Calculations concerning interstellar isomeric abundance ratios for C3H and C3H2.
Calculations concerning interstellar isomeric abundance ratios for C3H and C3H2.
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关于 C3H 和 C3H2 星际异构体丰度比的计算。
作者：
S. Maluendes;A. D. McLean;E. Herbst
星际c-C3 H2、H2 CCC、c-C3 H和HCCC的合成，其中“c”代表环状异构体，被认为是通过前体离子c-C3 H3+和H2 CCCH+的解离复合进行的，它们本身主要通过C3 H+和H2之间的辐射缔合反应产生。利用从头算方法研究了线性离子C_3H ~+与H_2缔合生成异构体c-C_3 H ~（3+）和H_2CCCH ~+的势能面。通过相空间方法计算了辐射缔合的总速率系数作为温度的函数。我们的从头计算表明，H2 CCCH+异构体直接形成反应物的活化势垒，并通过一个低能量的途径，包括两个过渡态（鞍点上的PES）和一个中间体（本地最小值上的PES），这两种异构体之间的异构化可以很容易地发生。异构化H2 CCCH + <->c-C3 H3+作为能量的函数的平衡系数的计算表明，这两种离子的相等丰度应产生作为松弛进行，在高压下的实验测量一致。我们的结果证实了一个重要的观点，即一个简单的离子-分子缔合反应可以产生一个环烃。如果涉及c-C3 H3+和H2 CCCH+的解离重组反应保持离子的碳骨架结构并产生大致相似的C3 H/C3 H2分支比，则通过这种机制在C3 H和C3 H2的环状和非环状异构体之间产生丰度比。在TMC-1中观察到的c-C3 H2与H2 CCC的大丰度比可以通过不同的破坏率来解释。
The syntheses of interstellar c-C3H2, H2CCC, c-C3H, and HCCC, where "c" stands for the cyclic isomer, are thought to proceed via dissociative recombination of the precursor ions c-C3H3+ and H2CCCH+, which are themselves produced mainly via the radiative association reaction between C3H+ and H2. We have utilized ab initio methods to study the potential energy surface (PES) for the association of the linear ion C3H+ and H2 to form the isomers c-C3H3+ and H2CCCH+. The overall rate coefficient for radiative association has been calculated as a function of temperature via the phase space method. Our ab initio calculations show that the H2CCCH+ isomer is formed directly without an activation barrier from reactants, and that isomerization between the two isomers can occur readily via a low-energy pathway consisting of two transition states (saddle points on the PES) and one intermediate (local minimum on the PES). Calculations of the equilibrium coefficient for the isomerization H2CCCH+ <-> c-C3H3+ as a function of energy shows that equal abundances of these two ions should be produced as relaxation proceeds, in agreement with experimental measurements at high pressure. Our results confirm the important point that a simple ion-molecule association reaction can produce a cyclic hydrocarbon. If dissociative recombination reactions involving c-C3H3+ and H2CCCH+ maintain the carbon skeletal structure of the ions and produce roughly similar C3H/C3H2 branching ratios, then abundance ratios of unity are produced between the cyclic and noncyclic isomers of C3H and C3H2 via this mechanism. The large abundance ratio of c-C3H2 to H2CCC observed in TMC-1 can then be explained by differential destruction rates.