Singlet diradical structures

When Heitler-London AO-type wavefunctions (i.e.. ..aabP +. ..baaP in which a and b are AOs) are used to represent electron-pair 7i c(CN) and 7i y(CN) bonds, it can be deduced [2,4,16, cf. also Eq.(ll) below] that VB structure 7 is equivalent to resonance between the Kekule Lewis structure 3 and the Dewar or "long-bond" Lewis structures 11-13. Only nearest-neighbour spin-pairing is indicated in increased-valence structures [2-5,10]. When the "long" or formal bonds are omitted from structures 11-13, these structures are designated as singlet diradical structures [2-4]. 

X (-) rW Y (-) and X " R Y are included in the valence-bond descriptions of the reactants and products. It is needed to help form the reactant-like and productlike complexes. Therefore unless the singlet diradical structure is included in the valence-bond formulation for the + R-Y -> X-R + Y reaction, reactant-like and product-like complexes with intermolecular one-electron bonds will not be formed. A non-concerted SnI reaction mechanism, formulated as... 

Further consideration for the singlet-diradical mechanism for 1,3-dipolar cycloadditions is provided in Refs. 25a-c. It is to be noted that, as discussed for Sn2 reactions in the Chapter 21 Addendum, a constribution from the singlet-diradical structure to the valence-bond resonance scheme is needed in order that the cycloaddition reaction be concerted. This occurs in the valence-bond formulations of both (31) (28) and (34) (36), but not for (24) —> (25). 

As discussed in both volumes, singlet diradical structures help the standard Kekule-type structures to internet. Resonance between these two types of Lewis structures generates electronic hypervalence - for example a possible valence greater than four for the central nitrogen atom of N2O - and is needed to ensure that valence-bond formulations of mechanisms for Sn2 and 1,3-dipolar cycloaddition reactions are those for concerted reactions. Also, as will be shown in Chapter 25, Pauling 3-electron bonds are components of increased-valence struetures for (non electron-rich) 3-electron 3-centre bonding units. 

An alternative description of the singlet excited state is a cyclopropylmethyl singlet diradical. Only one of the terminal carbons would be free to rotate in such a structure. 

In connection with Chichibabin s hydrocarbon it is appropriate to mention that there is little or no resonance between states of different multiplicity.4 Thus the singlet-triplet transition represented by the covalent and diradical structures of the hydrocarbon is a true equilibrium and not an example of resonance. 

The intermediate produced by loss of nitrogen from a l.ff-triazole can be written as an imino-carbene, as a zwitterion, or as a diradical. Subsequent reactions of the intermediate which have been observed (Scheme 57) include (a) photochemical WoUf rearrangement (suggesting that the intermediate may have a singlet carbene structure) ... 

Now we turn to selected examples of interest. One involves a multiplicity dependence of the regioselectivity. This is illustrated in Scheme 4. While one simplistically might write a single intermediate cyclopropyldicarbinyl diradical structure for both singlet and triplet multiplicity, the regioselectivity of unzipping depends on which species is involved37. 

Table 8.3 Results of attempted geometry optimization of the CH2CH2CH2 singlet diradical by different model chemistries the 6-31G basis was used in all cases. See Fig. 8.7 for the input structures and text for clarification...

The importance of Dewar-type (or singlet diradical) canonical Lewis structures, as well as the Kekule-type (or zwitterionic) Lewis structures for VB descriptions of the electronic structures of 1,3-dipolar (or zwitterionic diradical hybrid [19]) molecules has been often stressed in our publications [2-4,10,16], and a similar type of conclusion has been obtained by others [20]. In contrast, the results of spin-coupled [21] and biorthogonal [22] VB calculations have provided a contrary result However because the orbitals used in the latter calculations are primarily either 2-centre or multicentre in form, they disguise the presence of the singlet-diradical character [3(a),4,13,23]. 

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