Examination of intramolecular potential surfaces of flexible conjugated molecules by calculation of crystal structures. equilibrium geometries of chalc

A method for refinement of intramolecular potential surfaces of flexible molecules is proposed based on the knowledge of the equilibrium molecular geometry in the crystal and on a good first approximation to the intra- and intermolecular surfaces. The equilibrium conformation in the crystal is calculated by minimization of the total potential with respect to all Cartesian coordinates of the asymmetric unit as well as the components of the unit cell parameters. The initial approximations to the potential function are further refined with the purpose of obtaining better agreement between the calculated crystal equilibrium geometry and the corresponding X-ray results. The previously proposed QCFF/PI potential surfaces for conjugated hydrocarbons are re-examined by calculation of the crystal structures of chalcone, p-methylchalcone, p,p'-dimethylchalcone and α,ω-diphenyloctatetraene. The resulting potentials of the present refinement are somewhat shallower than the previous QCFF/PI surfaces. It is suggested that the application of the method as a model calculation may be considered a reliable “quasi-experimental” technique for the determination of gas phase equilibrium conformations.     

Grouping and trapping of evaporating droplets in an oscillating gas flow

A new approach to the analysis of droplet grouping in an oscillating gas flow is suggested. This is based on the investigation of droplet trajectories in the frame of reference moving with the phase velocity of the wave. Although the equations involved are relatively simple, the analysis shows distinctive characteristics of grouping and non-grouping cases. In the case of grouping, droplet trajectories converge to the points for which the ratio of flow velocity in this frame of reference and the amplitude of flow oscillations is less than 1, and the cosine of the arc sine of this ratio is positive. In the case of non-grouping, droplet trajectories in this frame of reference oscillate around the translational velocity close to the velocity of flow in the same frame of reference. The effect of droplet size on the grouping pattern is investigated. It has been pointed out that for the smaller droplets much more stable grouping is observed. The effect of droplet evaporation is studied in the limiting case when the contribution of the heat-up period can be ignored. It is shown that evaporation can lead to droplet grouping even in the case when the non-evaporating droplets are not grouped. This is related to the reduction in droplet diameter during the evaporation process. Coupling between gas and droplets is shown to decrease the grouping tendency. A qualitative agreement between predictions of the model and in-house experimental observations referring to Diesel engine sprays has been demonstrated.