Pyramidal Isomer Research Articles

Iron carbonyl complexes of a rigid chelating dicarbene: A density functional theory study

The series of vegiR dicarbenes with a phenanthroline-like backbone synthesized by Kunz and co-workers provide rigid chelating bidentate ligands with carbon donor atoms. Such ligands can, in principle, replace two carbonyl groups in a metal carbonyl derivative leading to analogues of metal carbonyls but with weaker-field carbon-based ligands. Density functional theory has been used to investigate the structures and energetics of the iron carbonyl complexes (vegiMe)Fe(CO)n (n = 3, 2) and (vegiMe)Fe2(CO)n (n = 7, 6, 5) of the simplest such ligands with methyl substituents. Replacement of two carbonyl groups in Fe(CO)5 with one vegiMe ligand is predicted to give trigonal bipyramidal and tetragonal pyramidal isomers of the tricarbonyl (vegiMe)Fe(CO)3 having similar energies within ~2 kcal/mol suggesting a fluxional system. Removal of a carbonyl group from (vegiMe)Fe(CO)3 is predicted to give singlet and triplet (vegiMe)Fe(CO)2 dicarbonyl structures with similar energies having a hole in the coordination sphere for the “missing” carbonyl group. A quintet (vegiMe)Fe(CO)2 structure with tetrahedral FeC4 coordination is a higher energy isomer by ~10 kcal/mol. The three lowest energy (vegiMe)Fe2(CO)7 structures, obtained by replacing two carbonyl groups in Fe2(CO)9 with one vegiMe ligand, have the vegiMe ligand bridging a Fe–Fe single bond to form a six-membered Fe2CN2C ring. Isomeric (vegiMe)Fe2(CO)7 structures with the vegiMe ligand chelated to a single iron atom forming a five-membered FeC2N2 ring lie at least 10 kcal/mol above the lowest energy isomer. The lowest energy isomers of the unsaturated (vegiMe)Fe2(CO)n (n = 6, 5) are triplet and quintet spin state structures reflecting the lower field strength of the vegiR ligands relative to carbonyl groups.

Probing Isomers of the Benzene Dication in a Low-Temperature Trap

The structure of doubly ionized benzene has been spectroscopically studied for the first time. Helium-tagged complexes were prepared at temperatures below 4 K and analyzed using infrared predissociation spectroscopy. Double ionization of benzene yields primarily high-energy dications with a six-membered-ring structure. Some of the dications undergo rearrangement to a more stable pyramidal isomer with a C5H5 base and CH at the apex. By means of isomer-selective heating by a CO2 laser, infrared predissociation spectra of both the classical and pyramidal dications were obtained.

CO Oxidation on TiO2(110) Supported Subnanometer Gold Clusters: Size and Shape Effects

We performed a comprehensive study of catalytic activities of subnanometer Au clusters supported on TiO2(110) surface (Aun/TiO2, n = 1-4, 7, 16-20) by means of density functional theory (DFT) calculations and microkinetics analysis. The creditability of the chosen DFT/microkienetics methodologies was demonstrated by the very good agreement between predicted catalytic activities with experimental measurement (J. Am. Chem. Soc, 2004, 126, 5682-5483) for the Au1-4/TiO2 and Au7/TiO2 benchmark systems. For the first time, the size- and shape-dependent catalytic activities of the subnanometer Au clusters (Au16-Au20) on TiO2 supports were systematically investigated. We found that catalytic activities of the Aun/TiO2 systems increase with the size n up to Au18, for which the hollow-cage Au18 isomer exhibits highest activity for the CO oxidation, with a reaction rate ∼30 times higher than that of Au7/TiO2 system. In stark contrast, the pyramidal isomer of Au18 exhibits much lower activity comparable to the Au3-4/TiO2 systems. Moreover, we found that the hollow-cage Au18 is robust upon the soft-landing with an impact velocity of 200 m/s to the TiO2 substrate, and also exhibits thermal stability upon CO and O2 co-adsorption. The larger pyramidal Au19 and Au20 clusters (on the TiO2 support) display much lower reaction rates than the pyramidal Au18. Results of rate of reactions for unsupported (gas-phase) and supported Au clusters can be correlated by a contour plot that illustrates the dependence of the reaction rates on the CO and O2 adsorption energies. With the TiO2 support, however, the catalytic activities can be greatly enhanced due to the weaker adsorption of CO on the TiO2 support than on the Au clusters, thereby not only the ratio of O2/CO adsorption energy and the probability for the O2 to occupy the Ti sites are increased but also the requirement for meeting the critical line becomes weaker. The obtained contour plot not only can provide guidance for the theoretical investigation of catalytic activity on other metal cluster/support systems, but also assist experimental design of optimal metal cluster/support systems to achieve higher catalytic efficiency.

CO chemisorption on the surfaces of the golden cages

We report a joint experimental and theoretical study of CO chemisorption on the golden cages. We find that the Au(17)(-) cage is highly robust and retains its cage structure in Au(17)(CO)(-). On the other hand, the Au(16)(-) cage is transformed to a structure similar to Au(17)(-) upon the adsorption of CO. Au(18)(-) is known to consist of two nearly degenerate structures, i.e., a cage and a pyramidal isomer, which coexist in the cluster beam. However, upon CO chemisorption only the cage isomer is observed while the pyramidal isomer no longer exists due to its less favorable interaction with CO, compared to the cage isomer. We find that inclusion of the spin-orbit effects is critical in yielding simulated spectra in quantitative agreement with the experimental data and providing unequivocal structural information and molecular insights into the chemical interactions between CO and the golden cages.

Structural Transition of Gold Nanoclusters: From the Golden Cage to the Golden Pyramid

How nanoclusters transform from one structural type to another as a function of size is a critical issue in cluster science. Here we report a study of the structural transition from the golden cage Au(16)(-) to the pyramidal Au(20)(-). We obtained distinct experimental evidence that the cage-to-pyramid crossover occurs at Au(18)(-), for which the cage and pyramidal isomers are nearly degenerate and coexist experimentally. The two isomers are observed and identified by their different interactions with O(2) and Ar. The cage isomer is observed to be more reactive with O(2) and can be preferentially "titrated" from the cluster beam, whereas the pyramidal isomer has slightly stronger interactions with Ar and is favored in the Au(18)Ar(x)(-) van der Waals complexes. The current study allows the detailed structural evolution and growth routes from the hollow cage to the compact pyramid to be understood and provides information about the structure-function relationship of the Au(18)(-) cluster.

Use of Spectra Resolution Methodology to Investigate Surfactant/β-Cyclodextrin Mixed Systems

A study of the spectral behavior of crystal violet in mixed systems formed by hexadecyltrimethylammonium chloride (CTACl) and beta-cyclodextrin (beta-CD) has been carried out. The spectra resolution methodology was used to decompose the absorption spectra of crystal violet in the sum of constituent sub-bands centered at 17,953, 16,807, and 16,474 cm(-1). The sub-bands centered at upsilon = 17,953 and 16,807 cm(-1) correspond to planar and pyramidal isomers of the dye in water and the same isomers associated with the cyclodextrin and micelle. The sub-band centered at upsilon = 16,474 cm(-1) is due to the formation of an inclusion complex between the dye and the cyclodextrin, where solvation of the carbocation takes place through the hydroxyl groups of the cyclodextrin and the association of the crystal violet to micelles of CTACl through the formation of ion pairs in the Stern layer. The results obtained in the mixed systems are compatible with the results obtained in the previously studied simple systems, thus supporting the situation where interactions do not exist between cyclodextrins and micelles once these have formed.

Influence of Single Impurity Atoms on the Structure, Electronic, and Magnetic Properties of Ni5Clusters

With a gradient-corrected density functional method, we have studied computationally the influence of single impurity atoms on the structure, electronic, and magnetic properties of Ni5 clusters. The square-pyramidal isomer of bare Ni5 with six unpaired electrons was calculated 23 kJ/mol more stable than the trigonal bipyramid in its lowest-energy electronic configuration with four unpaired electrons. In a previous study on the cluster Ni4, we had obtained only one stable isomer with an O or an H impurity, but we located six minima for ONi5 and five minima for HNi5. In the most stable structures of HNi5, the H atom bridges a Ni-Ni edge at the base or the side of the square pyramid, similarly to the coordination of an H atom at the tetrahedral cluster Ni4. The most stable ONi5 isomers exhibit a trigonal bipyramidal structure of the Ni5 moiety, with the impurity coordinated at a facet, (micro3-O)Ni5, or at an apex edge, (micro-O)Ni5. We located four stable structures for a C impurity at a Ni5 cluster. As for CNi4, the most stable structure of the corresponding Ni5 complex comprises a four-coordinated C atom, (micro4-C)Ni5, and can be considered as insertion of the impurity into a Ni-Ni bond of the bare cluster. All structures with C and five with O impurity have four unpaired electrons, while the number of unpaired electrons in the clusters HNi5 varies between 3 and 7. As a rough trend, the ionization potentials and electron affinities of the clusters with impurity atoms decrease with the coordination number of the impurity. However, the position of the impurity and the shape of the metal moiety also affect the results. Coordination of an impurity atom leads to a partial oxidation of the metal atoms.

Relative stabilities of all-metal clusters [formula omitted] (M = Li +, Na +, K +, Rb +, Cs +, Cu +, Ag +, Au +)

Upon having made use of relative energy, relative polarizability and relative harness as three descriptors of relative stability, we test their behaviors in the description of the lowest energy isomers of all-metal clusters MAl 4 - (M = Li +, Na +, K +, Rb +, Cs +, Cu +, Ag +, Au +). It has been shown that the stability of the pyramidal isomer decreases with the addition of atomic number. Due to this reason, finally the planar isomer of the AuAl 4 - cluster is more stable than its pyramidal isomer. Compared to the minimum energy principle, the minimum polarizability principle is worth recommendation in assessing relative stabilities of isomers.

Theoretical evidence of d‐orbital aromaticity in anionic metal X (X = Sc, Y, La) clusters

AbstractThe equilibrium geometries, total electronic energies, and vibrational frequencies for singlet, triplet, and quinted states of three all‐metal X (X = Sc, Y, and La) anions and nine relevant neutral singlet MX3 (M = Li, Na, K, X = Sc, Y, La) clusters are investigated with four density functional theory (DFT) and correlated ab initio methods B3LYP, B3PW91, MP2, and CCSD(T). To our knowledge, the theoretical study on these clusters composed of the transition metal Sc, Y, La is first reported here. The calculated results show that for the X clusters the singlet states with trigonal D3h structures are the lowest energetically, while the neutral singlet MX3 clusters each have two stable isomers: one trigonal pyramidal C3v and one bidentate C2v structures with the pyramidal C3v isomer being ground state. In addition, we calculate the resonance energies (RE) and nucleus‐independent chemical shift (NICS) for the singlet trigonal X rings and show that these singlet trigonal X rings exhibit higher degree of aromaticity. The detailed molecular orbital (MO) analyses reveal that the singlet trigonal X anions have one delocalized σ‐type and one delocalized π‐type MOs, which follow the 4n + 2 electron counting rule, respectively and play an important role in rendering these species two‐fold aromaticity. Here, an explicit theoretical evidence is given to prove that the contribution to the two‐fold aromaticity of the singlet trigonal X (X = Sc, Y, and La) rings originates primarily from the d‐orbital bonding interactions of these component transition metal X atoms. © 2007 Wiley Periodicals, Inc. Int J Quantum Chem, 2007

Experimental and Computational Studies of Alkali-Metal Coinage-Metal Clusters

Coinage and alkali metal mixed clusters, M4Na- (M = Cu, Au) have been investigated experimentally using photoelectron spectroscopy and computationally at correlated ab initio levels. The related Cu4Li-, Ag4Li-, Ag4Na-, and Au4Li- clusters as well as the neutral Cu4Li2 and Cu4Na2 clusters have also been studied computationally. The calculations show that the two lowest isomers of the negatively charged clusters include a pyramidal C4v structure and a planar C2v species. For Cu4Li- and Cu4Na-, the C4v structure is calculated at correlated ab initio level to be 30.9 and 16.9 kJ/mol below the planar C2v isomer, whereas the planar isomers of Au4Li- and Au4Na- are found to be 29.7 and 49.4 kJ/mol below the pyramidal ones. For Ag4Li- and Ag4Na-, the pyramidal isomers are the lowest ones. Comparison of the calculated and measured photoelectron spectra of Cu4Na- and Au4Na- shows that the pyramidal Cu4Na- cluster of C4v symmetry and the planar Au4Na- of C2v symmetry are detected experimentally. Calculations of the magnetically induced current density in Cu4Li- and Cu4Li2 using the Gauge-Including Magnetically Induced Current (GIMIC) method show that strong ring currents are sustained mainly by the highest-occupied molecular orbital primarily derived from the Cu 4s. The GIMIC calculations thus show that the Cu4(2-) ring is -aromatic and that the d orbitals do not play any significant role for the electron delocalization effects. The present study does not support the notion that the square-planar Cu4(2-) is the first example of d-orbital aromatic molecules.

Magnetic, Spectroscopic and Thermal Study of [Cu(NO3)(PyTz)2](NO3). A Copper(II) Complex Containing Thiazine Derivative Ligand

The copper(II) complex [Cu(NO3)(PyTz)2](NO3) has been previously characterized means X-ray powder diffraction and now studied by IR spectroscopy, UV-Vis-NIR diffuse reflectance, magnetic susceptibility measurement, electronic spin resonance (ESR) and thermal analysis. The results are correlated with a distorted square pyramidal coordination around copper(II) ion rather than the cis-distorted octahedral stereochemistry of a CuN4OO′ chromophore in good concordance with their structure. Likewise, in order to indicate towards what square pyramidal isomer the complex is distorted, the method proposed by Carugo and Bisi has been applied to the structural data of [Cu(NO3)(PyTz)2](NO3). It is deduced that there is a large distortion from the trigonal bipyramid geometry, close to a square pyramid geometry, being produced almost exclusively through the B route of the Berry mechanism.

Synthesis, Structure, Spectroscopy, and Reactivity of a Neutral Iridathiabenzene1

Treatment of the cationic iridathiabenzene (3) with sodium tert-butylthiolate leads to the production of a stable neutral iridathiabenzene, (4). Compound 4 exists in solution as an equilibrating mixture of two square pyramidal isomers, “cis” 4a and “trans” 4b. When 4 is cooled to −30 °C in acetonitrile, it precipitates as an acetonitrile adduct, (6), featuring an iridathiacyclohexa-1,3-diene ring system. Compound 6 reverts back to 4 upon redissolving and warming to room temperature. Treatment of 4 (or 6) with excess PMe3 results in addition of PMe3 to the iridium center and replacement of the bulky PEt3 ligands with PMe3's, producing (7) as a mixture of equilibrating isomers, “cis” 7a and “trans” 7b. Compound 7, like 6, includes a nonaromatic iridathiacyclohexa-1,3-diene ring. When 4 in acetonitrile solvent (or 6) is reacted with trifluoromethanesulfonic acid, protonation occurs at the thiolate sulfur, causing loss of thiol. The resulting cationic fragment dimerizes to produce (8), in which the two iridium centers are bridged by the two ring sulfurs. Finally, treatment of 4 (or 6) with [(η5-C5Me5)Ru(NCMe)3]+O3SCF3- results in production of the sandwich compound (9), which exists in solution as an equilibrating mixture of isomers, “cis” 9a and “trans” 9b. Compounds 4a, 7a, 8, and 9a have been structurally characterized by X-ray diffraction.

Structures and stabilities of Al 3B isomers

A theoretical study of the Al 3B system has been carried out by means of density functional (B3LYP) and ab initio methods (MP2) in combination with relatively large basis sets. The structures of several singlet and triplet species are reported, together with their vibrational frequencies and relative energies. There are two species lying quite close in energy: a pyramidal isomer, 4t ( 3A 1), a trigonal isomer, 1s ( 1A 1), in which boron is found to be three-fold coordinated and aluminum behave as a monovalent atom. The pyramidal triplet species is predicted to be the best candidate for the global minimum at all levels.

A density functional theory investigation of the polytopal rearrangements of nido square-based pyramidal clusters: the C 3B 2H 5− case

Three square-based pyramidal isomers are predicted, by our DFT calculations, to be stable for the C 3B 2H 5 − cluster. The most stable isomer is the only one, which bears a boron atom at the pyramid apex. The energy differences between the three isomers are less than 21 kcal/mol so that thermal rearrangement between them is possible. The first step of the rearrangement process between two structures is the breaking of the bond linking the atom situated at the apex of the pyramid and a basal atom. The variation of the energy barriers for these rearrangements follows the fact that, for a given isomer, the breaking of a B(apex)–B(base) or C(apex)–B(base) is easier than that of a B(apex)–C(base) or C(apex)–C(base) bond.

Pyramidal inversion isomers in the solid-state structure of the tricyclic antimony-tellurium imido complex TeSb(2)Cl(2)(N(t)Bu)(4).

The reaction of tBuNTe(μ-NtBu)2TeNtBu with ClSb(μ-NtBu)2SbCl in toluene produces the tricyclic complex {[tBuNSb(μ-NtBu)2TeNtBu](μ-SbCl)}Cl- (3). In the solid state, the cation in 3 exists as an approximately 3:1 mixture of pyramidal inversion isomers.

Beyond Classical Stoichiometry: Experiment and Theory

Most known molecules and compounds follow fixed stoichiometry and can be rationalized on the basis of classical valence theories. However, nonstoichiometric species, particularly in the gas phase, are common. These species cannot be easily understood by classical valence considerations because they do not have the full octet of valence electronssthey are valence unsaturated molecules with dangling bonds. We consider nonstoichiometric molecules consisting of only four or five atoms and show the great variety of molecules and bonding that can be derived from this class of seemingly simple species. We demonstrate that gas-phase photodetachment photoelectron spectroscopy using a laser vaporization source and ab initio quantum calculations provide an ideal approach to characterize and understand the structure and bonding of nonstoichiometric molecular and cluster species. Specifically, we review our recent progress in the design and characterization of the first pentaatomic tetracoordinate planar carbon molecules, CAl 4 - , CAl3Si - , CAl3Ge - , and a salt complex, Na + [CAl4 2- ] containing a planar carbon building block. We also review our recent discovery of an all-metal aromatic species, Al4 2- , in a series of bimetallic clusters, M + [Al4 2- ]( M) Cu, Li, Na), as well as the Ga4 2- and In4 2- analogues. We also show the existence of aromaticity in a series of isoelectronic singly charged anions, MAl3 - (M ) Si, Ge, Sn, Pb), and how aromaticity helps stabilize the heterocyclic structure over a pyramidal isomer. We show how, by pursuing and understanding the concept of nonstoichiometry, one can extend the classical valence theory and discover new structures and new types of bonding.

Amine conformational change and spin conversion induced by metal-assisted ligand oxidation: from the seven-coordinate iron(II)–TPAA complex to the two oxidized iron(II)–(py) 3tren isomers. Characterization, crystal structures, and density functional study

The tripodal heptadentate ligand TPAA (TPAA=tris[ N-(2-pyridylmethyl)-2-aminoethyl]amine) is found to undergo an iron(II)-assisted oxidative dehydrogenation with its three amine functions oxidized into three imine groups, giving the tripodal potentially heptadentate ligand (py) 3tren ((py) 3tren=tris[ N-(2-pyridylmethyl)-2-iminoethyl]amine). This oxidative process induces structural changes and spin conversion in the three identified iron(II) complexes, Fe–TPAA complex ( 1), Fe–py3tren complexes ( 2) and ( 3). X-ray crystallographic studies revealed differences in the coordination geometry of the bridging nitrogen atom and shifts in the coordination number from seven to six. Complex 1 is seven-coordinate and is characterized by a pyramidal environment of the tripodal centered nitrogen and a short N tripodalFe distance equal to 2.504 Å. Complex 3 is hexa-coordinate with a planar environment of the centered nitrogen located at 3.435 Å from iron(II) and therefore not bound to the iron. Complex 2 which is an isomeric form of complex 3 is found to be a structural intermediate between complexes 2 and 3 with a pseudo-coordination of six as shown by the N tripodalFe distance equal to 2.754 Å. Electronic changes were recognized from NMR studies in solution and EPR and SQUID measurements of susceptibilities in solid state. Complex 1 is high-spin with the S=2 state characterized by a g factor equal to 2.25. Complex 3 is low-spin but its isomer, the intermediate complex 2 exhibits a temperature-dependent spin conversion from the S=2 high-spin form at room temperature to a lower spin that we provisionally identify as a S=1 intermediate spin form. DFT Becke3LYP calculations were carried out on the two isomeric complexes 2 and 3. The planar complex was found to be 3.2 kcal mol −1 more stable than the pyramidal isomer, in agreement with the experiment.

Allylmetal-directed addition of 1O2: Scope, mechanism and synthetic utility

Allylic silyl and stannyl groups strongly influence the regio- and stereochemistry of alkene oxygenations by 1O2, even within functionalized systems. Allylstannanes undergo anti-SE2′ oxygenation to form both Z-stannylalkenyl hydroperoxides and 4-stannyl-1,2-dioxolanes; the ene-like reaction is generally preferred unless limited by allylic strain. The alkenylstannane products, as well as the derived iodides, are effective substrates for palladium-mediated cross-couplings, additions, carbonylations, and acylations to form peroxydienes, peroxyenones, and peroxyenoates. Allylsilanes are less effective directing groups, possessing reactivity surprisingly similar to simple alkenes, and undergoing oxidation to form regioisomeric mixtures of hydroperoxides. The different reactivities and product distributions observed for allylstannanes and allylsilanes reflect different nucleophilicities of the ground state alkenes as well as variable polarization of the developing perepoxides by the neighboring C–Sn or C–Si bond. The observed selectivity for production of Z-alkenylmetals appears to result from the preferential formation of a single perepoxide pyramidal isomer and the tendency for this perepoxide to abstract the inside hydrogen on the metal-bearing carbon at a rate which is faster than either perepoxide inversion or single bond rotation to deliver the outside hydrogen for abstraction.

A MODEL FOR CALCULATING CONFORMATIONAL ENERGIES IN PENTACOORDINATE PHOSPHORUS COMPOUNDS

Abstract A model is developed which allows the estimation of the relative stability of all trigonal bipyramidal (TP) and square pyramidal (SP) isomers of a given pentacoordinate phosphorus compound. It is based on the widely used pseudorotational hypothesis. The model makes use of activational energies governing fluxional behavior for simple acyclic derivatives of known structure and results of theoretical and spectroscopic studies defining TP-SP energy differences. Based on these criteria, apicophilicity scales are constructed for the TP and SP with the additional constraints that they have the same range and correlate linearly with each other as a function of ligand electronegativity. The resultant model which incorporates ring strain and steric terms has general applicability and should prove of considerable predictive value in quantifying pentacoordinate intermediates and transition states postulated in phosphorus reaction mechanisms. The model gives relative isomer energies in excellent agreement with those obtained from ab initio calculations and reproduces measured ΔG‡ for intramolecular ligand exchange on cyclic and acyclic derivatives to within ± 1.5 kcal/mol.

Two isomers of SF•5 and SF+5: Structures and energetics

The apparently conflicting results for the ionization energy of SF•5 and the standard enthalpy of formation of SF+5 reported over the last 25 years have been reinterpreted and brought into line in terms of the existence of square pyramidal (C4v) and trigonal bipyramidal (D3h) isomers of SF•5 and SF+5. New experimental results and theoretical calculations are reported which are consistent with this interpretation. Values are recommended for the ionization energy and standard enthalpies of formation for the two isomers of SF•5 and SF+5.