AN INTERESTING RELATIONSHIP BETWEEN INTERELECTRONIC DISTANCE AND THE CORRESPONDING COULOMB INTEGRAL
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A simple expression for the distance between two electrons, (δr 12 ) ab , has been defined from one-electron expectation values. This value is calculated for triplet and singlet systems of two electrons, and closed-shell molecules of up to 58 electrons. When (δr 12 ) ab is compared to the corresponding coulomb integral, J ab , an interesting relationship is observed. The relationship is followed extremely closely by all pairs of electrons, except for some deviations involving delocalized core–core electron pairs.Keywords:
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Photoinduced decarbonylation of 2,4-bis(spirocyclohexyl)-1,3-cyclobutanedione 1 in the crystalline solid state resulted in formation of a deep blue transient with λmax = 550 nm and a half-life of 42 min at 298 K, identified as kinetically stabilized oxyallyl. Support for an open-shell singlet species was obtained by spectroscopic analysis and (4/4) CASSCF calculations with the 6-31+G(d) basis set and multireference MP2 corrections. The electronic spectrum of the singlet biradical, confirmed by femtosecond pump−probe studies in solution, was matched by coupled cluster calculations with single and double corrections.
Decarbonylation
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Picosecond
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The stability of the singlet and triplet states of tetraoxocyclobutane (C4O4, the tetramer of carbon monoxide) has been investigated at various levels of theory. Second-order perturbation favors the singlet state, whereas the triplet and open-shell singlet states are computed to be more stable using the hybrid B3LYP density functional method. High-level CCSD(T) calculations reveal the perturbation theory to be insufficient, whereas the B3LYP method provides both qualitatively and quantitatively reliable results.
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Tetramer
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Localization and delocalization indices derived in the framework of the quantum Atoms in Molecules theory have recently been used to analyze the electron-pair structure of closed-shell molecules. Here we report calculations of localization and delocalization indices for open-shell molecules at the Hartree-Fock (HF) level. Several simple doublet and triplet radical molecules are studied. In general, interatomic delocalization between bonded atoms is heavily dependent on the order and polarity of the bond. Unpaired electrons also have a significant effect on the interatomic delocalization indices. Indeed, for many radicals, the analysis of the spin components reveals that the interatomic delocalization is very different for alpha and beta spin electrons in many cases. In general, at the HF level, the results can be rationalized in terms of orbital contributions. However, the definition of localization and delocalization indices is completely general, and they could be calculated at any level of theory, provided that the one- and two-electron densities are available.
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Electron localization function
Unpaired electron
Atoms in molecules
Electron Delocalization
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Partial ytterbium f-orbital occupancy (i.e., intermediate valence) and open-shell singlet formation are established for a variety of bipyridine and diazabutadiene adducts with decamethylytterbocene, (C(5)Me(5))(2)Yb, abbreviated as Cp*(2)Yb. Data used to support this claim include ytterbium valence measurements using Yb L(III)-edge X-ray absorption near-edge structure spectroscopy, magnetic susceptibility, and complete active space self-consistent field (CASSCF) multiconfigurational calculations, as well as structural measurements compared to density functional theory calculations. The CASSCF calculations indicate that the intermediate valence is the result of a multiconfigurational ground-state wave function that has both an open-shell singlet f(13)(pi*)(1), where pi* is the lowest unoccupied molecular orbital of the bipyridine or diazabutadiene ligands, and a closed-shell singlet f(14) component. A number of other competing theories for the unusual magnetism in these materials are ruled out by the lack of temperature dependence of the measured intermediate valence. These results have implications for understanding chemical bonding not only in organolanthanide complexes but also for f-element chemistry in general, as well as understanding magnetic interactions in nanoparticles and devices.
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Spin states
Functional theory
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Saddle point
Singlet fission
Zero-point energy
Potential energy surface
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We study the performance of the two-determinant (TD) coupled-cluster (CC) method which, unlike conventional ground-state single-reference (SR) CC methods, can, in principle, provide a naturally spin-adapted treatment of the lowest-lying open-shell singlet (OSS) and triplet electronic states. Various choices for the TD-CC reference orbitals are considered, including those generated by the multi-configurational self-consistent field method. Comparisons are made with the results of high-level SR-CC, equation-of-motion (EOM) CC, and multi-reference EOM calculations performed on a large test set of over 100 molecules with low-lying OSS states. It is shown that in cases where the EOMCC reference function is poorly described, TD-CC can provide a significantly better quantitative description of OSS total energies and OSS-triplet splittings.
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Triplet carbene H−C−SiHCl2 (1), prepared by a photochemical [1,2]H-shift from 1,1-dichlorosilaethylene, was identified by matching experimental and [CCSD(T)/cc-pVTZ] infrared absorptions. Parts of the potential energy surface were explored utilizing CCSD(T)/cc-pVTZ and B3LYP/6-311+G** computations. DFT reproduces the experimental features and CCSD(T) computations for the triplet surface but fails in the description of the open-shell singlet state of 1. We emphasize the notion of electropositive heterosubstitution for the generation of persistent ground-state triplet carbenes.
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Potential energy surface
Matrix Isolation
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Calculations were performed for closed shell singlet states, open shell singlet states and open-shell triplet states using DFT and B3LYP functional in oligoacenes. 6-31+G(d,p) basis set was used to calculate energies for polyacenes in Hartrees at various levels of spin-state configuration. For systems beyond pentacene, open shell sing let state is more stablilized for hexacene, heptacene, octacene and nonacene and singlet triplet gap decreases as size of polyacene increases. This work is used to describe unrestricted DFT calculations, spin contamination and quantum mechanical applications..
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Singlet fission
Pentacene
Spin states
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