Electron dynamics in double proton transfer

We next study the mechanism of proton transfer starting from the dimer molecular structure, which is the stage of C to D in Fig. 7.19. 

In Fig. 7.22 (panel (al) and (a2)), we show the dynamical change of unpaired electron populations on H5 and HIO, estimated with Eq. (7.27). From this figure, the unpaired electron populations on H5 and HIO are as small as 

2 Much electron density carried over by the proton nuclei 

As observed, the direction of electron flow from 03 to 06 (and from 08 to Ol) is the same as that of the transferring proton H5 (and HIO). This is qualitatively different from what we observed before in the proton transfer in Zundel cation [449] and water-molecule-mediated proton transfer in formamide [286], in which electrons flow in the reverse direction of proton motion. Therefore it appears that in the cychc moleciflar field like FAD, much electron carried by proton nuclei does not have to be compensated by the backward intermolecular flow but mainly by reorganization within the individual monomers. 

Thus we may conclude that the dimerization (Fig. 7.21) and concomitant proton transfer (Fig. 7.23) are smoothly continued in this regard. In other words, this smooth process is supported by the mechanism of concerted reaction, and there is not a conflicting factor in between the processes of dimerization and double proton transfer. As mentioned before, the STO-6G basis employed in this study tends to separate the dimerization and proton transfer as independent processes. However, these two processes can happen simultaneously in different basis sets. Even in such a case, the qualitative characteristics of the reaction mechanism must be essentially the same as we surveyed here, since the electron flow induced by these two processes are qualitatively equivalent to each other. (Of course, this does not justify the use of STO-6G basis for quantitative study to seek for accurate physical quantities.) 

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