Reactions in clusters of acetone and fluorinated acetones triggered by low energy electrons

Electron attachment to clusters of acetone (A), trifluoroacetone (TFA) and hexafluoroacetone (HFA) is studied in a crossed beam experiment with mass spectrometric detection of the anionic products. We find that the electron attachment properties in A change dramatically on going from isolated molecules to clusters. While single acetone is a very weak electron scavenger (via a dissociative electron attachment (DEA) resonance near 8.5 eV), clusters of A capture electrons at very low energy (close to 0 eV). The final ionic products consist of an ensemble of molecules (M) subjected to the loss of two neutral H₂ molecules ((M_n−2H₂)⁻, n ≥ 2). Their formation at low energies can only be explained by invoking new cyclic structures and polymers. In clusters of TFA, anionic complexes containing non-decomposed molecules (M_n⁻) including the monomer (M⁻) and ionic products formed by the loss of one and two HF molecules are observed. Loss of HF units is also interpreted by the formation of new cyclic structures in the anionic system. HFA is a comparatively stronger electron scavenger forming a non-decomposed anion via a narrow resonant feature near 0 eV in the gas phase. In HFA clusters, the non-decomposed parent anion is additionally observed at higher electron energies in the range 3–9 eV. The M⁻ signal carries signatures of self-scavenging processes, i.e., inelastic scattering by one molecule and capture of the completely slowed down electron by a second molecule within the same cluster. The scavenging spectrum is hence an image of the electronically excited states of the neutral molecule.     

In-Situ Electronegativity and the Bridging of Chemical Bonding Concepts

One challenge in chemistry is the plethora of often disparate models for rationalizing the electronic structure of molecules. Chemical concepts abound, but their connections are often frail. This work describes a quantum-mechanical framework that enables a combination of ideas from three approaches common for the analysis of chemical bonds: energy decomposition analysis (EDA), quantum chemical topology, and molecular orbital (MO) theory. The glue to our theory is the electron energy density, interpretable as one part electrons and one part electronegativity. We present a three-dimensional analysis of the electron energy density and use it to redefine what constitutes an atom in a molecule. Definitions of atomic partial charge and electronegativity follow in a way that connects these concepts to the total energy of a molecule. The formation of polar bonds is predicted to cause inversion of electronegativity, and a new perspective of bonding in diborane and guanine−cytosine base-pairing is presented. The electronegativity of atoms inside molecules is shown to be predictive of pK_a.     

Investigation of the distinction between van der Waals interaction and chemical bonding based on the PAEM‐MO diagram

In recent years, the basic problem of understanding chemical bonding, nonbonded, and/or van der Waals interactions has been intensively debated in terms of various theoretical methods. We propose and construct the potential acting on one electron in a molecule‐molecular orbital (PAEM‐MO) diagram, which draws the PAEM inserted the MO energy levels with their major atomic orbital components. PAEM‐MO diagram is able to show clear distinction of chemical bonding from nonbonded and/or vdW interactions. The rule for this is as follows. Along the line connecting two atoms in a molecule or a complex, the existence of chemical bonding between these two atoms needs to satisfy two conditions: (a) a critical point of PAEM exists and (b) PAEM barrier between the two atoms is lower in energy than the occupied major valence‐shell bonding MO which contains in‐phase atomic components (positive overlap) of the two considered atoms. In contrast to the chemical bonding, for a nonbonded interaction or van der Waals interaction between two atoms, both conditions (a) and (b) do not be satisfied at the same time. This is demonstrated and discussed by various typical cases, particularly those related to helium atom and H? H bonding in phenanthrene. There are helium bonds in HHeF and HeBeO molecules, whereas no H? H bonding in phenanthrene. The validity and limitation for this rule is demonstrated through the investigations of the curves of the PAEM barrier top and MO energies versus the internuclear distances for He₂, H₂, and He₂⁺ systems. © 2014 Wiley Periodicals, Inc.     

Electron Momentum Distributions for 4a1 Orbitals of CFxCl4-x in Low Momentum Region： a Possible Evidence of Molecular Geometry Distortion

Electron momentum distributions for 4a1 orbitals of serial freon molecules CFaC1, CF2Cl2, and CFCl3 （CFxC14-x, x=1-3） have been reanalyzed due to the severe discrepancies between theory and experiment in low momentum region. The tentative calculations using equilibrium geometries of molecular ions have exhibited a great improvement in agreement with the experimental data, which suggests that the molecular geometry distortion may be responsible for the observed high intensities at p〈0.5 a.u.. Further analyses show that the severe discrepancies at low momentum region mainly arise from the influence of molecular geometry distortion on C-Cl bonding electron density distributions.     

Silver nanoparticles active as surface-enhanced Raman scattering substrates prepared by high energy irradiation

In surface-enhanced Raman scattering (SERS) technique the preparation of metal substrates containing minimum hindrance from impurities is an important issue. The synthesis of silver nanoparticles (Ag NPs) active as SERS substrates and having the above-mentioned advantage, were obtained by electron beam irradiation of Ag⁺ aqueous solutions. Ag⁺ ions were reduced by free radicals radiolytically generated in solution without the addition of chemical reductants or stabilizing agents.The metal colloids were characterised by UV-Vis spectroscopy and scanning electron microscopy, monitoring the nanoparticles’ growth process that depends on the irradiation dose and the initial AgNO₃ concentration. Nanoparticles of long-time stability and with different size and shape, included silver nanocubes, were synthesised by varying the irradiation dose. Different tests on the SERS activity of Ag NPs obtained by electron beam irradiation were performed by using benzenethiol as a probing molecule, achieving a good magnification of the adsorbate Raman bands.     

Hydrogen bonding of single acetic acid with water molecules in dilute aqueous solutions

In separation processes, hydrogen bonding has a very significant effect on the efficiency of isolation of acetic acid (HOAc) from HOAc/H₂O mixtures. This intermolecular interaction on aggregates composed of a single HOAc molecule and varying numbers of H₂O molecules has been examined by using ab initio molecular dynamics simulations (AIMD) and quantum chemical calculations (QCC). Thermodynamic data in aqueous solution were obtained through the self-consistent reaction field calculations and the polarizable continuum model. The aggregation free energy of the aggregates in gas phase as well as in aqueous system shows that the 6-membered ring is the most favorable structure in both states. The relative stability of the ring structures inferred from the thermodynamic properties of the QCC is consistent with the ring distributions of the AIMD simulation. The study shows that in dilute aqueous solution of HOAc the more favorable molecular interaction is the hydrogen bonding between HOAc and H₂O molecules, resulting in the separation of acetic acid from the HOAc/H₂O mixtures with more difficulty than usual.     

Chemical bonding in the hydrogen molecule

The chemical bond in the hydrogen molecule is examined using the electron density and the generalized overlap amplitudes. Logarithmic derivatives of the electron density provide a clear picture of its behavior in the bonding region as well as in the outer region. The GOA expansion of the density is used to examine the dependence of the rate of decay of the density on the GOA ionization potentials. The increase in the electron density at the nuclei and in the bonding region coincides with the higher ionization potential of H₂ over the H atom. The density in the bonding region along the internuclear axis does not decay exponentially, but its shape is very nearly an inverted Gaussian. © 1996 John Wiley & Sons, Inc.     

The hierarchy of localization basins: a tool for the understanding of chemical bonding exemplified by the analysis of the VOx and VOx+ (x=1–4) systems

The hierarchy of the electron localization basins is a powerful tool of analysis of the bonding in molecules and solids within the “elfological” framework. It is a generalization of the molecular isodensity contour analysis originally proposed by Mezey. In this approach the basins are ordered with respect to the electron localization function values at the critical points which determine the reduction of the reducible localization domains. The procedure enabling the corresponding tree diagrams to be built is described and it is shown how the method can be used as a generator of mathematical definitions of chemical concepts. The possibility offered by this simple tool is illustrated by a study of the VO _x and VO _x ⁺ (x=1–4) oxides in their ground state and in some excited states. Received: 20 July 2000 / Accepted: 20 October 2000 / Published online: 23 January 2001     

Chemical bonding in refractory transition metal compounds with 8, 9, and 10 valence electrons

The chemical bonding in the refractory transition metal compounds TiC, TiN, and VN is investigated by experimental and theoretical techniques. High-precision X-ray diffraction is used to determine the electron densities in these three compounds experimentally. The X-ray structure factors and the respective valence electron densities are used twice, once to understand the chemical bonding and once to relate the experimental charge densities to those obtained from band structure calculations. These calculations, which in general are in very good agreement with experimental data, utilize the linearized augmented plane wave (LAPW) method. Theory and experiment lead to a detailed analysis of the chemical bonding in these compounds with 8, 9, and 10 valence electrons. By decomposition of the theoretical charge density into contributions from different states (energy regions), it was possible to show the strong covalent nonmetal p-metal d interaction, which is otherwise apparent only in TiC, but not in TiN or VN. In the latter two compounds the additional electrons occupy mainly metal d states with t_2g symmetry, so that in the total valence charge densities the most important bonding feature is covered. In addition to covalent interactions all compounds have a metallic bonding contribution as well as a considerable charge transfer from the metal to the nonmetal site. This mixture in chemical bonding accounts for the unusual combination of properties such as ultrahardness, high melting points, and good conductivity.     

Heteroallene Capture and Exchange at Functionalised Heptaphosphane Clusters

Despite being known for decades the chemical reactivity of homoatomic seven-atom phosphorus clusters towards small molecules remains largely unexplored. Here, we report that neutral tris(silyl) functionalised heptaphosphane (P₇(SiR₃)₃) cages are capable of heteroallene capture between the P−Si bonds of the cluster. A range of isocyanates and an isothiocyanate were investigated. In the case of isocyanates, silyl bonding at oxygen or nitrogen is regioselectively directed by the functional group on the isocyanate and substituents on the silyl moiety. Above all, we find that captured isothiocyanate molecules can be exchanged for isocyanate molecules, indicative of small molecule catch and release. Small molecule catch and release at these Zintl-derived clusters reveals their potential application as chemical storage materials or as reusable probes.     

X-Ray Diffraction Study of 2-Aminopropenenitrile at 97 K

2-Aminopropenenitrile crystallizes in the space group P2₁2₁2₁ with two molecules in the asymmetric unit. Both molecules show appreciable pyramidalization at the amino group. The crystal structure is built from approximately centrosymmetric dimers stabilized by hydrogen bonding between the amino group of each molecule and the nitrile group of its partner. The dimers are linked into chains by further hydrogen bonds in which the amino group of one molecule acts as donor, the amino group of the other as acceptor. The two types of molecule thus play different roles in the crystal structure. Electron density difference maps for the two independent molecules show characteristic bonding density features. The molecular structure as obtained by the low-temperature X-ray analysis is closely similar to that derived from ab initio molecular orbital calculations and leads to rotational constants close to those obtained from a microwave spectroscopic study.     

Trimethylaluminum: Bonding by Charge and Current Topology

The charge density distribution of the trimethylaluminum dimer was determined by high‐angle X‐ray diffraction of a single crystal and quantum‐chemical methods and analyzed using the quantum theory of atoms in molecules. The data can be interpreted as Al₂Me₆ being predominantly ionically bonded, with clear indications of topological asymmetry for the bridging Al? C bonds owing to delocalized multicenter bonding. This interpretation is supported by the calculated magnetic response currents. The data shed new light on the bonding situation in this basic organometallic molecule, which was previously described by contradicting interpretations of bonding.     

Cation influence on the structure and electron density of water in some Men+·H2O complexes

The cation influence on the water molecule in the Li⁺·H₂O, Be²⁺·H₂O, Mg²⁺·H₂O and A1³⁺·H₂O complexes has been studied by means of quantum-mechanical ab initio calculations. A number of general trends are noted. (1) The calculated equilibrium water O-H distances increase with increasing binding energies, i.e. in the order Li⁺, Mg²⁺, Be²⁺, Al³⁺. The H-O-H angles differ by about ±1 ° from the calculated equilibrium angle for the free H₂O molecule; the variation has no systematic trend. (2) The electron density redistribution accompanying the change in the internal H₂O geometry in these complexes is considerably smaller than the redistribution brought about by the direct influence of the external field. (3) The harmonic O-H stretching force constant decreases with increased cation-water bonding. (4) The qualitative features of the density changes are very similar for the four complexes. The magnitudes of the interactions follow the relation Li⁺ < Mg²⁺ < Be²⁺ Al³⁺. An increased polarization of the H₂O molecule occurs with electron migration from the H atoms towards the O atom and an accumulation of electron charge approximately at the centre of the Meⁿ⁺—O bond, especially in Be²⁺·H₂O and A1³⁺·H₂O. An electron deficiency is found in the lone-pair region.     

Electron difference densities and chemical bonding

In view of recent advances in X-ray technology it may be possible to deduce information regarding chemical bonding from experimentally determined electron densities. The construction of difference density maps represents a possible intermediate step in attaining this goal, but unresolved questions exist regarding appropriate definitions and interpretations of such maps. To shed light on these problems, theoretical difference densities are determined by ab-initio calculations for the molecules H₂, He₂, Li₂, Be₂, N₂ and F₂ at various internuclear distances. An examination of these difference density maps shows that the identification of those features of molecular electronic densities which are related to chemical bonding requires a judicious construction and a careful analysis of difference densities between molecules and their constituent atoms. Chemically relevant deformations can be small compared to density differences between different components of degenerate atomic groundstates and, consequently, chemical information can be swamped when difference densities are formed with spherically averaged atoms. To avoid such artifacts, oriented unaveraged atomic states must be subtracted for the formation of meaningful Chemical Difference Densities. The latter are explainable by means of a partitioning in terms of contributions from non-bonded inner shells, from lone pairs and from sigma and pi bonding shells. Such partitionings can be obtained through decompositions in terms of natural orbitals from correlated wavefunctions. Canonical SCF orbitals prove to be considerably less effective. Internuclear distances are found to have a great influence upon difference densities regardless of the attractive or repulsive nature of the interactions.     

Activation of Water in Titanium Dioxide Photocatalysis by Formation of Surface Hydrogen Bonds: An In Situ IR Spectroscopy Study

The hole‐driving oxidation of titanium‐coordinated water molecules on the surface of TiO₂ is both thermodynamically and kinetically unfavorable. By avoiding the direct coordinative adsorption of water molecules to the surface Ti sites, the water can be activated to realize its oxidation. When TiO₂ surface is covered by the H‐bonding acceptor F, the first‐layer water adsorption mode is switched from Ti coordination to a dual H‐bonding adsorption on adjacent surface F sites. Detailed in situ IR spectroscopy and isotope‐labeling studies reveal that the adsorbed water molecules by dual H‐bonding can be oxidized to O₂ even in the absence of any electron scavengers. Combined with theoretical calculations, it is proposed that the formation of the dual H‐bonding structure can not only enable the hole transfer to the water molecules thermodynamically, but also facilitate kinetically the cleavage of O? H bonds by proton‐coupled electron transfer process during water oxidation.     

Effects of water vapor on the radiolysis of methane over molecular sieve 5A

Effects of the addition of H₂O on the radiation-induced chemical reaction of methane over molecular sieve 5A at 460°C have been studied by product analysis. Hydrogen, carbon monoxide, carbon dioxide and hydrocarbons consisting mainly of C₂ and C₃ alkanes and alkenes were produced from CH₄ + H₂O mixtures at high conversion levels. The yields of hydrocarbons from 3:1 and 3:2 CH₄ + H₂O mixtures decreased slightly with time but those from 3:4 mixture showed no decrease with time. When the molecular sieve 5A that had been irradiated in flowing methane was reirradiated in the presence of H₂O, carbonaceous solid produced from methane on molecular sieve 5A was readily decomposed to carbon dioxide, carbon monoxide, hydrogen and hydrocarbons, mainly alkanes. Therefore, it is concluded that the suppression of decrease of product yields with time by the addition of H₂O is mainly ascribed to decomposition of the carbonaceous solid by H₂O under electron beam irradiation. The role of added H₂O is also discussed in connection with the conventional methane-steam reforming reaction.     

Comparative study on the nonadditivity of methyl group in lithium bonding and hydrogen bonding

Quantum chemical calculations at the second‐order Moeller–Plesset (MP2) level with 6‐311++G(d,p) basis set have been performed on the lithium‐bonded and hydrogen‐bonded systems. The interaction energy, binding distance, bond length, and stretch frequency in these systems have been analyzed to study the nonadditivity of methyl group in the lithium bonding and hydrogen bonding. In the complexes involving with NH₃, the introduction of one methyl group into NH₃ molecule results in an increase of the strength of lithium bonding and hydrogen bonding. The insertion of two methyl groups into NH₃ molecule also leads to an increase of the hydrogen bonding strength but a decrease of the lithium bonding strength relative to that of the first methyl group. The addition of three methyl groups into NH₃ molecule causes the strongest hydrogen bonding and the weakest lithium bonding. Although the presence of methyl group has a different influence on the lithium bonding and hydrogen bonding, a negative nonadditivity of methyl group is found in both interactions. The effect of methyl group on the lithium bonding and hydrogen bonding has also been investigated with the natural bond orbital and atoms in molecule analyses. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2009     

Radiation Chemical Studies of Protein Reactions: Effect of Irradiation Liquids Containing Aromatic Hydrocarbons and pH on the Breaking of Secondary Bonding in Protein

When protein in various liquids containing aromatic hydrocarbons, such as benzene, naphthalene, and phenanthrene, is irradiated by γ-rays from a ⁶⁰Co source, the breaking of secondary bonding in the protein molecule varies with the irradiation liquids containing aromatic hydrocarbons. Protein irradiated by γ-rays from a ⁶⁰Co source in air showed the effect of pH on the breaking of secondary bonding in the protein molecule. In both cases an empirical equation for the viscosity change was obtained, and the phenomena were explained on the basis of the molecular mechanism.