The impact of vertical π-extension on redox mechanisms of aromatic diimide dyes

The impact of vertical π-extension on redox mechanisms of aromatic diimides in the organic lithium-ion batteries have been carefully studied by a combined experiment and theoretical analyses.     

Crossing the phase boundary to study protein dynamics and function: combination of amide hydrogen exchange in solution and ion fragmentation in the gas phase

Protein dynamics are the key to understanding their behavior. The static protein structure alone in most cases is insufficient to describe the vast array of complex functions they perform in vivo. Until recently there were relatively few techniques available to investigate the dynamic nature of these proteins. Mass spectrometry has recently emerged as a powerful biophysical method, capable of providing both structural and dynamic information. By utilizing the labile nature of amide hydrogens as a marker of the backbone dynamics in solution, combined with gas-phase dissociation techniques, we now have a high-resolution tool to locate these exchanging hydrogens within the sequence of the protein and to probe the functional importance of its structural elements. In this paper we describe several applications of these methodologies to illustrate the importance of dynamics to the biological functions of proteins.     

Ring dehydration mechanisms and kinetics of polyamic acid models in solution and in the solid state

We have synthesized and studied the ring dehydration mechanisms and kinetics of polyamic acid models in solution and in the solid state using ¹³C-NMR (solid and liquid), HPLC, FTIR, and x-ray diffraction. Results obtained in solution show the role of temperature, catalysts, and the basicity of the amine in ring dehydration mechanisms and kinetics, as well as conformation and intramolecular bonds in the amic acid bond in the solid state. © 1993 John Wiley & Sons, Inc.     

Direct experimental observation of CS2OH.

The first experimental detection of CS(2)OH is reported. CS(2)OH was observed for about one microsecond after its formation, as an intact isolated species in the gas phase. It was generated by electron transfer to the CS(2)OH(+) ion, prepared in the source of a multisector mass spectrometer by suitable ion-molecule reactions. The vertical formation process allowed characterization of CS(2)OH by structural analysis of CS(2)OH(+). Theoretical calculations were performed at the B3LYP/6-311+G(2d,p) and CCSD(T)/aug-cc-pVTZ//B3LYP/6-311+G(2d,p) levels of theory. The computed structure and stability of CS(2)OH and CS(2)OH(+) as well as the energetics of the involved processes satisfactorily fit with the experimental results.     

Mechanism and kinetics of the reaction of OH radicals with glyoxal and methylglyoxal: a quantum chemistry + CVT/SCT approach.

A theoretical study of the mechanism and kinetics of the OH hydrogen abstraction from glyoxal and methylglyoxal is presented. Optimum geometries, frequencies, and gradients have been computed at the BHandHLYP/6-311++G(d,p) level of theory for all the stationary points, as well as for 12 additional points along the minimum energy path (MEP). Energies were obtained by single-point calculations at the above geometries using CCSD(T)/ 6-311++G(d,p) to produce the potential energy surface. The rate coefficients were calculated for the temperature range 200-500 K by using canonical variational theory (CVT) with small-curvature tunneling (SCT) corrections. Our analysis suggests a stepwise mechanism, which involves the formation of a reactant complex. The overall agreement between the calculated and experimental kinetic data is very good. This agreement supports the reliability of the Arrhenius parameters of the glyoxal + OH reaction that are proposed in this work for the first time. The Arrhenius expressions that best describe the studied reactions are k1 = (9.63 +/- 0.23) x l0(-13)exp[(517 +/- 7)/T] and k2 = (3.93 +/- 0.11) x 10(-13)exp[(1060 +/- 8)/T]cm3 molecule(-1)s(-1) for glyoxal and methylglyoxal, respectively.     

Structures and functions of reaction interfaces developed during solid-state dehydrations

The literature reveals that the mechanisms of some solid-state dehydrations are more complicated than has been generally accepted. Reactions at a thin advancing reactant-product interface provide the geometric models on which the most widely employed rate equations are based. For some systems, this thin interface model is a simplification of observed behaviour. Elimination of water from crystallographic sites may occur to a significant extent within a much thicker zone of reactant towards which the active interface is progressing. Consequently the region of chemical change may not coincide with the region of structural transformation. Limited initial dehydration may occur across all crystal faces prior to the onset of a nucleation and growth process that is usually regarded as the dominant rate process in the dehydrations of many large crystals. Experimental observations for solid-state dehydrations are discussed and reaction mechanisms with different rate controlling processes are distinguished. Studies of dehydrations have contributed substantially to the theory of solid-state reactivity, and advances in understanding may have wider application to other solid-state reactants.     

Competitive Reaction Pathways for the Gas‐Phase Reactivity of [Me2AlNH2]3

Reaction energy profiles for [Me₂AlNH₂]₃ have been computationally explored by using density functional theory. Both intra‐ and intermolecular methane elimination reactions, as well as Al?N bond‐breaking pathways, were considered. The results show that the energy required for Al?N bond breaking in cyclic [Me₂AlNH₂]₃ is of the same order of magnitude as the activation energies for the first (limiting) step of methane elimination (for both mono‐ and bimolecular mechanisms). Thus, dissociative and associative reaction pathways are competitive. Low‐temperature/high‐pressure conditions will favor the bimolecular pathway, whereas at high temperatures, either intramolecular methane elimination or Al?N bond‐breaking dissociative pathways will be operational.     

Calculations of Monomer Conversion and Radical Concentration in Reversible Addition‐Fragmentation Chain Transfer Radical Polymerization

Simple expressions are derived for the development of monomer conversion, as well as propagating radical, adduct radical, dormant chain, and dead chain concentrations in reverse addition‐fragmentation transfer polymerization (RAFT). The relations for the profiles of propagating radical concentration and conversion versus time are derived and depend on group parameters of rate constants and chemical recipe. The analytical equations are verified against numerical solutions of the mass‐balance differential equations. This derivation involves the steady‐state hypothesis for radical and RAFT agent concentrations. The errors introduced by these assumptions are negligible when the fragmentation rate constant, k_f, is higher than 10 s⁻¹ or when the cross‐termination rate constant, k_ct, is higher than 10⁵ L · mol⁻¹ s⁻¹.

Calculated concentration profiles (points: numerical, lines: analytical) of propagating radical R, adduct radical A, dormant T, and dead D (= P + P′) chains.