Juan J. Novoa

Strain index, lattice softness and superconductivity of organic donor-molecule salts: Crystal and electronic structures of three isostructural salts k-(BEDT- TTF)2Cu[N(CN)2]X (X=Cl, Br, I)

Abstract The recently discovered k -phase salts k -(BEDT-TTF) 2 ]Cu[N(CN) 2 ]X (X=Cl,Br, I) are isostructural but differ in their superconducting properties: the Br-salt is an ambient-pressure superconductor with T c =11.6K, the Cl-salt becomes superconducting ( T c =12.8K) under a slight applied pressure of 0.3 kbar, and the I-salt does not exhibit superconductivity under an applied-pressure of up to 5 kbar. We examine the structural and electronic properties of these three salts on the basis of their crystal structures determined at 127 K by single crystal X-ray diffraction measurements. The band electronic structure of the three salts are virtually the same and do not account for the differences in the conduction properties. The differences in the three salts with regard to superconductivity are examined from the viewpoint of lattice softness by calculating strain indices for short intermolecular contacts. The random potential arising from ethylene group conformational disorder prevents superconductivity in the iodide salt.

Interactions energies associated with short intermolecular contacts of C–H bonds. II. Ab initio computational study of the C–H⋅⋅⋅H–C interactions in methane dimer

On the basis of single‐and multireference ab initio calculations on (Be)2 and (H2)2, we suggest that van der Waals systems with large highest occupied molecular orbital–lowest unoccupied molecular orbital gap can be reasonably well described by single reference MPn (n=2–4) calculations if the basis set is large enough. The binding energies of C–H⋅⋅⋅H–C contacts are then examined by performing single reference MPn calculations on (CH4)2 dimer. Our study shows that (CH4)2 is bound in all possible CH4⋅⋅⋅CH4 arrangements, and that those arrangements with more than one C–H⋅⋅⋅H–C contact lead to a greater amount of stabilization than does the arrangement with one C–H⋅⋅⋅H–C contact. The potential energy curves obtained for staggered arrangements of (CH4)2 by the MP2 calculations with the 6‐311G(2d,2p) basis set are in close agreement with the experimentally deduced, isotropic potential curve.

Ab Initio Computational Study of the C-H…Donor and C-H…Anion Contact Interactions in Organic Donor Salts

Abstract By performing ab initio SCF-MO/MP2 calculations on simple model systems, we have examined the energetics of the π-framework bending and the six-membered ring conformational change in BEDT-TTF, the chalcogen-chalcogen contact interactions, the C-H-donor contact interactions, and the C-H—anion contact interactions. Our study shows that the chalcogen-chalcogen interactions are repulsive while the C-H-donor and C-H-anion interactions are attractive. The energy required for the π-framework bending and the six-membered ring conformational change is provided by the C-H-donor and C-H anion interactions. These attractive interactions involving the C-H bonds are important factors governing the packing patterns of donor-molecule layers and hence the electronic properties of organic donor salts.

Structural and electronic properties of TXF-TCNQ (X = S, Se, Te)

Abstract We report the crystal structure and physical properties of TTeF-TCNQ, compare the electrical conductivities of TXF-TCNQ (X ˭ S, Se, Te) by performing tight-binding band calculations on TXF-TCNQ, and examine the differences in the packing patterns of TXF-TCNQ (X ˭ S, Se, Te) by carrying out ab initio SCF-MO/MP2 calculations on appropriate model systems. The electrical conductivity of TXF-TCNQ increases as X varies from S to Se to Te. This increase is primarily caused by the conductivity enhancement in the donor stacks. In TXF-TCNQ (X ˭ S, Se) the C-H bonds of both the donor and the acceptor molecules make intermolecular contacts shorter than the van der Waals radii sums, but in TTeF-TCNQ only the C-H bonds of the donor molecules do. This difference in the packing patterns was rationalized on the basis of the C-H…X and N(sp)…X (X ˭ S, Se, Te) contact interaction energies estimated by ab initio SCF-MO/MP2 calculations on H3C-H…XH2 and H-C≡N…XH2.

Interaction energies associated with short intermolecular contacts of CH bonds. 4. Ab initio computational study of CH…anion interactions in CH4…X− (X=F, Cl, Br, I)

Abstract We carried out SCF and MP2 calculations on the corner-bridged (1, C3v), edge-bridged (2, C2v) and face-bridged (3, C3v) structures of CH4…X− (X=F, Cl, Br, I). Our study shows that the relative stabilities of the structures 1–3 decrease in the order 1>2>>3, that 1 is the only minimum-energy structure, and that the stabilization energies of the structure 1 decrease in the order X− = F−2 > Cl−> Br− > I−. The CH…X− interaction energies are estimated to be 6.1, 2.7, 2.5 and 2.0 kcal/mol for X=F, Cl, Br and I, respectively.

Interaction energies associated with short intermolecular contacts of C—H bonds. Structure and energetics of the interaction between CH4 and CN−

Abstract On the basis of SCF and single reference MP2 calculations, the full potential energy surface of the interaction between CH 4 and CN − was studied using extended basis sets of up to near Hartree-Fock limit quality. Collinear arrangements CN − ⋯HCH 3 and NC − ⋯ HCH 3 are found to be the only two energy minima. The binding energies of these two structures are calculated to be 2.5 and 2.1 kcal/mol, respectively,at the MP2 level. The full vibrational analyses of the two structures show a red-shift of about 30 cm −1 for the ν s CH stretching.

Importance of C-H⋯Donor and C-H⋯Anion Contact Interactions for the Crystal Packing, the Lattice Softness and the Superconducting Transition Temperatures of Organic Conducting Salts

The organic donor molecule BEDT-TTF and its analogs 2--4 have yielded a number of ambient-pressure superconducting salts. What structural and electronic factors govern the magnitudes of their superconducting transition temperature {Tc} has been a topic of intense studies. Examination of the band electronic structures of closely related superconducting salts shows, that the magnitudes of their {Tc}s are primarily determined by the softness of their crystal lattices. The crystal packing and the lattice softness of organic donor salts are strongly influenced by the donor{hor ellipsis}donor and donor{hor ellipsis}anion contact interactions involving the donor-molecule C-H bonds. In the present work, we briefly review the electronic structures of some representative organic salt superconductors and discuss the softness of their crytsal lattices on the basis of the interaction energies calculated for the C-H{hor ellipsis}donor and C-H{hor ellipsis}anion contact interactions. 34 refs., 14 figs., 8 tabs.

On the structures, stabilities and fragmentation patterns of carbon clusters including Buckminsterfullerene

Abstract The heats of formation and optimum structures of various neutral C n clusters ( n s ⩽∼60) and linear C3 n + cations ( n ⩽40) were determined by quantum chemical calculations. Buckminsterfullerene is thermodynamically more stable than the next stable form of C 60 by about 600 kcal/mol. C 60 and C 70 fullerenes belong to a series of fullerenes C 10 m +40 ( m ⩾2) that are derived from C 10 m cylinders and C 20 capping units. In the fragmentation of C n and C n + clusters, the preference for the C 2 or C 3 loss is predicted to depend upon whether the clusters have structures with mainly two- or three-coordinate carbon atoms.

Recent Progress in the Development of Structure-Property Correlations for κ-Phase Organic Superconductors

Abstract An examination of the molecular packing arrangements of the electron-donor molecules in κ-phase organic conductors and superconductors [κ-(BEDT-TTF)2X (X ˭ I3−, Cu(NCS)2−), κ-(MDT-TTF)2AuI2, and κ-(BMDT-TTF)2Au(CN)2] reveal that the main predictors of superconductivity appear to be (i) bond-over-ring intradimer molecular packing and (ii) relatively short (∼ 3.35A) intradimer separations. These structural features in newly discovered κ-phase organic conductors may be requirements for the occurrence of ambient pressure superconductivity.

Nature of Chalcogen⋯Chalcogen Contact Interactions in Organic Donor-Molecule Salts

The nature of chalcogen... chalcogen contact interactions in organic donor-molecule salts was examined by performing ab initio SCF-MO/MP2 calculations on H2X⋯XH2 (X=O, S, Se,Te) and MM2 calculations on donor dimers (TXF)2 (X=S, Se, Te) and (BEDX-TTF)2 (X=O, S).