Deriving Global OH Abundance and Atmospheric Lifetimes for Long-Lived Gases: A Search for CH 3 CCl 3 Alternatives

An accurate estimate of global hydroxyl radical (OH) abundance is important for projections of air quality, climate, and stratospheric ozone recovery. As the atmospheric mixing ratios of methyl chloroform (CH₃CCl₃) (MCF), the commonly used OH reference gas, approaches zero, it is important to find alternative approaches to infer atmospheric OH abundance and variability. The lack of global bottom‐up emission inventories is the primary obstacle in choosing a MCF alternative. We illustrate that global emissions of long‐lived trace gases can be inferred from their observed mixing ratio differences between the Northern Hemisphere (NH) and Southern Hemisphere (SH), given realistic estimates of their NH‐SH exchange time, the emission partitioning between the two hemispheres, and the NH versus SH OH abundance ratio. Using the observed long‐term trend and emissions derived from the measured hemispheric gradient, the combination of HFC‐32 (CH₂F₂), HFC‐134a (CH₂FCF₃, HFC‐152a (CH₃CHF₂), and HCFC‐22 (CHClF₂), instead of a single gas, will be useful as a MCF alternative to infer global and hemispheric OH abundance and trace gas lifetimes. The primary assumption on which this multispecies approach relies is that the OH lifetimes can be estimated by scaling the thermal reaction rates of a reference gas at 272 K on global and hemispheric scales. Thus, the derived hemispheric and global OH estimates are forced to reconcile the observed trends and gradient for all four compounds simultaneously. However, currently, observations of these gases from the surface networks do not provide more accurate OH abundance estimate than that from MCF

Enantioselective chemisorption on model chirally modified surfaces: 2-Butanol on r-(1-naphthyl)ethylamine/Pd(111)

The enantioselective adsorption of propylene oxide and 2-butanol is explored on α-(1-naphthyl)-ethylamine (NEA) covered Pd(111) using temperature-programmed desorption (TPD), reflection-absorption infrared spectroscopy (RAIRS), and scanning tunneling microscopy (STM). The saturation coverage of NEA is ∼0.1 monolayers (ML), and it thermally decomposes to desorb hydrogen and HCN. 2-Butanol adsorbs enantioselectively on NEA-modified Pd(111), while propylene oxide does not, emphasizing the importance of hydrogen-bonding interactions. An enantioselectivity ratio, ER, of ∼2 (corresponding to an ee of ∼33%) is found at an NEA coverage of ∼0.055 ML, where 2-butanol adsorbs on the Pd(111) substrate. A second regime is found in which 2-butanol adsorbs on an NEA-covered surface with a maximum ER ∼ 1.8 (corresponding to an ee of ∼29%). This interaction appears to cause the NH2 group to reorient to facilitate hydrogen bonding interactions between 2-butanol and the amine group, and the heat of adsorption of ∼35 kJ/mol is typical of - OH... NH2 hydrogen bond strengths. © 2009 American Chemical Society

An Infrared Spectroscopic and Temperature-Programmed Desorption Study of 1,1-Difluoroethylene on Clean and Hydrogen-Covered Pd(111)

The surface chemistry of 1,1-difluoroethylene was studied on clean and hydrogen-covered Pd(111) using a combination of temperature-programmed desorption and reflection absorption infrared spectroscopy (RAIRS) to explore whether the larger infrared absorbance of 1,1-difluoroethylene than ethylene may be used to examine reactions under realistic catalytic conditions using RAIRS. It was found that the chemistry of 1,1-difluoroethylene on Pd(111) surfaces is similar to that of ethylene with bonding occurring in both the π- and di-σ-forms. However, due to the presence of C–F bonds in the molecule, the infrared absorbances for 1,1-difluoroethylene were much larger than those for ethylene. This provides the potential for using RAIRS for in situ studies of catalytic reactions that involve alkenes

Surface Segregation of Gold for Au/Pd(111) Alloys Measured by LowEnergy Electron Diffraction and Low-Energy Ion Scattering

Abstract 11 The surface composition of a Au/Pd(1 1 1) alloy formed by depositing five monolayers of gold onto clean Pd(1 1 1) at 300 K in an 12 ultrahigh vacuum and heating to various temperatures is measured from an analysis of the low-energy electron diffraction (LEED) inten

The structure of alanine anionic-zwitterionic dimers on Pd(111); formation of salt bridges

The structure of alanine zwitterion-anion dimers previously proposed to form on Pd(111) is investigated using low-energy electron diffraction (LEED). Because of the propensity of amino acids on Pd(111) to undergo electron-beam-induced decomposition, LEED intensity versus beam voltage (I-V) curves were measured using a delay line detector LEED (DLD-LEED) system, which enables the complete LEED I-V curves to be obtained for an electron exposure of less than one electron per adsorbate. Since no long-range order is found following alanine adsorption on Pd(111), the adsorbate structure is determined from the substrate diffraction spots. The I-V data were analyzed using the CLEED: Automated Surface Structure package for an initial input structure from previous DFT calculations of the alanine zwitterion-anion dimer, the best fit structure yielded a satisfactory Pendry R-factor of 0.249, thus confirming the correctness of the originally proposed structure. Such anionic-zwitterionic dimers are a class of hydrogen-bonding intermolecular interactions in proteins that are dominated by direct electrostatic interactions, in particular for residues such a lysine and arginine, known as salt bridges. While such salt bridges interactions are relatively weak in biological systems, the attractive interaction energy between the anion and the zwitterion in the dimer on Pd(111) in vacuo is found to be ∼95 kJ/mol. It is proposed that the weaker binding biological systems occurs because the electrostatic screening in aqueous media weakens the electrostatic interaction between the anion and zwitterion, causing their structures to relax

Identifying Molecular Species on Surfaces by Scanning Tunneling Microscopy: Methyl Pyruvate on Pd(111)

The structures of low coverages of methyl pyruvate on a Pd(111) surface at 120 K were studied using scanning tunneling microscopy in ultrahigh vacuum. The experimentally observed images were assigned to adsorbate structures using a combination of density functional theory calculations and by simulating the images using the Bardeen method. Two forms of methyl pyruvate were identified. The first, previously found using reflection–absorption infrared spectroscopy, was a flat-lying, keto form of <i>cis</i>-methyl pyruvate. It was characterized by elongated, two-lobed images with the long axes of the images oriented at ∼0 and ∼30° to the close-packed directions. The structure was simulated using clean, CO- and methyl-functionalized gold tips, and the simulated images agreed well with those found experimentally. The simulated structures were not strongly dependent on the tip structure or tip bias. This approach was used to identify the nature of the second species as the enol form of <i>cis</i>-methyl pyruvate with the carbonyl groups located over atop and bridge sites. Again, the orientation of the image with respect to the underlying Pd(111) lattice as well as the calculated image shape agreed well with the experimental images