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At least 991 records · Page 55Linked to original sources

PubMed2001

[Me3SiN(PPh3)-ICN]: a new labile donor-acceptor complex.

The reaction behavior of trimethylsilyl phosphanimine, Me3SiNPPh3, toward the pseudohalogen species XCN (X = Cl, Br, and I), especially the intermediate formation of [Me3SiN(PPh3)-XCN] adduct complexes, was investigated in solution. Only the ICN adduct was shown to be metastable in solution, with respect to further reaction into Ph3PNCN and Me3SiX, and can be intercepted. Raman and X-ray data of the ICN adduct revealed a very labile donor-acceptor complex with the iodine atom of the ICN moiety loosely bound to the nitrogen atom of Me3SiNPPh3. There are two distinct rather long N...I bonds with bond lengths of 2.634(1) and 2.739(14) A. The structure and bonding are discussed on the basis of natural bond orbital analysis and valence bond considerations.

Journal Article↗

PubMed2002

Bulk weak ferromagnet in ferrimagnetic chains of organic-inorganic hybrid materials based on BDH-TTP and paramagnetic thiocyanato complex anions: (BDH-TTP)[M(isoq)(2)(NCS)(4)], M = Cr(III), Fe(III).

The preparation, X-ray crystal structures, and magnetic properties of two new isostructural charge transfer salts, (BDH-TTP)M(isoq)(2)(NCS)(4) (M = Cr(III) (1), Fe(III) (2), BDH-TTP = 2,5-bis(1,3-dithiolan-2-ylidene)-1,3,4,6-tetrathiapentalene, isoq = isoquinoline), are reported. Crystal data for 1: monoclinic, space group C2/c (#15), a = 16.1363(9) A, b = 19.0874(12) A, c = 12.5075(6) A, beta = 95.70(4) degrees, V = 3833.2(4) A(3), Z = 4, R = 0.0516 for 2844 reflections with I > 2 sigma(I); for 2: monoclinic, C2/c (#15), a = 16.1938(8) A, b = 19.1117(11) A, c = 12.5100(10) A, beta = 94.265(3) degrees, V = 3861.0(4) A(3), Z = 4, R = 0.0479 for 2969 reflections with I > 2 sigma(I). The crystal structure consists of zigzag mixed organic and inorganic layers, and each layer is formed by mixed columns of BDH-TTP radical cations and paramagnetic metal complex anions. Short intermolecular atomic contacts between donor and anion are observed within the column in the c-direction. The two compounds have weak room-temperature electrical conductivities. ESR measurements show a single signal without separating the donor and anion spins, suggesting a pi interaction between the d and pi electrons. For both compounds ferrimagnetic interactions are observed between the nonequivalent donor and anion spins. These materials exhibit bulk canted weak ferromagnetism below 7.6 K for both 1 and 2.

Journal Article↗

PubMed2003

Donor atom dependent geometric isomers in mononuclear oxo-molybdenum(V) complexes: implications for coordinated endogenous ligation in molybdoenzymes.

We have previously demonstrated that the complex [(L1O)MoOCl(2)], where L1OH = (2-hydroxy-3-tert-butyl-5-methylphenyl)bis(3,5-dimethylpyrazolyl)methane, exists as both cis and trans isomers (Kail, B.; Nemykin, V. N.; Davie, S. R.; Carrano, C. J.; Hammes, B. S.; Basu, P. Inorg. Chem. 2002, 41, 1281-1291). Here, the cis isomer is defined as the geometry with the heteroatom in the equatorial position, and the trans isomer is designated as the geometry with the heteroatom positioned trans to the terminal oxo group. The trans isomer represents the thermodynamically more stable geometry as indicated by its spontaneous formation from the cis isomer. In this report, we show that for complexes of [(LO)MoOCl(2)], where LOH is the sterically less restrictive (2-hydroxyphenyl)bis(3,5-dimethylpyrazolyl)methane, only the trans isomer could be isolated, while in the corresponding thiolate containing ligand (2-dimethylethanethiol)bis(3,5-dimethylpyrazolyl)methane (L3SH) only the cis isomer could be observed. In addition, we have isolated and structurally characterized the complex [(L1O)MoO(OPh)(Cl)], a rare example of a species possessing both cis and trans phenolates. Using DFT calculations, we have investigated the origins of the differences in stability between the cis and trans isomers in these complexes and suggest that they are related to the trans influence of the oxo-group. Crystal data for [(LO)MoOCl(2)] (1) include that it crystallizes in the triclinic space group P(-)1 with cell dimensions a = 8.9607 (12) A, b = 10.596 (4) A, c = 13.2998 (13) A, alpha = 98.03 (2) degrees, beta = 103.21 (2) degrees, gamma = 110.05(2) degrees, and Z = 2. [(L1O)MoO(OPh)Cl].2CH(2)Cl(2) (2.2CH(2)Cl(2)) crystallizes in the triclinic space group P(-)1 with cell dimensions a = 12.2740 (5) A, b = 13.0403 (5) A, c = 13.6141 (6) A, alpha = 65.799 (2) degrees, beta = 64.487 (2) degrees, gamma = 65.750 (2) degrees, and Z = 2. [(L3S)Mo(O)Cl(2)] (3) crystallizes in the orthorhombic space group Pna2(1), with cell dimensions a = 13.2213 (13) A, b = 8.817 (2) A, c = 15.649 (4) A, and Z = 4. The implications of these results on the function of mononuclear molybdoenzymes such as sulfite oxidase, and the DMSO reductase, are discussed.

Chemical Phenomena↗

PubMed2005

Cobalt(III) carbonate and bicarbonate chelate complexes of tripodal tetraamine ligands containing pyridyl donors: the steric basis for the stability of chelated bicarbonate complexes.

The synthesis and characterization (X-ray crystallography, UV/vis spectroscopy, electrochemistry, ESI-MS, and (1)H, (13)C, and (59)Co NMR) of the complexes [Co(L)(O(2)CO)]ClO(4)xH(2)O (L = tpa (tpa = tris(2-pyridylmethyl)amine) (x = 1), pmea (pmea = bis((2-pyridyl)methyl)-2-((2-pyridyl)ethyl)amine) (x = 0), pmap (pmap = bis(2-(2-pyridyl)ethyl)(2-pyridylmethyl)amine) (x = 0), tepa (tepa = tris(2-(2-pyridyl)ethyl)amine) (x = 0)) which contain tripodal tetradentate pyridyl ligands and chelated carbonate ligands are reported. The complexes display different colors in both the solid state and solution, which can be rationalized in terms of the different ligand fields exerted by the tripodal ligands. Electrochemical data show that [Co(tepa)(O(2)CO)](+) is the easiest of the four complexes to reduce, and the variation in E(red.) values across the series of complexes can also be explained in terms of the different ligand fields exerted by the tripodal ligands, as can the (59)Co NMR data which show a chemical shift range of over 2000 ppm for the four complexes. [Co(pmea)(O(2)CO)](+) is fluxional in aqueous solution, and VT NMR spectroscopy ((1)H and (13)C) in DMF-d(7) (DMF = dimethylformamide) over the temperature range -25.0 to 75.0 degrees C are consistent with inversion of the unique six-membered chelate ring. This process shows a substantial activation barrier (DeltaG(#) = 58 kJ mol(-1)). The crystal structures of [Co(tpa)(O(2)CO)]ClO(4)xH(2)O, [Co(pmea)(O(2)CO)]ClO(4).3H(2)O, [Co(pmap)(O(2)CO)]ClO(4), and [Co(tepa)(O(2)CO)]ClO(4) are reported, and the complexes containing the asymmetric tripodal ligands pmea and pmap both crystallize as the 6-isomer. The carbonate complexes all show remarkable stability in 6 M HCl solution, with [Co(pmap)(O(2)CO)](+) showing essentially no change in its UV/vis spectrum over 4 h in this medium. The chelated bicarbonate complexes [Co(pmea)(O(2)COH)]ZnCl(4), [Co(pmap)(O(2)COH)][Co(pmap)(O(2)CO)](ClO(4))(3), [Co(pmap)(O(2)COH)]ZnCl(4)xH(2)O, and [Co(pmap(O(2)COH)]ZnBr(4)x2H(2)O can be isolated from acidic aqueous solution, and the crystal structure of [Co(pmap)(O(2)COH)]ZnCl(4)x3H(2)O is reported. The stability of the carbonate complexes in acid is explained by analysis of the crystallographic data for these, and other slow to hydrolyze chelated carbonate complexes, which show that the endo (coordinated) oxygen atoms are significantly hindered by atoms on the ancillary ligands, in contrast to complexes such as [Co(L)(O(2)CO)](+) (L = (NH(3))(4), (en)(2), tren, and nta), which undergo rapid acid hydrolysis and which show no such steric hindrance.

Journal Article↗

PubMed2005

Facile syntheses of unsolvated UI3 and tetramethylcyclopentadienyl uranium halides.

In the course of comparing the reaction chemistry of (C5Me5)3U, 1, and its slightly less crowded analogue (C5Me4H)3U, 2, new syntheses of UI3, (C5Me4H)3U, (C5Me4H)3UCl, 3, and (C5Me5)3UCl, 4, have been developed. Additionally, (C5Me4H)3UI, 5, and (C5Me4H)2UCl2, 6, have been identified for the first time. A facile synthesis of unsolvated UI3 is reported that proceeds in high yield with inexpensive equipment from iodine and hot uranium turnings. Both UI3 and UI3(THF)4 react with KC5Me4H in toluene to make unsolvated (C5Me4H)3U in higher yield than in previous reports that involve reduction of tetravalent (C5Me4H)3UCl, 3. A more atom-efficient synthesis of complex 3 is also reported that proceeds from reduction of t-BuCl, PhCl, or HgCl2 by 2. Similarly, (C5Me4H)3U reacts with PhI or HgI2 to generate (C5Me4H)3UI. These studies also provided a basis to improve the synthesis of (C5Me5)3UCl from 1 by employing t-BuCl or HgCl2 as the halide source. Like (C5Me5)3UCl, the (C5Me4H)3UCl complex reacts with HgCl2 to form (C5Me4H)2 and (C5Me4H)2UCl2, 6, but unlike (C5Me5)3UX (X = Cl or I), the less substituted (C5Me4H)3UX complexes do not reduce t-BuCl or PhX. The synthesis of 6 from (C5Me4H)MgCl x THF and UCl4 is also included.

Journal Article↗

PubMed2005

Systematic investigation of the hydrothermal syntheses of Pr(III)-PDA (PDA = pyridine-2,6-dicarboxylate anion) metal-organic frameworks.

A series of novel two-dimensional (2D) and three-dimensional (3D) praseodymium coordination polymers, namely, {[Pr3(PDA)4(HPDA)(H2O)8] x 8H2O}n (2), {[Pr2(PDA)3(H2O)3] x H2O}n (3), {[Pr(PDA)(H2O)4] x ClO4}n (4), and { [Pr2(PDA)2(H2O)5SO4] x 2H2O}n (5) (PDA = pyridine-2,6-dicarboxylic anion), was designed and synthesized under hydrothermal conditions. Complexes 1-3 (chainlike polymer, {[Pr(PDA)(HPDA)(H2O)2] x 4H2O}n (1) was also obtained independently by us, although it has been reported recently by Ghosh et al.) were fabricated successfully by simply tuning the Pr/PDA ratio and exhibited various and intriguing topological structures from a 1D chain to a 3D network. While the synthetic strategy of 5 was triggered and further performed only after 1 was structurally characterized. The complexes were characterized by X-ray single-crystal determination, spectroscopic, and variable-temperature magnetic susceptibility analyses. In complex 2 an unusual nanosized square motif as a building block constructed by eight Pr ions was further assembled into a highly ordered 2D grid compound. In complex 3 the decanuclear Pr metal-based structure as a repeat unit interpenetrated to form a novel 3D polymer. Complex 4 was a 3D network polymer fabricated through a hexanuclear Pr ring as a building block, and ClO4- anions as guests were trapped in the cavity. In complex 5 six Pr atoms, two SO4(2-) anions, and carboxylic oxygen bridges constructed an intriguing rectangle structure as a repeat unit in the grid to form a 2D coordination polymer in which the unique bi-bidentate coordination mode of SO4(2-) anion was observed.

Journal Article↗

PubMed1996

Reactivity of the Coordinated Hydroperoxo Ligand.

The reactivity of the hydroperoxo complex [Co(CN)(5)OOH](3)(-) has been studied in aqueous solution. The complex undergoes acid-catalyzed aquation (k = 1.89(5) x 10(-)(2) s(-)(1), pK(a) = 5.21(4), T = 20 degrees C, I = 0.1 M). Assuming an I(d) mechanism, this allows the relative affinity for Co(III) to be deduced as H(2)O(2) < H(2)O < HO(2)(-) and implies H(2)O(2) to be a very weak ligand. At neutral pH the hydroperoxo complex effects efficient oxygen atom transfer to L-methionine to give an intermediate identified as [Co(CN)(5)(L-methionine S-oxide)](2)(-), which then dissociates to [Co(CN)(5)OH(2)](2)(-) and L-methionine S-oxide. The reaction is acid catalyzed and is proposed to take place via nucleophilic attack of sulfur on the proximal oxygen of the hydroperoxo ligand with concerted loss of water. The significance of these results for the interaction of hydrogen peroxide with labile metal ions is discussed.

Journal Article↗

PubMed1997

Spin Transition in a Manganese(III) Porphyrin Cation Radical, Its Transformation to a Dichloromanganese(IV) Porphyrin, and Chlorination of Hydrocarbons by the Latter.

Chemical oxidation of (TMP)Mn(III)(Cl) (TMP = the tetramesitylporphyrinato dianion) by Fe(ClO(4))(3) leads to the porphyrin-oxidized product (TMP(*)(+))Mn(III)(ClO(4))(2). Magnetic measurements and EPR spectroscopy show that the total spin of the complex changes from S = (5)/(2) at high temperature to S = (3)/(2) at low temperature. Ligand exchange of the perchlorato ligands in (TMP(*)(+))Mn(III)(ClO(4))(2) by chloride anions is accompanied by a change of the oxidation site from porphyrin to metal, resulting in (TMP)Mn(IV)(Cl)(2). This high-valent-metal complex can effect chlorine atom transfer to olefins, as well as to dimedone and chlorodimedone, natural substrates of chloroperoxidases.

Journal Article↗

PubMed1997

Copper(II) Complexes of Bis(1,4,7-triazacyclononane) Ligands with Polymethylene Bridging Groups: An Equilibrium and Structural Study.

Copper(II) complexation by a series of ligands containing two 1,4,7-triazacyclononane, [9]aneN(3), groups conjoined by polymethylene chains two to six carbons in length is described. Equilibrium modeling studies in aqueous solution using pH-potentiometry indicate that the smallest homologue of the series, EM2, forms only Cu(EM2)(2+) in dilute aqueous solutions. All other ligands of the series form stable 1:1 (protonated and nonprotonated) and 2:1 dicopper(II) (hydroxo and non-hydroxo) complexes. Those ligands that contain bridging chains of four or more carbon atoms likely form dimeric or oligomeric complex species in solution. The EM ligands with the shortest polymethylene bridging groups form the most stable 1:1 species. There is little difference among the ligands (n = 3-6) in complex stability of the protonated, CuH(2)(EMn)(4+), and dicopper(II), Cu(2)(EMn)(4+), species. UV-vis spectroscopic continuous variation studies at pH 4.0 and 7.5 are interpreted on the basis of the principal equilibrium species obtained from the equilibrium models. Single-crystal X-ray diffraction studies on four complexes ([Cu(EM2)]SO(4).6H(2)O (1), [Cu(2)(EM2)Cl(4)].2H(2)O (2), [Cu(2)(EM6)Cl(4) ] (3), and [Cu(EM3)][ZnBr(4)].H(2)O (4)) characterize structural features of several 1:1 monomeric and dicopper(II) complexes in the crystalline solid. The monomeric compounds contain CuN(6) chromophores while the dicopper(II) compounds contain square pyramidal CuN(3)Cl(2) coordination geometry. Compound 1 crystallizes in space group P&onemacr; with a = 7.849(2) Å, b = 9.783(2) Å, c = 16.919(5) Å, alpha = 78.42(3) degrees, beta = 85.76(3) degrees, gamma = 73.06(3) degrees, and Z = 2. 2: P2(1)/n with a = 9.689(3) Å, b = 11.733(3) Å, c = 10.124(3) Å, beta = 98.20(2) degrees, and Z = 2. 3: P2(1)/n with a = 7.278(2) Å, b = 12.416(3) Å, c = 13.781(2) Å, beta = 90.15(2) degrees, and Z = 2. 4: P2(1)/c with a = 9.295(3) Å, b = 16.233(4) Å, c = 16.544(5) Å, beta = 92.62(2) degrees, and Z = 4. Cyclic voltammograms of aqueous solutions prepared by dissolving [Cu(2)(EM2)Cl(4)].2H(2)O confirm its dissociation to Cu(EM2)(2+). Aqueous solutions containing 1:1 molar ratios of Cu(II) and EM2 in 0.1 mol dm(-)(3) KCl at 25 degrees C show a one-electron chemically reversible reduction at scan rates of 500 mV s(-)(1) with E(1/2) (Cu(II)-Cu(I)) = -868 mV relative to SCE. EPR (X- and Q- band) spectra of frozen solutions (1:1 DMSO/H(2)O and glycerol/H(2)O) of Cu(EM2)(2+) at 100 K are typical of axial copper(II) features (X-band parameters: g( parallel) =2.225 (A( parallel) = 164 x 10(-)(4)) and g( perpendicular) = 2.045).

Journal Article↗

PubMed2002

Lewis acid-base interactions between metal atoms and their applications for the synthesis of bimetallic cluster complexes.

A series of new palladium-ruthenium cluster complexes have been prepared by adding Pd(PBu(t)(3)) fragments to the ruthenium-ruthenium bonds of ruthenium carbonyl complexes. Reaction of Pd(PBu(t)(3))(2) with Ru(3)(CO)(12) yielded the tripalladium adduct Ru(3)(CO)(12)[Pd(PBu(t)(3))](3) 1. Compound 1 contains three Pd(PBu(t)(3)) groups symmetrically disposed with each one acting as a bridge across one Ru-Ru bond of the former Ru(3)(CO)(12) molecule. Reaction of Pd(PBu(t)(3))(2) with Ru(6)(CO)(17)(C) yielded the dipalladium adduct Ru(6)(CO)(17)(C)[Pd(PBu(t)(3))](2) 2. Compound 2 exists as two isomers in the solid state. One isomer has Pd(PBu(t)(3)) groups bridging two edges of the Ru(6) octahedron. The other isomer has a Pd(PBu(t)(3)) group bridging one edge of the Ru(6) octahedron, and the other is a triple bridge. The reaction of Pd(PBu(t)(3))(2) with Ru(CO)(5) yielded the dipalladium-diruthenium complex Ru(2)(CO)(9)[Pd(PBu(t)(3))](2) 3, a dipalladium adduct of the unstable molecule Ru(2)(CO)(9).

Journal Article↗

PubMed2004

Oxidation of alkylarenes by nitrate catalyzed by polyoxophosphomolybdates: synthetic applications and mechanistic insights.

Alkylarenes were catalytically and selectively oxidized to the corresponding benzylic acetates and carbonyl products by nitrate salts in acetic acid in the presence of Keggin type molybdenum-based heteropolyacids, H(3+)(x)()PV(x)()Mo(12)(-)(x)()O(40) (x = 0-2). H(5)PV(2)Mo(10)O(40) was especially effective. For methylarenes there was no over-oxidation to the carboxylic acid contrary to what was observed for nitric acid as oxidant. The conversion to the aldehyde/ketone could be increased by the addition of water to the reaction mixture. As evidenced by IR and (15)N NMR spectroscopy, initially the nitrate salt reacted with H(5)PV(2)Mo(10)O(40) to yield a N(V)O(2)(+)[H(4)PV(2)Mo(10)O(40)] intermediate. In an electron-transfer reaction, the proposed N(V)O(2)(+)[H(4)PV(2)Mo(10)O(40)] complex reacts with the alkylarene substrate to yield a radical-cation-based donor-acceptor intermediate, N(IV)O(2)[H(4)PV(2)Mo(10)O(40)]-ArCH(2)R(+)(*). Concurrent proton transfer yields an alkylarene radical, ArCHR(*), and NO(2). Alternatively, it is possible that the N(V)O(2)(+)[H(4)PV(2)Mo(10)O(40)] complex abstracts a hydrogen atom from alkylarene substrate to directly yield ArCHR(*) and NO(2). The electron transfer-proton transfer and hydrogen abstraction scenarios are supported by the correlation of the reaction rate with the ionization potential and the bond dissociation energy at the benzylic positions of the alkylarene, respectively, the high kinetic isotope effect determined for substrates deuterated at the benzylic position, and the reaction order in the catalyst. Product selectivity in the oxidation of phenylcyclopropane tends to support the electron transfer-proton transfer pathway. The ArCHR(*) and NO(2) radical species undergo heterocoupling to yield a benzylic nitrite, which undergoes hydrolysis or acetolysis and subsequent reactions to yield benzylic acetates and corresponding aldehydes or ketones as final products.

Acetates↗

PubMed2006

Efficient energy transfer from peripheral chromophores to the self-assembled zinc chlorin rod antenna: a bioinspired light-harvesting system to bridge the "green gap".

An artificial light-harvesting rod aggregate based on zinc chlorin and covalently linked naphthalene bisimide chromophore has been realized by self-assembly. Efficient energy transfer (phiET >/= 0.99) takes place upon excitation at 620 nm from peripheral naphthalene bisimides to the zinc chlorin rod aggregate backbone. The appended naphthalene bisimide dyes improve the total LH efficiency of the rod aggregate by 26%. Thus, the present bioinspired antenna system is promising for application in nanodevices for the effective utilization of solar energy by bridging the "green gap".

Energy Transfer↗

PubMed2006

Conformational equilibrium of cytochrome P450 BM-3 complexed with N-palmitoylglycine: a replica exchange molecular dynamics study.

UV-vis absorbance measurements and associated studies of cytochrome P450 BM-3 in complex with N-palmitoylglycine (NPG) indicate that a conformational change occurs in the active site of the complex where the terminal atoms of the ligand move from a site distant from the heme iron, as seen in the low temperature crystal structure to a site proximal to the heme iron at biological temperatures. We employ replica exchange molecular dynamics simulations to study this conformational change. The population of the proximal state is found to increase with temperature in agreement with UV-vis absorbance and NMR measurements. In addition to the conformations characterized by X-ray crystallography and computer modeling, this study shows that a new conformational state is significantly populated at room temperature. The observed increase in the population of conformations where the terminal atoms of NPG are proximal to the heme iron with increasing temperature indicates that the proximal state is stabilized by conformational entropy. A proposal for the origin of this entropic stabilization is provided on the basis of the structure of the newly identified state. We use the temperature weighted histogram (T-WHAM) method to characterize the transition state regions of the conformational ensemble and propose a mechanism of interconversion between these low free energy conformational states.

Crystallography, X-Ray↗

PubMed2006

Tautomerism in the guanyl radical.

Despite a few decades of intense study, a full description of tautomers of one-electron-oxidized guanine remains to be achieved. Here we show that two of these tautomers are produced by the protonation of an 8-haloguanine electron adduct. The rate constants for the reactions of hydrated electrons (e(aq)(-)) with a variety of 8-substituted guanine derivatives have been measured by a pulse radiolysis technique and correlated with both inductive and resonance components of the substituents. The fate of electron adducts was investigated by radiolytic methods coupled with product studies and addressed computationally by means of time-dependent DFT (TD-B3LYP/6-311G**//B1B95/6-31+G**) calculations. The reaction of e(aq)(-) with 8-haloguanosine or 8-halo-2'-deoxyguanosine produces the first observable transient species that decay unimolecularly (k = 1 x 10(5) s(-)(1) at 22 degrees C) to give the one-electron oxidized guanosine or 2'-deoxyguanosine. Theory suggests that the electron adducts of 8-bromoguanine derivatives protonated at C8 form a pi-complex, with the Br atom situated above the molecular plane, that is prompt to eject Br(-). The two short-lived intermediates, which show a substantial difference in their absorption spectra, are recognized to be the two purine tautomers (i.e., iminic 7 and aminic 3 forms). The spin density distributions of the two tautomers are quite different at the O6 and N10 positions, whereas they are very similar at the N3, C5, and C8 positions. The resonance structures of the two tautomers are discussed in some detail. B1B95/6-31+G calculations show also that the tautomerization from the iminic (7) to the aminic (3) arrangement is a water-assisted process.

Journal Article↗

PubMed2006

Further insight into the DNA recognition mechanism of trabectedin from the differential affinity of its demethylated analogue ecteinascidin ET729 for the triplet DNA binding site CGA.

Trabectedin and its N12-demethylated analogue ET729 bind covalently to the central guanine of selected DNA triplets. Although both drugs equally target several sites, including AGA, we show that covalent modification of CGA is only achieved by ET729. By means of molecular dynamics simulations of the precovalent complexes, we explain in atomic detail how such a simple structural modification brings about this notable change in the DNA-binding selectivity profiles of these two drugs.

Antineoplastic Agents, Alkylating↗

PubMed2005

Adsorption and vibrational spectroscopy of CO on mordenite: Ab initio density-functional study.

We present a periodic density-functional investigation of the adsorption and the vibrational spectroscopy of CO in mordenite. Our results highlight a pronounced sensitivity of the strength of the hydrogen bond between the acidic hydroxyl groups and the adsorbed molecule, and hence of the induced red shift of the OH, and the blue shift of the CO stretching mode on the choice of the exchange-correlation functional. The popular Perdew-Wang (PW) gradient-corrected functional strongly overestimates the frequency shifts and the interaction energies. We demonstrate that the revised Perdew-Burke-Ernzerhof (RPBE) functional leads to an improved description of hydrogen bonding. For bridging OH groups, terminal silanol groups and for Lewis sites formed by tricoordinated Al atoms, we predict adsorption energies and frequency shifts in good agreement with experiment. The calculated difference in the binding energies of CO in purely siliceous mordenite and at Brønsted acid sites in the main channel agrees very well with microcalorimetry data. We find that Brønsted acid sites in the small channels (the side-pockets) of mordenite do not adsorb CO, which is adsorbed only in the main channel via the C atom. For these adsorption complexes we find reasonable (though not perfect) agreement of the predicted blue shift of the CO-mode and of the red shift of the OH-mode with experiment. Our prediction that the side-pockets are inaccessible to CO correlates well with the microcalorimetric studies and with experimental observation of the adsorption of O(2), N(2) and H(2) molecules but contradicts the current interpretation of experimental adsorption studies for CO.

Journal Article↗

PubMed2005

A numerical simulation of nonadiabatic electron excitation in the strong field regime: Linear polyenes.

Time-dependent Hartree-Fock theory has been used to study of the electronic optical response of a series of linear polyenes in strong laser fields. Ethylene, butadiene, and hexatriene have been calculated with 6-31G(d,p) in the presence of a field corresponding to 8.75 x 10(13) W/cm2 and 760 nm. Time evolution of the electron population indicates not only the pi electrons, but also lower lying valence electrons are involved in electronic response. When the field is aligned with the long axis of the molecule, Löwdin population analysis shows large charges at each end of the molecule. For ethylene, the instantaneous dipole moment followed the field adiabatically, but for hexatriene, nonadiabatic effects were very pronounced. For constant intensity, the nonadiabatic effects in the charge distribution, instantaneous dipole, and orbital populations increased nonlinearly with the length of the polyene. These calculations elucidate the mechanism of the strong field nonadiabatic electron excitation of polyatomic molecules leading to their eventual ionization and fragmentation. The described computational methods are a viable tool for studying the complex processes in multielectron atomic and molecular systems in strong laser fields.

Journal Article↗

PubMed2006

A self-consistent field quantum hydrodynamic approach for molecular clusters.

We present a novel self-consistent orbital-free method useful for quantum clusters. The method uses a hydrodynamical approach based on the de Broglie-Bohm description of quantum mechanics to satisfy an orbital-free density functional-like Euler-Lagrange equation for the ground state of the system. In addition, we use an information theoretical approach to obtain the optimal density function derived from a series of statistical sample points in terms of density approximates. These are then used to calculate an approximation to the quantum force in the hydrodynamic description. As a demonstration of the utility and flexibility of the approach, we compute the lowest-energy structures for small rare-glass clusters of argon and neon with 4, 5, 13, and 19 atoms. Extension to more complex systems is straightforward.

Journal Article↗