Thermal population, of low-lying

In several types of compounds, excited triplet levels lie very close to the ground states, as has been mentioned already. Such thermal population of low-lying triplet levels occurs frequently in inorganic complexes as well as with organic biradicals. Thermochromism of bianthrone has been explained as due to thermal conversion to a triplet state.129... 

Since t(2E) is essentially independent of temperature for Cr(teta)(CN)2> it is clear that thermal population of low lying electronic components of the higher energy doublet state do not provide an efficient relaxation pathway in this system. 

Thus, in order to understand such environments it is necessary to calculate complete network of the competitions between neutron capture and beta decay as well as their corrections for the thermal population of excited states. With regard to this latter correction it is particularly important to know the low-energy level structure of nuclei away from stability. This structure will affect the beta decay properties differently from the neutron capture properties. In a separate contribution to this conference, [TAK85] we will discuss the corrections for beta decay. Basically this becomes important if a low-lying excited state can undergo a Gamow-Teller allowed decay. The... 

More systematic calculations with the present method will certainly hel to clarify our understanding of the 0-decay properties of spherical nuclei fa off the line of stability, which are needed in r-process studies. In particular, a study of the effects of the 0-decay of low-lying states thermally populated in the high temperature r-process environment is due. Sue effects have not been included in any r-process model yet attempted. Finally we mention that a different approach (i.e. RPA) is probably called for in order to deal with deformed nuclei effectively [BRA85]. ... 

A recent study of the photochemistry of [IrCb (phen) 2] Cl and [IrCl2-(5,6-mephen)2]Cl attributes the photochemical activity of these complexes to thermal population of a low-lying set of levels of d d orbital parentage (29). We have now studied the time-resolved emission spectroscopy of these complexes between —196° and 0°C in glycerol, and have concluded that the emission around 0°C is primarily d-d in character whereas the emission at —196°C is mainly d-7r [IrCl2(phen)2]Cl or 7r-7T [IrCl2(5,6-mephen)]Cl (30). We attribute the lack of d-d emission at — 196°C to a thermal barrier for the radiationless process which leads from the d-w or tt-tt levels to the d-d levels. 

On this basis the observed temperature dependence of the moment could be reproduced, on the assumption that the low-spin 2A (a2 53) state lay lower with the higher 6 + level being thermally populated as the temperature was increased. Since the esr studies also showed a 2A ground state for Mn(MeCp)2, and a similar ground level may also be obtained for the unsubstituted Mn(Cp)2 (68), it is reasonable to conclude that in both cases the 2A state lies lower. For Mn(Cp)2 the results of Ammeter et al. (68) show that the observed temperature dependence of the moment may be replicated asuming (62+) - E(2A) ... 

Attempts to detect a thermally populated triplet state ( A ) of 8 by ESR spectroscopy were unsuccessful. This was attributed to the high reactivity of the diradical, which presumably easily abstracts hydrogen atoms in hydrocarbon matrices (to form p-quinodimethane), even at very low temperatures. In this context, the triplet state of /7-phenylenebis(phenylmethylene) has been observed. Apparently, substitution of the carbenic hydrogens of 8 by phenyl groups confers sufficient stability (thermodynamic and perhaps kinetic) to the biradical, which allows its observation. According to variable-temperature ESR spectroscopy, the triplet state of /7-phenylenebis(phenylmethylene) is thermally populated and the singlet state lies 0.5-1 kcal/mol lower in energy [76-79]. 

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