Chemical Properties
silvery metal; α: ortho-rhomb, a=0.4721 nm, b=0.4888 nm, c=0.6661nm, stable from room temp to 280°C; β: tetra, a=0.4895 nm, c=0.3386nm, stable from 280–577°C; γ: a=0.3518 nm; bcc, stable from 577–637°C; enthalpy of vaporization 418 kJ/mol; enthalpy of fusion 3.20 kJ/mol; 237Np, t1/2=2.14×10+6 years, t1/2 of 236Np 1.29×10+6 years; discovered in 1940; produced in kg amounts as a by-product of plutonium production [MER06] [KIR78] [CRC10]
Uses
Source material for production of 238U (power source).
Hazard
A radioactive poison.
Description
ML300 is a selective non-covalent inhibitor of SARS-CoV 3CL protease (IC50 = 4.11 μM).1,2
Physical properties
The chemistry of neptunium (93Np) is somewhat similar to that of uranium (92U) and plutonium(94Pu), which immediately precede and follow it in the actinide series on the periodictable. The discovery of neptunium provided a solution to a puzzle as to the missing decayproducts of the thorium decay series, in which all the elements have mass numbers evenlydivisible by four; the elements in the uranium series have mass numbers divisible by fourwith a remainder of two. The actinium series elements have mass numbers divisible by fourwith a remainder of three. It was not until the neptunium series was discovered that a decayseries with a mass number divisible by four and a remainder of one was found. The neptuniumdecay series proceeds as follows, starting with the isotope plutonium-241: Pu-241→Am-241→Np-237→Pa-233→U-233→Th-229→Ra-225→Ac-225→Fr-221→At-217→Bi-213→Ti-209→Pb-209→Bi-209.
Neptunium is a silvery-white radioactive, heavy metal. Its melting point is 644°C, its boilingpoint is 3,902°C, and its density is 20.25g/cm3.
Isotopes
There are a total of 23 isotopes of neptunium. None are stable. All are radioactivewith half-lives ranging from two microseconds to 2.144×10+6years for the isotopeNp-237, which spontaneously fissions through alpha decay.
Origin of Name
Named for the planet Neptune.
Occurrence
At one time, neptunium’s entire existence was synthesized by man. Sometime later, in themid-twentieth century, it was discovered that a very small amount is naturally produced inuranium ore through the actions of neutrons produced by the decay of uranium in the orepitchblende. Even so, a great deal more neptunium is artificially produced every year than everdid or does exist in nature. Neptunium is recovered as a by-product of the commercial productionof plutonium in nuclear reactors. It can also be synthesized by bombarding uranium-238with neutrons, resulting in the production of neptunium-239, an isotope of neptunium witha half-life of 2.3565 days.
Characteristics
Neptunium is the first of the subseries of the actinide series known as the transuranic elements—those heavy, synthetic (man-made) radioactive elements that have an atomic numbergreater than uranium in the actinide series of the periodic table. An interesting fact is thatneptunium was artificially synthesized before small traces of it were discovered in nature. Moreis produced by scientists every year than exists in nature.
Neptunium has an affinity for combining with nonmetals (as do all transuranic elements)such as oxygen, the halogens, sulfur, and carbon.
Production Methods
Neptunium-237 is obtained as a by-product of making plutonium from uranium isotopes in nuclear reactors. Significant amounts of this element may be recovered from plutonium plant nuclear wastes. Both the recovery and purification of neptunium can be carried out by various chemical processes, including precipitation, solvent extraction and ion exchange.
Neptunium-237 may be synthesized by bombarding uranium-235 or uranium-238 with neutrons:
Neptunium-239 may be obtained from uranium-238 by neutron bombardment as it was first produced:
Neptunium may be prepared in the metallic state by the reduction of its trifluoride with barium vapor at 1,200°C followed by rapid cooling. Its tetrafluoride may be reduced with excess calcium metal at about 750°C under argon atmosphere.
References
Turlington et al. (2012) Non-covalent triazole-based inhibitors of the SARS main protease 3CLpro; In: Probe Reports from the NIH Molecular Libraries Program
Turlington et al. (2013) Discovery of N-(Benzo[1,2,3]triazol-1-yl)-N-(benzyl)acetamido)phenyl)carboxamides as severe acute respiratory syndrome coronavirus (SARS-CoV) 3CLpro inhibitors: Identification of ML300 and noncovalent nanomolar inhibitors with an induced-fit binding; Bioorg. Med. Chem. Lett. 23 6172
