Vibrational Mode Assignment of α-Pinene by Isotope Editing: One Down, Seventy-One To Go

This study aims to reliably assign the vibrational sum frequency generation (SFG) spectrum of alpha-pinene at the vapor/solid interface using a method involving deuteration of various methyl groups. The synthesis of five deuterated isotopologues of alpha-pinene is presented to determine the impact that removing contributions from methyl group C-H oscillators has on its SFG response. 0.6 cm(-1) resolution SFG spectra of these isotopologues show varying degrees of differences in the C-H stretching region when compared to the SFG response of unlabeled alpha-pinene. The largest spectral changes were observed for the isotopologue containing a fully deuterated vinyl methyl group. Noticeable losses in signal intensities allow us to reliably assign the 2860 cm(-1) peak to the vinyl methyl symmetric stretch. Furthermore, upon removing the vinyl methyl group entirely by synthesizing apopinene, the steric influence of the unlabeled C9H14 fragment on the SFG response of alpha-pinene SFG can be readily observed. The work presented here brings us one step closer to understanding the vibrational spectroscopy of alpha-pinene.