In situ spectroscopic study of the oxidation and reduction of Pd(111)

Using a photoemission spectroscometer that operates close to ambient conditions of pressure and temperature we have determined the Pd-O phase diagram and the kinetic parameters of phase transformations. We found that on the (111) surface oxidation proceeds by formation of stable and metastable structures. As the chemical potential of O2 increases chemisorbed oxygen forms followed by a thin surface oxide. Bulk oxidation is a two-step process that starts with the metastable growth of the surface oxide into the bulk, followed by a first-order transformation to PdO.     

The breakdown of the minimum polarizability principle in vibrational motions as an indicator of the most aromatic center

The vibrational motions that disobey the minimum polarizability principle (MPP) in pi-conjugated molecules are distortions of the equilibrium geometry that produce a reduction in the polarizability due to the localization of pi electrons. For aromatic species, this electronic localization is responsible for the subsequent reduction in the aromaticity of the system. In the present work, we diagonalize the Hessian matrix of the polarizability with respect to the vibrational nontotally symmetric normal coordinates, to calculate the nontotally symmetric distortions that produce the maximum breakdown of the MPP in a series of twenty polycyclic aromatic hydrocarbons. It is shown that the nuclear displacements that break the MPP have larger components in those rings that possess the highest local aromaticity. Thus, these vibrational motions can be used as an indicator of local aromaticity.     

A comparative study on the photochemistry of two bipyridyl derivatives: [2,2'-bipyridyl]-3,3'-diamine and [2,2'-bipyridyl]-3,3'-diol.

The two isoelectronic bipyridyl derivatives, [2,2'-bipyridyl]-3,3'-diamine and [2,2'-bipyridyl]-3,3'-diol, are experimentally known to undergo very different excited-state double-proton-transfer processes, which result in fluorescence quantum yields that differ by four orders of magnitude. Herein, density functional theory (DFT), time-dependent DFT (TDDFT), and complete active space self-consistent field (CASSCF) calculations are used to study the double-proton-transfer processes in the ground and first singlet pi-->pi* excited state. The quantum-chemistry calculations indicate 1) the existence of only one energy minimum in the ground electronic state corresponding to reactants (thus avoiding the possibility of a fast fluorescent relaxation process from the photoproducts region), 2) an endoergic process of the complete double proton transfer, and 3) the presence of a conical intersection in the excited intermediate region of [2,2'-bipyridyl]-3,3'-diamine. These facts explain the very low fluorescence quantum yield in [2,2'-bipyridyl]-3,3'-diamine compared to [2,2'-bipyridyl]-3,3'-diol.     

Electronic Structure of Rh(2)(&mgr;-CO)(CO)(2)(H(2)PCH(2)PH(2))(2). An Example of a Non-A-Frame Structure

Calculations based on density functional theory (DFT) and Hartree-Fock configuration interaction (HF-CI) methodology have been carried out to investigate the rhodium-rhodium coupling in Rh(2)(CO)(2)(dppm)(2), 1 (dppm = Ph(2)PCH(2)PPh(2)) and in Rh(2)(&mgr;-CO)(CO)(2)(dppm)(2), 2. DFT geometries, obtained with the Dgauss program, are in good agreement with those determined from X-ray, but HF geometries, calculated using the same basis sets, yield bond distances systematically too long. Calculations indicate that the rhodium atoms in 1 are linked by a single bond. The insertion of a semibridging carbonyl between the two metal atoms leads to a shortening of the rhodium-rhodium distance and also to a noticeable weakening of the metal-metal interaction. Both effects, and also the stabilization of the HOMO of 2, are related to an observed change from square planar to tetrahedral of the ligand environment of the Rh atom proximal to the inserted CO. Both MO analysis and bond characterization from the topology of the charge density confirm the existence of a bonding interaction between the semibridging carbonyl and the distal rhodium atom. The electronic structures of the dicationic complex [Rh(2)(CO)(3)(dppm)(2)](2+) and of the A-frame-like, isoelectronic system Rh(2)Br(2)(&mgr;-CO) (dppm)(2) are also discussed. The electron deformation density is derived from 2 by means of several methodological approaches, namely, HF, HF-CI, DFT, and DFT + gradient corrections. The HF deformation density obtained in the plane containing the metals and the three CO ligands is discussed, as well as the "correlation density" obtained from the difference maps DFT - HF and CI - HF.     

Acetylene diethers: Dineopentyloxyethyne

Dineopentyloxyethyne has been synthesized fromtrans-2,3-dichloro-1,4-dioxane according to a general procedure already described. In contrast with alkyl neopentyloxyacetylenes, RC=COCH₂Bu^t, which only polymerize when are strongly heated (150°), dineopentyloxyethyne is a kinetically unstable acetylene diether that polymerizes readily at room temperature. However, the compound has been trapped with Co₂(CO)₈ to give either the corresponding hexacarbonyl complex, m.p. 74–75°, or the cyclic trimer, m.p. 205–206°, depending upon the experimental conditions.     

Synthesis, characterization and electrochemical studies of the heterometallic diclusters [{Fe2(μ-CO)(CO)6(μ-PPh2)Au}2(diphosphine)] (diphosphine=dppm, dppip, dppe, dppp)

Treatment of [(ClAu)₂(diphosphine)] {diphosphine=bis(diphenylphosphino)methane (dppm), bis(diphenylphosphino)isopropane (dppip), 1,2-bis(diphenylphosphino)ethane (dppe), 1,3-bis(diphenylphosphino)propane (dppp)} with two equivalents of the anion [Fe₂(μ-CO)(CO)₆(μ-PPh₂)]⁻ in the presence of TlBF₄ gives the new heterometallic diclusters [{Fe₂(μ-CO)(CO)₆(μ-PPh₂)Au}₂(diphosphine)] that have been isolated and characterized. Their ³¹P-NMR spectra show different patterns as a function of the diphosphine ligand. The electrochemical behavior of these compounds has been investigated and compared with that of the mono- [Fe₂(μ-CO)(CO)₆(μ-PPh₂)(μ-AuPPh₃)] and tricluster [{Fe₂(μ-CO)(CO)₆(μ-PPh₂)Au}₃(triphos)] derivatives.     

TEMPO-promoted Pauson-Khand reaction. Single-electron activation of cobalt-carbonyl bonds?

[reaction: see text] The Pauson-Khand reaction is notably accelerated by TEMPO. According to DFT calculations, TEMPO could trigger a radical, low-energy pathway for the reaction by facilitating the decarbonylation of doublet complexes arising either from a CO/nitroxide exchange or from nitroxide addition to a CO ligand.     

Ab initio and DFT modeling of stereoselective deamination of aziridines by nitrosyl chloride

The stereochemical course of the deamination of cis‐2,3‐dimethylaziridine by nitrosyl chloride was investigated at the QCISD/6‐31G(d) level. Calculations reveal that the reaction takes place in two steps. In the first step, the reactants form a pre‐reactive complex, followed by a spiro‐type bicyclic transition state, which on dissociative cycloelimination gives the N‐nitrosoaziridine intermediate. In the second step, this intermediate undergoes cycloreversion through a slightly asynchronous concerted transition state to form an alkene with the same stereochemistry, which is in total agreement with experiment. In the whole reaction, the denitrosation step is found to be rate‐determining. For comparison, geometry optimizations and energies were also obtained at the B3LYP/6‐31G(d) level. It was found that the B3LYP energy results differed significantly from the QCISD ones. To analyze the reason for this difference, B3LYP calculations were repeated by varying the contribution of exact exchange in the Becke functional. With respect to the QCISD results, it has been shown that the functional with 0% exact exchange yields the best activation barriers, whereas the functional with 30% exact exchange is the most suitable one to carry out the complexation and reaction energy calculations. © 2004 Wiley Periodicals, Inc. Int J Quantum Chem, 2005     

Etude dans la série des pyrrolidinones. IV. Formation de quelques dérivés de l'acide L méthylène bis N-(oxo-5 pyrrolidine carboxylique-2)

Starting from the readily available 5-oxo-2-carboxypyrrolidine, methylene-bis-(N-5-oxo-2-carboxypyrrolidine) was prepared and transformed into a diamine, then into two isomeric diols, which cyclised into one ether. The meso configuration has been assigned for this ether. The reduction of both diols and ether was complete and gave rise to methylene-bis-N-pyrrolidine.     

Ring-closing metathesis of chiral allylamines. Enantioselective synthesis of (2S,3R,4S)-3,4-dihydroxyproline

The ring-closing metathesis (RCM) of two types of unsaturated chiral allylamines III, easily available from enantiomerically enriched epoxy alcohols, has been studied. Fully protected allylamines IIIa [(1)R = CH(2)-(CH(2))(n)()-CH=CH(2); (2)R = Boc; (3)R = PMB] have been prepared from unsaturated epoxy alcohols, whereas bis-allylamines IIIb ((1)R = Ph, (2)R = allyl,(3)R = Boc or PMB) have been prepared from 2,3-epoxy-3-phenylpropanol. Both types have been subjected to RCM to provide either cyclic allylamine I or II. The synthetic potential of these intermediates has been demonstrated by the enantioselective synthesis of (2S,3R,4S)-3,4-dihydroxyproline.