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Hoffmann, Roald

Publications and source records attributed to Hoffmann, Roald.

30 records · Page 2
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DOE OSTI2016

Distinguishing Bonds

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catalysis (heterogeneous)↗
DOE OSTI2016

The Dimerization of H 2 NO

H 2 NO is the prototype of aminoxyls, kinetically persistent free radicals. The potential dimerization and reaction modes of H 2 NO are examined. The dimer potential energy surface features a barely metastable O–O bound species and several locally bound dimeric structures. One of these, a rectangular or rhomboid O–N–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 2 NO dimers from a balance of four-electron repulsions between NO π electrons, and two-electron attractive interaction between the singly occupied π* orbitals of the diradical. The most stable diradical structure is a surprisingly strongly hydrogen bonded dimer diradical. Lastly, the barriers separating the other isomers from this global minimum are calculated to be small.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2016

AuO: Evolving from Dis- to Comproportionation and Back Again

The structural, electronic, and dynamic properties of hypothetical gold(II) oxide (AuO) are studied theoretically, at atmospheric and elevated pressures, with the use of hybrid density functional theory. At $p$ = 1 atm, hypothetical AuO (metastable with respect to the elements) is predicted to crystallize in a new structure type, unique among the late-transition-metal monoxides, with disproportionation of the Au ions to Au I/III and featuring aurophilic interactions. Under pressure, familiar structure types are stabilized: a semiconducting AgO-type structure at ~2.5 GPa and, with a further increase of the pressure up to ~80 GPa, an AuSO 4 -type structure containing Au 2 pairs. Finally, above 105 GPa, distorted NaCl- and CsCl-type Au II O structures dominate, and metallization is predicted at 329 GPa.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2015

Chemical bonding in hydrogen and lithium under pressure

Though hydrogen and lithium have been assigned a common column of the periodic table, their crystalline states under common conditions are drastically different: the former at temperatures where it is crystalline is a molecular insulator, whereas the latter is a metal that takes on simple structures. On compression, however, the two come to share some structural and other similarities associated with the insulator-to-metal and metal-to-insulator transitions, respectively. To gain a deeper understanding of differences and parallels in the behaviors of compressed hydrogen and lithium, we performed an ab initio comparative study of these systems in selected identical structures. Both elements undergo a continuous pressure-induced s-p electronic transition, though this is at a much earlier stage of development for H. The valence charge density accumulates in interstitial regions in Li but not in H in structures examined over the same range of compression. Moreover, the valence charge density distributions or electron localization functions for the same arrangement of atoms mirror each other as one proceeds from one element to the other. Application of the virial theorem shows that the kinetic and potential energies jump across the first-order phase transitions in H and Li are opposite in sign because of non-local effects in the Li pseudopotential. Finally, the common tendency of compressed H and Li to adopt three-fold coordinated structures as found is explained by the fact that such structures are capable of yielding a profound pseudogap in the electronic densities of states at the Fermi level, thereby reducing the kinetic energy. These results have implications for the phase diagrams of these elements and also for the search for new structures with novel properties.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2015

Toward an Experimental Quantum Chemistry: Exploring a New Energy Partitioning

Following the work of L. C. Allen, the work reported here begins by relating the central chemical concept of electronegativity with the average binding energy of electrons in a system. The average electron binding energy, $\overline{χ}$, is in principle accessible from experiment, through photoelectron and X-ray spectroscopy. It can also be estimated theoretically. $\overline{χ}$ 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 nuclear–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 2 from atoms, orbital interactions dominate nearly canceling nuclear–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 nuclear–nuclear contributions one can judge when more traditional electronegativity arguments can be justifiably invoked in the rationalization of a particular chemical event.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2014

Quantum interference in polyenes

The explicit form of the zeroth Green's function in the Hückel model, approximated by the negative of the inverse of the Hückel matrix, has direct quantum interference consequences for molecular conductance. We derive a set of rules for transmission between two electrodes attached to a polyene, when the molecule is extended by an even number of carbons at either end (transmission unchanged) or by an odd number of carbons at both ends (transmission turned on or annihilated). These prescriptions for the occurrence of quantum interference lead to an unexpected consequence for switches which realize such extension through electrocyclic reactions: for some specific attachment modes the chemically closed ring will be the ON position of the switch. Normally the signs of the entries of the Green's function matrix are assumed to have no physical significance; however, we show that the signs may have observable consequences. In particular, in the case of multiple probe attachments – if coherence in probe connections can be arranged – in some cases new destructive interference results, while in others one may have constructive interference. One such case may already exist in the literature.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2014

Lithium hydroxide, LiOH, at elevated densities

We discuss the high-pressure phases of crystalline lithium hydroxide, LiOH. Using first-principles calculations, and assisted by evolutionary structure searches, we reproduce the experimentally known phase transition under pressure, but we suggest that the high-pressure phase LiOH-III be assigned to a new hydrogen-bonded tetragonal structure type that is unique amongst alkali hydroxides. LiOH is at the intersection of both ionic and hydrogen bonding, and we examine the various ensuing structural features and their energetic driving mechanisms. At P = 17 GPa, we predict another phase transition to a new phase, Pbcm-LiOH-IV, which we find to be stable over a wide pressure range. Eventually, at extremely high pressures of 1100 GPa, the ground state of LiOH is predicted to become a polymeric structure with an unusual graphitic oxygen-hydrogen net. However, because of its ionic character, the anticipated metallization of LiOH is much delayed; in fact, its electronic band gap increases monotonically into the TPa pressure range.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗