Spectroscopy Reference Tables

Spectroscopy interrogates molecules with light and listens to what they absorb. Every interaction between radiation and matter is quantized — a bond, an electron, or a nucleus accepts only the exact energy matching the gap between its allowed states — so a spectrum is a machine-readable fingerprint of a structure. The five panels on this page cover the five questions a chemist asks of an unknown: IR (which bonds are present), ¹H and ¹³C NMR (which atoms sit in which environments), MS (how much does the molecule weigh), and UV-Vis (how does its electron cloud respond to visible and ultraviolet light).

A bond vibrates like a mass on a spring: heavier atoms and looser bonds oscillate slower. IR reports positions in wavenumbers (cm⁻¹) — the inverse of wavelength, used because it is directly proportional to energy — running from 4000 to 400 in the panel's 41 bands. The diagnostic anchors: a C=O stretch near 1700 is strong and unmistakable (esters 1750-1735, aldehydes 1740-1720, ketones 1725-1705, acids 1725-1700); a broad O–H smear from 3550-3200 signals hydrogen bonding; a sharp pair of aromatic ring bands at 1600/1500 betrays a benzene ring. Below 1500 lies the fingerprint region, where the whole pattern — not any single peak — identifies the molecule by comparison with a reference. Wavenumbers run the IR because they are proportional to photon energy and keep bond absorptions in a tidy 400–4000 cm⁻¹ range.

Proton NMR sorts hydrogens by electronic neighborhood, reported as chemical shift δ in ppm from the reference TMS (assigned 0.00 because its silicon shields its protons more than almost anything). Electronegative neighbors withdraw electron density, deshield the proton, and push it downfield to higher δ: alkyl CH₃ sits at 0.7-1.3, attach an oxygen and the same protons land at 3.3-3.8, an aromatic ring carries them to 6.5-8.5, an aldehyde proton to 9-10, a carboxylic O–H all the way to 10-13. Each signal's integration counts the protons producing it, and its multiplicity follows the n+1 rule — a proton with n equivalent neighbors splits into n+1 lines, which is how connectivity is read. The panel's 23 environments note D₂O exchange for O–H and N–H: shake with heavy water and those signals vanish, a standard confirmation trick.

Carbon NMR spans 0-220 ppm and maps the framework directly, one line per distinct carbon. The landmarks are chemical common sense: sp³ carbons 8-55, carbons bearing oxygen or nitrogen pushed downfield to 35-70, sp² and aromatic 100-155, and every carbonyl 160-220 — aldehyde and ketone C=O highest (190-220), esters, acids, amides and anhydrides clustered at 160-185. Two quirks: the natural isotope ¹²C is NMR-silent (nuclear spin 0), which is why ¹³C — only 1.1% abundant — must carry the signal and needs far more scans; and broadband decoupling from the protons collapses what would be messy splitting into clean singlets, one per carbon.

Electron impact ionizes the molecule, and the instrument sorts fragments by mass-to-charge ratio (m/z). The molecular ion M⁺ gives the molecular weight; the fragmentation pattern then works like a postal address — the panel's 30 fragments are the common ones: m/z 43 (C₃H₇⁺ or acetyl), 91 (the tropylium/benzyl cation, hallmark of an alkylbenzene), 105 (benzoyl). Neutral losses read from the same table: M−18 means an alcohol just dehydrated, M−31 a methoxy leaving. Halogens announce themselves by isotopes: chlorine's ³⁵Cl/³⁷Cl (3:1) and bromine's ⁷⁹Br/⁸¹Br (1:1) give M and M+2 peak pairs in those exact ratios. Rearrangements like the McLafferty (γ-H transfer across a carbonyl) produce diagnostic peaks from whole structural units.

UV-Vis watches electrons jump between orbitals: π→π* for unsaturated systems (allowed, intense) and n→π* for lone pairs into antibonding orbitals (forbidden-ish, weak, ε of only 10-100). The absorbing unit is the chromophore, and conjugation is the lever: each added double bond narrows the HOMO–LUMO gap and shifts λ_max to longer wavelength — the table shows the ladder: isolated alkene below 200 nm, diene 217, triene 258, benzene's forbidden B-band at 256, anthracene at 375, into the visible. Intensity obeys the Beer–Lambert law, A = ε·b·c — absorbance is the molar absorptivity ε (read straight from the table) times path length times concentration — which is what makes UV-Vis a quantitative instrument, not just an identifier.

• An IR peak always sits at one fixed number. Hydrogen bonding shifts and broadens O–H dramatically; conjugation drops C=O by 20-40 cm⁻¹. The tables give ranges, not laws.
• Bigger NMR shift means more protons. Position reports environment; the integration reports count. A tall peak at 0.9 can be forty methyls; a lonely one at 9.8 is one aldehyde proton.
• The molecular ion is the biggest or tallest peak. The tallest (base peak) is whatever fragment forms most easily — often m/z 43 or 91. M⁺ can be tiny or absent entirely.
• A UV spectrum's color is what the compound absorbs. Observed color is the complementary color: a substance absorbing at 400 nm (violet) looks yellow.

Related tools: Organic Chemistry (the functional groups these tables identify), Redox & Electrochemistry (electron transfer, the other way molecules change), and Chemical Data.