Geometrical structures diatomic molecules

The H-atom chain is to solid-state structures as the diatomic molecule is to polyatomic molecules, e.g., clusters. The geometric-structure problems for H2 and Hoc are so simple that one can focus on the electronic structure problem exclusively. However, real solid-state structures, e.g., a solid with linked clusters or even bulk elemental Al, are not found in the form of a linear chain, square sheet or simple cubic structures so we need a way to treat solids with more complex structures, i.e., define a repeat unit that is more than a single atom. 

The dihydrogen molecule (s1 + s1) is formed through the interaction of s-electrons only. There is no angular momentum to impart structure to the diatomic molecule, which hence cannot be described in classical geometrical terms other than a spherically symmetrical distribution of electron and proton densities. It has no shape, no bond and, unless it interacts with external fields, no geometrical features. Compounds such as LiH, Li2, etc. belong to the same class of amorphous molecules. 

Although our discussion in the next seven chapters will center on simple tetrahedral structures, in which all electrons form simple two-electron bonds, it is desirable to introduce two other types of structures in these, some of the electrons form two-electron bonds (and are understandable in the same terms used for the tetrahedral solids) and other electrons are accommodated in pure p slates, similar to the 71 states discussed in Chapter 1 for diatomic molecules. For a discussion of the stability of these structures, see Friedel (1978). The two-electron bonds are simpler geometrically in these systems than in the tetrahedral solids and will provide very good problems. (Many of the problems at the ends of the following chapters will explore the theory of the bonds for these simpler systems.)... 

Vaara and Pyykko presented a theory for the magnetic-field-dependent quadrupole splitting in the Xe NMR spectra in isotropic media and tested it by ab initio electronic structure calculations. Evidence exists only for even-power magnetic field dependence. The dominant mechanism is verified to be the electric field gradient caused by the diamagnetic distortion of the atomic electron cloud, quadratic in the magnetic field. NQCC for diatomic molecules were calculated by Bryce and Wasylishen. Turner et al performed a systematic computational study of the geometrical dependence of the deuteron quadrupole interaction parameters (DQCC and asymmetry parameter) for the water-formaldehyde model system. Bematowicz and Szymanski studied NMR spectra of a spin nucleus scalar coupled to two equivalent spin-1 nuclei... 

Geometrical Structure of Simple Radicals 2.1 Diatomic Molecules... 

This volume contains data on the geometric parameters (intemuclear distances, bond angles, dihedral angles of internal rotation etc.) of free polyatomic molecules including free radicals and molecular ions. (For the diatomic stractures measured by high-resolution spectroscopy, see [1], and for molecular stmctures in crystals, see [2]. For the structures of free molecules published before 1960, see [3].)... 

The range of problems to which electronic structure theory is applied includes potential energy surfaces for chemical dynamics studies and calculations in a small geometrical region for spectroscopic comparisons. Potential energy surface problems involve some level of symmetry for diatomic and triatomic cases and may involve very little for larger cases. Spectroscopic problems frequently involve symmetry in an essential way, but may not, depending on the molecule of interest. 

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