4Stretching 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.
7H2O: Stretching and Bending Vibrations Fig Source: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 bondsHeavier atoms move slower than lighter ones
9Selection 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.Vibrations which do not change the dipole moment are Infrared Inactive (homonuclear diatomics).Homonuclear diatomic molecules (O2, N2, H2, Cl2) – IR InactiveHeteronuclear diatomic molecules (HF, HCl) – IR ActiveDipole 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
10Asymmetric Stretching Vibration of the CO2 Fig. The asymmetric stretching vibration of the CO2 molecule, showing the fluctuation in the dipole moment.
11Bending motion of the CO2 Fig. The bending motion of the carbon dioxide molecule and its associated dipole fluctuation.