Comparison of resolution in microemulsion EKC and MEKC employing suppressed electroosmosis: application to bisphenol-A-diglycidyl ether and its derivatives

The resolution (R(s)) of hydrophobic analytes in microemulsion EKC (MEEKC) and MEKC with suppressed electroosmosis was investigated using bisphenol-A-diglycidyl ether and its derivatives (BADGEs) as test analytes. Separation scales were compared using our equation for the resolution, R(s)= (square rootN/4)(alpha-1)/(1+K(2)),where k is the retention factor, alpha the selectivity (alpha = k(2)/k(1) for k(2) > or = k(1)>0), and N the average efficiency. At given concentrations of SDS and organic cosolvent in the buffer, in comparison with MEKC, MEEKC was found to provide better resolution of BADGEs, mainly due to the significantly smaller k in MEEKC, but not the greater alpha in MEEKC, while a comparable range of N. Significantly improved resolution of BADGEs was obtained with increase in the concentration of organic cosolvent in the MEEKC and MEKC buffers, while small change in R(s) with the SDS concentration in a range of 100-180 mM. In addition, a decrease in temperature or voltage resulted in slightly better R(s).     

Polarographic studies of cadmium, zinc and manganese(II) iodide complexes in acetonitrile

Cadmium, zinc and manganese(II) iodide complexes have been studied polarographically in acetonitrile and the electrode reactions for these complexes discussed. The overall stability constants of the iodide complexes of these metal ions were evaluated and corrected for the effect of the ion-pairing electrolyte. The values for log beta(4) of CdI(4)(2-) and ZnI(4)(2-) are 26.2 and 18.4 respectively and the values found for the Mn(II) iodide complex are log beta(1) = 3.5, log beta(2) = 5.6, log beta(3) = 7.8, log beta(4)= 10.0, log beta(5) = 12.2 and log beta(6) = 14.4. Within certain limits, the wave-height for each complex is proportional to the metal concentration.     

Mixed hydroxide complex formation and solubility of bismuth in nitrate and perchlorate medium

From the precipitation borderlines in the pBi'-pH diagram, determined experimentally under CO(2)-free conditions, the stability constants of bismuth hydroxide, bismuthoxynitrate and bismuthoxyperchlorate have been established. The following values have been found Nitrate-medium: Perchlorate-medium: log *K(SO)(OH) = 5.2, log *K(SO)(OH) = 5.2; log *K(SO)(NO(3)) = -1.2, log*K(SO)(ClO(4)) = -0.9; log *beta(2) = -4.0, log *beta(2) = -4.1; log *beta(3) = -10.0, log *beta(3)= -9.9; log *beta(4) = -21.5, log *beta(4) = -21.5; log *beta(1,0,1) = 1.2, log *beta(1,0,1) = 3.5. The constants refer to precipitates equilibrated for 30 min, prepared at room temperature (23 +/- 0.5 degrees) in sodium perchlorate or sodium nitrate medium with an ionic strength of 1.00 +/- 0.01. Concerning error propagation it is stated that pBi' values calculated with these constants will have a standard deviation of about 0.1 log unit.     

Sequestering ability of polycarboxylic ligands towards dioxouranium(VI)

In this paper we report a comparison on the sequestering ability of some polycarboxylic ligands towards dioxouranium(VI) (UO(2)(2+), uranyl). Ligands taken into account are mono- (acetate), di- (oxalate, malonate, succinate and azelate), tri- (1,2,3-propanetricarboxylate) and hexa-carboxylate (1,2,3,4,5,6-benzenehexacarboxylate). The sequestering ability of polycarboxylic ligands towards UO(2)(2+) was quantified by a new approach expressed by means of a sigmoid Boltzman type equation and of a empirical parameters (pL(50)) which defines the amount of ligand necessary to sequester 50% of the total UO(2)(2+) concentration. A fairly linear correlation was obtained between pL(50) or log K(110) (log K(110) refers to the equilibrium: UO(2)(2+)+L(z-)=UO(2)L((2-z)); L=generic ligand) and the polyanion charges. In order to complete the picture, a tetra-carboxylate ligand (1,2,3,4-butanetetracarboxylate) was studied in NaCl aqueous solutions at 0相似文献   

Release of nortriptyline hydrochloride from oil-water microemulsions

The release of nortritptyline hydrochloride from oil-in-water (o/w) microemulsions (isopropyl myristate as oil, propylene glycol as cosurfactant, polysorbate 80 as surfactant and phosphate buffer, pH 7.4, as the continuous phase) containing increasing concentrations of polyethylene glycol 400, used to facilitate the diffusion of a drug from the inner oily phase of the microemulsion to the outer aqueous phase of such a dispersion system, was studied by determining the permeability constants of the drug through hydrophilic and lipophilic membranes separating the o/w microemulsions from the receiving aqueous phase (phosphate buffer pH 7.4). The permeability of nortriptyline hydrochloride from microemulsions through the lipophilic membrane increased as the concentration of polyethylene glycol 400 in the disperse system increased. The apparent permeability constant for nortriptyline hydrochloride, from the microemulsion without polyethylene glycol, was 1.36 x 10(-3) cm x h(-1), it increased up to 7.80 x 10(-3) cm x h(-1) in the presence of polyethylene glycol at a concentration of 50% (v/v) of the initial volume of the aqueous phase.     

Use of electrospray mass spectrometry (ESI-MS) for the study of europium(III) complexation with bis(dialkyltriazinyl)pyridines and its implications in the design of new extracting agents

ESI mass spectrometry was used to investigate the europium complexation by tridentate ligands L identical with 2,6-bis(5,6-dialkyl-1,2,4-triazin-3-yl)-pyridines (DATP) that have shown unique separation properties of actinides(III) from lanthanides(III) in nitric acid solutions. Complexes of three ligands, namely methyl (DMTP), n-propyl (DnPTP), and iso-propyl (DiPTP), have been investigated in acidic solutions to check the aqueous-phase stability of Eu(L)(3)(3+) ions identified previously in the solid state. The data obtained show, first, the presence of stable Eu(L)(3)(3+) ions with DnPTP (log beta(3)(app) = 12.0 +/- 0.5) and DiPTP (log beta(3)(app) = 14.0 +/- 0.6) in methanol/water (1:1 v/v) solutions under pH range 2.8-4.6 and, second, a mechanism whereby alkyl moieties contribute to a self-assembling process leading to the formation of Eu(L)(3)(3+) ions. Other complexes such as Eu(L)(2)(3+) ions are only observed for DnPTP (log beta(2)(app) = 6.7 +/- 0.5) and DMTP (log beta(2)(app) = 6.3 +/- 0.1) and Eu(L)(3+) only for DMTP (log beta(1)(app) = 2.9 +/- 0.2). The log beta(n)(app) values for the Eu(L)(n)(3+) (n = 1-3) complexes were determined at pH 2.8. Better insight was given in this study concerning the role of the hydrophobic exterior of the ligands for the design of a new range of extracting agents.     

Systematic approach to the quantitative voltammetric analysis of the FeIII/FeII component of the [alpha2-Fe(OH2)P2W17O61]7-/8- reduction process in buffered and unbuffered aqueous media

The one-electron reduction of [alpha(2)-Fe(III)(OH(2))P(2)W(17)O(61)](7-) at a glassy carbon electrode was investigated using cyclic and rotating-disk-electrode voltammetry in buffered and unbuffered aqueous solutions over the pH range 3.45-7.50 with an ionic strength of approximately 0.6 M maintained. The behavior is well-described by a square-scheme mechanism P + e(-) <--> Q (E(1)(0/) = -0.275 V, k(1)(0/) = 0.008 cm s(-1), and alpha(1) = 1/2), PH(+) + e(-) <--> QH(+) (E(2)(0/) = -0.036 V, k(2)(0/) = 0.014 cm s(-1), and alpha(2) = 1/2), PH(+) <--> P + H(+) (K(P) = 3.02 x 10(-6) M), and QH(+) <--> Q + H(+) (K(Q) = 2.35 x 10(-10) M), where P, Q, PH(+), and QH(+) correspond to [alpha(2)-Fe(III)(OH)P(2)W(17)O(61)](8-), [alpha(2)-Fe(II)(OH)P(2)W(17)O(61)](9-), [alpha(2)-Fe(III)(OH(2))P(2)W(17)O(61)](7-), and [alpha(2)-Fe(II)(OH(2))P(2)W(17)O(61)](8-), respectively; E(1)(0)' and E(2)(0)' are the formal potentials, k(1)(0)' and k(2)(0)' are the formal (standard) rate constants, and K(P) and K(Q) are the acid dissociation constants for the relevant reactions. The analysis for the buffered media is based on the approach of Laviron who demonstrated that a square scheme with fully reversible protonations, reversible or quasi reversible electron transfers with the assumption that alpha(1) = alpha(2), can be well-described by the behavior of a simple redox couple, ox + e(-) <--> red, whose formal potential, E(app)(0)', and standard rate constant, k(app)(0)', are straightforwardly derived functions of pH, as are the values of E(1)(0)', k(1)(0)', E(2)(0)', k(2)(0)', and K(P) (only three of the four thermodynamic parameters in a square scheme can be specified). It was assumed that alpha(app) = 1/2, and the simulation program DigiSim was used to determine the values of E(app)(0)' and k(app)(0)', which are required to describe the cyclic voltammograms obtained in buffered media in the pH range from 3.45 to 7.52 (buffer-related reactions which effect general acid-base catalysis are included in the simulations). DigiSim simulations of cyclic voltammograms obtained in unbuffered media yielded the values of E(1)(0)' and k(1)(0)'; K(Q) was then directly computed from thermodynamic constraints. These simulations included additional reactions between the redox species and H(2)O. The value of the diffusion coefficient of the [alpha(2)-Fe(III)(OH(2))P(2)W(17)O(61)](7-), 2.92 x 10(-6) cm(2) s(-1), was determined using DigiSim simulations of voltammograms at a rotating disk electrode in buffered and unbuffered media at pH 3.45. The diffusion coefficients of all redox species were assumed to be identical. When the pH is greater than 6, instability of P (i.e., [alpha(2)-Fe(III)(OH)P(2)W(17)O(61)](8-)) led to the loss of the reactant and precluded lengthy experimentation.     

Characterization of 2,3-dihydroxyterephthalamides as M(IV) chelators

The ligand N,N'-diethyl-2,3-dihydroxyterephthalamide (ETAM) has been characterized as a chelator for Zr(IV), Ce(IV), and Th(IV). The K(+) salts of the complexes [Zr(ETAM)(4)](4)(-), [Ce(ETAM)(4)](4)(-), and [Th(ETAM)(4)](4)(-) were prepared in a MeOH solution containing H(2)ETAM, the corresponding M(acac)(4), and 4 equiv of KOH. Single-crystal X-ray diffraction analyses are reported for K(4)[Zr(ETAM)(4)] (C2/c, Z = 8, a = 27.576(3) A, b = 29.345(3) A, c = 15.266(2) A, alpha = 90 degrees, beta = 118.688(4) degrees, gamma = 90 degrees ), [Me(3)BnN](4)[Th(ETAM)(4)] (P, Z = 2, a = 13.7570(3) A, b = 13.9293(3) A, c = 26.9124(6) A, alpha = 99.941(1) degrees, beta = 94.972(1) degrees, gamma = 103.160(1) degrees ), and the dimeric (NMe(4))(4)[Th(ETAM)(3)MeOH](2) (P2(1)/c, Z = 4, a = 18.2603(9) A, b = 18.5002(9) A, c = 19.675(1) A, beta = 117.298(1) degrees ). Solution thermodynamic studies were used to determine formation constants (log K(f) and esd) for Th(IV)-ETAM log K(110) =17.47(1), log K(120) = 13.23(1), log K(130) = 8.28(3), log K(140) = 6.57(6), and log beta(140) = 45.54(5). These results support the hypothesis that the terephthalamides are high-affinity chelators for the actinide(IV) ions and thus promising ligands for use in nuclear waste remediation.     

Use of phosphorus ligand NMR probes to investigate electronic and second-sphere solvent effects in ligand substitution reactions at manganese(II) and manganese(III)

Manganese/ligand association dynamics were studied using a series of structurally related anionic phosphorus ester ligand probes [CH(3)OP(O)(X)(Y)(-), where X = CH(3)O, CH(3)CH(2), or H and Y = O, S, or BH(3)]. Reactions of the probe ions with Mn(H(2)O)(6)(2+) and a manganese(III) porphyrin (Mn(III)TMPyP(5+)) were studied in aqueous solution by paramagnetic (31)P NMR line-broadening techniques. A satisfactory linear free energy relationship for reactions of the probe ions with Mn(H(2)O)(6)(2+) and Mn(III)TMPyP(5+) required consideration of both the basicity and solvent affinity of the probe ligands: log(k(app)) = log(k(0)) + alpha pK(a) + beta log(K(ext)), where k(0), alpha, and beta are metal complex dependent parameters and pK(a) and K(ext) represent the measured Bronsted acidity and water/n-butanol extraction constant for the probe anions, respectively. Reactions of Mn(H(2)O)(6)(2+) were relatively insensitive to changes in ligand basicity (alpha = -0.04) and favored the more hydrophilic anions (beta = -0.54). These observations are consistent with a dissociative ligand exchange mechanism wherein the outer-sphere complex is stabilized by hydrogen bonding between Mn(H(2)O)(6)(2+) and the incoming ligand. In contrast, reactions with Mn(III)TMPyP(5+) are accelerated by decreases in both the basicity (alpha = -0.43) and the hydrophilicity (beta = +0.97) of the probe. We conclude that reactions of Mn(III)TMPyP(5+) are also dissociative but that the aromatic groups of the porphyrin provide a hydrophobic environment surrounding the ligand binding site in Mn(III)TMPyP(5+). Thus, the probe/water solvent interactions must be significantly weakened in order to form the outer-sphere complex that leads to ligand substitution. This work demonstrates the utility of phosphorus relaxation enhancement (PhoRE) techniques for characterizing the second coordination sphere environment of metal complexes leading to ligation and will allow comparison of the second coordination spheres of Mn(H(2)O)(6)(2+) and Mn(III)TMPyP(5+) to those of other metal complexes.     

Experimental correlation between the pKa value of sulfonphthaleins with the nature of the substituents groups

This work presents the results obtained from a spectrophotometry study performed on some indicators of the sulfonphtaleins like phenol red (PR), thymol blue (TB), bromothymol blue (BTB), xylenol orange (XO) and methylthymol blue (MTB). During the first stage the acidity constants of some of the indicators were determined using the data from spectrophotometry, potentiometry and with the use of the software SQUAD. These were as follows: for the equilibrium 2H+BTB<-->H(2)BTB, log beta(2)=15.069+/-0.046 and for H+BTB<-->HBTB, log beta(1)=8.311+/-0.044. For the XO and the MTB five values were calculated for each, namely, for MTB: log beta(5)=42.035, log beta(4)=38.567+/-0.058, log beta(3)=32.257+/-0.057, log beta(2)=23.785+/-0.057, and log beta(1)=12.974+/-0.045 while for XO: log beta(5)=40.120+/-0.102, log beta(4)=35.158+/-0.062, log beta(3)=29.102+/-0.053, log beta(2)=21.237+/-0.044, and log beta(1)=11.682+/-0.044. During the second stage, a study was conducted on the effect of the substituents present in the indicators to determine the effect of different functional groups on the pK(a) value corresponding to the last indicator's dissociation.     

Molecular switch triggered by solvent polarity: synthesis,Acid-base behavior,alkali metal ion complexation,and crystal structure

The synthesis and characterization of the new tetraazamacrocycle L, bearing two 1,1'-bis(2-phenol) groups as side-arms, is reported. The basicity behavior and the binding properties of L toward alkali metal ions were determined by means of potentiometric measurements in ethanol/water 50:50 (v/v) solution (298.1+/-0.1 K, I=0.15 mol dm(-3)). The anionic H(-1)L(-) species can be obtained in strong alkaline solution, indicating that not all of the acidic protons of L can be removed under the experimental conditions used. This species behaves as a tetraprotic base (log K(1)=11.22, log K(2)=9.45, log K(3)=7.07, log K(4)=5.08), and binds alkali metal ions to form neutral [MH(-1)L] complexes with the following stability constants: log K(Li)=3.92, log K(Na)=3.54, log K(K)=3.29, log K(Cs)=3.53. The arrangement of the acidic protons in the H(-1)L(-) species depends on the polarity of the solvents used, and at least one proton switches from the amine moiety to the aromatic part upon decreasing the polarity of the solvent. In this way two different binding areas, modulated by the polarity of solvents, are possible in L. One area is preferred by alkali metal ions in polar solvents, the second one is preferred in solvents with low polarity. Thus, the metal ion can switch from one location to the other in the ligand, modulated by the polarity of the environment. A strong hydrogen-bonding network should preorganize the ligand for coordination, as confirmed by MD simulations. The crystal structure of the [Na(H(-1)L)].CH(3)CN complex (space group P2(1)/c, a=12.805(1), b=20.205(3), c=14.170(2) A, beta=100.77(1) degrees, V=3601.6(8) A(3), Z=4, R=0.0430, wR2=0.1181), obtained using CH(2)Cl(2)/CH(3)CN as mixed solvent, supports this last aspect and shows one of the proposed binding areas.     

Thermodynamics, kinetics, and mechanism of the stepwise dissociation and formation of Tris(L-lysinehydroxamato)iron(III) in aqueous acid

pK(a) values for the hydroxamic acid, alpha-NH(3)(+), and epsilon-NH(3)(+) groups of L-lysinehydroxamic acid (LyHA, H(3)L(2+)) were found to be 6.87, 8.89, and 10.76, respectively, in aqueous solution (I = 0.1 M, NaClO(4)) at 25 degrees C. O,O coordination to Fe(III) by LyHA is supported by H(+) stoichiometry, UV-vis spectral shifts, and a shift in nu(CO) from 1648 to 1592 cm(-1) upon formation of mono(L-lysinehydroxamato)tetra(aquo)iron(III) (Fe(H(2)L)(H(2)O)(4)(4+)). The stepwise formation of tris(L-lysinehydroxamato)iron(III) from Fe(H(2)O)(6)(3+) and H(3)L(2+) was characterized by spectrophotometric titration, and the values for log beta(1), log beta(2), and log beta(3) are 6.80(9), 12.4(2), and 16.1(2), respectively, at 25 degrees C and I = 2.0 M (NaClO(4)). Stopped-flow spectrophotometry was used to study the proton-driven stepwise ligand dissociation kinetics of tris(L-lysinehydroxamato)iron(III) at 25 degrees C and I = 2.0 M (HClO(4)/NaClO(4)). Defining k(n) and k(-n) as the stepwise ligand dissociation and association rate constants and n as the number of bound LyHA ligands, k(3), k(-3), k(2), k(-2), k(1), and k(-1) are 3.0 x 10(4), 2.4 x 10(1), 3.9 x 10(2), 1.9 x 10(1), 1.4 x 10(-1), and 1.2 x 10(-1) M(-1) s(-1), respectively. These rate and equilibrium constants are compared with corresponding constants for Fe(III) complexes of acetohydroxamic acid (AHA) and N-methylacetohydroxamic acid (NMAHA) in the form of a linear free energy relationship. The role of electrostatics in these complexation reactions to form the highly charged Fe(LyHA)(3)(6+) species is discussed, and an interchange mechanism mediated by charge repulsion is presented. The reduction potential for tris(L-lysinehydroxamato)iron(III) is -214 mV (vs. NHE), and a comparison to other hydroxamic acid complexes of Fe(III) is made through a correlation between E(1/2) and pFe.     

UV/vis, 1H, and 13C NMR spectroscopic studies to determine mangiferin pKa values

The acid constants of mangiferin (a natural xanthonoid) in aqueous solution were determined through an UV/vis spectroscopic study employing the SQUAD program as a computational tool. A NMR study complements the pK(a) values assignment and evidences a H-bridge presence on 1-C. The chemical model used was consistent with the experimental data obtained. The pK(a) values determined with this procedure were as follows: H(4)(MGF)=H(3)(MGF)(-)+H(+), pKa1 (6-H)=6.52+/-0.06; H(3)(MGF)(-)=H(2)(MGF)(2-)+H(+), pKa2 (3-H)=7.97+/-0.06; H(2)(MGF)(2-)=H(MGF)(3-)+H(+), pKa3 (7-H)=9.44+/-0.04; H(MGF)(3-)=(MGF)(4-)+H(+), pKa4 (1-H)=12.10+/-0.01; where it has been considered mangiferin C(19)H(18)O(11) as H(4)(MGF). Mangiferin UV/vis spectral behavior, stability study in aqueous solution as well as NMR spectroscopy studies: one-dimensional (1)H,(13)C, 2D correlated (1)H/(13)C performed by (g)-HSQC and (g)-HMBC methods; are also presented. pK(a) values determination of H(4)(MGF) in aqueous solution is a necessary contribution to subsequent pharmacokinetic study, and a step towards the understanding of its biological effects.     

The B 1Pi state of NaCs: high resolution laser induced fluorescence spectroscopy and potential construction

The lowest (1)Pi state of the NaCs molecule, the B(1)(1)Pi state, was studied using a dye laser for inducing fluorescence that was resolved by a high resolution Fourier-transform spectrometer. The presence of argon buffer gas yielded rich rotational relaxation spectra allowing to enlarge the data set for the B(1)(1)Pi state, to obtain Lambda-splittings and to reveal numerous local perturbations. 543 weakly perturbed energy levels for rotational quantum numbers from J(')=5 to 168 and vibrational quantum numbers from v(')=0 to 25, which cover about 87% of the potential well depth, were used for a direct pointwise fit of the potential energy curve applying the inverted perturbation approach method. The resulting potential reproduces the term values for v(')=0-7 with an experimental accuracy of about 0.01-0.02 cm(-1), whereas for v(')=8-25 the deviations increase due to the perturbations, going to the order of 1 cm(-1); an extrapolation is made to the dissociation asymptote.     

Synthesis, structures, and magnetic properties of heterodimetal bis(mu-hydroxo)chromium(III)nickel(II) complexes with Tpa derivatives having sterically bulky substituents

A series of heterodinuclear bis(mu-hydroxo)chromium(III)nickel(II) complexes was newly prepared: [(phen)(2)Cr(mu-OH)(2)Ni(tpa)](ClO(4))(3) x 0.5H(2)O (1), [(phen)(2)Cr(mu-OH)(2)Ni(Me-tpa)](ClO(4))(3) x 2H(2)O (2), [(phen)(2)Cr(mu-OH)(2)Ni(Me(2)-tpa)](ClO(4))(3) x 2H(2)O (3), and [(phen)(2)Cr(mu-OH)(2)Ni(Me(3)-tpa)](ClO(4))(3) x 3H(2)O (4), where phen is 1,10-phenanthroline and tpa, Me-tpa, Me(2)-tpa, and Me(3)-tpa are tris(2-pyridylmethyl)amine, [(6-methyl-2-pyridyl)methyl]bis(2-pyridylmethyl)amine, bis[(6-methyl-2-pyridyl)methyl](2-pyridylmethyl)amine, and tris[(6-methyl-2-pyridyl)methyl]amine, respectively. X-ray crystallography revealed that the structures of 1-4 resemble one another having an edge-shared bioctahedral structure with a Cr(mu -OH)(2)Ni unit (crystal data: 1 x C(2)H(5)OH, triclinic, P1, a = 13.179(4) A, b = 13.685(4) A, c = 14.260(4) A, alpha = 84.95(2) degrees, beta = 77.65(1) degrees, gamma = 90.21(2) degrees, V = 2502(1) A(3), Z = 2, R = 0.103, R(w) = 0.097; 2 x C(2)H(5)OH, triclinic, P1, a = 13.214(2) A, b = 13.657(2) A, c = 14.417(3) A, alpha = 95.205(5) degrees, beta = 102.583(4) degrees, gamma =90.720(3) degrees, V = 2527.3(8) A(3), Z = 2, R = 0.090, R(w) = 0.122; 3 x C(2)H(5)OH, triclinic, P1, a = 13.276(2) A, b =13.696(2) A, c = 14.454(2) A, alpha = 95.640(3) degrees, beta = 102.821(4) degrees, gamma = 90.174(3) degrees, V = 2549.5(6) A(3), Z = 2, R= 0.087, R(w)= 0.119; 4, triclinic, P1, a = 10.8916(9) A, b = 14.268(2) A, c = 17.522(2) A, alpha = 84.498(9) degrees, beta = 74.313(7) degrees, gamma = 72.402(7) degrees, V = 2498.6(5) A(3), Z = 2, R = 0.060, R(w)= 0.088). Chromium and nickel ions are coordinated by two phen's and Me(n)-tpa, respectively, to complete a distorted octahedral coordination sphere. Introduction of the 6-methyl group(s) onto the pyridyl group(s) results in the elongation of the Ni-N bond distances due to an unfavorable steric interaction between the methyl group and the bridging hydroxide group: systematic elongation of the Ni-N bond distances and the Cr ...Ni separations accompanied by an increase in the Cr-O-Ni angles was observed as the number of the methyl groups increases. Variable-temperature magnetic susceptibility measurements of 1-4 (4.2-300 K) indicated that magnetic interactions between Cr(III) and Ni(II) ions are systematically modulated from a very weak antiferromagnetic interaction to a ferromagnetic interaction as the number of the methyl groups increases; the exchange integrals J's for 1-4 are estimated to be -1.4, +0.0, +4.1, and +7.4 cm(-1), respectively. The magneto-structural relationship is discussed in terms of the change in the magnetic orbital energies of nickel(II) centers arising from the change in the Ni-N bond distances.     

The d (3)Pi(g)-c (3)Sigma(u) (+) band system of C2

A two-dimensional fluorescence (excitation/emission) spectrum of C2 produced in an acetylene discharge was used to identify and separate emission bands from the d (3)Pi(g)<--c (3)Sigma(u) (+) and d (3)Pi(g)<--a (3)Pi(u) excitations. Rotationally resolved excitation spectra of the (4<--1), (5<--1), (5<--2), and (7<--3) bands in the d (3)Pi(g)<--c (3)Sigma(u) (+) system of C2 were observed by laser-induced fluorescence spectroscopy. The molecular constants of each vibrational level, determined from rotational analysis, were used to calculate the spectroscopic constants of the c (3)Sigma(u) (+) state. The principal molecular constants for the c (3)Sigma(u) (+) state are B(e)=1.9319(19) cm(-1), alpha(e)=0.018 55(69) cm(-1), omega(e)=2061.9 cm(-1), omega(e)x(e)=14.84 cm(-1), and T(0)(c-a)=8662.925(3) cm(-1). We report also the first experimental observations of dispersed fluorescence from the d (3)Pi(g) state to the c (3)Sigma(u) (+) state, namely, d (3)Pi(g)(v=3)-->c (3)Sigma(u) (+)(v=0,1).     

Equilibria between alpha- and beta-agostic stabilized rotamers of secondary alkyl niobium complexes.

The isopropyl chloro complex Tp(Me2)NbCl(i-Pr)(PhC&tbd1;CMe) (2) [Tp(Me2) = hydrotris(3,5-dimethylpyrazolyl)borate] exhibits a beta-agostic structure in the crystal. The conformation of the alkyl group is such that the agostic methyl group lies in the Calpha-Nb-Cl plane and the nonagostic one, in a wedge formed by two pyrazole rings. As observed by solution NMR spectroscopy, restricted rotation about the Nb-C bond allows the observation of an equilibrium between this species, 2beta, and a minor alpha-agostic rotamer 2alpha. A putative third rotamer which would have the secondary hydrogen in the wedge is not observed. Similar behavior is observed for related Tp'NbCl(i-Pr)(R(2)C=CMe) [Tp' = Tp(Me2), R(2) = Me (3); Tp' = Tp(Me2,4Cl), R(2) = Ph (4)]. The two diastereomers of the sec-butyl complex Tp(Me2)NbCl(sec-Bu)(MeC=CMe) (5) have been separated. In the crystal, 5CR-AS has a beta-agostic methyl group with the ethyl group located in the wedge formed by two pyrazole rings. The same single beta-agostic species is observed in solution. The other diastereomer, 5AR-CS has a beta-agostic methylene group in the solid state, and the methyl group sits in the wedge. In solution, an equilibrium between this beta-agostic methylene complex 5AR-CSbeta and a minor alpha-agostic species 5AR-CSalpha, where the ethyl substituent of the sec-Bu group is located in the wedge between two pyrazole rings, is observed. NMR techniques have provided thermodynamic parameters for these equilibria (K = 2beta/2alpha = 4.0 +/- 0.1 at 193 K, DeltaG(o)(193) = -2.2 +/- 0.1, DeltaH(o) = -7.4 +/- 0.1 kJ mol(-)(1), and DeltaS(o) = -27 +/- 1 J K(-)(1) mol(-)(1)), as well as kinetic parameters for the rotation about the Nb-C bond (at 193 K, DeltaG(2)= 47.5 +/- 2.5, DeltaH= 58.8 +/- 2.5 kJ mol(-)(1), and DeltaS = 59.0 +/- 10 J K(-)(1) mol(-)(1)). Upon selective deuteration of the beta-methyl protons in Tp(Me2)NbCl[CH(CD(3))(2)](PhC=CMe) (2-d(6)), an expected isotope effect that displaces the equilibrium toward the alpha-agostic rotamer is observed (K = 2-d(6)beta/2-d(6)alpha = 3.1 +/- 0.1 at 193 K, DeltaG(o)(193) = -1.8 +/- 0.1, DeltaH(o) = -8.3 +/- 0.4 kJ mol(-)(1) and DeltaS(o)= -34 +/- 2 J K(-)(1) mol(-)(1)). The anomalous values for DeltaH(o) and DeltaS(o) are discussed. Hybrid quantum mechanics/molecular mechanics calculations (IMOMM (B3LYP:MM3)) on the realistic model Tp(Me2)NbCl(i-Pr)(HC=CMe) have reproduced the energy differences between the alpha- and beta-agostic species with remarkable accuracy. Similar calculations show that Tp(Me2)NbCl(CH(2)Me)(HC=CMe) is alpha-agostic only and that Tp(5)(-)(Me)NbCl(CH(2)Me)(HC=CMe), which has no methyl groups at the 3-positions of the pyrazole rings, is beta-agostic only. Analysis and discussion of the computational and experimental data indicate that the unique behavior observed for the secondary alkyl complexes stems from competition between electronic effects favoring a beta-agostic structure and steric effects directing a bulky substituent in the wedge between two pyrazole rings of Tp(Me2). All of the secondary alkyl complexes thermally rearrange to the corresponding linear alkyl complexes via a first-order reaction.     

Prediction of 31P nuclear magnetic resonance chemical shifts for phosphines

Quantitative relationships of the (31)P NMR chemical shifts of the phosphorus atoms in 291 phosphines with the atomic ionicity index (INI) and stereoscopic effect parameters (epsilon(alpha), epsilon(beta), epsilon(gamma)) were primarily investigated in this paper for modeling some fundamental quantitative structure-spectroscopy relationships (QSSR). The results indicated that the (31)P NMR chemical shifts of phosphines can be described as the quantitative equation by multiple linear regression (MLR): delta(p)(ppm)= -174.0197-2.6724INI+40.4755epsilon(alpha)+15.1141epsilon(beta)-3.1858epsilon(gamma), correlation coefficient R=0.9479, root mean square error (rms)=13.9, and cross-validated predictive correlation coefficient was found by using the leave-one-out procedure to be Q(2)=0.8919. Furthermore, through way of random sampling, the estimative stability and the predictive power of the proposed MLR model were examined by constructing data set randomly into both the internal training set and external test set of 261 and 30 compounds, respectively, and then the chemical shifts were estimated and predicted with the training correlation coefficient R=0.9467 and rms=13.4 and the external predicting correlation coefficient Q(ext)=0.9598 and rms=10.8. A partial least square model was developed that produced R=0.9466, Q=0.9407 and Q(ext)=0.9599, respectively. Those good results provided a new, simple, accurate and efficient methodology for calculating (31)P NMR chemical shifts of phosphines.     

Synthesis of Tris(1,2-dimethoxyethane-O,O')barium Bis(2,2,5,5-tetramethyl-2,5-disilaphospholanide) and the Monomer-Dimer Equilibrium in Toluene Solution

(1,2-Dimethoxyethane-O,O')lithium phosphanide (dme)LiPH(2) reacts with 1,2-bis(chloro-dimethylsilyl)ethane to give 2,2,5,5-tetramethyl-2,5-disilaphospholane, 1, as well as 1,1,4,4-tetramethyl-1,4-bis(2,2,5,5-tetramethyl-2,5-disilaphospholanyl)-1,4-disilabutane, 2 (P(2)Si(6)C(18)H(48), space group P&onemacr;, a = 943.3(2) pm, b = 1278.3(3) pm, c = 1413.3(2) pm, alpha = 72.45(1) degrees, beta = 78.13(1) degrees, gamma = 70.83(1) degrees, d = 1.081 g cm(-)(3), Z = 2, wR2 = 0.1553 at 6548 F(2) values). The reaction of 2,2,5,5-tetramethyl-2,5-disilaphospholane 1 and barium bis[bis(trimethylsilyl)amide] in 1,2-dimethoxyethane yields nearly quantitatively tris(1,2-dimethoxyethane-O,O')barium bis(2,2,5,5-tetramethyl-2,5-disilaphospholanide), 3A, which crystallizes in the monoclinic space group C2/c (BaP(2)Si(4)O(6)C(24)H(62), a = 2152.3(1) pm, b = 1381.5(1) pm, c = 1459.7(1) pm, beta = 113.73(1) degrees, d(calc) = 1.268 g cm(-)(3), Z = 4, wR2 = 0.0989 at 5220 F(2) values). Due to the high coordination number of eight of the barium center, rather long Ba-P distances of 333 pm are observed. With loss of the complexating ether solvent this compound forms a dimer 3B of the type R(dme)Ba(&mgr;-R)(3)Ba(dme)(2) in toluene or benzene solution as can be proven by (31)P{(1)H}-NMR spectroscopy ((2)J(P-P) = 6.7 Hz) and by X-ray structure analysis (Ba(2)P(4)Si(8)O(6)C(48)H(106), space group P2(1)/n, a = 1256.3(2) pm, b = 2000.0(3) pm, c = 2986.9(2) pm, beta = 98.929(9) degrees, d(calc) = 1.257 g cm(-)(3), Z = 4, wR2 = 0.1334 at 11580 F(2) values). The Ba-P bond lengths vary between 318 and 338 pm.     

Coordination polymers based on cobridging of rigid and flexible spacer ligands: syntheses, crystal structures, and magnetic properties of [Mn(bpy)(H2O)(C4H4O4)].0.5bpy, Mn(bpy)(C5H6O4), and Mn(bpy)(C6H8O4)

Reactions of 4,4'-bipyridine (bpy) with Mn(C(4)H(4)O(4)).4H(2)O and Mn(C(5)H(6)O(4)).4H(2)O in methanolic aqueous solutions yielded [Mn(bpy)(H(2)O)(C(4)H(4)O(4))].0.5bpy (1) and Mn(bpy)(C(5)H(6)O(4)) (2), respectively, and reactions of freshly prepared Mn(OH)(2)(-)(2)(x)(CO(3))(x).yH(2)O, adipic acid and 4,4'-bipyridine in a methanolic aqueous solution afforded Mn(bpy)(C(6)H(8)O(4)) (3). The six-coordinate Mn atoms in 1 are interlinked by flexible succinato ligands to form layers, which are sustained by rigid bpy ligands into an 3D open framework with the free bpy molecules in tunnels. The ribbonlike chains in 2 result from Mn atoms bridged by glutarato ligands and are connected by bpy ligands into open layers. In 3, the Mn atoms are bridged by both bpy and adipato ligands to form 3D nanoporous frameworks and 2-fold interpenetration of the resulting 3D frameworks completes the crystal structure. In comparison with 1 and 2, compound 3 displays significant antiferromagnetic behavior at low temperature. The antiferromagnetic exchange becomes stronger from 1 through 2 to 3, and the antiferromagnetic ordering of Mn(2+) centers is related to the syn-syn bridging mode of the terminal carboxylate groups of alpha,omega-dicarboxylate anions. Crystal data: C(19)H(18)MnN(3)O(5) (1), monoclinic P2(1)/c, a= 11.686(2) A, b = 17.847(2) A, c = 8.852(1) A, beta = 99.67(1) degrees, V = 1819.9(4) A(3), Z = 4, D(c) = 1.545 g.cm(-3); C(15)H(14)MnN(2)O(4) (2), triclinic P, a = 8.145(2) A, b = 9.574(2) A, c = 10.180(1) A, alpha = 108.01(3) degrees, beta = 93.55(3) degrees, gamma = 105.30(1) degrees, V = 719.2(2) A(3), Z = 2, D(c) = 1.576 g.cm(-3); C(15)H(14)MnN(2)O(4) (3), triclinic P, a = 8.544(1) A, b= 8.881(1) A, c = 10.949(2) A, alpha = 108.81(1) degrees, beta = 95.40(1) degrees, gamma = 101.94(1) degrees, V = 757.7(2) A(3), Z = 2, D(c) = 1.557 g.cm(-3).