46 research outputs found
Toward an Experimental Quantum Chemistry: Exploring a New Energy Partitioning
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, ĻĢ
, is in principle accessible from experiment,
through photoelectron and X-ray spectroscopy. It can also be estimated
theoretically. ĻĢ
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 nuclearānuclear 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 nuclearānuclear 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 nuclearānuclear
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
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
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 OāO bound species and several locally bound
dimeric structures. One of these, a rectangular or rhomboid OāNāO-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
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
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>āAnion Adducts with Transition Metal Pentacarbonyls
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 anionācation
case. āEnd-onā coordination is favored by 5ā13
kcal/mol over āside-onā (to the central I of I<sub>3</sub><sup>ā</sup>), with a ā¼10 kcal/mol barrier for isomerization.
A developing asymmetry in the IāI bonding of āend-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
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 Ļ*Ā(IāI) 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 ābentā 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 Ļ*Ā(RhāRh) 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 donorāacceptor
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
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
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 āequalizeā,
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
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