Synthesis and surface spectroscopy of α-pinene isotopologues and their corresponding secondary organic material.

Atmospheric aerosol-cloud interactions remain among the least understood processes within the climate system, leaving large uncertainties in the prediction of future climates. In particular, the nature of the surfaces of aerosol particles formed from biogenic terpenes, such as α-pinene, is poorly understood despite the importance of surface phenomena in their formation, growth, radiative properties, and ultimate fate. Herein we report the coupling of a site-specific deuterium labeling strategy with vibrational sum frequency generation (SFG) spectroscopy to probe the surface C-H oscillators in α-pinene-derived secondary organic aerosol material (SOM) generated in an atmospheric flow tube reactor.

Partially (resp. fully) reversible adsorption of monoterpenes (resp. alkanes and cycloalkanes) to fused silica.

This work compares the extent of reversibility and the thermodynamics of adsorption (K, ΔG°) of room-temperature vapors of common environmentally relevant monoterpenes (α-pinene, β-pinene, limonene, and 3-carene) and industrially relevant cyclic and acyclic non-terpene hydrocarbons (cyclohexane, hexane, octane, and cyclooctane) to fused silica surfaces. Vibrational sum frequency generation spectroscopy carried out in the C-H stretching region shows negligible surface coverage-dependent changes in the molecular orientation of all species surveyed except for cyclohexane. The group of monoterpenes studied here distinctly exhibits partially reversible adsorption to fused silica surfaces compared to the group of non-terpene hydrocarbons, demonstrating a link between molecular structure and adsorption thermodynamics.

Single-component supported lipid bilayers probed using broadband nonlinear optics.

Broadband SFG spectroscopy is shown to offer considerable advantages over scanning systems in terms of signal-to-noise ratios when probing well-formed single-component supported lipid bilayers formed from zwitterionic lipids with PC headgroups. The SFG spectra obtained from bilayers formed from DOPC, POPC, DLPC, DMPC, DPPC and DSPC show a common peak at ∼2980 cm, which is subject to interference between the C-H and the O-H stretches from the aqueous phase, while membranes having transition temperatures above the laboratory temperature produce SFG spectra with at least two additional peaks, one at ∼2920 cm and another at ∼2880 cm. The results validate spectroscopic and structural data from SFG experiments utilizing asymmetric bilayers in which one leaflet differs from the other in the extent of deuteration.

Unanticipated Stickiness of α-Pinene.

The adsorption of α-pinene to solid surfaces is an important primary step during the chemical conversion of this common terpene over mesoporous materials, as well as during the formation of atmospheric aerosols. We provide evidence of tight and loose physisorbed states of α-pinene bound on amorphous SiO as determined by their adsorption entropy, enthalpy, and binding free energies characterized by computational modeling and vibrational sum frequency generation (SFG) spectroscopy. We find that adsorption is partially (40-60%) irreversible over days at 294-342 K and 1 ATM total pressure of helium, which is supported by molecular dynamics (MD) simulations.

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 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.

Assessment of DFT for Computing Sum Frequency Generation Spectra of an Epoxydiol and a Deuterated Isotopologue at Fused Silica/Vapor Interfaces.

We assess the capabilities of eight popular density functional theory (DFT) functionals, in combination with several basis sets, as applied to calculations of vibrational sum frequency generation (SFG) spectra of the atmospherically relevant isoprene oxidation product trans-β-isoprene epoxydiol (IEPOX) and one of its deuterated isotopologues at the fused silica/vapor interface. We use sum of squared differences (SSD) and total absolute error (TAE) calculations to estimate the performance of each functional/basis set combination in producing SFG spectra that match experimentally obtained spectra from trans-β-IEPOX and one of its isotopologues. Our joined SSD/TAE analysis shows that while the twist angle of the methyl C3v symmetry axis of trans-β-IEPOX relative to the surface is sensitive to the choice of DFT functional, the calculated tilt angle relative to the surface normal is largely independent of the functional and basis set.

Beyond local group modes in vibrational sum frequency generation.

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.