Study of the isomers of isoelectronic C(4), (C(3)B)(-), and (C(3)N)(+): rearrangements through cyclic isomers

Optimized structures of the isoelectronic cumulenes (CCCB)−, CCCC, and (CCCN)+ and of their isomers formed by rearrangement have been calculated at the B3LYP/6-311+ G(3df) level of theory with relative energies and electronic states determined at the CCSD(T)/aug-cc-pVTZ level of theory. The ground states of CCCC and (CCCN)+ are triplets, whereas the ground state of (CCCB)− is a quasi-linear singlet structure that is only 0.6 kcal mol−1 more negative in energy than the linear triplet. When energized, both triplet and singlet CCCC cyclize to planar rhomboids, of which the singlet is the lowest-energy configuration. Ring-opening of rhomboid C₄ reforms CCCC with the carbons partially randomized. Similar rearrangements occur for (CCCB)− and (CCCN)+, but the reactions are different in the detail. In the case of (CCCN)+, rearrangement of atoms is supported both experimentally and theoretically. Because (CCCB)− and (CCCN)+ are not symmetrical, two fully cyclized forms are possible; the one more resembling a rhomboid structure is called a “kite” structure, and the other is called a “fan” structure. The rearrangement of (CCCB)− is more favored via the triplet with equilibrating kite and fan structures being formed, whereas the singlet (CCCN)+ ring closes to give the singlet kite structure, which may ring open to give a mixture of (CCCN)+ and (CCNC)+. Intersystem crossing may occur for the triplet and singlet forms of CCCC and (CCCB)− but not for (CCCN)+.Tianfang Wang, Mark A. Buntine and John H. Bowi