Thermal isomerization in cyclopenta[fg]aceanthrylene

Abstract

Abstract Two mechanisms for the isomerization ofcyclopenta[fg]aceanthrylene to acefluoranthene wererevealed. The first pathway occurs via a cyclobutyl inter-mediate, whereas the second pathway involves a transitionstate that contains an sp 3 -hybridized carbon atom. Bothpatterns show that the Stone-Wales rearrangement requiresextremely high activation energy and indicate that theisomerization process can occur only under a drastic tem-perature regime.Keywords Stone-Wales rearrangement Density functional calculations Electronic structure Reaction mechanismsIntroductionCyclopenta-fused polycyclic aromatic compounds (CP-PAHs) are recently attracting much attention fromexperimental [1–4] and theoretical chemists [5–7]. One ofthe reasons for this lies in their important role in theprocesses of incomplete combustion of hydrocarbon-con-taining fuel sources [1, 3, 4]. Namely, among otherpolycyclic aromatic compounds, these are considered tobe responsible for the genotoxicity of respective com-bustion mixtures [8–10]. It was found that certaintemperature intervals permit particular isomerisation andintraconversion pathways of various CP-PAHs, whereasmany other reactions do not even occur [11–18]. Forexample, a series of FVP experiments has been performedto prepare cyclopenta[fg]aceanthrylene (1) under variousexperimental conditions [11]. However, these attemptshave yielded mixtures of isomeric cyclopenta[de]acea-nthrylene (2) and cyclopenta[de]phenanthrylene (3). It hasbeen suggested that 1 did form, but, due to its instability,it underwent isomerization to 2 via ethynylaceanthrylene[11]. This assumption has been recently confirmed bymeans of density-functional-theory-based calculations[19].Various computational approaches have been employedto provide additional information on possible thermalrearrangement processes, involving different intraconver-sions of CP-PAHs, and rearrangements of ethynylarenes toCP-PAHs [12, 17–21]. Stone-Wales rearrangement, i.e.,the isomerisation followed with the simultaneous rear-rangement of two carbon atoms, has been widely discussed[22–25]. The Stone-Wales transformation has originallybeen suggested as a hypothetical mechanism useful forderiving fullerene isomers [26]. Scott et al. [27] haveclaimed that pyracylene does not rearrange according tothe Stone-Wales pathway at 1,100 C. On the other hand,Nimlos et al. [22] have proposed two mechanisms forthe Stone-Wales rearrangement of pyracylene. The firstmechanism occurs via a cyclobutyl intermediate, and thesecond contains one transition state with an sp