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Computational Level (computational + level)
Selected AbstractsSolid-State and Solution Structure of Lanthanide(III) Complexes with a Flexible Py-N6 Macrocyclic LigandEUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 8 2009Cristina Núñez Abstract Lanthanide complexes of a hexaaza macrocyclic ligand containing a pyridine head unit (L) were synthesized (Ln = La,Lu, except Pm). The solid-state structures of the corresponding La, Ce, Pr, Nd, and Lu complexes were determined by single-crystal X-ray crystallography, and they reveal the presence of three different mononuclear complexes with three different conformations of the macrocycle and coordination environments around the metal ions. In all complexes the lanthanide ion is coordinated in an endomacrocyclic manner to the six nitrogen donor atoms of the ligand. In the La, Ce, and Pr complexes the metal ions show a 12-coordinate mononuclear environment in which 3 nitrate anions coordinate in a bidentate fashion. However, in the Nd analogue the metal ion displays a 10-coordinated environment with the coordination of 2 bidentate nitrate groups, whereas Lu shows a 9-coordinate environment interacting with 2 nitrate ligands, one of them acting as bidentate and the second one coordinating in a monodentate fashion. The 1H and 13C NMR spectra of the complexes recorded in CD3CN suggest that the complexes adopt in solution a similar structure to that observed for the Nd complex in the solid state. The [Ln(L)(NO3)3] and [Ln(L)(NO3)2]+ complexes were characterized by density functional theory (DFT) calculations (B3LYP model). The structures obtained from these calculations for La, Ce, Pr, and Nd are in good agreement with the experimental solid-state structures. The relative stabilities of the [Ln(L)(NO3)2]+ complexes with respect to the [Ln(L)(NO3)3] ones (Ln = La, Nd, Gd, Ho, or Lu) were studied both in vacuo and in acetonitrile solution (PCM model) at the same computational level. Our calculations indicate that in solution the [Ln(L)(NO3)2]+ species is the most stable one along the whole lanthanide series, in agreement with the NMR spectroscopic data.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009) [source] Nitrogen Trifluoride as a Bifunctional Lewis Base: Implications for the Adsorption of NF3 on Solid SurfacesEUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 5 2004Paola Antoniotti Abstract The structure, stability, and thermochemistry of isomeric adducts between NF3 and the Lewis acids BH3,nFn (n = 0,3) have been investigated at the coupled-cluster and at the Gaussian-3 (G3) level of theory. At the CCD/cc-pVDZ level both the nitrogen- and the fluorine-coordinated structures of all BH3,nFn,(NF3) (n = 0,3) adducts were characterized as true minima on the potential energy surface, thus providing the first theoretical evidence for the behavior of NF3 as a bifunctional Lewis base when interacting with neutral Lewis acids. At the G3 level, and 298.15 K, including the contribution of the entropy term, the H3B,NF3 adduct is predicted to be more stable than H3B,F,NF2 by 4.3 kcal mol,1; this free energy difference is 3.7 kcal mol,1 at the CCSD(T)/cc-pVTZ//CCD/cc-pVDZ level of theory. Conversely, at the latter computational level, the fluorine-coordinated isomers of the BH2F,(NF3), BHF2,(NF3), and BF3,(NF3) adducts are practically degenerate with the nitrogen-coordinated ones. BH3,nFn,(NF3) (n = 0,3) complexes feature typical bond dissociation energies of ca. 1,2 kcal mol,1, and are predicted to be thermodynamically stable only at low temperatures. However, the appreciable influence of the basis set superposition error (BSSE) prevents a quantitative assessment of these small computed dissociation energies. Finally, we briefly discuss the implications of our calculations for the adsorption of NF3 on solid surfaces. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004) [source] A CSOV study of the difference between HF and DFT intermolecular interaction energy values: The importance of the charge transfer contributionJOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 10 2005Jean-Philip Piquemal Abstract Intermolecular interaction energy decompositions using the Constrained Space Orbital Variation (CSOV) method are carried out at the Hartree,Fock level on the one hand and using DFT with usual GGA functionals on the other for a number of model complexes to analyze the role of electron correlation in the intermolecular stabilization energy. In addition to the overall stabilization, the results provide information on the variation, with respect to the computational level, of the different contributions to the interaction energy. The complexes studied are the water linear dimer, the N -methylformamide dimer, the nucleic acid base pairs, the benzene,methane and benzene-N2 van der Waals complexes, [Cu+ -(ImH)3]2, where "ImH" stands for the Imidazole ligand, and ImH-Zn++. The variation of the frozen core energy (the sum of the intermolecular electrostatic energy and the Pauli repulsion energy) calculated from the unperturbed orbitals of the interacting entities indicates that the intramolecular correlation contributions can be stabilizing as well as destabilizing, and that general trends can be derived from the results obtained using usual density functionals. The most important difference between the values obtained from HF and DFT computations concerns the charge transfer contribution, which, in most cases, undergoes the largest increase. The physical meaning of these results is discussed. The present work gives reference calculations that might be used to parametrize new correlated molecular mechanics potentials. © 2005 Wiley Periodicals, Inc. J Comput Chem 26: 1052,1062, 2005 [source] Synthesis and Preferred All- syn Conformation of C3 -Symmetrical N -(Hetero)arylmethyl TriindolesCHEMISTRY - A EUROPEAN JOURNAL, Issue 28 2008Abstract A new series of C3 -symmetrical N -(hetero)arylmethyl triindoles has been synthesized in a straightforward procedure. The structure and conformation in the solid state have been determined for three derivatives (3, 4, and 6) by X-ray crystallographic analysis. In all three cases, the molecules adopt a tripodal conformation with all of the flexible arms directed towards the same side, thereby delimiting an inner cavity. Compound 6 crystallizes and forms C3 -symmetric dimeric cagelike complexes. Guest molecules of chloroform and water are confined within the resulting cavities with stabilization by different intermolecular interactions; this highlights the potential of these compounds in the construction of supramolecular systems. A computational analysis has been performed to predict the most stable conformers. As a general trend, a preference for a conformation with all branches directed to the same side has been predicted. Comparison between theoretical and experimental results indicates that the computational level selected for the present study, B3LYP/6-31G*, is able to reproduce both the minimum energy conformations and the rotation barriers about the NCH2 bond. [source] How Should a Speech Recognizer Work?COGNITIVE SCIENCE - A MULTIDISCIPLINARY JOURNAL, Issue 6 2005Odette Scharenborg Abstract Although researchers studying human speech recognition (HSR) and automatic speech recognition (ASR) share a common interest in how information processing systems (human or machine) recognize spoken language, there is little communication between the two disciplines. We suggest that this lack of communication follows largely from the fact that research in these related fields has focused on the mechanics of how speech can be recognized. In Marr's (1982) terms, emphasis has been on the algorithmic and implementational levels rather than on the computational level. In this article, we provide a computational-level analysis of the task of speech recognition, which reveals the close parallels between research concerned with HSR and ASR. We illustrate this relation by presenting a new computational model of human spoken-word recognition, built using techniques from the field of ASR that, in contrast to current existing models of HSR, recognizes words from real speech input. [source] Novel Superalkali Superhalogen Compounds (Li3)+(SH), (SH=LiF2, BeF3, and BF4) with Aromaticity: New Electrides and AlkalidesCHEMPHYSCHEM, Issue 5 2006Fang-Fang Wang Dr. Abstract Optimized structures, with all real frequencies, of superalkali superhalides (Li3)+(SH), (SH=LiF2, BeF3, and BF4), are obtained, for the first time, at the B3LYP/aug-cc-pVDZ and MP2/aug-cc-pVDZ computational levels. These superalkali superhalides possess three characteristics that are significantly different from normal alkali halides. 1) They have a variety of structures, which come from five bonding mode types: edge,face, edge,edge, face,face, face,edge, and staggered face,edge. We find that the bonding mode type closely correlates with the Li3SH bond energy. 2) The valence electrons on the Li3 ring are pushed out by the (SH), anion, and become excess electrons, conferring alkalide or electride characteristics on these Li3SH species, depending on the bonding mode type. 3) The highest occupied molecular orbital of each Li3SH species is a doubly occupied delocalized , bonding orbital on the Li3 ring, which indicates its aromaticity. It is noticeable that the maximum negative nucleus-independent chemical shift value (about ,10 ppm) moves out from the center of the Li3 ring, owing to repulsion by the SH, anion. We find that these superalkali superhalides are not only complicated "supermolecules", but are also a new type of alkalide or electride, with aromaticity. [source] | ||||||||||