Polyetherimides via nitro-displacement polymerization: Monomer synthesis and 13C-NMR analysis of monomers and polymers

The synthetic route to polyetherimides by displacement of nitro-groups from disubstituted bis-imides by the dianion of bisphenols are described. The prepration of bis-imides and bisphenol dianions, their polymerization, and some of the properties of the polymers are presented in detail. The ¹³C-NMR spectra of intermediates and products were used to establish compositions and determine molecular weights.     

Pyrolysis of trifluoroacetaldehyde. The formation of trifluoromethane and hexafluoroethane

The thermal decomposition of trifluoroacetaldehyde at temperatures f 471 to 519°C has been studied by measuring the rates of formation of CF₃H and C₂F₆. It is concluded that the high-pressure reaction mechanism involves a Rice-Herzfeld reaction scheme with first-order initiation and second-order termination via CF₃ combination. However, a falloff in reaction rates is observed at pressures below 100 mmHg. The Arrhenius parameters of the three rate constants corresponding to the overall reaction, the initiation reaction, and an abstraction reaction have been evaluated.     

Monte Carlo method for computing density of states and quench probability of potential energy and enthalpy landscapes

The thermodynamics and kinetics of a many-body system can be described in terms of a potential energy landscape in multidimensional configuration space. The partition function of such a landscape can be written in terms of a density of states, which can be computed using a variety of Monte Carlo techniques. In this paper, a new self-consistent Monte Carlo method for computing density of states is described that uses importance sampling and a multiplicative update factor to achieve rapid convergence. The technique is then applied to compute the equilibrium quench probability of the various inherent structures (minima) in the landscape. The quench probability depends on both the potential energy of the inherent structure and the volume of its corresponding basin in configuration space. Finally, the methodology is extended to the isothermal-isobaric ensemble in order to compute inherent structure quench probabilities in an enthalpy landscape.     

Non-equilibrium entropy of glasses formed by continuous cooling

We propose a generalized definition of entropy accounting for the continuous breakdown of ergodicity at the laboratory glass transition. Our approach is applicable through all regimes of glass forming, from the equilibrium liquid state through the glass transition range and into the glassy state at low temperatures. The continuous loss of ergodicity during the laboratory glass transition is accompanied by a loss of entropy as the system gradually becomes trapped in a subset of the configurational phase space. Using a hierarchical master equation approach, we compute the configurational entropy of selenium, a simple but realistic glass-former, for cooling rates covering 25 orders of magnitude, viz., 10^?12 to 10¹² K/s. In all cases, the entropy of glass is zero in the limit of absolute zero temperature, since here the system is necessarily confined to a single microstate.     

A simplified eigenvector-following technique for locating transition points in an energy landscape

We derive an eigenvector-following technique for locating transition points in an N-dimensional energy landscape. A separate Lagrange multiplier is used for each eigendirection to provide maximum flexibility in determining step sizes. In contrast to previous techniques based on a similar approach, we provide a simple algorithm for choosing specific values of these Lagrange multipliers. We demonstrate the robustness of the algorithm using two-dimensional Cerjan-Miller and Adams landscapes. The technique has also been applied to the S(12) molecular cluster.