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This laboratory experiment introduces rocket science from a chemistry perspective. The focus is set on chemical propulsion, including its environmental impact and future development. By combining lecture-based teaching with practical, theoretical, and computational exercises, the students get to evaluate different propellant alternatives. To complete the task, they need to use several important curricular concepts, such as the breaking and formation of bonds, redox reactions, and thermodynamics. They also apply basic computational electronic structure calculations to investigate the energetic content of hitherto nonexisting alternatives. Finally, actual chemical rocket propulsion is demonstrated through the assembly and testing of a model rocket motor, employing a commercially available kit. The full experiment was developed for upper-level high school classes and is completed in a 3-h lab period. The experiment, or parts of it, has also been successfully used both in undergraduate programs and continuing education for teachers. (Contains 3 figures.)

In this work, a scale for pressure‐adapted atomic electronegativity is used to make general predictions of bond polarity in H‐, C‐, N‐ and O‐based compounds experiencing shock conditions. The qualitative picture that emerges is one of increasing polarity of several bonds common in energetic materials. The general predictions made are compared to, and found to support, claims of ionic decomposition routes in compressed nitromethane and nitrate esters at high pressure. Changing electronegativity is also suggested as a factor driving the ionic disproportionation of various molecular phases with compression. Calculations using the eXtreme‐Pressure Polarizable Continuum Model (XP‐PCM) predict increasing energy differences between ground and excited states in non‐bonded H, C, N, and O atoms as a function of pressure. This data enables for a discussion on the reliability of electronegativity‐based rationales at more extreme thermodynamic conditions. [ABSTRACT FROM AUTHOR]

One challenge in chemistry is the plethora of often disparate models for rationalizing the electronic structure of molecules. Chemical concepts abound, but their connections are often frail. This work describes a quantum‐mechanical framework that enables a combination of ideas from three approaches common for the analysis of chemical bonds: energy decomposition analysis (EDA), quantum chemical topology, and molecular orbital (MO) theory. The glue to our theory is the electron energy density, interpretable as one part electrons and one part electronegativity. We present a three‐dimensional analysis of the electron energy density and use it to redefine what constitutes an atom in a molecule. Definitions of atomic partial charge and electronegativity follow in a way that connects these concepts to the total energy of a molecule. The formation of polar bonds is predicted to cause inversion of electronegativity, and a new perspective of bonding in diborane and guanine−cytosine base‐pairing is presented. The electronegativity of atoms inside molecules is shown to be predictive of pKa. [ABSTRACT FROM AUTHOR]

A palladium-catalyzed domino arylation-cyclization of biocatalytically derived cyclic 1,3-dienes is demonstrated. The reaction introduces a high degree of structural complexity in a single step, giving access to tricyclic tetrahydrofluorenones with full regio- and stereoselectivity. The transformation proceeds through a novel acylation-terminated Heck-type sequence, and quantum chemical calculations indicate that C–H activation is involved in the terminating acylation step. [ABSTRACT FROM AUTHOR]

We present a general protocol for the formal Michael addition of acetone to α,β‐unsaturated esters and amides, a transformation difficult to perform using current methods. The protocol comprises of an amidine catalyzed relay ring‐opening and fragmentation of 3,4‐dihydropyranones. The reaction proceeds under mild conditions, has a broad substrate scope and the products can be isolated in good to excellent yields. The method can be applied to homochiral substrates with total preservation of chiral information, generating products in high optical purity. Kinetic experiments supported by quantum chemical modeling indicate a mechanism in which the catalyst takes a bifunctional role, acting both as a Brønsted base and as a hydrogen‐bond donor. [ABSTRACT FROM AUTHOR]

Fluorination is a proven method for challenging the limits of chemistry, both structurally and electronically. Here we explore computationally how pressures below 300 GPa affect the fluorination of several transition metals. A plethora of new structural phases are predicted along with the possibility for synthesizing four unobserved compounds: TcF7, CdF3, OsF8, and IrF8. The Ir and Os octaflourides are both predicted to be stable as quasi‐molecular phases with an unusual cubic ligand coordination, and both compounds formally correspond to a high oxidation state of +8. Electronic‐structure analysis reveals that otherwise unoccupied 6p levels are brought down in energy by the combined effects of pressure and a strong ligand field. The valence expansion of Os and Ir enables ligand‐to‐metal F 2p→M 6p charge transfer that strengthens M−F bonds and decreases the overall bond polarity. The lower stability of IrF8, and the instability of PtF8 and several other compounds below 300 GPa, is explained by the occupation of M−F antibonding orbitals in octafluorides with a metal‐valence‐electron count exceeding 8. [ABSTRACT FROM AUTHOR]

The mechanism of a base-catalyzed one-pot reaction of 2-cyanobenzaldehyde and primary nitroalkanes, to produce 3-substituted isoindolinones, is examined. A route starting with a nitroaldol (Henry) reaction, followed by a subsequent cyclization and rearrangement, is supported by intermediate analogous synthesis and direct diastereoselective crystallization from the reaction mixture is achieved.

The effect of supramolecular control in carbohydrate epimerization is described based on a new anion-recognition system. The effects are shown to be sensitive to the carbohydrate structure, which has needed a H1-, H3-, H5-cis pattern for efficient complexation.

A study was conducted to analyze 1,3 dipolar cycloaddition reactions of dinitraminic acid (HDN) and its proton transfer isomer (HO(O)NNN[O.sub.2]). The reactivity trends observed in HDN could be used to design unsaturated compounds that are highly reactive toward azides and chemically inert toward dinitramides.

The paper collects the answers of the authors to the following questions:Is the lack of precision in the definition of many chemical concepts one of the reasons for the coexistence of many partition schemes?Does the adoption of a given partition scheme imply a set of more precise definitions of the underlying chemical concepts?How can one use the results of a partition scheme to improve the clarity of definitions of concepts?Are partition schemes subject to scientific Darwinism? If so, what is the influence of a community's sociological pressure in the "natural selection" process?To what extent does/can/should investigated systems influence the choice of a particular partition scheme?Do we need more focused chemical validation of Energy Decomposition Analysis (EDA) methodology and descriptors/terms in general?Is there any interest in developing common benchmarks and test sets for cross‐validation of methods?Is it possible to contemplate a unified partition scheme (let us call it the "standard model" of partitioning), that is proper for all applications in chemistry, in the foreseeable future or even in principle?In the end, science is about experiments and the real world. Can one, therefore, use any experiment or experimental data be used to favor one partition scheme over another? © 2019 Wiley Periodicals, Inc. [ABSTRACT FROM AUTHOR]

We show that the pressure acting on atoms and molecular systems within the compression cavity of the eXtreme‐Pressure Polarizable Continuum method can be expressed in terms of the electron density of the systems and of the Pauli‐repulsion confining potential. The analytical expression holds for spherical cavities as well as for cavities constructed from van der Waals spheres of the constituting atoms of the molecular systems. © 2018 Wiley Periodicals, Inc. The eXtreme‐Pressure Polarizable Continuum model is a quantum chemical method aimed to describe the effects of the extreme‐high pressure (> 1 GPa) on the properties of atomic and molecular systems, by confining the systems within a var der Waals cavity of an external transmitting medium. This article explains the pressure acting on atoms and molecular systems within the cavity can be analytically expressed in terms of the electron density ρ (r) of the atomic or molecular systems and of the confining potential Z(f). [ABSTRACT FROM AUTHOR]

Reactions of the BH4− anion with equimolar amounts of HN(NO2)2 or of BH3⋅THF with K[N(NO2)2]− produced a mono-substituted [BH3N(NO2)2]− anion, which contains a B−N connected dinitramido ligand. The reaction of BH4− with two equivalents of HN(NO2)2 afforded the di-substituted borate anion consisting of two isomers, one with both nitramido ligands attached to B through N and the other one with one ligand attached through N and the other one through O. The disubstituted dinitramidoborates are marginally stable under ambient conditions, and the isomer with two N-connected ligands was characterized by its crystal structure. A tri-substituted borate was tentatively identified by NMR in the reaction of BH4− with a large excess of HN(NO2)2. All of the anions are highly energetic. Theoretical calculations show that the energy differences between the B−N and B−O tautomers are small, explaining the formation of both. [ABSTRACT FROM AUTHOR]

The Td-symmetric [CsO4]+ ion, featuring Cs in an oxidation state of 9, is computed to be a minimum. Cs uses outer core 5s and 5p orbitals to bind the oxygen atoms. The valence Cs 6s orbital lies too high to be involved in bonding, and contributes to Rydberg levels only. From a molecular orbital perspective, the bonding scheme is reminiscent of XeO4: an octet of electrons to bind electronegative ligands, and no low-lying acceptor orbitals on the central atom. In this sense, Cs+ resembles hypervalent Xe. [ABSTRACT FROM AUTHOR]

Die Reaktionen von Nitramin, N-Methylnitramin und N,N-Dimethylnitramin mit wasserfreiem HF und den binären supersauren Systemen HF/MF5 (M=As, Sb) wurden bei Temperaturen unterhalb von −40 °C untersucht. Mittels multinuklearer NMR-Spektroskopie konnte in Lösung ausschließlich die O-Protonierung beobachtet werden, wobei aus reiner HF-Lösung selbst bei −78 °C kein Feststoff isoliert werden konnte. Im HF/MF5-System konnte jedoch protoniertes Nitramin zum ersten Mal in Form feuchtigkeitsempfindlicher MF6−-Salze isoliert werden, die sich bei Temperaturen oberhalb von −40 °C zersetzen. Im Feststoff resultiert abhängig vom Gegenion ein O-protoniertes oder N-protoniertes Kation. Dies ist in Einklang mit Rechnungen, die einen nur minimalen Energieunterschied zwischen O-Protonierung und N-Protonierung aufzeigen. Die Salze [H2N-NO2H][AsF6], [H3N-NO2][SbF6], [MeHNNO2H][SbF6] und [Me2NNO2H][SbF6] wurden durch ihre Röntgenkristallstrukturen charakterisiert. [ABSTRACT FROM AUTHOR]

The reactions of nitramine, N-methyl nitramine, and N, N-dimethyl nitramine with anhydrous HF and the superacids HF/MF5 (M=As, Sb) were investigated at temperatures below −40 °C. In solution, exclusive O-protonation was observed by multinuclear NMR spectroscopy. Whereas no solid product could be isolated from the neat HF solutions even at −78 °C, in the HF/MF5 systems, protonated nitramine MF6− salts were isolated for the first time as moisture-sensitive solids that decompose at temperatures above −40 °C. In the solid state, depending on the counterion, O-protonated or N-protonated cations can be formed, in accord with theoretical calculations which show that the energy differences between O-protonation and N-protonation are very small. The salts [H2N-NO2H][AsF6], [H3N-NO2][SbF6], [MeHNNO2H][SbF6], and [Me2NNO2H][SbF6] were characterized by their X-ray crystal structures. [ABSTRACT FROM AUTHOR]

The CF2H group, a potential surrogate for the OH group, can act as an unusual hydrogen bond donor, as confirmed by crystallographic, spectroscopic, and computational methods. Here, we demonstrate the bioisosterism of the OH and CF2H groups and the important roles of CF2-H···O hydrogen bonds in influencing intermolecular interactions and conformational preferences. Experimental evidence, corroborated by theory, reveals the distinctive nature of CF2H hydrogen bonding interactions relative to their normal OH hydrogen bonding counterparts. [ABSTRACT FROM AUTHOR]

Atomic and cationic radii have been calculated for the first 96 elements, together with selected anionic radii. The metric adopted is the average distance from the nucleus where the electron density falls to 0.001 electrons per bohr3, following earlier work by Boyd. Our radii are derived using relativistic all-electron density functional theory calculations, close to the basis set limit. They offer a systematic quantitative measure of the sizes of non-interacting atoms, commonly invoked in the rationalization of chemical bonding, structure, and different properties. Remarkably, the atomic radii as defined in this way correlate well with van der Waals radii derived from crystal structures. A rationalization for trends and exceptions in those correlations is provided. [ABSTRACT FROM AUTHOR]

α-Fluorocarbanions are key intermediates in nucleophilic fluoroalkylation reactions. Although frequently discussed, the origin of the fluorine effect on the reactivity of α-fluorinated CH acids has remained largely unexplored. We have now investigated the kinetics of a series of reactions of α-substituted carbanions with reference electrophiles to elucidate the effects of α-F, α-Cl, and α-OMe substituents on the nucleophilic reactivities of carbanions. [ABSTRACT FROM AUTHOR]

The energy change per electron in a chemical or physical transformation, ΔE/n, may be expressed as Δχ̅ + Δ (VNN + ω)/n, where Δχ̅ is the average electron binding energy, a generalized electronegativity, ΔVNN is the change in nuclear repulsions, and Δω is the change in multielectron interactions in the process considered. The last term can be obtained by the difference from experimental or theoretical estimates of the first terms. Previously obtained consequences of this energy partitioning are extended here to a different analysis of bonding in a great variety of diatomics, including more or less polar ones. Arguments are presented for associating the average change in electron binding energy with covalence, and the change in multielectron interactions with electron transfer, either to, out, or within a molecule. A new descriptor Q, essentially the scaled difference between the Δχ̅ and Δ (VNN + ω)/n terms, when plotted versus the bond energy, separates nicely a wide variety of bonding types, covalent, covalent but more correlated, polar and increasingly ionic, metallogenic, electrostatic, charge-shift bonds, and dispersion interactions. Also, Q itself shows a set of interesting relations with the correlation energy of a bond. [ABSTRACT FROM AUTHOR]

Two ammonia-(dinitramido)boranes were synthesized by the reaction of dinitroamine with ammonia-borane. These compounds are the first reported examples of (dinitramido)boranes. Ammonia-mono(dinitramido)borane is a perfectly oxygen-balanced high-energy-density material (HEDM) composed of an ammonia-BH2 fuel group and a strongly oxidizing dinitramido ligand. Although it is thermally not stable enough for practical applications, its predicted specific impulse as a solid rocket propellant would be 333 s. Its predicted performance as an explosive matches that of pentaerythtritol tetranitrate (PETN) and significantly exceeds that of trinitrotoluene (TNT). Its structure was established by X-ray crystallography and vibrational and multinuclear NMR spectroscopy. Additionally, the over-oxidized ammoniabis(dinitramido)borane was detected by NMR spectroscopy. [ABSTRACT FROM AUTHOR]

Following the work of L. C. Allen, this work begins by relating the central chemical concept of electronegativity with the average binding energy of electrons in a system. The average electron binding energy, χ, is in principle accessible from experiment, through photoelectron and X-ray spectroscopy. It can also be estimated theoretically. χ has a rigorous and understandable connection to the total energy. That connection defines a new kind of energy decomposition scheme. The changing total energy in a reaction has three primary contributions to it: the average electron binding energy, the nuclear--nuclear repulsion, and multielectron interactions. This partitioning allows one to gain insight into the predominant factors behind a particular energetic preference. We can conclude whether an energy change in a transformation is favored or resisted by collective changes to the binding energy of electrons, the movement of nuclei, or multielectron interactions. For example, in the classical formation of H2 from atoms, orbital interactions dominate nearly canceling nuclear--nuclear repulsion and two-electron interactions. While in electron attachment to an H atom, the multielectron interactions drive the reaction. Looking at the balance of average electron binding energy, multielectron, and nuclear--nuclear contributions one can judge when more traditional electronegativity arguments can be justifiably invoked in the rationalization of a particular chemical event. [ABSTRACT FROM AUTHOR]

NF3 and N(NO2)3 are known compounds, whereas the mixed fluoronitroamines, FN(NO2)2 and F2NNO2, have been unknown thus far. One of these, FN(NO2)2, has now been prepared and characterized by multinuclear NMR and Raman spectroscopy. FN(NO2)2 is the first known example of an inorganic fluoronitroamine. It is a thermally unstable, highly energetic material formed by the fluorination of the dinitramide anion using NF4+ salts as the preferred fluorinating agent. [ABSTRACT FROM AUTHOR]

NF3 and N(NO2)3 are known compounds, whereas the mixed fluoronitroamines, FN(NO2)2 and F2NNO2, have been unknown thus far. One of these, FN(NO2)2, has now been prepared and characterized by multinuclear NMR and Raman spectroscopy. FN(NO2)2 is the first known example of an inorganic fluoronitroamine. It is a thermally unstable, highly energetic material formed by the fluorination of the dinitramide anion using NF4+ salts as the preferred fluorinating agent. [ABSTRACT FROM AUTHOR]

The trifluoromethanide anion is the postulated key intermediate in nucleophilic trifluoromethylation reactions. However, for more than six decades, the trifluoromethanide anion was widely believed to exist only as a short-lived transient species in the condensed phase. It has now been prepared in bulk for the first time in THF solution. The trifluoromethanide anion with the [K(18-crown-6)]+ cation was unequivocally characterized by low-temperature 19F and 13C NMR spectroscopy. Its intermediacy in nucleophilic trifluoromethylation reactions was directly evident by its reaction chemistry with various electrophilic substrates. Variable-temperature NMR spectroscopy, along with quantum mechanical calculations, support the persistence of the trifluoromethanide anion. [ABSTRACT FROM AUTHOR]

We propose a principle for H-bonding activation in acylation of hydroxyl groups, where the acylation is activated by the formation of hydrogen bonds between hydroxyl groups and anions. With the guidance of this principle, we demonstrate a method for the selective acylation of carbohydrates. By this method, diols and polyols are regioselectively acetylated in high yields under mild conditions using catalytic amounts of acetate. In comparison to other methods involving reagents such as organotin, organoboron, organosilicon, organobase, and metal salts, this method is more environmentally friendly, convenient, and efficient and is also associated with higher regioselectivity. We have performed a thorough quantum chemical study to decipher the mechanism, which suggests that acetate first forms a dual H-bond complex with a diol, which enables subsequent monoacylation by acetic anhydride under mild conditions. The regioselectivity appears to originate from the inherent structure of the diols and polyols and their specific interactions with the coordinating acetate catalyst. [ABSTRACT FROM AUTHOR]

The elusive nitryl cyanide, NCNO2, has been synthesized and characterized. It was prepared in good yield, isolated by fractional condensation, characterized by NMR and vibrational spectroscopy, and studied by theoretical calculations. Nitryl cyanide holds promise as a high energy density material (HEDM) and might also prove useful as a HEDM building block. The simplicity and inherent stability of nitryl cyanide, together with the known multitude of nitriles in interstellar space, suggest that the compound might also be a potential candidate for observations in atmospheric and interstellar chemistry. [ABSTRACT FROM AUTHOR]

The elusive nitryl cyanide, NCNO2, has been synthesized and characterized. It was prepared in good yield, isolated by fractional condensation, characterized by NMR and vibrational spectroscopy, and studied by theoretical calculations. Nitryl cyanide holds promise as a high energy density material (HEDM) and might also prove useful as a HEDM building block. The simplicity and inherent stability of nitryl cyanide, together with the known multitude of nitriles in interstellar space, suggest that the compound might also be a potential candidate for observations in atmospheric and interstellar chemistry. [ABSTRACT FROM AUTHOR]