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.

The ultraviolet photochemistry of vinylacetylene (C4H4) was studied under temperature and pressure conditions similar to Titan's atmosphere by exciting the molecule in a constrained expansion that opens into the ion source region of a time-of-flight mass spectrometer. The primary dissociation products detected by vacuum-ultraviolet ionization were found to be C4H3 and C4H2, in a ratio of 3-10 : 1. Subsequent reaction of the C4H3 radicals with the parent C4H4 produced two major secondary products: C8H6 and C6H4. The former was spectroscopically identified as phenylacetylene, confirming that photochemical reactions of C4H4 can produce aromatic molecules. The primary dissociation reaction was also studied computationally. The results were consistent with the experimental findings for C4H2 and C4H3. However, the major product is C2H2, which is undetected by 118 nm photoionization in the present experiment but should account for roughly two-thirds of the products. Simulations were also performed to confirm that the present experiment accurately represents the 220 nm photochemistry of vinylacetylene at the temperature and pressure of Titan's atmosphere, with a product yield of C2H2 : C4H2 : C4H3 of 66 : 7 : 27. Accounting for the wavelength dependent solar flux on Titan, the estimated absorption cross section of vinylacetylene in the ultraviolet, and the slightly wavelength dependent product distribution, the overall product yield predicted by the simulations for ultraviolet photolysis of vinylacetylene on Titan is C2H2 : C4H2 : C4H3 = 65 : 8 : 27. Finally, a simulation was performed under conditions of a shock tube experiment to examine the differences between thermal and photochemical dissociation. The product yield of this simulation was C2H2 : C4H2: C4H3 = 61 : 1 : 38.