Collision induced dissociation of laser irradiated silicon tetrafluoride

The time development of visible fluorescence from CO₂-laser-irradiated SiF₄ at various pressures is compared to the predictions of a simple model. This comparison leads to the conclusion that dissociation is a result of binary collisions between molecules that are vibrationally excited but translationally cool.     

Routing in a National Distribution Network

This paper describes the construction of a mathematical model and the results of its use in planning the pattern of routing in a national, complex distribution network; the model's particular feature being its ability to balance return loads on routes where two-way flow is possible.     

Reaction of hexachloropropene with hydroxy-coumarins and chromones

The reaction of hexachloropropene with hydroxy-coumarins and chromones has been utilized to synthesise dichloro-benzodipyrandiones. The structures of the latter have been deduced from their spectral-analytical data. The reaction with 4-hydroxy-coumarin derivatives affords dichloropyrano-benzpyrandiones.     

Subtle white matter volume differences in children treated for medulloblastoma with conventional or reduced dose craniospinal irradiation

Medulloblastoma is the most common malignant brain tumor in children, and approximately seventy percent of average-risk patients will achieve long-term survival. Craniospinal irradiation (CSI), combined with chemotherapy and surgery, is currently the mainstay of treatment but places children who survive at risk for serious neurocognitive sequelae. These sequelae are intensified with a younger age at treatment, greater elapsed time following treatment, and an increased radiation dose. Many newer treatment approaches have attempted to address this problem by reducing the dose of the CSI component of radiation therapy while maintaining the current survival rates. This study evaluates longitudinal MR imaging during therapy to assess the impact of the two CSI doses (conventional [36 Gy] and reduced [23.4 Gy]) on normal appearing white matter volumes (NAWMV) evaluated in a single index slice. Twenty-six children and young adults at least three years of age enrolled on an institutional protocol for newly diagnosed, previously untreated primary medulloblastoma had at least four MR examinations over a minimum nine month period following CSI. These serial volumes were evaluated as a function of time since CSI in three analyses: 1) all subjects, 2) subjects stratified by age at CSI, and 3) subjects stratified by CSI dose. The first analysis demonstrated that medulloblastoma patients treated with CSI have a significant loss of NAWMV in contradistiction to normally expected maturation. Stratifying the patients by age at CSI found no significant differences in the rate of NAWMV loss. The final analysis stratified the patients by CSI dose and revealed that the rate of NAWMV loss was 23% slower in children receiving reduced-dose. Serial quantitative MR measures of NAWMV may provide a neuroanatomical substrate for assessing functional impact of CSI on normal brain function following treatment for medulloblastoma.     

Water coordination structures and the excess free energy of the liquid

We assess the contribution of each coordination state to the hydration free energy of a distinguished water molecule, the solute water. We define a coordination sphere, the inner-shell, and separate the hydration free energy into packing, outer-shell, and local, solute-specific (chemical) contributions. The coordination state is defined by the number of solvent water molecules within the coordination sphere. The packing term accounts for the free energy of creating a solute-free coordination sphere in the liquid. The outer-shell contribution accounts for the interaction of the solute with the fluid outside the coordination sphere and it is accurately described by a Gaussian model of hydration for coordination radii greater than the minimum of the oxygen-oxygen pair-correlation function: theory helps identify the length scale to parse chemical contributions from bulk, nonspecific contributions. The chemical contribution is recast as a sum over coordination states. The nth term in this sum is given by the probability p(n) of observing n water molecules inside the coordination sphere in the absence of the solute water times a factor accounting for the free energy, W(n), of forming an n-water cluster around the solute. The p(n) factors thus reflect the intrinsic properties of the solvent while W(n) accounts for the interaction between the solute and inner-shell solvent ligands. We monitor the chemical contribution to the hydration free energy by progressively adding solvent ligands to the inner-shell and find that four-water molecules are needed to fully account for the chemical term. For a chemically meaningful coordination radius, we find that W(4) ≈ W(1) and thus the interaction contribution is principally accounted for by the free energy for forming a one-water cluster, and intrinsic occupancy factors alone account for over half of the chemical contribution. Our study emphasizes the need to acknowledge the intrinsic solvent properties in interpreting the hydration structure of any solute, with particular care in cases where the solute-solvent interaction strength is similar to that between the solvent molecules.     

Ion-water clusters, bulk medium effects, and ion hydration

Thermochemistry of gas-phase ion-water clusters together with estimates of the hydration free energy of the clusters and the water ligands are used to calculate the hydration free energy of the ion. Often the hydration calculations use a continuum model of the solvent. The primitive quasichemical approximation to the quasichemical theory provides a transparent framework to anchor such efforts. Here we evaluate the approximations inherent in the primitive quasichemical approach and elucidate the different roles of the bulk medium. We find that the bulk medium can stabilize configurations of the cluster that are usually not observed in the gas phase, while also simultaneously lowering the excess chemical potential of the ion. This effect is more pronounced for soft ions. Since the coordination number that minimizes the excess chemical potential of the ion is identified as the optimal or most probable coordination number, for such soft ions the optimum cluster size and the hydration thermodynamics obtained with and without account of the bulk medium on the ion-water clustering reaction can be different. The ideas presented in this work are expected to be relevant to experimental studies that translate thermochemistry of ion-water clusters to the thermodynamics of the hydrated ion and to evolving theoretical approaches that combine high-level calculations on clusters with coarse-grained models of the medium.     

Synthesis of a new heterocyclic ring system 2,3,5,6,7,9,10,11-octahydro-lH-cyclopenta[9,H)]benzo[i,j] quinolizine-1,9-dione

Reaction of 1,2,3,4-tetrahydroquinoline (1) with different α,β-unsaturated acids in the presence of PPA has been investigated. With acrylic acid, 1 affords two compounds identified as 1-keto-benzo[i,j]quinolizine (III) and 2,3,5,6,7,9,10,1 1-octahy dro-1H-cy el open ta [9,10] benzo [i,j]-quinolizine-1,9-dione (IV). A similar reaction of 1 with a-methyl acrylic and crotonie acids gives compounds V and VI whose structures are analogous to that of IV. These were deduced from their spectral and analytical data.     

Synthesis of some benzopyranocyclopentapyridine and pyranoacridine derivatives

Aminocoumarins react with ethyl cyclopentanone-and ethyl cyclohexanone-2-carboxylates to afford intermediates which on thermal cyclisation yield the title compounds.