The Relationship Between Dipole Moment Changes and Infrared Absorption Bands

The Relationship Between Dipole Moment Changes and Infrared Absorption Bands 

One further concept can be introduced at this juncture. The asymmetric and symmetric stretching vibrations of carbon dioxide are described as parallel ( ) vibrations, that is, the change in dipole moment which occurs during these vibrations is parallel to the axis of symmetry of the molecule. The bending vibration is described as a perpendicular (i.) motion since the change of dipole moment 

Vibrations for Molecules Containing Four or More Atoms 

While it is generally easy to picture the vibrations of a simple molecule, it is often more difficult to visualize the complex motions of large and/or asymmetric molecules. In this section, we present a simple method for visualizing molecular motion involving four or more atoms. 

For NH3, the first type of vibration, v, consists of a motion in which the nitrogen moves up and down the axis of symmetry of the molecule and the hydrogens move back and forth along the axes of the chemical bonds between the nitrogen and the hydrogens. This is a symmetric motion which produces a parallel band and is classified as an vibration. (The significance of this and other classifications or types will be discussed later in this chapter.) 

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