Heavy elements mainly refer to elements with higher atomic numbers and relative atomic masses.Heavy element is a treasure house of new functional materials and new energy, which is of great significance to the national economy and national defense construction.For example, the active centers of most catalysts are heavy elements;Steel series elements such as uranium and plutonium are the core components of nuclear energy and nuclear weapons;rare earth elementIt is also the source of new materials.[1]
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Due to the large heavy element system and the complex electronic structure system, it has always been a hot and difficult spot in physical chemistry and other related scientific research.Especially the transitional ethnic groups developed in recent decadesOrganometallic compoundThe research on the chemical properties of carbon dioxide is one of the frontier fields of chemistry today.Since ferrocene was synthesized in the early 1950s, a variety of new transition metal organic compounds have sprung up, and have been enduring.A large number of papers and reports also reflect the research heat of transition metal organic compounds.The further development of transition metal organic compounds is not only of great scientific significance, but also of great importance to the development of new energy sources, the discovery of new chemical reactions, the development of new drugs and environmental protection.[1]
Relativistic effect of heavy element system
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An important research point of heavy element system is the influence of relativistic effect. This is because the motion speed of electrons in the inner shell of heavy element atoms is greatly increased, so that it is close to the speed of light. Therefore, the relativistic effect of electrons is relatively significant.In addition, the cascade effect will transmit the influence of the relativistic effect to the valence electrons with lower motion speed.Generally speaking, the relativistic effects in atoms and molecules can be divided into direct relativistic effects and indirect relativistic effects.The former includesElectron spin magnetic momentThe coupling with the orbital magnetic moment (spin orbit coupling) and the space contraction and energy reduction of s and p orbitals caused by the high-speed movement of electrons near the atomic nucleus.It should be pointed out that the contraction of the valence s and p orbitals is also a direct relativistic effect, rather than due to the fact that these orbitals are orthogonal to the inner orbitals.Because the s and p valence electrons will penetrate near the nucleus.On the other hand, the so-called indirect relativistic effect means that the shrinking inner orbital forms a better shielding for the atomic nucleus, which leads to the outer d and f orbitals extending in space and the energy rising.[2]
For a long time, it is generally believed that the relativistic effect of valence electrons can be ignored, because the inner electrons have a significant shielding effect on the nucleus, which makes the motion speed of valence electrons far less than the speed of light.However, with the development of experimental techniques and theoretical methods, people gradually realize that the relativistic effects of heavy atoms and molecules containing heavy elements are actually quite important.
Research Methods of Relativistic Effects of Heavy Element Systems
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The theoretical basis for studying the relativistic effect of heavy element system is Dirac equation, and the theoretical tool isRelativistic quantum chemistrycalculation.According to the different approximation degree of Dirac Hamiltonian, the relativistic quantum chemical method is divided into four component, two component and single component (scalar) relativistic methods.European countries have always attached great importance to the research of relativistic quantum chemical calculation.In about 15 years after the mid-1980s, European countries led by Germany, with strong financial support, carried out in-depth and systematic research on relativistic quantum chemistry, made great progress in theoretical methods, procedures and applications, and greatly affected all aspects of theoretical and computational chemistry.
Amsterdam density functional (ADF) software is a mature commercial software developed under such a historical background, which supports scalar and two-componentRelativistic quantum chemistrycalculation.Compared with other quantum chemistry software, it has unique advantages in dealing with transition metal systems and heavy element compounds.
The experimental research on the relativistic effect of heavy element system lags behind the theoretical research.In experiment, the research of relativistic effects mainly includes the methods of photoelectron spectroscopy (PES), nuclear magnetic resonance (NMR) and Compton scattering.Where PES is measuredPhotoelectron spectroscopyThe relativistic effect is studied by measuring the spin orbit splitting energy and the relationship between the branching ratio of the spin orbit splitting component and the photon energy, while the relativistic effect is studied by measuring the relative shielding constant, Compton profile, etc. for NMR and Compton scattering.
EMS has unique advantages in studying the electronic structure of atoms and molecules because it can obtain the radial electron density distribution in the space of discrete orbital momentum.Cook et al. were the first to apply EMS to the study of relativistic effects.[3]In 1984, they used electron momentum spectroscopy to study the effect of relativistic effects on the electronic wave function of atoms.
