chemistry Chemistry is the scientific study of the properties and behavior of matter. It is a natural science that covers the elements that make up matter to the compounds made of atoms, molecules and ions: their composition, structure, proper ...

, the pentazenium

cation An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by conven ...

(also known as pentanitrogen) is a positively-charged

polyatomic ion A polyatomic ion, also known as a molecular ion, is a covalent bonded set of two or more atoms, or of a metal complex, that can be considered to behave as a single unit and that has a net charge that is not zero. The term molecule may or may no ...

with the

chemical formula In chemistry, a chemical formula is a way of presenting information about the chemical proportions of atoms that constitute a particular chemical compound or molecule, using chemical element symbols, numbers, and sometimes also other symbols, ...

and

structure A structure is an arrangement and organization of interrelated elements in a material object or system, or the object or system so organized. Material structures include man-made objects such as buildings and machines and natural objects such a ...

. Together with solid nitrogen polymers and the

azide In chemistry, azide is a linear, polyatomic anion with the formula and structure . It is the conjugate base of hydrazoic acid . Organic azides are organic compounds with the formula , containing the azide functional group. The dominant appli ...

anion, it is one of only three poly-

nitrogen Nitrogen is the chemical element with the symbol N and atomic number 7. Nitrogen is a nonmetal and the lightest member of group 15 of the periodic table, often called the pnictogens. It is a common element in the universe, estimated at se ...

species obtained in bulk quantities.

History

Within the High Energy Density Matter research program, run by the U.S. Air Force since 1986, systematic attempts to approach polynitrogen compounds began in 1998, when

Air Force Research Laboratory The Air Force Research Laboratory (AFRL) is a scientific research organization operated by the United States Air Force Materiel Command dedicated to leading the discovery, development, and integration of aerospace warfighting technologies, pl ...

at Edwards AFB became interested in researching alternatives to the highly toxic

hydrazine Hydrazine is an inorganic compound with the chemical formula . It is a simple pnictogen hydride, and is a colourless flammable liquid with an ammonia-like odour. Hydrazine is highly toxic unless handled in solution as, for example, hydrazine ...

-based rocket fuel and simultaneously funded several such proposals. Karl O. Christe, then, a senior investigator at AFRL, chose to attempt building linear out of and , based on the proposed bond structure: : The reaction succeeded, and was created in sufficient quantities to be fully characterized by NMR, IR and Raman spectroscopy in 1999. The salt was highly explosive, but when was replaced by , a stronger Lewis acid, much more stable was produced, shock-resistant and thermally stable up to 60–70 °C. This made bulk quantities, easy handling, and X-ray crystal structure analysis possible.

Preparation

Reaction of and in dry HF at −78 °C is the only known method so far: : :

Chemistry

is capable of oxidizing water, NO, and , but not or ; its electron affinity is 10.44 eV (1018.4 kJ/mol). For this reason, must be prepared and handled in a dry environment: : : Due to stability of the fluoroantimonate, it is used as the precursor for all other known salts, typically accomplished by metathesis reactions in non-aqueous solvents such as HF, , , or , where suitable hexafluoroantimonates are insoluble: : The most stable salts of decompose when heated to 50–60 °C: , , and , while the most unstable salts that were obtained and studied, and were extremely shock and temperature sensitive, exploding in solutions as dilute as 0.5 mmol. A number of salts, such as fluoride, azide, nitrate, or perchlorate, cannot be formed.

Structure and bonding

valence bond theory In chemistry, valence bond (VB) theory is one of the two basic theories, along with molecular orbital (MO) theory, that were developed to use the methods of quantum mechanics to explain chemical bonding. It focuses on how the atomic orbitals of ...

, pentazenium can be described by six

resonance structure In chemistry, resonance, also called mesomerism, is a way of describing bonding in certain molecules or polyatomic ions by the combination of several contributing structures (or ''forms'', also variously known as ''resonance structures'' or ' ...

s: :, where the last three structures have smaller contributions to the overall structure because they have less favorable

formal charge In chemistry, a formal charge (F.C. or q), in the covalent view of chemical bonding, is the charge assigned to an atom in a molecule, assuming that electrons in all chemical bonds are shared equally between atoms, regardless of relative electro ...

states than the first three. According to both ''ab initio'' calculations and the experimental X-ray structure, the cation is planar, symmetric, and approximately V-shaped, with bond angles 111° at the central atom (angle N2–N3–N4) and 168° at the second and fourth atoms (angles N1–N2–N3 and N3–N4–N5). The bond lengths for N1–N2 and N4–N5 are 1.10 Å and the bond lengths N2–N3 and N3–N4 are 1.30 Å.

References

{{reflist Cations Nitrogen Explosive chemicals

History

Preparation

Chemistry

Structure and bonding

See also

References