Singlet-triplet behavior

Very often, however, these dyads are not isolated in the solid state and interact with neighboring ones at least along one preferential direction. In this case, we can distinguish two important situations the alternated spin chain and the spin ladder. As shown in Scheme 5, the alternated spin chain is characterized with two different magnetic interactions, noted / and aJ with 0 < a < 1. Note that if a = 0, one recovers the singlet-triplet behavior while if a = 1, we are in the presence of a uniform spin chain. The spin ladder is also characterized by two J values, noted J// and J in the following. 

A singlet-triplet behavior is found for those complexes such as [CpNi(oxdt)] [71]or [CpNi(F2pdt)] [72] associated into dyads (Fig. 12), where the strong distortions from planarity of the dithiolate ligands hinder any other interdyad intermolecular contacts. The large Ni S and S S intermolecular distances lead to weak intermolecular interactions with / values of —29 or —8 cnC1 for [CpNi(oxdt)] and [CpNi(F2pdt)] respectively. 

When paramagnetic complexes are condensed into a solid, magnetic interactions of several types may occur. The simplest interaction is that within isolated dimers in an insulator. In such cases a singlet-triplet behavior may be observed as for example in [NEt4][Pt(S2C2(CN)2)2], Fig. 18. Insulating columnar systems composed of paramagnetic complexes can have intrachain... 

The usual reactivity indices, such as elements of the first-order density matrix, are also incapable of distinguishing properly between singlet and triplet behavior. Recently, French authors 139,140) have discussed the problem and shown how electron repulsion terms can be introduced to obtain meaningful results. The particular case of interest to them was excited state basicity, but their arguments have general applicability. In particular, the PMO approach, which loses much of its potential appeal because of its inability to distinguish between singlet and triplet behavior 25,121) coui(j profit considerably from an extension in this direction. 119,122)... 

The iron(II) complexes [Fe(pppp)X]BPh4 (X=Br , I") showed unusual magnetic behavior, which was interpreted as being due to a singlet triplet spin transition [16]. 

The quenching of a triplet carbene reaction with methanol is frequently used as the standard means of probing the singlet-triplet gap. It is widely believed that singlet carbenes insert readily into the O—H bonds of methanol, while the triplet states undergo hydrogen abstraction from the C—H bonds." The behavior of diarylcar-... 

The parent nitrenium ion (NH2) is firmly established as a ground-state triplet both extensive ab initio calculations as well as PES experiments all agree that the singlet-triplet energy gap is 30 kcal/mol. There have been several investigations on its behavior in solution. Takeuchi et al. " showed that this species could be generated by photolysis of l-amino-(2,4,6-triphenylpyridinium) ion. These photolyses were carried out in the presence of various aromatic compounds. It was found that the triplet state abstracted hydrogen atoms from traps such as toluene... 

This behavior matches quite well to the scenario that has recently been developed for 2D spin systems in the proximity to a quantum critical point [16] and is a direct fingerprint of the tendency to form local correlations. It has to be mentioned, however, that the lowest energy excitation still has triplet character. For compounds with closer proximity to quantum criticality, such as the 1/6 depleted triangular lattice (Kagome) [6] and the 1/5 depleted square lattice, also exists a dense singlet spectrum within the singlet triplet gap [17-20], Unfortunately no Raman scattering data for the related compounds are available up to now. 

Looking more closely at the evolution of IIT1 at low temperatures, it is interesting to note that there seems to be a coexistence between the insulating and metallic behavior at 8 kbar. For low temperatures, the behavior is linear like in a metallic state with a relatively low density of states (this would correspond to y=3.10 1 emu/mol) but it increases more steeply up to room temperature, as if the singlet-triplet transitions were still present in the metal. 

Since modern structural determination depends heavily upon H and C NMR analysis, information from IR and UV spectroscopies is less abundant than formerly. However, these spectroscopies are still useful to study photophysical properties, chemical behavior, and electronic orientation of the substituents in heteroaromatics. The pH dependence of both electronic and IR spectra of pyrazine has been examined to obtained p. values and information about the solute-solvent interactions, respectively <84JST(i 14)367). Electronic absorption taken in conjunction with fluorescence spectra indicate that 1,4-dihydro-2,3-quinoxalinedione exists in its diketo form irrespective of the nature of the solvent, by comparison with the spectra of the parent quinoxaline and 1,4-dimethyl-2,3-quinoxalinedione <87MI 603-01). New measurements of the singlet-triplet absorption spectra of pyrazine and pyrazine-d 4 at longer pathlengths and higher vapor pressures have been recorded <93aci537). 

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