Theoretical study of the reaction mechanism of CH<Subscript>3</Subscript>NO<Subscript>2</Subscript> with NO<Subscript>2</Subscript>, NO and CO: the bimolecular reactions that cannot be ignored

The intriguing decompositions of nitro-containing explosives have been attracting interest. While theoretical investigations have long been concentrated mainly on unimolecular decompositions, bimolecular reactions have received little theoretical attention. In this paper, we investigate theoretically the bimolecular reactions between nitromethane (CH₃NO₂)—the simplest nitro-containing explosive—and its decomposition products, such as NO₂, NO and CO, that are abundant during the decomposition process of CH₃NO₂. The structures and potential energy surface (PES) were explored at B3LYP/6-31G(d), B3P86/6-31G(d) and MP2/6-311?+?G(d,p) levels, and energies were refined using CCSD(T)/cc-pVTZ methods. Quantum chemistry calculations revealed that the title reactions possess small barriers that can be comparable to, or smaller than, that of the initial decomposition reactions of CH₃NO₂. Considering that their reactants are abundant in the decomposition process of CH₃NO₂, we consider bimolecular reactions also to be of great importance, and worthy of further investigation. Moreover, our calculations show that NO₂ can be oxidized by CH₃NO₂ to NO₃ radical, which confirms the conclusion reached formerly by Irikura and Johnson [(2006) J Phys Chem A 110:13974–13978] that NO₃ radical can be formed during the decomposition of nitramine explosives.     

Root proteomics reveals cucumber 24-epibrassinolide responses under Ca(NO<Subscript>3</Subscript>)<Subscript>2</Subscript> stress

Key message

The application of exogenous 24-epibrassinolide promotes Brassinosteroids intracellular signalling in cucumber, which leads to differentially expressed proteins that participate in different life process to relieve Ca(NO ₃ ) ₂ damage.

Abstract

NO₃ ^? and Ca²⁺ are the main anion and cation of soil secondary salinization during greenhouse cultivation. Brassinosteroids (BRs), steroidal phytohormones, regulate various important physiological and developmental processes and are used against abiotic stress. A two-dimensional electrophoresis gel coupled with MALDI-TOF/TOF MS was performed to investigate the effects of exogenous 24-epibrassinolide (EBL) on proteomic changes in cucumber seedling roots under Ca(NO₃)₂ stress. A total of 80 differentially accumulated protein spots in response to stress and/or exogenous EBL were identified and grouped into different categories of biological processes according to Gene Ontology. Under Ca(NO₃)₂ stress, proteins related to nitrogen metabolism and lignin biosynthesis were induced, while those related to cytoskeleton organization and cell-wall neutral sugar metabolism were inhibited. However, the accumulation of abundant proteins involved in protein modification and degradation, defence mechanisms against antioxidation and detoxification and lignin biosynthesis by exogenous EBL might play important roles in salt tolerance. Real-time quantitative PCR was performed to investigate BR signalling. BR signalling was induced intracellularly under Ca(NO₃)₂ stress. Exogenous EBL can alleviate the root indices, effectively reduce the Ca²⁺ content and increase the K⁺ content in cucumber roots under Ca(NO₃)₂ stress. This study revealed the differentially expressed proteins and BR signalling-associated mRNAs induced by EBL in cucumber seedling roots under Ca(NO₃)₂ stress, providing a better understanding of EBL-induced salt resistance in cucumber seedlings. The mechanism for alleviation provides valuable insight into improving Ca(NO₃)₂ stress tolerance of other horticultural plants.

     

Minor Changes in Vegetation and Carbon Gas Balance in a Boreal Mire under a Raised CO<Subscript>2</Subscript> or NH<Subscript>4</Subscript>NO<Subscript>3</Subscript> Supply

Increasing concentrations of carbon dioxide (CO₂) in the atmosphere or continuous nitrogen (N) deposition might alter the carbon (C) cycle in boreal mires and thus have significant impacts on the development of climate change. The atmospheric impact of the C cycle in mires is twofold: C accumulation attenuates and CH₄ release strengthens the natural greenhouse effect. We studied the effects of an increased supply of CO₂ or NH₄NO₃ on the vegetation and annual CO₂ exchange in lawns of a boreal oligotrophic mire in eastern Finland over a 2-year period. Ten study plots were enclosed with mini-FACE (Free Air Carbon Dioxide Enrichment) rings. Five plots were vented with CO₂-enriched air (target 560 ppmv), while their controls were vented with ambient air; five plots were sprayed with NH₄NO₃, corresponding to a cumulative addition of 3 g N m⁻² a⁻¹, while their controls were sprayed with distilled water only. A raised NH₄NO₃ supply seemed to affect the composition of the moss layer. Raised CO₂ did not affect the vegetation, but gross photosynthesis increased significantly. The change in net CO₂ exchange depended on the annual weather conditions. Our results suggest that C accumulation may increase in wet years and compensate for the warming effect caused by the increase in CH₄ release from this mire. In contrast, a relatively dry and warm growing period favors decomposition and can even make the CO₂ balance negative. Along with the increased CH₄ release under raised CO₂, the decreased C accumulation then increases the radiative forcing of boreal mires. Received 22 October 2001; accepted 13 May 2002.     

Microbial N Turnover and N-Oxide (N<Subscript>2</Subscript>O/NO/NO<Subscript>2</Subscript>) Fluxes in Semi-arid Grassland of Inner Mongolia

Gross rates of N mineralization and nitrification, and soil–atmosphere fluxes of N₂O, NO and NO₂ were measured at differently grazed and ungrazed steppe grassland sites in the Xilin river catchment, Inner Mongolia, P. R. China, during the 2004 and 2005 growing season. The experimental sites were a plot ungrazed since 1979 (UG79), a plot ungrazed since 1999 (UG99), a plot moderately grazed in winter (WG), and an overgrazed plot (OG), all in close vicinity to each other. Gross rates of N mineralization and nitrification determined at in situ soil moisture and soil temperature conditions were in a range of 0.5–4.1 mg N kg⁻¹ soil dry weight day⁻¹. In 2005, gross N turnover rates were significantly higher at the UG79 plot than at the UG99 plot, which in turn had significantly higher gross N turnover rates than the WG and OG plots. The WG and the OG plot were not significantly different in gross ammonification and in gross nitrification rates. Site differences in SOC content, bulk density and texture could explain only less than 15% of the observed site differences in gross N turnover rates. N₂O and NO _x flux rates were very low during both growing seasons. No significant differences in N trace gas fluxes were found between plots. Mean values of N₂O fluxes varied between 0.39 and 1.60 μg N₂O-N m⁻² h⁻¹, equivalent to 0.03–0.14 kg N₂O-N ha⁻¹ y⁻¹, and were considerably lower than previously reported for the same region. NO _x flux rates ranged between 0.16 and 0.48 μg NO _x -N m⁻² h⁻¹, equivalent to 0.01–0.04 kg NO _x -N ha⁻¹ y⁻¹, respectively. N₂O fluxes were significantly correlated with soil temperature and soil moisture. The correlations, however, explained only less than 20% of the flux variance.     

Si-doped graphene: an ideal sensor for NO- or NO<Subscript>2</Subscript>-detection and metal-free catalyst for N<Subscript>2</Subscript>O-reduction

Exploring and evaluating the potential applications of two-dimensional graphene is an increasingly hot topic in graphene research. In this paper, by studying the adsorption of NO, N₂O, and NO₂ on pristine and silicon (Si)-doped graphene with density functional theory methods, we evaluated the possibility of using Si-doped graphene as a candidate to detect or reduce harmful nitrogen oxides. The results indicate that, while adsorption of the three molecules on pristine graphene is very weak, Si-doping enhances the interaction of these molecules with graphene sheet in various ways: (1) two NO molecules can be adsorbed on Si-doped graphene in a paired arrangement, while up to four NO₂ molecules attach to the doped graphene with an average adsorption energy of −0.329 eV; (2) the N₂O molecule can be reduced easily to the N₂ molecule, leaving an O-atom on the Si-doped graphene. Moreover, we find that adsorption of NO and NO₂ leads to large changes in the electronic properties of Si-doped graphene. On the basis of these results, Si-doped graphene can be expected to be a good sensor for NO and NO₂ detection, as well as a metal-free catalyst for N₂O reduction.     

24-Epibrassinolide-induced alterations in the root cell walls of <Emphasis Type="Italic">Cucumis sativus</Emphasis> L. under Ca(NO<Subscript>3</Subscript>)<Subscript>2</Subscript> stress

Brassinosteroids (BRs) can effectively alleviate the oxidative stress caused by Ca(NO₃)₂ in cucumber seedlings. The root system is an essential organ in plants due to its roles in physical anchorage, water and nutrient uptake, and metabolite synthesis and storage. In this study, 24-epibrassinolide (EBL) was applied to the cucumber seedling roots under Ca(NO₃)₂ stress, and the resulting chemical and anatomical changes were characterized to investigate the roles of BRs in alleviating salinity stress. Ca(NO₃)₂ alone significantly induced changes in the components of cell wall, anatomical structure, and expression profiles of several lignin biosynthetic genes. Salt stress damaged several metabolic pathways, leading to cell wall reassemble. However, EBL promoted cell expansion and maintained optimum length of root system, alleviating the oxidative stress caused by Ca(NO₃)₂. The continuous transduction of EBL signal thickened the secondary cell wall of casparian band cells, thus resisting against ion toxicity and maintaining water transport.     

Effects of Ca(NO<Subscript>3</Subscript>)<Subscript>2</Subscript> stress on oxidative damage,antioxidant enzymes activities and polyamine contents in roots of grafted and non-grafted tomato plants

The effects of Ca(NO₃)₂ stress on biomass production, oxidative damage, antioxidant enzymes activities and polyamine contents in roots of grafted and non-grafted tomato plants were investigated. Results showed that when exposed to 80 mM Ca(NO₃)₂ stress, the biomass production reduction in non-grafted plants was more significant than that of grafted plants. Under Ca(NO₃)₂ stress, superoxide anion radical (O₂•⁻) producing rate, hydrogen peroxide (H₂O₂) and malondialdehyde (MDA) contents of non-grafted plants roots were significantly higher than those of grafted plants, however, nitrate (NO₃ ⁻), ammonium (NH₄ ⁺) and proline contents, superoxide dismutase (SOD, EC1.15.1.1), peroxidase (POD, EC1.11.1.7), catalase (CAT, EC1.11.1.6) and arginine decarboxylase (ADC, EC 4.1.1.19) activities of grafted plants roots were significantly higher than those of non-grafted plants. Regardless of stress, free, conjugated and bound polyamine contents in roots of grafted plants were significantly higher than those of non-grafted plants. The possible roles of antioxidant enzymes, prolines and polyamines in adaptive mechanism of tomato roots to Ca(NO₃)₂ stress were discussed. Gu-Wen Zhang and Zheng-Lu Liu contributed equally to this work.     

The effects of grafting on glycolysis and the tricarboxylic acid cycle in Ca(NO<Subscript>3</Subscript>)<Subscript>2</Subscript>-stressed cucumber seedlings with pumpkin as rootstock

In this research, we investigated the effects of grafting on intermediate metabolites and key enzymes of glycolysis and the tricarboxylic acid (TCA) cycle in self-grafted and salt-tolerant pumpkin rootstock-grafted cucumber seedlings supplied with nutrient solution and subjected to 80 mM Ca(NO₃)₂ stress for 6 days. Ca(NO₃)₂ stress induced accumulation of 3-phosphoglycerate (3-PGA) and phosphoenolpyruvate (PEP) in the leaves of self-grafted cucumber seedlings and enhanced the activities of phosphoenolpyruvate carboxylase (PEPC) and enolase (ENO). Succinic acid and malic acid contents and isocitrate dehydrogenase, succinate dehydrogenase (SDH), and malate dehydrogenase (MDH) activities in self-grafted seedlings were significantly decreased by Ca(NO₃)₂ stress. In addition, activities of PEPC, ENO, SDH, and MDH and contents of glycolysis intermediate metabolites (citric, succinic, and malic acids) were significantly higher in leaves of rootstock-grafted seedlings compared with those in self-grafted seedlings under saline conditions. Furthermore, leaf adenosine triphosphate (ATP) content of rootstock-grafted seedlings was relatively higher than that in self-grafted plants under salt stress, with an opposite effect observed on adenosine diphosphate content. These results indicate that rootstock grafting alleviates Ca(NO₃)₂ stress-induced inhibition of the glycolytic pathway and the TCA cycle in cucumber seedling leaves, which may aid the respiratory metabolism of cucumber seedlings and help maintain a high ATP synthesis level, thereby increasing the biomass of cucumber seedlings and enhancing their salt tolerance.     

Room Temperature NO<Subscript>2</Subscript> Gas Sensor Based on SnO<Subscript>2</Subscript>-WO<Subscript>3</Subscript> Thin Films

Metal oxide semiconductors (MOS) are important and promising materials in optoelectronics, and it has been widely used in various catalytic applications such as gas sensing due to its high reactivity with many gases. In current work, mixtures of SnO₂-WO₃ (1:1) were prepared to synthesize nanostructured thin films by pulsed laser deposition as gas sensors. The sensitivity of sensors was measured for a relatively low concentration (200 ppm) of NO₂ gas at room temperature; sensors prepared with target exposed to (200) laser shots have higher sensitivity with a maximum value of 96.49 % at time 65 s as compared with the sensors prepared with (150) laser shots where the sensitivity has a maximum value 71.82 % at time 110 s; XRD pattern shows a better crystalline and high intensity with increasing laser shots up to 200; scanning electron microscopy (SEM) micrographs show approximate homogeneity of grains that cover the substrate without cracks and pinholes with nanoparticles fall in micro and nanometer range 50–200 nm. The values of the direct band gap were found to be 2.07143 eV for films prepared with 150 laser shots and 2.02899 eV for films prepared with 200 laser shots which have higher absorbance than the former films due to the increment in thickness and particle size. Empirical equations between sensitivity and gas exposure time have been formulated with great coincidence with the experimental data.     

Synchronous trends in N–NO<Subscript>3</Subscript> export from N-saturated river catchments in relation to climate

Long term trends (1978–2005) of N–NO₃ concentrations in river water were investigated for 10 rivers draining forested catchments in Piedmont, North-Western Italy, and Canton Ticino, Switzerland. All the river catchments come into the category of the medium-high stage of N saturation (levels 2–3 of the Stoddard’s classification). The seasonal signal in N–NO₃ concentrations and its changes in time over the course of the study period was also evaluated. Single trends were analysed for significance and magnitude; statistical techniques for the detection of common trends were then applied to identify a common pattern in the N–NO₃ time series. Both the increasing NO₃ levels and the limited seasonal pattern in recent years indicate an aggrading level of N saturation in time. Synchronous trends of N–NO₃ export were found for 8 rivers. The main common trend was used to test relationships with: (i) temperature, (ii) precipitation, and (iii) N deposition. Step-changes in the data series were also assessed, and the main points of change are discussed in relation to meteorological factors and response to the N saturation status. Temperature proved to be the main factor affecting the temporal pattern of N–NO₃ concentrations: warm periods were usually followed by an N–NO₃ increase in river water due to enhanced mineralisation and nitrification in soil.     

The addition reactions of germylenoid H<Subscript>2</Subscript>GeAlCl<Subscript>3</Subscript> with ethylene: a theoretical investigation

Theoretical calculations using the M062X and QCISD methods were performed on the addition reactions of the aluminum germylenoid H₂GeAlCl₃ with ethylene. The most two stable structures of germylenoid H₂GeAlCl₃, i.e., the p-complex and three-membered ring structures, respectively, were employed as reactants. The calculated results indicate that, for the p-complex, H₂GeAlCl₃ there are two pathways, I and II, of which path I involves just one transition state, while path II involves two transition states between reactants and products. Comparing the reaction barrier heights of path I (44.6 kJ mol^?1) and II (37.6 kJ mol^?1), the two pathways are competitive, with similar barriers under the same conditions, while for the three-membered ring structure, another two pathways, III and IV, also exist. Path III has one transition state; however, in path IV, two transition states exist. By comparing their barrier heights, path III (barrier height 39.2 kJ mol^?1) could occur more easily than path IV (barrier height 92.8 kJ mol^?1). Considering solvent effects on these addition reactions, the PCM model and CH₂Cl₂ solvent were used in calculations, and the calculated results demonstrate that CH₂Cl₂ solvent is unfavorable for the reactions, except for path II. In CH₂Cl₂ solvent, paths II and III are more favorable than paths I and IV.     

Salt-induced NO<Subscript>3</Subscript><Superscript>-</Superscript> uptake inhibition in cowpea roots is dependent on the ionic composition of the salt and its osmotic effect

Salinity remarkably inhibits NO₃ ^- uptake but the mechanisms are not well understood. This study was addressed to elucidate the role of ionic and osmotic components of salinity on NO₃ ^- influx and efflux employing classic kinetics involving a low affinity transport system (LATS) and a high affinity transport system (HATS). In the presence of KCl, NaCl, and Na₂SO₄ at 100 mM concentrations, in both LATS and HATS, Michaelis constant (K_m) was similar for the three salts and maximum rate (V_max) decreased as follows: KCl > NaCl > Na₂SO₄, compared to control indicating a non-competitive interaction with NO₃ ^-. Unexpectedly, iso-osmotic solutions (osmotic potential Ψ_π = -0.450) of polyethylene glycol (PEG, 17.84 %, v/v) and mannitol (100 mM) remarkably increased K_m in both the LATS and the HATS, but V_max did not change indicating a competitive inhibition. Under the PEG and mannitol treatments, K_m and V_max were higher than under the salt treatments. The salts increased slightly NO₃ ^- efflux in the following order KCl > NaCl > Na₂SO₄. In contrast, mannitol strongly stimulated and the PEG inhibited NO₃ ^- efflux. The obtained data reveal that salinity effects were not dependent on the anion type (Cl^- versus SO₄ ^2-) indicating a non-competitive inhibition mechanism between Cl^- and NO₃ ^-. In contrast, the cation types (K⁺ versus Na⁺) had a pronounced effect. The osmotic component is important to net NO₃ ^- uptake affecting remarkably the influx in both LATS and HATS components of cowpea roots.     

Transient-state biodegradation behavior of a horizontal biotrickling filter in co-treating gaseous H<Subscript>2</Subscript>S and NH<Subscript>3</Subscript>

A horizontal biotrickling filter (HBTF) was used to inoculate autotrophic sulfide-oxidizing and ammonia-oxidizing microbial consortiums over H₂S-exhausted carbon for co-treating H₂S and NH₃ waste gas in a long-term operation. In this study, several aspects (i.e., pH change, shock loading and starvation) of the dynamic behavior of the HBTF were investigated. The metabolic products of N and S bearing species in recycling liquid and biological activities of the biofilm were analyzed to explain the observed phenomena and further explore the fundamentals behind. In the pH range of 4–8.5, although the removal efficiencies of H₂S and NH₃ remained 96–98% and 100%, respectively, the metabolic products demonstrated different removal mechanisms and pathways. NH₄-N and NO₂/NO₃-N were dominated at pH ≤6 and ≥7, respectively, indicating the differentiated contributions from physical/chemical adsorption and bio-oxidation. Moreover, the HBTF demonstrated a good dynamic stability to withstand shock loadings by recovering immediately to the original. During shock loading, only 15.4% and 17.9% of captured H₂S and NH₃ was biodegraded, respectively. After 2, 11, and 48 days of starvation, the HBTF system reached a full performance within reasonable re-startup times (2–80 h), possibly due to the consumption of reduced S and N species in biomass or activated carbon thus converted into SO₄-S and NO₃-N during starvation period. The results helped to understand the fundamental knowledge by revealing the effects of pH and transient loadings linked with individual removal mechanism for H₂S and NH₃ co-treatment in different conditions.     

Effects of Litter Inputs on N<Subscript>2</Subscript>O Emissions from a Tropical Rainforest in Southwest China

Litter inputs are expected to have a strong impact on soil N₂O efflux. This study aimed to assess the effects of the litter decomposition process and nutrient efflux from litter to soil on soil N₂O efflux in a tropical rainforest. A paired study with a control (L) treatment and a litter-removed (NL) treatment was followed for 2 years, continuously monitoring the effects of these treatments on soil N₂O efflux, fresh litter input, decomposed litter carbon (LCI) and nitrogen (LNI), soil nitrate (NO₃ ^?–N), ammonium (NH₄ ⁺–N), dissolved organic carbon (DOC), and dissolved nitrogen (DN). Soil N₂O flux was 0.48 and 0.32 kg N₂O–N ha^?1 year^?1 for the L and NL treatments, respectively. Removing the litter caused a decrease in the annual soil N₂O emission by 33%. The flux values from the litter layer were higher in the rainy season as compared to the dry season (2.10 ± 0.28 vs. 1.44 ± 0.35 μg N m^?2 h^?1). The N₂O fluxes were significantly correlated with the soil NO₃ ^?–N contents (P < 0.05), indicating that the N₂O emission was derived mainly from denitrification as well as other NO₃ ^? reduction processes. Suitable soil temperature and moisture sustained by rainfall were jointly attributed to the higher soil N₂O fluxes of both treatments in the rainy season. The N₂O fluxes from the L were mainly regulated by LCI, whereas those from the NL were dominated jointly by soil NO₃ ^? content and temperature. The effects of LCI and LNI on the soil N₂O fluxes were the greatest in the 2 months after litter decomposition. Our results show that litter may affect not only the variability in the quantity of N₂O emitted, but also the mechanisms that govern N₂O production. However, further studies are still required to elucidate the impacting mechanisms of litter decomposition on N₂O emission from tropical forests.     

Nitrite/Nitrate (NO<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>) and Zinc Concentrations in Influenza-associated Encephalopathy in Children with Different Sequela

NOx (NO₂ and NO₃) in CSF obtained from 22 patients with influenza-associated encephalopathy were higher than those of a control group. Within the different prognosis, there were no significant differences in NO_x levels. By analyzing the serum obtained from patients infected with influenza, including encephalopathy, with others, the serum zinc levels did show marked differences between them. Four out of eleven patients with influenza-associated encephalopathy showed low zinc levels below the normal range. However, there were no significant differences in the zinc levels between the group with sequela and without sequela. These results indicate that the increase of NO_x levels detected in influenza-associated encephalopathy relates to the low zinc levels, and both low molecules might play an important role for the cause of encephalopathy.     

Ab initio studies on the decomposition kinetics of CF<Subscript>3</Subscript>OCF<Subscript>2</Subscript>O radical

The present study deals with the decomposition of CF₃OCF₂O radical formed from a hydrofluoroether, CF₃OCHF₂ (HFE-125), in the atmosphere. The study is performed using ab initio quantum mechanical methods. Two plausible pathways of decomposition of the titled species have been considered, one involving C-O bond scission and the other occurring via F atom elimination. The geometries of the reactant, products and transition states involved in the decomposition pathways are optimized and characterized at DFT (B3LYP) level of theory using 6-311G(d,p) basis set. Single point energy calculations have been performed at G2M(CC,MP2) level of theory. Out of the two prominent decomposition channels considered, the C-O bond scission is found to be dominant involving a barrier height of 15.3 kcal mol⁻¹ whereas the F-elimination path proceeds with a barrier of 26.1 kcal mol⁻¹. The thermal rate constants for the above two decomposition pathways are evaluated using canonical transition state theory (CTST) and these are found to be 1.78 × 10⁶ s⁻¹ and 2.83 × 10⁻⁷ s⁻¹ for C-O bond scission and F-elimination respectively at 298 K and 1 atm pressure. Transition states are searched on the potential energy surfaces involved during the decomposition channels and each of the transition states is characterized. The existence of transition states on the corresponding potential energy surface is ascertained by performing intrinsic reaction coordinate (IRC) calculation.     

On-line estimation of O<Subscript>2</Subscript> production,CO<Subscript>2</Subscript> uptake,and growth kinetics of microalgal cultures in a gas-tight photobioreactor

Growth of the green algae Chlamydomonas reinhardtii and Chlorella sp. in batch cultures was investigated in a novel gas-tight photobioreactor, in which CO₂, H₂, and N₂ were titrated into the gas phase to control medium pH, dissolved oxygen partial pressure, and headspace pressure, respectively. The exit gas from the reactor was circulated through a loop of tubing and re-introduced into the culture. CO₂ uptake was estimated from the addition of CO₂ as acidic titrant and O₂ evolution was estimated from titration by H₂, which was used to reduce O₂ over a Pd catalyst. The photosynthetic quotient, PQ, was estimated as the ratio between O₂ evolution and CO₂ up-take rates. NH₄ ⁺, NO₂ ⁻, or NO₃ ⁻ was the final cell density limiting nutrient. Cultures of both algae were, in general, characterised by a nitrogen sufficient growth phase followed by a nitrogen depleted phase in which starch was the major product. The estimated PQ values were dependent on the level of oxidation of the nitrogen source. The PQ was 1 with NH₄ ⁺ as the nitrogen source and 1.3 when NO₃ ⁻ was the nitrogen source. In cultures grown on all nitrogen sources, the PQ value approached 1 when the nitrogen source was depleted and starch synthesis became dominant, to further increase towards 1.3 over a period of 3–4 days. This latter increase in PQ, which was indicative of production of reduced compounds like lipids, correlated with a simultaneous increase in the degree of reduction of the biomass. When using the titrations of CO₂ and H₂ into the reactor headspace to estimate the up-take of CO₂, the production of O₂, and the PQ, the rate of biomass production could be followed, the stoichiometrical composition of the produced algal biomass could be estimated, and different growth phases could be identified.     

Insight into the gas phase dissociation of CF<Subscript>3</Subscript>CH<Subscript>2</Subscript>I and its reactions with H and OH by first principles

The Arrhenius kinetic parameters of dissociation reactions and reactions of CF₃CH₂I with radicals like H, O, and OH are determined using highly accurate first principles calculations. Thermophysical properties like molar heat capacity (C_p), thermal stability index, and the bond dissociation energies are also determined for the CF₃CH₂I molecule under the PBE/DNP formalism. Since, there are no theoretical study or experimental investigation reports available regarding the dissociation reactions of CF₃CH₂I and reactions of this molecule with the H and OH radical, a parallel comparative analysis is done with similar iodoalkanes to ascertain the precision of the results obtained. The atmospheric lifetime of 0.54 years is obtained for this molecule.     

Mechanism leading to N<Subscript>2</Subscript>O production in wastewater treating biofilm systems

Wastewater treatment plants are known to be important point sources for nitrous oxide (N₂O) in the anthropogenic N cycle. Biofilm based treatment systems have gained increasing popularity in the treatment of wastewater, but the mechanisms and controls of N₂O formation are not fully understood. Here, we review functional groups of microorganism involved in nitrogen (N) transformations during wastewater treatment, with emphasis on potential mechanism of N₂O production in biofilms. Biofilms used in wastewater treatment typically harbour aerobic and anaerobic zones, mediating close interactions between different groups of N transforming organisms. Current models of mass transfer and biomass interactions in biofilms are discussed to illustrate the complex regulation of N₂O production. Ammonia oxidizing bacteria (AOB) are the prime source for N₂O in aerobic zones, while heterotrophic denitrifiers dominate N₂O production in anoxic zones. Nitrosative stress ensuing from accumulation of NO₂ ^? during partial nitrification or denitrification seems to be one of the most critical factors for enhanced N₂O formation. In AOB, N₂O production is coupled to nitrifier denitrification triggered by nitrosative stress, low O₂ tension or low pH. Chemical N₂O production from AOB intermediates (NH₂OH, HNO, NO) released during high NH₃ turnover seems to be limited to surface-near AOB clusters, since diffusive mass transport resistance for O₂ slows down NH₃ oxidation rates in deeper biofilm layers. The proportion of N₂O among gaseous intermediates (NO, N₂O, N₂) in heterotrophic denitrification increases when NO or nitrous acid (HNO₂) accumulates because of increasing NO₂ ^?, or when transient oxygen intrusion impairs complete denitrification. Limited electron donor availability due to mass transport limitation of organic substrates into anoxic biofilm zones is another important factor supporting high N₂O/N₂ ratios in heterotrophic denitrifiers. Biofilms accommodating Anammox bacteria release less N₂O, because Anammox bacteria have no known N₂O producing metabolism and reduce NO₂ ^? to N₂, thereby lowering nitrosative stress to AOB and heterotrophs.     

Theoretical investigation of reactivities of amines in the <Emphasis Type="Italic">N</Emphasis>-nitrosation reactions by N<Subscript>2</Subscript>O<Subscript>3</Subscript>

N-nitrosamine is a class of carcinogenic, mutagenic, and teratogenic compounds, which can be produced from N-nitrosation of amine by nitrosating agents. N-nitrosation of 19 amines (eight acyclic amines, five heterocyclic amines, and six amines with unsaturated groups) by N₂O₃ was investigated at the CBS-QB3 level of theory. The results indicate that generally the heterocyclic amines have the highest reactivities among the three kinds of amines, whereas the reactivities of the amines with unsaturated and electron-withdrawing groups are relatively low. Frontier molecular orbital analysis indicates that the energy gap between the HOMO of an amine and the LUMO of N₂O₃ has a close connection with the reactivity of an amine. A structure-reactivity relationship of amines in the N-nitrosation reactions by N₂O₃ was established using the stepwise multivariate linear regression. The results indicate that the reactivity of an amine has a definite relationship (R_adj² = 0.947) with the heterolytic bond dissociation energy of R₁R₂N-H bond, energy of HOMO, NBO occupancy of the natural lone pair orbital of N atom, the NBO charge of the N atom, and the pyramidalization angle of an amine. This work will be helpful to gain more insight into the N-nitrosation reactions.