Continuum electrostatics for electronic structure calculations in bulk amorphous polymers: application to polylactide.

The bond rotational energy landscapes of polylactide (PLA) oligomers were estimated using electron density functional theory (DFT) at the B3LYP/6-31G** level, both in vacuo and with a self-consistent reaction field (SCRF) method to simulate the electronic environment within the condensed phase. The SCRF method was evaluated for application to polymeric systems, and we demonstrate the difficulties involved in applying the method to bulk amorphous polymers with specific attention to the selection of the solvent probe radius. In addition, rotational isomeric states (RIS) calculations were performed, showing the effect of accounting for the bulk phase reaction field on the bond rotational energetics and characteristic ratio. We conclude that present methods of accounting for bulk environments in electronic structure calculations are not well suited for use with polymeric systems, and the development of improved methods is needed in this area.

An improved replica-exchange sampling method: temperature intervals with global energy reassignment.

In a molecular dynamics (MD) simulation, representative sampling over the entire phase space is desired to obtain an accurate canonical distribution at a given temperature. For large molecules, such as proteins, this is problematic because systems tend to become trapped in local energy minima. The extensively used replica-exchange molecular dynamics (REMD) simulation technique overcomes this kinetic-trapping problem by allowing Boltzmann-weighted configuration exchange processes to occur between numerous thermally adjacent and compositionally identical simulations that are thermostated at sequentially higher temperatures. While the REMD method provides much better sampling than conventional MD, there are two substantial difficulties that are inherent in its application: (1) the large number of replicas that must be used to span a designated temperature range and (2) the subsequent long time required for configurations sampled at high temperatures to exchange down for potential inclusion within the low-temperature ensemble of interest. In this work, a new method based on temperature intervals with global energy reassignment (TIGER) is presented that overcomes both of these problems. A TIGER simulation is conducted as a series of short heating-sampling-quenching cycles. At the end of each cycle, the potential energies of all replicas are simultaneously compared at the same temperature using a Metropolis sampling method and then globally reassigned to the designated temperature levels. TIGER is compared with regular MD and REMD methods for the alanine dipeptide in water. The results indicate that TIGER increases sampling efficiency while substantially reducing the number of central processing units required for a comparable conventional REMD simulation.

Supercritical water oxidation of the PCB congener 2-chlorobiphenyl in methanol solutions: a kinetic analysis.

Incineration is commonly used to destroy polychlorinated biphenyl (PCB) wastes, but this method of treatment is not ideal for all mixed liquid wastes, especially those containing radioactive materials. Therefore, other remediation technologies are needed to efficiently treat these waste forms. This study examined the supercritical water oxidation (SCWO) of 2-chlorobiphenyl (2-PCB), using hydrogen peroxide as the oxidant and methanol as a cosolvent at 2 vol %. Kinetic studies were carried out in a plug flow reactor at temperatures from 686 to 789 K and a pressure of 250 bar, with reactor residence times ranging from 1.1 to 5.8 s. Least-squares regression of the collective reaction rate data revealed that 2-PCB exhibited second-order kinetics, with an Arrhenius frequency factor, A, equal to 10(18.2+/-2.3) L x mol(-1) x s(-1) and activation energy, Ea, of 181.7 +/- 33.2 kJ/mol. The primary organic reaction products from the SCWO of 2-PCB were biphenyl at low temperatures (<700 K) and CO2 at elevated temperatures. No dioxins or chlorinated dibenzofurans were ever detected in any of the effluent samples. Additional experiments with higher organic feed concentrations (4 vol %) illustrated how the exothermic nature of the organic oxidation reactions can be used to render the process self-sustaining. Finally, SCWO destruction rates greater than 99.98% were achieved for a simulated PCB-contaminated job control waste similar to that encountered at many U.S. Department of Energy sites.