Ground and first excited states electrostatic molecular potentials of ketene and diazomethane

INDO wavefunctions for ¹A₁, ¹A″ and ³A″ states of ketene and diazomethane, obtained with a RHF technique, after some geometry optimization for the excited states, are used to obtain electrostatic molecular potentials under ZDO assumptions. Ground-state results agree with the experimental behaviour and also with other theoretical calculations for both molecules.     

Photogeneration of o-quinone methides from o-cycloalkenylphenols

6-Alkylidenecyclohexa-2,4-dienones (o-quinone methides II) have been generated by photolysis of 2-(2'-cycloalkenyl)phenols 1 and trapped by methanol to give the ring-opened products 2. The best results have been obtained with the cyclohexenyl derivatives 1a, 1e, and 1f. In the case of the cyclopentenyl derivative 1b, photoproduct 2b was not observed, whereas only small amounts of 2c and 2d were formed from the seven- and eight-membered ring analogues 1c and 1d. Thus, ring size appears to be a key factor in the formation of o-quinone methides. This experimental result has been rationalized by means of density-functional theory (DFT) calculations. On the other hand, phenol substitution also appears to play a role in the process. Thus, electron-withdrawing groups such as CF(3) (1f) accelerate the reaction, whereas the opposite is true for electron-donating groups such as OCH(3) (1e). This is explained by an excited-state intramolecular proton transfer (ESIPT) mechanism, as the above results are consistent with the excited-state acidities of the different phenols. The lack of reactivity in the case of ketone 1g, where the intersystem crossing quantum yield is close to unity, allows us to rule out a mechanism involving the triplet state.     

Effect of ionic strength and ionic interactions on the oxidation of Fe(II)

The rates of oxidation of Fe(II) in NaCl and NaClO ₄ solutions were studied as a function of pH (6 to 9), temperature (5 to 25°C), and ionic strength (0 to 6m). The rates are second order with respect to [H⁺] or [OH^–] and independent of ionic strength and temperature. The overall rate of the oxidation is given by

Theoretical calculations on the cycloreversion of oxetane radical cations

The molecular mechanism for the cycloreversion of oxetane radical cations has been studied at the UB3LYP/6-31G* level. Calculations support that the cycloreversion takes place via a concerted but asynchronous process, where C-C bond breaking at the transition state is more advanced than O-C breaking. This allows a favorable rearrangement of the spin electron density from the oxetane radical cation (with the spin density located mainly on the oxygen atom) to the alkene radical cation which is one of the final products. Inclusion of solvent effects does not modify the gas-phase results.     

Reaction of 5-hydroxy-L-tryptophan with alkyl isocyanates

5-Hydroxy-L-tryptophan 4 with potassium cyanate gives 5-(5-hydroxy-3-indolylmethyl)hydantoin 5 or N_b-carbamoyl-5-hydroxy-L-tryptophan 6 depending on the reaction conditions. Reaction of 4 with methyl iso-cyanate in acetone provides 5-hydroxy-N_b-methylcarbamoyl-L-tryptophan 7 . Treatment of 4 with ethyl, propyl and isopropyl isocyanates in acetone gives rise to the formation of the corresponding 2-[(3-alkyl-4,4-dimethyl-2-oxo)-1,3-diazetidinyl]-3-(5-hydroxy-3-indolyl)propionic acids 8–10 .     

Pore size distribution analysis of selected hexagonal mesoporous silicas by grand canonical Monte Carlo simulations

We combine here a regularization procedure with individual adsorption isotherms obtained from grand canonical Monte Carlo simulations in order to obtain reliable pore size distributions. The methodology is applied to two hexagonal high-ordered silica materials: SBA-15 and PHTS, synthesized in our laboratory. Feasible pore size distributions are calculated through an adaptable procedure of deconvolution over the adsorption integral equation, with two necessary inputs: the experimental adsorption data and individual adsorption isotherms, assuming the validity of the independent pore model. The application of the deconvolution procedure implies an adequate grid size evaluation (i.e., numbers of pores and relative pressures to be considered for the inversion, or kernel size), the fulfillment of the discret Picard condition, and the appropriate choice of the regularization parameter (L-curve criteria). Assuming cylindrical geometry for both porous materials, the same set of individual adsorption isotherms generated from molecular simulations can be used to construct the kernel to obtain the PSD of SBA-15 and PHTS. The PSD robustness is measured imposing random errors over the experimental data. Excellent agreement is found between the calculated and the experimental global adsorption isotherms for both materials. Molecular simulations provide new insights into the studied systems, pointing out the need of high-resolution isotherms to describe the presence of complementary microporosity in these materials.     

Different sites of insertion in the reaction of isocyanates with [Re(N(R)Ar)(CO)3(bipy)] (R = H or Me): N-H vs. Re-N

The reactions of isocyanates with [Re(N(R)Ar)(CO)3(bipy)] complexes lead to R'NCO insertion into the Re-N bond (for R = Me) or the N-H bond (R = H).     

Oxamato-bridged trinuclear NiIICuIINiII complexes: a new (NiIICuIINiII)2 hexanuclear complex and supramolecular structures. Characterization and magnetic properties

Eight oxamato-bridged heterotrinuclear Ni(II)Cu(II)Ni(II) complexes of formula ([Ni(H(2)O)(dpt)](2)(mu-Cu(H(2)O)(opba)))(ClO(4))2 (1), ([Ni(H(2)O)(dien)](2)(mu-Cu(pba)))(ClO(4))(2).6H(2)O (2), ([Ni(H(2)O)(Medpt)](2)(mu-Cu(OHpba)))(ClO(4))(2).4H(2)O (3), ([Ni(H(2)O)(dien)](2)(mu-Cu(Me(2)pba)))(ClO(4))(2).2.5H(2)O (4), ([Ni(H(2)O)(dpt)](2)(mu-Cu(Me(2)pba)))(ClO(4))(2).2H(2)O (5), ([Ni(H(2)O)(dien)](2)(mu-Cu(OHpba)))(ClO(4))(2).4H(2)O (6), ([Ni(2)(dpt)(2)(mu-Cu(H(2)O)(pba))](2)(mu-N(3))(2))Na(2)(ClO(4))(4).6H(2)O (7), and ([Cu(H(2)O)(2)(dpt)Ni(2)(H(2)O)(dpt)(2)](mu-H(2)Me(2)pba(2-)))(ClO(4))(4).3H(2)O (8) in which opba = o-phenylenbis(oxamato), pba = 1,3-propylenebis(oxamato), OHpba = 2-hydroxy-1,3-propylenebis(oxamato), Me(2)pba = 2,2-dimethyl-1,3-propylenbis(oxamato), dpt = 3,3'-diaminodipropylamine, dien = 2,2'-diaminodiethylamine, and Medpt = 3,3'-diamino-N-methyldipropylamine were synthesized and characterized. The crystal structures of 1, 7, and 8 were solved. For complex 1, the trinuclear entities are linked by hydrogen bonds forming a one-dimensional system, and for complex 8, the presence of van der Waals interactions gives a one-dimensional system, too. For complex 7, the trinuclear entities are self-assembled by azido ligands, given a hexanuclear system; each of these hexanuclear entities are self-assembled through two [Na(O)(3)(H(2)O)(3)] octahedral-sharing one-edge entities, given a one-dimensional system. The magnetic behavior of complexes 2-7 was investigated by variable-temperature magnetic susceptibility measurements. Complexes 2-6 exhibit the minimum characteristic of this kind of polymetallic species with an irregular spin state structure. The Jvalue through the oxamato bridge varied between -88 cm(-1) (for 6) and -111.2 cm(-1) (for 5). For complex 7, the values obtained were J(1) = -101.7 cm(-1) (through the oxamato ligand) and J(2) = -3.2 cm(-1) (through the azido ligand).     

Solid-phase extraction of soy isoflavones

An automated method using solid-phase extraction (SPE) for the concentration and clean-up of soy isoflavone extracts is proposed in this work. Using a standardized sample (0.1 g of a freeze dried soybean extract/25 mL of water); eight SPE cartridges with a wide range of sorbents (C18, divinylbenzene and modified divinylbenzene) from different suppliers were evaluated and compared. A large variation on SPE cartridges performance was observed, especially regarding retention and breakthrough volume of isoflavones during sample load and washing steps. The most effective cartridges were the divinylbenzene based cartridges, especially Strata X (from Phenomenex) and HLB oasis (from Waters). Using Strata X cartridges, several extraction parameters, such as sample loading flow (5-15 mL min(-1)), extracting solvent volume (2-6 mL of methanol), pH of the extracting solvent and the necessity of drying the sorbent before elution, were evaluated to provide a fast, specific, quantitative and reproducible SPE method. The optimized method consists of conditioning the cartridge with 10 mL of methanol and 10 mL of water (10 mL min(-1)), loading 25 mL of the standardized extract onto the cartridges (5 mL min(-1)), washing the cartridge with 10 mL of water (10 mL min(-1)) and finally eluting with 4 mL of methanol (10 mL min(-1)). Mean isoflavones recovery was 99.37% and mean intra- and inter-day reproducibility was higher than 98%. The developed sample clean-up/concentration (6.25:1) method takes less than 10 min and can be used in the analysis of isoflavones from soy extracts.