Cremer Dieter, Kraka Elfriede, Joo Hyun, Stearns Jaime A, Zwier Timothy S
Department of Chemistry, University of the Pacific, Stockton, CA 95211-2271, USA.
Phys Chem Chem Phys. 2006 Dec 7;8(45):5304-16. doi: 10.1039/b609284e.
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.
利用量子化学方法（DFT、MP2、MP4、GVB - MP2、CCSD(T)、G2M、CBSQ/APNO）对C4H4的势能面（PES）进行了探索，并确定了位于全局和局部极小值处的43种不同结构。这些结构中的大多数对应于卡宾，少数对应于闭壳层体系和双自由基（未研究卡拜结构）。虽然闭壳层体系如乙烯基乙炔（1）、丁三烯（2）、亚甲基环丙烯（3）、环丁二烯（5）或四面体烷（15）的化学性质是众所周知的，但卡宾代表着不寻常的结构实体。1A基态卡宾碳上具有σ2π0电子构型的2 - 甲基 - 环丙 - 2 - 烯 - 1 - 亚基（4）（相对于1，ΔΔH(298) = 36.2 kcal/mol）与环丁二烯（5）（ΔΔH(298) = 33.4 kcal/mol；实验值：由于芳香性2π离域为32.1 kcal/mol）具有相当的稳定性；卡宾3 - 亚乙烯基环丙烯（13）（ΔΔH(298) = 53.9 kcal/mol）不具有C(2v)对称性，而是亚乙烯基基团向三元环弯曲（C(s)对称性），因此代表了:C=C:与乙烯进行亲烯加成的一个冻结路径点。烯丙基卡宾（14）具有三重态基态以及两个低能级的闭壳层激发单重态（高2.5 kcal/mol）和开壳层特征激发单重态（高14.1 kcal/mol）。卡宾14是C4H4势能面上连接闭壳层体系的一个交叉点。由于1的稳定性，其重排反应都伴随着高达66至92 kcal/mol的高活化焓，以至于它们在能量上与分解反应（从90 kcal/mol向上）典型的活化焓重叠。考虑到1的重排反应与泰坦大气条件下该分子化学行为的相关性，对其可能的重排反应进行了研究。
