3D Isomers Research Articles

Probing the 2D to 3D Structural Transition in Gold Cluster Anions Using Argon Tagging

Different physisorption properties by 2D and 3D isomers of Au_{n};{-} clusters are observed and used to probe the 2D to 3D structural transition. Strong Ar clustering occurs on planar Au_{n};{-} and the planar faces of the pyramidal Au20-. An abrupt change of Ar clustering at Au12- confirms the 2D to 3D structural transition at this size, where both isomers coexist. The minor 2D isomer can be titrated out by Ar to produce a clean 3D-Au12- beam and beams of Au_{12}Ar_{m};{-} with enhanced 2D isomers. Using the Ar titration and tagging, isomer-specific photoelectron spectra for the 2D and 3D Au12- are obtained.

Splendid symmetry: crystallization of an unbridged isomer of Co2(CO)8 in Co2(CO)8·C60

Cocrystallization of Co2(CO)8 with C60 traps the cobalt carbonyl molecule as the D(3d) isomer that otherwise is not available for crystallographic analysis.

Supramolecular Isomerism in Coordination Polymers of Isophthalato‐Bridged MnII Complexes Using 4,4′‐Dipyridyl N,N′‐Dioxide

AbstractA one‐pot reaction of three components [MnII, isophthalate dianion (isop) and 4,4′‐dipyridyl N,N′‐dioxide (dpyo)] in water/methanol (1:1) causes self‐assembly resulting in two 3D supramolecular isomers of the same stoichiometry, {[Mn(isop)(H2O)2]·(dpyo)·(H2O)}n (colorless) (1) and {[Mn(isop)(H2O)(dpyo)0.5]·(dpyo)0.5·(H2O)2}n (pink) (2). Both isomers crystallize in the same crystal system (triclinic) and space group (P$\bar {1}$). The structural determination of 1 reveals an isophthalato‐bridged 1D coordination polymer, {[Mn(isop)(H2O)2]n, having 16‐ and 8‐membered rings where isophthalate acts simultaneously as chelating and bridging ligand connected to three metal centers. In isomer 2 the 1D chains transform to a 2D arrangement through bridging dpyo that replaces a water molecule at metal axial position. Both the crystal structures result in a 3D supramolecular network achieved through H‐bonding among the 1D (in 1) or 2D (in 2) coordination polymers with lattice water and dpyo molecules and also by means of π‐π interactions in 1. Low‐temperature magnetic measurement indicates antiferromagnetic interaction in both the isomers and the best fits are given by the exchange parameters J = –0.56 ± 0.02 cm–1, g = 2.01 ± 0.01 and R = 3.1 × 10–5 for 1 and J = –1.06 ± 0.03 cm–1, g = 2.01 ± 0.01 and R = 2.8 × 10–5 for 2.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)

Bonding in Cu, Ag, and Au clusters: relativistic effects, trends, and surprises.

Electronic structure and bonding in anionic coinage metal clusters are investigated via density-functional calculations, focusing on an extensive set of isomers of Cu(-)(7), Ag(-)(7), and Au(-)(7). While the ground states of Cu(-)(7) and Ag(-)(7) are three dimensional (3D), that of Au(-)(7) is planar, separated from the optimal 3D isomer by 0.5 eV. The simulated thermally weighted photoabsorption spectrum of Au(-)(7) is dominated by planar structures, and it agrees well with the measured one. The propensity of Au(-)(N) clusters to favor planar structures (with N as large as 13) is correlated with strong hybridization of the atomic 5d and 6s orbitals due to relativistic effects.

The vibrational spectroscopy of C 60H 36: An experimental and theoretical study

Several samples of C 60H 36 have been prepared by the transfer hydrogenation method and studied by infrared and Raman spectroscopy. The results are compared with published infrared spectra of C 60H 36 samples synthesized by other techniques such as high pressure hydrogenation and zinc reduction of C 60. Raman spectra of C 60H 36 have not previously been reported. Based on previous data in the literature, five isomers of low-energy were selected for semiempirical quantum chemical calculations. They are one each of S 6, T and T h symmetry and two isomers of D 3d symmetry. The calculations predict that very few IR lines will be coincident with Raman bands, even in the case of the T isomer for which t symmetry vibrations can be both infrared and Raman active, and for which the few coincident lines should be extremely weak. The interplay between theory and experiment places some strong constraints on the possible symmetries and structures of the various samples of the C 60H 36 molecule. The results indicate that each of two samples prepared by transfer hydrogenation contains a mixture of two principal isomers, of D 3d and S 6 symmetries, the former being the major component in one, and the latter in the other, sample. The analysis also confirms a D 3d isomer as being the major component of a sample of C 60H 36 prepared by high pressure hydrogenation and proposes the S 6 isomer as the major component of a sample prepared by zinc reduction of C 60 in aromatic solvents.

Spectroscopy and photophysics of C 60H 18 and C 60H 36

Ground and triplet state spectroscopic and photophysical properties of C 60H 18 and C 60H 36 have been studied using the complementary techniques of nanosecond pulse radiolysis and laser flash photolysis to determine triplet-triplet (T-T) absorption spectra, triplet lifetimes, T-T molar absorption coefficients and triplet quantum yields, in particular for C 60H 18. The spectroscopic and photophysical propperties of C 60H 18 are similar to those of an sp 2 network species such as C 60. whereas those of C 60H 36 resemble benzene. No T-T absorption was observed for C 60H 36 at λ > 300 nm, so that its triplet state parameters could not be measured. The most probable assignment of the C 60H 36 samples studied, on the basis of vibrational and electronic spectra and structure, is to an S 6 symmetry isomer, but the lowest energy T and D 3d isomers are not entirely excluded. Our partially hydrogenated fullerene samples were prepared by a transfer hydrogenation method. Other preparation techniques may have produced other isomers of these species, thus accounting for some differences in reported spectroscopic results.

‘Dynamic’ QSAR For Semicarbazide-induced Mortality in Frog Embryos

The conventional quantitative-structure-activity relationship (QSAR) provides only a single three-dimensional (3D) model for a given two-dimensional (2D) item in the modeling process. However, in complex reaction environments with solvents of different polarity, especially biological systems, the molecules can take the form of different conformers depending on the particular interaction. Therefore, chemical behavior, e.g. toxicity, may be considered the integral effect of a set of conformers rather than the property of a single 3D isomer. The 'dynamic' QSAR method is unique in that it provided for the calculation of a set of conformers for 2D representation of each chemical of the series under investigation. Moreover, these conformers can be selected interactively according to the hypothesized mechanism of toxic action. The acute lethality of 36 semicarbazides and thiosemicarbazides, evaluated using the Frog Embryo Teratogenesis Assay: Xenopus (FETAX), was modeled by using the 'dynamic' QSAR method. The assumed mode of action, osteolathyrism, was defined by the failure of connective tissue to polymerize properly due to interference with lysyl oxidase. Conformer screenings were based on parameter distribution according to the frontier orbital energies and volume polarizability, conditioning their reactivity and hydrophobicity, respectively. The best results were obtained by the selection of conformers providing prevailing values of electron acceptor properties. Moreover, the best two-parameter QSARs encompassing all the evaluated compounds incorporate a geometric parameter, the geometric analog of the Wiener topological index, and the local electronic characteristics of the C=O or C=S group, superdelocalizabilities and charges.

The performance of a variety of density functional and ab initio methods on the structural problem of beryllium borohydride

The D 2d and D 3d isomers of beryllium borohydride (BeB 2H 8) have been studied with different ab initio and density functional techniques. Optimized geometries were obtained with a polarized split valence basis set and electronic energies were calculated at the optimum geometry with an extended polarized basis set containing f functions. Both MP2 and MP4 calculations with the extended basis set indicate the D 3d conformer to be slightly more stable at the basis set level. The performance of many combinations of local and non-local exchange and correlation density functionals have been tested on this difficult structural problem. Only the non-local functionals approached the accuracy of ab initio MP2 calculations.

Theoretical studies on the stability of M8C12 clusters

Ab initio calculations with effective core potentials were completed on M 8 C 12 (M=Y, Zr, Nb, and Mo) clusters. Three different isomers (T h , D 3d , and T d ) have been investigated. The T h and D 3d isomers are derived from a dodecahedral geometry. In the former, the faces of a cube of the metal atoms are capped by ethylene-like C 2 units, while in the latter the 12 carbons form a cylindrical, closed chain capped by two M 4 units. In the T d isomer, six acetylene-like C 2 units link the vertices of a tetracapped tetrahedral metal framework in a butterfly fashion

Self-consistent cluster calculations of local magnetic properties of non-equilibrium crystalline FCC Cu-Fe and FCC Ag-Fe alloys

The first-principles spin-polarized discrete variational Xalpha method was used to calculate the electronic structures at a central Fe site in a number of embedded clusters representing FCC Cu-Fe and FCC Ag-Fe alloys. The magnetic moment and isomer shift were derived from the calculations and compared with the existing experimental data. The calculated results show that the cluster model yields a magnetic moment and isomer shift consistent with experimental magnetization and Mossbauer data. The Fe 3d magnetic moment and isomer shift in Cu-Fe alloys decrease slightly on increase in the Cu concentration. The Fe 3d magnetic moment in Ag-Fe alloys is larger than that in Cu-Fe alloys. There is an enhancement of the magnetic moment in Ag-Fe alloys.