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.[1] It has allowed the study of a vast number of otherwise inaccessible compounds and reactions.[2][3][4] It has also provided much of our current knowledge about the chemistry of the helium hydride ion [HeH]+.[2]

Carbocation generation

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]+.[2]

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]+.[2]

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.[2]

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.[2] 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.[5][6]

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 170 K (−103 °C).[5][6]

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 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]+).[2]

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.[7][8]

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 ion was through beta decay of the selenium-83 atom in selenate:[9]

83
SeO2−
4 → 83
BrO−
4 + β−

Decay of iodine-133 to give xenon is reported as a route to phenylxenonium, and likewise decay of bismuth-210 in a variety of structures is reported as a route to organopolonium structures.[10]

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 [(R,R′,R″)C]+.[2]

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.[2]

Scientific literature

Many papers have been published by about this technique, chiefly by Cacace and his successors at La Sapienza.[1][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27][3][28][29] An exhaustive survey was provided by M. Speranza in 1993.[2]

References
  1. 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.
  2. Maurizio Speranza (1993): "Tritium for generation of carbocations". Chemical Reviews, volume 93, issue 8, pages 2933–2980. doi:10.1021/cr00024a010.
  3. Fulvio Cacace (1990): "Nuclear Decay Techniques in Ion Chemistry". Science, volume 250, issue 4979, pages 392-399. doi:10.1126/science.250.4979.392.
  4. 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).
  5. 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. doi:10.1021/ja01027a057
  6. 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. doi:10.1063/1.167326952
  7. Cacace, Fulvio (1990). "Nuclear Decay Techniques in Ion Chemistry". Science. 250 (4979): 392–399. Bibcode:1990Sci...250..392C. doi:10.1126/science.250.4979.392. PMID 17793014. S2CID 22603080.
  8. Ikuta, Shigeru; Yoshihara, Kenji; Shiokawa, Takanobu (1977). "Fragmentation by Beta-Decay in Tritium-Labelled Compounds, (III): Potential Energy Curves of LiHe+, BeHHe+ FHe+ Resulting from LiT, BeHT and FT". Journal of Nuclear Science and Technology. 14 (10): 720–722. doi:10.1080/18811248.1977.9730829.
  9. Appelman, E. H. (1973). "Nonexistent compounds. Two case histories". Accounts of Chemical Research. 6 (4): 113–117. doi:10.1021/ar50064a001.
  10. Nefedov, V. D.; Toropova, M. A.; Sinotova, E. N. (1989). Usp. Khim. 88: 883. {{cite journal}}: Missing or empty |title= (help) as cited by Appelman (1973)
  11. 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. doi:10.1021/ja00965a004
  12. Fulvio Cacace (1970): "Gaseous Carbonium Ions from the Decay of Tritiated Molecules". Advances in Physical Organic Chemistry, volume 8, pages 79-149. doi:10.1016/S0065-3160(08)60321-4
  13. 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. doi:10.1021/ja00799a002
  14. 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.doi:10.1021/ja00423a029
  15. 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. doi:10.1021/ja00458a040.
  16. 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. doi:10.1021/ja00457a022.
  17. Fulvio Cacace and Pierluigi Giacomello (1978): "Aromatic substitutions by [3H3]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. doi:10.1039/P29780000652
  18. 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. doi:10.1039/P29790000891
  19. 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. doi:10.1021/ja00534a032
  20. 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. doi:10.1021/ja00396a035
  21. 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. doi:10.1039/P29820001129.
  22. 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. doi:10.1021/ja00372a025
  23. 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. doi:10.1039/P29820001129
  24. 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. doi:10.1021/ja00343a009
  25. 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. doi:10.1016/S0040-4020(01)91321-3
  26. 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. doi:10.1021/ja00292a017
  27. 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. doi:10.1021/ja00265a006
  28. 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+ [tert-butyl and trimethylsilyl] cations". Journal of the American Chemical Society, volume 114, issu 17, pages 6776–6784. doi:10.1021/ja00043a024
  29. 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. doi:10.1021/ja00056a029
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