Mebel A M, Bandrauk A D
Department of Chemistry and Biochemistry, Florida International University, Miami, Florida 33199, USA.
J Chem Phys. 2008 Dec 14;129(22):224311. doi: 10.1063/1.3037204.
Ab initio coupled clusters and multireference perturbation theory calculations with geometry optimization at the density functional or complete active space self-consistent-field levels have been carried out to compute ionization energies and to unravel the dissociation mechanism of allene and propyne cations, C(3)H(4)(n+) (n=1-3). The results indicate that the dominant decomposition channel of the monocation is c-C(3)H(3)(+) + H, endothermic by 37.9 kcal/mol and occurring via a barrier of 43.1 kcal/mol, with possible minor contributions from H(2)CCCH(+) + H and HCCCH(+) + H(2). For the dication, the competing reaction channels are predicted to be c-C(3)H(3)(+) + H(+), H(2)CCCH(+) + H(+), and CCCH(+) + H(3)(+), with dissociation energies of -20.5, 8.5, and 3.0 kcal/mol, respectively. The calculations reveal a H(2)-roaming mechanism for the H(3)(+) loss, where a neutral H(2) fragment is formed first, then roams around and abstracts a proton from the remaining molecular fragment before leaving the dication. According to Rice-Ramsperger-Kassel-Marcus calculations of energy-dependent rate constants for individual reaction steps, relative product yields vary with the available internal energy, with c-C(3)H(3)(+) + H(+) being the major product just above the dissociation threshold of 69.6 kcal/mol, in the energy range of 70-75 kcal/mol, and CCCH(+) + H(3)(+) taking over at higher energies. The C(3)H(4)(3+) trication is found to be not very stable, with dissociation thresholds of 18.5 and 3.7 kcal/mol for allene and propyne, respectively. Various products of Coulomb explosion of C(3)H(4)(3+), H(2)CCCH(2+) + H(+), CHCHCH(2+) + H(+), C(2)H(2)(2+) + CH(2)(+), and CCH(2)(2+) + CH(2)(+) are highly exothermic (by 98-185 kcal/mol). The tetracation of C(3)H(4) is concluded to be unstable and therefore no more than three electrons can be removed from this molecule before it falls apart. The theoretical results are compared to experimental observations of Coulomb explosions of allene and propyne.
已进行了从头算耦合簇和多参考微扰理论计算，并在密度泛函或完全活性空间自洽场水平上进行几何优化，以计算电离能并阐明丙二烯和丙炔阳离子C(3)H(4)(n+)（n = 1 - 3）的解离机制。结果表明，单阳离子的主要分解通道是环丙烯基阳离子（c-C(3)H(3)(+)） + H，吸热37.9 kcal/mol，通过43.1 kcal/mol的势垒发生，H(2)CCCH(+) + H和HCCCH(+) + H(2)可能有较小贡献。对于双阳离子，预测竞争反应通道为环丙烯基阳离子（c-C(3)H(3)(+)） + H(+)、H(2)CCCH(+) + H(+)和CCCH(+) + H(3)(+)，解离能分别为 - 20.5、8.5和3.0 kcal/mol。计算揭示了H(3)(+)损失的H(2)漫游机制，即首先形成中性H(2)片段，然后四处漫游并从剩余分子片段中夺取一个质子，然后离开双阳离子。根据对各个反应步骤的能量相关速率常数的赖斯 - 拉姆齐格 - 卡塞尔 - 马库斯计算，相对产物产率随可用内能而变化：在略高于69.6 kcal/mol的解离阈值时，环丙烯基阳离子（c-C(3)H(3)(+)） + H(+)是主要产物；在70 - 75 kcal/mol的能量范围内，CCCH(+) + H(3)(+)在更高能量时占主导。发现丙二烯和丙炔的C(3)H(4)(3+)三阳离子不太稳定，丙二烯和丙炔的解离阈值分别为18.5和3.7 kcal/mol。C(3)H(4)(3+)库仑爆炸的各种产物，H(2)CCCH(2+) + H(+)、CHCHCH(2+) + H(+)、C(2)H(2)(2+) + CH(2)(+)和CCH(2)(2+) + CH(2)(+)都是高度放热的（98 - 185 kcal/mol）。得出结论，C(3)H(4)的四阳离子不稳定，因此在该分子分解之前，最多只能从该分子中移除三个电子。将理论结果与丙二烯和丙炔库仑爆炸的实验观测结果进行了比较。
