Local modes

The historical background for the development of the local mode approach is reviewed, including the critical role of the radiation field. The harmonically coupled anharmonic oscillator model is described. Local modes have been used in the interpretation of XH stretching overtone spectra. These spectra can be used as sensitive probes of molecular properties such as bond length and molecular conformation. Examples of these spectral features are discussed. An intensity theory has been developed that permits the accurate calculation of relative and absolute overtone intensities. In molecules with low barriers to methyl rotation, torsional states contribute to the XH stretching band profile. A successful theoretical approach is presented that accounts for these effects and provides insight into the vibrational dynamics. The application of calculated local mode overtone spectra to atmospheric chemistry is discussed. (author)