Open-Shell Electronic Structures

CASSCF and CASPT2 calculations both overestimate the stability of the open-shell electronic structure of singlet nitrenes 8c-f by about 3 kcal/moP as in the case of parent The ring opening is also computed to require... 

A first group of compounds Ce Cg2, Gd Cg2, Y Cg2, and the major [C2v] and minor [Cs] isomers of La Cg2 and Pr Cg2 showed two oxidation steps, the first reversible and the second irreversible, even at scan rates up to 1 V/s. The low potential of the first oxidation step, close to that of the ferrocene/ferrocenium couple (see Table 8.3 and Fig. 8.4), made these compounds rather good electron donors. These compounds could also be reduced in four to six distinct steps, most of them reversible, and their reducing ability was found even higher than that of and similar to that of the major isomer of Cg2 (C2). Noticeably, all these compounds had a very low electrochemical HOMO-LUMO gap (A ,gap<0.50 V). In addition, similar UV/Visible spectra were obtained for all of them,28 suggesting also similar electronic structures. ESR showed that Y Cg229 and both isomers of La CX249 52 are radical species and consequently that the formal oxidation state of the metal in these structures is probably + 3. Therefore, their low HOMO-LUMO gap is probably a consequence of their open-shell electronic structure. 

We now proceed to consider the magnetic interactions involving the electron spin S in X states with open shell electronic structures. The magnetic dipole moment arising from electron spin is... 

The reduction of Ni ions and the stability of the corresponding NF ions can be explained by arguments based on structure and electrostatic theory. But the chemical activity of NF, particularly the highly increased bond energy with carbon monoxide, shows that the properties of the open-shell electronic structure are of great importance also. 

At the present state of the art, it seems that the available ab initio vibrational spectroscopy codes cannot yet be applied routinely to open-shell systems. In the case of CC-VSCF, the problem seems to be due to difficulties of convergence of the electronic structure codes employed, for at least some of the points used in the CC-VSCF calculations. A complete solution for this problem may well come from the development of improved and more versatile open-shell electronic structure codes. There are also, however, examples of successful applications. Table 9.5 lists the CC-VSCF/MP2-TZP calculations of Chaban... 

CASSCF and CASPT2 calculations both overestimate the stability of the open-shell electronic structure of singlet nitrenes 62a-d by about 3 kcal/mol, as in the case of parent 149 101 pjj g opening is computed to require passage over a 2 to 3 kcal/mol lower energy barrier than reversion of the intermediates to the reactants. Therefore, cyclization is the rate-determining step in the ring-expansion reactions of cyano-substituted phenylnitrenes 62a-d to derivatives of 64 and 64 (Scheme 5.3). 

A more interesting case is the genuine open-shell electronic structure of the H2O+ molecule for which the data is displayed in the above fragment of code (14 spin-basis functions, 9 electrons). 

As far as we know there has not been any earlier theoretical analysis of the electronic spectrum of PbO. Many of the methods used to study this molecule are such that they cannot be applied to excited states. The present approach, on the other hand, is constructed to be able to deal with complex open shell electronic structures. With the development of the RASSI... 

A different - and the most popular - approach to describe open-shell electronic structures is the density functional theory (DFT) [106, 107], where the system and its properties are completely determined by the electron density p(r). The most widespread implementation of DFT is the Kohn-Sham formalism [108] in which an artificial reference system of noninteracting electrons is introduced to yield exactly the same electron density as the fully interacting system. In Kohn-Sham DFT, the electronic energy is a functional of p(r) and can be partitioned into several contributions. 

Recently, two developments have taken place in this field one concerns the quantum chemical level used to solve open shell electronic structures [37], the other one is an extention of the homogeneous model to treat an anisotropic surrounding medium [38]. 

After having received his PhD in 1985 at KU Leuven, Marc F. A. Hendrickx was the second researcher to join the theoretical chemistry group of this university as a permanent member of the academic staff. Since then, the main focus of his research activities has been on the study of properties of a wide variety of transition metal compounds. His recent research activity is mainly directed toward applying quantum chemical methods on small transition metal-containing clusters. Their frequently complicated open-shell electronic structures are studied in relation to their magnetic and spectroscopic properties. 

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