107 research outputs found
Fusing a Planar Group to a π-Bowl: Electronic and Molecular Structure, Aromaticity and Solid-State Packing of Naphthocorannulene and its Anions
No full textMolecular and electronic structure, reduction electron transfer and coordination abilities of a polycyclic aromatic hydrocarbon (PAH) having a planar naphtho-group fused to the corannulene bowl have been investigated for the first time using a combination of theoretical and experimental tools. A direct comparison of naphtho[2,3-a]corannulene (C28H14, 1) with parent corannulene (C20H10, 2) revealed the effect of framework topology change on electronic properties and aromaticity of 1. The presence of two reduction steps for 1 was predicted theoretically and confirmed experimentally. Two reversible one-electron reduction processes with the formal reduction potentials at −2.30 and −2.77 V versus Fc+/0 were detected by cyclic voltammetry (CV) measurements, demonstrating accessibility of the corresponding mono- and dianionic states of 1. The products of the singly and doubly reduced napththocorannulene were prepared using chemical reduction with Group 1 metals and isolated as sodium and rubidium salts. Their X-ray diffraction study revealed the formation of “naked” mono- and dianions crystallized as solvent-separated ion products with one or two sodium cations as [Na+(18-crown-6)(THF)2][C28H14 −] and [Na+(18-crown-6)(THF)2]2[C28H14 2−] (3⋅THF and 4⋅THF, respectively). The dianion of 1 was also isolated as a contact-ion complex with two rubidium countercations, [{Rb+(18-crown-6)}2(C28H14 2−)] (5⋅THF). The structural consequences of adding one and two electrons to the carbon framework of 1 are compared for 3, 4 and 5. Changes in aromaticity and charge distribution stemming from the stepwise electron acquisition are discussed based on DFT computational study
CHEMISTRY OF BUCKYBOWL FROM CLOSED-SHELL TO OPEN-SHELL
No full textBuckybowl is an open geodesic polyaromatic molecule with unevenly distributed π-electron on its convex and concave surface, which leads to a readily accessible π-surface for substitution reactions and complexation with various metals. Despite the diverse structures of buckybowl complexes observed in the previous experimental study, our computational work has shown that the π-surface of buckybowl always plays the most important role in the bonding. Modification of the π-surface by changing the size of conjugation and the curvature enable us to tune the bonding preference of the buckybowl surface and the stability of the complex. Our continued study has shown similar importance of the π-surface in functionalization of buckybowl with different electrophilic groups. Surprisingly, our investigation on buckybowl cations intrigued an original perspective of aromatic behavior of the π-surface. Our results have revealed an intrinsic nature of aromatic stabilization in polyaromatic cations, which is mainly attributed to the depletion of anti-aromaticity at the center ring. Further study showed an explicit correlation between the curvature of π-surface and the stability of adducts, aromatic behavior at center ring, as well as the spin distribution over polyaromatic moiety. By curving the π-surface, we have proposed several buckybowl radical adducts and confirmed their stability. These models provide an alternative strategy of developing polyaromatic spin carriers, which have a great potential in the manufacture of quantum bits. We believe our comprehensive theoretical study on versatile chemistry of buckybowl and related polyaromatic hydrocarbons can offer fundamental understanding and essential guidance for developing buckybowl-based electrode materials in the lithium-ion battery, organometallic building block, and spin electronic devices
THE INTERACTION BETWEEN COINAGE OR ALKALI METALS AND POLYAROMATIC HYDROCARBONS
No full textTheoretical study on versatile chemistry of buckybowls and related polyaromatic hydrocarbons has been comprehensively accomplished and documented. Polyaromatic hydrocarbons from simple double bond to fullerene C60, as one of major family in buckybowls has shown a wide potential in development of various specifically purposed materials. Complexes with coinage metals evidenced tunable donor ability of related polyaromatic systems’ π-surface. Moreover, functionalization with small ligands cations interact with these π-surface also show some patterns which have certain enlightenment to the experiment. By adding the methyl group on corannulene, to pursue the relationship between geometry and stabilization which provide an alternative strategy of developing. Further study of alkali metals interacts with annulene, continuously adding with crown ether to mimic experiment environment display an interesting pattern. In the end, extended topics of some applications with computational chemistry, such as the help of Raman spectrum of L-focus
CORANNULENE η(ETA)5-COORDINATION WITH TRANSITION METALS: A THEORETICAL STUDY
No full textPlanar polyaromatic molecules η5-coordinated to transition metal complexes are well-studied and applied in organometallic chemistry. Much less attention is paid to curved polyaromatic systems, so-called buckybowls or fullerene fragments. Here we present results of theoretical investigation of η5-adducts of corannulene as representative model for buckybowls with VIIIB group transition metals (Co, Rh, Ir and Fe, Ru, Os). Natural bonding orbital (NBO) analysis revealed that corannulene acts as a π-donor whereas transition metal center behaves as an acceptor, using its empty dxy orbital. Additionally, relatively weak back-donation from transition metals to corannulene was also observed. Energetics of the bonding was further explored with help of Energy Decomposition Analysis (EDA), which includes repulsive Pauli and attractive electrostatic (ionic) and orbital (covalent) components. Combined with EDA, NOCV (Natural Orbitals for Chemical Valence) analysis of donor-acceptor pairs associated with shape of charge flow and quantification of its magnitude in terms of energy confirmed the aforementioned nature of the orbital interactions. For comparison, η6-coordination mode of corannulene was also analyzed.M.S. in Chemistry, May 201
Pre-implant Brain Activation Modeling to Drive Placement of Depth Leads in White Matter for Direct Neurostimulation Therapy in Epilepsy
No full textA critical step towards applying direct brain stimulation therapy in focal onset epilepsy is to effectively interface with epileptogenic neural circuits using a limited set of active contacts. This takes special relevance when interacting with networks that exhibit two or more foci. A strategy to influence the maximum extent of the epileptogenic circuit is to stimulate white matter pathways to enhance propagation to distant epileptic tissue.A significant number of elements must be considered in the clinical response to stimulation delivered directly to neuronal populations. These variables include: stimulation parameter settings, number and interdependence of anatomical targets, electrode number, electrode location and orientation, geometry or shape of the electrode contacts, contact polarity, biophysical properties of stimulated medium, andtrajectory of axonal bundles adjacent to the stimulation site.This document addresses the development of a computational model which takes into consideration all the mentioned variables to predict activation of distant sites via white matter pathways. A method to calculate the extracellular potential field, induced by the application of time-dependent stimulation waveforms, is discussed. Such a method considers both the anisotropic conductivity nature of neural tissue and the electrochemical phenomena of the electrode-tissue interface. The response of white matter fibers is then evaluated by solving a compartmental cable model based in the Hodgkin and Huxley membrane description.The model was integrated into a pre-surgical workflow and was used prospectively to guide stereotactic implantation of depth leads to apply direct neurostimulation therapy in four patients with refractory focal onset epilepsy
Iodine (I 2 ) as a Janus-Faced Ligand in Organometallics
No full textThe 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 d8 Pt(II) complex, the dominant contributor to the bonding is a σ*(I-I) orbital as the acceptor orbital, while a mainly dz 2 orbital centered on the metal center is the corresponding donor. That this kind of bonding is characteristic of axial bonding in d8 complexes was supported by model calculations with incoming donors and acceptors, NH3 and BH3. In contrast, the distinct "bent" coordination of the I2 bound at the axial position of the [Rh2(O2CCF3)4] paddle-wheel complex is associated with a dominant donation from a p-type lone pair localized on one of two iodine atoms, the σ*(Rh-Rh) antibonding orbital of the metal complex acting as an acceptor orbital. We check the donor capabilities of I2 in some hypothetical complexes with Lewis acids, H+, AlCl3, B(CF3)3. Also, we look at the weakly bound donor-acceptor couple [(I2)·(I2)]. We explore the reasons for the paucity of I2 complexes and propose candidates for synthesis. © 2013 American Chemical Society.We are grateful to G. van Koten for bringing to our attention a number of important papers and useful comments, and to P. Wolzanski for steering us to the work of A. Heyduk. Our work was supported by the National Science Foundation, Research Grant CHE-0910623. Computational facilities provided by KAUST (King Abdullah University of Science and Technology) Supercomputing Laboratory are gratefully acknowledged
Hypervalent Compounds as Ligands: I 3 -Anion Adducts with Transition Metal Pentacarbonyls
No full textJust 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 I3 - with model organometallic fragments, [Cr(CO)5] and [Mn(CO) 5]+. Bonding energy computations were augmented by a Natural Bond Orbital (NBO) perturbation theory analysis and Energy Decomposition Analysis (EDA). The bonding between I3 - and the organometallic fragment is substantial, especially for the electrostatically driven anion-cation case. "End-on" coordination is favored by 5-13 kcal/mol over "side-on" (to the central I of I3 -), with a ∼10 kcal/mol barrier for isomerization. A developing asymmetry in the I-I bonding of "end-on" coordinated I 3 - led us to consider in some detail the obvious fragmentation to a coordinated I- and free I2. While the signs of incipient fragmentation in that direction are there, these is a definite advantage to maintaining some I- to I2 bonding in triiodide complexes. © 2013 American Chemical Society.Our work was supported by the National Science Foundation, Research Grant CHE-0910623. Computational facilities provided by KAUST (King Abdullah University of Science and Technology) Supercomputing Laboratory are gratefully acknowledged
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