Crystal structure of ammonia dihydrate II

We have used density-functional-theory (DFT) methods together with a structure searching algorithm to make an experimentally constrained prediction of the structure of ammonia dihydrate II (ADH-II). The DFT structure is in good agreement with neutron diffraction data and verifies the prediction. The structure consists of the same basic structural elements as ADH-I, with a modest alteration to the packing, but a considerable reduction in volume. The phase diagram of the known ADH and ammonia monohydrate + water-ice structures is calculated with the Perdew-Burke-Ernzerhof density functional, and the effects of a semi-empirical dispersion corrected functional are investigated. The results of our DFT calculations of the finite-pressure elastic constants of ADH-II are compared with the available experimental data for the elastic strain coefficients.     

Liquid-vapor interfacial properties of water-ammonia mixtures: dependence on ammonia concentration

The equilibrium and dynamical properties of the liquid-vapor interfaces of water-ammonia mixtures are investigated by means of molecular-dynamics simulations. Altogether, we have simulated seven different systems of different concentration of ammonia. The inhomogeneous density, anisotropic orientational profiles, surface tension, and the pattern of hydrogen bonding are calculated for both water and ammonia molecules in order to characterize the location, width, thermodynamic aspects, and microscopic structure of the liquid-vapor interfaces of each of the water-ammonia systems. The dynamical aspects of the interfaces are investigated in terms of the anisotropic diffusion and dipole orientational relaxation of water and ammonia molecules. The properties of the interfaces are compared with those of the corresponding bulk phases. The present theoretical results are also compared with experimental findings wherever available.     

Direct measurement of ammonia in simulated human breath using an inkjet-printed polyaniline nanoparticle sensor

A sensor fabricated from the inkjet-printed deposition of polyaniline nanoparticles onto a screen-printed silver interdigitated electrode was developed for the detection of ammonia in simulated human breath samples. Impedance analysis showed that exposure to ammonia gas could be measured at 962 Hz at which changes in resistance dominate due to the deprotonation of the polymer film. Sensors required minimal calibration and demonstrated excellent intra-electrode baseline drift (≤1.67%). Gases typically present in breath did not interfere with the sensor. Temperature and humidity were shown to have characteristic impedimetric and temporal effects on the sensor that could be distinguished from the response to ammonia. While impedance responses to ammonia could be detected from a single simulated breath, quantification was improved after the cumulative measurement of multiple breaths. The measurement of ammonia after 16 simulated breaths was linear in the range of 40–2175 ppbv (27–1514 μg m⁻³) (r² = 0.9963) with a theoretical limit of detection of 6.2 ppbv (4.1 μg m⁻³) (SN⁻¹ = 3).     

Hierarchical modeling of ammonia adsorption in functionalized metal-organic frameworks

The adsorption of ammonia in four metal-organic frameworks modified with different functional groups (-OH, -C=O, -Cl, -COOH) was investigated using a hierarchical molecular modeling approach. To describe the hydrogen bonding and other strong interactions between NH(3) and the surface functional groups, a set of Morse potential parameters were obtained by fitting to energies from quantum chemical calculations at the MP2 level of theory. We describe a systematic force field parameterization process, in which the Morse parameters were fitted using simulated annealing to match a large number of single-point MP2 energies at various distances and angles. The fitted potentials were then used in grand canonical Monte Carlo simulations to predict ammonia adsorption isotherms and heats of adsorption in functionalized MIL-47, IRMOF-1, IRMOF-10, and IRMOF-16. The results show that ammonia adsorption can be significantly enhanced by using materials with appropriate pore size, strongly interacting functional groups, and high density of functional groups.     

Computational study of water and ammonia dimers with density functional theory methods

The structures and energies for the dimerization of water and ammonia molecules were computed with density functional theory (DFT) and ab initio methods. For all studies the same 6-311+G(2d,2p) basis set was used. Two linear hydrogen-bonded and cyclic ammonia dimer structures were computed and their relative stability is discussed. From the systematic studies, hybrid DFT methods were selected as reliable for computing the parameters of these types of van der Waals' complex.     

The vibrational levels of ammonia

A new six-dimensional potential energy function (PEF) of ammonia expressed in internal coordinates is determined by fitting to points evaluated by Density Functional Theory with the B97-1 functional. The C3v and D3h structures are treated on an equal footing. The inversion barrier is 1820 cm(-1), which is in very good agreement with the experimental value of 1834 cm(-1). The minimum 'reaction path' is well defined by the analytic function up to 40 degrees for the umbrella angle. Using this PEF, the vibrational levels are calculated variationally using three different methods. The first employs the internal kinetic energy operator developed for ammonia by Handy, Carter and Colwell (Mol. Phys. 96 (1999) 477). The second uses the code MULTIMODE (J. Chem. Phys. 107 (1997) 10458), which involves the kinetic energy operator as expressed in normal coordinates by Watson. The third uses an implementation of the reaction path hamiltonian (J. Chem. Phys. 72 (1980) 99) within the MULTIMODE code. All three approaches give similar energies for the vibrational energies of ammonia, and these agree with experiment to within 15 cm(-1) for the fundamental vibrations.     

Ultramicrocoulometric determination of ammonia in serum, plasma, and blood

A method has been developed for the ultramicrodetermination of ammonia in as little as 0.01 ml of serum, plasma, or blood by coulometric titration with electrogenerated hypobromite using a sensitive amperometric endpoint. The only accurate volumetric measurement required is the pipetting of the sample. The sample is added to a 33-mm microdiffusion cell for 10 min to separate the ammonia; the ammonia is collected in 0.1 ml of 0.0025 M sulfuric acid in the center compartment. The separated ammonia is added to a coulometric generation cell where an excess amount of hypobromite has been generated. The decrease in amperometric current due to the utilization of the hypobromite by the ammonia is taken as a measure of the ammonia content; comparison is made with diffused standards. An optimum pH of 8.6 for the titration has been found to give reproducible results at all levels encountered. One-tenth-milliliter blood samples are routinely analyzed. When elevated ammonia levels are encountered, 0.01-ml samples can be used with accurate results. The time for the analysis is 10–15 min after initiating the diffusion.     

Femtosecond relaxation dynamics of solvated electrons in liquid ammonia.

The ultrafast relaxation dynamics of the well-known solvated electron in liquid ammonia solutions are investigated with femtosecond near-infrared pump-probe absorption spectroscopy. Immediately after photoexcitation, the dynamic absorption spectrum of the electron is substantially red-shifted with respect to its stationary spectrum. A subsequent dynamic blue shift of the pump-probe spectrum occurs on a timescale of 150 fs. The data are understood in terms of ground-state "cooling" and can be quantitatively simulated by an intuitive temperature-jump model employing a dynamically evolving Kubo line shape for the electronic resonance. A simple estimate implies that, on average, the electron in the liquid is coordinated to six nearest-neighbor ammonia molecules. An equivalent analysis of the data based on a bubble-formation/cavity-contraction mechanism is briefly outlined.     

Absorption of ammonia in the melt of ammonium nitrate

The paper deals with the absorption of ammonia in a model fertilizer of ammonium nitrate. Volumetric overall mass transfer coefficient quantitatively characterizing the process was estimated on the basis of experimental data and kinetics modeling. The volumetric overall mass transfer coefficient depends on the temperature, content of water in the fertilizer and hydrodynamic conditions. Different hydrodynamic conditions were simulated by different speeds of the propeller stirrer. The empirical equation describing the volumetric overall mass transfer coefficient was proposed.     

Improvement of ammonia sensing properties of polypyrrole by nanocomposite with graphitic materials

The more sensitive and rapid ammonia gas sensors were prepared with nanocomposites of polypyrrole (PPy) and graphitic materials such as graphite, graphite oxide (GO), and reduced graphene oxide (RGO). Pyrrole was polymerized uniformly on the surface of graphitic materials by in situ polymerization method. The structures of nanocomposites were studied by scanning electron microscopy, X-ray diffraction, and X-ray photoelectron spectroscopy indicating the well-exfoliated GO and RGO in PPy matrix with favorable interfacial interaction. PPy/RGO nanocomposite showed the highly improved response in detecting ammonia gas mainly due to the effective electron charge transfer between PPy and ammonia and the efficient transfer of electrical resistance variation by the uniformly dispersed conductive RGO in PPy. PPy/RGO nanocomposite gas sensor also showed the excellent reproducibility in ammonia sensing behavior during the recovery process at lower temperature of 373 K.     

Theoretical study of the ammonia trimer

Open-chain (I) and cyclic (II) structures of the ammonia trimer (NH₃)₃ have been optimized using the 4-3IG extended basis set. The- cyclic structure (II) is found to be the most stable, in agreement with recent experimental results.     

Investigation of proton transport tautomerism in clusters of protonated nucleic acid bases (cytosine, uracil, thymine, and adenine) and ammonia by high-pressure mass spectrometry and ab initio calculations

The energetics of the ion-molecule interactions and structures of the clusters formed between protonated nucleic acid bases (cytosine, uracil, thymine, and adenine) and ammonia have been studied by pulsed ionization high-pressure mass spectrometry (HPMS) and ab initio calculations. For protonated cytosine, uracil, thymine, and adenine with ammonia, the measured enthalpies of association with ammonia are -21.7, -27.9, -22.1, and -17.5 kcal mol-1, respectively. Different isomers of the neutral and protonated nucleic acid bases as well as their clusters with ammonia have been investigated at the B3LYP/6-31+G(d,p) level of theory, and the corresponding binding energetics have also been obtained. The potential energy surfaces for proton transfer and interconversion of the clusters of protonated thymine and uracil with ammonia have been constructed. For cytosine, the experimental binding energy is in agreement with the computed binding energy for the most stable isomer, CN01-01, which is derived from the enol form of protonated cytosine, CH01, and ammonia. Although adenine has a proton affinity similar to that of cytosine, the binding energy of protonated adenine to ammonia is much lower than that for protonated cytosine. This is shown to be due to the differing types of hydrogen bonds being formed. Similarly, although uracil and thymine have similar structures and proton affinities, the binding energies between the protonated species and ammonia are different. Strikingly, the addition of a single methyl group, in going from uracil to thymine, results in a significant structural change for the most stable isomers, UN01-01 and TN03-01, respectively. This then leads to the difference in their measured binding energies with ammonia. Because thymine is found only in DNA while uracil is found in RNA, this provides some potential insight into the difference between uracil and thymine, especially their interactions with other molecules.     

Enantioselective iridium-catalyzed allylic amination of ammonia and convenient ammonia surrogates

Iridium-catalyzed, asymmetric allylation of ammonia as a nucleophile occurs with stereoselectivity to form a symmetric diallylamine, and related allylation of the inexpensive ammonia equivalent potassium trifluoroacetamide or the highly reactive ammonia equivalent lithium di-tert-butyliminodicarboxylate forms a range of conveniently protected, primary, alpha-branched allylic amines in high yields, high branched-to-linear regioselectivities, and high enantiomeric excess. The reactions of ammonia equivalents were conducted with a catalyst generated from a phosphoramidite containing a single stereochemical element.     

Structures and dynamics of protonated ammonia clusters

The structures and infrared spectra of protonated ammonia clusters NH(4+)(NH3)n, for n < or = 8, are investigated using density functional-theory (DFT) calculations and semiempirical DFT/molecular dynamics simulations. For n < 5 the clusters are found to be mostly stable up to 100 K, while the larger clusters (n > or = 5) isomerize. Temperature effects are taken into account by performing ab initio molecular dynamics simulations with the computationally tractable self-consistent charges density functional tight-binding method. The infrared spectra at 10 K for the most stable isomers for n = 3-8 compare qualitatively with predissociation experiments, and using a common scaling factor almost quantitative agreement is found. For n > or = 6 the notion of multiple isomers present under the experimental conditions is supported. Of the 13 stable structures for n = 8 only three are found to survive at 100 K. All other clusters isomerize. Cluster structures are inferred from the analysis of the cumulative radial distribution function of the ammonia molecules surrounding the NH(4+) core. The infrared spectra are found to be typical for the structure of the clusters, which should help to relate the experimentally measured infrared spectra to the number and identity of the contributing isomers. For clusters that reorganize to a more stable isomer during the dynamics, the infrared spectrum is generally similar to that of the stable isomer itself. The clusters are found to preferably form globular structures, although chain-like arrangements are also among the low-energy configurations.     

Excited state hydrogen transfer in fluorophenol.ammonia clusters studied by two-color REMPI spectroscopy

Two-color (1 + 1') REMPI mass spectra of o-, m- and p-fluorophenol.ammonia (1 ration) clusters were measured with a long delay time between excitation and ionization lasers. The appearance of NH(4)(NH(3))(n-1)(+) with 100 ns delay after exciting the S(1) state is a strong indication of generation of long-lived species via S(1). In analogy with the phenol.ammonia clusters, we conclude that an excited state hydrogen transfer reaction occurs in o-, m- and p-fluorophenol.ammonia clusters. The S(1)-S(0) transition of o-, m- and p-fluorophenol.ammonia (1 : 1) clusters were measured by the (1 + 1') REMPI spectra, while larger (1 ration) cluster (n = 2-4) were observed by monitoring the long-lived NH(4)(NH(3))(n-1) clusters action spectra. The vibronic structures of m- and p-fluorophenol.ammonia clusters are assigned based on vibrational calculations in S(0). The o-fluorophenol.ammonia (1 : 1) cluster shows an anharmonic progression that is analyzed by a one-dimensional internal rotational motion of the ammonia molecule. The interaction between the ammonia molecule and the fluorine atom, and its change upon electronic excitation are suggested. The broad action spectra observed for the o-fluorophenol.ammonia (1 : n) cluster (n>== 2) suggest the excited state hydrogen transfer is faster than in m- and p-fluorophenol.ammonia clusters. The different reaction rates between o-, m- and p-fluorophenol.ammonia clusters are found from comparison between the REMPI and action spectra.     

Pretreatment with ammonia water for enzymatic hydrolysis of corn husk, bagasse, and switchgrass

Bagasse, corn husk, and switchgrass were pretreated with ammonia water to enhance enzymatic hydrolysis. The sample (2 g) was mixed with 1–6 mL ammonia water (25–28% ammonia) and autoclaved at 120°C for 20 min. After treatment, the product was vacuum-dried to remove ammonia gas. The dried solid could be used immediately in the enzymatic hydrolysis without washing. The enzymatic hydrolysis was effectively improved with more than 0.5 and 1 mL ammonia water/g for corn husk and bagasse, respectively. In bagasse, glucose, xylose, and xylobiose were the main products. The adsorption of CMCase and xylanase was related to the initial rate of enzymatic hydrolysis. In corn husks, arabinoxylan extracted by pretreatment was substantially unhydrolyzed because of the high ratio of arabinose to xylose (0.6). The carbohydrate yields from cellulose and hemicellulose were 72.9% and 82.4% in bagasse, and 86.2% and 91.9% in corn husk, respectively. The ammonia/water pretreatment also benefited from switchgrass (Miscanthus sinensis and Solidago altissima L.) hydrolysis.     

A polypyrrole-based amperometric ammonia sensor

An ammonia sensor is described in this work. The sensing membrane is a thin layer of oxidized polypyrrole (PPy) on a platinum substrate. This sensor is used as the working electrode in a conventional three-electrode system for amperometric measurement of ammonia in aqueous solutions in the potential range of + 0.2 to + 0.4 V (vs. Ag/AgCl). Contact with ammonia causes a current to flow through the electrode. This current is proportional to the concentration of free ammonia in the solution and ammonium ions do not contribute to the measured signal. The signal is due to reduction of PPy by ammonia with subsequent oxidation of PPy by the external voltage source. The sensor is able to detect ammonia reproducibly at the muM level. The main interference is the doping effect of small anions such as Cl(-) and NO(3)(-), also giving a response on PPy at the mM level. This anionic response can, to a certain degree, be reduced by covering the polymer surface with dodecyl sulfate. The sensor gradually loses its activity when exposed to ammonia concentrations greater than 1 mM. The sensor has been tested by the flow injection analysis technique.     

Reaction of 3,6-disubstituted 4-nitropyridazine 1-oxides with methanolic ammonia and liquid ammonia

Reaction of 3-chloro-6-methyW-nitropyridazine 1-oxide ( 5 ) with methanolic ammonia at 0° led to a replacement of the chlorine atom by a methoxy group as well as by an amino group. Reaction of the 3-methoxy-6-methyl-4-nitropyridazine 1-oxide ( 6 ) with the same reagent led to amino-demethoxylation; this replacement reaction was very slow. Attempts to perform these reactions with liquid ammonia failed. Pmr spectroscopy of solutions of compound 6 in methanolic ammonia revealed that no σ-adduct was present. However in liquid ammonia a 1:1 σ-adduct at C-5 i.e. 12b was formed. 3,6-Dimethoxy-4-nitropyridazine 1-oxide (7) gave with methanolic ammonia an amino-demethoxylation at C-6. No σ-adduct could be detected by pmr spectroscopy. However, in liquid ammonia convincing pmr data were obtained showing the presence of a 1:1 σ-adduct at C-5.     

Influence of additives including amine and hydroxyl groups on aqueous ammonia absorbent for CO2 capture

Aqueous ammonia absorbent (10 wt %) was modified with four kinds of additives (1 wt %) including amine and hydroxyl groups, i.e., 2-amino-2-methyl-1-propanol (AMP), 2-amino-2-methyl-1,3-propandiol (AMPD), 2-amino-2-ethyl-1,3-propandiol (AEPD), and tri(hydroxymethyl) aminomethane (THAM), for CO(2) capture. The loss of ammonia by vaporization was reduced by additives, whereas the removal efficiency of CO(2) was slightly improved. These results were attributed to the interactions between ammonia and additives or absorbents and CO(2) via hydrogen bonding, as verified by FT-IR spectra and computational calculation. Molecular structures as well as binding energies were obtained from the geometries of (ammonia + additives) and (ammonia + additives + CO(2)) at the optimized state. These experimental and theoretical findings demonstrate that additives including amine and hydroxyl group are suitable for modifying aqueous ammonia absorbent for CO(2) removal.     

Monte Carlo simulation for chemical reaction equilibrium of ammonia synthesis in MCM-41 pores and pillared clays

Reactive canonical Monte Carlo (RCMC) method was performed to simulate the chemical reaction equilibrium of ammonia synthesis in two important porous materials: MCM-41 pores and pillared clays. First, our results were compared with those in slit pores in the literature. Then, the effect of other factors such as pore size, pressure and temperature on the chemical equilibrium was investigated. A parameter of the absolute increase of ammonia mole fraction in the pores against that in the bulk phase, Δ_abs, is introduced to describe the effect of confinement on the chemical equilibrium. The yield of ammonia increases with the decrease of pore size, but this increase becomes pronounced at pore sizes of 1.5 nm for MCM-41 pores and 1.02 nm for pillared clays. The yield of ammonia also increases with pressure. In addition, the maximum ammonia mole fraction is attained at 100 bar and 573 K in both MCM-41 pores and pillared clays. When the feed mole ratio of N:H of the bulk phase declines from 4:13 to 4:15, the yield of ammonia in the pore phase also decreases. In addition, the effect of porosity in pillared clays on the chemical equilibrium was simulated.