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Stable Isomers (stable + isomer)
Selected AbstractsAb initio and DFT studies on van der Waals trimers: The OCS · (CO2)2 complexesJOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 4 2002H. Valdés Abstract Ab initio calculations [MP2, MP4SDTQ, and QCISD(T)] using different basis sets [6-31G(d,p), cc-pVXZ (X = D, T, Q), and aug-cc-pVDZ] and density functional theory [B3LYP/6-31G(d,p)] calculations were carried out to study the OCS · (CO2)2 van der Waals trimer. The DFT has proved inappropriate to the study of this type of systems where the dispersion forces are expected to play a relevant role. Three minima isomers (two noncyclic and one cyclic) were located and characterized. The most stable isomer exhibits a noncyclic barrel-like structure whose bond lengths, angles, rotational constants, and dipole moment agree quite well with the corresponding experimental values of the only structure observed in recent microwave spectroscopic studies. The energetic proximity of the three isomers, with stabilization energies of 1442, 1371, and 1307 cm,1, respectively, at the CBS-MP2/cc-pVXZ (X = D, T, Q) level, strongly suggests that the two unobserved structures should also be detected as in the case of the (CO2)3 trimer where both noncyclic and cyclic isomers have been reported to exist. The many-body symmetry-adapted perturbation theory is employed to analyze the nature of the interactions leading to the formation of the different structures. The three-body contributions are small and stabilizing for the two most stable structures and almost negligible for the cyclic isomer. © 2002 Wiley Periodicals, Inc. J Comput Chem 23: 444,455, 2002; DOI 10.1002/jcc.10041 [source] Electrophilic Attack on Sulfur,Sulfur Bonds: Coordination of Lithium Cations to Sulfur-Rich Molecules Studied by Ab Initio MO MethodsCHEMISTRY - A EUROPEAN JOURNAL, Issue 4 2005Yana Steudel Dr. Abstract Complex formation between gaseous Li+ ions and sulfur-containing neutral ligands, such as H2S, Me2Sn (n = 1,5; Me = CH3) and various isomers of hexasulfur (S6), has been studied by ab initio MO calculations at the G3X(MP2) level of theory. Generally, the formation of LiSn heterocycles and clusters is preferred in these reactions. The binding energies of the cation in the 29 complexes investigated range from ,88 kJ,mol,1 for [H2SLi]+ to ,189 kJ,mol,1 for the most stable isomer of [Me2S5Li]+ which contains three-coordinate Li+. Of the various S6 ligands (chair, boat, prism, branched ring, and triplet chain structures), two isomeric complexes containing the S5S ligand have the highest binding energies (,163±1 kJ,mol,1). However, the global minimum structure of [LiS6]+ is of C3v symmetry with the six-membered S6 homocycle in the well-known chair conformation and three LiS bonds with a length of 256 pm (binding energy: ,134 kJ,mol,1). Relatively unstable isomers of S6 are stabilized by complex formation with Li+. The interaction between the cation and the S6 ligands is mainly attributed to ion,dipole attraction with a little charge transfer, except in cations containing the six sulfur atoms in the form of separated neutral S2, S3, or S4 units, as in [Li(S3)2]+ and [Li(S2)(S4)]+. In the two most stable isomers of the [LiS6]+ complexes, the number of SS bonds is at maximum and the coordination number of Li+ is either 3 or 4. A topological analysis of all investigated complexes revealed that the LiS bonds of lengths below 280 pm are characterized by a maximum electron-density path and closed-shell interaction. [source] Semi-Empirical and DFT Studies on Structures and Spectra for C78(CH2)2CHINESE JOURNAL OF CHEMISTRY, Issue 2 2007Shi Wu Abstract Eighteen possible isomers of C78(CH2)2 were investigated by the INDO method. It was indicated that the most stable isomer was 42,43,62,63-C78(CH2)2, where the ,CH2 groups were added to the 6/6 bonds located at the same hexagon passed by the longest axis of C78 (C2v), to form cyclopropane structures. Based on the most stable four geometries of C78(CH2)2 optimized at B3LYP/3-21G level, the first absorptions in the electronic spectra calculated with the INDO/CIS method and the IR frequencies of the C,C bonds on the carbon cage computed using the AM1 method were blue-shifted compared with those of C78 (C2v) because of the bigger LUMO-HOMO energy gap and the less conjugated carbon cage after the addition. The chemical shifts of 13C NMR for the carbon atoms on the added bonds calculated at B3LYP/3-21G level were moved upfield thanks to the conversion from sp2 -C to sp3 -C. [source] Photoswitches: From Molecules to MaterialsADVANCED MATERIALS, Issue 31 2010Maria-Melanie Russew Abstract Small organic molecules, capable of undergoing efficient and reversible photochemical reactions to switch them between (at least) two (meta)stable isomers associated with markedly different properties, continue to impact the materials world. Such photoswitches are being implemented in a variety of materials for applications ranging from optical devices to "smart" polymers. All approaches exploit the photoswitching molecular entities as gates, which translate an incoming light stimulus to trigger macroscopic property changes of the materials. In this progress report, the most promising recent examples in this field are highlighted and put in perspective. Moving from supramolecular systems in solution to surfaces and finally to bulk materials, important design concepts are discussed, emphasizing both the challenges as well as the great promise of such truly advanced materials. [source] Multiple-Well, multiple-path unimolecular reaction systems.INTERNATIONAL JOURNAL OF CHEMICAL KINETICS, Issue 4 2001Vibrationally excited 2-methylhexyl radicals formed by shock wave activation or by chemical activation can isomerize by multiple pathways to form any of six stable isomers, can fragment by multiple CH and CC bond fission pathways, and can be collisionally stabilized. Master equation simulations of chemical activation and of shock wave activation are used to explore the generic behavior of this complicated coupled system. Selecting the argon pressure in chemical activation systems that produce the 2-methyl-1-hexyl radical isomer (1) can control the yield of specific isomers. Shock heating of 1 also shows a highly regular sequence of isomer formation. This regular behavior is because the first isomerization steps are faster than subsequent steps. Other radical isomers, such as 2-methyl-3-hexyl (3), do not show such regular behavior, because the first isomerization step is slower than subsequent steps. Incubation and unimolecular rate-constant fall-off are observed in the shock wave simulations. The unimolecular rate-constant fall-off for the coupled system produces low-pressure limiting rate constants proportional to [M]n, where n can be greater than unity. The fact that n can be greater than unity is a natural feature of multichannel coupled unimolecular reaction systems, but detection of the effect in experiments may be very demanding. © 2001 John Wiley & Sons, Inc. Int J Chem Kinet 33: 246,261, 2001 [source] Aromatic stabilization in heterofullerenes C48X12 (X,=,N, P, B, Si),JOURNAL OF PHYSICAL ORGANIC CHEMISTRY, Issue 10 2003Zhongfang Chen Abstract B3LYP density functional calculations were performed on two S6 symmetrical isomers (I and II) of C48X12 (X=N, B, P, Si) heterofullerenes, and their global and local aromaticity were evaluated by nucleus-independent chemical shifts (NICS). Despite the unfavorable heteroatom repulsive interactions, isomer II is more stable than I owing to the combination of global and local aromaticity. The latter arises from the presence of triphenylene units in isomer II. The aromatic stabilization effects found in this study should be taken into account when predicting the most stable isomers of heterofullerenes. The same is true for predictions of the isomers of fullerene adducts such as C60Cl12. Copyright © 2003 John Wiley & Sons, Ltd. [source] Shape transition of medium-sized neutral silicon clustersPHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 3 2003A. Sieck Abstract Addressing the shape transition of silicon clusters, indicated by mobility experiments on silicon cluster cations with 24 to 30 atoms, we investigate the structure of low energy neutral silicon clusters with 25, 29, and 35 atoms within a density-functional based tight-binding approach. Since there is strong evidence for several nearly degenerate low-energy isomers for clusters of this size, we perform an extensive, but limited global search with Simulated Annealing and statistically analyze for each cluster size the 100 clusters with the lowest energy. We find different dominant shapes in the set of low energy clusters for each size. For neutral silicon clusters with 25 atoms, both prolate and spherical structures with low cohesive energies exist. For clusters containing 29 or 35 atoms, the low-energy isomers exhibit a spherical shape. For each cluster size several stable isomers with similar shapes, and hence similar mobilities, but different bonding patterns exist. The most stable 25 atom cluster resulting from our global search has the lowest energy within DFT-GGA known so far. Finally, we investigate the transition to diamond-like bonding patterns expected for larger silicon clusters. Clusters with up to 239 atoms resemble amorphous silicon rather than the diamond structure and contain several highly coordinated atoms. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Hydrogen-Atom Abstraction from Methane by Stoichiometric Vanadium,Silicon Heteronuclear Oxide Cluster CationsCHEMISTRY - A EUROPEAN JOURNAL, Issue 37 2010Dr. Xun-Lei Ding Abstract Vanadium,silicon heteronuclear oxide cluster cations were prepared by laser ablation of a V/Si mixed sample in an O2 background. Reactions of the heteronuclear oxide cations with methane in a fast-flow reactor were studied with a time-of-flight (TOF) mass spectrometer to detect the cluster distribution before and after the reactions. Hydrogen abstraction reactions were identified over stoichiometric cluster cations [(V2O5)n(SiO2)m]+ (n=1, m=1,4; n=2, m=1), and the estimated first-order rate constants for the reactions were close to that of the homonuclear oxide cluster V4O10+ with methane. Density functional calculations were performed to study the structural, bonding, electronic, and reactivity properties of these stoichiometric oxide clusters. Terminal-oxygen-centered radicals (Ot.) were found in all of the stable isomers. These Ot. radicals are active sites of the clusters in reaction with CH4. The Ot. radicals in [V2O5(SiO2)1,4]+ clusters are bonded with Si rather than V atoms. All the hydrogen abstraction reactions are favorable both thermodynamically and kinetically. This work reveals the unique properties of metal/nonmetal heteronuclear oxide clusters, and may provide new insights into CH4 activation on silica-supported vanadium oxide catalysts. [source] Structures and Vibrational Spectra of the Sulfur-Rich Oxides SnO (n = 4,9): The Importance of ,*,,* InteractionsCHEMISTRY - A EUROPEAN JOURNAL, Issue 2 2007Wah Wong Prof. Abstract The structures of a large number of isomers of the sulfur oxides SnO with n = 4,9 have been calculated at the G3X(MP2) level of theory. In most cases, homocyclic molecules with exocyclic oxygen atoms in an axial position are the global minimum structures. Perfect agreement is obtained with experimentally determined structures of S7O and S8O. The most stable S4O isomer as well as some less stable isomers of S5O and S6O are characterized by a strong ,*,,* interaction between SO and SS groups, which results in relatively long SS bonds with internuclear distances of 244,262,pm. Heterocyclic isomers are less stable than the global minimum structures, and this energy difference approximately increases with the ring size: 17 (S4O), 40 (S5O), 32 (S6O), 28 (S7O), 45 (S8O), and 54,kJ,mol,1 (S9O). Owing to a favorable ,*,,* interaction, preference for an axial (or endo) conformation is calculated for the global energy minima of S7O, S8O, and S9O. Vapor-phase decomposition of SnO molecules to SO2 and S8 is strongly exothermic, whereas the formation of S2O and S8 is exothermic if n<7, but slightly endothermic for S7O, S8O, and S9O. The calculated vibrational spectra of the most stable isomers of S6O, S7O, and S8O are in excellent agreement with the observed data. [source] Electrophilic Attack on Sulfur,Sulfur Bonds: Coordination of Lithium Cations to Sulfur-Rich Molecules Studied by Ab Initio MO MethodsCHEMISTRY - A EUROPEAN JOURNAL, Issue 4 2005Yana Steudel Dr. Abstract Complex formation between gaseous Li+ ions and sulfur-containing neutral ligands, such as H2S, Me2Sn (n = 1,5; Me = CH3) and various isomers of hexasulfur (S6), has been studied by ab initio MO calculations at the G3X(MP2) level of theory. Generally, the formation of LiSn heterocycles and clusters is preferred in these reactions. The binding energies of the cation in the 29 complexes investigated range from ,88 kJ,mol,1 for [H2SLi]+ to ,189 kJ,mol,1 for the most stable isomer of [Me2S5Li]+ which contains three-coordinate Li+. Of the various S6 ligands (chair, boat, prism, branched ring, and triplet chain structures), two isomeric complexes containing the S5S ligand have the highest binding energies (,163±1 kJ,mol,1). However, the global minimum structure of [LiS6]+ is of C3v symmetry with the six-membered S6 homocycle in the well-known chair conformation and three LiS bonds with a length of 256 pm (binding energy: ,134 kJ,mol,1). Relatively unstable isomers of S6 are stabilized by complex formation with Li+. The interaction between the cation and the S6 ligands is mainly attributed to ion,dipole attraction with a little charge transfer, except in cations containing the six sulfur atoms in the form of separated neutral S2, S3, or S4 units, as in [Li(S3)2]+ and [Li(S2)(S4)]+. In the two most stable isomers of the [LiS6]+ complexes, the number of SS bonds is at maximum and the coordination number of Li+ is either 3 or 4. A topological analysis of all investigated complexes revealed that the LiS bonds of lengths below 280 pm are characterized by a maximum electron-density path and closed-shell interaction. [source] | ||||||||||||||||||||||