Abstract
Introduction The relationship between chemical reactions and ion formation at the junction between the electrode and the molecular film is considered an important issue for improving the performance of rechargeable batteries and realizing many types of micro-molecular devices. In this study, we focus on hydrogen atom (H) addition reactions in imidazole-terminated alkanthiolate self-assembled monolayers (Im-SAMs) on Au substrates. While hydrogenated imidazoles stabilize as imidazolium cations in acidic solutions, it is not clear whether charge transfer from the Au substrate, across the alkyl insulating layer, to the imidazole groups is possible in non-aqueous systems. In our previous experiments, the H-addition reaction proceeded when the Im-SAM was irradiated with hydrogen atoms [1]. To characterize the products, we calculated the total energies and molecular orbitals of the chemical species likely to be produced in this reaction. The results were useful to examine the possibility of charge transfer.
Results and Discussions For the molecular orbital calculations, B3LYP/6-31++G(d,p) level of calculation was used as the density functional and basis set. To simplify the calculations, we picked up molecular model based on 1-methylimidazole. Two corrections were made when comparing the total energies of each molecular model. First, the energies of the unadded hydrogen atoms alone were added together. When electrons were transferred to the substrate, a negative value was added to the total energy as stabilizing energy for the work function φ of the substrate . In terms of total energy, the first radical species produced (Fig. 1b) is lower than the imidazole form (Fig. 1a), suggesting that the H addition reaction proceeds sufficiently. In contrast, the cationic species (Fig. 1c) were estimated to have a higher energy of the individual molecule. However, when the stabilization energy for the work function φ due to the transfer of excess electrons to the substrate was taken into account, the energy was found to be lower than that of the radical species. In other words, the formation of cationic species with electron transfer was confirmed to be possible. Moreover, the reaction involving the addition of another H atom (Figs. 1d and 1e) was suggested, while the reaction had an activation energy. For this reaction, it is expected to involve the transfer of electrons from the substrate and further progress to a stable neutral state.
REFERENCES
[1] R. Muneyasu et al. (in preparation for publication).
