chemistry Chemistry is the science, scientific study of the properties and behavior of matter. It is a natural science that covers the Chemical element, elements that make up matter to the chemical compound, compounds made of atoms, molecules and ions ...

, the decay technique is a method to generate

chemical species A chemical species is a chemical substance or ensemble composed of chemically identical molecular entity, molecular entities that can explore the same set of molecular energy levels on a characteristic or delineated time scale. These energy levels ...

such as

radicals Radical may refer to: Politics and ideology Politics *Radical politics, the political intent of fundamental societal change *Radicalism (historical), the Radical Movement that began in late 18th century Britain and spread to continental Europe and ...

carbocation A carbocation is an ion with a positively charged carbon atom. Among the simplest examples are the methenium , methanium and vinyl cations. Occasionally, carbocations that bear more than one positively charged carbon atom are also encountere ...

s, and other potentially unstable

covalent A covalent bond is a chemical bond that involves the sharing of electrons to form electron pairs between atoms. These electron pairs are known as shared pairs or bonding pairs. The stable balance of attractive and repulsive forces between atoms ...

structures by

radioactive decay Radioactive decay (also known as nuclear decay, radioactivity, radioactive disintegration, or nuclear disintegration) is the process by which an unstable atomic nucleus loses energy by radiation. A material containing unstable nuclei is consid ...

of other compounds. For example, decay of a

tritium Tritium ( or , ) or hydrogen-3 (symbol T or H) is a rare and radioactive isotope of hydrogen with half-life about 12 years. The nucleus of tritium (t, sometimes called a ''triton'') contains one proton and two neutrons, whereas the nucleus o ...

-labeled molecule yields an ionized

helium Helium (from el, ἥλιος, helios, lit=sun) is a chemical element with the symbol He and atomic number 2. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas and the first in the noble gas group in the periodic table. ...

atom, which might then break off to leave a cationic molecular fragment. The technique was developed in 1963 by the

Italian Italian(s) may refer to: * Anything of, from, or related to the people of Italy over the centuries ** Italians, an ethnic group or simply a citizen of the Italian Republic or Italian Kingdom ** Italian language, a Romance language *** Regional Ita ...

chemist A chemist (from Greek ''chēm(ía)'' alchemy; replacing ''chymist'' from Medieval Latin ''alchemist'') is a scientist trained in the study of chemistry. Chemists study the composition of matter and its properties. Chemists carefully describe th ...

Fulvio Cacace Fulvio Cacace (died 1 December 2003) was an Italian chemist. In 1963, while at the Sapienza University of Rome, he devised the decay technique for the study of organic radicals and carbenium cations. The technique is based on the preparation of c ...

at the University of Rome. It has allowed the study of a vast number of otherwise inaccessible compounds and reactions. It has also provided much of our current knowledge about the chemistry of the

helium hydride ion The helium hydride ion or hydridohelium(1+) ion or helonium is a cation (positively charged ion) with chemical formula HeH+. It consists of a helium atom bonded to a hydrogen atom, with one electron removed. It can also be viewed as protonated h ...

Carbocation generation

In the basic method, a molecule is prepared where the vacant bond of the desired radical or ion is satisfied by an atom of tritium , the radioactive

isotope Isotopes are two or more types of atoms that have the same atomic number (number of protons in their nuclei) and position in the periodic table (and hence belong to the same chemical element), and that differ in nucleon numbers (mass numbers) ...

hydrogen Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-toxic, an ...

with

mass number The mass number (symbol ''A'', from the German word ''Atomgewicht'' tomic weight, also called atomic mass number or nucleon number, is the total number of protons and neutrons (together known as nucleons) in an atomic nucleus. It is approxima ...

3. As the tritium undergoes

beta decay In nuclear physics, beta decay (β-decay) is a type of radioactive decay in which a beta particle (fast energetic electron or positron) is emitted from an atomic nucleus, transforming the original nuclide to an isobar of that nuclide. For ...

(with a

half-life Half-life (symbol ) is the time required for a quantity (of substance) to reduce to half of its initial value. The term is commonly used in nuclear physics to describe how quickly unstable atoms undergo radioactive decay or how long stable ato ...

of 12.32 years), it is transformed into an ion of

helium-3 Helium-3 (3He see also helion) is a light, stable isotope of helium with two protons and one neutron (the most common isotope, helium-4, having two protons and two neutrons in contrast). Other than protium (ordinary hydrogen), helium-3 is the ...

, creating the cation . In the decay, an

electron The electron ( or ) is a subatomic particle with a negative one elementary electric charge. Electrons belong to the first generation of the lepton particle family, and are generally thought to be elementary particles because they have no kn ...

and an

antineutrino A neutrino ( ; denoted by the Greek letter ) is a fermion (an elementary particle with spin of ) that interacts only via the weak interaction and gravity. The neutrino is so named because it is electrically neutral and because its rest mass is ...

are ejected at great speed from the tritium nucleus, changing one of the neutrons into a proton with the release of 18,600

electronvolt In physics, an electronvolt (symbol eV, also written electron-volt and electron volt) is the measure of an amount of kinetic energy In physics, the kinetic energy of an object is the energy that it possesses due to its motion. It is defi ...

s (eV) of energy. The neutrino escapes the system; the electron is generally captured within a short distance, but far enough away from the site of the decay that it can be considered lost from the molecule. Those two particles carry away most of the released energy, but their departure causes the nucleus to recoil, with about 1.6 eV of energy. This recoil energy is larger than the

bond strength In chemistry, bond energy (''BE''), also called the mean bond enthalpy or average bond enthalpy is the measure of bond strength in a chemical bond. IUPAC defines bond energy as the average value of the gas-phase bond-dissociation energy (usually a ...

of the carbon–helium bond (about 1 eV), so this bond breaks. The helium atom almost always leaves as a neutral , leaving behind the

. These events happen very quickly compared to typical molecular relaxation times, so the carbocation is usually created in the same conformation and electronic configuration as the original neutral molecule. For example, decay of tritiated methane, (R = R′ = R″ = H) produces the

carbenium ion A carbenium ion is a positive ion with the structure RR′R″C+, that is, a chemical species with a trivalent carbon that bears a +1 formal charge. In older literature the name carbonium ion was used for this class, but now it refers exclusivel ...

in a tetrahedral conformation, with one of the orbitals having a single unpaired electron and the other three forming a trigonal pyramid. The ion then relaxes to its more favorable

trigonal planar In chemistry, trigonal planar is a molecular geometry model with one atom at the center and three atoms at the corners of an equilateral triangle, called peripheral atoms, all in one plane. In an ideal trigonal planar species, all three ligands a ...

form, with release of about 30

kcal The calorie is a unit of energy. For historical reasons, two main definitions of "calorie" are in wide use. The large calorie, food calorie, or kilogram calorie was originally defined as the amount of heat needed to raise the temperature of on ...

/ mol of energy—that goes into vibrations and rotation of the ion. The carbocation then can interact with surrounding molecules in many reactions that cannot be achieved by other means. When formed within a rarefied gas, the carbocation and its reactions can be studied by

mass spectrometry Mass spectrometry (MS) is an analytical technique that is used to measure the mass-to-charge ratio of ions. The results are presented as a ''mass spectrum'', a plot of intensity as a function of the mass-to-charge ratio. Mass spectrometry is use ...

techniques. However the technique can be used also in condensed matter (liquids and solids). In liquid phase, the carbocation is initially formed in the same solvation state as the parent molecule, and some reactions may happen before the solvent shells around it have time to rearrange. In a crystalline solid, the cation is formed in the same crystalline site; and the nature, position, and orientation of the other reagent(s) are strictly constrained.

Radical formation

In a condensed phase, the carbocation can also gain an electron from surrounding molecules, thus becoming an electrically neutral radical. For example, in crystalline naphthalene, a molecule with tritium substituted for hydrogen in the 1 (or 2) position will be turned by decay into a cation with a positive charge at that position. That charge will however be quickly neutralized by an electron transported through the lattice, turning the molecule into the 1-naphthyl (or 2-naphthyl) radical; which are stable, trapped in the solid, below .

Persistent bound structures

Whereas the carbon–helium-ion bond breaks spontaneously and immediately to yield a carbocation, bonds of other elements to helium are more stable. For example, molecular tritium or tritium-hydrogen . On decay, these form a stable

(respectively or ), which is stable enough to persist. This cation is claimed to be the strongest acid known, and will protonate any other molecule it comes in contact with. This is another route to creating cations that are not obtainable in other ways. In particular (or ) will protonate methane to the

carbonium ion In chemistry, a carbonium ion is any cation that has a pentavalent carbon atom. The name carbonium may also be used for the simplest member of the class, properly called methanium (), where the five valences are filled with hydrogen atoms. The nex ...

(or ). Other structures that are expected to be stable when formed by beta-decay of tritium precursors include ³HeLi⁺, B₂H₅³He⁺, and BeH³He⁺ according to theoretical calculations.

Other nuclear decay processes

Radioisotopic decay of other elements besides tritium can yield other stable covalent structures. For example, the first successful synthesis of the

perbromate In chemistry, the perbromate ion is the anion having the chemical formula . It is an oxyanion of bromine, the conjugate base of perbromic acid, in which bromine has the oxidation state +7. Unlike its chlorine () and iodine () analogs, it is diff ...

ion was through beta decay of the

selenium-83 Selenium (34Se) has six natural isotopes that occur in significant quantities, along with the trace isotope 79Se, which occurs in minute quantities in uranium ores. Five of these isotopes are stable: 74Se, 76Se, 77Se, 78Se, and 80Se. The last thr ...

atom in

selenate The selenate ion is . Selenates are analogous to sulfates and have similar chemistry. They are highly soluble in aqueous solutions at ambient temperatures. Unlike sulfate, selenate is a somewhat good oxidizer; it can be reduced to selenite or ...

: : → + β⁻ Decay of iodine-133 to give

xenon Xenon is a chemical element with the symbol Xe and atomic number 54. It is a dense, colorless, odorless noble gas found in Earth's atmosphere in trace amounts. Although generally unreactive, it can undergo a few chemical reactions such as the ...

is reported as a route to phenylxenonium, and likewise decay of

bismuth-210 Bismuth (83Bi) has 41 known isotopes, ranging from 184Bi to 224Bi. Bismuth has no stable isotope, stable isotopes, but does have one very long-lived isotope; thus, the standard atomic weight can be given as . Although bismuth-209 is now known to b ...

in a variety of structures is reported as a route to organopolonium structures. as cited by Appelman (1973)

Practical considerations

A major difficulty in using this method in practice is that the energetic electron released by the decay of one atom of tritium can break apart, modify, ionize, or excite hundreds of other molecules in its path. These fragments and ions can further react with the surrounding molecules producing more products. Without special precautions, it would be impossible to distinguish these "radiolytic" products and reactions from the "nucleogenic" ones due to mutation and reactions of the cation . The technique developed by Cacace and his team to overcome this problem is to use a starting compound that has at least two tritium atoms substituted for hydrogens, and dilute it in a large amount of an unsubstituted compound. Then the radiolytic products will be all unlabeled, whereas the nucleogenic ones will be still labeled with tritium. The latter then can be reliably extracted, measured, and analyzed, in spite of the much larger number of radiolytic products. The high dilution also ensures that the beta electron will almost never hit another tritiated molecule.

Scientific literature

Many papers have been published by about this technique, chiefly by Cacace and his successors at La Sapienza. An exhaustive survey was provided by M. Speranza in 1993.

References

Roger Vaughan Lloyd, Frank A. Magnotta, and David Eldon Wood (1968): "Electron paramagnetic resonance study of free-radical reactions initiated by radioactive decay in solid naphthalene-1-t". ''Journal of the American Chemical Society'', volume 90, issue 25, pages 7142–7144. V. Lloyd and D. E. Wood (1970): "EPR Studies of 1-Naphthyl and 2-Naphthyl Radicals Produced by Tritium Decay". ''Journal of Chemical Physics'', volume 52, pages 2153-2154. G. P. Akulov (1976): "Ion-molecular reactions initiated by β-decay of tritium in tritiated compounds" ("Ionn-molekulyarnye reaktsii, initsiirovannye β-raspadom tritiya v tritirovannykh soedineniyakh"). ''Uspekhi Khimii'' (USSR), volume 45, issue 2, pages 1970-1999. (No DOI). Marina Attina, Fulvio Cacace, Giovanna Ciranni, and Pierluigi Giacomello (1977): "Aromatic substitution in the gas phase. Ambident behavior of phenol toward t-C4H9+ cations". ''Journal of the American Chemical Society'', volume 99, issue 15, pages 5022–5026. . Marina Attinà, Fulvio Cacace, Giovanna Ciranni, and Pierluigi Giacomello (1979): "Gas-phase reaction of free isopropyl ions with phenol and anisole". ''Journal of the Chemical Society, Perkin Transactions 2'', issue 7, pages 891-895. Marina Attina, Fulvio Cacace, and Pierluigi Giacomello (1980): "Aromatic substitution in the gas phase. A comparative study of the alkylation of benzene and toluene with C3H7+ ions from the protonation of cyclopropane and propene". ''Journal of the American Chemical Society'', volume 102, issue 14, pages 4768–4772. Marina Attina, and Fulvio Cacace (): "Aromatic substitution in the gas phase. Intramolecular selectivity of the reaction of aniline with charged electrophiles". ''Journal of the American Chemical Society'', volume 105, issue 5, pages 1122–1126. Marina Attina, Fulvio Cacace, and Giulia De Petris (1085): "Intramolecular selectivity of the alkylation of substituted anilines by gaseous cations". ''Journal of the American Chemical Society'', volume 107, issue 6, pages 1556–1561. H. Colosimo, M. Speranza, F. Cacace, G. Ciranni (1984): "Gas-phase reactions of free phenylium cations with C3H6 hydrocarbons", ''Tetrahedron'', volume 40, issue 23, pages 4873-4883. Fulvio Cacace (1964): ''Proceedings of the 1963 Conference on the Methods for Preparing and Storing Marked Molecules'', Bruxelles, page 179. Euratom report EUR.1625.e. Fulvio Cacace, Giovanna Ciranni, and Angelo Guarino (1966): "A Tracer Study of the Reactions of Ionic Intermediates Formed by Nuclear Decay of Tritiated Molecules. I. Methane-t4". ''Journal of the American Chemical Society'', volume 88, issue 13, pages 2903–2907. Fulvio Cacace (1970): "Gaseous Carbonium Ions from the Decay of Tritiated Molecules". ''Advances in Physical Organic Chemistry'', volume 8, pages 79-149. Fulvio Cacace and Pierluigi Giacomello (1973): "Gas-phase reaction of tert-butyl ions with arenes. Remarkable selectivity of a gaseous, charged electrophile". ''Journal of the American Chemical Society'', volume 95, issue 18, pages 5851–5856. Fulvio Cacace and Pierluigi Giacomello (1977): "Aromatic substitution in the liquid phase by bona fide free methyl cations. Alkylation of benzene and toluene". ''Journal of the American Chemical Society'', volume 99, issue 16, pages 5477–5478. . Fulvio Cacace and Pierluigi Giacomello (1978): "Aromatic substitutions by []methyl decay ions. A comparative study of the gas- and liquid-phase attack on benzene and toluene". ''Journal of the Chemical Society, Perkin Transactions 2'', issue 7, pages 652-658. Fulvio Cacace, Giovanna Ciranni, and Pierluigi Giacomello (1981): "Aromatic substitution in the gas phase. Alkylation of arenes by gaseous C4H9+ cations". ''Journal of the American Chemical Society'', volume 103, issue 6, pages 1513–1516. Fulvio Cacace (1982): "On the formation of adduct ions in gas-phase aromatic substitution". ''Journal of the Chemical Society, Perkin Transactions 2'', issue 9, pages 1129-1132. . Fulvio Cacace, Giovanna Ciranni, and Pierluigi Giacomello (1982): "Alkylation of nitriles with gaseous carbenium ions. The Ritter reaction in the dilute gas state". ''Journal of the American Chemical Society'', volume 104, issue 8, pages 2258–2261. Fulvio Cacace, Giovanna Ciranni and Pierluigi Giacomello (1982): "Aromatic substitution in the gas phase. Alkylation of arenes by C4H9+ ions from the protonation of C4 alkenes and cycloalkanes with gaseous Brønsted acids". ''Journal of the Chemical Society, Perkin Transactions 2'', issue 11, pages 1129-1132. Fulvio Cacace, and Giovanna Ciranni (1986): "Temperature dependence of the substrate and positional selectivity of the aromatic substitution by gaseous tert-butyl cation". ''Journal of the American Chemical Society'', volume 108, issue 5, pages 887–890. Fulvio Cacace (1990): "Nuclear Decay Techniques in Ion Chemistry". ''Science'', volume 250, issue 4979, pages 392-399. . Fulvio Cacace, Maria Elisa Crestoni, and Simonetta Fornarini (1992): "Proton shifts in gaseous arenium ions and their role in the gas-phase aromatic substitution by free Me3C+ and Me3Si+ ert-butyl and trimethylsilylcations". ''Journal of the American Chemical Society'', volume 114, issu 17, pages 6776–6784. Fulvio Cacace, Maria Elisa Crestoni, Simonetta Fornarini, and Dietmar Kuck (1993): "Interannular proton transfer in thermal arenium ions from the gas-phase alkylation of 1,2-diphenylethane". ''Journal of the American Chemical Society'', volume 115, issue 3, pages 1024–1031. Pierluigi Giacomello and Fulvio Cacace (1976): "Gas-phase alkylation of xylenes by tert-butyl(1+) ions". ''Journal of the American Chemical Society'', volume 98, issue 7, pages 1823–1828. Maurizio Speranza (1993): "Tritium for generation of carbocations". ''Chemical Reviews'', volume 93, issue 8, pages 2933–2980. . Laboratory techniques Chemistry