DFT M06-2X investigation of alkaline hydrolysis of nitroaromatic compounds.

The nitroaromatic compounds 2,4,6-trinitrotoluene (TNT), 2,4-dinitrotoluene (DNT) and 2,4-dinitroanisole (DNAN) are potential environmental contaminants and their transformations under a variety of environmental conditions are consequently of great interest. One possible method to safely degrade these nitrocompounds is alkaline hydrolysis. A mechanism of the initial stages of this reaction was investigated computationally. Simulations of UV-VIS and NMR spectra for this mechanism were also produced. The results obtained were compared to available experimental data on the alkaline hydrolysis of TNT and suggest that the formation of Meisenheimer complexes and an anion of TNT are potential first-step intermediates in the reaction path. As the reaction proceeds, computational results indicate that polynegative complexes dominate the degradation pathway, followed by cycles of carbon chain opening and breaking. A second possible pathway was identified that leads to polymeric products through Janovsky complex formation. Results from this study indicate that the order of increasing resistance to alkaline hydrolysis is TNT, DNT and DNAN.

Kinetics of charge separation in poly(A)-poly(T) DNA hairpins.

A kinetics model is designed to investigate the charge separation (CT) process in stilbene-capped DNA hairpins composed of AT base pairs. This model combines standard tunneling and hopping electron transport with exciplex formation upon photoexcitation of the acceptor stilbene and its neighboring adenine and is capable of interpreting the CT rate and yield data within experimental accuracy. An analysis of hopping transport within the framework of a 1-D diffusion model results in a calculation of the nearest-neighbor CT rate to be approximately 1.2 ns(-1). In agreement with previous experimental and theoretical work, it is ascertained through a novel application of an extension to classical Marcus theory that the nearest-neighbor CT is adiabatic with reorganization energy lambda approximately 0.83 eV. The kinetics model can be extended to accurately characterize CT in other poly(A)-poly(T) systems with different hole donors (naphthaldiimide and 2-aminopurine) and acceptors (phenothiazine and guanine).

Interference of guided modes in a two-port ring waveguide composed of dielectric nanoparticles.

The interference is considered of guided polariton modes in a two-port ring waveguide composed of dielectric nanospheres. The dependence of the guided polariton intensity on the relative orientation of the input and output channels is investigated. It is shown that, if the frequency of the external light source corresponds to one of the resonant modes of the waveguide ring segment, the guided polariton may be treated as two optical beams running along the ring segment in the opposite directions and interfering with each other. The multisphere Mie scattering formalism is used. The dipole approximation is shown to grasp the essence of physics. Our simple interpretation of the results is obtained in terms of scalar waves. The applications of the interference revealed in the manuscript are discussed as well.

Charge recombination in DNA hairpins controlled by counterions.

The slow charge recombination in DNA hairpins of various lengths is investigated. The distance dependence for the charge recombination rate between stilbene donor (Sd(+)) and stilbene acceptor (Sa(-)) linkers separated by AT bridges of various lengths has the double exponential form. We suggest that this puzzling dependence is associated with two tunneling channels distinguished by the presence or absence of a Cl(-) counterion bound to an Sd(+) linker. When Cl(-) is associated with Sd(+) the energy of positive charge is lower compared to Sd(+) without a counterion. Therefore the potential barrier for charge recombination is higher in the presence of Cl(-) bound to Sd(+), and the recombination is characterized by the larger tunneling exponent than the tunneling exponent in the absence of counterions. Experiment-based estimates of binding parameters agree within reasonable expectations. The control experiment replacing the Cl(-) ion with other ions is suggested. This replacement enables the control of the charge recombination rate modifying it in either direction by orders of magnitude.

Guiding optical modes in chains of dielectric particles.

We have investigated low frequency guiding polariton modes in finite linear chains of closely packed dielectric spherical particles of different optical materials. These guiding (chain bound) modes cannot decay radiatively, because photon emission cannot take place with simultaneous conservation of energy and momentum. For extending previous work on infinite chains of spherical particles [1] and infinite rods [2, 3], we were able to apply the multisphere Mie scattering formalism to finite chains of dielectric particles to calculate quality factors of most bound modes originating from the first two Mie resonances depending on the number of particles N and the material's refractive index nr. We found that, in agreement with the earlier work [4], guiding modes exist for n(r) > 2 and the quality factor of the most bound mode scales by N(3). We interpreted this behavior as the property of "frozen" modes near the edges of guiding bands with group velocity vanishing as N increases. In contrast with circular arrays, longitudinal guiding modes in particle chains possess a higher quality factor compared to the transverse ones.