Non-disjoint

Attempts to photoswitch the intramolecular magnetic interaction based on photo-chromism of diarylethenes have been overviewed. The switching between the disjoint and non-disjoint structures caused a change in the interaction between the separated spins. Photochromic diarylethene is one of the most favorable photoswitching units for magnetic interactions. This system has the possibility to be applied to the molecular-scale information processing system [73-77]. 

A non-disjoint mechanism, which at least in principle allows simultaneous linear-quadratic kinetics, is shown in Fig. 8. This arises exclusively in the monometallic case that M = M when the core CBER structure is shared with a unicyclic mechanism. In this case, there are two pathways for product formation, and this arises... 

Fig. 9 A heterobimetallic non-disjoint mechanism arising when a core CBER stmcture is shared with a unicyclic mechanism. The orange shaded region involves mononuclear species of M, and the two shades of blue are used to represent the mononuclear sequences involving M ...

M. If ra t4, the CBER mechanism dominates and the rate of product formation can be either linear in M or M or bilinear in the product of M and M as limiting cases. A general expression for the total rate of product formation from a non-disjoint heterobimetallic [ = M, — M ]cber+uni mechanism... 

Closer consideration of the NBMOs of these systems explains the differing spin preferences. For a degenerate pair, any linear combination of the MOs is acceptable. For CBD, we can make a linear combination of the NBMOs such that they have no atoms in common (Figure 14.32 A). One NBMO is confined to atoms 1 and 3, while the other is confined to atoms 2 and 4. Such orbitals are said to be disjoint they occupy different sets of atoms. In contrast, it is not possible to find a linear combination of the NBMOs of TMM that are disjoint (Figure 14.32 B). No matter what you do, there will always be atoms in common, and the NBMOsare termed non-disjoint. 

Earlier in this chapter we introduced the concept of alternate hydrocarbons (AH). TMM and CBD are both even AHs, but there is a significant difference. While CBD has two and two non- atoms, TMM has three and one non-. It is not a coincidence that these two systems with differing spin preferences also appear different in the /non- analysis. The / non- rule is basically a topology rule—it reflects the connectivity of the system. The topology of the system also influences the nature of the NBMOs. In fact, it can be shown that a system with an excess of over non- atoms will in general have non-disjoint NBMOs, while a system with equal numbers of and non- atoms will have disjoint NBMOs. 

Figure 14.32 C shows the NBMOs of m-xylylene, another prototype high-spin system. This structure also has two more than non- atoms. As such, its NBMO are non-disjoint, and a triplet state for the biradical is preferred. In our discussion of organic magnetic materials in Section 17.3 we will see that this /non- approach to predicting spin states can be extended to a remarkable degree, allowing the rational design of very high-spin organic molecules.

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