179 research outputs found
Continuous symmetry of C60 fullerene and its derivatives
Conventionally, the Ih symmetry of fullerene C60 is accepted which is
supported by numerous calculations. However, this conclusion results from the
consideration of the molecule electron system, of its odd electrons in
particular, in a close-shell approximation without taking the electron spin
into account. Passing to the open-shell approximation has lead to both the
energy and the symmetry lowering up to Ci. Seemingly contradicting to a
high-symmetry pattern of experimental recording, particularly concerning the
molecule electronic spectra, the finding is considered in the current paper
from the continuous symmetry viewpoint. Exploiting both continuous symmetry
measure and continuous symmetry content, was shown that formal Ci symmetry of
the molecule is by 99.99% Ih. A similar continuous symmetry analysis of the
fullerene monoderivatives gives a reasonable explanation of a large variety of
their optical spectra patterns within the framework of the same C1 formal
symmetry exhibiting a strong stability of the C60 skeleton.Comment: 11 pages. 5 figures. 6 table
Step-Wise Computational Synthesis of Fullerene C60 derivatives. 1.Fluorinated Fullerenes C60F2k
The reactions of fullerene C60 with atomic fluorine have been studied by
unrestricted broken spin-symmetry Hartree-Fock (UBS HF) approach implemented in
semiempirical codes based on AM1 technique. The calculations were focused on a
sequential addition of fluorine atom to the fullerene cage following indication
of the cage atom highest chemical susceptibility that is calculated at each
step. The effectively-non-paired-electron concept of the fullerene atoms
chemical susceptibility lays the foundation of the suggested computational
synthesis. The obtained results are analyzed from energetic, symmetry, and the
composition abundance viewpoints. A good fitting of the data to experimental
findings proves a creative role of the suggested synthesis methodology.Comment: 33 pages, 11 figures, 2 tables, 2 chart
Entanglement Measures for Single- and Multi-Reference Correlation Effects
Electron correlation effects are essential for an accurate ab initio
description of molecules. A quantitative a priori knowledge of the single- or
multi-reference nature of electronic structures as well as of the dominant
contributions to the correlation energy can facilitate the decision regarding
the optimum quantum chemical method of choice. We propose concepts from quantum
information theory as orbital entanglement measures that allow us to evaluate
the single- and multi-reference character of any molecular structure in a given
orbital basis set. By studying these measures we can detect possible artifacts
of small active spaces.Comment: 14 pages, 4 figure
Global hybrids from the semiclassical atom theory satisfying the local density linear response
We propose global hybrid approximations of the exchange-correlation (XC)
energy functional which reproduce well the modified fourth-order gradient
expansion of the exchange energy in the semiclassical limit of many-electron
neutral atoms and recover the full local density approximation (LDA) linear
response. These XC functionals represent the hybrid versions of the APBE
functional [Phys. Rev. Lett. 106, 186406, (2011)] yet employing an additional
correlation functional which uses the localization concept of the correlation
energy density to improve the compatibility with the Hartree-Fock exchange as
well as the coupling-constant-resolved XC potential energy. Broad energetical
and structural testings, including thermochemistry and geometry, transition
metal complexes, non-covalent interactions, gold clusters and small
gold-molecule interfaces, as well as an analysis of the hybrid parameters, show
that our construction is quite robust. In particular, our testing shows that
the resulting hybrid, including 20\% of Hartree-Fock exchange and named hAPBE,
performs remarkably well for a broad palette of systems and properties, being
generally better than popular hybrids (PBE0 and B3LYP). Semi-empirical
dispersion corrections are also provided.Comment: 12 pages, 4 figure
Benchmark thermochemistry of the C_nH_{2n+2} alkane isomers (n=2--8) and performance of DFT and composite ab initio methods for dispersion-driven isomeric equilibria
The thermochemistry of linear and branched alkanes with up to eight carbons
has been reexamined by means of W4, W3.2lite and W1h theories. `Quasi-W4'
atomization energies have been obtained via isodesmic and hypohomodesmotic
reactions. Our best atomization energies at 0 K (in kcal/mol) are: 1220.04
n-butane, 1497.01 n-pentane, 1774.15 n-hexane, 2051.17 n-heptane, 2328.30
n-octane, 1221.73 isobutane, 1498.27 isopentane, 1501.01 neopentane, 1775.22
isohexane, 1774.61 3-methylpentane, 1775.67 diisopropyl, 1777.27 neohexane,
2052.43 isoheptane, 2054.41 neoheptane, 2330.67 isooctane, and 2330.81
hexamethylethane. Our best estimates for are: -30.00
n-butane, -34.84 n-pentane, -39.84 n-hexane, -44.74 n-heptane, -49.71 n-octane,
-32.01 isobutane, -36.49 isopentane, -39.69 neopentane, -41.42 isohexane,
-40.72 3-methylpentane, -42.08 diisopropyl, -43.77 neohexane, -46.43
isoheptane, -48.84 neoheptane, -53.29 isooctane, and -53.68 hexamethylethane.
These are in excellent agreement (typically better than 1 kJ/mol) with the
experimental heats of formation at 298 K obtained from the CCCBDB and/or NIST
Chemistry WebBook databases. However, at 0 K a large discrepancy between theory
and experiment (1.1 kcal/mol) is observed for only neopentane. This deviation
is mainly due to the erroneous heat content function for neopentane used in
calculating the 0 K CCCBDB value. The thermochemistry of these systems,
especially of the larger alkanes, is an extremely difficult test for density
functional methods. A posteriori corrections for dispersion are essential.
Particularly for the atomization energies, the B2GP-PLYP and B2K-PLYP
double-hybrids, and the PW6B95 hybrid-meta GGA clearly outperform other DFT
functionals.Comment: (J. Phys. Chem. A, in press
Aluminum Complexes of N<sub>2</sub>O<sub>2</sub><sup>3â</sup> Formazanate Ligands Supported by Phosphine Oxide Donors
The synthesis and characterization
of a new family of phosphine oxide supported aluminum formazanate
complexes (<b>7a</b>,<b>b</b>, <b>8a</b>, <b>9a</b>) are reported. X-ray diffraction studies showed that the
aluminum atoms in the complexes adopt an octahedral geometry in the
solid state. The equatorial positions are occupied by an N<sub>2</sub>O<sub>2</sub><sup>3â</sup> formazanate ligand, and the axial
positions are occupied by L-type phosphine oxide donors. UVâvis
absorption spectroscopy revealed that the complexes were strongly
absorbing (Δ â 30000 M<sup>â1</sup> cm<sup>â1</sup>) between 500 and 700 nm. The absorption maxima in this region were
simulated using time-dependent density functional theory. With the
exception of 3-cyano-substituted complex <b>7b</b>, which showed
maximum luminescence intensity in the presence of excess phosphine
oxide, the title complexes are nonemissive in solution and the solid
state. The electrochemical properties of the complexes were probed
using cyclic voltammetry. Each complex underwent sequential one-electron
oxidations in potential ranges of â0.12 to 0.29 V and 0.62
to 0.97 V, relative to the ferrocene/ferrocenium redox couple. Electrochemical
reduction events were observed at potentials between â1.34
and â1.75 V. In combination with tri-<i>n</i>-propylamine
as a coreactant, complex <b>7b</b> acted as an electrochemiluminescence
emitter with a maximum electrochemiluminescence intensity at a wavelength
of 735 nm, red-shifted relative to the photoluminescence maximum of
the same compound
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