Singlet−Triplet Gap in 1,2,3-Butatriene and Its Consequences on the Mechanism of Its Spontaneous Polymerization

Abstract

The reactivity of 1,2,3 butatriene, C4H4, has been postulated to be due to the presence of low-lying triplet states. However, there have been no experimental or theoretical determinations of the energy separations or geometries of these states. We have examined the structures, energetics, and possible mechanisms involved in the self-polymerization of the title compound using single-determinant ab initio molecular orbital theory. We find that the magnitude of the singlet−triplet gap for the monomer and its relatively high positive heat of formation does not allow for a mechanism involving the direct formation of an isolated triplet species. We speculate on an alternative mechanism and show its relationship to recent work on related reactive monomers.

Peralkynylated Buta‐1,2,3‐Trienes: Exceptionally Low Rotational Barriers of Cumulenic CC Bonds in the Range of Those of Peptide CN Bonds

“…Computational study : The comparison between the rotational barriers Δ G ≠ for 1,4‐dimethylbutatriene (31.8 kcal mol −1 ),6 1,4‐dialkynylbutatriene 2 d (≈25 kcal mol −1 ), and 1 a / 1 d (≈20.2 kcal mol −1 ) clearly demonstrates the dramatic stabilizing effect of the alkynyl substituents on the transition state of the isomerization process. Computational results at the unrestricted B3LYP6‐31G(d) level of theory calculated with Gaussian 98,21 show that the rotation of ethynyl‐, methyl‐, and ethenyl‐substituted butatriene, as well as the parent butatriene,10c proceeds via perpendicular singlet diradical transition states (Scheme ).…”

Spin–Spin Interactions in One‐Dimensional Assemblies of a Cumulene‐Based Singlet Biradical

“…Crystals were precipitated when the mixed dilute solution was cooled under light shielding.Aplausible interpretation is that this decomposition is due to cumulene dimerization, as described in previous reports. [25][26][27] Attempts to utilize phenyl shielding groups instead of 3,5-di-tert-butylphenyl groups failed due to rapid product decomposition. Therefore,3,5-ditert-butylphenyl groups were deemed effective for accessing the cumulene structure.…”

“…Thetwo sterically bulky aryl groups are expected to protect the diacetylene moiety from chemical reactions,s uch as dimerization. [25][26][27] The3 ,5-di-tert-butylphenyl group was introduced by the reaction of 2-(3,5-di-tertbutylphenyl)malononitrile 4 with 1-chloromethylnaphthalene 5.A fter hydrolysis of 6,F riedel-Crafts cyclization of 7 with oxalyl chloride yielded dihydrophenalenone 8.A ddition of lithium trimethylsilylacetylide to 8 followed by O-methylation and TMS-deprotection afforded 9 as ad iastereomeric mixture.A fter deprotection of the TMS group,E glinton coupling of 10 gave phenalane dimer 11 in good yield. Elimination of methanol with ac atalytic amount of TsOH•H 2 Oa fforded phenalene precursor 12 as an air-sensitive solid.…”

Spin–Spin Interactions in One‐Dimensional Assemblies of a Cumulene‐Based Singlet Biradical

“…3,5‐Di‐ tert ‐butylphenyl derivative 1 b was synthesized as outlined in Scheme 2. The two sterically bulky aryl groups are expected to protect the diacetylene moiety from chemical reactions, such as dimerization [25–27] . The 3,5‐di‐ tert ‐butylphenyl group was introduced by the reaction of 2‐(3,5‐di‐ tert ‐butylphenyl)malononitrile 4 with 1‐chloromethylnaphthalene 5 .…”