Exploration of the potential energy surface of C4H4 for rearrangement and decomposition reactions of vinylacetylene: A computational study. Part I

Abstract

The potential energy surface (PES) of C4H4 was explored using quantum chemical methods (DFT, MP2, MP4, GVB-MP2, CCSD(T), G2M, CBSQ/APNO) and 43 different structures located at global and local minima were identified. The majority of these structures correspond to carbenes, a minority to closed shell systems and biradicals (carbyne structures were not investigated). Whereas the chemistry of the closed shell systems such as vinylacetylene (1), butatriene (2), methylenecyclopropene (3), cyclobutadiene (5) or tetrahedrane (15) is well known, the carbenes represent unusual structural entities. 2-Methyl-cycloprop-2-en-1-ylidene (4) (DeltaDeltaH(298) = 36.2 kcal mol(-1) relative to 1) in its sigma2pi0 electron configuration at the carbene C of the 1A ground state is of comparable stability to cyclobutadiene (5) (DeltaDeltaH(298) = 33.4 kcal mol(-1); exp. value: 32.1 kcal mol(-1) as a result of aromatic 2pi-delocalization; carbene 3-vinylidenecyclopropene (13) (DeltaDeltaH(298) = 53.9 kcal mol(-1) does not possess C(2v) symmetry but has the vinylidene group bent toward the three-membered ring (C(s)-symmetry) thus representing a frozen path point of the chelotropic addition of :C=C: to ethene. Allenyl carbene (14) has a triplet ground state and two low lying excited singlet states of closed shell (2.5 kcal mol(-1) higher) and open shell character (14.1 kcal mol(-1)). Carbene 14 is a crossing point on the C4H4 PES connecting closed-shell systems with each other. Because of the stability of 1, its rearrangement reactions are all connected with high activation enthalpies requiring 66 up to 92 kcal mol(-1) so that they energetically overlap with the activation enthalpies typical of decomposition reactions (from 90 kcal mol(-1) upward). The possible rearrangement reactions of 1 are investigated with a view to their relevance for the chemical behavior of the molecule under the conditions of Titan's atmosphere.

Dehydropericyclic Reactions: Symmetry-Controlled Routes to Strained Reactive Intermediates

“…A more recent comprehensive study on the C 4 H 4 surface by Zwier and co-workers does not appear to have located structure 35. 34 In the present study, we have explored the potential surface around 32 and 35 to help define their role in enyne photochemistry. Our computed energetics of the CCSD-(T)//DFT surface connecting 35 with vinylacetylene (4) and the singlet diradical structure 32 are shown in Figure 2.…”

Finding the Transition State of Quasi-Barrierless Reactions by a Growing String Method for Newton Trajectories: Application to the Dissociation of Methylenecyclopropene and Cyclopropane

“…[19,25] In line with this, the energy of TS2 lies only 2.9 kcal/mol above that of the 1 A excited state of anti allenylcarbene (the ground state is a triplet [19,25]). The reaction leading from MCP to allenylcarbene is endothermic by 41.9 kcal/mol Table 1).…”

“…Hence, results confirm the experimental observation that the reaction proceeds without an activation enthalpy. [19] Nevertheless, we can use TS1 as a meaningful starting point to carry out a mechanistic analysis of the reaction, which will be described in a forthcoming paper.…”

“…Dissociation can happen in a single step (TS1) or a two step process where in the latter case first allenylcarbene is formed (TS2) and then allenylcarbene dissociates in an endothermic reaction (∆ H(298 o ) = 61.7 kcal/mol) without barrier. [19] …”

A heavy analogue of the smallest bridgehead alkene stabilized by a base

“…1a) is a bridgehead alkene with the smallest number of bridge atoms and several possible canonical structures containing a strained double bond 2 , even though BBE has been considered to be an exceptional bridgehead alkene ('zero-bridge alkene') because of the lack of a twisted double-bond geometry observed in typical bridgehead alkenes. Although BBE and its derivatives have been proposed as reactive intermediates 6 and theoretically investigated as the isomers of C 4 H 4 molecules present in important fundamental organic molecules such as tetrahedrane and cyclobutadiene [7][8][9][10][11] , the synthesis and experimental observation of BBE and its derivatives have not been reported yet.…”