Generation from Closed-Shell Species

Most free radicals are not kinetically stable—they tend to react with one another to give electron-sufficient species—so free radicals that are to be used in reactions must usually be generated from closed-shell species in situ. Free radicals can be generated from closed-shell species in four ways u-bond homolysis, photochemical excitation of a tt bond, one-electron reduction or oxidation, and cy-cloaromatization. 

Benzoyl peroxide and AIBN are two of the compounds most widely used to generate free radicals in a reaction mixture. The 0-0 bond in benzoyl peroxide and the C-N bonds in AIBN homolyze under thermal or photochemical conditions. 

The movement of unpaired electrons in free radical reactions is shown with single-headed curved arrows. 

The likelihood of bond homolysis is directly related to the BDE for that bond. The BDE for the H-H bond is 104 kcal/mol, whereas the BDE for the Br Br bond is 46 kcal/mol, so the likelihood of H-H homolysis is much smaller than the likelihood of Br-Br homolysis. Sigma bonds that are particularly prone to homolysis include N-O and 0-0 bonds, bonds between C and very heavy atoms like Pb and I, halogen-halogen bonds, and very strained bonds. 

Upon photolysis of a diazo compound, the C N bond cleaves in heterolytic fashion, N2 is lost, and a carbene is generated, usually in the triplet form. Triplet carbenes have one electron in each of two orbitals and may be thought of as 1,1-diradicals. Alkyl and acyl azides also undergo loss of N2 upon photolysis. The products are highly reactive nitrenes, the N analogs of carbenes. 

---

Where do the thermochemical data that are used to determine the energetics of a reaction come from For closed-shell species that can be generated chemically via proton transfer, gas phase acidities (reaction [2]) and basicities (reaction [3]) are the principal sources. If the acidity or basicity for a reaction leading to a given ion is known, then the heat of formation for that ion can be calculated via Equations (4) and (5). This latter point is important, because this is the source for much of the ionic thermochemical data that are used for application of the no endothermic reactions tool. 

The cage effect can be a source of great frustration in matrix isolation studies of monoradicals, because such species are usually formed by homolysis from closed-shell compounds, and hence any radical generated in situ is invariably accompanied by another radical that will be trapped in the same matrix cage. 

Recent advances in the techniques of photoelectron spectroscopy (7) are making it possible to observe ionization from incompletely filled shells of valence elctrons, such as the 3d shell in compounds of first-transition-series elements (2—4) and the 4/ shell in lanthanides (5, 6). It is certain that the study of such ionisations will give much information of interest to chemists. Unfortunately, however, the interpretation of spectra from open-shell molecules is more difficult than for closed-shell species, since, even in the simple one-electron approach to photoelectron spectra, each orbital shell may give rise to several states on ionisation (7). This phenomenon has been particularly studied in the ionisation of core electrons, where for example a molecule (or complex ion in the solid state) with initial spin Si can generate two distinct states, with spin S2=Si — or Si + on ionisation from a non-degenerate core level (8). The analogous effect in valence-shell ionisation was seen by Wertheim et al. in the 4/ band of lanthanide tri-fluorides, LnF3 (9). More recent spectra of lanthanide elements and compounds (6, 9), show a partial resolution of different orbital states, in addition to spin-multiplicity effects. Different orbital states have also been resolved in gas-phase photoelectron spectra of transition-metal sandwich compounds, such as bis-(rr-cyclo-pentadienyl) complexes (3, 4). 

In summary, alkenes are reactive compounds and are removed rapidly from the atmosphere by a variety of processes. Reaction with OH radicals, ozone, and NO3 radicals all play important roles. These reactions proceed via addition to the unsaturated bond giving an adduct which decomposes and/or reacts with 02 leading to the generation of a variety of transient radical species which react to form the first generation closed-shell products (principally carbonyl compounds). 

It is helpful in certain cases to further subdivide these clusters into closed-shell and open-shell species. This is generally not an issue in neutral clusters so far studied because most are closed shell, except for some notable exceptions such as ICI—Ne (Drobits and Lester, 1988), OH—Ar (Berry et al., 1990), and benzyl-X (Disselkamp and Bernstein, 1993). On the other hand, it is important in ionic clusters because an ionic cluster generated from a neutral cluster is an open-shell species. These tend to rearrange into stable clusters with strong bonds that often do not resemble their neutral counterparts. By contrast, ionic closed shell clusters such as Cs+(X) are expected to be similar in structure and energy to the isoelectronic neutral cluster Xe(X) . 

---