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Spectroscopy

Read the fingerprints molecules leave in light.

Follow the cause1 / 3

Radiation meets a molecule. A photon interacts when its energy matches an allowed molecular transition.

molecular fingerprintsIR · 4 diagnostic peaks
Focus the idea
A spectrum reveals molecular structure through diagnostic peaksThe IR axis runs from 4000 to 500 cm⁻¹, left to right (high ν̃ · higher energy on the left). The highlighted peak is at 1714.5 cm⁻¹, with photon energy 20.5 kJ/mol.40003000200015001000500O–HC–HC=OC–O← high ν̃ · higher energylow ν̃ · lower energy →ν̃ (cm⁻¹) · E = hcν̃E = 20.5 kJ/mol
methodIR
highlighted peak1715 cm⁻¹
photon energy20.5 kJ/mol
largest signal70%

The C=O stretch sits at 1715 cm⁻¹; E = hcν̃ = 20.5 kJ/mol. IR axes run from high wavenumber (higher energy) on the left to low on the right; a vibration absorbs strongly when it changes the dipole.

The plotted peaks are teaching signals, not an instrument trace. Real spectra depend on calibration, solvent, coupling, line width, concentration, and instrument resolution. IR is often drawn as transmittance dips; here absorption is drawn upward.

04 / PRACTICE

Make the chemistry yours.

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Answer from the model, then use the explanation to connect the particle picture to the chemical rule.

CHECK 1 / 3

What does an NMR integration mainly estimate?