Heavy Ion Radiolysis of Liquid Pyridine

Prediction of Intramolecular Polarization of Aromatic Amino Acids Using Kriging Machine Learning

Present computing power enables novel ways of modeling polarization. Here we show that the machine learning method kriging accurately captures the way the electron density of a topological atom responds to a change in the positions of the surrounding atoms. The success of this method is demonstrated on the four aromatic amino acids histidine, phenylalanine, tryptophan, and tyrosine. A new technique of varying training set sizes to vastly reduce training times while maintaining accuracy is described and applied to each amino acid. Each amino acid has its geometry distorted via normal modes of vibration over all local energy minima in the Ramachandran map. These geometries are then used to train the kriging models. Total electrostatic energies predicted by the kriging models for previously unseen geometries are compared to the true energies, yielding mean absolute errors of 2.9, 5.1, 4.2, and 2.8 kJ mol(-1) for histidine, phenylalanine, tryptophan, and tyrosine, respectively.

Combinations of Aromatic and Aliphatic Radiolysis

The production of H(2) in the radiolysis of benzene, methylbenzene (toluene), ethylbenzene, butylbenzene, and hexylbenzene with γ-rays, 2-10 MeV protons, 5-20 MeV helium ions, and 10-30 MeV carbon ions is used as a probe of the overall radiation sensitivity and to determine the relative contributions of aromatic and aliphatic entities in mixed hydrocarbons. The addition of an aliphatic side chain with progressively from one to six carbon lengths to benzene increases the H(2) yield with γ-rays, but the yield seems to reach a plateau far below that found from a simple aliphatic such as cyclohexane. There is a large increase in H(2) with LET (linear energy transfer) for all of the substituted benzenes, which indicates that the main process for H(2) formation is a second-order process and dominated by the aromatic entity. The addition of a small amount of benzene to cyclohexane can lower the H(2) yield from the value expected from a simple mixture law. A 50:50% volume mixture of benzene-cyclohexane has essentially the same H(2) yield as cyclohexylbenzene at a wide variation in LET, suggesting that intermolecular energy transfer is as efficient as intramolecular energy transfer.

Nanobubbles and the nanobubble bridging capillary force

Interactions between hydrophobic surfaces at nanometer separation distances in aqueous solutions are important in a number of biological and industrial processes. Force spectroscopy studies, most notably with the atomic force microscope and surface-force apparatus, have found the existence of a long range hydrophobic attractive force between hydrophobic surfaces in aqueous conditions that cannot be explained by classical colloidal science theories. Numerous mechanisms have been proposed for the hydrophobic force, but in many cases the force is an artifact due to the accumulation of submicroscopic bubbles at the liquid-hydrophobic solid interface, the so called nanobubbles. The coalescence of nanobubbles as hydrophobic surfaces approach forms a gaseous capillary bridge, and thus a capillary force. The existence of nanobubbles has been highly debated over the last 15 years. To date, experimental evidence is sound but a theoretical understanding is still lacking. It is the purpose of this review to bring together the many experimental results on nanobubbles and the resulting capillary force in order to clarify these phenomena. A review of pertinent nanobubble stability and formation theories is also presented.

Electronic structure of the benzene dimer cation

The benzene and benzene dimer cations are studied using the equation-of-motion coupled-cluster model with single and double substitutions for ionized systems. The ten lowest electronic states of the dimer at t-shaped, sandwich, and displaced sandwich configurations are described and cataloged based on the character of the constituent fragment molecular orbitals. The character of the states, bonding patterns, and important features of the electronic spectrum are explained using qualitative dimer molecular orbital linear combination of fragment molecular orbital framework. Relaxed ground state geometries are obtained for all isomers. Calculations reveal that the lowest energy structure of the cation has a displaced sandwich structure and a binding energy of 20 kcal/mol, while the t-shaped isomer is 6 kcal/mol higher. The calculated electronic spectra agree well with experimental gas phase action spectra and femtosecond transient absorption in liquid benzene. Both sandwich and t-shaped structures feature intense charge resonance bands, whose location is very sensitive to the interfragment distance. Change in the electronic state ordering was observed between sigma and piu states, which correlate to the B and C bands of the monomer, suggesting a reassignment of the local excitation peaks in the gas phase experimental spectrum.