Octet™ system

In spin relaxation theory (see, e.g., Zweers and Brom[1977]) this quantity is equal to the correlation time of two-level Zeeman system (r,). The states A and E have total spins of protons f and 2, respectively. The diagram of Zeeman splitting of the lowest tunneling AE octet n = 0 is shown in fig. 51. Since the spin wavefunction belongs to the same symmetry group as that of the hindered rotation, the spin and rotational states are fully correlated, and the transitions observed in the NMR spectra Am = + 1 and Am = 2 include, aside from the Zeeman frequencies, sidebands shifted by A. The special technique of dipole-dipole driven low-field NMR in the time and frequency domain [Weitenkamp et al. 1983 Clough et al. 1985] has allowed one to detect these sidebands directly. 

Experimental evidence for the six electron systems has been described in Sect. 2.1.4. Skancke reproduced the relative stabihty of the cross conjugated systems relative to the linear isomers by calculating the trimethylenemethane and buta-l,4-diyl dianions [27] and their dilithio salts [28]. For the four electron systems butadiene is more stable than trimethylenemethane. Experimental examination of the relative stabihties of two electron systems using the trimethylenemethane and buta-14-diyl dications needs to overcome the intrinsic instabihties of dications dissatisfying the octet rule. 

What experimental evidence supports the idea that an octet in the valence level of atoms within a molecule often corresponds to a minimum total energy for the system ... 

Given that text/plain and application/octet-stream messages can carry information in arbitrary languages and that database languages exist, HTTP networks have the ability to support distributed information systems. This is very useful. 

The dipole is a triad of atoms which has a n system of four electrons and for which a dipolar resonance form provides an important component of its bonding description. The requirements have also been described as atom A must have a sextet of outer electrons and C has an octet with at least one unshared pair ([119], p. 743). The substrate is an olefin and an alkyne or a carbonyl. The reaction therefore is of n4 + n2 type like the related Diels-Alder reaction and has proved to be very useful synthetically for the construction of 5-membered ring heterocycles [268, 269]. Evidence suggests the reaction is concerted and regioselective. Dipoles fall into two general categories [119] ... 

Compound 129 was investigated in a general study of cyclic systems of alternating atoms A and B.142 The salient result was that the requirements for aromaticity in compounds in which the conjugated system is formed of pir- and tin-orbitals differ substantially from those for the more usual cyclic systems composed of atoms with onlypn-orbitals, e.g. sometimes the necessary condition for aromaticity is the presence of an octet rather than a sextet of conjugated electrons. Craig143 has... 

In comparison to carbanions, which maintain a full octet of valence electrons, carbenium ions are deficient by two electrons and are much less stable. Therefore, the controlled cationic polymerization requires specialized systems. The instability or high reactivity of the carbenium ions facilitates undesirable side reactions such as bimolecular chain transfer to monomer, /1-proton elimination, and carbenium ion rearrangement. All of that limits the control over the cationic polymerization. 

This synthesis of a variety of five-membered heterocycles involves the reaction of a neutral 4rr-electron-three-atom system, the dipole, with a 2TT-electron system, the dipolarophile (Scheme 2). Table 3 illustrates 1,3-dipoles with a double bond and with internal octet stabilization, the propargyl-allenyl anion type. 

Fig. 32 shows on the left a conventional, localized domain model of the electronic environment of an atom that satisfies the Octet Rule. Each domain is occupied by two electrons. It is well known, however, that the assumption of two electrons per orbital is unnecessarily restrictive 27>122>. Better energies are obtained in quantum mechanical calculations if different orbitals are used for electrons of different spins, a fact first demonstrated in quantitative calculations on helium by Hylleraas 123> and Eckart 124>. Later, this "split-orbital method was applied to 71-electron systems 27,125) Its general application to chemical systems has been developed by Linnett 126>. 

The influence and impact of these semi-empirical calculations and absolute reaction rate theory on the thinking of physical organic chemists was profound. It makes clear, for example, the electronic basis for some of Ingold s broad generalizations, e.g. In bimolecular eliminations, E2, in systems H—Cp—Ca—X, where X may be neutral or charged, the ]8-CH electrons, independently of the electrostatic situation, enter the Ca octet on the side remote from X, because repulsive energy between electron-pairs in the transition state can thus be minimized the result is anti-elimination, independently of the structural details of the system (Ingold, 1953). 

Carbon is known with all coordination numbers from 0 to 8. Some typical examples are given in Table 14.3.2, and their structures are shown in Fig. 14.3.3. In these examples, the compounds with the high coordination numbers, such as > 5, do not belong to the class of hypervalent compounds, but rather to electron-deficient systems. Hypervalent molecules usually have a central atom which requires the presence of more than an octet of electrons to form more than four 2c-2e bonds, such as the S atom in SF6. 

Holmium has a nucleus with 7 = 7/2 spin in a natural abundance of 100%. Each sublevel of the J = 8 ground multiplet is split into an octet by the hyperfine interaction between the (4/)10 system and the nucleus. All observed step positions are... 

A 1,3-dipole is a compound of the type a—Het—b that may undergo 1,3-dipolar cycloadditions with multiply bonded systems and can best be described with a zwitterionic all-octet Lewis structure. An unsaturated system that undergoes 1,3-dipolar cycloadditions with 1,3-dipoles is called dipolarophile. Alkenes, alkynes, and their diverse hetero derivatives may react as dipolarophiles. Since there is a considerable variety of 1,3-dipoles—Table 15.2 shows... 

As early as 1967, Huisgen and coworkers [37] had shown that, upon photolysis, certain aziridines of type 96 undergo C—C bond fragmentation stereospecifkally to produce octet-stabilized azomethine ylides which, on cycloaddition with electron-deficient dipolarophiles, produce pyrrolidine ring systems (Scheme 8.29). 

---