www.asiachem.news December 2021 | 59 connecting them. The stable structures were categorized based on the bonding pattern, and the nodes and the edges in the network represent the groups and the reaction path connecting them, respectively. When the RCMC method was applied to the reaction path network, we predicted cyanohydrin (Me2C(OH)CN) would be produced in 97.49 % yield at 250K, while at 300 K and 350 K, the targeted aminonitrile (Me2C(NH2)CN) would be produced in 97.96 % and 93.28 % yield, respectively. In addition, the examination of the most feasible reaction path revealed that first cyanohydrin was formed even at 300K and 350K, then it returned to the reactant by the retro-cyanation, and finally, aminonitrile was formed. The white arrows on the reaction path network in Fig. 2(a) illustrate the path from the reactant to aminonitrile, while Fig. 2(b) shows the structural changes along this path. The reactant is a complex consisting of acetone, ammonium cation, cyanoanion, sodium cation, and chloroanion. The reactant undergoes the following steps on the reaction path: (1) the addition of cyanoanion to the carbonyl carbon followed by the proton transfer from the ammonium cation to the carbonyl oxygen generates cyanohydrin, (2) a retro reaction of the step (1) regenerates the reactant complex, (3) the proton transfers from the ammonium cation to the cyanoanion generates ammonia and hydrogen cyanide, (4) ammonia is added to acetone, (5) the proton transfers from hydrogen cyanide to the carbonyl oxygen, (6) the proton elimination by the chloroanion generates a hemiaminal intermediate, (7) the dissociation of water from the hemiaminal intermediate generates an iminium cation, and (8) the addition of cyanoanion to the iminium cation generates aminonitrile. The results of this simulation reproduce the known features of the Strecker synthesis very well, including the detailed reaction mechanism. It is interesting to note that this simulation was performed without any known informat ion. In other words, based on a priori DFT calculations, we have succeeded in reproducing the reaction mechanism of the Strecker synthesis hitherto proposed. Thermal Structural Transition of Amorphous Carbon The AFIR method can be appl ied to periodic systems using periodic boundary conditions. Here, we present the results of calculating the structural changes of the interfacial amorphous carbon induced by the annealing of carbon nanotube (CNT) yarns.21 In this application, the structure of the interfacial amorphous carbon was represented by 96 carbon atoms sandwiched between two parallel CNTs, as illustrated in Fig. 3(a). We assumed a one-dimensional periodicity along the two CNTs. As a reference, the same calculations were performed for the system without CNTs. For these systems, the reaction path network consisting of more than 10000 stable structures was obtained by the AFIR search using the DFTB3 method with pbc-0-3 parameters. Fig. 3(b) illustrates the reaction path network for the structural transition of the interfacial amorphous carbon structure sandwiched between two CNTs. Among the various reaction paths predicted on the network, we found the most kinetical ly favorable path, passing through the region highlighted by the arrow in Fig. 3(c), was the one for the transition to a structure with more sp2-bonds. The results of the kinetic simulation using this reaction network shown in Fig. 3(d) also indicate that the transition to the sp2-bond-rich structure occurs during annealing at 1500 K or higher. On the other hand, in the results of the kinetic simulation for the system without CNTs shown in Fig. 3(e), which was performed as a reference calculation, we found that the transitions to the sp2-bond-rich structure were suppressed. Amorphous carbon (without CNTs) is known to transform into the sp2-bond-rich structure only when annealed at 2500 K or Figure 2(a) Reaction path network of the Strecker synthesis. Each node is colored based on the free energy value of each stable structure. White arrows indicate the path from the reactant to the product. (b) Reaction mechanism of the Strecker synthesis found in the reaction path network as the path highlighted by white arrows.
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