Insights into Heterogeneous Atmospheric Oxidation Chemistry:  Development of a Tailor-Made Synthetic Model for Studying Tropospheric Surface Chemistry

Abstract

The synthesis, characterization, and oxidation reaction of a tropospherically relevant terpene bound to a glass surface are reported. Vibrational broadband sum frequency generation (SFG) is used to characterize the various terpene-modified glass surfaces and track their interaction with ozone. SFG spectra indicate that, although orientations of the surface-bound terpenes depend on the linker strategies employed, the CdC double bond is accessible to gas-phase ozone regardless of the strategy applied. Exposure of the terpene-functionalized surface to ppm levels of ozone at 1 atm and 300 K yields an initial reaction probability of approximately 1 × 10 -5 per surface collision, which is significantly higher than the corresponding gas-phase reaction involving 1-methyl-1-cyclohexene (5 × 10 -7 from gas-phase collision theory). The interaction of ozone with a saturated octyl silane-functionalized glass surface leads to a slight molecular reorientation, or tilting, of the terminal CH 3 groups on a much slower time scale. Our work demonstrates that SFG spectroscopy can be used to determine reaction probabilities of heterogeneous atmospheric reactions and bridges the gap between atmospheric chemistry and surface functionalization.

Reaction of gas phase OH with unsaturated self-assembled monolayers and relevance to atmospheric organic oxidations

“…An enhanced reaction probability for the surface reaction is consistent with the fact that the terminal double bond should be the part of the SAM that OH first encounters in the structured monolayer, as also proposed for the NO 3 reaction, 57 and with previous studies showing enhanced reactivity at interfaces compared to the gas phase. [19][20][21][22][23][24][25][26][27][28] Reaction on every collision has also been observed for the reaction of Cl atoms with a C3Q SAM. 22 Although there are a number of studies of the OH oxidation of organics on surfaces and in or on particles, there are 25 measured g = 0.29 for the same surface, indicating that even for saturated alkyl chains, the surface reaction is fast.…”

Experimental and theoretical studies of the interaction of gas phase nitric acid and water with a self-assembled monolayer

“…44 Self-assembled monolayers (SAMs) are used as a model for organic compounds that are adsorbed on atmospheric surfaces. [45][46][47][48][49][50][51][52][53] We show, via a combination of experimental spectroscopic data and molecular dynamics simulations, that some of the gas phase HNO 3 intercalates between the hydrocarbon chains and is not fully available to interact with water that is added subsequently. However, simultaneous exposure of the SAM to HNO 3 and H 2 O only generates protons and nitrate ions.…”

Contrasting organic aerosol particles from boreal and tropical forests during HUMPPA-COPEC-2010 and AMAZE-08 using coherent vibrational spectroscopy

“…1a, we use a broadband infrared source for our SFG experiments (Voges et al, 2005b(Voges et al, , 2007bHayes et al, 2009;Stokes et al, 2009a, c). Briefly, a regeneratively amplified Ti:sapphire laser (Spitfire Pro, Spectra Physics) produces ∼120 fsec pulses of 800 nm wavelength at a 1 kHz repetition rate.…”

“…Vibrational SFG spectra of aliphatic hydrocarbons can show up to five peaks in the CH stretching region, and the assignment of the vibrational resonances is very well established (Dollish et al, 1974;Bellamy, 1975;Guyot-Sionnest et al, 1987b;Roeges, 1994;Sefler et al, 1995;Nanjundiah et al, 2005;Esenturk et al, 2006;Voges et al, 2007a). Specifically important for this work, methyl CH stretches are identified as follows: asymmetric CH 3 stretching modes generally appear in the 2945 cm −1 to 2975 cm −1 range, symmetric CH 3 stretching modes appear between 2870 and 2890 cm −1 , and CH 3 Fermi resonances are located between 2915 and 2945 cm −1 .…”

“…SFG has become a popular method for addressing scientific questions regarding heterogeneous atmospheric chemistry in laboratory model systems consisting of surfaces of flat oxide solids functionalized with atmospherically relevant molecules (Konek et al, 2004;Voges et al, 2005aVoges et al, , 2007bStokes et al, 2009a, b, c), and of liquids, notably water (Allen et al, 1999;Liu et al, 2004;Mucha et al, 2005;Gopalakrishnan et al, 2006;Tarbuck et al, 2006;Voss et al, 2007). Hallmarks of the method are an exquisite sensitivity to molecular structure within complex environments, a very high selectivity for environments where symmetry is broken, and intrinsic heterodyne detection of weak vibrational responses, which allows for the analysis of nanoand sub-nanogram amounts of sample.…”