Singlet structure isomers

The following spectra for A and B correspond to two structural isomers. The NMR singlet at 51.16 in spectrum A disappears when the sample is shaken with DiO. The singlet at S0.6 ppm in the spectrum of B disappears on shaking with D2O. Propose structures for these isomers, and show how your structures correspond to the spectra Show what cleavage is responsible for the base peak at m/z 44 in the mass spectrum of A and the prominent peak at m/z 58 in the mass spectrum of B. 

All the above-mentioned acyclic 1,3-diradicals are less stable than the a-bonded isomers. Therefore, in addition to using various substituents, other factors should be further considered in our design of persistent singlet 1,3-diradicals. In Sect. 5.2, ring structure is taken into account. Strain prevents the ring closure in the singlet state. Two linkers between the radical centers multiply the through-bond interactions. 

Hexacoordinate, hypervalent sulfur fluorides have an octahedral geometry that is symmetrical for SF6, which appears as a sharp singlet at +56 ppm, but which has magnetically nonequivalent (axial and equatorial, ab4 system) fluorines for compounds of the structure R-SF5. Compounds with the general structure R-SF4-X can exist as cis- and trans-isomers, the former having three types of fluorine, and the latter only one (Scheme 7.17). 

Rauhut and Semsel [141] and Steinfatt [142] have suggested an unusual mechanism whereby the proposed intermediate 1,2-dioxetandione forms a structural (isoelectronic) isomer as outlined in route B in Figure 6. Route A represents a dark reaction. The isomerized molecule is then thought to decompose into singlet oxygen and carbon trioxide [143],... 

When ortho substituents interfere with the coplanar structure, which is the most stable shape for the singlet isomer, the amount of diradical is very much increased. 

As has already been discussed (Section m.B.3) we were able to demonstrate that the three C3H2 isomers cyclopropenylidene (2), propargylene (36), and vinylidenecarbene (37), interconvert photochemically in low-temperature matrices. Unlike 36 vinylidenecarbene (37) was predicted to be a singlet.108-110 To aid the spectroscopic identification of S-37 we calculated (MP2/6-31G ) IR frequencies and intensities of this species.26 Comparison with the experimental IR spectrum (most intense band at 1952 cm-1) confirmed the allenic structure S-37. For T-37 a completely different IR spectrum was expected. An additional structural proof for S-37 was its reversible transformation into the other two C3H2 isomers S-2 and T-36. 

Aminonitrene (isodiazene) (145) is the least stable of all isomers of the elemental composition N2H2 and was isolated in an argon matrix for the first time in 1984.195 Theoreticians also gave much attention to aminonitrene (145).196 This molecule is exceptional in so far, that it is one of the few nitrenes with a singlet electronic ground state and is best described by the dipolar structure 145. ... 

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