Zyubina T S, Dyakov Y A, Lin S H, Bandrauk A D, Mebel A M
Institute of Atomic and Molecular Sciences, Academia Sinica, P. O. Box 23-166, Taipei 10764, Taiwan.
J Chem Phys. 2005 Oct 1;123(13):134320. doi: 10.1063/1.2050649.
Ab initio calculations employing the configuration interaction method including Davidson's corrections for quadruple excitations have been carried out to unravel the dissociation mechanism of acetylene dication in various electronic states and to elucidate ultrafast acetylene-vinylidene isomerization recently observed experimentally. Both in the ground triplet and the lowest singlet electronic states of C2H2(2+) the proton migration barrier is shown to remain high, in the range of 50 kcal/mol. On the other hand, the barrier in the excited 2 3A" and 1 3A' states decreases to about 15 and 34 kcal/mol, respectively, indicating that the ultrafast proton migration is possible in these states, especially, in 2 3A", even at relatively low available vibrational energies. Rice-Ramsperger-Kassel-Marcus calculations of individual reaction-rate constants and product branching ratios indicate that if C2H(2)2+ dissociates from the ground triplet state, the major reaction products should be CCH+(3Sigma-)+H+ followed by CH+(3Pi)+CH+(1Sigma+) and with a minor contribution (approximately 1%) of C2H+(2A1)+C+(2P). In the lowest singlet state, C2H+(2A1)+C+(2P) are the major dissociation products at low available energies when the other channels are closed, whereas at Eint>5 eV, the CCH+(1A')+H+ products have the largest branching ratio, up to 70% and higher, that of CH+(1Sigma+)+CH+(1Sigma+) is in the range of 25%-27%, and the yield of C2H++C+ is only 2%-3%. The calculated product branching ratios at Eint approximately 17 eV are in qualitative agreement with the available experimental data. The appearance thresholds calculated for the CCH++H+, CH++CH+, and C2H++C+ products are 34.25, 35.12, and 34.55 eV. The results of calculations in the presence of strong electric field show that the field can make the vinylidene isomer unstable and the proton elimination spontaneous, but is unlikely to significantly reduce the barrier for the acetylene-vinylidene isomerization and to render the acetylene configuration unstable or metastable with respect to proton migration.
已采用包含戴维森四重激发校正的组态相互作用方法进行了从头算计算，以揭示乙炔双阳离子在各种电子态下的解离机制，并阐明最近通过实验观察到的超快乙炔 - 亚乙烯基异构化过程。在C2H2(2+)的基态三重态和最低单重态电子态中，质子迁移势垒均显示保持在较高水平，在50千卡/摩尔范围内。另一方面，在激发的2 3A"和1 3A'态中，势垒分别降至约15和34千卡/摩尔，这表明在这些态中，特别是在2 3A"态，即使在相对较低的可用振动能量下，超快质子迁移也是可能的。对各个反应速率常数和产物分支比的赖斯 - 拉姆施佩格 - 卡塞尔 - 马库斯计算表明，如果C2H(2)2+从基态三重态解离，主要反应产物应为CCH+(3Σ-)+H+，其次是CH+(3Π)+CH+(1Σ+)，C2H+(2A1)+C+(2P)的贡献较小（约1%）。在最低单重态中，当其他通道关闭时，C2H+(2A1)+C+(2P)是低可用能量下的主要解离产物，而在Eint>5 eV时，CCH+(1A')+H+产物具有最大的分支比，高达70%及以上，CH+(1Σ+)+CH+(1Σ+)的分支比在25% - 27%范围内，C2H++C+的产率仅为2% - 3%。在Eint约为17 eV时计算得到的产物分支比与现有实验数据在定性上一致。计算得到的CCH++H+、CH++CH+和C2H++C+产物的出现阈值分别为34.25、35.12和34.55 eV。在强电场存在下的计算结果表明，电场可使亚乙烯基异构体不稳定并使质子消除自发进行，但不太可能显著降低乙炔 - 亚乙烯基异构化的势垒，也不太可能使乙炔构型相对于质子迁移变得不稳定或亚稳。
