Cyclopropenylidenes, structure

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. 

In agreement with earlier theoretical work, non-empirical structure theory predicts that cyclopropenylidene (390) has a singlet ground state ca. 290 kJ mol below the lowest triplet. The expected high dipole moment (3.40 D) suggests a significant contribution of... 

A particularly interesting singlet carbene is cyclopropenylidene, 32 (Figure 5.29), which may be the most abundant cyclic hydrocarbon in interstellar space. The structure is thought to be stabilized because the nonbonded pair of electrons is in an orbital with considerable s character, while the p orbital of the carbenic center is stabilized by electron donation from the adjacent olefinic system. Cyclopropenylidene itself is not stable in the condensed phase, but the thermally stable bis(diisopropylamino) derivative 33 was reported to be stable. ... 

Is 101 aromatic The C—C and Si—C bond distances in 101 [at CCSD(T)/TZ(2df,2pd), the best available level of theory for this system], of 1.350 A and 1.833 A respectively , are in good agreement with the experimental values of 1.346 A and 1.820 A, respectively The C—C bond is somewhat longer than in the analogous cyclopropenylidene (1.328 A ) and the C—Si bond distance is somewhat shorter than a regular single C—Si bond. These structural data and the results of Mulliken" and population analyses indicate some degree of 27r-aromatic delocalization... 

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