Decomposition of the Magnetic Coupling

The production of accurate electronic structure parameters is of course an important result for computational chemistry. However, it should not be the final goal and one has to go one step further on the road towards understanding. The qualitative valence methods described in the first sections of the previous chapter of this book are mainly focused on this understanding of the coupling, but here we discuss three approaches to analyse the results of the computational schemes that aim at a quantitative agreement with experiment. In this way quantitative accuracy can be combined with qualitative understanding. 

The binuclear complex [L2Cu2(m-13-N3)2] (L=N,N ,N -trimethyl-l,4,7-triaza-cyclononane) shows a large antiferromagnetic coupling with J = —800 cm, nicely reproduced with a DDCI calculation using a CAS(2,2)SCF reference wave function on the model complex [(NH3)6Cu2()u-l,3-N3)2] [1]. In this section, we will closely follow the work of Calzado and co-workers, decompose this 800 cm into small pieces and ascribe each individual contribution to well defined physical mechanisms [2, 3]. 

Graaf and R. Broer, Magnetic Interactions in Molecules and Solids, 

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Fig. 5.11 Schematic representation of the decomposition of the magnetic coupling for computa-tioneil schemes based on a single determinant. The orbittds and electrons marked in red are subject to chtmges with respect to the previous step, the rest is kept frozen...

Table 5.5 Decomposition of the magnetic coupling of the binuclear Ni-azido complex with 5 = 0...

Thomas reported a very similar study (decomposition of Co2(CO)8 in a hydrocarbon in the presence of a terpolymer), which resulted in the formation of nanoparticies of 2-30 nm according to the reaction conditions. The author was particularly interested in the modification of the magnetic properties due to dipolar couplings as a function of the separation of the particles.Similar procedures are still used for the production of cobalt particles in organic matrices. For example, a recent paper reports thermolysis reactions of Co2(CO)8 in a block co-polymer, PS2S3oo- -MMA2S9oo, hence producing cobalt nanoparticies embedded in the poly(methylmethacrylate) (PMMA) blocks. ... 

Generally, in MS an ion source is coupled with a mass analyser equipped with a detection system. The ion source generates gas-phase ions of the molecules of interest. The generation of analyte ions is a prerequisite because mass analyzers usually apply either magnetic or electrical fields for the molecular mass determination. Second, the process of desorption and ionization is a crucial step. It needs to proceed as gently as possible to avoid decomposition of the analyte, and the lack of appropriate methods to produce intact ions of large biomolecules, such as nucleic acids and proteins, initially has hampered the application of MS. 

Table 5.1 Decomposition of the CAS(2,2) magnetic coupling in the binuclear Cu + complex with a double azido bridge ...

In practice, one typically employs so-called relativistic SDFT (R-SDFT), in which a Gordon decomposition of the four-current is performed to separate orbital and spin degrees of freedom, and only the spin magnetization is maintained as a fundamental variable. This reduced RDFT is not Lorentz-invariant (which is not essential in solid-state physics and quantum chemistry, where a preferred frame is provided by the laboratory) and does not account for orbital magnetism (except, possibly, induced by spin-orbit coupling, which can be treated as a perturbation). Relativistic CDFT (R-CDFT) can overcome both limitations, but is less frequently used since it is more complicated to implement, and less is known about approximate four-current functionals. 

The flame retardant mechanism of PC/ABS compositions using bisphenol A bis(diphenyl phosphate) (BDP) and zinc borate have been investigated (54). BDP affects the decomposition of PC/ABS and acts as a flame retardant in both the gas and the condensed phase. The pyrolysis was studied by thermogravimetry coupled with fourier transform infrared spectroscopy (FUR) and nuclear magnetic-resonance spectroscopy. Zinc borate effects an additional hydrolysis of the PC and contributes to a borate network on the residue. 

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