Homonuclear, definition

Homonuclear 19F-19F experiments are the most commonly carried out, and they are also the most easily implemented on conventional NMR spectrometers. Among such experiments, 19F COSY correlation spectroscopy is probably the 2D 19F NMR technique most frequently encountered, mainly because of through space couplings that can make it otherwise difficult to infer definitive structural information from the presence or magnitude of observed correlations. It has been found to be particularly useful in the analysis of fluoropolymers. 

Herein we describe the synthesis and characterization of small-size homonuclear gold cluster compounds with a nuclearity ranging from 3 to 39 gold atoms. These types of dusters are usually characterized through definitive structural techniques such as single-crystal X-ray diffraction, enabling in-depth study of the structural arrangements found in the solid state. 

For homonuclear diatomic molecules, the electronic wave functions have definite parity (g or w), and since del is of odd parity, we must have a change in parity of f/el (corresponding to the Laporte rule in atoms) ... 

The unique ligating behavior of the bridging 2,6-dimethoxyphcnyl ligand with respect to promoting a substantial decrease in the metal atom separation for molybdenum(II) dimers is even more prominent in the case of chromium. The chromium-chromium distance of 1.847(1) A in Cr2(DMP)4 (90) is more than 0.1 A less than the corresponding value in any other chromous dimer yet reported. To compare homonuclear multiple bonds among elements with inherently different atomic radii, Cotton, Koch, and Millar proposed a normalized value for intemuclear distances based on Pauling s atomic radius of the element in question (209). A simple definition of formal shortness as t/(M—M)/2r(M) then follows as a measure of the relative compactness of the attractive interaction (90). The formal shortness ratio of 0.778 for the quadruple bond in... 

Single-crystal X-ray crystallography remains the only definitive technique for the structural characterization of heteronuclear gold cluster compounds, although other techniques, in particular Mossbauer, NMR, IR, and fast atom bombardment mass spectroscopies (FABMS), have yielded valuable information, especially concerning the nature of these species in solution. Electron spectroscopy, which has proved to be of great value in the identification of homonuclear gold cluster compounds (210) has received little attention by workers in this area,... 

Good examples are the core hole excited states of homonuclear molecules. When one electron is removed from a core orbital, the original Dooh symmetry is lowered to C v The D h group can be decomposed into two CooV components related by a C, or Cs operation, so it is fair to consider that the core-hole excited states are described by resonance between the two structures. The adiabatic subsystems have, by definition, zero overlap in the real space. Their interaction is defined only in complex space through the explicit overlap between the many-electron states. 

Klug and Schaefer105 extended CEDRA (Controlled-SEDRA, Simple Excitation for the Dephasing of Rotation-Echo Amplitude), which was used for heteronuclear systems, to homonuclear systems. This is named T-CEDRA. Distance can be measured but no definite information about the orientations can be obtained with T-CEDRA. 

The term Zintl phase is applied to solids formed between either an alkali- or alkaline-earth metal and a main group p-block element from group 14, 15, or 16 in the periodic table. These phases are characterized by a network of homonuclear or heteronuclear polyatomic clusters (the Zintl ions), which carry a net negative charge, and that are neutralized by cations. Broader definitions of the Zintl phase are sometimes used. Group 13 elements have been included with the Zintl anions and an electropositive rare-earth element or transition element with a filled d shell (e.g. Cu) or empty d shell (e.g. Ti) has replaced the alkali- or alkaline-earth element in some reports. Although the bonding between the Zintl ions and the cations in the Zintl phases is markedly polar, by our earlier definition those compounds formed between the alkali- or alkaline-earth metals with the heavier anions (i.e. Sn, Pb, Bi) can be considered intermetallic phases. 

Figure 2 Examples of global and local axis systems, (a) Molecular axis system for a homonuclear diatomic. Wth this system, all central multipoles with k 0 otl odd will be zero, and no S functions with k 0 oxl odd can appear in a molecule-molecule expansion of U(R, Q). The atomic multipoles Q q (all / 0 allowed) on the two atoms will be related by Qio = (-l) Qio- ( ) Local atomic axis system for a homonuclear diatomic molecule. With this definition QJq = Qio- (c) Molecular axis system for water. The nonzero atomic multipole moments for the O atom would be QoO> QlO) QzOj Qz2c = (Q22 + Qz-2)1 > QsOJ Q32c = (Qs2 + on the hydrogen atoms Qjo = Qoo. Qio = Qio> Qiic = (-Q11 + Qi-i) = -Qiic...

There are several definitions and values assigned to covalent radii. Pauling created an early scale using bond lengths in simple homonuclear compounds as the starting point. An extended version of this scale is listed as covalent in Table 1.5. A related, but more comprehensive, approach is to examine structural data to determine covalent radii that best correlate with observed bond distances. This approach was developed by Slater. An extensive tabulation of bond lengths derived from structural data was published in 1987. These values are labeled stmctural in Table 1.5. A set of... 

Inspection of Tables IV and V, in which are presented known heteronuclear associative ionization reactions, leads to the conclusion that usually the metastable levels of the reactant with the higher ionization potential are involved, a characteristic not encountered in the homonuclear reactions. However, this need not be true, as exemplified by the existence of reaction (28), in which metastable levels are definitely not involved. A particularly interesting case is the formation of HeAr. Munson et reported that the onset potential of this ion is 17.9 eV and that the ionization-efficiency curve of HeAr shows a sharp upward break at about 19.9 eV corresponding to the expected process... 

The coverage of group 2 homobimetallics that is given here is largely restricted to the chemistry of isolable molecular compounds with metal-metal bonds. With that said, a brief summary of metal- metal bonded species that are only fleetingly stable in the gas phase, or at very low temperatures, is warranted and is addressed first. It should be noted that systems containing homonuclear group 2 metal-metal bonds are by definition low oxidation state compounds. A number of excellent recent reviews have covered the known chemistry in this area [70-75], and the reader should consult these for more detailed information. 

Homonuclear diatomic molecules do not absorb or emit radiation due to vibrational state transitions. By definition, homonuclear diatomic molecules have no dipole moment and no changing dipole moment as the two atoms vibrate. Other methods are necessary to observe vibrational energy levels directly. 

These combinations therefore give pure bonding and antibonding diatomic molecular spinors, which also have pure spin. For homonuclear diatomic molecules, the spinors have a definite inversion symmetry a = ofg, a = a , ti = 7i , n = Ttg, and so on. The Kramers partners of these spinors are found by interchanging the spin parts. The first one is... 

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