3.11 Spectroscopy

Molecular Spectroscopy

A wavelength-scaled EM spectrum paired with the energy-level ladder behind each transition. Send a photon through HCl and see why electronic gaps need UV/vis, vibrational need IR, and rotational need microwaves.

UV-Vis → ElectronicIR → VibrationalMicrowave → RotationalΔE = hν
Topic 3.11

Spectroscopy and the Electromagnetic Spectrum

Explain the relationship between a region of the electromagnetic spectrum and the types of molecular or electronic transitions associated with that region.

Different regions of the electromagnetic spectrum carry different photon energies, and each energy range drives a different kind of transition. The CED lists exactly three:

  • Microwave radiation → molecular rotational transitions. Lowest energy; the molecule tumbles faster.
  • Infrared radiation → molecular vibrational transitions. Intermediate energy; bonds stretch and bend. IR spectroscopy identifies functional groups from their characteristic vibration frequencies.
  • Ultraviolet/visible radiation → electronic transitions. Highest of the three; valence electrons are promoted between energy levels. This is what a colorimeter or spectrophotometer measures.

The ordering follows the energy ordering: rotational energy gaps are smallest, vibrational gaps are larger, and electronic gaps are larger still. Match the gap to the photon energy and you know which region is required.

Absorption and color. A solution appears the complementary color of the visible light it absorbs. A solution that absorbs orange-red light looks blue. This matters when choosing the wavelength for a Beer–Lambert experiment (3.13): you set the instrument to the wavelength of maximum absorbance, which is not the color you see.

Emission spectra. When an excited electron falls back to a lower level, it emits a photon whose energy equals the gap between levels. Because atomic energy levels are quantized and unique to each element, the resulting line spectrum is an elemental fingerprint. Absorption spectra are the negative image: dark lines exactly where the emission lines would be.

Key points

  • Microwave → rotation, infrared → vibration, UV/vis → electronic. Memorize this mapping.
  • Energy of a transition equals the energy of the photon absorbed or emitted.
  • Quantized energy levels are why spectra are lines rather than continuous bands.
  • The color you see is complementary to the color absorbed.

Equations

  • not on the sheetConservation of energy — the photon supplies exactly the gap.

Common mistakes

  • Do not confuse absorption with emission. Absorption raises the electron; emission is the drop back down.
  • Longer wavelength means lower energy. λ and E are inversely related.
  • Color observed ≠ color absorbed. They are complementary.
  • PES uses far higher energy than UV/vis — it ejects electrons entirely rather than promoting them.

Worked example

A student wants to (a) identify whether a sample contains a C=O group, and (b) measure the concentration of a blue Cu²⁺ solution. Which region of the spectrum is appropriate for each, and why?

(a) Infrared. Identifying a functional group means detecting a characteristic bond vibration. The C=O stretch absorbs strongly near 1700 cm⁻¹ in the infrared, because IR photon energies match vibrational energy gaps. UV/vis would give no functional-group information.

(b) Visible. Measuring the concentration of a colored ion relies on an electronic transition, which falls in the UV/visible region. The Cu²⁺ solution looks blue, meaning it transmits blue and absorbs the complementary orange-red light around 600–650 nm. The student should set the spectrophotometer near that absorbance maximum (λ_max) to get the greatest sensitivity, then apply the Beer–Lambert law.

Full notes for topic 3.11 →