The pure Coulomb explosions of the methane clusters (CA4)n, (light atom A = H or D) have been investigated by a simplified electrostatic model for both a single cluster and an ensemble of clusters with a given cluster size distribution. The dependence of the energy of ions produced from the explosions on cluster size and the charge state of the carbon ions has been analysed. It is found that, unlike the average proton energy which increases with the charge q of the carbon ions, the average deuteron energy tends to saturate as q becomes larger than 4. This implies that when the laser intensity is sufficiently high for the (CD4)n to be ionized to a charge state of (C4+D+4)n, the neutron yield from a table-top laser-driven Coulomb explosion of deuterated methane clusters (CD4)n could be increased significantly by increasing the interaction volume rather than by increasing the laser intensity to produce the higher charge state (C6+D+4)n. The flight-time spectra of the carbon ions and the light ions have also been studied.Print publication: Issue 10 (28 May 2008)Received 2 November 2007, in final form 14 April 2008Published 6 May 2008
Theoretical molecular descriptors were tested against log KOW values for polybrominated diphenyl ethers (PBDEs) using the Partial Least-Squares Regression method which can be used to analyze data with many variables and few observations. A quantitative structure–property relationship (QSPR) model was successfully developed with a high cross-validated value (Qcum2)">(Qcum2) of 0.961, indicating a good predictive ability and stability of the model. The predictive power of the QSPR model was further cross-validated. The values of log KOW for PBDEs are mainly governed by molecular surface area, energy of the lowest unoccupied molecular orbital and the net atomic charges on the oxygen atom. All these descriptors have been discussed to interpret the partitioning mechanism of PBDE chemicals. The bulk property of the molecules represented by molecular surface area is the leading factor, and KOW values increase with the increase of molecular surface area. Higher energy of the lowest unoccupied molecular orbital and higher net atomic charge on the oxygen atom of PBDEs result in smaller KOW. The energy of the lowest unoccupied molecular orbital and the net atomic charge on PBDEs oxygen also play important roles in affecting the partition of PBDEs between octanol and water by influencing the interactions between PBDEs and solvent molecules.
Recent evidence has suggested that heterogeneous chemistry of oxygenated hydrocarbons, primarily carbonyls, plays a role in the formation of secondary organic aerosol (SOA); however, evidence is emerging that direct uptake of alkenes on acidic aerosols does occur and can contribute to SOA formation. In the present study, significant uptake of monoterpenes, oxygenated monoterpenes and sesquiterpenes to acidic sulfate aerosols is found under various conditions in a reaction chamber. Proton transfer mass spectrometry is used to quantify the organic gases, while an aerosol mass spectrometer is used to quantify the organic mass uptake and obtain structural information for heterogeneous products. Aerosol mass spectra are consistent with several mechanisms including acid catalyzed olefin hydration, cationic polymerization and organic ether formation, while measurable decreases in the sulfate mass on a per particle basis suggest that the formation of organosulfate compounds is also likely. A portion of the heterogeneous reactions appears to be reversible, consistent with reversible olefin hydration reactions. A slow increase in the organic mass after a fast initial uptake is attributed to irreversible reactions, consistent with polymerization and organosulfate formation. Uptake coefficients (gamma) were estimated for a fast initial uptake governed by the mass accommodation coefficient (alpha) and ranged from 1 x 10(-6)-2.5 x 10(-2). Uptake coefficients for a subsequent slower reactive uptake ranged from 1 x 10(-7)-1 x 10(-4). These processes may potentially lead to a considerable amount of SOA from the various biogenic hydrocarbons under acidic conditions, which can be highly significant for freshly nucleated aerosols, particularly given the large array of atmospheric olefins.