Stretching and Bending Types of Molecular Vibrations: Vibrations fall into the basic categories of stretching and bending. Stretching vibration involves a continuous change in the inter-atomic distance along the axis of the bond between two atoms. Bending vibrations are characterized by a change in the angle between two bonds and are of four types: scissoring, rocking, wagging, and twisting.
H 2 O: Stretching and Bending Vibrations Bending is easier than stretching -- happens at lower energy (lower wavenumber) Bond Order is reflected in ordering -- triple > double > single (energy) with single bonds easier than double easier than triple bonds Heavier atoms move slower than lighter ones Fig Source: bend
Vibrations which do not change the dipole moment are Infrared Inactive (homonuclear diatomics). Selection Rules Selection rule: A molecule will absorb infrared radiation if the change in vibrational states is associated with a change in the dipole moment ( ) of the molecule. Dipole moment is greater when electronegativity difference between the atoms in a bond is greater. Some electronegativity values are: H 2.2; C 2.55; N 3.04; O 3.44; F 3.98; P 2.19; S 2.58; Cl 3.16 Homonuclear diatomic molecules (O 2, N 2, H 2, Cl 2 ) – IR Inactive Heteronuclear diatomic molecules (HF, HCl) – IR Active
Fig. The asymmetric stretching vibration of the CO2 molecule, showing the fluctuation in the dipole moment. Asymmetric Stretching Vibration of the CO 2
Fig. The bending motion of the carbon dioxide molecule and its associated dipole fluctuation. Bending motion of the CO 2