Empirical Method for the Prediction of Heat of Formation of Organic High Energy Molecules

An empirical method based on additive procedures is proposed for estimating the heats offormation of aliphatic, aromatic, and ring molecules containing nitro and other energetic groupsat standard state. The method uses only molecular structural information. Calculation of heatof formation is carried out in three stages, first the heat of formation for gaseous state is calculated,followed by incorporation of heat of vapourisation/sublimation, and finally, corrections are donefor interactions. Some interaction terms, based on hydrogen bonding strength for variouscompounds and experimental heat of formation of isomeric compounds, are also proposed. Theresults are in good agreement with the experimentally determined values. The method providesquick and sufficiently accurate values of heat of formation of organic high energy molecules

Secondary organic aerosol 2. Thermodynamic model for gas/particle partitioning of molecular constituents

A model that predicts secondary organic aerosol (SOA) formation based on the thermodynamic equilibrium partitioning of secondary organic oxidation products has been developed for implementation into atmospheric models. Hydrophobic secondary products are assumed to partition to an absorbing organic aerosol consisting of primary organic aerosol (POA) and other secondary hydrophobic organics according to an equilibrium partitioning coefficient calculated iteratively for each secondary compound present. The hydrophobic module is evaluated by studying the partitioning of octadecanoic acid to surrogate POA species. As expected, the amount of octadecanoic acid predicted to be present in the aerosol phase increases as the total amount of absorbing material increases or as the total amount of acid present increases. Hydrophilic secondary compounds partition to an aqueous phase via Henry's law; the fraction of each compound's mass that partitions is determined by its Henry's law constant and its acid dissociation constant(s). The available liquid water content (LWC) of the aerosol is determined iteratively between an inorganic aerosol module and the hydrophilic module, which is evaluated by studying the partitioning of glyoxalic and malic acids. While glyoxalic acid tends to remain in the gas phase, malic acid partitions strongly to the aqueous phase, with ions being the dominant form in the aqueous phase. As expected, an increase in relative humidity increases the amount of water associated with the organics (ΔLWC), and a lower aerosol pH favors molecular solutes over ionized forms. Increasing pH results in higher effective Henry's law constants for the acids, yielding higher organic aerosol concentrations. Results also indicate that increasing ΔLWC induces additional partitioning of inorganics to the aqueous phase

Predictive Model Development for Adsorption of Organic Contaminants by Carbon Nanotubes

The main objective of the study was to investigate mechanisms and statistical modeling of synthetic organic contaminant (SOC) adsorption by carbon nanotubes (CNTs). First, predictive models were developed for adsorption of low molecular weight aromatic compounds by multi-walled carbon nanotubes (MWCNTs) using experimental data for 59 compounds. Quantitative structure-activity relationship (QSAR) and linear solvation energy relationship (LSER) approaches were employed and developed models were externally validated using an independent dataset obtained from the literature. Up to date, no QSAR model has been reported for predicting adsorption of organics by CNTs. No LSER model is available which comprehensively investigates the adsorption of organics on CNTs. Only recently, one study reported an LSER equation for the modeling of their experimental adsorption data on one MWCNT. Then, adsorption of ten environmentally relevant halogenated aliphatic SOCs by a single walled (SWCNT) and MWCNT was tested experimentally for the first time in the literature. Several LSER models were developed to further examine the adsorption mechanisms. The LSER equations constitute the first predictive models generated for adsorption of aliphatic SOCs by CNTs. In addition, the poly-parameter LSER model was compared to those previously generated for adsorption of aromatic SOCs by CNTs. The LSER model generated in this research is currently the most comprehensive models available in the literature. Finally, the role of carbon nanotube morphology (i.e. surface area, diameter, and length) on the adsorption of phenanthrene (PNT) was investigated by analyzing the adsorption isotherms obtained with several SWCNTs and MCWNTs in the laboratory and the literature. The QSAR (r2 = 0.88), and LSER (r2 = 0.83) equations and their external validation accuracies indicated the success of parameter selection, data fitting ability, and the prediction strength of the developed models. These models were developed for adsorption of low-molecular weight (/mol) aromatic SOCs by MWCNTs (with less than 5% oxygen content) in distilled and deionized water. For aromatic SOC adsorption models, the molecular volume term (V) of the LSER model was the most influential descriptor controlling adsorption at all concentrations. At higher equilibrium concentrations, hydrogen bond donating (A) and hydrogen bond accepting (B) terms became significant in the models. For halogenated aliphatic SOC adsorption models, at higher concentrations, the B parameter, capturing hydrogen bond accepting ability, was the most influential descriptor both for SWCNT and MWCNT. The negative dependence on B indicates that as the hydrogen bond accepting ability of an aliphatic compound increases, it becomes less likely to be adsorbed by CNTs. The other important LSER parameters were V (size) followed by P (polarizability), and they were positively correlated with adsorption, indicating that size and polarizability favors adsorption. The contribution of these parameters was 2 - 3 times less than the B parameter. However, there was no single parameter predominant in the aliphatic SOC models. The number of data points for aliphatic SOCs were much smaller than aromatic models. These results indicated that adsorption of aromatic SOCs by CNTs strongly depend on adsorbate hydrophobicity; while for aliphatic SOCs, in addition to hydrophobic driving force, other interactions (i.e., hydrogen bond accepting ability) also play a role. Additional investigation of CNT properties on adsorption of PNT showed that at low (e.g., 1 μg/L) equilibrium concentrations, MWCNTs with the larger outer diameters exhibit higher adsorption capacity on a specific surface area basis than those with smaller diameters. With increasing equilibrium concentration, adsorption on a specific surface area basis becomes independent of MWCNT diameter, and maximum adsorption capacity was controlled by the total surface area. A similar analysis for the adsorption of naphthalene (NPT), a planar molecule with one less benzene ring but twenty times higher solubility than PNT, showed no correlation with respect to MWCNT outer diameter at both low and high equilibrium concentrations. The results indicated that the surface curvature of MWCNT was more important on the adsorption of PNT than on the adsorption of NPT due to its smaller molecular size and lower adsorption capacity than PNT. Specific surface area normalized isotherms did not show a correlation between PNT adsorption and lengths of SWCNTs and MWCNTs. Carbon nanotube characterization results showed that the morphology of CNTs impacts their aggregation and plays an important role on the available surface area and pore volume for adsorption. Manufacturer\u27s data may not always represent the characteristics of CNTs in a particular batch. Therefore, accurate characterization of CNTs is essential to systematically examine the behavior of CNTs (e.g., adsorption, transport) in environmental systems. A fundamental understanding of CNT-SOC adsorption interactions is important to (i) assess the environmental implications of CNT releases and spills to natural waters, and their roles as the contaminant carriers in the environment and, (ii) evaluate the potentials of CNTs as adsorbents in water and wastewater treatment applications. Predictive LSER modeling can be used to gain insight to the adsorption mechanisms by examining the individual contribution of intermolecular interactions to overall adsorption. This study examined and showed adsorption mechanisms and CNT properties (such as surface area, pore volume, outer diameter, and surface oxygen content) on the adsorption behavior of different classes of SOCs by CNTs

Profiling the Organic Emissions from a Light-Duty Direct Injection Diesel Engine over a range of Speeds and Loads

Diesel engines account for a large percentage of the particulates in urban city environments. Polycyclic aromatic compounds (PAC), some proven carcinogens, have been found on diesel particulates. The trace level nitro-PAC emissions, such as 1-nitropyrene and dinitropyrenes, contribute a large proportion of the mutagenicity in the particulates; in the case of 1-nitropyrene between 10 to 40% of the total mutagenicity of the particulate has been claimed. The potential health hazards of PAC require the levels and sources of such emissions to be evaluated over a range of speeds and loads. PAC emissions are dependant on the engine specification, such as normally aspirated compared with turbocharged, and the operating conditions (speed and load). The effect of such variables can be determined using emission profiling, in which profiles of the exhaust are compared at varying engine powers. In this way the effect of speed and load on the combustion efficiency can be established. Identification of PAC sources may be further complicated when engine sampling systems, such as the conventional dilution tunnel/filter system, are prone to artefact formation. This is especially relevant to secondary nitro-PAC emissions, which are prone to forming as artefacts of the filter installed in the dilution tunnel. In this study, organic emission maps were constructed using the unique total exhaust solvent-stripping apparatus (TESSA) developed at the University of Plymouth. TESSA allowed rapid sampling with a minimum potential for artefact formation. The close proximity of TESSA to the engine allowed the role of the combustion chamber in the formation of emissions to be evaluated. Primary organic emissions, such as pyrene, are derived from survival of compounds in the fuel/oil and by combustion generation. Establishment of emission maps for the primary emissions are vital to resolving the formation of secondary emissions, such as 1-nitropyrene. Profiling of primary emissions sampled 26 different speeds and loads using TESSA (sampling times as low as IS seconds). Following simple work-up, quantification of the o-alkanes was by gas chromatography with flame ionisation detection and PAH by gas chromatography/mass spectrometry operated in electron impact mode. Emissions were expressed as a recovery of the compound emitted as a percentage of the same compound entering the chamber in the fuel. The o-alkane and PAH emission maps correlated with the gaseous unburnt hydrocarbon emissions, indicating that fuel survival was an important source of emissions, whereas lubricating oil contributions were minimal. Fuel survival contributions decreased with load; at 1000 rpm the average PAH survival of 0.95% at idling decreased to 0.2% at full load. High survivals under idling was a consequence of the low chamber temperatures and air:fuel ratios mixed beyond the lean flammability limits, whereas at full load, the high temperatures resulted in the greatest combustion efficiency. The o-alkane emission trends replicated those of PAH; at 1000 rpm the average o-alkane survival was 0.48% at idle compared with 0.084% at full load. Correlations between the distribution of the emissions and fuel at high load, suggested fuel survival unchanged was responsible. At low loads the exhaust/fuel PAH ratios were more varied, with the range of percentage recoveries at low loads increasing with speed (difference between percentage recovery of fluorene and phenanthrene at idling for 1000 rpm and 3000 rpm was 0.05 and 0.18 respectively). At high load, the combustion environment can be envisaged as producing areas under which complete combustion and survival unchanged occur. In between the complete combustion and unburnt fuel zones, a narrow range of temperatures and time for combustion reduce the opportunity for combustion generation reactions and/or preferential survival. Low load at 3000 rpm may increase the intermediate zone, allowing preferential survival and/or combustion generation reactions to evolve. Possible pyrolytic cracking of o-alkanes and demethylation of PAH at low loads and 3000 rpm was evident. The optimum time, swirl, and temperature for efficient combustion at low loads was generated at 2000 rpm to 2500 rpm, whereas at the higher temperatures corresponding to high loads, the effect of speed was much smaller. The primary emissions map show engineering improvements, particularly at low loads, could be implemented to lower the PAH emissions. The correlation between the emissions and fuel input suggest modifications to the PAH content of fuels may lower emissions. The formation of secondary nitro- and oxy-PAC emissions is by transformation of primary emissions. In the case of nitro-PAC, nitration has been proposed to occur by free radical processes between PAH and oxides of nitrogen, NO,, within the chamber and by electrophilic substitution of PAH surviving combustion by nitrogen dioxide and nitric acid, via the nitronium ion. The combustion contribution to nitro-PAC emissions was investigated using an upgraded TESSA system, and 3 speeds at low, mid, and high NO, for each speed were sampled. Following initial extraction, concentration and clean-up of the samples, the nitro-PAC fractions of interest were isolated by normal phase high performance liquid chromatography. The nitro-PAC were identified and quantified by gas chromatography with electron capture detection, and gas chromatography/mass spectrometry operated in the negative ion chemical ionisation mode (the detection limits for both analytical systems was of the order of 40-50 pg of nitro-PAC standards injected). The profiling indicated that a proportion of the fuel underwent nitration within the combustion chamber across a range of speeds and loads. The extract concentrations (average of 5.3 ppm) found in this study were much lower than those previous found (ranging from 55 to 2280 ppm). The majority of the previous studies relied on sampling using dilution tunnel/filter systems, for which post combustion contributions are simulated; suggesting that a major source of nitro-PAC is derived from post-combustion nitration of PAH surviving combustion, some of which may be artefacts of the filter. Different speeds produced different trends for nitro-PAC emissions with respect to engine load. It was not until the high temperature speed of 3500 rpm was reached that both NO, and nitro-PAC increased with load (R-sq= 0.989 & p=0.067 for 1-nitronaphthalene & NO,). Nitro-PAC emissions at 3500 rpm were primarily the result of combustion chamber nitration of PAH at high NO,. In the case of 1-nitropyrene, there was strong evidence to support pyrosynthetic contributions to the pyrene mass, which in turn became nitrated. The nitro-PAC emissions at low loads were the result of post-combustion nitration of PAH surviving combustion with the nitronium ion. The correlation of the PAH precursors to nitro-PAC in the fuel and nitro-PAC emissions suggest fuel modifications may to some extent lower the nitro-PAC emissions. The combustion generation of nitro-PAC at high engine powers may require post-combustion after treatment.Perkins Technology, Peterboroug

Tautomeric Equilibria Studies by Mass Spectrometry

Tautomerism in organic chemistry has been extensively studied in condensed phase by spectrometric methods, mainly by IR and NMR techniques. Mass spectrometry studies start 40 years ago but just recently it has been recognized the importance of the mass spectral data for the study of tautomerism in the gas phase.
Mass spectrometry can provide valuable information in regard to tautomeric equilibria when studying mass spectra among the members of different families of organic compounds.
The relevance of the mass spectral data resides on several facts but there are two that are of key importance:
1-	Mass spectral fragmentation assignments should be tautomer specific since the corresponding abundances ratios are supposed to be correlated to the keto/enol contents.
2-	Ionization in the ion source is supposed to have no effect on the position of the equilibrium so that the results reflect the tautomers content in the gas phase previous to ionization.
Some of the carbonylic compounds do not exhibit noticeable tautomerism so the fragment abundances assigned to the enol form is very low or not measurable. Since enolization is more noticeable in the case of thio-derivatives (which correlates adequately with the oxygenated analogues), the study of their mass spectra is an interesting choice to reach some degree of generalization. 
In addition, experimental findings are supported by semiempirical theoretical calculations, which probed to be adequate not only for supporting tendency correlations among the members of a compound family but also to calculate heats of tautomerization in gas phase.
Reports using mass spectrometry for tautomerism are becoming less common. One of the reasons is that now it would appear that the interpretation of MS results is not as straightforward as it was once believed, even though in a recent review it was written that: “Mass spectrometry is the most informative and practical method for studying and identifying tautomers in the gas phase” [1]. 
In fact, mass spectrometry seems to be very informative for studying and identifying tautomers, because in this case external factors like solvents, intermolecular interactions, etc., can be excluded by transferring the tautomeric system into gas phase, where the process becomes truly unimolecular [1].
This review covers the study of Tautomerism by Mass Spectrometry in the last four decades. 
&#xa