Impact of a hydrophobic ion on the early stage of atmospheric aerosol formation

Atmospheric aerosols are one of the major factors affecting planetary climate, and the addition of anthropogenic molecules into the atmosphere is known to strongly affect cloud formation. The broad variety of compounds present in such dilute media and their specific underlying thermalization processes at the nanoscale make a complete quantitative description of atmospheric aerosol formation certainly challenging. In particular, it requires fundamental knowledge about the role of impurities in water cluster growth, a crucial step in the early stage of aerosol and cloud formation. Here, we show how a hydrophobic pyridinium ion within a water cluster drastically changes the thermalization properties, which will in turn change the corresponding propensity for water cluster growth. The combination of velocity map imaging with a recently developed mass spectrometry technique allows the direct measurement of the velocity distribution of the water molecules evaporated from excited clusters. In contrast to previous results on pure water clusters, the low-velocity part of the distributions for pyridinium-doped water clusters is composed of 2 distinct Maxwell–Boltzmann distributions, indicating out-of-equilibrium evaporation. More generally, the evaporation of water molecules from excited clusters is found to be much slower when the cluster is doped with a pyridinium ion. Therefore, the presence of a contaminant molecule in the nascent cluster changes the energy storage and disposal in the early stages of gas-to-particle conversion, thereby leading to an increased rate of formation of water clusters and consequently facilitating homogeneous nucleation at the early stages of atmospheric aerosol formation.

Nucleophilic Dearomatization of Activated Pyridines

Amongst nitrogen heterocycles of different ring sizes and oxidation statuses, dihydropyridines (DHP) occupy a prominent role due to their synthetic versatility and occurrence in medicinally relevant compounds. One of the most straightforward synthetic approaches to polysubstituted DHP derivatives is provided by nucleophilic dearomatization of readily assembled pyridines. In this article, we collect and summarize nucleophilic dearomatization reactions of - pyridines reported in the literature between 2010 and mid-2018, complementing and updating previous reviews published in the early 2010s dedicated to various aspects of pyridine chemistry. Since functionalization of the pyridine nitrogen, rendering a (transient) pyridinium ion, is usually required to render the pyridine nucleus sufficiently electrophilic to suffer the attack of a nucleophile, the material is organized according to the type of N-functionalization. A variety of nucleophilic species (organometallic reagents, enolates, heteroaromatics, umpoled aldehydes) can be productively engaged in pyridine dearomatization reactions, including catalytic asymmetric implementations, providing useful and efficient synthetic platforms to (enantioenriched) DHPs. Conversely, pyridine nitrogen functionalization can also lead to pyridinium ylides. These dipolar species can undergo a variety of dipolar cycloaddition reactions with electron-poor dipolarophiles, affording polycyclic frameworks and embedding a DHP moiety in their structures.

The DFT calculations of pKa values of the cationic acids of aniline and pyridine derivatives in common solvents

The theoretical pKa values of the derivatives of anilinium and pyridinium ions in 7 solvents are presented. For this purpose, the usage of isodesmic reaction scheme using the DFT/B3LYP approach with IEFPCM solvation was evaluated. We have shown that the suitable selection of reference species has the primary influence on the resulting data. For the studied anilinium ion derivatives the nonsubstituted anilinium ion seems to be a satisfactory reference system. The calculated values are in good accordance with the available experimental data with the RMS error of 1.00 and 0.99 pKa units in water and THF, respectively. The highest error in predicted pKa value is less than 2.0 pKa units in all cases. The chemical accuracy of the applied treatment is limited in the case of nitroaniline ions and the maximal therotetical uncertainty for derivatives of the pyridinium ion is within 2.1 pKa units. Our theoretical results enable us to predict the values of pKa for the solvents, where the experimental data are not completely available. Also the influence of the chemical structure on the accuracy of the applied method was discussed.