Intramolecular couplings and the secondary structure of C [tbnd]C stretching bands

Abstract

The band shape analysis of the acetylenic Raman bands of phenylacetylene (2109 cm −1), diphenylacetylene (2220 cm −1), trimethylsilylacetylene (2035 cm −1) and di(trimethylsilyl)acetylene (2106 cm −1) vs. concentration and the IR bands of phenylacetylene and trimethylsilylacetylene vs. temperature has been performed. All the bands are asymmetric being broader at the low-frequency side with the first band moments measured around 2–3 cm −1. Since the differences between the normalized spectra in CCl 4 (with mole fractions of solute roughly from 0.01 to 0.2) were within the spectral noise, the Raman band asymmetry was shown to be concentration independent. The temperature-dependent IR spectra in C 2Cl 4 revealed increase of full widths at half height by around 50% and decrease of heights by the same percent for all cases. The centres of gravity were also shown to be temperature dependent rising from around 2 cm −1 at 25 °C to around 5 cm −1 at 105 °C. The isosbestic-point like temperature evolution of the band shapes is qualitatively explained by the absorption temperature factor. Supported by the previous and present findings, as a common cause of the Raman and IR band asymmetry, the sequences of hot bands with slightly shifted wavenumbers has been accepted. However, contrary to what is usually assumed we argue that the anharmonic coupling is not rigorously selective and the asymmetric band shape is the result of the anharmonic interactions with more than one of the modes lying low enough for their excited states to be significantly populated at the given temperature. This view is further corroborated by the calculated anharmonic constants for phenylacetylene. It is also suggested that the observed band shape is not a sum of uncorrelated individual symmetric band shapes but the result of indirect vibrational dephasing.