PNNL

Abstract

A rotationally fluid state of a-pinene at fused silica/vapor interfaces is revealed by computational and experimental vibrational sum frequency generation (SFG) studies. We report the first assignment of the vibrational modes in the notoriously congested C-H stretching region of a-pinene and identify its bridge methylene group on the four-membered ring ("ßCH2") as the origin of its dominant spectral feature. We find that the spectra are perfused with Fermi resonances that need to be accounted for explicitly in the computation of vibrational spectra of strained hydrocarbons such a-pinene. The preferred orientations of a-pinene are consistent with optimization of van der Waals contacts with the silica surface that results in a bimodal distribution of highly fluxional orientations in which the ßCH2 group points "towards" or "away from” the surface. The reported findings are particularly relevant to the exposure of a-pinene to primary oxidants in heterogeneous catalytic pathways that exploit a-pinene as a sustainable feedstock for fine chemicals and polymers.