Biradicaloids

1 Homocyclic Systems. Cyclobutanediyls exhibit spin states that are very close in energy. The triplet state is preferred by only l.Vkcal mol it can be observed by EPR spectroscopy because the ring closure to a bicyclo[l. 1. Ojbutane is spin-forbidden. Singlet cyclobutanediyls are predicted as very short-lived transition states for the ring inversion of bicyclo[1.1.0]butanes (AF 50kcal mol ). Quantum chemical calculations predict that the heavier group 14 

Interestingly, for the parent molecule (H2PBH)2 calculations show that the planar form would be the transition state (at 16.4kcalmol ) for the inversion of the l,3-diphospha-2,4-diborabicyclo[1.1.0]butane 5.17. Hence the choice of the sterically demanding substituents on P and B ( Pr and Bu, respectively) appears to be the determining factor in the isolation of the biradicaloid 5.17. A more detailed discussion of the influence of the exocyclic substituents on the relative stabilities of cyclic B2P2 biradicaloid systems can be found in Section 9.3.2. 

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The concept of biradicals and biradicaloids was often used in attempts to account for the mechanism of photochemical reactions [2,20,129-131]. A biradical (or diradical) may be defined as [132] an even-electron molecule that has one bond less than the number permitted by the standard rules of valence. 

The first biradicaloid, l,3-diaza-2,4-digermacyclobutane-l,3-diyl, 53, was simply prepared by the reaction of the digermyne ArGeGeAr with trimethylsilyl azide in hexane (Scheme 2.40). ... 

The second stable biradicaloid, l,3-diaza-2,4-distannacyclobutane-l,3-diyl, 54, was unexpectedly obtained by the reaction of chloro(amino)stannylene dimer [Sn N(SiMe3)2 (n-Cl)]2 and AgOCN in diethyl ether (Scheme 2.41). ... 

Now in addition to all these Si and Ti minima which have counterparts in the ground state surface, minima (or funnels) in Si and Ti can also be expected at biradicaloid geometries. These are geometries at which the molecule, in the MO description, has two approximately... 

Many types of biradicaloid geometries can be envisaged for a given molecule.19 They can be derived from its ordinary geometry (minimum... 

On a somewhat higher level of qualitative MO argumentation, one can allow for the fact that the total energy of the molecule is not related only to the sum of energies of occupied orbitals, but also to certain electron repulsion terms. This leads to a better understanding of the nature of electronic states of molecules at biradicaloid geometries 19>U2> and, in particular, of the difference between Si and Ti hypersurfaces.19) We... 

Now, the energy differences between the four states, Ti, So, Si, and S2, will depend on the conformation of the biradicaloid molecule.19 If the two localized non-bonding orbitals are far apart in space, the cost... 

It is easy to see how simple MO arguments lead one to expect that two non-bonding orbitals will result if a single bond is stretched sufficiently (Fig. 6), as a double bond is twisted by 90 degrees (Fig. 5), or as atoms are rearranged to make a carbene. Return through such biradicaloid... 

The preceding argument works also for the T i hypersurface, but exactly in the opposite sense. Now loose geometries are more favorable and this is where the biradicaloid minima should be sought (for instance, 2, not 4). These minima in Ti will typically allow considerable freedom of motion such as bond rotation, since there now is no rigid geometrical requirement such as a need for a cyclic array of orbitals was in the singlet case. Also, return to So is spin-forbidden and may be relatively slow,... 

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