Ten electron species

In this section, results of ab initio electronic structure calculations on endohedral complexes are discussed. The complexes studied include those with noble gases, ten-electron species, the lithium and sodium cations, and diatomic molecules. 

Table 7. The calculated properties of endohedral complexes of the Ceo cluster with the ten-electron species.

As in complexes with the noble gases, the orbital energies of the ten-electron species undergo shifts upon encagement. In the absence of the dipolar term, the magnitude of the shift can be deduced from the effective Hartree-Fock Hamiltonian, Eq.(25), to be approximately equal to... 

For comparison, let us also consider the ten-electron M(ligand)+ complexes for M+ = Au+ as a prototype one-pair-acceptor species. (For convenience, we refer to these species as formal Au+ complexes, but it should be understood that the actual bonding is far more covalent than the ionic label Au+ might seem to suggest.) Optimized structures for the AuL2+ complexes are shown in Fig. 4.87 for direct comparison with the Ir+ complexes in Fig. 4.86. Similarly, the energetic, geometrical, and NBO descriptors of these Au+ complexes are summarized in... 

A structural investigation of [Ta(=CH Bu)(CH2 Bu)3] grafted on siUca partially dehydroxylated at 700 °C using EXAFS [18] has revealed a short-range interaction of 2.64 between the O-atom from a siloxane bridge and tantalum. This O-atom acts as a two-electron donor ligand to stabiUze the formally ten-electron surface complex [(=SiO)Ta(=CH Bu)(CH2 Bu)2j, yielding the more stabiUzed twelve-electron species [(=Si0)Ta(=CH Bu)(CH2 Bu)2(=Si-0-Si j (Scheme 11.9). 

To analyze the bonding first remove the four BHB bridging H as protons to give [BgHg]4-. This species has a total of 32 AOs and 30 valence electrons. If we remove 16 orbitals and 16 electrons to form the eight two-center-two-electron B-H bonds, we are left with 16 orbitals and 14 electrons. Hence, two three-center-two-electron bonds (six orbitals and four electrons) plus five two-center-two-electron bonds (ten orbitals and ten electrons) nicely utilizes them all and is consistent with the fact that there are two four-connect vertices in the framework. 

MARA is an innovative bioassay devised for the evaluation of toxicity of chemicals and environmental samples. The assay utilizes a taxonomically diverse array of ten bacterial species (prokaryotes) and a yeast (eukaryote). The assay is performed in a 96 well micro titre plate and involves exposure of the microorganisms provided in a freeze-dried state. The toxicity of the test sample using a concentration gradient is determined with the employment of the redox dye tetrazolium red (TZR). The dye is transformed from a soluble colourless state to a red insoluble form upon reduction. The dye is a growth indicator and detects enzyme systems by acting as an electron acceptor. 

A trivalent carbon atom surrounded by six electrons, and bearing a positive charge (known as a carbocation, Figure 4.3) is relatively unstable. Such species have been observed spectroscopically and, in special cases can be isolated. However because the elements of the first row of the Periodic Table have small nuclei, they can only have eight valence electrons at most. Apentavalent carbon surrounded by ten electrons is thus extremely rare and we can discount the associative mechanism on chemical grounds — a concerted process is much more likely. As you will see in the CD-ROM, examination of the stereochemistry of the reactants and products of substitution confirms that the second-order substitutions proceed via the concerted mechanism. 

The species PF5, PFg, SFg are exceptions to the octet rule. In PF5 there are ten electrons in the valency group around the phosphorus atom in PF and SFg there are twelve electrons in the valency groups. The elements in the first row, on the other hand, are bound rigidly to the rule of eight. ... 

No experimental results on the PH ion have been published so far. The species has been the subject of a few quantum-chemical ab initio calculations. CASSCF and MRD Cl calculations of all ten electronic states correlating with the four lowest asymptotes, D, S)+ H " and p2+(2p ) + H(2S), reveal three quasi-bound and seven repulsive states. The quasi-bound states are the previously known [1 to 3] ground state X (KL (4a)2 (5a)2) correlating with the... 

Other elements show varying tendencies to obtain a filled shell by the sharing of electrons with other atoms, or by the actual gain or loss of electrons to form charged species, called ions. For example, sodium (symbol Na, atomic number 11) readily loses one electron to form the sodium ion, Na+, with ten electrons. By losing one... 

Some n.m.r. spectra for benzo- and dibenzo-cyclobutadiene dications indicate that these species are fully delocalised six- and ten-electron aromatic systems respectively. Flash-pyrolytic extrusion of nitrogen from benzocinnoline analogues has been employed in the generation of the fused cyclobutadiene derivatives (52), (53), and (54). 1,2-Dimethylbenzocyclobutadiene gives the... 

Dicarbocyanine and trie arbo cyanine laser dyes such as stmcture (1) (n = 2 and n = 3, X = oxygen) and stmcture (34) (n = 3) are photoexcited in ethanol solution to produce relatively long-Hved photoisomers (lO " -10 s), and the absorption spectra are shifted to longer wavelength by several tens of nanometers (41,42). In polar media like ethanol, the excited state relaxation times for trie arbo cyanine (34) (n = 3) are independent of the anion, but in less polar solvent (dichloroethane) significant dependence on the anion occurs (43). The carbocyanine from stmcture (34) (n = 1) exists as a tight ion pair with borate anions, represented RB(CgH5 )g, in benzene solution photoexcitation of this dye—anion pair yields a new, transient species, presumably due to intra-ion pair electron transfer from the borate to yield the neutral dye radical (ie, the reduced state of the dye) (44). 

The elemental composition of the fish otoliths is a potential source of the useful information to recreate environment history of the individual fish in some of the species. In-depth study of the chemical composition of the otolith center (formed eaidy in fish life) and otolith edge (formed later in fish life) ensures chronological and environmental information stored in the otoliths [1]. This infoiTnation may be achieved by X-ray electron probe microanalysis (EPMA). EPMA is the analytical method to determine the elemental composition of different otolith s parts, their sizes varying from ten up to some tens of microns. 

Perhaps the most notable difference between S-N and N-O compounds is the existence of a wide range of cyclic compounds for the former. As indicated by the examples illustrated below, these range from four- to ten-membered ring systems and include cations and anions as well as neutral systems (1.14-1.18) (Sections 5.2-5.4). Interestingly, the most stable systems conform to the well known Htickel (4n -1- 2) r-electron rule. By using a simple electron-counting procedure (each S atom contributes two electrons and each N atom provides one electron to the r-system in these planar rings) it can be seen that stable entities include species with n = 1, 2 and 3. 

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