46 research outputs found

    Toward an Experimental Quantum Chemistry: Exploring a New Energy Partitioning

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    Following the work of L. C. Allen, this work begins by relating the central chemical concept of electronegativity with the average binding energy of electrons in a system. The average electron binding energy, Ļ‡Ģ…, is in principle accessible from experiment, through photoelectron and X-ray spectroscopy. It can also be estimated theoretically. Ļ‡Ģ… has a rigorous and understandable connection to the total energy. That connection defines a new kind of energy decomposition scheme. The changing total energy in a reaction has three primary contributions to it: the average electron binding energy, the nuclearā€“nuclear repulsion, and multielectron interactions. This partitioning allows one to gain insight into the predominant factors behind a particular energetic preference. We can conclude whether an energy change in a transformation is favored or resisted by collective changes to the binding energy of electrons, the movement of nuclei, or multielectron interactions. For example, in the classical formation of H<sub>2</sub> from atoms, orbital interactions dominate nearly canceling nuclearā€“nuclear repulsion and two-electron interactions. While in electron attachment to an H atom, the multielectron interactions drive the reaction. Looking at the balance of average electron binding energy, multielectron, and nuclearā€“nuclear contributions one can judge when more traditional electronegativity arguments can be justifiably invoked in the rationalization of a particular chemical event

    BH<sub>3</sub> under Pressure: Leaving the Molecular Diborane Motif

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    Molecular and crystalline structures of (BH<sub>3</sub>)<sub><i>n</i></sub> have been theoretically studied in the pressure regime from 1 atm to 100 GPa. At lower pressures, crystals of the familiar molecular dimer are the structure of choice. At 1 atm, in addition to the well-characterized Ī² diborane structure, we suggest a new polymorph of B<sub>2</sub>H<sub>6</sub>, fitting the diffraction lines observed in the very first X-ray diffraction investigation of solid diborane, that of Mark and Pohland in 1925. We also find a number of metastable structures for oligomers of BH<sub>3</sub>, including cyclic trimers, tetramers, and hexamers. While the higher oligomers as well as one-dimensional infinite chains (bent at the bridging hydrogens) are less stable than the dimer at ambient pressure, they are stabilized, for reasons of molecular compactness, by application of external pressure. Using periodic DFT calculations, we predict that near 4 GPa a molecular crystal constructed from discrete trimers replaces the Ī² diborane structure as the most stable phase and remains as such until 36 GPa. At higher pressures, a crystal of polymeric, one-dimensional chains is preferred, until at least 100 GPa

    The Dimerization of H<sub>2</sub>NO

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    H<sub>2</sub>NO is the prototype of aminoxyls, kinetically persistent free radicals. The potential dimerization and reaction modes of H<sub>2</sub>NO are examined. The dimer potential energy surface features a barely metastable Oā€“O bound species and several locally bound dimeric structures. One of these, a rectangular or rhomboid Oā€“Nā€“O-N ring, is a characteristic structural feature of more stable aminoxyls in the solid state. Its electronic structure is related to other four-center six-electron systems. A general picture of the weak dimer binding is constructed for these and other H<sub>2</sub>NO dimers from a balance of four-electron repulsions between NO Ļ€ electrons, and two-electron attractive interaction between the singly occupied Ļ€* orbitals of the diradical. The most stable diradical structure is a surprisingly strongly hydrogen bonded dimer diradical. The barriers separating the other isomers from this global minimum are calculated to be small

    High Pressure Electrides: A Predictive Chemical and Physical Theory

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    ConspectusElectrides, in which electrons occupy interstitial regions in the crystal and behave as anions, appear as new phases for many elements (and compounds) under high pressure. We propose a unified theory of high pressure electrides (HPEs) by treating electrons in the interstitial sites as filling the quantized orbitals of the interstitial space enclosed by the surrounding atom cores, generating what we call an interstitial quasi-atom, ISQ.With increasing pressure, the energies of the valence orbitals of atoms increase more significantly than the ISQ levels, due to repulsion, exclusion by the atom cores, effectively giving the valence electrons less room in which to move. At a high enough pressure, which depends on the element and its orbitals, the frontier atomic electron may become higher in energy than the ISQ, resulting in electron transfer to the interstitial space and the formation of an HPE.By using a He lattice model to compress (with minimal orbital interaction at moderate pressures between the surrounding He and the contained atoms or molecules) atoms and an interstitial space, we are able to semiquantitatively explain and predict the propensity of various elements to form HPEs. The slopes in energy of various orbitals with pressure (s > p > d) are essential for identifying trends across the entire Periodic Table. We predict that the elements forming HPEs under 500 GPa will be Li, Na (both already known to do so), Al, and, near the high end of this pressure range, Mg, Si, Tl, In, and Pb. Ferromagnetic electrides for the heavier alkali metals, suggested by Pickard and Needs, potentially compete with transformation to d-group metals

    High-Pressure Electrides: The Chemical Nature of Interstitial Quasiatoms

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    Building on our previous chemical and physical model of high-pressure electrides (HPEs), we explore the effects of interaction of electrons confined in crystals but off the atoms, under conditions of extreme pressure. Electrons in the quantized energy levels of voids or vacancies, interstitial quasiatoms (ISQs), effectively interact with each or with other atoms, in ways that are quite chemical. With the well-characterized Na HPE as an example, we explore the ionic limit, ISQs behaving as anions. A detailed comparison with known ionic compounds points to high ISQ charge density. ISQs may also form what appear to be covalent bonds with neighboring ISQs or real atoms, similarly confined. Our study looks specifically at quasimolecular model systems (two ISQs, a Li atom and a one-electron ISQ, a Mg atom and two ISQs), in a compression chamber made of He atoms. The electronic density due to the formation of bonding and antibonding molecular orbitals of the compressed entities is recognizable, and a bonding stabilization, which increases with pressure, is estimated. Finally, we use the computed Mg electride to understand metallic bonding in one class of electrides. In general, the space confined between atoms in a high pressure environment offers up quantized states to electrons. These ISQs, even as they lack centering nuclei, in their interactions with each other and neighboring atoms may show anionic, covalent, or metallic bonding, all the chemical features of an atom

    Hypervalent Compounds as Ligands: I<sub>3</sub>ā€‘Anion Adducts with Transition Metal Pentacarbonyls

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    Just a couple of transition metal complexes of the familiar triiodide anion are known. To investigate the bonding in these, as well as isomeric possibilities, we examined theoretically adducts of I<sub>3</sub><sup>ā€“</sup> with model organometallic fragments, [CrĀ­(CO)<sub>5</sub>] and [MnĀ­(CO)<sub>5</sub>]<sup>+</sup>. Bonding energy computations were augmented by a Natural Bond Orbital (NBO) perturbation theory analysis and Energy Decomposition Analysis (EDA). The bonding between I<sub>3</sub><sup>ā€“</sup> and the organometallic fragment is substantial, especially for the electrostatically driven anionā€“cation case. ā€œEnd-onā€ coordination is favored by 5ā€“13 kcal/mol over ā€œside-onā€ (to the central I of I<sub>3</sub><sup>ā€“</sup>), with a āˆ¼10 kcal/mol barrier for isomerization. A developing asymmetry in the Iā€“I bonding of ā€œend-onā€ coordinated I<sub>3</sub><sup>ā€“</sup> led us to consider in some detail the obvious fragmentation to a coordinated I<sup>ā€“</sup> and free I<sub>2</sub>. While the signs of incipient fragmentation in that direction are there, these is a definite advantage to maintaining some I<sup>ā€“</sup> to I<sub>2</sub> bonding in triiodide complexes

    Iodine (I<sub>2</sub>) as a Janus-Faced Ligand in Organometallics

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    The four known diiodine complexes have distinct geometries. These turn out, as we demonstrate by a bonding analysis, to be a direct consequence of diiodine acting as an acceptor in one set, the van Koten complexes, and as a donor in the Cotton, Dikarev, and Petrukhina extended structure. The primary analytical tool utilized is perturbation theory within the natural bond orbital (NBO) framework, supported by an energy decomposition analysis. The study begins by delineating the difference between canonical molecular orbitals (MOs) and NBOs. When iodine acts as an acceptor, bonding collinearly in the axial position of a square-planar d<sup>8</sup> PtĀ­(II) complex, the dominant contributor to the bonding is a Ļƒ*Ā­(Iā€“I) orbital as the acceptor orbital, while a mainly d<sub><i>z</i></sub><sup>2</sup> orbital centered on the metal center is the corresponding donor. That this kind of bonding is characteristic of axial bonding in d<sup>8</sup> complexes was supported by model calculations with incoming donors and acceptors, NH<sub>3</sub> and BH<sub>3</sub>. In contrast, the distinct ā€œbentā€ coordination of the I<sub>2</sub> bound at the axial position of the [Rh<sub>2</sub>(O<sub>2</sub>CCF<sub>3</sub>)<sub>4</sub>] paddle-wheel complex is associated with a dominant donation from a p-type lone pair localized on one of two iodine atoms, the Ļƒ*Ā­(Rhā€“Rh) antibonding orbital of the metal complex acting as an acceptor orbital. We check the donor capabilities of I<sub>2</sub> in some hypothetical complexes with Lewis acids, H<sup>+</sup>, AlCl<sub>3</sub>, BĀ­(CF<sub>3</sub>)<sub>3</sub>. Also, we look at the weakly bound donorā€“acceptor couple [(I<sub>2</sub>)Ā·(I<sub>2</sub>)]. We explore the reasons for the paucity of I<sub>2</sub> complexes and propose candidates for synthesis

    The Low-Lying Electronic States of Pentacene and Their Roles in Singlet Fission

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    We present a detailed study of pentacene monomer and dimer that serves to reconcile extant views of its singlet fission. We obtain the correct ordering of singlet excited-state energy levels in a pentacene molecule (<i>E</i> (<i>S</i><sub>1</sub>) < <i>E</i> (<i>D</i>)) from multireference calculations with an appropriate active orbital space and dynamical correlation being incorporated. In order to understand the mechanism of singlet fission in pentacene, we use a well-developed diabatization scheme to characterize the six low-lying singlet states of a pentacene dimer that approximates the unit cell structure of crystalline pentacene. The local, single-excitonic diabats are not directly coupled with the important multiexcitonic state but rather mix through their mutual couplings with one of the charge-transfer configurations. We analyze the mixing of diabats as a function of monomer separation and pentacene rotation. By defining an oscillator strength measure of the coherent population of the multiexcitonic diabat, essential to singlet fission, we find this population can, in principle, be increased by small compression along a specific crystal direction

    Surface Activation of Transition Metal Nanoparticles for Heterogeneous Catalysis: What We Can Learn from Molecular Dynamics

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    Many heterogeneous reactions catalyzed by nanoparticles occur at relatively high temperatures, which may modulate the surface morphology of nanoparticles during reaction. Inspired by the discovery of dynamic formation of active sites on gold nanoparticles, we explore theoretically the nature of the highly mobile atoms on the surface of nanoparticles of various sizes for 11 transition metals. Using molecular dynamics simulations, on a 3 nm Fe nanoparticle as an example, the effect of surface premelting and overall melting on the structure and physical properties of the nanoparticles is analyzed. When the nanoparticle is heated up, the atoms in the outer shell appear amorphous already at 900 K. Surface premelting is reached at 1050 K, with more than three liquid atoms, based on the Lindemann criterion. The activated atoms may transfer their extra kinetic energy to the rest of the nanoparticle and activate other atoms. The dynamic studies indicate that the number of highly mobile atoms on the surface increases with temperature. Those atoms with a high Lindemann index, usually located on the edges or vertices, attain much higher kinetic energy than other atoms and potentially form different active sites in situ. When the temperature passes the surface premelting temperature, a drastic change in the coordination number (SCN) of the surface atoms occurs, with attendant dramatic broadening of the distribution of the SCN, suppling active sites with more diverse atomic coordination numbers. The electronic density of states of a nanoparticle tends to ā€œequalizeā€, due to the breaking of the translational symmetry of the atoms in the nanoparticle, and the d-band center of the nanoparticle moves further away from the Fermi level as the temperature increases. Besides Au, other nanoparticles of the transition metals, such as Pt, Pd, and Ag, may also have active sites easily formed in situ

    Seeking Small Molecules for Singlet Fission: A Heteroatom Substitution Strategy

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    We design theoretically small molecule candidates for singlet fission chromophores, aiming to achieve a balance between sufficient diradical character and kinetic persistence. We develop a perturbation strategy based on the captodative effect to introduce diradical character into small Ļ€-systems. Specifically, this can be accomplished by replacing pairs of not necessarily adjacent C atoms with isoelectronic and isosteric pairs of B and N atoms. Three rules of thumb emerge from our studies to aid further design: (i) Lewis structures provide insight into likely diradical character; (ii) formal radical centers of the diradical must be well-separated; (iii) stabilization of radical centers by a donor (N) and an acceptor (B) is essential. Following the rules, we propose candidate molecules. Employing reliable multireference calculations for excited states, we identify three likely candidate molecules for SF chromophores. These include a benzene, a napthalene, and an azulene, where four C atoms are replaced by a pair of B and a pair of N atoms
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