Uncovering the mechanism of selective stabilization of high-energy diastereoisomers via inclusion.

Unlabelled: Supramolecular systems may be used to stabilize otherwise unstable isomers to find alternative synthetic pathways. It has been reported that cucurbit[8]uril can stabilize -I and -II Cu cyclam, whereas -III is the only non-substituted Cu cyclam diastereoisomer found outside of the host molecule experimentally. Quantum chemistry methods can provide valuable insight into the intermolecular interactions involved in these inclusion complexes.

Machine Learned Composite Methods for Electronic Structure Theory.

Because of the prohibitive scaling of ab initio techniques for modeling chemical species with high accuracy, they are not generally tractable for large systems. It is therefore of considerable interest to develop high-accuracy computational models with low computational cost that can afford predictions of electronic structure and properties of macromolecular species. Composite methods, as first introduced by Pople [Pople, J.

A shared-weight neural network architecture for predicting molecular properties.

Quantum chemical methods scale poorly with increasing molecular size and machine learning models have emerged as a promising, computationally-efficient alternative. We present a shared-weight neural network architecture based on modified atom-centered symmetry functions (ACSFs) and show that it performs similarly to the more computationally expensive per-element neural networks of previous work with ACSFs. The model achieves chemically accurate predictions, with a mean absolute error as low as 0.

Novel Bonding Mode in Phosphine Haloboranes.

We have predicted, using a wide range of theoretical models, the potential energy surfaces of dative bond stretching in some phosphine haloboranes and closely associated analogues. It is shown that these dative complexes demonstrate unusual bond stretching potentials that are characterized by having multiple inflection points and are not able to be fit to any traditional Morse or Lennard-Jones-type curve. Specifically, in the case of ClB-PH, this effect is so pronounced that the surface actually exhibits two distinct minima.

Revealing Electron-Electron Interactions within Lewis Pairs in Chemical Systems.

The so-called "Lewis pair" is a ubiquitous phenomenon in chemistry and is often used as an intuitive construct to predict and rationalize chemical structure and behavior. Concepts from the very general Valence Shell Electron Pair Repulsion (VSEPR) model to the most esoteric reaction mechanism routinely rely on the notion that electrons tend to exist in pairs and that these pairs can be thought of as being localized to a particular region of space. It is precisely this localization that allows one to intuit how these pairs might behave, generally speaking, so that reasonable predictions may be made regarding molecular structure, intermolecular interactions, property trends, and reaction mechanisms, etc.

Exploring electron pair behaviour in chemical bonds using the extracule density.

We explore explicit electron pair behaviour within the chemical bond (and lone pairs) by calculating the probability distribution for the center-of-mass (extracule) of an electron pair described by single localized orbitals. Using Edmiston-Ruedenberg localized orbitals in a series of 61 chemical systems, we demonstrate the utility of the extracule density as an interpretive tool in chemistry. By accessing localized regions of chemical space we simplify the interpretation of the extracule density and afford a quantum mechanical interpretation of "chemically intuitive" features of electronic structure.

Assessment and Application of Density Functional Theory for the Prediction of Structure and Reactivity of Vanadium Complexes.

We assess the performance of six density functionals, each paired with one of five basis sets (a total of 30 model chemistries) for the prediction of geometrical parameters in the coordination sphere of nine vanadium complexes (for a total of 270 structural analyses). We find that results are generally consistent over the range of functionals tested and that none fail drastically. For bond lengths, the model chemistry PBE0/QZVP performed the best overall (having a MAD of only 0.

Predicting bond strength from a single Hartree-Fock ground state using the localized pair model.

We present an application of the recently introduced Localized Pair Model (LPM) [Z. A. Zielinksi and J.

Isolation and structure elucidation of satosporin A and B: new polyketides from Kitasatospora griseola.

Satosporins A and B, two novel glucosylated polyketides, were isolated from the actinomycete Kitasatospora griseola MF730-N6. The polyketides possess an unprecedented tricyclic ring system that was fully characterized using a combination of spectroscopic analyses and computational calculations. Satosporin A was quantitatively converted into its aglycon homologue, satosporin C, using a β-glucosidase.

A simultaneous probability density for the intracule and extracule coordinates.

We introduce the intex density X(R,u), which combines both the intracular and extracular coordinates to yield a simultaneous probability density for the position of the center-of-mass radius (R) and relative separation (u) of electron pairs. One of the principle applications of the intex density is to investigate the origin of the recently observed secondary Coulomb hole. The Hartree-Fock (HF) intex densities for the helium atom and heliumlike ions are symmetric functions that may be used to prove the isomorphism 2I(2R)=E(R), where I(u) is the intracule density and E(R) is the extracule density.

Isolation, biomimetic synthesis, and cytotoxic activity of bis(pseudopterane) amines.

LC-MS/MS-based screening of the dichloromethane extract of the gorgonian coral Pseudopterogorgia acerosa led to the isolation of a novel bis(pseudopterane) amine (1). The structural assignment of 1 was achieved by 1D and 2D NMR and mass spectrometry analysis. A biomimetic synthesis of 1 and the known symmetrical diterpene 2 from pseudopterolide (3) is described in this report.

Intracule functional models. IV. Basis set effects.

We have calculated position and dot intracules for a series of atomic and molecular systems, starting from an unrestricted Hartree-Fock wave function, expanded using the STO-3G, 6-31G, 6-311G, 6-311++G, 6-311++G(d,p), 6-311++G(3d,3p), and 6-311++G(3df,3pd) basis sets as well as the nonpolarized part of Dunning's cc-pV5Z basis. We find that the basis set effects on the intracules are small and that correlation energies from the dot intracule ansatz are remarkably insensitive to the basis set quality. Mean absolute errors in correlation energies across the G1 data set agree to within 2 mE(h) for all basis sets tested.

Understanding the electronic structure, reactivity, and hydrogen bonding for a 1,2-diphosphonium dication.

The experimental charge density for hexamethyldiphosphonium ditriflate has been determined from low-temperature high-resolution X-ray diffraction data. These results have been compared with theoretically calculated values for the isolated gas-phase compound. Analysis of the topological and atomic basin properties has provided insight into the exact nature of the P-P bond in both the crystalline and the gas-phase structures.

Effect of substituents on the GPx-like activity of ebselen: steric versus electronic.

The direct oxidation of ebselen and several derivatives by hydrogen peroxide is investigated using the B3LYP/6-31G(d,p) method to elucidate the effects of substituents on GPx-like activity. While previous studies have attributed the differences in GPx activity of substituted ebselen compounds to the electronic nature of the substituents, the influence of functional groups is poorly understood. The effects of various solvents are incorporated by employing the CPCM method.

Density functional theory study of the reaction mechanism and energetics of the reduction of hydrogen peroxide by ebselen, ebselen diselenide, and ebselen selenol.

Density functional theory calculations at the B3LYP/6-311++G(3df,3pd)//B3LYP/6-31G(d,p) level have been performed to elucidate the mechanism and reaction energetics for the reduction of hydrogen peroxide by ebselen, ebselen diselenide, ebselen selenol, and their sulfur analogues. The effects of solvation have been included with the CPCM model, and in the case of the selenol anion reaction, diffuse functions were used on heavy atoms for the geometry optimizations and thermochemical calculations. The topology of the electron density in each system was investigated using the quantum theory of atoms in molecules, and a detailed interpretation of the electronic charge and population data as well as the atomic energies is presented.

Modeling the reduction of hydrogen peroxide by glutathione peroxidase mimics.

Theoretical calculations have been performed on three model reactions representing the reduction of hydrogen peroxide by ebselen, ebselen selenol, and ebselen diselenide. The reaction surfaces have been investigated at the B3PW91/6-311G(2df,p) level, and single-point energies were calculated using the 6-311++G(3df,3pd) basis set. Solvent effects were included implicitly with the conductor-like polarizable continuum model and in one case with explicit inclusion of three water molecules.

An evaluation of various computational methods for the treatment of organoselenium compounds.

A reliable computational method for the prediction of organoselenium geometries and bond dissociation energies (BDEs) has been determined on the basis of the performance of density functional theory (DFT: B3LYP and B3PW91) and ab initio molecular orbital procedures (Hartree-Fock (HF)) in conjunction with various Pople basis sets including (but not limited to) the 6-31G(d), 6-31G(d,p), 6-311G(d), 6-311G(d,p), 6-311G(2df,p), and 6-311G(3df,3pd) sets. Predicted geometries and BDEs are compared with available experimental data and quadratic configuration interaction including single and double substitutions (QCISD) results. The B3PW91/6-311G(2df,p) level of theory is recommended for the prediction of the geometries and energetics of organoselenium compounds.