Reactivity properties of the HOSO and HSO2 isomers in liquid medium: a sequential Monte Carlo/quantum mechanics study.

CONTEXT: The rationalization of acid rain formation steps is fundamental for mitigating its effects. It is believed the hydroxysulfinyl radical is an intermediate species for the production of atmospheric sulfuric acid. Two stable configurations HOSO and HSO2 have been reported for such a radical in the gas phase. This work aims at studying these isomers in the aqueous medium. The electrical and reactivity quantities - electronic chemical potential ([Formula: see text]), chemical hardness ([Formula: see text]), and electrophilicity ([Formula: see text]) - are here calculated and compared. Considering first solvation shells (15 H2O for HSO2 and 23 H2O for HOSO), an increase of 25% in the dipole moment of HSO2 was obtained, while the dipole moment of HOSO decreases in 11%. Both solvated isomers grow softer ([Formula: see text] decreases) contrasted to the gas phase. METHODS: HOSO and HSO2 are studied through a sequential Monte Carlo/quantum mechanics approach. Lennard-Jones plus the Coulomb potentials were used to represent intermolecular potential interaction in the frame of the DICE package. Molecular structure calculations were performed at CASPT2/aug - cc - pV(T + d)Z level of theory using the MOLPRO suite of programs.

Supported or unsupported three-center two-electron bonds? A criterion based on Interference Energy Analysis.

The classification of three-center two-electron (3c2e) bonds into supported (closed) or unsupported (open) was proposed by Lipscomb in his work on boranes and extended to transition metal complexes by Bau and co-workers. The species in which the interactions of the terminal atoms are negligible are called "unsupported bonds." Examples of chemical species that are said to exhibit such bonds are Li2H+, Na2H+, B2H7 -, Al2(CH3)7 -, and [(µ2-H)Cr2(CO)10]- although the general criterion for distinguishing these types of bonds is somewhat qualitative. Besides providing a unifying view of the nature of the chemical bond, in terms of quantum interference among electronic states, the Generalized Product Function Energy Partitioning method through the Interference Energy Analysis (IEA) is also potentially capable of providing a rigorous ground to the concept of supported bonds by looking at the specific interference energies between the orbital pairs associated with the bond. The IEA was performed in the species Li2H+, Na2H+, B2H7 -, C2H7 -, Al2H7 -, and [(µ2-H)Cr2(CO)10]-, as well as along the reaction path Li2H+ → Li2 + + H. The results shown that in all studied A-B-C bonds, the A-C interactions are as important as the A-B/B-C ones, leading to the conclusion that all studied 3c2e bonds are "supported," in the sense that the A-C interaction is not negligible. The particularity of those species in preferring linear geometry is completely explained by quasi-classical effects, more specifically, by minimization of the electron-electron and nucleus-nucleus repulsions.

Three-centre two-electron bonds from the quantum interference perspective.

The nature of the three-center two-electron (3c2e) chemical bond is investigated by the Interference Energy Analysis (IEA) method and using a SC(2, 3) (spin coupled wave function with two electrons and three orbitals) approach for describing 3c2e bonds. In this approach, each center involved in the bonding contributes with a one-electron state for the interference process. The species H3+, Li3+, LiH2+, C3H5+, C3H3+, R2CBeCR2 (R = H, CH3), C7H11+ and CH5+ are considered in the study. The results show that 3c2e bonds have the same features of a 2c2e bond: the stability of the chemical systems exhibiting 3c2e bonds derives from quantum interference among electronic states. Other (quasi-classical) factors are always overall destabilizing, mostly because of the nuclear repulsion. The interference energy of a 3c2e bond is about three times higher than that of a 2c2e bond involving atoms of the same elements. In particular, concerning Li3+ and C3H3+ we found no evidence that the 'aromatic' character attributed to those species imparts any special features to their chemical structures, compared to other 3c2e bonds. Therefore, these species exhibit multicenter bonds, essentially equivalent to the other studied cases.

Flame-Retarding Properties of Injected and 3D-Printed Intumescent Bio-Based PLA Composites: The Influence of Brønsted and Lewis Acidity of Montmorillonite.

The influence of processing intumescent bio-based poly(lactic acid) (PLA) composites by injection and fused filament fabrication (FFF) was evaluated. A raw (ANa) and two acidic-activated (AH2 and AH5) montmorillonites were added to the intumescent formulation, composed by lignin and ammonium polyphosphate, in order to evaluate the influence of the strength and the nature (Brønsted or Lewis) of their acidic sites on the fire behavior of the composites. The thermal stability and the volatile thermal degradation products of the composites were assessed. The injected and 3D-printed composites were submitted to cone calorimeter (CC), limit oxygen index (LOI), and UL-94 flammability tests. A similar tendency was observed for the injected and 3D-printed samples. The high density of strong Lewis sites in AH2 showed to be detrimental to the fire-retarding properties. For the CC test, the addition of the intumescent composite reduced the peak of heat released (pHRR) in approximately 49% when compared to neat PLA, while the composites containing ANa and AH5 presented a reduction of at least 54%. However, the addition of AH2 caused a pHRR reduction of around 47%, close to the one of the composite without clay (49%). In the LOI tests, the composites containing ANa and AH5 achieved the best results: 39% and 35%, respectively, for the injected samples, and 35 and 38% for the 3D-printed samples. For the composite containing AH2 the LOI values were 34% and 32% for injected and 3D-printed samples, respectively. Overall, the best performance in the flammability tests was achieved by the composites containing clays with only weak and moderate strength acidic sites (ANa and AH5).

The Valence-Bond (VB) Model and Its Intimate Relationship to the Symmetric or Permutation Group.

VB and molecular orbital (MO) models are normally distinguished by the fact the first looks at molecules as a collection of atoms held together by chemical bonds while the latter adopts the view that each molecule should be regarded as an independent entity built up of electrons and nuclei and characterized by its molecular structure. Nevertheless, there is a much more fundamental difference between these two models which is only revealed when the symmetries of the many-electron Hamiltonian are fully taken into account: while the VB and MO wave functions exhibit the point-group symmetry, whenever present in the many-electron Hamiltonian, only VB wave functions exhibit the permutation symmetry, which is always present in the many-electron Hamiltonian. Practically all the conflicts among the practitioners of the two models can be traced down to the lack of permutation symmetry in the MO wave functions. Moreover, when examined from the permutation group perspective, it becomes clear that the concepts introduced by Pauling to deal with molecules can be equally applied to the study of the atomic structure. In other words, as strange as it may sound, VB can be extended to the study of atoms and, therefore, is a much more general model than MO.

Substituent Effects on the Quantum Interference of Two-Center One-Electron Bonds: [B2X6]- (X = H, F, Cl, CN, OH, CH3, and OCH3).

The interference energy analysis (IEA) provided by the generalized product function energy partitioning (GPF-EP) method was applied to investigate the influence of the neighboring atoms on the nature of the two-center one-electron (2c1e) bonds in the anion dimers of BX3 species (X = H, F, Cl, CN, OH, CH3, and OCH3). The species were studied at the GVB-PP(6/12).SC(1,2)/6-31**G++ level of calculation. The IEA has revealed that there is a balance between two main factors determining the chemical stability of the species. Quantum interference acts as the sole stabilizing effect in the formation of the chemical bonds, particularly as the result of the drop in kinetic energy, and the electronegativity of the substituent has a direct influence on the magnitude of this effect. The quasi-classical energy is responsible for the destabilizing factors, mainly the group bulkiness, and the "electron-withdrawing" effect in the case of the cyano group.

Splitting of multiple hydrogen molecules by bioinspired diniobium metal complexes: a DFT study.

Splitting of molecular hydrogen (H2) into bridging and terminal hydrides is a common step in transition metal chemistry. Herein, we propose a novel organometallic platform for cleavage of multiple H2 molecules, which combines metal centers capable of stabilizing multiple oxidation states, and ligands bearing positioned pendant basic groups. Using quantum chemical modeling, we show that low-valent, early transition metal diniobium(ii) complexes with diphosphine ligands featuring pendant amines can favorably uptake up to 8 hydrogen atoms, and that the energetics are favored by the formation of intramolecular dihydrogen bonds. This result suggests new possible strategies for the development of hydrogen scavenger molecules that are able to perform reversible splitting of multiple H2 molecules.

Synergistic Action of Montmorillonite with an Intumescent Formulation: The Impact of the Nature and the Strength of Acidic Sites on the Flame-Retardant Properties of Polypropylene Composites.

A raw montmorillonite (Mt) was submitted to different acidic activation times in order to investigate the influence of the strength and the nature (Brønsted and Lewis) of acidic sites on the synergistic action with an intumescent formulation (IF) composed of ammonium polyphosphate (APP) and pentaerythritol (PER) when incorporated into a polypropylene (PP) matrix. The acidity of the Mt samples was quantified by ammonia temperature-programmed desorption (TPD-NH3) and Fourier transform infrared spectroscopy (FTIR) with pyridine adsorption. The mineral clays were also characterized by X-ray fluorescence (XRF), X-ray diffraction (XRD), nitrogen adsorption analysis and particle size distribution. Thermogravimetric analysis (TGA), limit oxygen index (LOI) and UL-94 were performed to evaluate the flame-retardant properties and the thermal stability. The TGA results show that the final residue increased 2 to 3 fold in comparison to the values predicted theoretically. The flammability properties achieved a maximum for the system containing an excess of moderate-strength Brønsted sites relative to the Lewis ones, reaching 38% in the LOI test. This result suggests that the presence of these Brønsted acidic sites is important, as they take part in the esterification reaction between APP and PER which gives rise to the char formation. The FTIR-Pyr adsorption and flammability results indicate that both the nature and strength of the acidic sites influence the flame-retardant properties.

Are disulfide bonds resilient to double ionization? Insights from coincidence spectroscopy and ab initio calculations.

Disulfide bonds (-S-S-) are commonly present in biomolecules and have also been detected in astrophysical environments. In this work, the stability of the disulfide bond towards double ionization is investigated using quantum chemical calculations and photoelectron photoion photoion coincidence (PEPIPICO) spectroscopy measurements on the prototype dimethyl disulfide (CH3SSCH3, DMDS) molecule. The experiments were performed using high energy synchrotron radiation photons before (2465.0 eV) and at (2470.9 eV) the first sigma resonance around the S 1s edge. We applied the multivariate normal distribution analysis to identify the most plausible ionic fragmentation mechanisms from the doubly ionized DMDS. By mapping the minimum energy structures on the dicationic C2H6S2 2+ potential energy surface, we show that disulfide bonds are only present in high-lying isomers, in contrast to their analogous neutral systems. Our results also indicate that the number of fragment ions containing a disulfide bond for both photon energies is negligible. Taken together, our results reveal that the disulfide bond is severely damaged as a consequence of sulfur core-shell ionization processes, due to the lowering of its thermodynamic stability in multiply-charged systems.

Unexpected reversal of stability in strained systems containing one-electron bonds.

Ring strain energy is a very well documented feature of neutral cycloalkanes, and influences their structural, thermochemical and reactivity properties. In this work, we apply density functional theory and high-level coupled cluster calculations to describe the geometry and relative stability of C6H12+˙ radical cations, whose cyclic isomers are prototypes of singly-charged cycloalkanes. Molecular ions with the mentioned stoichiometry were produced via electron impact experiments using a gaseous cyclohexane sample (20-2000 eV). From our calculations, in addition to structures that resemble linear and branched alkenes as well as distinct conformers of cyclohexane, we have found low-lying species containing three-, four- and five-membered rings with the presence of an elongated C-C bond. Remarkably, the stability trend of these ring-bearing radical cations is anomalous, and the three-membered species are up to 11.3 kcal mol-1 more stable than the six-membered chair structure. Generalized Valence Bond calculations and the Spin Coupled theory with N electrons and M orbitals were used in conjunction with the Generalized Product Function Energy Partitioning (GPF-EP) method and Interference Energy Analysis (IEA) to describe the chemical bonding in such moieties. Our results confirm that these elongated C-C motifs are one-electron sigma bonds. Our calculations also reveal the effects that drive thermochemical preference of strained systems over their strained-free isomers, and the origin of the unusual stability trend observed for cycloalkane radical cations.

Molecular inner-shell photoabsorption/photoionization cross sections at core-valence-separated coupled cluster level: Theory and examples.

Oxygen, nitrogen, and carbon K-shell photoabsorption and photoionization cross sections have been calculated within core-valence-separated coupled cluster (CC) linear response theory for a number of molecular systems, namely, water, ammonia, ethylene, carbon dioxide, acetaldehyde, furan, and pyrrole. The cross sections below and above the K-edge core ionization thresholds were obtained, on the same footing, from L2 basis set calculations of the discrete electronic pseudospectrum yielded by an asymmetric-Lanczos-based formulation of CC linear response theory at the CC singles and doubles (CCSD) and CC singles and approximate doubles (CC2) levels. An analytic continuation procedure for both discrete and continuum cross sections as well as a Stieltjes imaging procedure for the photoionization cross section were applied and the results critically compared.

One-electron bonds are not "half-bonds".

Despite the success of the molecular orbital (MO) and valence-bond (VB) models to describe the electronic structure and properties of molecules, neither MO nor VB provides an explanation for the nature of the chemical bond. The first to address this problem was Ruedenberg, who showed that chemical bonds result from quantum interference. He developed a method to calculate the interference contribution to the total electronic energy and density and applied it to molecules containing typical two-centre two-electron (2c-2e) covalent bonds. To test the generality of Ruedenberg's hypothesis, we developed a powerful Interference Energy Analysis (IEA) method to calculate the interference contributions of individual chemical bonds to the total energy of diatomic and polyatomic molecules, and showed that any two-electron bond, despite its polarity, results from quantum interference. Nevertheless, many stable molecules are experimentally known whose chemical structures clearly indicate the existence of two-centre one-electron bonds (2c-1e). Therefore, the question remains if quantum interference will be the dominant effect for these systems. This work describes the extension of the IEA for treating two-centre one-electron bonds, making use of a Generalised Product Function (GPF) built from spin coupled wave functions of N electrons in M orbitals, SC(N,M). Several diatomic and polyatomic molecules were analysed and whenever possible the results were compared with the analogous case of a two-electron bond. The results indicate that interference is the dominant effect for the one-electron bonds, which reinforces the role of quantum interference as the central element in chemical bonding theory.

Theoretical study of the absolute inner-shell photoionization cross sections of the formic acid and some of its hydrogen-bonded clusters.

Inner-shell absolute photoabsorption and photoionization cross sections of the formic acid, HCOOH, and its small hydrogen-bonded clusters, i.e., (HCOOH)2, HCOOH2 +, HCOHOH+, and HCOOH·H3O+, were calculated at the time-dependent density functional theory (TDDFT) level, and the results were used to analyze the effect of the formic acid clustering on the carbon and oxygen K-edge photoionization cross sections. The discrete electronic pseudospectra obtained with square-integrable (L2) basis set calculations were used in an analytic continuation procedure based on continued fraction functions to obtain the photoabsorption cross sections. Symmetry adapted cluster configuration interaction calculations on the small formic acid clusters have also been performed at the oxygen K-edge to assign the discrete transitions and ionization potentials in support to the TDDFT results.

Mechanistic Insights into the Formation of Lithium Fluoride Nanotubes.

Born-Oppenheimer molecular dynamics (BOMD) and periodic density functional theory (DFT) calculations have been applied for describing the mechanism of formation of lithium fluoride (LiF) nanotubes with cubic, hexagonal, octagonal, decagonal, dodecagonal, and tetradecagonal cross-sections. It has been shown that high energy structures, such as nanowires, nanorings, nanosheets, and nanopolyhedra are transient species for the formation of stable nanotubes. Unprecedented (LiF)n clusters (n≤12) were also identified, some of them lying less than 10 kcal mol-[1] above their respective global minima. Such findings indicate that stochastic synthetic techniques, such as laser ablation and chemical vapor deposition, should be combined with a template-driven procedure in order to generate the nanotubes with adequate efficiency. Apart from the stepwise growth of LiF units, the formation of nanotubes was also studied by rolling up a planar square sheet monolayer, which could be hypothetically produced from the exfoliation of the FCC crystal structure. It was shown that both pathways could lead to the formation of alkali halide nanotubes, a still unprecedented set of one-dimensional materials.

Soft X-ray Chlorine Photolysis on Chlorobenzene Ice: An Experimental and Theoretical Study.

An experimental and theoretical study of the photoinduced homolysis of the carbon-chlorine bond in an ice matrix of chlorobenzene is presented. A condensed chlorobenzene film has been grown in situ and near edge X-ray fine structure (NEXAFS) spectra were collected after exposing the condensed film to a monochromatic photon beam centered at the 2822 eV resonant excitation of chlorine and at 2850 eV. The photoabsorption to the Cl 1s → σ* and Cl 1s → π* states has been measured and the hypothesis of free radical coupling reactions was investigated via time-dependent density functional theory (TD-DFT) and complete active space self-consistent field (CASSCF) calculations. Also, potential energy pathways to the C-Cl cleavage have been obtained at the CASSCF level to the Cl 1s → σ*, 1s → π*, and 1s → ∞ states. A strong dissociative character was only found for the Cl 1s → σ* resonance.

Coupled Cluster and Time-Dependent Density Functional Theory Description of Inner Shell Photoabsorption Cross Sections of Molecules.

Near K-edge photoabsorption cross section spectra of a number of molecules, namely, water, ammonia, acetone, acetaldehyde, furan, and pyrrole, were obtained at the nitrogen, oxygen, and carbon K-edges with the Coupled Cluster ansatz (CC) and with the Time-Dependent Density Functional Theory (TDDFT) by treating the inner shell excitations as individual channels, separated from the valence part of the spectrum. The discretized electronic pseudospectrum, obtained with quadratically integrable basis sets ( a.k.a. L2) at the CC or TDDFT level, is used to reconstruct the complex dipole polarizability function, from which the photoabsorption cross section near the K-edge is obtained by a continued fraction based analytic continuation procedure. The CC2 and CCSD results are in good agreement with experimental data, while the TDDFT results yield reliable cross sections. Overall, the results obtained in this work indicate that our method can be used for the treatment of the NEXAFS spectra, with the advantage that the electronic excitations in the K-edges can be easily obtained at low computational cost using TDDFT.

Diboryne Nanostructures Stabilized by Multitopic N-Heterocyclic Carbenes: A Computational Study.

Different families of nanomaterials produced from the stabilization of diboryne (B≡B) units by multitopic N-heterocyclic carbenes (NHCs), such as nanowires, nanorings, and nanotents, were studied by computational methods. Density functional theory calculations with and without periodic boundary conditions were applied to estimate the dependence of the electronic and thermochemical properties of different diboryne macromolecules with respect to the nature of the bridging ligand. Our results show that all diboryne nanostructures studied herein are viable candidates for synthesis. The Janus-type multitopic naphthobis(imidazolylidene) (5), anthrobis(imidazolylidene) (10), and pyracenetetrakis(imidazolylidene) (16) compounds are the best candidates for generating diboryne nanowires. A path to covalent organic frameworks, nanocages, and nanotubes from the optimized diboryne nanostructures is also described. Rather than just scientific curiosity, diboryne nanostructures emerge as interesting targets for the synthesis of main-group nanomaterials.

Doubly and Triply Charged Species Formed from Chlorobenzene Reveal Unusual C-Cl Multiple Bonding.

In free-radical halogenation of aromatics, singly charged ions are usually formed as intermediates. These stable species can be easily observed by time-of-flight mass spectrometry (TOF-MS). Here we used electron and proton beams to ionize chlorobenzene (C6H5Cl) and investigate the ions stability by TOF-MS. Additionally to the singly charged parent ion and its fragments, we find a significant yield of doubly and triply charged parent ions not previously reported. In order to characterize these species, we used high-level theoretical methods based on density functional theory (DFT), coupled-cluster (CC), and generalized valence bond (GVB) to calculate the structure, relative stabilities, and bonding of these dications and trications. The most stable isomers exhibit unusual carbon-chlorine multiple bonding: a terminal C═Cl double bond in a formyl-like CHCl moiety (1, rC-Cl = 1.621 Å) and a ketene-like C═C═Cl cumulated species (2, rC-Cl = 1.542 Å). The calculations suggest that an excited state of 2 has a nitrile-like C≡Cl triple bond structure.

Time-dependent density functional theory description of total photoabsorption cross sections.

The time-dependent version of the density functional theory (TDDFT) has been used to calculate the total photoabsorption cross section of a number of molecules, namely, benzene, pyridine, furan, pyrrole, thiophene, phenol, naphthalene, and anthracene. The discrete electronic pseudo-spectra, obtained in a L2 basis set calculation were used in an analytic continuation procedure to obtain the photoabsorption cross sections. The ammonia molecule was chosen as a model system to compare the results obtained with TDDFT to those obtained with the linear response coupled cluster approach in order to make a link with our previous work and establish benchmarks.

Quantum Interference Contribution to the Dipole Moment of Diatomic Molecules.

The interference energy partitioning analysis method developed by our group and used to study the nature of the chemical bond was extended to partition the electric dipole moment in quasi-classical and interference contributions. Our results show that interference participates in charge displacement in polar molecules, providing, directly or indirectly, a relevant contribution for the total dipole moment. A linear correlation was found between the interference contribution of the dipole moment from the bond electron group, µINT(bond), and the difference of electronegativity of the atoms which form the bond, ΔXAB. This interesting result reinforces the fact that electronegativity is not a property of an atom alone, but rather a property of the atom in the molecule and that ΔXAB can only be associated with that part of the total charge displacement resulting from the formation of the chemical bond. The partitioning of the total dipole moment into quasi-classical and interference contributions provides new insights about the reasons for the failure of the ΔXAB criterion in predicting the correct orientation of the dipole moment in several molecules. The results of the present work also bring additional evidence for the previously proposed mechanism of formation of polar bonds.