Tetramethyleneethane

On the basis of the simple Hiickel theory, the two NMBOs of both 33 and 34 will be degenerate. Hund s rule therefore dictates a triplet state. However, Borden and Davidson argued that since the electrons in the two NBMOs of 33 are specially isolated and have reduced the electrostatic repulsion, the singlet state will not violate the Pauli s exclusion principle and will be energy competitive with the triplet 

In fact, they suggested that electron correlation will lower the singlet state below the triplet. 

Over the following 25 years, significant computational and experimental effort has been expended to determine if TME is in fact a triplet, and therefore conforms 

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Among the non-Kekule diradicals, tetramethyleneethane (TME, 7) has evoked lasting attention during the last two decades due to the controversy over its spin preference in the ground state between experiments and theoretical predictions [48-59], Now TME is known to be a slightly favored singlet diradical with a negligible... 

Fig. 13 Selected cyclic It-conjugated diradicals on the basis of tetramethyleneethane (TME) unit...

Scheme 6.25 shows the formulas of the compounds 88 [68], 89 [70], 90 [78], 91 and 92 [70], which emerge from the dimerization of 74, 76, 77 and 79, respectively. The structure is of the same type as that of the dimer 38 of 6 (Scheme 6.10). The configuration has not been determined, but it is assumed to be trans as in the case of 38. Hence the cyclization of the tetramethyleneethane diradicals of type 37 (Scheme 6.10), the immediate precursors of the isolated dimers, should proceed as a least-motion process at the unsubstituted radical centers. Only the trimethylsilyl group causes a predominantly alternative course, since the dimers 91 and 92 were obtained in a ratio of 1 2.

The dimer 352 of 351 was isolated from the product mixtures of two experiments conducted to trap 351 by alkenes, one with 350 and the other with 354 as substrate. Although no cycloadduct with the alkene was observed in one case, the yield of 352 amounted to only 0.8%. Nevertheless, the structure of 352 is interesting, since it suggests that the tetramethyleneethane diradical assumed to be the intermediate undergoes ring closure preferentially between two different allyl-radical termini. 

Tetramethylene-ethane (TME), or 2,2/-bis-allyl diradical 81, was suggested as an intermediate in the thermal dimerization of allene, as well as in the interconversions of 1,2-dimethylenecyclobutane 82, methylenespiropentane 83, bis-cyclopropylidene 84 and other bicyclic systems (equation 30)45. The isolation of two different isomeric dimethylene cyclobutanes 87 and 88 (in a ca 2 I ratio) after the thermal rearrangement of the deuteriated 1,2-dimethylene cyclobutane 85 suggests that the rearrangement proceeds via a perpendicular tetramethyleneethane diradical (2,2/-bisallyl) 86 (equation 31)45. 

More recently, however, the ground spin states of some alternant hydrocarbons have been found to be contradictory to the Longuet-Higgins prediction. For example, whereas the ground state is predicted to be a triplet for tetramethyleneethane [12], and triplet signals have been observed for the parent and the derivatives (Dowd, 1979 Dowd et al., 1896, 1989 Roth et al., 1987), it is difficult to establish triplet ground states experimentally (Du and Borden, 1987). /M,w -Biphenyldicarbene [13] is predicted to have a ground... 

Upon y-irradiation of 1 in a CF3CCI3 matrix at 77 K [78], a radical cation was formed, the ESR spectrum of which consisted of nine broad hyperfine components spaced by ca. 0.75 mT (g = 2.0029 0.002), and the corresponding proton END OR spectrum exhibited two essentially isotropic signals at 25.83 and 24.58 MHz. The detailed analysis of the ESR and END OR spectra disclosed that the initially formed radical cation 1+ had transformed into the tetramethyleneethane radical cation 94+ (Scheme 17). In CFCI3 and CF2CICFCI2 matrices 1+ persists up to 100 K [79]. On going from 1 to l+, the set of eight equivalent protons splits... 

Scheme 17. Tetramethyleneethane radical cation 94+ formed from bicyclopropylidene (1) and bonding linear combinations of the Walsh orbitals for 1 [78-80]...

Also dating from that period was the synthesis of another of the Longuet-Higgins molecules, tetramethyleneethane (5), which Dowd " reported in 1970 and assigned a triplet ground state in 1986 in view of its linear Curie plot. We discuss this molecule and its derivatives in Section 7. 

THE DEPENDENCE OF SPIN STATE ON MOLECULAR CONNECTIVITY THE CASE OF TETRAMETHYLENEETHANE... 

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