Electronic structure bifurcations

Hypervalent carbocations have received some attention this year.14 The concept of three-centre, two-electron bonding in these entities is supported by a topological bifurcation analysis of the electronic structure of CH5 15 this and the related species CHg+ and CH3+ are also the subject of a review.16 The CHj + H2 reaction has been studied theoretically.17... 

It is actually very difficult to solve the entire scheme down to Eq. (6.5) for systems of chemical interest, even if a very good set of >/) is available. (Note that electronic structure theory (quantum chemistry) can handle far larger molecular systems within the Born-Oppenheimer approximation) than the nuclear dynamics based on Eq. (6.5) can do.) This is because the short wavelength natme of nuclear matter wave blocks accurate computation and brings classical nature into the nuclear dynamics, in which path (trajectory) representation is quite often convenient and useful than sticking to the wave representation. Then what do the paths of nuclear dynamics look like on the occasion of nonadiabatic transitions, for which it is known that the nuclear wavepackets bifurcate, reflecting purely quantum nature. 

The recent X-ray crystal structure of ascorbate oxidase [6] has indicated the relative positions of type 1, 2 and 3 Cu centers. The type 1 center is in a plastocyanin like domain, and is the primary acceptor of electrons from substrate. The shortest pathway for electron transfer from the type 1 to type 3 Cu s is the bifurcated path via Cys508 and either His 507 or His509. The two histidines are part of the plastocyanin-like domain, and serve also to coordinate the type 3 Cu s, Fig. 2. The His507 to Cys508 bonding is similar to that of Tyr83... 

The key step of the Qcycle is the electron bifurcation at the Qp site. The FeS cluster has a far higher redox potential (+300 mV) than that of heme bl (—90 mV). Considering the redox potential, both electrons could be transferred to the FeS cluster, although this never occurs in reality. The first electron is always transferred to the FeS cluster and the second electron to heme b. There was no clear answer how electrons could be bifurcated against the redox potential until the structures of the bc complex were revealed. 

This was the first complete structure of the bc complex. The structure provided information about all 11 subunits and revealed that subunit 9, the mitochondrial targeting presequence of ISP, exists between two core subunits, which are most likely a mitochondrial targeting presequence peptidase. We have solved the structures of the bc complex in two different crystal forms. Surprisingly, the conformation of the Rieske FeS protein was totally different between two crystal forms, and this provided a crucial insight of the electron bifurcation mechanism at the Qp site (see Section II,F). 

Now that it is substantiated that the [2Fe 2S] domain of the Rieske iron-sulfur protein is not static but moves between domains of cytochrome-c, and cytochrome-/ subunits, and that it is likely that such movement may provide a novel mechanism to allow catalysis of all the reactions involved in the oxidation of hydroquinone at the Qo site and the subsequent bifurcated pathway of electron transfer. It has been found that during the movement, the mobile [2Fe 2S] domain retains essentially the same tertiary structure, and the anchoring N-terminal tail of the R-ISP molecule remains in the same fixed position. The movement occurs through an extension of a helical segment in the short linking span. 

Note The experimental rotational constants of the fluoroform-oxirane complex measured in Ref [89a] are consistent with the two Cj stationary structures. One of them is 2 whereas the other has a bifurcated C-H- - -F- - -C-H bond. 2 is the global minimum at the MP2/6-311++G(2(/f,2/ ) level [89a]. The latter is the transition structure, distinguished by the energy offset of only 0.22 kcal - moF [89a]. The topological analysis of the electron density in 1 is investigated in Ref [103]... 

In this chapter we will consider molecular crystals with normal hydrogen bonds in which the donor A H interacts with an acceptor B. The so-called bifurcated and trifurcated H-bonds [1] as well as the new multiform unconventional H-bonds [2] are beyond the scope of the present chapter. We will focus on the proton dynamics in molecular crystals with strong and moderate H-bonds [3] in the ground electronic state. Attention will be focused on the interpretation of the structural and spectroscopic manifestations of the dynamics of the bridging proton as established in X-ray, neutron diffraction, infrared, and inelastic neutron scattering (INS) studies of H-bonded crystals. 

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