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 (Kads, ΔG°ads) of room-temperature vapors of common environmentally relevant monoterpenes (α-pinene, ß-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. The standard Gibbs free energy of adsorption is nonlinearly correlated with the equilibrium vapor pressure of the compounds surveyed.

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-1, 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-1 and another at ∼2880 cm-1. 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. Differences in H2O-D2O exchange experiments reveal that the lineshapes of the broadband SFG spectra are significantly influenced by interference from OH oscillators in the aqueous phase, even when those oscillators are not probed by the incident infrared light in our broadband setup. In the absence of spectral interference from the OH stretches of the solvent, the alkyl chain terminal methyl group of the bilayer is found to be tilted at an angle of 15° to 35° from the surface normal.

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 SiO2 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. The distribution of α-pinene orientation remains invariant with temperature and partial pressure of α-pinene. Using the Redlich-Peterson adsorption model in conjunction with a van't Hoff analysis of adsorption isotherms recorded for up to 2.6 Torr α-pinene in 1 ATM total pressure of helium, we obtain ΔS°ads, ΔH°ads, and ΔG°ads values of -57 (±7) J mol-1 K-1, -39 (±2) kJ mol-1, and -22 (±5) kJ mol-1, respectively, associated with the reversibly bound population of α-pinene. These values are in good agreement with density functional theory (DFT)-corrected force field calculations based on configurational sampling from MD simulations. Our findings are expected to have direct implications on the conversion of terpenes by silica-based catalysts and for the synthesis of secondary organic aerosol (SOA) in atmospheric chambers and flow tubes.

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. 0.6 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.

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-ß-isoprene epoxydiol (trans-ß-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-ß-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.

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. Three α-pinene isotopologues with methylene bridge, bridgehead methine, allylic, and vinyl deuteration were synthesized and their vapor phase SFG spectra were compared to that of unlabeled α-pinene. Subsequent analysis of the SFG spectra of their corresponding SOM revealed that deuteration of the bridge methylene C-H oscillators present on the cyclobutane ring in α-pinene leads to a considerable signal intensity decrease (ca. 30-40%), meriting speculation that the cyclobutane moiety remains largely intact within the surface bound species present in the SOM formed upon α-pinene oxidation. These insights provide further clues as to the complexity of aerosol particle surfaces, and establish a framework for future investigations of the heterogeneous interactions between precursor terpenes and particle surfaces that lead to aerosol particle growth under dynamically changing conditions in the atmosphere.

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-ß-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-ß-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-ß-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. Moreover, we report that hybrid functionals such as B3LYP, ωB97X-D, PBE0, and B97-1 in combination with a modest basis set, such as 6-311G(d,p), provides good agreement with experimental data and much better performance than pure functionals such as PBE and BP86. However, improving the quality of the basis set only improves agreement with experimental data for calculations based on pure functionals. A conformational analysis, based on comparisons of calculated and experimental SFG spectra, suggests that trans-ß-IEPOX points all of its oxygen atoms toward the silica/vapor interface.