Trimethylenemethane state

The Franck-Condon question is also an issue in diradical studies. It is particularly relevant in nonrigid systems, such as trimethylenemethane (TMM) and tetramethyleneethylene (TME), °° in which neutral and/or anion states can be planar or nonplanar. Thus, transitions can occur from planar anions to nonplanar neutral states, or from nonplanar anions to planar neutrals. In these cases, the energy differences between the planar and nonplanar states have generally been estimated by using quality electronic structure calculations. 

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... 

In the same group, Feustel and G. Muller have observed an interesting reaction between two equivalents of the tris(phosphinomethyl)methanol 52 and five equivalents of n-butyl-lithium . Formally, the elimination of lithium oxide leads to the formation of a mixed aggregate of the lithium aUtoxide 53 and the dilithiated trimethylenemethane dianion 54 in the solid state (Scheme 19, Figure 15). The mechanism of the formal elimination of lithium oxide, which could also be part of a further mixed aggregate, has not been cleared up yet. 

Trimethylenemethane (14) has intrigued chemists for many years. HUckel theory predicts that 14 should have a triplet ground state, and this has been supported by more sophisticated calculations. Until recently the only direct evidence for the existence of 14 was its ESR spectrum obtained when it was prepared in low-temperature matrices. Maier originally attempted to obtain the IR spectrum of 14 by irradiation of 4-methylenedihydropyrazole (15) in an argon matrix, a proce-... 

A crucial methodological step forward was the discovery " that one could observe weU-defined electron spin resonance (ESR) spectra of frozen solutions of triplet species in random orientation. By the early 1960s, spectra of the triplet states of a number of carbenes had been recorded. Thus, when Dowd showed that photolysis of frozen matrices of the diazene (11) or the ketone (12) (Scheme 5.1) gave TMM (4), the spectroscopic tools for the characterization of this key non-Kekule compound lay to hand. Trimethylenemethane was the first non-Kekule molecule to be identified by ESR spectroscopy. 

Slipchenko, L. V. and Krylov, A. I. 2003. Electronic Structure of the Trimethylenemethane Diradical in its Ground and Electronically Excited States Bonding, Equilibrium Geometries, and Vibrational Frequencies , J. Chem. Phys., 118, 6874. 

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... 

Ground-state triplet dicarbenes in photochemical systems have also been extensively studied by ESR. Dowd and coworkers (304) examined the interesting, frequently postulated ground triplet trimethylenemethane obtained by photolysis of either 4-methylene-A -pyrazoline or a single crystal of 3-methylene-cyclobutanone. This triplet molecule is axially symmetrical and the proton hyperfine splittings of 8.9 gauss observed in single-crystal experiments indicate that all protons in the molecule are equivalent when the axis perpendicular to the plane... 

---