New particle formation from sulfuric acid and amines : Comparison of monomethylamine, dimethylamine, and trimethylamine

Amines are bases that originate from both anthropogenic and natural sources, and they are recognized as candidates to participate in atmospheric aerosol particle formation together with sulfuric acid. Monomethylamine, dimethylamine, and trimethylamine (MMA, DMA, and TMA, respectively) have been shown to enhance sulfuric acid-driven particle formation more efficiently than ammonia, but both theory and laboratory experiments suggest that there are differences in their enhancing potentials. However, as quantitative concentrations and thermochemical properties of different amines remain relatively uncertain, and also for computational reasons, the compounds have been treated as a single surrogate amine species in large-scale modeling studies. In this work, the differences and similarities of MMA, DMA, and TMA are studied by simulations of molecular cluster formation from sulfuric acid, water, and each of the three amines. Quantum chemistry-based cluster evaporation rate constants are applied in a cluster population dynamics model to yield cluster concentrations and formation rates at boundary layer conditions. While there are differences, for instance, in the clustering mechanisms and cluster hygroscopicity for the three amines, DMA and TMA can be approximated as a lumped species. Formation of nanometer-sized particles and its dependence on ambient conditions is roughly similar for these two: both efficiently form clusters with sulfuric acid, and cluster formation is rather insensitive to changes in temperature and relative humidity. Particle formation from sulfuric acid and MMA is weaker and significantly more sensitive to ambient conditions. Therefore, merging MMA together with DMA and TMA introduces inaccuracies in sulfuric acid-amine particle formation schemes.Peer reviewe

Mechanism of the Oxidation of Heptafulvenes to Tropones Studied by Online Mass Spectrometry and Density Functional Theory Calculations

We have identified the most likely reaction mechanism for oxidizing heptafulvenes to the corresponding tropones by experimental and theoretical investigations. The experimental studies were done by coupling a three-dimensional printed miniaturized reactor with an integrated electrospray ionization needle to a mass spectrometer. Using the experimentally observed ions as a basis, nine alternative reaction pathways were investigated with density functional theory calculations. The lowest energy reaction pathway starts with the formation of an epoxide that is opened upon the addition of a second equivalent of the oxidizing species meta-chloroperoxybenzoic acid. The adduct formed then undergoes a Criegee-like rearrangement to yield a positively charged hemiketal, which on deprotonation dissociates into acetone resembles a Hock-like rearrangement. and tropone. Overall, the reaction mechanism resembles a Hock-like rearrangement.Peer reviewe

Molecular understanding of sulphuric acid-amine particle nucleation in the atmosphere

4 pages 359-363 in the print version, additional 7 pages online.Peer reviewe

(H2O)10 and (H2O)12 on a Virtual Metal Surface: The Growth of Ice

(H2O)10 and (H2O)12 are used to investigate the growth of ice on metal surfaces with hexagonal symmetry. The model of the virtual metal surface was used to separate the electronic structure of the metal from that of the water cluster while maintaining the geometric constraints imposed by the metal surface on the water cluster. To complement the ab initio calculations on the water cluster, an additional multicenter analysis was done to analyze the hydrogen bonds within the clusters. These calculations suggested that the water bilayer structure adjacent to the virtual metal surface effectively shields the growing ice cystal from the metal surface

An Intuitively Understandable Quality Measure for Theoretical Vibrational Spectra

Computational chemistry has become a central tool in spectroscopic studies in most of chemical science. The quality of a calculated vibrational spectrum is commonly expressed as the deviation of the peak position from the experimental reference. With the increasing application of vibrational spectroscopy to complex (biological) systems, this is likely not sustainable. Here we present a quality measure for theoretical vibrational spectra based on matching the spectra to a reference database with the help of correlation coefficients. This approach can easily be applied to large sets of data and complex spectra without easily identifiable peak positions. We demonstrate this on a database of infrared spectra of 670 compounds using six different theoretical (DFT and force field) methods. Most importantly, it is intuitively understandable by both theoreticians and experimentalists