Decay technique

In chemistry, the decay technique is a method to generate chemical species such as radicals, carbocations, and other potentially unstable covalent structures by radioactive decay of other compounds. For example, decay of a tritium-labeled molecule yields an ionized helium atom, which might then break off to leave a cationic molecular fragment.

The technique was developed in 1963 by the Italian chemist Fulvio Cacace 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 [HeH]⁺.

In the basic method, a molecule (R,R′,R″)C−T is prepared where the vacant bond of the desired radical or ion is satisfied by an atom of tritium ³H, the radioactive isotope of hydrogen with mass number 3. As the tritium undergoes beta decay (with a half-life of 12.32 years), it is transformed into an ion of helium-3, creating the cation (R,R′,R″)C−[³He]⁺.

In the decay, an electron and an antineutrino are ejected at great speed from the tritium nucleus, changing one of the neutrons into a proton with the release of 18,600 electronvolts (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 of the carbon–helium bond (about 1 eV), so this bond breaks. The helium atom almost always leaves as a neutral ³He, leaving behind the carbocation [(R,R′,R″)C]⁺.

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, CH₃T (R = R′ = R″ = H) produces the carbenium ion H₃C⁺ 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 form, with release of about 30 kcal/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 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.

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 170 K (−103 °C).

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 T₂ or tritium-hydrogen HT. On decay, these form a stable helium hydride ion [HeH]⁺ (respectively [³HeT]⁺ or [³HeH]⁺), 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 [HeH]⁺ (or [HeT]⁺) will protonate methane CH₄ to the carbonium ion [CH₅]⁺ (or [CH₄T]⁺).