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A strategy is devised to screen molecules based on their radiative efficiency. The methodology should be useful as one additional constraint when determining the best molecule to use for an industrial application. The strategy is based on the results of a recent study where we examined molecular properties of global warming molecules using ab initio electronic structure methods to determine which fundamental molecular properties are important in assessing the radiative efficiency of a molecule. Six classes of perfluorinated compounds are investigated. For similar numbers of fluorine atoms, their absorption of radiation in the IR window decreases according to perfluoroethers>perfluorothioethers [approximately equal to] sulfur/carbon compounds > perfluorocarbons > perfluoroolefins > carbon/nitrogen compounds. Perfluoroethers and hydrofluorethers are shown to possess a large absorption in the IR window due to (i) the C--O bonds are very polar, (ii) the C-O stretches fall within the IR window and have large IR intensity due to their polarity, and (ii) the IR intensity for C-F stretches in which the fluorine atom is bonded to the carbon that is bonded to the oxygen atom is enhanced due to a larger C--F bond polarity. Lengthening the carbon chain leads to a larger overall absorption in the IR window, though the IR intensity per bond is smaller. Finally, for a class of partially fluorinated compounds with a set number of electronegative atoms, the overall absorption in the IR window can vary significantly, as much as a factor of 2, depending on how the fluorine atoms are distributed within the molecule. quantum chemistry calculations | climate change | fluorocarbons | infrared absorption | vibrational frequency doi/ 10.1073/pnas.0913590107

Prior to the Columbia accident, quantitative impact assessment tools were not available to analyze debris impacts onto the Shuttle's thermal protection system. Following the accident, the Columbia Accident Investigation Board recommended changes to increase the safety of future shuttle flights; one component was the development of physics-based analytical capabilities to evaluate damage due to debris impacts. This paper will present an overview of real time debris assessment impact analysis conducted by the Boeing Philadelphia Advanced Structural Analysis Impact Analysis Team in support of Space Shuttle missions since Return to Flight, the first mission after the Columbia accident. Specifically, analyses performed in support of missions STS-114, 121 and 117 will be presented. For each of these cases, an overview of the structural and material model development will be provided, and results of each analysis will be presented followed by a discussion of how the results lead into real time mission decisions. This work illustrates the importance of maintaining a physics-based real-time analysis capability as a vital instrument in supporting the safety of future spaceflight missions. DOI: 10.1061/(ASCE)0893-1321(2009)22:3(222) CE Database subject headings: Space shuttle orbiter; Debris; Impact; NASA.

The species responsible for the broad extended red emission (ERE), discovered in 1975 and now known to be widespread throughout the Galaxy, still is unidentified. Spanning the range from [approximately equal to] 540 to 900 nm, the ERE is a photoluminescent process associated with a wide variety of different interstellar environments. Over the years, a number of plausible candidates have been suggested, but subsequent observations ruled them out. The objects that present the ERE also emit the infrared features attributed to free polycyclic aromatic hydrocarbon (PAH) molecules, suggesting that closely related materials are plausible ERE carriers. Here, we show that the peculiar spectra and unique properties of closed-shell cationic PAH dimers satisfy the existing observational constraints and suggest that emission from mixtures of charged PAH clusters accounts for much of the ERE. This work provides a view into the structures, stabilities, abundances, and ionization balance of PAH-related species in the emission zones, which, in turn, reflects physical conditions in the emission zones and sheds fundamental light on the nanoscale processes involved in carbon-particle nucleation and growth and carbonaceous dust evolution in the interstellar medium. dimer | excited state | interstellar medium | nanoparticles

A study examining the singlet and triplet excited electronic states of several bromine compounds, using time dependent density functional theory, to access their photostability under stratospheric condition is conducted. The results show that several pentacoordinated bromine/oxygen compounds that could be photostable in the lower stratosphere, but it also shows that monovalent bromine compounds where the bromine atom is bounded to an atom with no lone-pair p-electrons is far and away the most photostable.

Bergman reaction and the energy splitting of the transformations of (Z)-hex-3-ene-1, 5-diyne (enediyence) into the singlet coupled biradical p-benzyne are reinvestigated at the CCSD(T) and CASPT2(g1) levels of theory for various basis sets using ab-initio. Underestimation of 5 Kcal/mol and 7 Kcal/mol is found for heat of formation on using small basis sets and CASPT2(g1) methods as compared to CCSD(T) level and method respectively. There are no significant differences in CCSD(T) and CASPT2(g1) methods for the energy splitting.

Ab initio quantum mechanical methods were used to study the equilibrium structures, dipole moments, harmonic vibrational frequencies and infrared intensities of methyl nitrate, methanol and ten structures of protonated methylnitrate. The calculated proton affinity (PA) methyl nitrate agrees with experimental data. In addition, higher PA was observed for nitric acid than formethyl nitrate, and explanations for this is discussed.

A theoretical study of the possible statospheric chlorine reservoir species is presented. Their equilibrium structures, dipole moments, rotational constants, harmonic vibrational frequencies, and infrared intensities are examined.