A collision theory-based derivation of semiempirical equations for modeling dispersive kinetics and their application to a mixed-phase crystal decomposition

In recent works, the author has shown the utility of new, semiempirical kinetic model equations for treating dispersive chemical processes ranging from slow (minute/hour time scale) solid-state phase transformations to ultrafast (femtosecond) reactions in the gas phase. These two fundamental models (one for homogeneous/deceleratory sigmoidal conversion kinetics and the other for heterogeneous/acceleratory sigmoidal kinetics; isothermal conditions), based on the assumption of a "Maxwell-Boltzmann-like" distribution of molecular activation energies, provide a novel, quantum-based interpretation of the kinetics. As an extension to previous work, it is shown here that the derivation of these dispersive kinetic equations is supported by classical collision theory (i.e., for gas-phase applications). Furthermore, the successful application of the approach to the kinetic modeling of the solid-state decomposition of a binary system, CO2.C2H2, is demonstrated. Finally, the models derived appear to explain some of the (solid-state) kinetic data collected using isoconversional techniques such as those often reported in the thermal analysis literature.     

Application of the Šesták-Berggren Equation to Organic and Inorganic Materials of Practical Interest

Reactions that have an initial acceleratory period are common in both organic and inorganic systems. The Šesták-Berggren equation, dx/dt= -kx ⁿ(1-x)^m[-ln(x)]^p, with p set to zero (also called the extended Prout-Tompkins (PT) equation) is an excellent empirical kinetic law for many of these systems. In this work, it is shown to fit both isothermal and constant heating rate pyrolysis data for a well-preserved algal kerogen in a petroleum source rock and two synthetic polymers (polycarbonate and poly-ether-etherketone), dehydration of calcium oxalate monohydrate, decomposition of ammonium percholorate, and diffusive release of gas implanted in materials. Activation energies derived by non-linear regression to multiple experiments are consistent with those derived by simple isoconversional methods. Errors caused by misapplication of first-order kinetics to single-heating-rate data are discussed briefly. This revised version was published online in August 2006 with corrections to the Cover Date.     

Semiempirical equations for modeling solid-state kinetics based on a Maxwell-Boltzmann distribution of activation energies: applications to a polymorphic transformation under crystallization slurry conditions and to the thermal decomposition of AgMnO4 crystals

Many solid-state reactions and phase transformations performed under isothermal conditions give rise to asymmetric, sigmoidally shaped conversion-time (x-t) profiles. The mathematical treatment of such curves, as well as their physical interpretation, is often challenging. In this work, the functional form of a Maxwell-Boltzmann (M-B) distribution is used to describe the distribution of activation energies for the reagent solids, which, when coupled with an integrated first-order rate expression, yields a novel semiempirical equation that may offer better success in the modeling of solid-state kinetics. In this approach, the Arrhenius equation is used to relate the distribution of activation energies to a corresponding distribution of rate constants for the individual molecules in the reagent solids. This distribution of molecular rate constants is then correlated to the (observable) reaction time in the derivation of the model equation. In addition to providing a versatile treatment for asymmetric, sigmoidal reaction curves, another key advantage of our equation over other models is that the start time of conversion is uniquely defined at t = 0. We demonstrate the ability of our simple, two-parameter equation to successfully model the experimental x-t data for the polymorphic transformation of a pharmaceutical compound under crystallization slurry (i.e., heterogeneous) conditions. Additionally, we use a modification of this equation to model the kinetics of a historically significant, homogeneous solid-state reaction: the thermal decomposition of AgMnO4 crystals. The potential broad applicability of our statistical (i.e., dispersive) kinetic approach makes it a potentially attractive alternative to existing models/approaches.     

Dipyridinocalixcrown/diiodoperfluorocarbon binary host systems for CsI: structural studies and fluorous phase extraction of caesium

A mixture of partial cone 2,4-bis[(2-pyridylmethyl)oxy]-(1,3)-p-tert-butylcalix[4]crown-6, hexadecafluoro-1,8-diiodooctane and CsI in EtOH produces a crystalline ‘supramolecular salt’. In the solid state the caesium ion of the ‘supercation’ is encapsulated inside the cavity created by the crown ether loop, the picolyl and the inverted phenyl moieties, while the two iodide ions of the ‘superanion’ form a discrete five-component aggregate held together by co-existing hydrogen and halogen bonds. Similar calix/CsI/iodofluorocarbon adducts are shown to exist also in solution. In a prototypical study, a combination of this calixcrown and liquid octafluoro-1,4-diiodobutane acts as an effective ‘binary host’ system for the selective extraction of CsI from aqueous to fluorous phase.     

Analysis of thermal reactions of photochromic species in glassy matrices based on the concept of dispersive processes

Thermal decoloration reactions of photochromic spiroindolinonaphthoxazine in glassy polymer matrices have been studied. The non-exponential kinetic behaviour in these systems was analyzed using the phenomenological equation with an exp(-t^α) dependency describing dispersive processes in glassy solids. The dispersive parameter α represents the molecular environmental characteristics of matrix polymers.     

Synthesis of chiral β-3-amino-2-hydroxyalkanoates and 3-alkyl-3-hydroxy-β-lactams by double asymmetric induction

Reactions of chiral (2S)-enolates of dioxolan-4-ones, derived from lactic, mandelic, and phenyllactic acids, with aliphatic (S_S)- and (S_R)-tert-butylsulfinyl aldimines afforded conformationally restrained C2-disubstituted N,O-orthogonally protected 3-amino-2-hydroxyalkanoates in the form of N-sulfinyl protected 1′-aminodioxolan-4-ones. The product distribution showed that there is significant kinetic selectivity, due to the presence of ‘matched’ and ‘mismatched’ components, between the (S)- or (R)-tert-butylsulfinyl aldimines and the (2S)-enolates of the 1,3-dioxolan-4-ones. Selective methoxide-induced removal of the acetal group of the N-sulfinyl-1′-aminodioxolanones yielded the corresponding N-sulfinyl protected methyl alkanoates. In addition, the selective acid-induced removal of the sulfinyl group of the N-sulfinyl-1′-aminodioxolanones provided the corresponding N-unprotected 1′-aminodioxolanones, whose base-induced cyclization afforded the corresponding β-lactams.     

Synthesis of cyclic peptides via O-N-acyl migration

We describe here a novel and convenient synthesis of head-to-tail cyclic peptide avoiding racemization. Linear depsipeptides including a serine residue as the key element for ester bond formation and acyl transfer were synthesized on 2-chlorotrityl chloride resin. After cleavage from the resin, intramolecular head-to-tail cyclization was performed in solution by C-terminal activation of urethane protected O-acyl serine residue. After removal of the N^α-serine protecting group, the final step consisted in O-N-acyl migration reaction on the ‘switch’ or ‘click’ element to restore native cyclic peptides.     

Use of UV-visible spectroscopy to monitor nitrocellulose degradation in thin films

Activation energies for nitrocellulose (NC) degradation have been determined from Arrhenius plots constructed using first-order rate constants measured at 40, 50 and 60 °C. The rate constants were obtained by monitoring the absorbance (A) at a wavelength in the visible region of an anthraquinone dye dispersed in NC thin films. The dye acts as a stabilizer and is slowly depleted as a result of its reaction with NO_x from the breakdown of the nitrate ester groups on NC. The data produced two linear regions in the first-order plots of ln(A₀/A_t) vs aging time. The first-region is attributed to the reaction of the dye with NO_x desorbed from the NC surface. The activation energy (∼73.5 kJ mol⁻¹) is in line with that found for NO_x surface desorption processes. The second linear region is thought to be due to the reaction of NO_x from the breakdown of the nitrate ester groups on the NC molecule. The activation energy (∼104.0 kJ mol⁻¹) is consistent with that for nitrate ester hydrolysis. The use of UV-visible spectroscopy has in this way made it possible to monitor the degradation of NC non-destructively without the need for stabilizer extraction and analysis.     

New oxypnictide superconductors: PrOFe1−xCoxAs

Oxypnictides of the type PrOFe_1−xCo_xAs (x≤0.3) were synthesized for the first time by the sealed tube method. All the compounds were found to be monophasic and crystallize in the tetragonal ZrCuSiAs type structure (space group=P4/nmm) and the lattice parameters (a and c) decrease with increase in cobalt content. Mössbauer measurements of the compounds indicate low spin Fe²⁺ in tetrahedral coordination. Resistivity and magnetization studies reveal superconducting transitions in compounds with ‘x’=0.05, 0.10 and 0.15, with maximum transition temperature (T_c) at ∼14 K in the compound with ‘x’=0.1. The variation of resistivity with temperature under different magnetic field has been studied to estimate the upper critical field (H_c2) (∼50.2 T for the ‘x’=0.1 composition). The Seebeck and Hall coefficient (R_H) suggests electron type charge carriers in these compound and the charge carrier density increases with increase in Co-doping.     

Chemical and electrocatalytic cascade cyclization of salicylaldehyde with three molecules of malononitrile: ‘one-pot’ simple and efficient way to the chromeno[2,3-b]pyridine scaffold

А new type of catalytic cascade cyclization was found: the direct ‘one-pot’ simple and efficient transformation of salicylaldehyde and three molecules of malononitrile into the chromeno[2,3-b]pyridine systems. Chemical and electrochemical methods result in direct ‘one-pot’ formation of chromeno[2,3-b]pyridines in 60–90% yields. The implication of electrocatalysis in complex cascade cyclization reaction is an efficient approach to medicinally relevant chromeno[2,3-b]pyridines—the promising small-molecule ligands with pronounced antihistamic, antirheumatic and antiasthmatic activities. The electrocatalytic method is beneficial from the viewpoint of diversity-oriented large-scale processes and represents fast efficient and environmentally benign synthetic concept for cascade reactions strategy.     

X-ray and neutron powder diffraction studies of Ba(NdxY2−x)CuO5

Ba(R,R′)₂CuO₅ (R,R′=lanthanides and Y) plays an important role as a flux-pinning agent in enhancing the superconducting properties of the Ba₂(R,R′)Cu₃O_6+x (R,R′=lanthanides and Y) coated conductors. Using X-ray diffraction and neutron diffraction, we found that the Ba(Nd_xY_2−x)CuO₅ solid solution adopts two structure types. In the Nd-rich region (1.8?x?2.0), the materials are of brown color (commonly referred to as the ‘brown phase’), and the structure is tetragonal with space group I4/mbm (no. 127). In the Y-rich region (0.0?x?1.4), the materials are green (commonly referred to as the ‘green phase’) and the structure is orthorhombic with space group Pnma (no. 62). A two-phase region (1.4<x<1.8) exists between the orthorhombic and tetragonal phases. The crystal chemistry and crystallography of the orthorhombic ‘green phase’ series, Ba(Nd_xY_2−x)CuO₅ (isostructural to BaY₂CuO₅), are discussed in this paper.     

A theoretical and experimental study of the racemization process of hexaaza[5]helicenes

A dynamic ¹H NMR study, together with DFT calculations, of bis-([1,2,3]triazolo)[1,5-a:5′,1′-k][1,10]phenanthroline 2 has allowed to identify the ring and open forms of a new example of ring/chain tautomerism, as well as their interconversion barriers (ring/ring and ring/open). The barrier of the exchange process between the chain forms and the ring form was found higher than the ‘racemization’ process in the closed form, so the ring opening does not contribute to the ‘racemization’. The di-1,10-methyl and di-1,10-iodo derivatives have been prepared and their properties calculated.     

Synthesis and biological evaluation of anthranilamide-based non-peptide mimetics of ω-conotoxin GVIA

Non-peptide mimetics based on an anthranilamide ‘scaffold’ possessing fragments that mimic Lys2, Tyr13 and Arg17 in ω-conotoxin GVIA have been prepared. Compounds were assayed for binding to the voltage-gated calcium channels Ca_v2.2 (‘N-type’) and Ca_v2.1 (‘P/Q-type’) in rat brain. The primary synthetic target, 2-(6-amino-hexanoylamino)-5-(3-guanidino-propoxy)-N-[4-(4-hydroxyphenoxy)-phenyl]-benzamide (2a), exhibited low μM binding to Ca_v2.2 and was more than 30-fold selective for Ca_v2.2 over Ca_v2.1.     

Asymmetric aziridine synthesis by aza-Darzens reaction of N-diphenylphosphinylimines with chiral enolates. Part 1: Formation of cis-aziridines

The aza-Darzens (‘ADZ’) reactions of N-diphenylphosphinyl (‘N-Dpp’) imines with chiral enolates derived from oxazolidinones and camphorsultam have been studied. Whilst oxazolidinone enolates reacted poorly in terms of aziridination, the use of the chiral enolate derived from both antipodes of N-bromoacetyl 2,10-camphorsultam, 2R-(5) and 2S-(5), with N-diphenylphosphinyl aryl and tert-butylimines proceeded in generally good yield to give, respectively, (2′R,3′R)- or (2′S,3′S)-cis-N-diphenylphosphinyl aziridinoyl sultams of high de.     

Gas-sensing properties of Fe2−xTixO3+γ (x = 0–1.4)

As part of a systematic study of mechanisms of response of semiconducting oxides as trace gas sensors, we have explored the behaviour of iron–titanium oxide solid solutions Fe_2−xTi_xO₃ (x = 0.1–1.4). The materials were single-phase for x = 0.1 with increasing proportions of a pseudobrookite second phase at higher degree of substitution. Unmodified, pure iron oxide does not show sensitivity to CO. A significant signal was developed for x = 0.1, that then diminished with increasing x and was lost for x = 1.4. Three effects have been deduced important for the gas response: significant surface segregation of Ti at low Ti content; grain growth inhibition and agglomeration into more massive, non-porous lumps as Ti content increased; and the appearance of a band-gap state associated with Fe(II) at higher Ti content. The effects of microstructure change have been analyzed by fitting the data to a simple 2-resistor model of gas-insensitive ‘grains’ in series with gas-sensitive ‘grain boundaries’. A Mars–van Krevelen type model for the response is presented, based on reactions at surface-segregated defect clusters, to develop and remove electrically-active surface trap states.     

Regiodivergent Baeyer-Villiger oxidation of fused ketone substrates by recombinant whole-cells expressing two monooxygenases from Brevibacterium

Microbial Baeyer-Villiger oxidations of fused bicyclic ketones with a cyclobutanone structural motif were investigated using recombinant Escherichia coli cells expressing two monooxygenases from Brevibacterium. In a kinetic resolution process fused ketones were transformed to regioisomeric lactones: ‘normal’ lactones were generated by migration of the more substituted carbon atom and ‘abnormal’ lactones resulted from migration of the less substituted carbon atom. The two Baeyer-Villigerases demonstrated a significantly different stereoselectivity for the regiodivergent biotransformation.     

Double asymmetric induction as a mechanistic probe: the doubly diastereoselective conjugate addition of enantiopure lithium amides to enantiopure α,β-unsaturated esters and enantiopure α,β-unsaturated hydroxamates

The doubly diastereoselective conjugate addition of the antipodes of lithium N-benzyl-N-(α-methylbenzyl)amide to a range of enantiopure α,β-unsaturated esters [derived from Corey’s 8-phenylmenthol chiral auxiliary] and enantiopure α,β-unsaturated hydroxamates [derived from our ‘chiral Weinreb amide’ auxiliary (S)-N-1-(1′-naphthyl)ethyl-O-tert-butylhydroxylamine] has been used as a mechanistic probe to determine the reactive conformations of these acceptors.     

Improved solid-phase peptide synthesis of ‘difficult peptides’ by altering the microenvironment of the developing sequence

Three types of resins, related to the spacer, environmental and microenvironmental models were prepared by grafting commercial AMP polymer with 2-[2-(2-aminoethylamino)-2-oxoethoxy]acetic acid. All resins were highly loaded and functionalized with Rink-amide linker. A comparative synthesis of the classic difficult sequence ACP (65-74) on the prepared resins by Fmoc/t-Bu chemistry is presented. The ‘microenviromental’ model resin afforded the crude peptide in the highest purity (98%).