Theoretical study of isomerism of compounds of CO and CO2 molecules with aluminum clusters Al13, Al12Ti,and Al12Ni

Structural characteristics, vibrational frequencies, and energies of isomers of compounds of CO and CO₂ molecules with the centered aluminum cluster Al₁₃ and its doped analogues Al₁₂M (M = Ti and Ni) have been calculated by the density functional theory method. For the Al₁₂MCO compounds, the most favor-able are two “fragment” isomers in which the C and O atoms are separated and built into the cluster cage, completing it to a 14-vertex polyhedron. In one of them, the C and O atoms are in the capping positions over adjacent trigonal MAl₂ faces; in the second isomer, there is the five-coordinate C* atom located in the center of a tetragonal MAl3 face and bound to the central Al atom through the long fifth bond. The “coordinated” isomers, in which the CO molecule is coordinated as a ligand to a cluster vertex, edge, or face, are unstable to removal of CO for Al₁₃CO, close in energy to the fragment isomers for Al₁₂NiCO, and considerably higher on the energy scale than the fragment isomers but remain stable to CO removal for Al₁₂TiCO. For the Al₁₂MCO₂ compounds, the most favorable is the fragment isomer in which both oxygen atoms are in the capping positions over adjacent faces and the C* atom is five-coordinate. The alternative oxo carbonyl isomer Al₁₂MO(CO) is close to the lowest-lying one in the case of M = Ni and is ～56 kcal/mol higher on the energy scale in the case of M= Ti. The less stable Al₁₂M(CO₂) isomer is the complex in which the CO₂ ligand is coordinated to an M-Al edge. According to calculations, addition of CO to Al₁₂MO and addition of CO₂ to Al₁₂M to form, respectively, Al₁₂MO(CO) and Al₁₂M(CO₂) can occur without noticeable barrier. The Al₁₂M(CO₂) and Al₁₂MO(CO) isomers are separated by a barrier, moderate for M = Ti (～16 kcal/mol) and small for M = Ni (～6 kcal/mol).     

Theoretical study of Al(n) and Al(n)O (n = 2-10) clusters

The stable structures, energies, and electronic properties of neutral, cationic, and anionic clusters of Al(n) (n = 2-10) are studied systematically at the B3LYP/6-311G(2d) level. We find that our optimized structures of Al5(+), Al9(+), Al9(-), Al10, Al10(+), and Al10(-) clusters are more stable than the corresponding ones proposed in previous literature reports. For the studied neutral aluminum clusters, our results show that the stability has an odd/even alternation phenomenon. We also find that the Al3, Al7, Al7(+), and Al7(-) structures are more stable than their neighbors according to their binding energies. For Al7(+) with a special stability, the nucleus-independent chemical shifts and resonance energies are calculated to evaluate its aromaticity. In addition, we present results on hardness, ionization potential, and electron detachment energy. On the basis of the stable structures of the neutral Al(n) (n = 2-10) clusters, the Al(n)O (n = 2-10) clusters are further investigated at the B3LYP/6-311G(2d), and the lowest-energy structures are searched. The structures show that oxygen tends to either be absorbed at the surface of the aluminum clusters or be inserted between Al atoms to form an Al(n-1)OAl motif, of which the Al(n-1) part retains the stable structure of pure aluminum clusters.     

Theoretical study on the structures and electronic spectra of TCNE2-.

Investigations into the charge-separated states and electron-transfer transitions in tetracyanoethylene (TCNE) complexes have recently generated much interest. In this work we present theoretical calculations showing that the most stable structure of the dianion TCNE2- has D2d symmetry in vacuum as well as in the solvents dichloromethane and acetonitrile. By means of the coupled cluster linear response, we compute the vertical electronic spectrum in both the gas phase and solution. The theoretical results are compared to the experimental data and good agreement is achieved.     

DFT calculation of molecular structures of Al2Fe3 and Al2Cu3 heterobinuclear clusters

Structural Chemistry - By using DFT method in the OPBE/TZVP level, key parameters of molecular structures of five-atomic heteronuclear clusters having Al2M3 composition where M?=?Fe or...     

Theoretical study of isomerism of compounds of N2 and N2H2 molecules with aluminum clusters Al13 and Al12Ti

Segments of the potential energy surfaces corresponding to successive elementary stages of multistep fragmentation of nitrogen and diimine molecules upon their reaction with the aluminum cluster Al₁₃ and its doped analogue Al₁₂Ti have been calculated by the density functional theory method. The minimum energy pathways of these reactions have been calculated for the stages of physisorption, chemisorption, and N₂ and N₂H₂ fragmentation with different ways of insertion of fragments into the Al₁₃ and Al₁₂Ti cages. Relative energies, structural characteristics, and vibrational frequencies of coordinated and fragment isomers have been calculated, the barriers separating these isomers have been evaluated, and molecular diagrams of the reactions have been constructed. The effect of doping on the relative energies of intermediates and activation barriers has been considered. A conclusion has been drawn that doping with titanium should facilitate the reactions of molecular nitrogen with aluminum clusters. Unusual isomers with a five- and six-coordinate nitrogen atom N* have been localized. The results are compared with the data of analogous previous calculations of the PES of isomers corresponding to stepwise fragmentation of an acetylene molecule in related Al₁₃C₂H₂ and Al₁₂TiC₂H₂ derivatives.     

Theoretical study of hydrogenated Mg, Ca@Al12 clusters

The studies on the structure and electronic properties of hydrogenated metal embedded Al(12) cage clusters have been performed by density functional theory calculations. We have investigated aluminum cluster hydrides with 12 and 14 hydrogen atoms, respectively. Insertion of the Mg, Ca alkali metals remarkably enhances the stability of the aluminum clusters. The hydrogen atom prefers to occupy on-top sites along the surface of the clusters. Mulliken population analysis indicates that significant charge transfer occurs between the Mg and Ca atoms and the Al atoms. Our computations suggest that these clusters appear to be physically and chemically stable.     

Quantum-chemical calculation of molecular structures of Al2Mn3 and Al2Zn3 clusters by using DFT method

Structural Chemistry - By using DFT method in the OPBE/TZVP level, key parameters of molecular structures of five-atomic heteronuclear clusters having Al2M3 composition where M = Mn or...     

Microwave spectra and structures of 1,2-(ortho)- and 1,7-(meta)-carborane, C2B10H12

The microwave spectra of 1,2- and 1,7-dicarba-closo-dodecaborane(12), C(2)B(10)H(12) (ortho- and meta-carborane), have been recorded for the first time at room temperature in the 32-88 and 24-80 GHz spectral ranges, respectively. The spectra of the parent species (1,2-C(2)(11)B(10)H(12) and 1,7-C(2)(11)B(10)H(12)) have been assigned, together with those of four monosubstituted ((10)B) 1,2-C(2)(10)B(11)B(9)H(12) and 1,7-C(2)(10)B(11)B(9)H(12) isotopologues. The microwave spectra confirm that the structures of each of these two molecules are slightly distorted icosahedrons of C(2v) symmetry. A previous determination of the gaseous structures of these two carboranes by the gas electron-diffraction method was based on several assumptions about the B-B bond length differences. All B-B bond lengths have now been redetermined using the substitution (r(s)) method, which is independent of such restraints. Although several of the r(s) and electron-diffraction bond lengths are in good agreement, there are also differences of up to 0.026 ?. Quantum chemical calculations at the B3LYP/6-311++G(3df,3pd) level of theory have also been performed.     

Intense laser-induced decomposition of mass-selected 2-, 3-, and 4-methylaniline cations

Photodecomposition processes of 2-, 3-, and 4-methylaniline cations induced by a moderately intense (10¹¹ W/cm²) visible nanosecond laser field and an intense (10¹⁵ W/cm²) UV femtosecond laser field have been investigated using a tandem mass spectrometer. Highly unsaturated fragment cations such as and are dominantly produced by the nanosecond laser, while less unsaturated smaller fragment cations such as , and are produced mainly by the femtosecond laser. Ab initio calculations have also been performed to estimate the stable geometrical structures of and and those of possible intermediate ring compounds for discussing the photodecomposition pathways in intense laser fields.     

Theoretical study of the adsorption of H on Si n clusters, (n=3-10)

A recently proposed local Fukui function is used to predict the binding site of atomic hydrogen on silicon clusters. To validate the predictions, an extensive search for the more stable SinH (n=3-10) clusters has been done using a modified genetic algorithm. In all cases, the isomer predicted by the Fukui function is found by the search, but it is not always the most stable one. It is discussed that in the cases where the geometrical structure of the bare silicon cluster suffers a considerable change due to the addition of one hydrogen atom, the situation is more complicated and the relaxation effects should be considered.     

Symmetries, vibrational instabilities, and routes to stable structures of clusters of Al, Sn, and As

We investigate the stability of small clusters using density functional theory to compute the total energy, forces, and vibrational frequencies using linear response. We exhibit an efficient and computationally low-cost route to finding stable structures, by starting with high-symmetry structures and distorting them according to their unstable modes. We illustrate this by application to 4-, 6-, and 13-atom clusters of Al, Sn, and As. This technique also naturally elucidates the origins of stability of the lower symmetry structures, which is variously due to the linear or pseudo Jahn-Teller effect, combined with a lowering of various contributions to the total energy. We show that the situation is more complex than has generally been appreciated.     

Synthesis of (+)-2R,3R,11R,12R- and (−)-2S,3S,11S,12S-tetraphenyl-18-crown-6

The synthesis of the title crown ethers starting from optically active hydrobenzoins is described. R(+)-1,in CDCl₃ ,preferentially extracts R(+)-phenylglycine methyl ester hydroperchlorate from an aqueous solution of the racemate with a chiral recognition factor of 1.5 as shown by nmr measurements.     

Theoretical studies on photoelectron and IR spectral properties of Br2.-(H2O)n clusters

We report vertical detachment energy (VDE) and IR spectra of Br2.-.(H2O)n clusters (n=1-8) based on first principles electronic structure calculations. Cluster structures and IR spectra are calculated at Becke's half-and-half hybrid exchange-correlation functional (BHHLYP) with a triple split valence basis function, 6-311++G(d,p). VDE for the hydrated clusters is calculated based on second order Moller-Plesset perturbation (MP2) theory with the same set of basis function. On full geometry optimization, it is observed that conformers having interwater hydrogen bonding among solvent water molecules are more stable than the structures having double or single hydrogen bonded structures between the anionic solute, Br2.-, and solvent water molecules. Moreover, a conformer having cyclic interwater hydrogen bonded network is predicted to be more stable for each size hydrated cluster. It is also noticed that up to four solvent H2O units can reside around the solute in a cyclic interwater hydrogen bonded network. The excess electron in these hydrated clusters is localized over the solute atoms. Weighted average VDE is calculated for each size (n) cluster based on statistical population of the conformers at 150 K. A linear relationship is obtained for VDE versus (n+3)(-1/3) and bulk VDE of Br2.- aqueous solution is calculated as 10.01 eV at MP2 level of theory. BHHLYP density functional is seen to make a systematic overestimation in VDE values by approximately 0.5 eV compared to MP2 data in all the hydrated clusters. It is observed that hydration increases VDE of bromine dimer anion system by approximately 6.4 eV. Calculated IR spectra show that the formation of Br2.--water clusters induces large shifts from the normal O-H stretching bands of isolated water keeping bending modes rather insensitive. Hydrated clusters, Br2.-.(H2O)n, show characteristic sharp features of O-H stretching bands of water in the small size clusters.     

A photoelectron spectroscopic and computational study of sodium auride clusters, NanAun- (n = 1-3)

Negatively charged sodium auride clusters, NanAun- (n = 1-3), have been investigated experimentally using photoelectron spectroscopy and ab initio calculations. Well-resolved electronic transitions were observed in the photoelectron spectra of NanAun- (n = 1-3) at several photon energies. Very large band gaps were observed in the photoelectron spectra of the anion clusters, indicating that the corresponding neutral clusters are stable closed-shell species. Calculations show that the global minimum of Na2Au2- is a quasi-linear species with Cs symmetry. A planar isomer of D2h symmetry is found to be 0.137 eV higher in energy. The two lowest energy isomers of Na3Au3- consist of three-dimensional structures of Cs symmetry. The global minimum of Na3Au3- has a bent-flake structure lying 0.077 eV below a more compact structure. The global minima of the sodium auride clusters are confirmed by the good agreement between the calculated electron detachment energies of the anions and the measured photoelectron spectra. The global minima of neutral Na2Au2 and Na3Au3 are found to possess higher symmetries with a planar four-membered ring (D2h) and a six-membered ring (D3h) structure, respectively. The chemical bonding in the sodium auride clusters is found to be highly ionic with Au acting as the electron acceptor.     

Pseudopotential and electron propagator methods for the calculation of the photoelectron spectra of anionic silicon clusters: predictions on Si10-

Photoelectron spectra of anionic clusters of silicon require reliable theoretical calculations for their assignment and interpretation. Electron propagator calculations in the outer valence Green's-function approximation with two well-characterized, all-electron basis sets on vertical electron detachment energies (VEDEs) of anions are compared to similar calculations that employ Stuttgart pseudopotentials. Tests on Si(n) (-) clusters with n=3-7 exhibit an encouraging agreement between the all-electron and pseudopotentials results and between electron propagator predictions and experiments and values obtained from coupled-cluster calculations. To illustrate the capabilities of the new approach based on a Si pseudopotential and electron propagator methods, VEDE calculations on Si(10) (-) are presented.     

Theoretical characterization of four distinct isomer types in hydrated-electron clusters, and proposed assignments for photoelectron spectra of water cluster anions

Water cluster anions, (H(2)O)(N)(-), are examined using mixed quantum/classical molecular dynamics based on a one-electron pseudopotential model that incorporates many-body polarization and predicts vertical electron detachment energies (VDEs) with an accuracy of ~0.1 eV. By varying the initial conditions under which the clusters are formed, we are able to identify four distinct isomer types that exhibit different size-dependent VDEs. On the basis of a strong correlation between the electron's radius of gyration and its optical absorption maximum, and extrapolating to the bulk limit (N → ∞), our analysis supports the assignment of the "isomer Ib" data series, observed in photoelectron spectra of very cold clusters, as arising from cavity-bound (H(2)O)(N)(-) cluster isomers. The "isomer I" data reported in warmer experiments are assigned to surface-bound isomers in smaller clusters, transitioning to partially embedded isomers in larger clusters. The partially embedded isomers are characterized by a partially formed solvent cavity at the cluster surface, and they are spectroscopically quite similar to internalized cavity isomers. These assignments are consistent with various experimental data, and our theoretical characterization of these isomers sheds new light on a long-standing assignment problem.     

Theoretical investigation of photoelectron spectra of furan,pyrrole, thiophene,and selenole

The ionization spectra of furan, pyrrole, thiophene, and selenophene have been calculated within the framework of the nonempirical quantum-chemical method with the Green's one-particle function in the approximation of the third order algebraic diagram construction [ADC(3)]. The calculated energies and the intensity of vertical transitions pertaining to the ionization of outer and inner shells are compared with the newest experimental data. The good agreement of theoretical and experimental results enabled a detailed assignment and interpretation of the observed photoelectron spectra to be carried out. Problems of disturbing the picture of orbital ionization are considered; the mechanism of formation of low-lying photoelectron satellites is explained. Certain general rules and trends of the behavior of the spectra of the systems studied are considered. Dedicated to Academician of the Russian Academy of Sciences B. A. Trofimov on his 70^th jubilee. Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 9, pp. 1366–1379. September, 2008.     

He I photoelectron spectra and gas-phase electronic structures of end-functionalized [3]- and [5]-ladderanes

[3]- and [5]-ladderanes obtained by way of template-controlled syntheses conducted in the organic solid state have been characterized via He I photoelectron (PE) spectroscopy. The results provide a first correlation with X-ray crystallographic structure data and establish the reliability of quantum chemical DFT (B3LYP/6-31G*) and ab initio HF calculations in predicting geometrical and electronic structures of molecular ladder frameworks.