Ab-Initio Anharmonic Analysis of Complex Vibrational Spectra of Phenylacetylene and Fluorophenylacetylenes in the Acetylenic and Aromatic C–H Stretching Region

Vibrational spectra in the acetylenic and aromatic C–H stretching region of phenylacetylene and fluorophenylacetylenes viz., 2-fluorophenylacetylene, 3-fluorophenylacetylene and 4-fluorophenylacetylene, were measured using IR-UV double resonance spectroscopic method. The spectra, in both acetylenic and aromatic C–H stretching regions, were complex exhibiting multiple bands. Ab-initio anharmonic calculations with quartic potential (QP) using B97D3/6-311++G(d,p) and vibrational configuration interaction (VCI) were able to capture all important spectral features, in both the regions of the experimentally observed spectra for all four molecules considered in the present work. Interestingly, for phenylacetylene, the spectrum in the acetylenic C–H stretching region emerges due to anharmonic coupling of mode localized on the acetylenic moiety along with the other ring modes which also involve displacements on the acetylenic group, which is in contrast to what has been proposed and propagated in the literature. In general, this coupling scheme is invariant to the fluorine atom substitution. For the aromatic C–H stretching region the observed spectrum emerges due to the coupling of the C–H stretching with C–C stretching and C–H in-plane bending modes.