1) Mass of the Atoms
Influence: Heavier atoms vibrate at lower frequencies compared to lighter atoms.
Hooke's Law:
ν = frequency of vibration
c = speed of light
k = force constant (bond strength)
μ = reduced mass of the two atoms
2) Bond Strength
Stronger Bonds: Higher force constants, leading to higher vibrational frequencies.
Order of Frequencies:
Single bonds < Double bonds < Triple bonds
3) Molecular Environment
Electron Delocalization: Conjugation can lower the frequency of carbonyl groups.
Ring Strain: In cyclic compounds, ring strain can affect bond angles and frequencies.
4) Hydrogen Bonding
Effect: Hydrogen bonding can shift vibrational frequencies to lower values (red shift) and broaden absorption bands.
Intramolecular vs. Intermolecular: Both types can impact the IR spectrum.
5) Substituent Effects
Electron Donating Groups: Can increase electron density, affecting bond strength and frequencies.
Electron Withdrawing Groups: Can decrease electron density, altering vibrational frequencies.
6) Isotopic Substitution (Factors affecting vibrations)
Isotopes: Replacing an atom with its isotope changes the reduced mass (μ\muμ) and hence the vibrational frequency.
Applications: Used to study reaction mechanisms and molecular structure.