Spiroconjugation in 1-, 2-, and 3-Dimensions: The Foundations of a Spiro Quantum Chemistry

A recent paper has described the structure of a hypothetical 3-,4-connected net termed glitter. This is a model of an allotrope of carbon in the form of a synthetic metal. That paper pointed to the importance of through-space p interactions of adjacent olefin units in the net in understanding the electronic structure at the Fermi level. The present communication elucidates the role of spiroconjugation in understanding features of the electronic band structure and density of states of glitter. With this analysis of spiroconjugation in the 1-dimensional polyspiroquinoid polymer and the 3-dimensional glitter lattice, the foundations have been laid for a new type of quantum chemistry herein called spiro quantum chemistry. Spiro quantum chemistry complements traditional quantum chemistry which is focused on linear polyenes, circular annulenes, polyhexes, 2-dimensional graphene sheets and related structures including fullerenes, by focusing on spiroconjugated hydrocarbon structures in 1-, 2- and 3-D, including linear spiro[n]quinoids and polyspiroquinoid in 1-D, circular cyclospiro[n]quinoids, spiro[m,n]graphene fragments and spirographene in 2-D and [m,n,o]glitter fragments and glitter in 3-D.     

Selective Oxygen Transfers with Iron(III) Porphyrin Nitrite

The reaction of octaethylporphyrin iron(III) chloride with potassium crown ether (18-crown-6) nitrite in N-methylpyrrolidone-1% acetic acid under argon generates the iron(III) nitrite salt (PFeNO(2)). The latter is a unique and selective oxygen atom transfer reagent. The reaction of a broad range of substrates (S) proceeds quantitatively to yield the oxidized substrate and the iron(II) porphyrin-nitrosyl adduct: PFeNO(2) + S --> PFeNO + SO. Diatomic molecules to which oxygen is directly transferred from PFeNO(2) are NO, CO, and O(2). The conversion NO to NO(2) is shown via (15)NO(2)(-) labeling experiments to proceed exclusively by the O atom transfer process. The ozone, generated from dioxygen, was trapped with nitrite ion and the two olefins 2-methyl-2 butene and 2,3-dimethyl-2 butene. These substances are inert to PFeNO(2) under argon. However, in an oxygen-saturated reaction mixture, nitrite produced nitrate. The olefins, following reduction of the reaction mixture with Zn/HOAc, yielded 1 mol of acetone and acetaldehyde and 2 mol of acetone, respectively. Other simple O atom transfers under argon were observed with dimethyl sulfide and triphenylphosphine. The PFeNO(2) reagent shows a preference for O insertion into allylic, benzylic, and aldehydic C-H bonds. Thus, no olefin containing these moieties is epoxidized. However, styrene and cis-stilbene are converted to styrene oxide and cis-stilbene oxide, respectively. The double oxidation of allylbenzene to trans-cinnamaldehyde entails an allylic rearrangement that suggests radical character to the O insertion process. However, no kinetic evidence for this was obtained. The reaction is an overall third-order process, rate = k(PFe(III))(NO(2)(-))(S). There was no correlation of observed rates with relevant C-H bond dissociation energies of substrates. The fastest reacting substrate was nitric oxide (k(22) degrees = 52 M(-)(2) s(-)(1)) and the slowest was toluene (k(50) degrees = 6.3 x 10(-)(4) M(-)(2) s(-)(1)). The range and selectivity of these O atom transfers sets them apart from the catalytic oxidations brought about by reactions of iron(III) porphyrins with peroxides, iodosoaryls, hypochlorite, and other oxidants. The driving force for the relatively mild oxidations with PFeNO(2) resides in the thermodynamic stability of the heme-NO adduct. Given the broad presence of nitrite in the environment and the ubiquity of porphyrins in the biosphere, the activation of nitrite by iron porphyrins has both an environmental and biochemical significance.     

Zeta-potential measurements as a tool to quantify the effect of charged drugs on the surface potential of egg phosphatidylcholine liposomes

The binding of charged drugs to neutral phosphatidylcholine membranes was assessed by measuring their zeta-potential values in the presence of different drug concentrations. This methodology was applied to the study of the concentration effects of two nonsteroidal antiinflammatory drugs (NSAIDs). Results revealed an intense membrane charging that was proportional to the amount of negatively charged drug in the media. A mathematical formalism was adapted and an analytical expression derived to calculate directly surface potentials from zeta-potential data. The membrane loading state, expressed as the number of molecules per unit area, was calculated for the negative and for the neutral forms of the drugs. An approach was also developed that allows the determination of the maximum number of molecules per unit area by fitting a binding isotherm to the dependence of the number molecules per unit area with the drug concentration. The calculation of the maximum mol lipid/drug ratio can also be estimated and related to the binding stoichiometry, as well as to the maximum lipid loading capacity. Furthermore, the concentration profiles for both drugs can be established in terms of the distance to the liposome surface. The developed methodology allowed for the simultaneous determination of partition coefficients (Kp) for the NSAIDs in lipid/aqueous media because zeta-potential values can be related to the drug concentration at the lipid/ aqueous media interface. Alternative independent methodologies were used to determine Kp: spectrophotometric and centrifugation assays. A mathematical relation was developed to compare the Kp values determined from the zeta-potential data with those obtained from the other techniques used because in the former case they are calculated on the basis of the number of molecules per unit area and in the latter on the basis of the total drug concentrations in solution, and the values of the partition coefficients obtained from all the techniques were found to be equal, within the experimental error. This methodology constitutes a more straightforward method than the other techniques used because partition coefficients for all drug forms (charged and noncharged) can be assessed with a minimum number of experimental determinations and it allows for a characterization of the electrostatic properties of neutral membranes upon binding of charged drugs.     

Direct introduction of solid samples into continuous-flow systems by use of ultrasonic irradiation

A new approach to the on-line treatment of solid samples based on the use of flow-injection analysis and acceleration of the partial dissolution (leaching) of the sample by using ultrasonic irradiation is proposed. Selection of a suitable leaching agent makes possible the solubilization of the component(s) of interest. This approach was applied to the determination of iron in plant material by acid dissolution followed by complex formation with 1,10-phenanthro- line. The results obtained are in agreement with those found by the conventional method using atomic absorption spectrometry after hot-acid treatment.     

Kinetics of ion-pair extraction in continuous flow systems

Kinetic aspects of ion-pair extraction in continuous-flow systems have been studied. Processes have been developed by using a new methodology based on reversal of the flow direction for a preselected number of cycles and on contiuous monitoring at the detector of a plug of organic phase inserted into a carrier stream (aqueous phase) by means of a conventional injection valve. The configuration used required none of the units typically associated with this separation technique. The information obtained, with clear kinetic connotations, can also be exploited for analytical purposes. The behaviour of different ion pairs formed between ferroin and 23 anionic counter ions has been studied. Individual and joint kinetic determinations of these anions from kinetic parameters are proposed.     

Thermal conductance measurements of superconducting bi-2212 rods and a bi-2212-based current leadmodule

We measured from 5 to 300 K the thermal conductance of different superconducting Bi-2212 thin rods and one superconducting module integrating a hybrid current lead based on such rods. The conductance values of the rods are lower than the measurable limits of the set-up and a detailed evaluation of the heat losses is required. The analysis of the heat exchanges permits to introduce a correction function to the measured values and allows to study low thermal conductivity materials. The dependence of the thermal conductivity of Bi-2212 rods on the sample microstructure is analyzed after this correction. We calculate analytically the time required to reach the steady-state for long bars for which extended relaxation times are expected. From the measured conductance data we estimate the heat load introduced by the current lead into a cryogenic bath in static conditions.     

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).     

Kinetics and mechanism of the aminolysis of 4-methylphenyl and 4-chlorophenyl 4-nitrophenyl carbonates in aqueous ethanol

Reactions of 4-methylphenyl 4-nitrophenyl carbonate (MPNPC) and 4-chlorophenyl 4-nitrophenyl carbonate (ClPNPC) with a series of quinuclidines (QUIN) and the latter carbonate with a series of secondary alicyclic amines (SAA) are subjected to a kinetic investigation in 44 wt % ethanol-water, at 25.0 degrees C and an ionic strength of 0.2 M. The reactions were followed spectrophotometrically at 330 or 400 nm (4-nitrophenol or 4-nitrophenoxide anion appearance, respectively). Under excess amine, pseudo-first-order rate coefficients (k(obsd)) are found. For all these reactions, plots of k(obsd) vs free amine concentration at constant pH are linear, the slope (k(N)) being independent of pH. The Br?nsted-type plots (log k(N) vs pK(a) of the conjugate acids of the amines) for the reactions of the series of QUIN with MPNPC and ClPNPC are linear with slopes (beta(N)) 0.88 and 0.87, respectively, which are explained by a stepwise process where breakdown of a zwitterionic tetrahedral intermediate (T(+/-)) to products is rate limiting. The Br?nsted-type plot for the reactions of the series of SAA with ClPNPC is biphasic with slopes beta(1) = 0.2 (high pK(a) region) and beta(2) = 0.9 (low pK(a) region) and a curvature center at pK(a)(0) = 10.6. This plot is in accordance with a stepwise mechanism through T(+/-) and a change in the rate-determining step, from T(+/-) breakdown to T(+/-) formation as the basicity of the SAA increases. Two conclusions arise from these results: (i) QUIN are better leaving groups from T(+/-) than isobasic SAA, and (ii) the non-leaving group effect on k(N) for these reactions is small, since beta(nlg) ranges from -0.2 to - 0.3. From these values, it is deduced that ClPNPC is ca. 70% more reactive than MPNPC toward SAA and QUIN, when expulsion of the leaving group from T(+/-) is the rate determining step.