Withdrawal Of Electron Density Research Articles

Temperature Dependence of Kinetic Isotope Effects for Enzymatic Carbon−Hydrogen Bond Cleavage

An analysis is reported of the anomalously weak temperature dependence of large kinetic isotope effects (KIEs) observed for enzymes that catalyze carbon−hydrogen bond cleavage. After a critical examination of the experimental data, rate expressions for proton tunneling are used to derive a universal relationship between KIEs and their temperature dependence for a model in which the tunneling coordinate, represented by two crossing Morse potentials or a quartic double-minimum potential, is assisted by a harmonic promoting mode. Since the reactions involve electron transfer to a redox system as well as proton transfer from a CH to an OH bond, both adiabatic and nonadiabatic mechanisms are considered. Model calculations are reported, which show that the derived relationship is valid under a wide range of conditions and depends on a minimum number of model parameters, namely almost exclusively on the proton transfer distance and the tunneling-mode anharmonicity. The results are used to derive proton transfer distances and promoting-mode force constants from the available data on temperature-dependent isotope effects for several enzymatic reactions effecting CH cleavage, including the reactions of linoleic acid catalyzed by soybean lipoxygenase-1 (SLO1), of primary amines catalyzed by methylamine and aromatic amine dehydrogenase (MADH and AADH), and of a dicopper complex that models the active site of monooxygenases and lacks a protein environment. These distances are found to be shorter and the force constants to be larger than those implied by van der Waals radii, but with anharmonicities suitably adjusted, they resemble those encountered in systems with hydrogen bonding. It is proposed that the strong redox system, which is always present in these enzymes, acts so as to withdraw electron density from the C···H···O transfer system. This will tend to shrink the tunneling barrier to a size that allows effective passage of H but not necessarily of D, a condition that may lead to a large KIE. It will also increase the enzyme−substrate force constant, thus reducing the temperature dependence of the KIE. For the reaction catalyzed by SLO1, a transfer distance of 0.9−1.0 Å is found. Similar results are obtained for three artificial mutants from which it is concluded that parts of the protein located outside the reaction site have little influence on the proton transfer. For the reactions catalyzed by MADH and AADH, the corresponding values range from 0.5 to 1.0 Å; however, the values much below 1.0 Å may be unrealistic since they are based on data that show anomalies suggesting that an incomplete kinetic model was used in their derivation. The analysis also yields a transfer distance of about 0.9 Å for the dicopper complex, a value similar to that of SLO1, although no protein is present. Since results for three different systems cluster about the same transfer distance of 0.9−1.0 Å, it is concluded that the large KIEs and their weak temperature dependence are due to a redox-induced C···H···O hydrogen bond.

Distinct molecular recognition of psychostimulants by human and Drosophila serotonin transporters.

In this study, human embryonic kidney (HEK)-293 cells stably expressing human, Drosophila, or a chimeric serotonin (5-hydroxytryptamine, 5-HT) transporter (hSERT, dSERT, and H(1-281)D(282-476)H(477-638), respectively) were used to explore the ability of two libraries of structurally distinct psychostimulants to inhibit 5-HT uptake. One library consisted of 3-phenyltropane analogs, whereas the second library consisted of several substituted amphetamines. hSERT exhibited a lower K(i) value for all the compounds in both libraries compared with dSERT, whereas the chimeric SERT exhibited properties more closely resembling those of dSERT. This species selectivity was explored using computer-generated comparative molecular field analysis to model the interactions of the cocaine analogs and substituted amphetamines at hSERT, dSERT, and the cross-species chimera. Models for the 3-phenyltropane analogs indicate that a region exists around the aromatic ring where decreased electron density is favored, particularly for hSERT. This finding may indicate pi-pi stacking with an aromatic amino acid residue in SERT. Also, electronegative substituents in the 4'-position provide favorable interactions. This structural feature was demonstrated by increased potency of analogs with electronegative substituents on the aromatic ring that withdraw electron density. For the substituted amphetamines, key areas for interaction exist around the amine, an electrostatic component surrounding the 3-position on the aromatic ring, and a steric component surrounding the 4-position.

Chain‐Like Tetra‐, Penta‐ and Heptanuclear Cyanide‐Bridged Complexes by Attachment of Organometallic Cyanides to M2, M3 and M5 Units

AbstractAttachment of the “ligands” [Cp(dppe)Fe‐CN], [Cp(PPh3)2Ru‐CN], [Cp(CO)2Fe‐CN] and [(CO)5M‐CN]−, (where M = Cr, Mo, W) to the polymetallic centres [Ru2(OAc)4]+, [Co3(dpa)4]2+ and [Ni5(tdpa)4]2+ yielded two tetra‐, three penta‐ and five heptanuclear complexes with chain‐like linear arrangements of the metal ions and the bridging cyanide ligands, as proved by the structure determination of [Ni5(tdpa)4{Cp(dppe)2Fe‐CN}2](PF6)2. IR and CV data show that the Ru2, Co3 and Ni5 centres of these complexes withdraw electron density from the external organometallic units. This prevents electronic interactions between the two terminal metal ions, yet allows multistep one‐ and two‐electron redox processes. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003)

Synthesis and Structural Characterization of Potassium Salts of Phosphane‐Substituted (Cyclopentadienyl)iron Dicyanides, and Their Use as Bridging Ligands for Copper(I) Phosphane Derivatives

AbstractPhosphane‐substituted CpFe(CN)2 anions have been synthesized by photochemical decarbonylation of KCpFe(CN)2CO in the presence of various mono‐ and diphosphanes. The synthesis and characterization of uniquely bridged diphosphane derivatives, [KCpFe(CN)2]2‐μ‐(Ph2P)2−(CH2)n (n = 2−4), are particularly noteworthy. These phosphane‐substituted CpFe(CN)2− units have afforded stable mixed‐metal complexes upon reaction with CuI in the presence of tricyclohexylphosphane in acetonitrile solution. X‐ray crystallography shows these derivatives to display two‐dimensional diamond‐shaped structures with copper(I) of the form [Fe(CN)2Cu]2, where the CuI centers exhibit both trigonal and tetrahedral geometries. Infrared data reveal that the withdrawal of electron density from the iron centers in these mixed metal cyanides via the cyanide ligands is enhanced upon replacing CO in CpFe(CN)2CO− with phosphanes. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003)

Carbon nanotube synthesis in a flame with independently prepared laser-ablated catalyst particles.

Laser ablation has been used ex situ to create metal nanoparticles for introduction into a reactive pyrolysis flame. By prior synthesis of the metal nanoparticles, the effects of the reactive gases can be clearly separated from the pyrolysis chemistry of a solvent carrier, as when nebulized solutions are used. Moreover, varying reactivity issues associated with particle growth and size are bypassed. Our results show that Fe selectively reacts with CO to produce nanotubes, whereas Ni selectively reacts with C2H2 to produce nanofibers. These observations are interpreted through the donation and withdrawal of electron density between the adsorbate's molecular orbitals and surface atoms of the metal nanoparticle. The rate of reaction of Ni with only C2H2 is found to be greater than the rate with C2H2 and CO. This suggests that CO inhibits the Ni-catalyzed reaction.

Self-activated supramolecular reactions: effects of host-guest recognition on the kinetics of the Diels-Alder reaction of open-chain oligoether quinones with cyclopentadiene.

Diels-Alder reactions of acyclic oligoether-substituted quinones 1b, 1c, 2b, and 2c with cyclopentadiene were accelerated by the addition of alkali and alkaline earth metal perchlorates, and scandium trifluoromethane sulfonate (k(c)/k(f) = 1.2-23 for univalent cations, 11-1160 for divalent cations, and 1700-192 000 for Sc(3+), where k(c) and k(f) are the rate constants for the metal complexed and uncomplexed quinones, respectively). The shorter-armed 1a, 2a, and 3, however, exhibited no such acceleration effects. The rate accelerations can be rationalized by the FMO consequence in which the bound guest cation withdraws electron density from the quinone dienophile and lowers the LUMO energy suitable for the orbital interaction with the HOMO of cyclopentadiene. Despite the poor cation selectivity, these acyclic oligoether quinones showed larger rate accelerations than the relevant quinocrown ethers 4 (k(c)/k(f) = 1.3-3.0 for univalent cations, 5.0-160 for divalent cations, and 100-2020 for Sc(3+)). The effective electron withdrawal, which leads to the enhanced rate acceleration, can be caused by the direct interaction between the metal cation accommodated in the pseudo-cyclic oligoether linkage and the quinone carbonyl oxygen, as indicated by (1)H NMR spectroscopy. In addition, the larger rate enhancement is rather achieved in the complex with low binding constant K, because the strong encapsulation of metal cation by the oligoether chain diminishes the crucial interaction to the quinone carbonyl oxygen. As a whole, the smaller and higher valent cations tend to bring about notable rate acceleration due to the more enhanced ion-dipole interaction with the quinone carbonyl oxygen. Spectroscopic titration (absorption and (1)H NMR) and kinetic experiments indicated that only the longest di-armed 2c constructs 1:1, and then 1:2, host/guest complexes with Ca(2+), Sr(2+), and Ba(2+). These 1:2 complexes exhibited the most effective acceleration for the respective metal cations.

Bond Dissociation Energies and Radical Stabilization Energies Associated with Substituted Methyl Radicals

Bond dissociation energies (BDEs) and radical stabilization energies (RSEs) associated with a series of 22 monosubstituted methyl radicals (•CH2X) have been determined at a variety of levels including, CBS-RAD, G3(MP2)-RAD, RMP2, UB3-LYP and RB3-LYP. In addition, W1‘ values were obtained for a subset of 13 of the radicals. The W1‘ BDEs and RSEs are generally close to experimental values and lead to the suggestion that a small number of the experimental estimates warrant reexamination. Of the other methods, CBS-RAD and G3(MP2)-RAD produce good BDEs. A cancellation of errors leads to reasonable RSEs being produced from all the methods examined. CBS-RAD, W1‘ and G3(MP2)-RAD perform best, while UB3-LYP performs worst. The substituents (X) examined include lone-pair-donors (X = NH2, OH, OCH3, F, PH2, SH, Cl, Br and OCOCH3), π-acceptors (X = BH2, CHCH2, C⋮CH, C6H5, CHO, COOH, COOCH3, CN and NO2) and hyperconjugating groups (CH3, CH2CH3, CF3 and CF2CF3). All substituents other than CF3 and CF2CF3 result in radical stabilization, with the vinyl (CHCH2), ethynyl (C⋮CH) and phenyl (C6H5) groups predicted to give the largest stabilizations of the π-acceptor substituents examined and the NH2 group calculated to provide the greatest stabilization of the lone-pair-donor groups. The substituents investigated in this work stabilize methyl radical centers in three general ways that delocalize the odd electron: π-acceptor groups (unsaturated substituents) delocalize the unpaired electron into the π-system of the substituent, lone-pair-donor groups (heteroatomic substituents) bring about stabilization through a three-electron interaction between a lone pair on the substituent and the unpaired electron at the radical center, while alkyl groups stabilize radicals via a hyperconjugative mechanism. Polyfluoroalkyl substituents are predicted to slightly destabilize a methyl radical center by inductively withdrawing electron density from the electron-deficient radical center.

Theoretical study on the electronic structure and properties of synthetic MoFe3S3 compounds*

AbstractWe examine the electronic structure of a series of MoFe3S3 compounds whose spectroscopic and structural parameters have been recently published (J. Han, K. Beck, N. Ockwig, D. Coucouvanis, J Am Chem Soc 1999, 121, 10448). The synthetic Mo/Fe/S compounds under examination are close structural and functional analogs of essential parts of the FeMo‐cofactor in the enzyme nitrogenase. Using the ZINDO program and the projected unrestricted Hartree–Fock (PUHF) method, we have demonstrated that these compounds exhibit antiferromagnetic coupling between the metal atoms, with calculated triplet and singlet ground states. The metal atoms are found in reduced oxidation states, namely Mo(III), Fe(II), and Fe(I), in agreement with the experimental isomer shifts and the Mössbauer data previously reported. The addition of carbonyl groups to the MoFe3S3 clusters leads to a withdrawal of electron density from the Mo/Fe/S core and thus affects the strength of the metal–metal and metal–sulfur bonds in the compounds. We also show that the removal of pyridine from the coordination of the Mo atom has a notable effect on the electron delocalization in the Mo/Fe/S core and may lead to preferential localization of the electron density on the iron atoms. © 2001 John Wiley & Sons, Inc. Int J Quantum Chem, 2001

Metal−Metal Bonding in Rh2(O2CCF3)4: Extensive Metal−Ligand Orbital Mixing Promoted by Filled Fluorine Orbitals

He I and He II gas-phase photoelectron spectra of Rh2(O2CCF3)4 are reported. The electron configuration of the metal−metal bond of Rh2(O2CCF3)4 is determined to be σ2 π4 δ2 δ*2 π*4 with an ionization energy order of σ ≈ π > δ > δ* ≈ π*. The δ* and π* ionization energies are similar within the range of vibrational energy separations. Assignment of the Rh−Rh δ ionization is assisted by previous observations that ionizations from δ orbitals in M2(O2CCF3)4 (M = Mo, W) and Mo2(O2CH)4 show enhanced intensity over ionizations from the σ and π orbitals with He II excitation. Changes in ionization energies from the dimolybdenum molecule to the dirhodium molecule and changes in ionization intensities from He I to He II excitation indicate greater metal−ligand mixing in these molecules than observed in other dimetal tetracarboxylates. Amsterdam density functional calculations agree with the observation that the Rh−Rh δ* and π* energies are similar. The calculations also indicate substantial ligand mixing into the metal−metal σ and π orbitals, which is enhanced by the trifluoroacetate ligand despite the inductive withdrawal of electron density by the electronegative fluorine atoms. It is found that a specific set of ligand orbitals that possess the same symmetries as the Rh−Rh σ and π orbitals are destabilized by overlap interactions with the filled fluorine p orbitals, resulting in greater metal−ligand mixing among these orbitals. The increased mixing explains the long-observed enhancement of the δ and δ* ionizations from He I to He II excitation for a number of M2(O2CCF3)4 (M = Mo, W, Ru) systems.

Redox and photochemical behaviour of ruthenium(II) complexes with H2dcbpy ligand (H2dcbpy = 2,2′-bipyridine-4,4′-dicarboxylic acid)

The electrochemical redox properties of the ruthenium(II) complexes Ru(H2dcbpy)(CO)2Cl21, Ru(H2dcbpy)2Cl22, Ru(H2dcbpy)2Br23, Ru(H2dcbpy)2(dcbpy) 4 and [Ru(H2dcbpy)3]Cl25 [H2dcbpy = 2,2′-bipyridine-4,4′-dicarboxylic acid] were studied in CH3CN and DMSO by cyclic voltammetry. The metal-based RuII–RuIII oxidations are observed at variable positive potentials (E1/2 = 0.15–1.62 V vs. Ag+–Ag). The H2dcbpy ligands withdraw electron density from the ruthenium centre and make the oxidation more difficult (seen as positive shifts in E1/2 values). At negative potentials the first irreversible ligand-based reductions (Epc = −1.37 to −1.57 V) are due to the deprotonation of carboxylic acid groups whereas the subsequent reversible reactions at high negative potentials are assigned as bipyridine ring based reductions. Based on CV and UV-Vis studies, photoirradiation in the co-ordinating solvent (CH3CN) revealed the tendency to light-induced CH3CN co-ordination for complexes 1–5. The photochemical products were also identified by using Lever’s E(L) parameters. For example, upon photoirradiation of 1, the principal product is Ru(H2dcbpy)(CO)(CH3CN)Cl2. The structures of 1 and 3–5 were determined by single-crystal X-ray crystallography.

Adhesion mechanisms in a cyanurate prepolymer on silicon and on aluminium

XPS, UPS, MIES, IR reflection spectroscopy and AFM are employed in a study of interaction mechanisms in thin films of a cyanurate prepolymer or various organic model compounds on different surface states of Si and oxidised Al. The trioxytriazine entity is identified as the only chemical part of the substances which can establish an adhesive interaction with the substrates. On the silicon surfaces, the mechanism is identified as a donor–acceptor interaction where the 2p electron lone pairs of N and O in the trioxytriazine act as donors for Si cations. On the Al oxide, the Lewis acidic OH-groups act as electron acceptors which withdraw electron density from the lone pairs at O and / or at N of the trioxytriazine. Moreover, a backdonation of electron density takes place from the metal Al beneath the oxide into the organic layer. The dicyanate monomer does not adhere at all, but desorbs quickly out of the interphase region on the substrates. Hence the thermosetting reaction of the prepolymer will be hampered and the resulting polymer network will be less dense near the substrate than in the bulk.

S-Cylindrophanes: From Metal Tweezers to Metal Sandwiches.

Inclusion of AgI and CuI ions by the preorganized cylindrophane cage compound 1 provides the first examples of bis(η6) sandwich complexes of these metals. Locked in a trigonal-planar heteroatom ligand field, the guest ions are shown by NMR spectroscopy to withdraw electron density from the aromatic rings. A statistical study of η6 coordination in the solid state supports the description of arene-metal contacts between 2.5 and 3.5 Å as bonds.

FEATURES OF A FLEXIBLE BACKBONE IN THE COORDINATION COMPOUNDS OF A DIOXIME LIGAND: THE CHARACTERIZATION OF SUPRAMOLECULAR AND DINUCLEAR METAL COMPLEXES

The grinding of a 2: 1 molar ratio mixture of isonitrosoacetylacetone and 1,3-diaminopropan-2-ol led to formation of the tribasic ligand (H3L), (1) with two oxime groups and a flexible alcoholic backbone. The 1:2 molar ratio reaction of (1) with CuX2 produced the planar dinuclear complexes LCu2(X) nH2O; × = acetate (2), phenylacetate (3), formate (4), monochloroacetate (5), dichloroacetate (6), trichloroacetate (7), benzoate (8), and p-hydroxybenzoate (9); n = 1 for (2) and (8); n = 2 for (3)-(7); and n = 4 for (9). The copper(II) ions are bridged by the carbox-ylate and the alcoholic oxygen. The strong antiferromagnetic interactions in (2)-(9) are impeded in (5)-(7) by the chloroacetate bridge withdrawing electron density from the carboxylate. The latter bridge is replaced by picrate in the 1:1 molar ratio reaction of (2) with picric acid (10). The 1:1 molar ratio reaction of (1) with copper(II) acetate produced the tetranuclear [HLCu]2[LCu2(OAc)] 5H2O (11), whereas the 2:1 molar ratio reaction, similar to the reaction which led to (8), produced HLCu (12). The latter complex reacted (1:1 molar ratio) with either copper(ll) acetate or nickel(II) acetate to produce complexes (2) and the heterodinuclear LNi-Cu(OAc) 2H2O (13), respectively. Similar reactions with (11) gave the same complexes (2) and (13). The acid adducts of (9) with p-hydroxybenzoic acid (14) and LCu2(X)-HX (15); × = p-aminobenzoic acid were isolated. The cobalt(II) analogue of the mononuclear (12), HLCo 2H2O (16) was obtained from the 1:1 molar reaction of (1) with cobalt(II) acetate. The supramolecular structure of (11), (12) and (16) took place via intermolecular hydrogen bonding of the alcoholic proton with the oximato oxygen of the adjacent molecule which mediated electron density and allowed for a magnetic exchange interaction. The suggested structures of the ligand and metal complexes are in accordance with analytical, spectral and magnetic moment data.

Hydrolytic breakdown of meso-diphenylamine substituted heptamethine dye IR140 and isomerism of the merocyanine breakdown product

Nucleophilic attack by hydroxyl ions is found to occur preferentially at the C 2 position of the benzothiazolium heterocyclic moiety of the laser dye IR140, and not at the eneamine functionality that consists of the mesocarbon atom and the adjacent diphenylamine moiety, which is the other electrophilic site on the dye. The temperature-dependence of the absorption spectrum of the merocyanine breakdown product suggests that similar ground state isomerism is present in the breakdown product, as is found in the initial dye IR140. It is concluded that the non-fluorescent majority conformer of the merocyanine breakdown product is the one in which the meso-amine substituent is twisted so as to withdraw electron density by induction, and that the fluorescent minority conformer is that in which themeso-amine substituent is conjugated to act as an electron donor by resonance.

Electronic Structure Perturbations of Substituted Ruthenocenes: The First Photoelectron Spectra of Perchloro- and Perfluorocyclopentadienyl Complexes

The electronic structure perturbations caused by cyclopentadienyl substitutions in the series of complexes (η5-C5H5)2Ru, (η5-C5Me5)(η5-C5H5)Ru, (η5-C5Me5)2Ru, (η5-C5Me5)(η5-C5Cl5)Ru, and (η5-C5Me5)(η5-C5F5)Ru are measured by gas-phase photoelectron spectroscopy. The shifts of the valence metal- and cyclopentadienyl-based ionizations give an indication of the overall electronic effects of methyl and halogen substitutions on the cyclopentadienyl rings. The halogen substituent interaction is an admixture of inductive σ-electron-withdrawing and filled−filled π-electron-overlap effects, which act in opposite directions. The π-overlap interaction is relatively weak in the case of chlorine substitution for hydrogen, and the combined σ and π interactions give rise to an overall withdrawal of electron density from the metal center and increase in the metal d-based ionization energies. Fluorine substituents on cyclopentadienyl make the ring only slightly more electron withdrawing than η5-C5Cl5, despite the much gre...

Why Do Cationic Carbon Monoxide Complexes Have High C−O Stretching Force Constants and Short C−O Bonds? Electrostatic Effects, Not σ-Bonding

A significant increase in the C−O stretching force constant (FCO) and a decrease in C−O bond length (rCO) result upon coordination of carbon monoxide to various cationic species. We report a study designed to elucidate the factors responsible for this effect. In particular, we distinguish between an explanation based on electrostatic effects and one based on withdrawal of electron density from the 5σ orbital of CO, an orbital generally considered to have some antibonding character. Ab initio electronic structure calculations on CO in the presence of a positive point charge (located on the carbon side of the bond axis) reveal that a simple Coulombic field increases the C−O stretching force constant and decreases the bond length. Coordination of CO to a simple cationic Lewis acid such as H+ or CH3+ is calculated to increase FCO (and decrease rCO) to extents slightly less than those engendered by a point charge at the same distance from the carbonyl carbon. These results indicate that electron donation from ...

Ionic organoboranes. Part 6. The ousenes revisited; 11B NMR spectra of two polyhedral borane anions, a nido-carborane anion, and a closo-carborane with carbocyclic cationic tropenyliumyl substituents

The ousenes are a class of ionic or zwitterionic organoboranes in which polyhedral borane anions or carboranes have carbocyclic cationic tropenyliumyl ( cyclo-C 7H + 6) substituents. 11B Fourier transform NMR studies have been used to probe ground state electronic interaction between cage and ring. Tropenyliumyl substitution on B 12H 2− 12 or a closo-C 2B 10H 12 results in relatively minor electron withdrawal from the cage by a − I effect, while such substitution on B 10H 2− 10 or a nido-C 2B 9H 2− 12 results in significant − I withdrawal of electron density at the point of attachment and, in addition, strong + T electron donation from cage to ring by borons other than at the point of attachment. Coupled with previous studies these observations confirm that modification of ousene structure can vary cage to ring electronic interaction from negligible, through excited state charge transfer, to significant electron delocalization from ground state electron density donation from cage to ring, and suggests that the electrical properties of these materials may well be worth further study.

The bimetallic Pt/Mo(110) surface: Structural and CO chemisorption studies

Auger electron spectroscopy (AES), low energy electron diffraction (LEED), and temperature-programmed desorption (TPD) measurements were performed to monitor the growth, thermal stability, and CO adsorption characteristics of Pt layers on Mo(110). Pt grows in a layer-by-layer or Frank-van der Merwe fashion on the molybdenum surface below 600 K. The first monolayer of Pt is stable and no significant structural change is observed at higher substrate temperatures. A monolayer of Pt eliminates the dissociative adsorption of CO observed on the Mo(110) surface. The thermal desorption of CO from this Pt overlayer occurs at lower temperatures (in comparison to CO desorption from the bulk Pt(111) and Mo(110) surfaces), indicating a withdrawal of electron density from the Pt monolayer due to interactions with the Mo(110) surface. As the Pt coverage is increased above a monolayer, LEED measurements indicate the growth of Pt(111) layers at 300 K. The multilayers of Pt are thermally unstable and agglomerate into three-dimensional (3D) clusters upon heating to above ~ 600 K. The 3D clusters contribute little to the AES signal. The CO desorption spectra from the annealed layers are very similar to the CO desorption spectra from one monolayer of Pt on the Mo(110) surface, suggesting that Pt clustering occurs on top of a single Pt monolayer on Mo(110). The spatial distribution of Pt clusters is estimated from Auger intensity changes following annealing of a 6 monolayer Pt film.

The stability of the proton in water + co-solvent mixtures

The Hammett acidity function H 0 gives an indication of the ability of a neutral molecule to be protonated and can be used to compare the relative stabilities of protons in solvents consisting of neutral molecules. The rise in H 0 when an organic, oxygen-containing base is introduced into water containing a mineral acid, accompanied by a decrease in conductivity, conforms to the existence of a solvent-sorting equilibrium around the solvated proton together with competing equilibria involving the protonation of the Hammett-indicator base by two different proton solvates. The proton solvate containing a co-solvent molecule is more stable than that without it, and the ionic mobility of the proton involved with the former is less than that in the latter. Free energies of transfer of the proton from water into the mixture Δ G t ⊖(H +) calculated using this solvent-sorting method and the reference ion method (Δ G t ⊖(Ph 4As +) = Δ G t ⊖(BPh 4 −) are compared with the rises in H 0 in water-rich media: those using the latter method are shown to be inconsistent with the rise in H 0 for some alcohols. From the extent of the conversion of the aqua-proton into the proton solvate containing the co-solvent, the factors influencing the proton-co-solvent solvate are discussed for a range of solvent compositions. The principal effect over the whole range of water-rich compositions is found to be the ability of substituent groups to supply or withdraw electron density from the basic site in the co-solvent molecule.

Stable platinum(II) complexed α-aminoethers derived from benzimidazole and benzoxazole

A series of Schiff base compounds derived from 2-aminobenzimidazole and 2-aminobenzoxazole react with Zeise's salt in CH 2Cl 2/ROH (R = CH 3, CH 2CH 3, CH(CH 3) 2,CH 2CH 2OCH 3) to form a series of relatively stable α-aminoether complexes with coordination to Pt(II) via the heterocyclic sp 2 nitrogen. The complexes, trans-PtCl 2(C 2H 4){2- N-(CH(OR)C 6H 4R 1)benzimidazole} ( 4) (R 13-NO 2, RCH 3, CH 2CH 3, CH(CH 3) 2; R 1 4-Cl, R CH 3) and trans-PtCl 2(C 2H 4){2- N(CH(OR)C 6H 4R 1)benzoxazole} ( 5) (R 13-NO 2, RCH 3, CH 2CH 3, CH 2CH 2OCH 3; R 14-Cl, 4-OCH 3, 4-H, RCH 3) arise through initial coordination of the heterocyclic sp 2 nitrogen followed by attack of ROH on the pendant 2- N-benzylidene moiety at the Schiff base carbon. It is suggested that withdrawal of electron density through complexation facilitates alcohol attack. In the absence of complexation the reaction can be 20-30 times slower. A Pd(II) phosphine complex can achieve the same result. 1H NMR spectroscopy, and specifically the chemical shift of the ‘CHN’ moiety, provides an indication of the polarization due to complexation. The structure of one complex, 5c, R 13-NO 2, RCH 2CH 2OCH 3, was determined by X-ray diffraction. A prismatic crystal belonging to the space group P1 with the following characteristics: a = 10.024(3), b= 11.242(9), c = 13.158(3) Å, α = 96.07(3), β = 107.01(2), γ = 109.52(3)°, V = 1302 Å 3, Z = 2, was used to determine the structure, which was refined to R = 0.044.