Bonding, relativistic effects Chemical shift

Relativistic effects on calculated NMR shieldings and chemical shifts have sometimes been divided into "direct" and "indirect" effects. According to this point of view, indirect effects are those that result from relativistic changes of the molecular geometry (the well-known relativistic bond contraction (55) in particular) whereas direct effects refer to a fixed geometry. 

Unlike aqueous clusters with predominating electrostatic interaction, the interactions in metallic clusters can be classified as either continuous transitions or metallic [95,96]. In the former interaction, there is a continuous shift from one type of bonding to another type as function of a chemical or physical variable. These types of bondings include covalent, ionic, metallic, and van der Waals interactions. Thus, the accuracy of the results on metallic clusters to a large extent depends on the ability of the theoretical method to describe several types of interactions. In the case of the clusters of higher transition metals, the theoretical method should also be able to describe relativistic effects accurately. 

Spin-orbit coupling has been shown to be particularly important for the chemical shifts of neighboring atoms of heavy halogen substituents, e.g for the shifts in iodomethanes (one speaks of a heavy-atom effect ). Early relativistic extended-Huckel calculations have indicated that even the shift of the heavy atom itself may be influenced by spin-orbit coupling (a heavy-atom effect on the heavy atom ). Thus, depending on the bonding situation of the molecule and on the position of the constituent atoms within the periodic table, spin-orbit corrections to chemical shifts have to be considered (the same holds for spin-spin coupling constants, a point we will not consider further here). 

The total electron density at the nucleus p(0) depends on the nature of the chemical bonding. Positive contributions to p(0) arise from the atomic 6s populations of the molecular orbitals on the gold ion, while a decrease in p(0) is caused by the atomic bd populations due to their shielding effects on s electrons. The relative magnitudes of the various contributions are known from the results of relativistic free-ion self-consistent field calculations for gold and for other d transition elements. The atomic 6p populations, on the other hand, yield only small contributions both by shielding of s electrons and by their relativistic density at the nucleus. Hence p(0) and consequently the isomer shift will mainly depend on the atomic 6s and 5[Pg.281]

Non-Bom-Oppenheimer (BODC), relativistic, and core-valence correlation corrections, tacitly neglected in most quantum chemical studies, result in small shifts of the calculated PES values. BODC and relativistic correction to the force constant are usually negligble for species involving first- and second-row. atoms. On the other hand, advances in the continuing development of quantitatively accurate ab initio methods have revealed the necessity of a full understanding of the consequences of core-core and core-valence electron correlation (see Core-Valence Correlation Effects) on calculated force fields. It has been found that (a) equilibrium bond distances of first-row diatomic molecules experience a considerable contraction, about 0.002 A for multiple bonds and 0.001 A for single bonds, reducing the errors in Rq predictions... 

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