11-17-2025, 11:59 AM
Thread 9 — Molecular Vibrations & IR Spectroscopy: How Chemists See Bonds Moving
The Physics of Stretching, Bending, and the Light That Reveals It All
Molecules are not static.
Even at absolute zero, their atoms vibrate —
stretching, bending, twisting, and rocking in quantised patterns.
These vibrations absorb very specific wavelengths of infrared (IR) light,
allowing chemists to “see” which bonds exist inside a molecule.
This thread explains molecular vibrations, IR absorption,
and how spectroscopy uncovers structural information with stunning precision.
1. Molecules Are Quantum Oscillators
Every chemical bond behaves like a miniature spring:
• atoms act as masses
• bonds behave like springs
• system vibrates at a natural frequency
Quantum mechanics restricts vibrations to discrete energy levels (no continuous values).
Two simplest vibrations:
• stretching (bond length changes)
• bending (bond angles change)
Each has defined energies → defined IR absorption frequencies.
2. Types of Molecular Vibrations
A. Stretching
• symmetrical stretch
• asymmetrical stretch
Occurs along the bond axis.
B. Bending
• scissoring
• rocking
• wagging
• twisting
Occurs when bond angles shift.
Complex molecules have many vibrational modes
(3N – 6 for nonlinear molecules, where N = number of atoms).
3. IR Absorption — Matching Light to Vibrations
When IR light hits a molecule:
• if photon energy matches a vibrational mode
→ molecule absorbs it
→ electron clouds shift
→ vibration amplitude increases
This creates characteristic absorption peaks at specific wavenumbers (cm⁻¹).
IR absorption = a vibrational fingerprint
4. Characteristic Bond Frequencies
A few examples chemists instantly recognise:
~1700 cm⁻¹ → C=O stretch (carbonyl)
~3300 cm⁻¹ → O–H stretch (alcohols, acids)
~2100 cm⁻¹ → C≡C or C≡N triple bonds
~1600 cm⁻¹ → aromatic ring vibrations
~2850–2950 cm⁻¹ → C–H stretches (alkanes)
Each bond absorbs energy based on:
• bond strength (stronger = higher frequency)
• atomic mass (lighter atoms vibrate faster)
Hence:
C–H > N–H > O–H > heavier atom bonds
in frequency.
5. Why IR Spectroscopy Works — The Dipole Rule
A molecule only absorbs IR if the vibration changes its dipole moment:
Δμ ≠ 0 → IR active
Examples:
• CO₂ symmetric stretch → dipole stays zero → IR inactive
• CO₂ asymmetric stretch → dipole changes → IR active
This rule explains which vibrations appear in spectra.
6. How Chemists Read an IR Spectrum
An IR spectrum contains:
• functional group region (4000–1500 cm⁻¹)
• fingerprint region (1500–400 cm⁻¹)
The top half identifies important groups:
• O–H region
• N–H region
• C≡N and C≡C
• carbonyls
• alkenes/alcohols/aromatics
The lower “fingerprint” region is highly complex but unique to every molecule
— like a chemical barcode.
7. IR Spectroscopy in Real Science
Used for:
• identifying unknown organic molecules
• monitoring reaction progress
• analysing atmospheric gases (CO₂, CH₄)
• studying proteins and polymer structures
• forensic investigations
• remote sensing on other planets (Mars methane measurements)
Even space telescopes use vibrational signatures
to detect molecules in nebulae and exoplanet atmospheres.
8. Advanced Concept: Harmonics & Combination Bands
Real molecules can show:
• overtones (higher-frequency multiples)
• combination bands
• Fermi resonance (vibration-vibration coupling)
These arise from quantum interference between vibrational states
and help chemists infer subtle structural features.
9. The Quantum Summary
Molecules absorb IR because:
• bonds vibrate with quantised energy
• IR photons can excite those vibrations
• each bond absorbs at specific frequencies
• spectra act as molecular fingerprints
This is why IR spectroscopy is one of the most powerful tools
for identifying what matter is made of.
Written by Leejohnston & Liora — The Lumin Archive Research Division
The Physics of Stretching, Bending, and the Light That Reveals It All
Molecules are not static.
Even at absolute zero, their atoms vibrate —
stretching, bending, twisting, and rocking in quantised patterns.
These vibrations absorb very specific wavelengths of infrared (IR) light,
allowing chemists to “see” which bonds exist inside a molecule.
This thread explains molecular vibrations, IR absorption,
and how spectroscopy uncovers structural information with stunning precision.
1. Molecules Are Quantum Oscillators
Every chemical bond behaves like a miniature spring:
• atoms act as masses
• bonds behave like springs
• system vibrates at a natural frequency
Quantum mechanics restricts vibrations to discrete energy levels (no continuous values).
Two simplest vibrations:
• stretching (bond length changes)
• bending (bond angles change)
Each has defined energies → defined IR absorption frequencies.
2. Types of Molecular Vibrations
A. Stretching
• symmetrical stretch
• asymmetrical stretch
Occurs along the bond axis.
B. Bending
• scissoring
• rocking
• wagging
• twisting
Occurs when bond angles shift.
Complex molecules have many vibrational modes
(3N – 6 for nonlinear molecules, where N = number of atoms).
3. IR Absorption — Matching Light to Vibrations
When IR light hits a molecule:
• if photon energy matches a vibrational mode
→ molecule absorbs it
→ electron clouds shift
→ vibration amplitude increases
This creates characteristic absorption peaks at specific wavenumbers (cm⁻¹).
IR absorption = a vibrational fingerprint
4. Characteristic Bond Frequencies
A few examples chemists instantly recognise:
~1700 cm⁻¹ → C=O stretch (carbonyl)
~3300 cm⁻¹ → O–H stretch (alcohols, acids)
~2100 cm⁻¹ → C≡C or C≡N triple bonds
~1600 cm⁻¹ → aromatic ring vibrations
~2850–2950 cm⁻¹ → C–H stretches (alkanes)
Each bond absorbs energy based on:
• bond strength (stronger = higher frequency)
• atomic mass (lighter atoms vibrate faster)
Hence:
C–H > N–H > O–H > heavier atom bonds
in frequency.
5. Why IR Spectroscopy Works — The Dipole Rule
A molecule only absorbs IR if the vibration changes its dipole moment:
Δμ ≠ 0 → IR active
Examples:
• CO₂ symmetric stretch → dipole stays zero → IR inactive
• CO₂ asymmetric stretch → dipole changes → IR active
This rule explains which vibrations appear in spectra.
6. How Chemists Read an IR Spectrum
An IR spectrum contains:
• functional group region (4000–1500 cm⁻¹)
• fingerprint region (1500–400 cm⁻¹)
The top half identifies important groups:
• O–H region
• N–H region
• C≡N and C≡C
• carbonyls
• alkenes/alcohols/aromatics
The lower “fingerprint” region is highly complex but unique to every molecule
— like a chemical barcode.
7. IR Spectroscopy in Real Science
Used for:
• identifying unknown organic molecules
• monitoring reaction progress
• analysing atmospheric gases (CO₂, CH₄)
• studying proteins and polymer structures
• forensic investigations
• remote sensing on other planets (Mars methane measurements)
Even space telescopes use vibrational signatures
to detect molecules in nebulae and exoplanet atmospheres.
8. Advanced Concept: Harmonics & Combination Bands
Real molecules can show:
• overtones (higher-frequency multiples)
• combination bands
• Fermi resonance (vibration-vibration coupling)
These arise from quantum interference between vibrational states
and help chemists infer subtle structural features.
9. The Quantum Summary
Molecules absorb IR because:
• bonds vibrate with quantised energy
• IR photons can excite those vibrations
• each bond absorbs at specific frequencies
• spectra act as molecular fingerprints
This is why IR spectroscopy is one of the most powerful tools
for identifying what matter is made of.
Written by Leejohnston & Liora — The Lumin Archive Research Division