Self-sorting: the exception or the rule?

In this paper, we pose the question of whether self-sorting in designed systems is exceptional behavior or whether it is likely to become a more general phenomenon governing molecular recognition and self-assembly. To address this question we prepared a mixture comprising two of Davis' self-assembled ionophores, Rebek's tennis ball and calixarene tetraurea capsule, Meijer's ureidopyrimidinone, Reinhoudt's calixarene bis(rosette), and two molecular clips in CDCl(3) solution and observed the behavior of this ensemble by (1)H NMR. As hypothesized, high-fidelity self-sorting behavior was observed. The influence of several key variables-temperature, concentration, equilibrium constants, and the presence of competitors-on the fidelity of self-sorting is described. These results show that self-sorting is neither the exception nor the rule. They suggest, however, that the subset of known molecular aggregates that exceed the criteria required for thermodynamic self-sorting is larger than previously appreciated and potentially quite broad.     

The σ− selection rule in electron attachment and autoionization of diatomic molecules

It is shown theoretically that Σ⁺ ⇐ Σ⁻ transitions in electron attachment and autionization of diatomic molecules are forbidden. For the verification of this selection rule, two experimental examples are given.     

Chirality: A superselection rule generated by the molecular environment?

The superposition of left- and right-handed forms of a chiral molecule does not exist or is at least very unstable. Frequently, this instability is traced back to the coupling of the molecule to its environment, e.g. the radiation field or collisions with neighbor molecules. The situation is not completely clear, neither theoretically nor experimentally. Here, the theoretical aspects and consequences of the coupling (molecule environment) are discussed.     

The theory of electron transfer reactions: what may be missing?

Molecular dynamics simulations are presented for condensed-phase electron transfer (ET) systems where the electronic polarizability of both the solvent and the solute is incorporated. The solute polarizability is allowed to change with electronic transition. The results display notable deviation from the standard free energy parabolas of traditional ET theories. A new three-parameter ET model is applied, and the theory is shown to accurately model the free energy surfaces. This paper presents conclusive evidence that the traditional theory for the free energy barrier of ET reactions requires modification.     

Chemical bonding in hypervalent molecules: is the octet rule relevant?

The bonding in a large number of hypervalent molecules of P, As, S, Se, Te, Cl, and Br with the ligands F, Cl, O, CH(3), and CH(2) has been studied using the topological analysis of the electron localization function ELF. This function partitions the electron density of a molecule into core and valence basins and further classifies valence basins according to the number of core basins with which they have a contact. The number and geometry of these basins is generally in accord with the VSEPR model. The population of each basin can be obtained by integration, and so, the total population of the valence shell of an atom can be obtained as the sum of the populations of all the valence basins which share a boundary with its core basin. It was found that the population of the V(A, X) disynaptic basin corresponding to the bond, where A is the central atom and X the ligand, varies with the electronegativity of the ligand from approximately 2.0 for a weakly electronegative ligand such as CH(3) to less than 1.0 for a ligand such as F. We find that the total population of the valence shell of a hypervalent atom may vary from close to 10 for a period 15 element and close to 12 for a group 16 element to considerably less than 8 for an electronegative ligand such as F. For example, the phosphorus atom in PF(5) has a population of 5.37 electrons in its valence shell, whereas the arsenic atom in AsMe5 has a population of 9.68 electrons in its valence shell. By definition, hypervalent atoms do not obey the Lewis octet rule. They may or may not obey a modified octet rule that has taken the place of the Lewis octet rule in many recent discussions and according to which an atom in a molecule always has fewer than 8 electrons in its valence shell. We show that the bonds in hypervalent molecules are very similar to those in corresponding nonhypervalent (Lewis octet) molecules. They are all polar bonds ranging from weakly to strongly polar depending on the electronegativity of the ligands. The term hypervalent therefore has little significance except to indicate that an atom in a molecule is forming more than four electron pair bonds.     

Reversed Hofmeister series—The rule rather than the exception

Over recent years, the supposedly universal Hofmeister series has been replaced by a diverse spectrum of direct, partially altered and reversed series. This review aims to provide a detailed understanding of the full spectrum by combining results from molecular dynamics simulations, Poisson–Boltzmann theory and AFM experiments. Primary insight into the origin of the Hofmeister series and its reversal is gained from simulation-derived ion–surface interaction potentials at surfaces containing non-polar, polar and charged functional groups for halide anions and alkali cations. In a second step, the detailed microscopic interactions of ions, water and functional surface groups are incorporated into Poisson–Boltzmann theory. This allows us to quantify ion-specific binding affinities to surface groups of varying polarity and charge, and to provide a connection to the experimentally measured long-ranged electrostatic forces that stabilize colloids, proteins and other particles against precipitation. Based on the stabilizing efficiency, the direct Hofmeister series is obtained for negatively charged hydrophobic surfaces. Hofmeister series reversal is induced by changing the sign of the surface charge from negative to positive, by changing the nature of the functional surface groups from hydrophobic to hydrophilic, by increasing the salt concentration, or by changing the pH. The resulting diverse spectrum reflects that alterations of Hofmeister series are the rule rather than the exception and originate from the variation of ion-surface interactions upon changing surface properties.     

Can a dipole-bound electron form a pseudo-atom? An atoms-in-molecules study of the hydrated electron

Non-nuclear local maxima, or attractors, of electron density are a rare but very interesting feature of the electron density distribution in molecules and solids. Recently, non-nuclear attractors (NNAs) and the corresponding pseudoatoms of electron density have been identified with the quantum theory of atoms in molecules for some anionic clusters formed by several polar solvent molecules and an excess electron bound in either a solvated-electron or dipole-bound fashion. This contribution reports a detailed study of the topology of the electron density for a series of dipole-bound water cluster anions, as calculated with Hartree-Fock, M?ller-Plesset perturbation theory, and coupled-cluster methods together with basis sets augmented with extra diffuse basis functions to accommodate the excess electron. For dipole-bound clusters, electron densities obtained with insufficient inclusion of electron correlation effects and tight basis sets feature a well-pronounced pseudoatom due to the excess electron, which ultimately disappears when a higher level of electronic structure theory and a more diffuse basis set are used. On the other hand, for solvated-electron clusters, where the excess electron is surrounded by solvent molecules, the existence of NNAs does not seem to be an artifact of the method employed, but rather a genuine feature of the electron density distribution. Pseudoatoms of electron density thus appear to be an exclusive feature of confined environments and are unlikely to be found on the tip of a cluster dipole or on solid surfaces.     

Single electron localisation on the cystine/cysteine couple: sulphur or carbon?

Protein sulphur functions can host a single electron on sulphur atoms in redox processes linking thiols to disulphides. However, experimental results have shown that the single electron can also reside on carbon atoms leading to protein damage. We have investigated this possibility on cystine for two initial conformations. The other site of electron fixation is always the carbonyl function. When there is no carbonyl, the electron remains on the sulphur atoms. In a model of the active site of thioredoxin (cystine, the carboxylic group of aspartic acid 30 and a water molecule), only the carbonyl group of the cystine is reactive.     

Paradigms and paradoxes: what are the 54 electron affinities of O2?

Only one electron affinity of oxygen, 43(1) kJ mol⁻¹ is generally cited since the molecular orbital theory anion bond order [3/4] gives an electron affinity, 14 kJ mol⁻¹. However, electron correlation rules predict 27 bonding and 27 antibonding spin orbital coupling states. The relative bond orders (RBOs), 12/13 to [1/4] and the 13 valence electrons of superoxide are used to calculate electron affinities 103 to −243 kJ mol⁻¹ consistent with experimental and theoretical values. These are used to construct 54 ionic Morse potentials.     

The regularities of the Rydberg energy levels of many-valence electron atom Al

Within the scheme of the weakest bound electron potential model theory, the concept of spectral-level-like series is presented by reasonably classifying the Rydberg energy level of atom Al. Based on this concept, the regularities of the Rydberg energy levels are systematically studied. The deviations of the calculated values from the experimental values are generally about several percent of 1 cm, which is of high accuracy.     

Is Se-Se bond cleavage the most favourable process in electron attachment to diselenides? The importance of asymmetry

Accurate ab initio calculations reveal that, in contrast with what has been commonly assumed up to now, electron attachment to diselenides does not always result in a Se-Se bond fission and, in general, the mechanism behind the bond fission may involve the crossing of several states.     

Exploring the relationship between cocrystal stability and symmetry: is Wallach's rule applicable to multi-component solids?

Comparison of structure and hydration stability of pairs of chiral and racemic binary cocrystals indicates that the racemic solid is more stable than the chiral one; we illustrate that this difference might arise from intermolecular (crystal packing) factors in one case, while intramolecular (molecular conformation) factors are more significant in the other.     

Fast electron density methods in the life sciences--a routine application in the future?

The understanding of mutual recognition of biologically interacting systems on an atomic scale is of paramount importance in the life sciences. Electron density distributions that can be obtained from a high resolution X-ray diffraction experiment can provide--in addition to steric information--electronic properties of the species involved in these interactions. In recent years experimental ED methods have seen several favourable developments towards successful application in the life sciences. Experimental and methodological advances have made possible on the one hand high-speed X-ray diffraction experiments, and have allowed on the other hand the quantitative derivation of bonding, non-bonding and atomic electronic properties. This has made the investigation of a large number of molecules possible, and moreover, molecules with 200 or more atoms can be subject of experimental ED studies, as has been demonstrated by the example of vitamin B12. Supported by the experimentally verified transferability concept of submolecular electronic properties, a key issue in Bader's The Quantum Theory of Atoms in Molecules, activities have emerged to establish databases for the additive generation of electron densities of macromolecules from submolecular building blocks. It follows that the major aims of any experimental electron density work in the life sciences, namely the generation of electronic information for a series of molecules in a reasonable time and the study of biological macromolecules (proteins, polynucleotides), are within reach in the near future.     

Where does the electron go? Electron distribution and reactivity of peptide cation radicals formed by electron transfer in the gas phase

We report the first detailed analysis at correlated levels of ab initio theory of experimentally studied peptide cations undergoing charge reduction by collisional electron transfer and competitive dissociations by loss of H atoms, ammonia, and N-C alpha bond cleavage in the gas phase. Doubly protonated Gly-Lys, (GK + 2H) (2+), and Lys-Lys, (KK + 2H) (2+), are each calculated to exist as two major conformers in the gas phase. Electron transfer to conformers with an extended lysine chain triggers highly exothermic dissociation by loss of ammonia from the Gly residue, which occurs from the ground ( X ) electronic state of the cation radical. Loss of Lys ammonium H atoms is predicted to occur from the first excited ( A ) state of the charge-reduced ions. The X and A states are nearly degenerate and show extensive delocalization of unpaired electron density over spatially remote groups. This delocalization indicates that the captured electron cannot be assigned to reduce a particular charged group in the peptide cation and that superposition of remote local Rydberg-like orbitals plays a critical role in affecting the cation-radical reactivity. Electron attachment to ion conformers with carboxyl-solvated Lys ammonium groups results in spontaneous isomerization by proton-coupled electron transfer to the carboxyl group forming dihydroxymethyl radical intermediates. This directs the peptide dissociation toward NC alpha bond cleavage that can proceed by multiple mechanisms involving reversible proton migrations in the reactants or ion-molecule complexes. The experimentally observed formations of Lys z (+*) fragments from (GK + 2H) (2+) and Lys c (+) fragments from (KK + 2H) (2+) correlate with the product thermochemistry but are independent of charge distribution in the transition states for NC alpha bond cleavage. This emphasizes the role of ion-molecule complexes in affecting the charge distribution between backbone fragments produced upon electron transfer or capture.     

Should negative electron affinities be used for evaluating the chemical hardness?

Despite recent advances in computing negative electron affinities using density-functional theory, it is an open issue as to whether it is appropriate to use negative electron affinities, instead of zero electron affinity, to compute the chemical hardness of atoms and molecules with metastable anions. We seek to answer this question using the accepted empirical rules linking the chemical hardness to the atomic size and the polarizability; we also propose a new correlation with the C6 London dispersion coefficient. For chemical reactivity in the gas phase, it seems to make no difference whether negative, or zero, electron affinities are used for systems with metastable anions. For reactions in solution the evidence that is presently available is insufficient to establish a preference. In addressing this issue, we noted that electron affinity data from which atomic chemical hardness values are computed are out of date; an update to Pearson's classic 1988 table [Inorg. Chem., 1988, 27, 734-740] is thus provided.     

Graphene: Safe or Toxic? The Two Faces of the Medal

Graphene is considered the future revolutionary material. For its development, it is of fundamental importance to evaluate the safety profile and the impact on health. Graphene is part of a bigger family which has been identified as the graphene family nanomaterials (GFNs). Clarifying the existence of multiple graphene forms allows better understanding the differences between the components and eventually correlating their biological effects to the physicochemical characteristics of each structure. Some in vitro and in vivo studies clearly showed no particular risks, while others have indicated that GFNs might become health hazards. This Minireview critically discusses the recent studies on the toxicity of GFNs to provide some perspective on the possible risks to their future development in materials and biomedical sciences.     

Pointing the way to the products? Comparison of the stress tensor and the second-derivative tensor of the electron density

The eigenvectors of the electronic stress tensor can be used to identify where new bond paths form in a chemical reaction. In cases where the eigenvectors of the stress tensor are not available, the gradient-expansion-approximation suggests using the eigenvalues of the second derivative tensor of the electron density instead; this approximation can be made quantitatively accurate by scaling and shifting the second-derivative tensor, but it has a weaker physical basis and less predictive power for chemical reactivity than the stress tensor. These tools provide an extension of the quantum theory of atoms and molecules from the characterization of molecular electronic structure to the prediction of chemical reactivity.