title: Beyond local group modes in vibrational sum frequency generation.
abstract: We combine deuterium labeling, density functional theory calculations, and experimental vibrational sum frequency generation spectroscopy into a form of "counterfactual-enabled molecular spectroscopy" for producing reliable vibrational mode assignments in situations where local group mode approximations are insufficient for spectral interpretation and vibrational mode assignments. We demonstrate the method using trans-β-isoprene epoxydiol (trans-β-IEPOX), a first-generation product of isoprene relevant to atmospheric aerosol formation, and one of its deuterium-labeled isotopologues at the vapor/silica interface. We use our method to determine that the SFG responses that we obtain from trans-β-IEPOX are almost exclusively due to nonlocal modes involving multiple C-H groups oscillating at the same frequency as one vibrational mode. We verify our assignments using deuterium labeling and use DFT calculations to predict SFG spectra of additional isotopologues that have not yet been synthesized. Finally, we use our new insight to provide a viable alternative to molecular orientation analysis methods that rely on local mode approximations in cases where the local mode approximation is not applicable.
authors.affiliation: †Department of Chemistry, Northwestern University, Evanston, Illinois 60208, United States.
authors.forename: Hilary M
authors.initials: HM
authors.lastname: Chase
authors.affiliation: ‡Department of Chemistry, Yale University, P.O. Box 208107, New Haven, Connecticut 06520-8107, United States.
authors.forename: Brian T
authors.initials: BT
authors.lastname: Psciuk
authors.affiliation: †Department of Chemistry, Northwestern University, Evanston, Illinois 60208, United States.
authors.forename: Benjamin L
authors.initials: BL
authors.lastname: Strick
authors.affiliation: †Department of Chemistry, Northwestern University, Evanston, Illinois 60208, United States.
authors.forename: Regan J
authors.initials: RJ
authors.lastname: Thomson
authors.affiliation: ‡Department of Chemistry, Yale University, P.O. Box 208107, New Haven, Connecticut 06520-8107, United States.
authors.forename: Victor S
authors.initials: VS
authors.lastname: Batista
authors.affiliation: †Department of Chemistry, Northwestern University, Evanston, Illinois 60208, United States.
authors.forename: Franz M
authors.initials: FM
authors.lastname: Geiger