydrogens). HSQC connects each hydrogen to its directly bonded carbon. HMBC identifies long-range hydrogen-carbon correlations (2-3 bonds). NOESY reveals hydrogens that are close in space regardless of bonding connectivity, providing three-dimensional structural information.

Mass Spectrometry (MS)

Mass spectrometry ionizes molecules and separates the resulting ions by their mass-to-charge ratio (m/z). The mass spectrum shows the relative abundance of each ion as a function of m/z. The molecular ion peak (M+) directly indicates the molecular weight of the compound. High-resolution mass spectrometry can measure mass with sufficient precision (to four or more decimal places) to determine the exact molecular formula, because each element has a unique atomic mass that is not exactly a whole number.

Fragmentation patterns provide structural information. When the molecular ion has excess energy, it breaks apart at the weakest bonds, producing characteristic fragment ions. Alkyl groups produce fragments at m/z 15 (CH3+), 29 (C2H5+), 43 (C3H7+), and so on. Carbonyl compounds often show loss of 28 (CO), alcohols lose 18 (H2O), and amines show an alpha cleavage adjacent to nitrogen. The base peak (most abundant fragment) often corresponds to the most stable carbocation that can form from the molecular ion.

Modern ionization methods like electrospray ionization (ESI) and matrix-assisted laser desorption/ionization (MALDI) allow the analysis of large, fragile, and nonvolatile molecules, including proteins, polymers, and supramolecular complexes. Tandem mass spectrometry (MS/MS) selects a specific ion, fragments it further, and analyzes the resulting fragments, providing detailed structural information for compounds in complex mixtures.

Combining Spectroscopic Data

No single spectroscopic technique provides complete structural information. The standard approach to structure determination combines molecular formula (from high-resolution MS or combustion analysis), functional group identification (from IR), carbon framework mapping (from 13C NMR), hydrogen environment analysis (from 1H NMR), and connectivity confirmation (from 2D NMR). The degree of unsaturation (calculated from the molecular formula) tells the chemist how many rings and double bonds are present, narrowing the structural possibilities before spectral analysis begins.