Knowledge Bank

Most chemical reactions occur under conditions where the reactants are in highly excited vibrational levels in the ground electronic state. As the vibrational energy content of the molecule increases and the molecular vibrations behave less like harmonic oscillators, the normal mode model fails to provide a satisfactory description of the molecular behavior. It is well known that overtone sepectra of highly excited anharmonic vibrations such as X-H stretching (X = C, O, N) show surprisingly simpler and narrower structures: in the near - IR and visible region than expected on the basis of a strongly-coupled normal mode model. Recently, the local mode model has been well established as a description for the highly excited vibrational states of X-H stretching. One significant result observed in the overtone transitions is that the overtone transition frequencies are highly sensitive to the bond nature and the environment of X-H bonds. Different frequencies for nonequivalent hydrogens in different steric environments can be easily observed in the overtone spectra. In this report, absorption spectra (from $\Delta v=4$ to $\Delta v=7)$ measured with thermal lensing and photoacoustic spectroscopy are studied for homologous series of alkenes, ketones, alcohols and amines. The overtone spectra are analyzed within the local node model as pure vibrational overtones of the nonequivalent X-H bonds on the molecule. The influence of the geometric environment on the X-H oscillators, such as axial-equatorial, cis-trans, or non-bonded $\sigma -\pi$ interactions, will be demonstrated in the overtone spectra. More importantly, the overtone study directly provides information about the strength of the chemical bond. Such a study allows us to establish systematic correlations of the overtone spectra with chemical bond strengths.