Kinetic study of the aminolysis and pyridinolysis of O-phenyl and O-ethyl O-(2,4-dinitrophenyl) thiocarbonates. A remarkable leaving group effect

The reactions of a series of secondary alicyclic (SA) amines with O-phenyl and O-ethyl O-(2,4-dinitrophenyl) thiocarbonates (1 and 2, respectively) and of a series of pyridines with the former substrate are subjected to a kinetic investigation in water, at 25.0 degrees C, ionic strength 0.2 M (KCl). Under amine excess over the substrate, all the reactions obey pseudo-first-order kinetics and are first-order in amine. The Br?nsted-type plots are biphasic, with slopes (at high pK(a)) of beta(1) = 0.20 for the reactions of SA amines with 1 and 2 and beta(1) = 0.10 for the pyridinolysis of 1 and with slopes (at low pK(a)) of beta(2) = 0.80 for the reactions of SA amines with 1 and 2 and beta(2) = 1.0 for the pyridinolysis of 1. The pK(a) values at the curvature center (pK(a)(0)) are 7.7, 7.0, and 7.0, respectively. These results are consistent with the existence of a zwitterionic tetrahedral intermediate (T++) and a change in the rate-determining step with the variation of amine basicity. The larger pK(a)(0) value for the pyridinolysis of 1 compared to that for 2 (pK(a)(0) = 6.8) and the larger pK(a)(0) value for the reactions of SA amines with 1 relative to 2 are explained by the greater inductive electron withdrawal of PhO compared to EtO. The larger pK(a)(0) values for the reactions of SA amines with 1 and 2, relative to their corresponding pyridinolysis, are attributed to the greater nucleofugalities of SA amines compared to isobasic pyridines. The smaller pK(a)(0) value for the reactions of SA amines with 2 than with O-ethyl S-(2,4-dinitrophenyl) dithiocarbonate (pK(a)(0) = 9.2) is explained by the greater nucleofugality from T(++) of 2,4-dinitrophenoxide (DNPO(-)) relative to the thio derivative. The stepwise reactions of SA amines with 1 and 2, in contrast to the concerted mechanisms for the reactions of the same amines with the corresponding carbonates, is attributed to stabilization of T(++) by the change of O(-) to S(-). The simple mechanism for the SA aminolysis of 2 (only one tetrahedral intermediate, T(++)) is in contrast to the more complex mechanism (two tetrahedral intermediates, T(++) and T(-), the latter formed by deprotonation of T(++) by the amine) for the same aminolysis of the analogous thionocarbonate with 4-nitrophenoxide (NPO(-)) as nucleofuge. To our knowledge, this is the first example of a remarkable change in the decomposition path of a tetrahedral intermediate T by replacement of NPO(-) with DNPO(-) as the leaving group of the substrate. This is explained by (i) the greater leaving ability from T(++) of DNPO(-) than NPO(-) and (ii) the similar rates of deprotonation of both T(++) (formed with DNPO and NPO).     

Structures and properties of neutral gallium clusters: a theoretical investigation

A systematic and unbiased structure search based on a genetic algorithm in combination with density functional theory (DFT) procedures has been carried out to locate low-energy isomers of Ga(n) up to n = 25. For the smaller clusters up to n = 8 results are checked by coupled cluster singles and doubles with perturbative triples corrections (CCSD(T)) employing a quadruple zeta type basis set. The CCSD(T) calculations confirm a (3)Π(u) ground state for the dimer. Ga(3) has a doublet ground state 0.2 eV below two quartet states, whereas two isoenergetic triplet states are predicted for Ga(4) with D(4h) and a rhombus structure (D(2h)). Three low-lying isomers with doublet electronic states are found for Ga(5): a W-structure (C(2v)), a planar envelope (C(s)) at 0.015 eV, and a non-planar envelope (C(1)) 0.086 eV above the ground state. A triplet state for a trigonal prism (D(3h)) and a singlet for an open prism (C(2v)) are computed with virtually identical energy for Ga(6). The global minimum for Ga(7) is a capped trigonal prism (C(s)) and that for Ga(8) a distorted cube in D(2h). DFT provides a fair agreement with CCSD(T), deviations in dissociation energies are up to 0.2 eV for n ≤ 8. The structures for Ga(n) are mostly irregular for n ≥ 9, those for Ga(12) to Ga(17) can be derived from the truncated decahedron with D(5h) symmetry though highly distorted by Jahn-Teller effects, for example. For Ga(18) to Ga(23) we find stacks of five- and six-membered rings as global minima, e.g., 5-1-5-1-6 for Ga(18). Ga(24) and Ga(25) consist of layers with packing sequence ABCBA similar to those found for clusters of aluminum. The most important feature of computed cohesive energies is a rapid increase with n: for Ga(25) it reaches 2.46 eV, the experimental bulk value is 2.84 eV. Particularly stable clusters for Ga(n) are seen for n = 7, 14, and 20.     

A sol-gel-modified poly(methyl methacrylate) electrophoresis microchip with a hydrophilic channel wall

A sol-gel method was employed to fabricate a poly(methyl methacrylate) (PMMA) electrophoresis microchip that contains a hydrophilic channel wall. To fabricate such a device, tetraethoxysilane (TEOS) was injected into the PMMA channel and was allowed to diffuse into the surface layer for 24 h. After removing the excess TEOS, the channel was filled with an acidic solution for 3 h. Subsequently, the channel was flushed with water and was pretreated in an oven to obtain a sol-gel-modified PMMA microchip. The water contact angle for the sol-gel-modified PMMA was approximately 27.4 degrees compared with approximately 66.3 degrees for the pure PMMA. In addition, the electro-osmotic flow increased from 2.13x10(-4) cm2 V(-1) s(-1) for the native-PMMA channel to 4.86x10(-4) cm2 V(-1) s(-1) for the modified one. The analytical performance of the sol-gel-modified PMMA microchip was demonstrated for the electrophoretic separation of several purines, coupled with amperometric detection. The separation efficiency of uric acid increased to 74,882.3 m(-1) compared with 14,730.5 m(-1) for native-PMMA microchips. The result of this simple modification is a significant improvement in the performance of PMMA for microchip electrophoresis and microfluidic applications.     

Accurate prediction of basicity in aqueous solution with COSMO-RS

The COSMO-RS method, a combination of the quantum chemical dielectric continuum solvation model COSMO with a statistical thermodynamics treatment for realistic solvation simulations, has been used for the prediction of base pK(a) constants. For a variety of 43 organic bases the directly calculated values of the free energies of dissociation in water showed a very good correlation with experimental base pK(a) values (r2 = 0.98), corresponding to a standard deviation of 0.56 pK(a) units. Thus, we have an a priori prediction method for base pK(a) with the regression constant and the slope as only adjusted parameters. In accord with recent findings for pK(a) acidity predictions, the slope of pK(a) vs. DeltaG(diss) was significantly smaller than the theoretically expected value of 1/RTln(10). The predictivity of the presented method is general and not restricted to certain compound classes, but systematic corrections of 1 and 2 pKa units for secondary and tertiary aliphatic amines are required, respectively. The pK(a) prediction method was validated on a set of 58 complex multifunctional drug-like compounds, yielding an RMS accuracy of 0.66 pK(a) units.     

Solvent-induced self-assembly of mixed poly(methyl methacrylate)/polystyrene brushes on planar silica substrates: molecular weight effect

The effect of molecular weight on the solvent-induced self-assembly of mixed poly(methyl methacrylate) (PMMA)/polystyrene (PS) brushes on silicon wafers was studied. For a series of mixed brushes with a fixed PMMA M(n) and systematically changed PS M(n), a transition in water advancing contact angle (theta(a)) from 74 degrees, the value for a flat PMMA surface, to 91 degrees, the value for a flat PS film, was observed with increasing PS M(n) after treatment with CHCl(3). Atomic force microscopy studies showed smooth surfaces for all samples. While no significant changes in surface morphologies were observed after treatment with cyclohexane, a selective solvent for PS, contact angle and XPS studies indicated that the mixed brushes with a PS M(n) slightly smaller than that of PMMA underwent self-reorganization, exhibiting a different theta(a). Intriguing surface morphologies composed of relatively ordered nanoscale domains were found from mixed brushes with PS M(n) slightly smaller than or similar to that of PMMA after treatment with acetic acid, a selective solvent for PMMA. The nanodomains are speculated to be of a micellar structure, with PS chains forming a core shielded by PMMA chains.     

Effect of changing electrophilic center from C=O to C=S on rates and mechanism: pyridinolyses of O-2,4-dinitrophenyl thionobenzoate and its oxygen analogue

Second-order rate constants have been measured spectrophotometrically for the reactions of O-2,4-dinitrophenyl thionobenzoate (1) and 2,4-dinitrophenyl benzoate (2) with a series of substituted pyridines in 80 mol % H(2)O/20 mol % DMSO at 25.0 +/- 0.1 degrees C. The Br?nsted-type plots obtained are nonlinear with beta(1) = 0.26, beta(2) = 1.07, and pK(a) degrees = 7.5 for the reactions of 1 and beta(1) = 0.40, beta(2) = 0.90, and pK(a) degrees = 9.5 for the reactions of 2, suggesting that the pyridinolyses of 1 and 2 proceed through a zwiterionic tetrahedral intermediate T(+/-) with a change in the rate-determining step at pK(a) degrees = 7.5 and 9.5, respectively. The thiono ester 1 is more reactive than its oxygen analogue 2 except for the reaction with the strongest basic pyridine studied (pK(a) = 11.30). The k(1) value is larger for the reactions of 1 than for those of 2 in the low pK(a) region, but the difference in the k(1) value becomes negligible with increasing the basicity of pyridines. On the other hand, 1 exhibits slightly larger k(2)/k(-1) ratio than 2 in the low pK(a) region but the difference in the k(2)/k(-1) ratio becomes more significant with increasing the basicity of pyridines. Pyridines are more reactive than alicyclic secondary amines of similar basicity toward 2 in the pK(a) above ca. 7.2 but less reactive in the pK(a) below ca. 7.2. The k(1) value is slightly larger, but the k(2)/k(-1) ratio is much smaller for the reactions of 2 with pyridines than with isobasic secondary amines in the low pK(a) region, which is responsible for the fact that the weakly basic pyridines are less reactive than isobasic secondary amines.     

Aminolysis of Y-substituted phenyl diphenylphosphinates and benzoates: effect of modification of electrophilic center from C=O to P=O

The effect of modification of the electrophilic center from C=O to P=O on reactivity and reaction mechanism has been investigated for aminolysis of Y-substituted phenyl diphenylphosphinates (1a-j) and benzoates (2a-i). The phosphinates 1a-j are less reactive than the benzoates 2a-i. The reactions of 2,4-dinitrophenyl diphenylphosphinate (1a) with alicyclic secondary amines resulted in a linear Br?nsted-type plot with a beta(nuc) value of 0.38, while the corresponding reactions of 2,4-dinitrophenyl benzoate (2a) yielded a curved Br?nsted-type plot. Similarly, a linear Br?nsted-type plot with a beta(lg) value of -0.66 was obtained for the reactions of 1a-j with piperidine, while the corresponding reactions of 2a-i gave a curved Br?nsted-type plot. The linear Br?nsted-type plots for the reactions of 1a-j have been taken as evidence for a concerted mechanism, while the curved Br?nsted-type plots for the reactions of 2a-i have been suggested to indicate a change in the rate-determining step of a stepwise mechanism. The Hammett plot for the reactions of 1b-j exhibited a poor correlation with sigma(-) constants (R(2) = 0.962) but slightly better correlation with sigma(o) (R(2) = 0.986). However, the Yukawa-Tsuno plot for the same reactions resulted in an excellent correlation (R(2) = 0.9993) with an r value of 0.30. The aminolysis of 1a-j has been suggested to proceed through a concerted mechanism with an early transition state on the basis of the small beta(nuc) and small r values.     

Aminolysis of Y-substituted phenyl X-substituted cinnamates and benzoates: effect of modification of the nonleaving group from benzoyl to cinnamoyl

A kinetic study is reported for reactions of Y-substituted phenyl X-substituted cinnamates (1a-e and 3a-g) and benzoates (2a-e and 4a-g) with a series of alicyclic secondary amines in 80 mol % H2O/20 mol % DMSO at 25.0 +/- 0.1 degrees C. Reactions of 2,4-dinitrophenyl X-substituted cinnamates (1a-e) and benzoates (2a-e) with amines result in linear Yukawa-Tsuno plots. The rho(X) values are much smaller for the reactions of 1a-e than for those of 2a-e. A distance effect and the nature of the reaction mechanism (i.e., a concerted mechanism for 1a-e) have been suggested to be responsible for the small rho(X) values. The Br?nsted-type plots for the reactions of 2,4-dinitrophenyl X-substituted cinnamates (1a, 1c, and 1e) with amines are curved with a decreasing betanuc value from 0.65 to 0.3-0.4. The reactions of Y-substituted phenyl cinnamates (3a-g) with morpholine also result in a curved Br?nsted plot, while the corresponding reactions of Y-substituted phenyl benzoates (4a-e) exhibit a linear Br?nsted plot. It has been concluded that the curved Br?nsted plots found for the reactions of the cinnamates (1a, 1c, 1e, and 3a-g) are not due to a change in the rate-determining step (RDS) but due to a normal Hammond effect for a concerted mechanism, that is, an earlier transition state (TS) for a more reactive amine or substrate.     

Mechanistic information from pressure acceleration of hydride formation via proton binding to a cobalt(I) macrocycle

The effect of pressure on proton binding to the racemic isomer of the cobalt(I) macrocycle, CoL(+) (L = 5,7,7,12,14,14-hexamethyl-1,4,8,11-tetraazacyclotetradeca-4,11-diene), has been studied for a series of proton donors using pulse radiolysis techniques. The second-order rate constants for the reaction of CoL(+) with proton donors decrease with increasing pK(a) of the donor acid, consistent with a reaction occurring via proton transfer. Whereas the corresponding volumes of activation (DeltaV) are rather small and negative for all acids (proton donors) with pK(a) values below 8.5, significantly larger negative activation volumes are found for weaker acids (pK(a) > 9.5) containing OH groups as proton donors. In the latter case, the observed DeltaV for these protonation reactions show a correlation with the reaction volumes (DeltaV degrees (ion)) for the ionization of the weak acids with a slope of 0.44, indicating that bond dissociation of the weak acid molecule bound to the metal center proceeds approximately halfway at the transition state along the reaction coordinate in terms of volume changes.     

A dynamic lattice searching method with interior operation for unbiased optimization of large Lennard-Jones clusters

For improving the efficiency of dynamic lattice searching (DLS) method for unbiased optimization of large Lennard-Jones (LJ) clusters, a variant of the interior operation (IO) proposed by Takeuchi was combined with DLS. The method is named as DLS-IO. In the method, the IO moves outer atoms with higher energy toward the coordinates center, i.e., (0, 0, 0), of a cluster and a local minimization (LM) follows each IO. This makes the interior atoms more compact and the outer atoms more uniformly distributed with lower potential energy. Therefore, the starting structure for DLS operations is closer to the global optimum compared with the randomly generated structures. On the other hand, a method to identify the central atom is proposed for the central vacancy problem. Optimizations of LJ(500), LJ(561), LJ(660), LJ(665), and LJ(670) were investigated with the DLS-IO, and the structural transition during the optimization was analyzed. It was found that the method is efficient and unbiased for optimization of large LJ clusters, and it may be a promising approach to be universally used for structural optimizations.     

Protonated benzofuran,anthracene, naphthalene,benzene, ethene,and ethyne: measurements and estimates of pK(a) and pK(R)

Aqueous solvolyses of acyl derivatives of hydrates (water adducts) of anthracene and benzofuran yield carbocations which undergo competitive deprotonation to form the aromatic molecules and nucleophilic reaction with water to give the aromatic hydrates. Trapping experiments with azide ions yield rate constants k(p) for the deprotonation and k(H2O) for the nucleophilic reaction based on the "azide clock". Combining these with rate constants for (a) the H(+)-catalyzed reaction of the hydrate to form the carbocation and (b) hydrogen isotope exchange of the aromatic molecule (from the literature) yields pK(R) = -6.0 and -9.4 and pK(a) = -13.5 and -16.3 for the protonated anthracene and protonated benzofuran, respectively. These pK values may be compared with pK(R) = -6.7 for naphthalene hydrate (1-hydroxy-1,2-dihydronaphthalene), extrapolated to water from measurements by Pirinccioglu and Thibblin for acetonitrile-water mixtures, and pK(a) = -20.4 for the 2-protonated naphthalene from combining k(p) with an exchange rate constant. The differences between pK(R) and pK(a) correspond to pK(H2O), the equilibrium constant for hydration of the aromatic molecule (pK(H2O) = pK(R) - pK(a)). For naphthalene and anthracene values of pK(H2O) = +13.7 and +7.5 compare with independent estimates of +14.2 and +7.4. For benzene, pK(a) = -24.3 is derived from an exchange rate constant and an assigned value for the reverse rate constant close to the limit for solvent relaxation. Combining this pK(a) with calculated values of pK(H2O) gives pK(R) = -2.4 and -2.1 for protonated benzenes forming 1,2- and 1,4-hydrates, respectively. Coincidentally, the rate constant for protonation of benzene is similar to those for protonation of ethylene and acetylene (Lucchini, V.; Modena, G. J. Am. Chem Soc. 1990, 112, 6291). Values of pK(a) for the ethyl and vinyl cations (-24.8) may thus be derived in the same way as that for the benzenonium ion. Combining these with appropriate values of pK(H2O) then yields pK(R) = -39.8 and -29.6 for the vinyl and ethyl cations, respectively.     

Isolation of poly(propylene glycol)s from water for quantitative analysis by reversed-phase liquid chromatography

Procedures for the isolation of poly(propylene glycol)s (PPGs) from a water matrix have been developed. Solid-phase extraction with an octadecylsilica cartridge and elution with methanol or with a graphitised carbon black cartridge and elution with a mixture of dichloromethane-methanol (4:1) or liquid-liquid extraction with chloroform were all suitable for model samples. However, only liquid-liquid extraction was suitable both for model and real environmental samples. Methods for reversed-phase liquid chromatographic determination of PPGs based on derivatisation and ultraviolet or fluorescence detection have been developed. Four derivatisation agents [3,5-dinitrobenzoyl chloride, phenyl isocyanate, 1-naphthoyl chloride and 1-naphthyl isocyanate (NIC)] were tested. Only NIC was found to give good reproducibility as well as a satisfactory detection limit. Finally, a method with liquid-liquid extraction with chloroform, derivatisation with NIC and liquid chromatographic separation with fluorescence detection was established. The developed method shows a highly correlated linearity of the analytical signals of particular homologues within a wide concentration range (approximately from 0.01 to 10 mg l(-1)). The precision of measurements is satisfactory for homologues having 5-9 oxypropylene subunits and becomes worse with an increase in the number of oxypropylene subunits. The limit of detection is 2 microg l(-1) for the majority of homologues. The method is suitable for the isolation and quantitative determination of PPGs in river water samples and as a tool for biodegradation testing.     

Carbon Dot−NaCl Crystals for White-Light Generation and Fabry-Perot Lasing

Phosphor materials with broad spectral range and an average emission lifetime (20 μs) have been achieved from carbon dots (CDs)−NaCl crystals. A one-pot synthesis pathway has been developed for CDs−NaCl crystals formation at room temperature. Precursor for CDs materials was screened at room temperature by oxidation methodology from different simple sugar molecules. CDs (size less than 10 nm) prepared from the fructose sugar exhibit most intense emission. Utilizing ripe banana peel (contains ∼27% of fructose) as a precursor for the carbon dot formation, white-light emission with a CIE index of (0.29, 0.34) has been achieved from the single source with CDs−NaCl crystals upon excitation at 430 nm. The crystals also function as Fabry-Perot (F−P) mode resonator for lasing, with a laser threshold value of 0.9 mW and a resonating Q-factor of 207. These results outline a new approach for realizing F−P lasing and white light emission from a non-toxic green source with a quick, facile and low-cost synthesis procedure.     

Palladium-catalyzed aminations of aryl halides with phosphine-functionalized imidazolium ligands

A series of 1-(2-diphenylphosphinoferrocenyl)ethyl-3-substituted imidazolium iodides [3-substituent = methyl (1a); isopropyl (1b); tert-butyl (1c); 1-adenosyl (1d); cyclohexyl (1e); 2,6-dimethylphenyl (1f); 2,4,6-trimethylphenyl (1g); 2,6-diisopropylphenyl (1h)] have been prepared and evaluated as ligands in the palladium-catalyzed aminations of aryl halides with various amines. The scope of the coupling process was carried out for various aryl bromides and chlorides with the catalysts generated in situ from a mixture of Pd(OAc)2 and in the presence of a base. NaO t Bu was found the choice of base in combination with dioxane, toluene, or DME as solvent, although NaOH or Cs2CO3 promoted the coupling of 4-bromotoluene with morpholine in moderate conversion. The steric hindrance from the 3-substituent of imidazolium in the hybrid-bidentate chelating system was found to be only beneficial to the substrates without ortho-substituents. The more sterically hindered 1d or 1h promoted the coupling of bromobenzene with morpholine in nearly quantitative conversion with 0.2 mol% of palladium loading in the presence of NaO t Bu at 110 degrees C, and 94% of conversion was afforded with the less sterical demanding 1a for a longer time. However, for the substrates with ortho-substituents, higher conversions were achieved with 1a. The Pd(OAc)2/1d catalytic system was also active for deactivated aryl chloride, and 71% isolated yield for the desired product was realized for coupling of 4-chloroanisole with morpholine at 2 mol% of catalyst loading. The developed catalyst system has been applied successfully to the synthesis of a key building block for a type of functional polymers.     

Binuclear homoleptic manganese carbonyls: Mn2(CO)x (x = 10, 9, 8, 7)

The unsaturated homoleptic manganese carbonyls Mn(2)(CO)(n)() (n = 7, 8, 9) are characterized by their equilibrium geometries, thermochemistry, and vibrational frequencies using methods from density functional theory (DFT). The computed metal-metal distances for global minima range from 3.01 A for the unbridged Mn(2)(CO)(10) with a Mn-Mn single bond to 2.14 A for a monobridged Mn(2)(CO)(7) formulated with a metal-metal quadruple bond. The global minimum for Mn(2)(CO)(9) has a four-electron bridging mu-eta(2)-CO group and a 2.96 A Mn-Mn distance suggestive of the single bond required for 18-electron configurations for both metal atoms. This structure is closely related to an experimentally realized structure for the isolated and structurally characterized stable phosphine complex [R(2)PCH(2)PR(2)](2)Mn(2)(CO)(4)(mu-eta(2)-CO). An unbridged (OC)(4)Mn-Mn(CO)(5) structure for Mn(2)(CO)(9) has only slightly (<6 kcal/mol) higher energy with a somewhat shorter metal-metal distance of 2.77 A. For Mn(2)(CO)(8) the lowest energy structure is a D(2)(d)() unbridged structure with a 2.36 A metal-metal distance suggesting the triple bond required for the favored 18-electron configuration for both metal atoms. However, the unbridged unsymmetrical (CO)(3)Mn-Mn(CO)(5) structure with a metal-metal bond distance of 2.40 A lies only 1 to 3 kcal/mol above this global minimum. The lowest energy structure of Mn(2)(CO)(7) is an unbridged C(s)() structure with a short metal-metal distance of 2.26 A. This is followed energetically by another C(s)() unbridged Mn(2)(CO)(7) structure with a somewhat longer metal-metal distance of 2.38 A.     

Addition of carbenes to the sidewalls of single-walled carbon nanotubes

The addition of carbenes CX(2) (X=H, Cl) to single-walled carbon nanotubes (SWNTs) was investigated by density functional theory and finite, hydrogen-terminated nanotube clusters or periodic boundary conditions in conjunction with basis sets of up to polarized triple-zeta quality. For armchair [(3,3) to (12,12)] and zigzag tubes [(3,0) to (18,0)], reaction of CH(2) with the C--C bond oriented along the tube axis (A) is less exothermic than with those C--C bonds having circumferential (C) orientation. This preference decreases monotonically with increasing tube diameter for armchair, but not for zigzag tubes; here, tubes with small band gaps have a very low preference for circumferential addition. Axial addition results in cyclopropane products, while circumferential addition produces "open" structures for both armchair and zigzag tubes. The barriers for addition of dichlorocarbene to a (5,5) SWNT, studied for a finite C(90)H(20) cluster, are higher than that for addition to C(60), in spite of similar diameters of the carbon materials. Whereas addition of CCl(2) to [60]fullerene proceeds in a concerted fashion, addition to a (5,5) armchair SWNT is predicted to occur stepwise and involve a diradicaloid intermediate according to B3LYP, PBE, and GVB-PP computations. Addition to C bonds of (5,5) armchair tubes resulting in the thermodynamically more stable insertion products is kinetically less favorable than that to A bonds yielding cyclopropane derivatives.     

Partitioning and adsorption in gas-liquid-solid chromatography: Models of temporal moments for retention and variance

Summary The resolution of gaseous chemical species in gasliquid-solid chromatography is influenced by absorption (partitioning) in the liquid and adsorption at the liquid-solid interface. We consider fundamental mass transfer models with adsorption and partitioning effects for solid chromatographic supports covered with thin films of stationary liquid. The dynamic models, based on mass-balance partial-differential equations, include the significant phenomena: convection, axial dispersion, gas-liquid mass transfer, intraparticle diffusion, and liquid-solid adsorption. Expressions for retention time and band variance (first and second temporal moments) are presented and evaluated for four distinct models: (1) capillary tube with inner surface covered with a uniform-thickness liquid film, (2) column of nonporous spheres covered with a uniform-thickness liquid film, (3) porous spherical particles with intraparticle pores covered with a uniform-thickness liquid film, (4) porous spherical particles with intraparticle pores completely filled with liquid.     

Molecule-Based Artificial Photosynthesis

Guidelines for the design of molecules with a long lifetime of the charge-separated state and for the formation of a self-assembled monolayer were studied by preparing various model compounds linking donor and acceptor with chemical bonds. Based on the obtained results we designed and prepared the SAMs of C₆₀-(porphyrin)-(ferrocene)-(CH₂)₁₁SH on a gold surface and observed a photocurrent with high efficiency (25% quantum yield). In addition, a well-defined, rigid-sheet-structured oligoporphyrin with 21 porphyrin chromophores was prepared as a model for antenna chlorophylls.     

Effects of amine nature and nonleaving group substituents on rate and mechanism in aminolyses of 2,4-dinitrophenyl X-substituted benzoates

Second-order rate constants have been measured for the reactions of 2,4-dinitrophenyl X-substituted benzoates (1a-f) with a series of primary amines in 80 mol % H(2)O/20 mol % DMSO at 25.0 +/- 0.1 degrees C. The Br?nsted-type plot for the reactions of 1d with primary amines is biphasic with slopes beta(1) = 0.36 at the high pK(a) region and beta(2) = 0.78 at the low pK(a) region and the curvature center at pK(a) degrees = 9.2, indicating that the reaction proceeds through an addition intermediate with a change in the rate-determining step as the basicity of amines increases. The corresponding Br?nsted-type plot for the reactions with secondary amines is also biphasic with beta(1) = 0.34, beta(2) = 0.74, and pK(a) degrees = 9.1, indicating that the effect of amine nature on the reaction mechanism and pK(a) degrees is insignificant. However, primary amines have been found to be less reactive than isobasic secondary amines. The microscopic rate constants associated with the aminolysis have revealed that the smaller k(1) for the reactions with primary amines is fully responsible for their lower reactivity. The electron-donating substituent in the nonleaving group exhibits a negative deviation from the Hammett plots for the reactions of 1a-f with primary and secondary amines, while the corresponding Yukawa-Tsuno plots are linear. The negative deviation has been ascribed to stabilization of the ground state of the substrate through resonance interaction between the electron-donating substituent and the carbonyl functionality.     

Mode selective tunneling dynamics observed by high resolution spectroscopy of the bending fundamentals of 14NH2D and 14ND2H

High resolution (0.004 and 0.01 cm(-1) instrumental bandwidth) interferometric Fourier transform infrared spectra of (14)NH2D and (14)ND2H were measured on a Bomem DA002 spectrometer in a supersonic jet expansion and at room temperature. We report the analysis of the bending fundamentals of (14)NH2D with term values Tv(s)=1389.9063(2) cm(-1) and Tv(a)=1390.4953(2) cm(-1) for the nu(4b) fundamental and Tv(s)=1605.6404(7) cm(-1) and Tv(a)=1591.0019(7) cm(-1) for the nu(4a) fundamental, and of (14)ND2H with term values of Tv(s)=1233.3740(2) cm(-1) and Tv(a)=1235.8904(2) cm(-1) for the nu(4a) fundamental and Tv(s)=1461.7941(9) cm(-1) and Tv(a)=1461.9918(19) cm(-1) for the nu(4b) fundamental. In all cases Tv(s) gives the position of the symmetric inversion sublevel (with positive parity) and Tv(a) the position of the antisymmetric inversion sublevel (with negative parity). The notation for the fundamentals nu(4a) and nu(4b) is chosen by correlation with the degenerate nu(4) mode in the C(3v) symmetric molecules NH3 and ND3. The degeneracy is lifted in Cs symmetry and a indicates the symmetric, b the antisymmetric normal mode with respect to the Cs symmetry plane in NH2D and ND2H. Assignments were established with certainty by means of ground state combination differences. About 20 molecular parameters of the effective S-reduced Hamiltonian could be determined accurately for each fundamental. In particular, the effect of Fermi resonances of the 2nu(2) overtone with the nu(4a) bending mode was observed, leading to an increased inversion splitting in the case of ND2H and to a strongly increased inversion splitting and an inverted order of the two inversion levels in NH2D. Rotational perturbations observed with the nu(4b) bending fundamentals are probably due to Coriolis interactions with the inversion overtone 2nu(2). The results are important for understanding isotope effects on the inversion in ammonia as well as its selective catalysis and inhibition by excitation of different vibrational modes, as treated by quantum dynamics on high dimensional potential hypersurfaces of this molecule.