IR Spectroscopy: Molecular Vibration Overview

IR-active molecules exhibit stretching and bending vibrations. Stretching vibrations occur along the bond line and change bond length, while bending vibrations change bond angles without occurring along the bond line. Both types result in characteristic IR absorption when bonds transition from ground state to the lowest energy excited state.

In symmetric stretching, two bonds elongate and contract together simultaneously. Asymmetric stretching involves one bond moving in one direction while another bond moves in the opposite direction. These different motions produce distinct peak splitting in IR spectra through IR spectrum peak splitting symmetric vs asymmetric vibrations.

Bending vibrations are classified as in-plane or out-of-plane modes. In-plane bending includes scissoring (symmetric) and rocking (asymmetric) motions. Out-of-plane bending comprises twisting (symmetric) and wagging (asymmetric) motions. These distinct bending modes contribute unique absorption frequencies to IR spectra and enable molecular identification.

Nonlinear molecules like ethanol containing n atoms allow 3n − 6 fundamental vibrations. Linear molecules such as HCl allow only 3n − 5 fundamental vibrations. This difference arises because linear molecules have one fewer rotational degree of freedom, reducing the number of independent vibrational modes available.

Overtones are integral multiples of fundamental absorption frequencies generated by any physical vibration in a molecule. Combination bands result from the fusion of two IR-active vibrational frequencies. Both phenomena appear in IR spectra and provide additional information about molecular structure beyond fundamental absorption peaks.

Fermi resonance is a coupled vibration created by the interaction of a fundamental band with either an overtone or combination band. This interaction produces spectral features that differ from simple superposition of individual bands, affecting peak positions and intensities in IR spectra.

Stretching and bending vibrations occur at characteristic fundamental absorption frequencies because bonds have specific energy requirements to transition from ground state to the lowest energy excited state. These frequencies depend on bond strength, atomic masses, and molecular geometry, making them unique identifiers for different functional groups.