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

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• 作者：Mary Alice Upshur ; Hilary M. Chase ; Benjamin F. Strick ; Carlena J. Ebben ; Li Fu ; Hongfei Wang ; Regan J. Thomson ; Franz M. Geiger
• 刊名：Journal of Physical Chemistry A
• 出版年：2016
• 出版时间：May 5, 2016
• 年：2016
• 卷：120
• 期：17
• 页码：2684-2690
• 全文大小：414K
• 年卷期：0
• ISSN：1520-5215

文摘

This study aims to reliably assign the vibrational sum frequency generation (SFG) spectrum of α-pinene at the vapor/solid interface using a method involving deuteration of various methyl groups. The synthesis of five deuterated isotopologues of α-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 α-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 α-pinene SFG can be readily observed. The work presented here brings us one step closer to understanding the vibrational spectroscopy of α-pinene.