Kinetics and mechanisms of S(IV) reductions of bromite and chlorite ions

The reaction of bromite with aqueous S(IV) is first order in both reactants and is general-acid catalyzed. The reaction half-lives vary from 5 ms (p[H+] 5.9) to 210 s (p[H+] 13.1) for 0.7 mM excess S(IV) at 25 degrees C. The proposed mechanism includes a rapid reaction (k(1) = 3.0 x 10(7) M(-1) s(-1)) between BrO(2)(-) and SO(3)(2-) to form a steady-state intermediate, (O(2)BrSO(3))(3-). General acids assist the removal of an oxide ion from (O(2)BrSO(3))(3-) to form OBrSO(3)(-), which hydrolyzes rapidly to give OBr(-) and SO(4)(2-). Subsequent fast reactions between HOBr/OBr(-) and SO(3)(2-) give Br(-) and SO(4)(2-) as final products. In contrast, the chlorite reactions with S(IV) are 5-6 orders of magnitude slower. These reactions are specific-acid, not general-acid, catalyzed. In the proposed mechanism, ClO(2)(-) and SO(3)H(-)/SO(2) react to form (OClOSO(3)H)(2)(-) and (OClOSO(2))(-) intermediates which decompose to form OCl(-) and SO(4)(2-). Subsequent fast reactions between HOCl/OCl(-) and S(IV) give Cl- and SO(4)(2-) as final products. SO(2) is 6 orders of magnitude more reactive than SO(3)H-, where k(5)(SO(2)/ClO(2)(-)) = 6.26 x 10(6) M(-1) s(-1) and k(6)(SO(3)H(-)/ClO(2)(-)) = 5.5 M(-1) s(-1). Direct reaction between ClO(2)(-) and SO(3)(2-) is not observed. The presence or absence of general-acid catalysis leads to the proposal of different connectivities for the initial reactive intermediates, where a Br-S bond forms with BrO(2)(-) and SO(3)(2-), while an O-S bond forms with ClO(2)(-) and SO(3)H-.     

Electronic effects in 12-metallacrown-3 complexes. A theoretical and experimental study

The Mulliken charges of the 12-metallacrown-3 complex [(C(6)H(6))Ru(C(5)H(3)NO(2))](3) were determined by a single point analysis at the HF level. For comparison, a Mulliken population analysis was carried out for the organic analogue 12-crown-3. The partial negative charges on the O-donor atoms of the metallamacrocycle were found to be larger than those on the O-donor atoms of 12-crown-3. The 12-metallacrown-3 complex [(cymene)Ru(C(5)H(2)ClNO(2))](3) with chloro-substituents in position 5 of the pyridonate ligand was synthesized to determine the effect of electron withdrawing groups on the structure and the host-guest properties of the receptor. The chloro-substituents were found to have only a small influence on the structures, but they reduce the binding affinity for LiCl and NaCl by approximately 2 orders of magnitude.     

Synthesis of cationic rhenium(VII) oxo imido complexes and their tunability towards oxygen atom transfer

A facile method is described for the synthesis of cationic Re(VII) cis oxo imido complexes of the form [Re(O)(NAr)(salpd)+] (salpd = N,N'-propane-1,3-diylbis(salicylideneimine)), 4, [Re(O)(NAr)(saldach)+] (saldach = N,N'-cyclohexane-1,3-diylbis(salicylideneimine)), 5, and [Re(O)(NAr)(hoz)2+] (hoz = 2-(2'-hydroxyphenyl)-2-oxazoline) (Ar = 2,4,6,-(Me)C(6)H(2); 4-(OMe)C(6)H(4); 4-(Me)C(6)H(4); 4-(CF3)C6H4; 4-MeC(6)H(4)SO(2)), 6, from the reaction of oxorhenium(V) [(L)Re(O)(Solv)+] (1-3) and aryl azides under ambient conditions. Unlike previously reported cationic Re(VII) dioxo complexes, these cationic oxo imido complexes can be obtained on a preparative scale, and an X-ray crystal structure of [Re(O)(NMes)(saldach)+], 5a, has been obtained. Despite the multiple stereoisomers that could arise from tetradentate ligation of salen ligands to rhenium, one major isomer is observed and isolated in each instant. The electronic rationalization for stereoselectivity is discussed. Investigation of the mechanism suggests that the reactions of Re(V) with aryl azides proceed through an azido adduct similar to the group 5 complexes of Bergman and Cummins. Treatment of the cationic oxo imido complexes with a reductant (PAr(3), PhSMe, or PhSH) results in oxygen atom transfer (OAT) and the formation of cationic Re(V) imido complexes. [(salpd)Re(NMes)(PPh(3))(+)] (7) and [(hoz)2Re(NAr)(PPh(3))(+)] (Ar = m-OMe phenyl) (9) have been isolated on a preparative scale and fully characterized including an X-ray single-crystal structure of 7. The kinetics of OAT, monitored by stopped-flow spectroscopy, has revealed rate saturation for substrate dependences. The different plateau values for different oxygen acceptors (Y) provide direct support for a previously suggested mechanism in which the reductant forms a prior-equilibrium adduct with the rhenium oxo (ReVII = O<--Y). The second-order rate constants of OAT, which span more than 3 orders of magnitude for a given substrate, are significantly affected by the electronics of the imido ancillary ligand with electron-withdrawing imidos being most effective. However, the rate constant for the most active oxo imido rhenium(VII) is 2 orders of magnitude slower than that observed for the known cationic dioxo Re(VII) [(hoz)2Re(O)(2)(+)].     

Reactivity of secondary metallocene alkyls and the question of dormant sites in catalytic alkene polymerization

The reactivity of [rac-(C2H4(1-indenyl)2)Zr(n-butyl)][MeB(C6F5)3] (4), [rac-(C2H4(1-indenyl)2)Zr(sec-butyl)][MeB(C6F5)3] (5), and [rac-(C2H4(1-indenyl)2)Zr(polypropenyl)][MeB(C6F5)3] with propene, ethene, and hydrogen was studied by low-temperature (<-40 degrees C) 1H and 13C NMR spectroscopy in toluene solutions. In contrast with previous suggestions that 2 degrees zirconium alkyl species such as 5 are dormant sites, these measurements demonstrate reactivity of 2 degrees zirconium alkyls with propene and ethene comparable to the 1 degrees zirconium alkyl species 4 and [rac-(C2H4(1-indenyl)2)Zr(polypropenyl)][MeB(C6F5)3]. Because 2,1-insertion of propene is an infrequent event, these results preclude significant accumulation of catalyst in the form of 2 degrees zirconium alkyls for this metallocene and counterion. The reactivity of 5 with hydrogen is at least 2 orders of magnitude faster than other 1 degrees zirconium alkyls. Such high reactivity accounts for the puzzlingly high fraction of butyl end groups in prior hydrooligomerization studies and implies that catalyst responsivity to H2 as a molecular weight control agent correlates with the regioselectivity of the catalyst.     

The kinetics of addition and fragmentation in reversible addition fragmentation chain transfer polymerization: An ab initio study

High-level ab initio calculations of the forward and reverse rate coefficients have been performed for a series of prototypical reversible addition fragmentation chain transfer (RAFT) reactions: R* + S=C(Z)SCH3 --> R-SC*(Z)SCH3, for R = CH3, with Z = CH3, Ph, and CH2Ph; and Z = CH3, with R = (CH3), CH2COOCH3, CH2Ph, and C(CH3)2CN. The addition reactions are fast (ca. 10(6)-10(8) L mol(-1) s(-1)), typically around three orders of magnitude faster than addition to the C=C bonds of alkenes. The fragmentation rate coefficients are much more sensitive to the nature of the substituents and vary from 10(-4) to 10(7) s(-1). In both directions, the qualitative effects of substituents on the rate coefficients largely follow those on the equilibrium constants of the reactions, with fragmentation being favored by bulky and radical-stabilizing R-groups and addition being favored by bulky and radical-stabilizing Z-groups. However, there is evidence for additional polar and hydrogen-bonding interactions in the transition structures of some of the reactions. Ab initio calculations were performed at the G3(MP2)-RAD//B3-LYP/6-31G(d) level of theory, and rates were obtained via variational transition state theory in conjunction with a hindered-rotor treatment of the low-frequency torsional modes. Various simplifications to this methodology were investigated with a view to identifying reliable procedures for the study of larger polymer-related systems. It appears that reasonable results may be achievable using standard transition state theory, in conjunction with ab initio calculations at the RMP2/6-311+G(3df,2p) level, provided the results for delocalized systems are corrected to the G3(MP2)-RAD level using an ONIOM-based procedure. The harmonic oscillator (HO) model may be suitable for qualitative "order-of-magnitude" studies of the kinetics of the individual reactions, but the hindered-rotor (HR) model is advisable for quantitative studies.     

Acid-base equilibria in nonpolar media. 4. Extension of the self-consistent basicity scale in THF medium. Gas-phase basicities of phosphazenes

Eleven new phenyl-substituted phosphazenes (P1-, P3-, and P4-bases) have been synthesized by the Staudinger or the Kirsanov reactions. The UV-vis spectrophotometric titration method was used to establish the relative basicity of them, and to extend the ion-pair basicity scale for THF medium. These measurements together with our previous work give a continuous basicity scale in THF ranging from 2.6 (2-MeO-pyridine) to 26.6 (2-Cl-C6H4P4(pyrr) phosphazene) in pKalpha units: that is for 24 orders of magnitude and containing 58 compounds (pyridines, anilines, amines, guanidines, amidines, phosphazenes). Ion-pair formation was taken into account by using the Fuoss equation. DeltapKip values of some phosphazene indicators estimated earlier by the 13C NMR method were revised. For some of the phosphazenes gas-phase basicities were measured.     

Bis(tridentate) Ruthenium-Terpyridine Complexes Featuring Microsecond Excited-State Lifetimes

A series of heteroleptic bis(tridentate) ruthenium(II) complexes, each bearing a substituted 2,2':6',2″-terpyridine (terpy) ligand, is characterized by room temperature microsecond excited-state lifetimes. This observation is a consequence of the strongly σ-donating and weakly π-accepting tridentate carbene ligand, 2',6'-bis(1-mesityl-3-methyl-1,2,3-triazol-4-yl-5-idene)pyridine (C(∧)N(∧)C), adjacent to the terpy maintaining a large separation between the ligand field and metal-to-ligand charge transfer (MLCT) states while also preserving a large (3)MLCT energy. The observed lifetimes are the highest documented lifetimes for unimolecular ruthenium(II) complexes and are four orders in magnitude higher than that associated with [Ru(terpy)(2)](2+).     

Kinetics of two-electron oxidations by the compound I derivative of chloroperoxidase, a model for cytochrome P450 oxidants

[structure: see text] Rate constants for two-electron oxidation reactions of Compound I from chloroperoxidase (CPO) with a variety of substrates were measured by stopped-flow kinetic techniques. The thiolate ligand of CPO Compound I activates the iron-oxo species with the result that oxidation reactions are 2 to 3 orders of magnitude faster than oxidations by model iron(IV)-oxo porphyrin radical cations containing weaker binding counterions.     

Gas phase reactions of NH2Cl with anionic nucleophiles: Nucleophilic substitution at neutral nitrogen

Reactions of chloramine, NH2Cl, with HO-, RO- (R = CH3, CH3CH2, CH3CH2CH2, C6H5CH2, CF3CH2), F- , HS- , and Cl- have been studied in the gas phase using the selected ion flow tube technique. Nucleophilic substitution (S(N)2) at nitrogen to form Cl- has been observed for all the nucleophiles. The reactions are faster than the corresponding S(N)2 reactions of methyl chloride; the chloramine reactions take place at nearly every collision when the reaction is exothermic. The thermoneutral identity S(N)2 reaction of NH2Cl with Cl-, which occurs approximately once in every 100 collisions, is more than two orders of magnitude faster than the analogous reaction of CH3Cl. The significantly enhanced S(N)2 reactivity of NH2Cl is consistent with a previous theoretical prediction that the barrier height for the S(N)2 identity reaction at nitrogen is negative relative to the energy of the reactants, whereas this barrier height for reaction at carbon is positive. Competitive proton abstraction to form NHCl- has also been observed with more highly basic anions (HO-, CH3O-, and CH3CH2O-), and this is the major reaction channel for HO- and CH3O-. Acidity bracketing determines the heat of deprotonation of NH2Cl as 374.4 +/- 3.0 kcal mol(-1).     

Fullerometallic ion chemistry: reactions of C(60)Fe+ and C(20)H(10)Fe+ in the gas phase

Fe+ has been attached to buckminsterfullerene, C(60), and corannulene, C(20)H(10), in the gas phase, and the reactivities of C(60)Fe+ and C(20)H(10)Fe+ have been measured with several small inorganic and organic molecules in helium bath gas at 0.35 Torr using a selected-ion flow tube (SIFT) mass spectrometer. Comparisons with measured reactivities of the bare Fe+ ion indicate that the presence of C(60) and C(20)H(10) leads to enhancements in reactivity at room temperature of up to 5 orders of magnitude. Ligation was the only chemistry observed with D(2), N(2), CO(2), CH(4), C(2)H(2), C(2)H(4), SO(2), C(6)D(6), NH(3), H(2)O, and CO, but other channels were observed to compete with adduct formation in the reactions with N(2)O and O(2). The number of molecules sequentially ligated to the ion was different: up to five molecules of ligand added sequentially to Fe+, up to four molecules of ligand were observed to attach to C(60)Fe+, while only up to three molecules added to C(20)H(10)Fe+. C(60)+ and C(20)H(10)+ were observed to be unreactive toward the same ligands. The kinetic results show the influence of carbonaceous surfaces on metal ion reactivity and are interpreted in terms of the nature of the coordination of Fe+ to the carbonaceous surface. Catalytic effects of the carbonaceous surfaces were identified for the reactions with N(2)O and O(2).     

Density functional theory predicts the barriers for radical fragmentation in solution

[reaction: see text] N-Methoxypyridyl radicals formed by one-electron reduction of the corresponding cationic heterocycles undergo N-O bond cleavage. Experimental activation free energies for a series of these bond fragmentations are compared to corresponding barriers determined from electronic structure calculations. The DFT barriers agree well with those from experiment, being smaller than the latter values by an average value of ca. 1 kcal/mol, for rate constants varying over almost 3 orders of magnitude, or within ca. 3 kcal/mol over 8 orders of magnitude of rate constant. For a model compound, the B3PW91/6-31+G hybrid density functional method is also found to be in good agreement with the MCSCF-MRMP2 method. One of the reactions is found by DFT to have no minimum for the reactant radical, consistent with a truly barrierless reaction.     

Lithium-alkyl, -aryl, and -amido ligand-exchange dynamics of dianionic zirconium(IV) complexes with chelating amidophenolate ligands

The synthesis, characterization, and solution behavior of a series of six-coordinate zirconium(IV) dianions [ZrX2(ap)2]2- (ap = 2,4-di-tert-butyl-6-(tert-butylamido)phenolate; X = Ph, 3a; X = p-tolyl, 3b; X = Me, 4; X = NMe2, 5) are described. Complexes 3-5 were prepared by treating the neutral zirconium complex Zr(ap)2(THF)2 (1) with 2 equiv of LiX or by the direct reaction of apLi2 and LiX with ZrCl4. The complexes were isolated as lithium-etherate salts, and they were characterized by NMR spectroscopy and single-crystal X-ray diffraction. In non-coordinating solvents such as benzene-d6, complexes 3-5 are robust in solution, but in coordinating solvents such as THF-d8, dissociation of LiX was observed. The rate of LiX loss was evaluated by exchange reactions; the reaction rate constants span nine orders of magnitude at 298 K, with the slowest reaction being the dissociation of PhLi from 3a (tau1/2 = 4 h) and the fastest reaction being the dissociation of LiNMe2 from 5 (tau1/2 = 53 mus). In the case of LiNMe2 dissociation from 5, activation parameters suggest that the rate-determining step is purely dissociative; however, for diphenyl and dimethyl complexes 3a and 4, respectively, activation parameters suggest a solvent-assisted rate-determining step.     

Sequence selective recognition in the minor groove of dsDNA by pyrrole,imidazole-substituted bis-benzimidazole conjugates

A series of pyrrole, imidazole-substituted bis-benzimidazole conjugates, Py-Py-Im-gamma-biBenz, Py-Py-gamma-biBenz, Py-Im-gamma-biBenz, and Im-Py-gamma-biBenz (1-4), were prepared in an attempt to target dsDNA sequences possessing both A/T and G/C bps. The dsDNA interactions and sequence specificity of the conjugates have been characterized via spectrofluorometric titrations and thermal melting studies. All conjugates form 1:1 complexes with dsDNA at subnanomolar concentrations. The Im moiety selectively recognizes a G/C bp embedded in the A/T-rich binding site. This represents the first clear example of sequence selective recognition in a 1:1 motif.(1) The equilibrium association constant (K(1)) for complexation of a specific nine-bp dsDNA site, 5'-gcggTATGAAATTcgacg-3', by conjugate 1 is approximately 2.6 x 10(9) M(-1). Displacement of the G/C position or G/C-->A/T substitution within the nine-bp site decreases the K(1) by approximately 8-fold, whereas two continuous G/C bps decrease the K(1) by approximately 50-fold magnitude. The K(1) values for seven-bp dsDNA, 5'-gcggtaTGAAATTcgacg-3' and 5'-gcggtaCAAAATTcgacg-3', binding sites by conjugates Py-Im-gamma-biBenz (3) and Im-Py-gamma-biBenz (4) are approximately 2.3 x 10(9) and approximately 1.2 x 10(9) M(-1), respectively. However, the conjugates with no Im moiety, Py-Py-gamma-biBenz (2) and Py-Py-Py-gamma-biBenz (5 and 6), are specific for seven- to nine-bp A/T-rich sites and single A/T-->G/C bp substitution within the binding site decreases the K(1) values by 1-2 orders of magnitude.     

Physical parameters and electron-transfer kinetics of the copper(II/I) complex with the macrocyclic sexadentate ligand [18]aneS6

The electron-transfer kinetics of the complex formed by copper(II/I) with the sexadentate macrocyclic ligand 1,4,7,10,13,16-hexathiacyclooctadecane ([18]aneS6) have been measured in acetonitrile with a series of three oxidizing agents and three reducing agents. These studies have been supplemented by determinations of the redox potential and the stability constants of the Cu(I)- and Cu(II)([18]aneS6) complexes in both acetonitrile and aqueous solution. The Marcus cross relationship has been applied to the cross-reaction rate constants for the six reactions studied to resolve the electron self-exchange rate constant for the Cu(II/I)([18]aneS6) complex. An average value of k11 = 3 x 10(3) M(-1) s(-1) was obtained at 25 degrees C, mu = 0.10 M in acetonitrile. This value is approximately 2 orders of magnitude smaller than the values reported previously for the corresponding Cu(II/I) complexes with the quadridentate and quinquedentate homoleptic homologues having all ethylene bridges, namely, 1,4,7,10-tetrathiacyclododecane ([12]aneS4) and 1,4,7,10,13-pentathiacyclopentadecane ([15]aneS5). This significant difference in reactivity is attributed to the greater rearrangement in the geometry of the inner-coordination sphere that accompanies electron transfer in the Cu(II/I)([18]aneS6) system, wherein two Cu-S bonds are ruptured upon reduction. In contrast to other Cu(II/I) complexes with macrocyclic polythiaethers that have self-exchange rate constants within the same range, no evidence for conformationally gated electron transfer was observed, even in the case of the most rapid oxidation reaction studied.     

Phosphodiester cleavage of guanylyl-(3',3')-(2'-amino-2'-deoxyuridine): rate acceleration by the 2'-amino function

Hydrolytic reactions of the structural analogue of guanylyl-(3',3')-uridine, guanylyl-(3',3')-(2'-amino-2'-deoxyuridine), having one of the 2'-hydroxyl groups replaced with an amino function, have been followed by RP HPLC in the pH range 0-13 at 90 degrees C. The results are compared to those obtained earlier with guanylyl-(3',3')-uridine, guanylyl-(3',3')-(2',5'-di-O-methyluridine), and uridylyl-(3',5')-uridine. Under basic conditions (pH > 8), the hydroxide ion-catalyzed cleavage of the P-O3' bond (first-order in [OH(-)]) yields a mixture of 2'-amino-2'-deoxyuridine and guanosine 2',3'-cyclic phosphate which is hydrolyzed to guanosine 2'- and 3'-phosphates. Under these conditions, guanylyl-(3',3')-(2'-amino-2'-deoxyuridine) is 10 times less reactive than guanylyl-(3',3')-uridine. Under acidic and neutral conditions (pH 3-8), where the pH-rate profile for the cleavage consists of two pH-independent regions (from pH 3 to pH 4 and from 6 to 8), guanylyl-(3',3')-(2'-amino-2'-deoxyuridine) is considerably reactive. For example, in the latter pH range, guanylyl-(3',3')-(2'-amino-2'-deoxyuridine) is more than 2 orders of magnitude more labile than guanylyl-(3',3')-(2',5'-di-O-methyluridine), while in the former pH range the reactivity difference is 1 order of magnitude. Under very acidic conditions (pH < 3), the isomerization giving guanylyl-(2',3')-(2'-amino-2'-deoxyuridine) and depurination yielding guanine (both first-order in [H(+)]) compete with the cleavage. The Zn(2+)-promoted cleavage ([Zn(2+)] = 5 mmol L(-)(1)) is 15 times faster than the uncatalyzed reaction at pH 5.6. The mechanisms of the reactions of guanylyl-(3',3')-(2'-amino-2'-deoxyuridine) are discussed, particularly focusing on the possible stabilization of phosphorane intermediate and/or transition state via an intramolecular hydrogen bonding by the 2'-amino group.     

Bisphosphonate derivatives of nucleoside antimetabolites: hydrolytic stability and hydroxyapatite adsorption of 5'-beta,gamma-methylene and 5'-beta,gamma-(1-hydroxyethylidene) triphosphates of 5-fluorouridine and ara-cytidine

Kinetics of the hydrolytic reactions of four bisphosphonate derivatives of nucleoside antimetabolites, viz., 5-fluorouridine 5'-beta,gamma-(1-hydroxyethylidene) triphosphate ( 4), 5-fluorouridine 5'-beta,gamma-methylene triphosphate ( 5), ara-cytidine 5'-beta,gamma-(1-hydroxyethylidene) triphosphate ( 6), and ara-cytidine 5'-beta,gamma-methylene triphosphate ( 7), have been studied over a wide pH range (pH 1.0-8.5) at 90 degrees C. With each compound, the disappearance of the starting material was accompanied by formation of the corresponding nucleoside 5'-monophosphate, the reaction being up to 2 orders of magnitude faster with the beta,gamma-(1-hydroxyethylidene) derivatives ( 4, 6) than with their beta,gamma-methylene counterparts ( 5, 7). With compound 7, deamination of the cytosine base competed with the phosphate hydrolysis at pH 3-6. The measurements at 37 degrees C (pH 7.4) in the absence and presence of divalent alkaline earth metal ions (Mg (2+) and Ca (2+)) showed no sign of metal ion catalysis. Under these conditions, the initial product, nucleoside 5'-monophosphate, underwent rapid dephosphorylation to the corresponding nucleoside. Hydrolysis of the beta,gamma-methylene derivatives ( 5, 7) to the corresponding nucleoside 5'-monophosphates was markedly faster in mouse serum than in aqueous buffer (pH 7.4), the rate-acceleration being 5600- and 3150-fold with 5 and 7, respectively. In human serum, the accelerations were 800- and 450-fold compared to buffer. In striking contrast, the beta,gamma-(1-hydroxyethylidene) derivatives did not experience a similar decrease in hydrolytic stability. The stability in human serum was comparable to that in aqueous buffer (tau 1/2 = 17 and 33 h with 4 and 6, respectively), and on going to mouse serum, a 2- to 4-fold acceleration was observed. To elucidate the mineral-binding properties of 4- 7, their retention on a hydroxyapatite column was studied and compared to that of zoledronate ( 1a) and nucleoside mono-, di-, and triphosphates.     

New findings on the Diels-Alder reactions. An analysis based on the bonding evolution theory

Two Diels-Alder type reactions, i.e., normal electron demand (NED) between 1,3-butadiene (BD) and acrolein (Acr) and inverse electron demand (IED) between 2,4-pentadienal (PDA) and methyl vinyl ether (MVE), have been investigated using the bonding evolution theory (BET). BET combines topological analysis of the electron localization function (ELF) and catastrophe theory. Catalyst effect has been incorporated through Lewis acid BH3. The B3LYP hybrid HF/DFT method along with 6-31G(d), 6-311++G(d,p) basis sets have been used. All reactions yield two-stage mechanism and there is no topological evidence that they might be concerted with two bonds partially formed during transition structure. A formation of six-membered ring requires 10 (or 11) steps separated by two types of catastrophes: fold and cusp. The first "intermolecular" bond (C1-C6) is formed at 1.93, 1.92 A (NED) and 1.92, 1.97 A (IED). The six-membered ring is "closed" at 2.11, 2.13 A (NED) and 2.5, 2.6 A (IED) via formation of the second bond C4-C5. All reactions begin with "reduction" of C=C bonds to single C-C (cusp catastrophes). Subsequently, the nonbonding electron density is concentrated (fold catastrophes) on terminal C atoms. Finally the new bonds, C1-C6 and C4-C5, are established (cusp catastrophes). Both magnitude and regularity of the electron redistribution, happening during reactions enable us to distinguish two effects: (1) the "ring effect", where a large amount of electron density is regularly transferred from double C=C bonds to intermolecular regions and single C-C bonds, (2) the "side chain effect"--usually weaker and irregular--involving substituents' bonds. In the transition structure, well formed bonding basin V(C1,C6), is observed only for the PDA...BH3/MVE reaction. For other reactions only the nonbonding basins: V(C1) and V(C6), are found in the interaction region C1...C6.     

Hydration process as an activation of trans- and cisplatin complexes in anticancer treatment. DFT and ab initio computational study of thermodynamic and kinetic parameters

The thermodynamic and kinetic aspects of hydration reactions of cis-/transplatin were explored. The polarizable continuum model was used for estimation of solvent effects. Using the B3LYP/6-31+G(d) method, the structures were optimized and vibrational frequencies estimated. Interaction energies and activation barriers were determined at the CCSD(T)/6-31++G(d,p) level within the COSMO approach. An associative mechanism was assumed with a trigonal-bipyramidal structure of the transition state. Within the applied model, all the hydration reactions are slightly endothermic. The Gibbs energies of cisplatin hydration amount to 7.0 and 14.2 kcal/mol for the chloride and ammonium replacement, respectively. Analogous values for the transplatin reactions are 6.8 and 11.9 kcal/mol. The determined rate constants are by several (three to four) orders of magnitude larger for the dechlorination process than for deammination. The cisplatin dechlorination rate constant was established as 1.3 x 10(-4) s(-1) in excellent accord with the experiment.     

Hydrolytic reactions of thymidine 5'-o-phenyl-N-alkylphosphoramidates, models of nucleoside 5'-monophosphate prodrugs

To obtain detailed data on the kinetics of hydrolytic reactions of triester-like nucleoside 5'-O-aryl-N-alkylphosphoramidates, potential prodrugs of antiviral nucleoside monophosphates, the hydrolysis of diastereomeric (Rp/Sp) thymidine 5'-{O-phenyl-N-[(1S)-2-oxo-2-methoxy-1-methylethyl]phosphoramidate} (3), a phosphoramidate derived from the methyl ester of L-alanine, has been followed by reversed-phase HPLC over the range from Ho=0 to pH 8 at 90 degrees C. According to the time-dependent product distributions, the hydrolysis of 3 proceeds at pH<4 by two parallel routes, namely by nucleophilic displacement of the alaninyl ester moiety by a water molecule and by hydrolysis of the carboxylic ester linkage that allows intramolecular attack of the carboxy group on the phosphorus atom, thereby resulting in the departure of either thymidine or phenol without marked accumulation of any intermediates. Both routes represent about half of the overall disappearance of 3. The departure of phenol eventually leads to the formation of thymidine 5'-phosphate. At pH>5, the predominant reaction is hydrolysis of the carboxylic ester linkage followed by intramolecular displacement of a phenoxide ion by the carboxylate ion and hydrolysis of the resulting cyclic mixed anhydride into an acyclic diester-like thymidine 5'-phosphoramidate. The latter product accumulated quantitatively without any indication of further decomposition. Hydroxide-ion-catalyzed P--OPh bond cleavage of the starting material 3 occurred as a side reaction. Comparative measurements with thymidine 5'-{N-[(1S)-2-oxo-2-methoxy-1-methylethyl]phosphoramidate} (4) revealed that, under acidic conditions, this diester-like compound is hydrolyzed by P--N bond cleavage three orders of magnitude more rapidly than the triester-like 3. At pH>5, the stability order is reversed, with 3 being hydrolyzed six times as rapidly as 4. Mechanisms of the partial reactions are discussed.     

Facile Fabrication of PBS/CNTs Nanocomposite Foam for Electromagnetic Interference Shielding

In order to reduce the pollutants of environment and electromagnetic waves, environment friendly polymer foams with outstanding electromagnetic interference shielding are imminently required. In this paper, a kind of electromagnetic shielding, biodegradable nanocomposite foam was fabricated by blending poly (butylene succinate) (PBS) with carbon nanotubes (CNTs) followed by foaming with supercritical CO₂. The crystallization temperature and melting temperature of PBS/CNTs nanocomposites with 4 wt % of CNTs increased remarkably by 6 °C and 3.1 °C compared with that of pure PBS and a double crystal melting peak of various PBS samples appeared in DSC curves. Increasing the CNT content from 0 to 4 wt % leads to an increase of approximately 3 orders of magnitude in storage modulus and nearly 9 orders of magnitude in enhancement of electrical properties. Furthermore, CNTs endowed PBS nanocomposite foam with adjustable electromagnetic interference (EMI) shielding property, giving a specific EMI shielding effectiveness of 28.5 dB cm³/g. This study provides a promising methodology for preparing biodegradable, lightweight PBS/CNTs foam with outstanding electromagnetic shielding properties.