Understanding the Underlying Field Evaporation Mechanism of Pure Water Tips in High Electrical Fields.

We investigated the field evaporation process of frozen water in atom probe tomography (APT) by density functional simulations. In previous experiments, a strong tailing effect was observed for peaks caused by the molecular structure (H2O)nH+, in contrast to other peaks. In purely field-induced and thermally assisted evaporation simulations, we found that chains of protonated water molecules were pulled out of the dielectric surface by up to 6 Å, which are stable over a wide range of field strengths. Therefore, the resulting water clusters experience only part of the acceleration after evaporation compared to molecules evaporating directly from the surface and, thus, exhibit an energy deficit, which explains the tailing effect. Our simulations provide new insight into the complex evaporation behavior of water in high electrical fields and reveal possibilities for adapting the existing reconstruction algorithms.

Predicting properties of periodic systems from cluster data: A case study of liquid water.

The accuracy of the training data limits the accuracy of bulk properties from machine-learned potentials. For example, hybrid functionals or wave-function-based quantum chemical methods are readily available for cluster data but effectively out of scope for periodic structures. We show that local, atom-centered descriptors for machine-learned potentials enable the prediction of bulk properties from cluster model training data, agreeing reasonably well with predictions from bulk training data. We demonstrate such transferability by studying structural and dynamical properties of bulk liquid water with density functional theory and have found an excellent agreement with experimental and theoretical counterparts.

Interlayer Interactions as Design Tool for Large-Pore COFs.

Covalent organic frameworks (COFs) with a pore size beyond 5 nm are still rarely seen in this emerging field. Besides obvious complications such as the elaborated synthesis of large linkers with sufficient solubility, more subtle challenges regarding large-pore COF synthesis, including pore occlusion and collapse, prevail. Here we present two isoreticular series of large-pore imine COFs with pore sizes up to 5.8 nm and correlate the interlayer interactions with the structure and thermal behavior of the COFs. By adjusting interlayer interactions through the incorporation of methoxy groups acting as pore-directing "anchors", different stacking modes can be accessed, resulting in modified stacking polytypes and, hence, effective pore sizes. A strong correlation between stacking energy toward highly ordered, nearly eclipsed structures, higher structural integrity during thermal stress, and a novel, thermally induced phase transition of stacking modes in COFs was found, which sheds light on viable design strategies for increased structural control and stability in large-pore COFs.

Evaporation and Fragmentation of Organic Molecules in Strong Electric Fields Simulated with DFT.

Atom probe tomography allows us to measure the three-dimensional composition of materials with up to atomic resolution by evaporating the material using high electric fields. Initially developed for metals, it is increasingly used for covalently bound structures. To aid the interpretation of the obtained fragmentation pattern, we modeled the fragmentation and desorption of self-assembled monolayers of thiolate molecules on a gold surface in strong electrostatic fields using density functional theory. We used a cluster model and a periodic model of amino-undecanethiolate, NH2(CH2)11S, and fluoro-decanethiolate, CF3(CF2)7(CH2)2S. In the former molecule, the fragment CH2NH2+ was found to evaporate at fields of 5.4-7.7 V/nm. It was followed by different hydrocarbon fragments. Fluoro-decanethiolate evaporates CF3+ at fields of 5.7-6.7 V/nm in the cluster model and at 15.4-23.1 V/nm in the periodic model, followed by CF2+ and C2F42+. Detailed analysis of the electronic structure during the evaporation process revealed a stepwise accumulation of the charge in the head groups exposed to the strongest fields, followed by dissociation of covalent bonds. These observations will facilitate the analysis of atom probe experiments of covalently bound structures.

Proton Affinities of N-Heterocyclic Olefins and Their Implications for Organocatalyst Design.

The proton affinity (PA) of a range of structurally different N-heterocycles with an exocyclic double bond (= N-heterocyclic olefins, NHOs) has been determined using DFT calculations on the BLYP/def2-TZVPP level. It was found that NHOs belong to the upper end of the superbasicity scale, covering PA values from 262 to 296 kcal/mol. Different types of NHOs are compared with each other and with frequently employed organocatalysts. To boost PA, (a) the ability to delocalize the positive charge and (b) steric pressure/ring strain which can be relieved after protonation were identified as key tuning parameters. Importantly, by analyzing PA alongside partial charges and molecular electrostatic potentials, it is shown that an increase of double bond polarization is not a necessary prerequisite for high PA. In contrast, the more basic, more sterically congested NHOs minimize unfavorable interactions by partly pyramidalyzing the nitrogen atoms, rendering the olefinic bond less electron rich and less polarized. These findings are in excellent agreement with experimental evidence on NHO catalysis, not only providing guidelines for a more rational design regarding PA/basicity but also suggesting that NHOs could be specifically tailored toward either nucleophilic or base-type reaction pathways.