Key Molecular Spectroscopy Techniques

Why This Matters

Spectroscopy is the backbone of molecular physics—it's how we "see" molecules without actually seeing them. Every technique on this list exploits a different physical interaction between electromagnetic radiation and matter, whether that's vibrational excitation, electronic transitions, nuclear spin flips, or rotational motion. When you're tested on these techniques, you're really being tested on your understanding of energy quantization, selection rules, and how molecular structure determines which wavelengths get absorbed, scattered, or emitted.

The key to mastering this material isn't memorizing wavelength ranges—it's understanding why each technique works and what molecular property it reveals. An IR spectrum tells you about bond vibrations because infrared photons have just the right energy to excite vibrational modes. NMR works because nuclei with spin behave like tiny magnets in a magnetic field. Don't just memorize facts—know what physical principle each technique illustrates and when you'd choose one over another.

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Vibrational Spectroscopy Techniques

These methods probe the quantized vibrational energy levels of molecules. When a molecule absorbs a photon matching the energy gap between vibrational states, we detect that absorption and learn about bond strengths, molecular geometry, and functional groups.

Infrared (IR) Spectroscopy

• Measures absorption of infrared light by molecular vibrations—photon energies in the IR range ($\sim 400-4000 \text{ cm}^{-1}$) match vibrational transition energies
• Requires a change in dipole moment for a vibration to be IR-active, making it ideal for polar bonds and asymmetric stretches
• Functional group identification through characteristic absorption bands—carbonyl stretch near $1700 \text{ cm}^{-1}$, O-H stretch near $3300 \text{ cm}^{-1}$

Raman Spectroscopy

• Based on inelastic (Stokes) scattering of monochromatic light—scattered photons lose energy to vibrational excitation
• Requires a change in polarizability rather than dipole moment, making it complementary to IR for symmetric vibrations
• Excellent for non-polar molecules and homonuclear diatomics like $N_2$ and $O_2$ that are IR-inactive

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Electronic Transition Spectroscopy

These techniques involve transitions between electronic energy levels, requiring higher-energy photons. Electronic spectroscopy reveals information about molecular orbital structure, conjugation, and chromophores.

UV-Visible Spectroscopy

• Measures electronic transitions in the $200-800 \text{ nm}$ range—typically $\pi \rightarrow \pi^*$ and $n \rightarrow \pi^*$ transitions
• Beer-Lambert Law ($A = \varepsilon bc$) enables quantitative concentration measurements from absorbance data
• Conjugation shifts absorption to longer wavelengths—extended $\pi$ systems appear colored because absorption enters the visible range

Fluorescence Spectroscopy

• Detects photon emission following electronic excitation—molecule relaxes vibrationally before emitting at longer wavelength than absorbed
• Stokes shift separates excitation and emission wavelengths, enabling high sensitivity detection of fluorophores
• Single-molecule sensitivity makes it essential for biological imaging, tracking biomolecules in live cells

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Nuclear and Magnetic Resonance Techniques

These methods exploit the magnetic properties of atomic nuclei or unpaired electrons. Energy splittings in magnetic fields are tiny, requiring radio-frequency or microwave radiation.

Nuclear Magnetic Resonance (NMR) Spectroscopy

• Exploits nuclear spin in magnetic fields—nuclei with $I \neq 0$ (like $^1H$ and $^{13}C$) split into energy levels proportional to field strength
• Chemical shift reveals electronic environment—shielded nuclei resonate at different frequencies than deshielded ones
• Spin-spin coupling (J-coupling) provides connectivity information through bond-mediated interactions between neighboring nuclei

Electron Spin Resonance (ESR) Spectroscopy

• Detects unpaired electrons by measuring transitions between spin states in a magnetic field—also called EPR (electron paramagnetic resonance)
• Hyperfine splitting from nuclear-electron interactions reveals the distribution of unpaired electron density
• Essential for studying free radicals, transition metal complexes, and reaction intermediates with paramagnetic character

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Rotational and Structural Techniques

These methods determine precise molecular geometry through rotational spectroscopy or diffraction patterns.

Microwave Spectroscopy

• Probes pure rotational transitions in gas-phase molecules—photon energies match gaps between rotational levels ($E_J = BJ(J+1)\hbar$)
• Requires permanent dipole moment for selection rule $\Delta J = \pm 1$ to allow transitions
• Yields precise bond lengths and angles from rotational constants—accuracy to $\pm 0.001 \text{ Å}$ for small molecules

X-ray Diffraction (XRD)

• Scatters X-rays from electron density in crystalline samples—constructive interference occurs when Bragg's law ($n\lambda = 2d\sin\theta$) is satisfied
• Determines complete 3D atomic positions in crystals with sub-angstrom resolution
• Requires crystalline samples—amorphous materials give broad, featureless patterns rather than sharp diffraction peaks

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Mass and Nuclear Gamma Techniques

These specialized methods provide information about molecular mass, composition, or nuclear environment through non-optical interactions.

Mass Spectrometry

• Measures mass-to-charge ratio ($m/z$) of ionized molecules—not technically spectroscopy but often grouped with these techniques
• Fragmentation patterns reveal molecular structure—characteristic breaking points identify functional groups
• Isotope patterns provide elemental composition—chlorine shows 3:1 ratio for $^{35}Cl:^{37}Cl$

Mössbauer Spectroscopy

• Uses recoil-free nuclear gamma absorption—only works for specific isotopes like $^{57}Fe$ embedded in solid lattices
• Isomer shift reveals oxidation state and electron density at the nucleus
• Quadrupole splitting indicates local symmetry around the Mössbauer-active nucleus—essential for iron chemistry

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Quick Reference Table

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Self-Check Questions

1. Which two techniques both probe vibrational modes but have complementary selection rules? What molecular property determines activity in each?

2. A molecule has a center of symmetry. If a particular vibration is IR-active, what can you conclude about its Raman activity, and why?

3. Compare NMR and ESR spectroscopy: what type of magnetic moment does each detect, and what limits ESR to certain samples?

4. You need to determine the precise bond length in a gas-phase diatomic molecule. Which technique would give the most accurate result, and what physical quantity would you measure?

5. An FRQ asks you to identify which spectroscopic technique would best distinguish between $Fe^{2+}$ and $Fe^{3+}$ in a solid sample. Which technique would you choose and what spectral feature would differ?