Theoretical study of the novel sandwich compound [Au<Subscript>3</Subscript>Cl<Subscript>3</Subscript>Tr<Subscript>2</Subscript>]<Superscript>2+</Superscript>

A theoretical study of a sandwich compound with a metal monolayer sheet between two aromatic ligands is presented. A full geometry optimization of the [Au₃Cl₃Tr₂]²⁺ (1) compound, which is a triangular gold(I) monolayer sheet capped by chlorines and bounded to two cycloheptatrienyl (Tr) ligands was carried out using perturbation theory at the MP2 computational level and DFT. Compound (1) is in agreement with the 18–electron rule, the bonding nature in the complex may be interpreted from the donation interaction coming from the Tr rings to the Au array, and from the back-donation from the latter to the former. NICS calculations show a strong aromatic character in the gold monolayer sheet and Tr ligands; calculations done with HOMA, also report the same aromatic behavior on the cycloheptatrienyl fragments giving us an insight on the stability of (1). The Au –Au bond lengths indicate that an intramolecular aurophilic interaction among the Au(I) cations plays an important role in the bonding of the central metal sheet. Figure (a) Ground state geometry of complex 1; (b) Top view of compound 1 and Wiberg bond orders computed with the MP2/B1 computational method; (c) Lateral view of compound 1 and NICS values calculated with the MP2/B1 method; the values in parenthesis were obtained at the VWN/TZP level     

On the existence of MHn species with M=Al, Ga and n=4, 5, 6. Computational study of structures, stabilities and bonding

Based on second-order perturbation theory (MP2) predictions with large 6-311++G(3df, 3pd) basis set we have reviewed the possible structures and stabilities of a series of neutral MH_n(M=Al, Ga; n=4, 5, 6) species. For AlH₄ and AlH₅, our results confirm the previous theoretical findings, which indicate the dihydrogen C_s complexes (²A′) AlH₂(H₂) and (¹A′) AlH₃(H₂), respectively, as the lowest energy isomers. We found, similarly, C_s (²A′) GaH₂(H₂) and (¹A′) GaH₃(H₂) van der Waals complexes as the most stable species of the gallium analogues GaH₄ and GaH₅. The calculated H₂ dissociation energies (D_e) for AlH₂(H₂) and AlH₃(H₂) are of the order 1.8–2.5 kcalmol¹, whereas this range of values for GaH₂(H₂) and GaH₃(H₂) is 1.4–1.8 kcalmol¹ . Symmetry-adapted perturbation theory (SAPT) was used to analyze the interaction energies of these dihydrogen complexes (for n=5) to determine why the Ga species show a smaller binding energy than the Al species. The SAPT partitioning of the interaction energy showed significant differences between AlH₃(H₂) and GaH₃(H₂), resulting from the much stronger “hydride” character of the aluminum species. The experimental observation of AlH₂(H₂) and AlH₃(H₂), and likely GaH₃(H₂), via low-temperature matrix isolation has been reported recently by Pullumbi et al. and Andrews et al., supporting the theoretical predictions. For n=6, we found the degenerate C₂(²A) and C_s(²A′) MH₂(H₂)₂ “double H₂” type van der Waals complexes as the lowest energy species for both M=Al and Ga.Electronic Supplementary Material Supplementary material is available for this article at     

Theoretical study on electronic spectra and interaction in [Au3]-L-[Au3] (L = C6F6,Ag+) complexes

The electronic structure and spectroscopic properties of [Au₃(μ-C(OEt) = NC₆H₄CH₃)₃]_n-(C₆F₆)_m and [Au₃(μ-C²,N³-bzim)₃]_n-(Ag⁺)_m were studied at the B3LYP, PBE and TPSS levels. The interaction between the [Au₃] cluster and L (C₆F₆, Ag⁺) was analyzed. Grimme’s dispersion correction is used for those functionals. Weak π-interactions (Au-C₆F₆) were found to be the main contribution short-range stability in the models; while in the models with Ag⁺, an ionic interaction is obtained. The absorption spectra of these models at the PBE level agree with the experimental spectra.     

Synthesis, structure, and antimycobacterial activity of 6-[1(3H)-isobenzofuranylidenemethyl]purines and analogs

6-Benzofuryl-, styryl, benzyl, and furfurylpurines as well as 6-[1(3H)-isobenzofuranylidenemethyl]purines have been synthesized and their activities against Mycobacterium tuberculosis (Mtb) determined. Several compounds displayed profound antimycobacterial activity in combination with low toxicity towards mammalian cells. NMR and X-ray crystallography were employed to determine the detailed structures and the results were supported by quantum chemical calculations.     

M2X (M= Li,Na; X= F,Cl): the smallest superalkali clusters with significant NLO responses and electride characteristics

Hypervalent M₂X (M = Li, Na; X = F, Cl) clusters are prototype species possessing lower ionisation potentials than Li, therefore classified as superalkalis. This study reveals some interesting properties of these small clusters using ab initio MP2/aug-cc-pVTZ and QCISD/aug-cc-pVTZ methods. These clusters are shown as an ionic species, composed of positively charged cage of alkali metals (M₂⁺) and halogen anion (X^?). Therefore, the stability of M₂X is governed by both ionic and covalent interactions. We show that the excess valence electron of (M₂⁺) is pushed out by anionic X^?, which allows M₂X clusters to possess ‘electride’ characteristics. It is also due to this excess electron that M₂X clusters exhibit significant non-linear optical (NLO) properties. The dipole moment, mean polarisability and hyperpolarisability suggest their significant NLO responses, which are explained on the basis of electronic transitions in crucial excited states using TD-B3LYP/aug-cc-pVTZ method. The first static hyperpolarisabilities of Li₂F and Na₂F take the values of order of 10⁴ a.u. due to their lower transition energies. This study should provide new insights into the design of novel materials with significant NLO responses useful for electro-optical applications.     

Computational Investigations of the Chalcogen-Substituted Carboxylic Acids RC(=O)XH and RC(=X)OH and their Dimers [RC(=O)XH]2 and [RC(=X)OH]2 (X=S, Se, Te)

Semiempirical and ab initio theoretical methods have been used to investigate molecular structures of the chalcogen-substituted carboxylic acid isomers RC(=O)XH (chalcogenol acid) and RC(=X)OH (chalcogenon acid). A recent experimental report suggests that the chalcogenon isomers, although less stable at room temperature, predominate at low temperature in polar solvents and that there is only a small barrier to isomerization between the isomers. Theoretical calculations have been used to locate minimum energy structures of chalcogen-substituted carboxylic acid isomers and to calculate energy differences between pairs of isomers. Carboxylic acids are well known to dimerize, especially in the gas phase and in non-polar solvents. We have, therefore, also calculated energies of dimerization of the chalcogen-substituted acids by optimizing the geometries of the symmetric dimers. We note that the PM3 level of theory is only qualitatively correct for sulfur- and selenium-containing species but fails even qualitatively for the tellurium-containing compounds. Ab initio results confirm the experimental observations and provide good estimates of both isomerization and dimerization energies. We conclude that for many functional groups with tautomers RC(=X)YH and RC(=Y)XH, the more acidic tautomer is the one with the acid proton on the smaller, more electronegative atom, although in many cases this may not be the more stable tautomer.Electronic Supplementary Material available.     

Synthesis, characterization and binding with DNA of four planaramineplatinum(II) complexes of the forms: trans-PtL2Cl2 and [PtL3Cl]Cl, where L = 3-hydroxypyridine, 4-hydroxypyridine and imidazo(1,2-alpha)pyridine

Four new trans-planaramineplatinum(II) complexes, three of the form: trans-PtCl2L2, code named CH1, CH2 and CH4 where L = 3-hydroxypyridine, 4-hydroxypyridine and imidazo[1,2-alpha]pyridine, respectively, and one of the form: PtClL3, code named CH3 where L = 3-hydroxypyridine, have been prepared and characterized by elemental analyses and IR, Raman, mass and 1H NMR spectral studies. The interactions of the compounds with salmon sperm and pBR322 plasmid DNAs have been investigated and their activity against human ovarian cancer cell lines: A2780, A2780cisR and A2780ZD0473R have also been determined. The compounds are believed to form mainly monofunctional N7(G) and bifunctional intrastrand N7(G)N7(G) adducts with DNA, causing a local distortion of DNA as a result of which gel mobility of the DNA changes. The compound containing three planaramine ligands per molecule (CH3) is found to be less reactive than the compounds containing two planaramine ligands per molecule (CH1, CH2 and CH4), which in turn are less reactive than compounds containing one of the same planaramine ligands per molecule. The decrease in reactivity is reflected in lower molar conductivity values (indicating lower degree of dissociation), less pronounced changes caused to DNA conformation (indicating decreased level of platinum-DNA binding) and lower activity. The decreased reactivity of the compounds is due to a greater steric crowding produced by the bulky planaramine ligands. Changes in DNA conformation are also found to be a function of the actual nature of the planaramine ligand. The results illustrate structure-activity relationship.     

Synthesis and reactions of [M(CO)4(Ph2PSiMe3)X] complexes (M = Mn,Re; X = Halogen)

The silylphosphine ligand Ph₂PSiMe₃ reacts readily with a slurry of [Re(CO)₅X] (X  Cl, Br) in polar and in non-polar solvents to yield soluble cis-[Re(CO)₄- (Ph₂PSiMe₃)X] (Ia, X  Cl;Ib, X  Br) via CO substitution. Compound I is readily hydrolyzed by water or silica gel to cis-[Re(CO)₄(Ph₂PH)X]. Compound Ib reacts with [Re(CO)₅Br] to yield [Re₂(CO)₈(μ-PPh₂)- (μ-Br)] (II), and with [Mn(CO)₅Br] to yield [MnRe- (CO)₈(μ-PPh₂)(μ-Br)] (III).The reaction of Ph₂PSiMe₃ with [Mn(CO)₅X] (X=Cl,Br,I) is highly dependent upon reaction conditions.In polar and in non-polar solvents, an excess of ligand gives mainly cis-[Mn(CO)₄(Ph₂PSiMe₃)X] (IVa, X  Cl;IVb, X  Br;IVc, X I). With ligand: [Mn(CO)₅X] reacting ratios in the range 0.5–1.0:1, the products from the three respective halomanganese complexes in THF were: (a) mainly [Mn₂(CO)₈(μ- PPh₂)(μ-Cl) (Va); (b) both [Mn(CO)₄(Ph₂PSiMe₃)Br] and [Mn₂(CO)₈(μ-PPh₂)(μ-Br)] (Vb); and (c) exclusively [Mn(CO)₄(Ph₂PSiMe₃)I]. The compounds IVa-c are stable in solution at ambient temperatures and are readily hydrolyzed by water or methanol to [Mn(CO)₄(Ph₂PH)X]. Compound IVb reacts at room temperature with [Mn(CO)₅Cl] to yield only [Mn₂- (CO)₈(μ-PPh₂)(μ-Br)] (Vb); compound IVc reacts in hot toluene with [Mn(CO)₅Cl] to yield mainly [Mn₂(CO)₈(μ-PPh₂)(μ-I)] (Vc), together with a small amount of the chloro-bridged analog.The dinuclear species II, III and Va-c appear to be formed mainly via an intermolecular elimination of Me₃SiX from the appropriate [M(CO)₄(Ph₂PSiMe₃)X] and metalpentacarbonylhalide (chloride or bromide) complexes.     

Metal ions in non-complementary DNA base pairs: an ab initio study of Cu(I), Ag(I), and Au(I) complexes with the cytosine-adenine base pair

 Ab initio calculations have been carried out to characterize the structure and energetics of a silver(I) complex with the cytosine-adenine DNA base pair and an aqua ligand in the coordination sphere of Ag. In addition, we have also studied analogous complexes with Cu(I) and Au(I), and structures in which adenine has been replaced by purine in order to investigate the structural role of the adenine amino group. The calculations revealed that all metal-modified structures are dominated by the metal-base interactions, while the water-metal ion interaction and many-body interligand repulsion are less important contributions. Nevertheless, the structural role of the water molecule in the complex is quite apparent and in agreement with an earlier crystallographic study. The metal-modified base pairs exhibit large conformational flexibility toward out-of-plane motions (propeller twist and buckle), comparable or, in some cases, even larger than that observed in the base pairs without metal ions. All structures have been optimized within the Hartree-Fock approximation, while interaction energies were evaluated with the inclusion of electron correlation. Received: 25 March 1999 / Accepted: 10 June 1999     

Monovalent Cation Permeation through the Connexin40 Gap Junction Channel : Cs,Rb, K,Na, Li,TEA, TMA,TBA, and Effects of Anions Br,Cl, F,Acetate, Aspartate,Glutamate, and NO3

The unitary conductances and permeability sequences of the rat connexin40 (rCx40) gap junction channels to seven monovalent cations and anions were studied in rCx40-transfected neuroblastoma 2A (N2A) cell pairs using the dual whole cell recording technique. Chloride salt cation substitutions (115 mM principal salt) resulted in the following junctional maximal single channel current-voltage relationship slope conductances (γ_j in pS): CsCl (153), RbCl (148), KCl (142), NaCl (115), LiCl (86), TMACl (71), TEACl (63). Reversible block of the rCx40 channel was observed with TBA. Potassium anion salt γ_j are: Kglutamate (160), Kacetate (160), Kaspartate (158), KNO₃ (157), KF (148), KCl (142), and KBr (132). Ion selectivity was verified by measuring reversal potentials for current in rCx40 gap junction channels with asymmetric salt solutions in the two electrodes and using the Goldman-Hodgkin-Katz equation to calculate relative permeabilities. The permeabilities relative to Li⁺ are: Cs⁺ (1.38), Rb⁺ (1.32), K⁺ (1.31), Na⁺ (1.16), TMA⁺ (0.53), TEA⁺ (0.45), TBA⁺ (0.03), Cl⁻ (0.19), glutamate⁻ (0.04), and NO₃− (0.14), assuming that the monovalent anions permeate the channel by forming ion pairs with permeant monovalent cations within the pore thereby causing proportionate decreases in the channel conductance. This hypothesis can account for why the predicted increasing conductances with increasing ion mobilities in an essentially aqueous channel were not observed for anions in the rCx40 channel. The rCx40 effective channel radius is estimated to be 6.6 Å from a theoretical fit of the relationship of relative permeability and cation radius.     

Systematic characterization on electronic structures and spectra for a series of complexes, M(IDB)Cl2 (M = Mn, Fe, Co, Ni, Cu and Zn): a theoretical study

Theoretical studies on the coordination stabilities, spectra and DNA-binding trend for the series of metal-varied complexes, M(IDB)Cl₂ (M = Mn, Fe, Co, Ni, Cu and Zn; IDB = N, N -bis(2-benzimidazolylmethyl) amine), have been carried out by using the DFT/B3LYP method and PCM model. The calculated coordination stabilities (S) for these complexes present a trend of S(Ni) > S(Co) > S(Fe) > S(Cu) > S(Zn) > S(Mn). It has been estimated from the molecular orbital energies of the complexes that the DNA-binding affinities (A) of the complexes are in the order of A(Zn) < A(Mn) < A(Fe) ≈ A(Co) < A(Ni) < A(Cu). The studied results indicate that the Cu, Ni and Co complexes with large coordination stabilities present the low virtual orbitals, consequently yielding to the favorable DNA-binding affinities. The spectral properties of excitation energies and oscillator strengths for M(IDB)Cl₂ in the ultraviolet region were calculated by TD-DFT/B3LYP method.     

Synthesis and characterization of [Fe2OX6]2tau; (X = Cl,Br, I) complexes. Crystal and molecular structure of (BzPh3P)2 [Fe2OCl6]

Improved and simple methods for the preparation of [Fe₂OX₆]^2tau; (X = Cl Br I) complexes are described. The complexes were obtained in high and purified yields as the BzPh₃P⁺ or R₄N⁺ salts. An X-ray crystallographic study of (BzPh₃P)₂ [Fe₂OCl₆] revealed a structure for the complex anion in which the two iron atoms are linked by gdot; -oxo bridge and the terminal coordination sites are occupied by the chloride ligands. Mcolon; ssbauer and infrared spectra for the complexes are reported.     

Structure-dependent in vitro cytotoxicity of the isomeric complexes [Ru(L)2Cl2] (L=o-tolylazopyridine and 4-methyl-2-phenylazopyridine) in comparison to [Ru(azpy)2Cl2]

The dichlorobis(2-phenylazopyridine)ruthenium(II) complexes, [Ru(azpy)(2)Cl(2)], are under renewed investigation due to their potential anticancer activity. The three most common isomers alpha-, beta- and gamma-[RuL(2)Cl(2)] with L= o-tolylazopyridine (tazpy) and 4-methyl-2-phenylazopyridine (mazpy) (alpha indicating the coordinating Cl, N(pyridine) and Nazo atoms in mutual cis, trans, cis positions, beta indicating the coordinating Cl, N(pyridine) and Nazo atoms in mutual cis, cis, cis positions, and gamma indicating the coordinating Cl, N(pyridine) and Nazo atoms in mutual trans, cis, cis positions) are synthesized and characterized by NMR spectroscopy. The molecular structures of gamma-[Ru(tazpy)(2)Cl(2)] and alpha-[Ru(mazpy)(2)Cl(2)] are determined by X-ray diffraction analysis. The IC(50) values of the geometrically isomeric [Ru(tazpy)(2)Cl(2)] and [Ru(mazpy)(2)Cl(2)] complexes compared with those of the parent [Ru(azpy)(2)Cl(2)] complexes are determined in a series of human tumour cell lines (MCF-7, EVSA-T, WIDR, IGROV, M19, A498 and H266). These data unambiguously show for all complexes the following trend: the alpha isomer shows a very high cytotoxicity, whereas the beta isomer is a factor 10 less cytotoxic. The gamma isomers of [Ru(tazpy)(2)Cl(2)] and [Ru(mazpy)(2)Cl(2)] display a very high cytotoxicity comparable to that of the gamma isomer of the parent compound [Ru(azpy)(2)Cl(2)] and to that of the alpha isomer. These biological data are of the utmost importance for a better understanding of the structure-activity relationships for the isomeric [RuL(2)Cl(2)] complexes.     

Molecular mechanic study of nerve agent O-ethyl S-[2-(diisopropylamino)ethyl]methylphosphonothioate (VX) bound to the active site of Torpedo californica acetylcholinesterase

Herein a molecular mechanic study of the interaction of a lethal chemical warfare agent, O-ethyl S-[2-(diisopropylamino)ethyl]methylphosphonothioate (also called VX), with Torpedo californica acetylcholinesterase (TcAChE) is discussed. This compound inhibits the enzyme by phosphonylating the active site serine. The chirality of the phosphorus atom induces an enantiomeric inhibitory effect resulting in an enhanced anticholinesterasic activity of the S_P isomer (VX^S) versus its R_P counterpart (VX^R). As formation of the enzyme-inhibitor Michaelis complex is known to be a crucial step in the inhibitory pathway, this complex was addressed by stochastic boundary molecular dynamics and quantum mechanical calculations. For this purpose two models of interaction were analyzed: in the first, the leaving group of VX was oriented toward the anionic subsite of TcAChE, in a similar way as it has been suggested for the natural substrate acetylcholine; in the second, it was oriented toward the gorge entrance, placing the active site serine in a suitable position for a backside attack on the phosphorus atom. This last model was consistent with experimental data related to the high inhibitory effect of this compound and the difference in activity observed for the two enantiomers. Proteins 28:543–555, 1997. © 1997 Wiley-Liss, Inc.     

Ab initio and DFT study on the electrophilic addition of bromine to endo-tricyclo[3.2.1.02,4]oct-6-ene

Full geometric optimization of endo-tricyclo[3.2.1.0^2,4]oct-6-ene (endo-TCO) by ab initio and DFT methods allowed us to investigate the structure of the molecule. The double bond is endo-pyramidalized and its two faces are no longer found to be equivalent. The exo face of the double bond has regions with far more electron density (q_i,HOMO) and more negative electrostatic potential. The endo-TCO-Br₂ system was investigated at the B3LYP/6-311+G** level and the endo-TCO···Br₂(exo) molecular complex was found to be relatively more stable than the endo-TCO···Br₂(endo) complex. The cationic intermediates of the reaction were studied by ab initio and DFT methods. The bridged exo-bromonium cation(I) is relatively more stable than the endo-bromonium cation(II). An absolute exo-facial selectivity should be observed in the addition reaction of Br₂ to endo-TCO, which is caused by steric and electronic factors. The nonclassical rearranged cation IV was found to be the most stable ion among the cationic intermediates and the ionic addition occurs via the formation of this cation. The mechanism of the addition reaction is also discussed.     

Theoretical study of the reaction of CH2XO (X = F, Cl, Br) radicals with the NO radical

In this paper, we focus on the multiple-channel reactions of CH₂XO (X = F, Cl, Br) radicals with the NO radical by means of direct dynamic methods. All structures of the stationary points were obtained at the MP2/6-311+G(d,p) level and vibrational frequency analysis was also performed at this level of theory. The minimum energy path (MEP) was obtained via the intrinsic reaction coordinate (IRC) theory at the MP2/6-311+G(d,p) level, and higher-level energetic information was refined by the MC-QCISD method. The rate constants for the three hydrogen abstraction reaction channels over the temperature range 200–1,500 K were calculated by the improved canonical variational transition state theory (ICVT) with a correction for small-curvature tunneling (SCT). The rate constants calculated in this manner were in good agreement with the available experimental data, and the three-parameter rate–temperature formulae for the temperature range 200–1,500 K were $ {k_{1{\text{a}} }}(T)=0.32\times {10^{-18 }}{T^{1.83 }}\exp \left( {1748.54/T} \right) $ , $ {k_{2{\text{a}} }}(T)=0.22\times {10^{-19 }}{T^{2.19 }}\exp \left( {1770.19/T} \right) $ , $ {k_{3{\text{a}} }}(T)=0.88\times {10^{-20 }}{T^{2.20 }}\exp \left( {1513.82/T} \right) $ (in units of cm³ molecule^?1?s^?1).     

Direct photoaffinity labeling of the primary region of the ouabain binding site of (Na+ + K+)-ATPase with [3H]ouabain, [3H]digitoxin and [3H]digitoxigenin.

The tritiated cardiotonic steroids, ouabain, digitoxin, and digitoxigenin are shown to photolabel the large polypeptide but not the glycoprotein or proteolipid component of the (Na+ + K+)-ATPase when they are bound to the inhibitory site and exposed to light of 220 or 254 nm. The extent of photolabeling is low, less than 1%, and is limited by photocross-linking of the enzyme. The mechanism of photoincorporation does not appear to be either photolysis of the lactone ring in ouabain or photolysis of tryptophan or tyrosine residues in the polypeptide.     

Theoretical study and rate constant calculations for the reactions of SiHX3 with CF3 and CH3 radicals (X = F, Cl)

Theoretical investigations were carried out on the multi-channel reactions CF₃ + SiHF₃, CF₃ + SiHCl₃, CH₃ + SiHF₃, and CH₃ + SiHCl₃. Electronic structures were calculated at the MP2/6-311+G(d,p) level, and energetic information further refined by the MC-QCISD (single-point) method. The rate constants for major reaction channels were calculated by the canonical variational transition state theory with small-curvature tunneling correction over the temperature range of 200–1,500 K. The theoretical rate constants were in good agreement with the available experimental data and were fitted to the three parameter expression: k _1a(T) = 2.93 × 10^?26 T ^4.25 exp (?318.68/T), and k _2a(T) = 3.67 × 10^?22 T ^2.72 exp (?1,414.22/T), k _3a (T) = 7.00 × 10^?24 T ^3.27 exp (?384.04/T), k _4a(T) = 6.35 × 10^?22 T ^2.59 exp (?603.18/T) (in unit of cm³molecule^?1s^?1) are given. Our calculations indicate that hydrogen abstraction channel is the major channel due to the smaller barrier height among four channels considered.

Reactivity, variable temperature solution H NMR studies and MO calculations of dimolybdenum allenylidene complexes of the type [(η-C5H5)2Mo2(CO)4(μ,η(4e)-C=C=CR1R2)]

The reactivity, towards nucleophiles and electrophiles, of dimolybdenum allenylidene complexes of the type [Cp₂Mo₂(CO)₄(μ,η²(4e)-C=C=CR1R2)] (Cp=η⁵-C₅H₅) has been investigated. The nucleophilic attacks occur at the C_γ carbon atom, while electrophiles affec the C atom. Variable temperature solution ¹H NMR studies show a dynamic behavior of these complexes consisting of an equilibrium between two enantiomers with a symmetrical [Cp₂Mo₂(CO)₄(μ-σ,σ(2e)-C=C=CR1R2)] transition state. Extended Hückel MO calculations have been carried out on the model [Cp₂Mo₂(CO)₄(μ,η²-C=C=CH₂]. The calculated charges of the allenylidene carbon atoms suggest that the electrophilic attacks are under charge control, while the nucleophilic attacks are rather under orbital control.