3 μ / X-H region
Best for O-H, N-H and C-H stretching. Shape matters: free O-H and N-H tend to be sharper, while hydrogen bonding broadens and lowers them. Very weak bands can be overtones.
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IR workspace
Infrared spectrum
The browser reproduces the plotting and peak-selection workflow from the supplied Python notebook: spectra are ordered by wavenumber, %T minima or absorbance maxima are detected, and the strongest peaks are selected by prominence and point spacing. Absorbance conversion uses A = −log₁₀(T/100). Hover inspection is available with a mouse or trackpad.
Reference guide
Best for O-H, N-H and C-H stretching. Shape matters: free O-H and N-H tend to be sharper, while hydrogen bonding broadens and lowers them. Very weak bands can be overtones.
Use for C≡N, C≡C, cumulative double bonds and some overtones. Nitriles and isocyanates are often diagnostic; symmetrical alkynes may be weak or absent.
Usually the most useful area for C=O, C=C and C=N stretching. Carbonyl position shifts predictably with conjugation, ring strain and nearby electronegative atoms.
Contains single-bond stretches, bends and complex coupled vibrations. Assign only stronger diagnostic bands; use the overall pattern to compare identity with a known spectrum.
Wavenumber
ν̄ = 1/λ = ν/cHigher wavenumber means higher vibrational frequency and energy.Transmittance
T = I/I₀%T = 100TCompares transmitted intensity with the incident beam.Absorbance
A = −log₁₀(T)Converts transmission dips into upward peaks and is proportional to concentration in the linear regime.Beer–Lambert law
A = εbcRelates absorbance to molar absorptivity ε, path length b and concentration c.Harmonic oscillator
ν̄ = [1/(2πc)]√(k/μ)Stronger bonds have larger force constants k and absorb at higher wavenumber.Reduced mass
μ = m₁m₂/(m₁ + m₂)Heavier bonded atoms lower the vibrational frequency; isotopic substitution therefore shifts bands.Vibrational levels
Eᵥ = (v + ½)hνIR absorption promotes a molecule between quantized vibrational states.Selection rule
Δv = ±1∂μdipole/∂Q ≠ 0A fundamental vibration must change molecular dipole moment to be IR-active.Practical rule: assign the interpretable high-wavenumber, triple-bond and double-bond regions first. Then use only selected medium or strong fingerprint peaks and confirm them through companion bands or comparison with a known spectrum. Complete assignment of every band is rarely justified.