Vibrational Overtone Intensities and Intramolecular Interactions

Abstract

Vibrational spectroscopy has been used to measure SH-, OH-, and NH-stretching vibrations in the fundamental and overtone regions from 2000 cm−1 to ~18000 cm−1. Both conventional and photoacoustic spectroscopic techniques were used.

The first section of this study is concerned with the intensities of the vibrational overtones of SH-, NH-, CH- and OH-stretching oscillators. It was observed that the oscillator strengths of SH-stretching transitions were markedly smaller than oscillator strengths for OH- and NH-stretching transitions. Using the local mode anharmonic oscillator method, the intensity was deconstructed into dipole and anharmonic components to find where the differences in intensity between similar moieties originates. In both cases, the dipole component of the oscillator strength was found to have a larger influence on the oscillator strength than the anharmonic component.

The second section involved observing weak intramolecular hydrogen bonds and intramolecular hydrogen bond-like interactions. Six different species were recorded to find spectral evidence of intramolecular hydrogen bonding between OH and S, SH and O, SH and S and OH and the pi-electrons that are associatedwith carbon-carbon multiple bonds. Geometry optimisations, along with the local mode anharmonic oscillator method were used to help assign the complicated XH-stretching vibration spectra. The quantum theory of atoms in molecules was also used to try and find additional computational evidence of intramolecular hydrogen bonding. However, no (3,-1) bond critical points were calculated between the hydrogen bonding moieties. Evidence of OH· · · S hydrogen bonding was observed in the vibrational overtone spectra of 2-mercaptoethanol. There was also evidence of weak intramolecular OH· · · pi hydrogen bonding in allyl carbinol and propargyl carbinol. This leads to the hypothesis that atoms in molecules does not accurately describe weak intramolecular hydrogen bonds, and other hydrogen bonding criteria should be investigated.