José Molina

A tight‐binding model for calculations of structures and properties of graphitic nanotubes

A parameterized tight‐binding model is applied to calculate the equilibrium structures, energies, band gaps and elastic properties of graphitic nanotubes with various helicities and radii. The structures, energies and band gaps for the tubes of large radii are in agreement with more accurate calculations. The elastic moduli depend only on the tube radius and follow the continuum model predictions.

Basis set superposition error-counterpoise corrected potential energy surfaces. Application to hydrogen peroxide⋯X (X=F−, Cl−,Br−, Li+, Na+) complexes

Mo/ller–Plesset (MP2) and Becke-3-Lee-Yang-Parr (B3LYP) calculations have been used to compare the geometrical parameters, hydrogen-bonding properties, vibrational frequencies and relative energies for several X− and X+ hydrogen peroxide complexes. The geometries and interaction energies were corrected for the basis set superposition error (BSSE) in all the complexes (1–5), using the full counterpoise method, yielding small BSSE values for the 6-311+G(3df,2p) basis set used. The interaction energies calculated ranged from medium to strong hydrogen-bonding systems (1–3) and strong electrostatic interactions (4 and 5). The molecular interactions have been characterized using the atoms in molecules theory (AIM), and by the analysis of the vibrational frequencies. The minima on the BSSE-counterpoise corrected potential-energy surface (PES) have been determined as described by S. Simon, M. Duran, and J. J. Dannenberg, and the results were compared with the uncorrected PES.

Stereochemistry of 14-hydroxy-β-caryophyllene and related compounds

The isomerization of β-caryophyllene (3), under treatment with SeO2, is described. Chemical correlations, between 3 14--hydroxy-β-caryophylllene (6z) from Juniperus oxycedrus, are establised. High resolution 1H NMR spectra and analysis by molecular mechanics of 3, 6 and 14-acetoxy-β-caryophyllene (7) indicate the existence of two conformational isomers, βα and ββ, in each compound. At 35°C, the βα conformer predominates in 3 and 7 but the ββ conformer predominates in 6. The higher precentage of 6ββ possibly derives from an intramolecullar hydrogen bond. The treatment of 3, 6 and 7 with m-CPBA generates, in each case, two diasteromeric 4,5-epoxi-derivatives. The epoxides obtained from 6 have been isolated and analysed separately.

Multiple Minima Hypersurfaces of Water Clusters for Calculations of Association Energy

Multiple minima of water cluster hypersurfaces are explored to find thermodynamic properties by means of the corresponding partition functions of their canonical distributions. The combination of semiempirical quantum chemical procedures for calculating the cluster energies in local minima of supermolecules and the statistical thermodynamics approach for both the evaluation of macroscopic association energies and the possible reduction by average of absolute errors intrinsic to the parametrized Hamiltonian are shown to provide an appropriate model for comparison between experimental and theoretical results. The method can explicitly take into account environmental effects due to intermolecular interaction. Water trimer and tetramer association energies of 10.9 and 14.1 kJ/mol obtained from virial coefficient calculations compare very well to the values of 10.5 and 16.4 kJ/mol, respectively, found for the theoretical association energies in this paper. c 2000 John Wiley & Sons, Inc. Int J Quantum Chem 79: 8-16, 2000

Structural and ab initio studies of AgAg bonding in the silver(I) dimer bis-μ-(5,7-dimethyl[1,2,4]triazolo[1,5-a]pyrimidine)dinitrato disilver(I)

Abstract This work is an attempt to address the issue of metal-metal bonding in d10 systems. The compound [Ag(dmtp)(NO3)]2, where dmtp represents the ligand 5,7-dimethyl[1,2,4]triazolo[1,5-a]pyrimidine, has been prepared, characterized by thermal analysis and IR spectroscopy and its structure has been solved by X-ray diffraction. The compound is triclinic, space group P 1 , a = 5.1295(4), b = 9.6009(7), c = 10.332(2) A , α = 103.73(1), β = 99.14(1), γ = 97.67(1)°, Z = 2, Dcal = 2.200 g cm−3, R = 0.029 for 4144 observed reflections. The compound exhibits a [Ag2(dmtp)2]2+ core structure with two silver atoms bridged by the dmtp ligands via N3 and N4 to give eight-membered Ag2N4C2 rings. The nitrato groups are coordinated in a semibridging and semichelating way (distances AgO, 2.816(2) A for non-bridging and 2.653(2) and 2.749(2) A for bridging oxygens). The AgAg separation is 3.058(1) A, only 0.17 A greater than in metallic silver. In order to make checking of the possible AgAg bonding, a theoretical analysis has been performed, based on the properties of the charge density distribution derived from ab initio MO calculations using STO-3G∗ and 3-21G(∗) wavefunctions, a significant direction interaction between the two metal centres having been found. Similar conclusions have been obtained for the model compound [Ag(NHCHNH)]2 after performing a geometry optimization and using improved LANL2DZ wavefunctions that take into account relativistic effect.

Ternary copper(II) complexes with the versatile 4,7-dihydro-5-methyl-7-oxo-[1,2,4]triazolo[1,5-a]pyridine ligand

Abstract The synthesis and structural characterization by X-ray crystallography, thermal, spectroscopic and magnetic methods of the ternary copper(II) complexes [Cu(Hmtpo-N3)2(H2O)2](NO3)2·2H2O (1), [Cu(mtpo-N3)2(η2-en)(H2O)]·1.5H2O (2), [ Cu(mtpo -N 3 )-(η 2 - mtpo -N′,O 7 )(η 2 - tn )]2 H 2 O ( 3 ), [ Cu(mtpo -N 3 ) 2 ( py ) 2 ( H 2 O ) 2 ] (4), [Cu(mtpo-N3)2(η2-bpy)(H2O)]·2H2O (5), and [Cu(mtpo)-N3)2(η2-phen)(H2O)2] (6) is reported (Hmtpo = 4,7-dihydro-5-methyl-7-oxo[1,2,4]triazolo[1,5-a]pyridine, en = 1,2-ethylenediamine, tn = 1,3-rmpropanodiamine, py = pyridine, bpy = 2,2′bipyridine, phen = 1,10-phenanthroline). Crystals of 1 are monoclinic, space group C2/c, with unit cell dimensions a = 19.473(2), b = 8.603(3), c = 15.198(3) A , β = 122.230(6)°, V = 2153.7(8) A3, Z = 4. The structure was refined to final R = 0.042 for 1659 reflections with I≥3.0σ(1). Crystals of 2 are monoclinic, P21/n with a = 13.029(4), b = 11.388(2), c = 13.518(4) A , β = 101.52(3)° V = 1965(1) A 3 , Z = 4, and R = 0.038 for 2707 reflections. Crystas of 3 are triclinic, P l , with a = 10.241(2), b = 12.271(2), c = 9.296(1) A , α = 107.69(1)°, β = 113.95(1)° γ = 84.421(1)°, V = 1016.7(3) A3, Z = 2, and R = 0.036 for 3554 reflections. Crystals of 4 are monoclinic, P21/c, with a = 8.561(1), b = 10645(1), c = 13.882(7) A , β = 103.530(7)°, V = 12.30.3(2) A 3 , Z = 2, and R = 0.056 for 1837 reflections. Crystals of 5 are triclinic, P l , with a = 13.236(6), b = 14.395(5), c = 9.033(2) A , α = 95.14(3)°, β = 106.27(2)°, γ = 68.31(3)°, V = 1534(1) A 3 , Z = 2, and R = 0.075 for 3252 reflections. Crystals of 6 are monoclinic. C2/c, with a = 11.439(1), b = 11.791(1), c = 19.718(3) A , β = 106.20(1)°, V = 2553.9(5) A 3 , Z = 4, and R = 0.048 for 2057 reflections. The X-ray crystal structure show that Hmtpo, both in its neural and monoanionic forms, binds the central atom preferentially through N3, nevertheless, compound 3 presents an unusual N1, O7 chelate, which is analogous to the rare 6-oxo-purines N7, 06 chelate. Further, the Cu(II) polyhedron in this set of complexes follows a trend to higher coordination numbers with increasing π-acid character of the auxilliary ligands. Finally, the coordination potential of the Hmtpo ligand is discussed on the basis of molecular orbital semi-empirical PM3 calculations, performed for the neutral and monoanionic forms of Hmtpo.

5-Oxo and 7-oxo derivatives of [1,2,4]triazolo-[1,5-a]pyrimidine: characterization and theoretical study

Abstract 4,5-Dihydro-5-oxo-[1,2,4]triazolo-[1,5-a]pyrimidine (5HtpO) and 4,7-dihydro-7-oxo-[1,2,4]triazolo-[1,5-a]pyrimidine (7HtpO) have been synthesized by the condensation of 3-amino-[1,2,4]triazole with a reagent bearing the three carbon atoms that close the six-membered ring in a strongly acidic (7HtpO) or strongly basic (5HtpO) medium. The crystal structures of these compounds have been determined by X-ray diffraction, exhibiting the N4–H tautomer, as expected from the RHF/AM1 calculations. The different possibilities for binding metal ions are discussed from the MO results.

Molecular orbital calculations on 6-amino-1,3 dimethyl-5-nitrosouracil. Crystal structure of its copper(II) complex

Abstract Molecular orbital calculations have been performed for 6-amino-1,3-dimethyl-5-nitrosouracil (HADNU) and its monoanion with the MOPAC program, discussing the role of the MOs of the latter in coordination. The copper complex of this derivative, [Cu(ADNU)2(H2O)2], has been synthesized and its structure has been determined by X-ray diffraction. The metal atom lies on a crystallographic inversion centre that relates the two bidentate [through N(5) and N(6)] pyrimidine moieties and the two water molecules that complete the octahedral environment. The characterization of the compound has been completed by the study of its vis-NIR diffuse reflectance and IR spectra, thermal behaviour and magnetic susceptibility measurements.

Cobalt(II) complexes of 5,7-dimethyl[1,2,4]-triazolo-[1,5-a]-pyrimidine. Spectroscopic characterization, XRD study and antimicrobial activity

SummarySeveral cobalt(II) complexes of 5,7-dimethyl-[1,2,4]-tria-zolo-[1,5-a]-pyrimidine (DMTP) have been prepared and characterized by thermal and spectroscopic techniques. The crystal structure of [Co(DMTP)2(H2O)4]Br2·2H2O has been determined by XRD; the metal ion is octahedrally coordinated by two DMTP ligands through the usual N(3) site and four water molecules. Metal binding to N(3) for DMTP is consistent with the electronic properties calculated with the MOPAC programme. All the complexes were screened for their activity against several types of bacteria, showing a broad-spectrum antimicrobial activity.

Semi-synthesis of triterpene A-ring derivatives from oleanolic and maslinic acids. Part II. Theoretical and experimental 13 C chemical shifts

Oleanolic and maslinic acids were isolated from solid waste from olive oil and several derivatives were semi-synthesised using typical reaction procedures. Experimental and theoretical (GIAO, B3LYP/6-31G*//MM+) 13C NMR chemical shifts for 21 compounds are given.