Singlet state trimethylenemethane

The relatively low-temperature conversion of MSD to MBD can reasonably be attributed to the formation of a trimethylenemethane biradical, presumably in its singlet state, which is further stabilized by a cyclopentadienyl moiety (Scheme 9.29). Of interest is the extent to which this species might be stabilized by electron transfer to form a species reminiscent of the oxaallyl zwitterion from cyclopropa-none (see Chapter 5, Section 2). Of further interest is the fact that the dimers formed are not those from a trimethylenemethane triplet state. 

Table 3 Energy Differences (kcal/mol) Between Three Low-Lying Singlet States of C2 Trimethylenemethane (TMM), Relative to the Ground State, Computed at Various Levels of Theory with the 6-3IG Basis Set...

The third member, trimethylenemethane (3), had some relevance to our studies on carbenes, since besides methylene and its simply substituted derivatives trimethylenemethane 3 is one of the few molecules having a triplet ground state.22 Also the experience with 3 could be of help in order to deal with the singlet/triplet differentiation in matrix-isolated carbenes. We learned that, if the calculated IR spectra of the singlet and triplet molecule are sufficiently different, it might be possible to determine the multiplicity of the matrix-isolated species by comparison with the experimental IR spectrum. In this context it is also worth mentioning that we were able to measure the matrix IR spectrum of 3, but a special technique (irradiation in halogen-doped xenon matrices) had to be developed in order to achieve a concentration of 3 sufficient for its IR detection.23... 

Nitroxides are the most common of the oxygen-centred biradicals to be reported. As a model for spin-crossover molecules, the nitronyl nitroxide (105) was prepared and by oxidation with PbC>2 afforded the triplet biradical (106) which was characterized by ESR (Scheme 15).242 The one-electron oxidation of (105) afforded the singlet cation (107) which was seen to exist in equilibrium with (106) in solution. The authors claim that pH-controlled interconversion between two species of different spin multiplicities in this way may provide die basis for novel magnetic switches or pH sensors. The N.N-dialkylamino nitronyl nitroxides (108) were prepared and afforded die diplet-state biradical cation species by one-electron oxidation with iodine.243 The authors propose that, by the similarity of die electronic structures, diese structures can be regarded as hetero-analogues of trimethylenemethane. A paper confirms die conversion of 3,3-dimethyldioxetane into die corresponding ring-opened 1,2-diol but refutes the... 

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