Orbital stabilization

Fig. 1.29. Interactions between two hydrogen Is orbitals and carbon 2p orbitals stabilize the eclipsed confonnation of propene.

The first experiments with the thermal electric engine were conducted in Russia in 1929 by its inventor, Valentin P. Glushko, who later became a world-famous authority in rocket propulsion. For more than forty years, the United States and Russia have devoted many resources to research and development of various kinds of EREs. First tested in space by the Russians in 1964, these engines have found some limited applications in modern space technology. For more than two decades Russian weather and communication satellites have regularly used electric rocket engines for orbital stabilization. The first spacecraft to employ ERE for main propulsion was the American asteroid exploration probe Deep Space 1, launched in 1998. The performance of... 

Figure 6.12 Stabilization of the ethyl carbocation, CH3CH2+, through hyperconjugation. Interaction of neighboring C H

If C is orbitally stable and, in addition, the distance between B and C tends to zero as t - oo, this form of stability is called asymptotic orbital stability. 

Table 1.3 contains values for two 3d cases. At KsCuFg, ATreplirreg) contributes about 40% of the total stabilization, but at Ni (aq) only 15%. This is because in the first transition series, the nephelauxetic effect increases substantially when the oxidation state increases from -i-2 to -e3. The relatively small contribution for the M (aq) ion explains why text books use this example to explain the double bowl shapes AErep(irreg) is almost exactly cancelled by the sum of AEso and ATrix, so the total stabilization is nearly equal to the orbital stabilization energy. In most other cases, A rep(irreg) is much more important and may play an important role in sustaining the Irving-WilUams rule in complex-ing reactions [32, 33]. 

It is striking that, despite its small size, the tetrad effect was discovered before the diad effect. This is because the diad effect occurs in d-electron systems and is therefore masked by the orbital stabilization energies produced by the stronger ligand field. 

The variations in orbital stability with n and I ensure that 1 S fills before 2 S and 2 S fills before 2 p. After 2, the next orbitals to fill are 3 S and 3 p, but then what Both 3 d and 4 S are less stable than 3, so... 

Accessible electrons are called valence electrons, and inaccessible electrons are called core electrons. Valence electrons participate in chemical reactions, but core electrons do not. Orbital size increases and orbital stability decreases as the principal quantum number n gets larger. Therefore, the valence electrons for most atoms are the ones in orbitals with the largest value of ti. Electrons in orbitals with lower tl values are core electrons. In chlorine, valence electrons have ft = 3, and core electrons have — 1 and — 2. In iodine, valence electrons have a = 5, and all others are core electrons. 

The minimum amount of energy needed to remove an electron from a neutral atom is the first ionization energy ij E ). Variations in ionization energy mirror variations in orbital stability, because an electron in a less stable orbital is easier to remove than one in a more stable orbital. 

The aufbau principle must be obeyed when an electron is added to a neutral atom, so the electron goes into the most stable orbital available. Hence, we expect trends in electron affinity to parallel trends in orbital stability. However, electron-electron repulsion and screening are more important for negative ions than for neutral atoms, so there is no clear trend in electron affinities as ft increases. Thus, there is only one general pattern ... 

Several studies have appeared showing that dimeric carbon-bridged dimers of gallium and of indium exist in solution these studies have provided the basis for the bonding model on which p-orbital stabilization was based since these species all contain vinyl or ethynyl bridging groups of unusual stability (91, 117). 

Additionally, Figure 6.4 shows the variation in the energy of the occupied 5d orbitals when the P-Au-P angle varies from 180° (linear coordination) to 120°. Thus, while the dzi orbital stabilizes slightly, the dxz destabilizes as a consequence of the interaction of this orbital with the 3px and 3pz orbitals of phosphorus. The main consequence is that, below 168°, the former HOMO (dz2 in linear molecules) is replaced by the dxz orbital, which displays a higher energy and, consequently, a lower energy is needed to reach the excited state. 

When an attempt is made to combine helium atoms, the pair of electrons assigned to the sigma orbital stabilizes the molecule, but the pair of electrons in the antibonding orbital increases the molecular energy to an extent that prevents molecular formation. 

Crystal field stabilization energy, CFSE. Each electron in a t2g orbital stabilizes a transition metal ion in octahedral coordination by 0.4Ao, whereas every electron in an eg orbital destabilizes it by 0.6Ao. The crystal field stabilization energy, CFSE, represents the algebraic sum of these factors. Cations may have... 

The [3 + 2] reaction of an allylsilane with an enone was proposed to proceed via a regiospecific electrophilic substitution (cf 80) of the enone at C-3 of the allylsilane followed by a cationic 1,2-silyl migration (equation 57). At the yyw-clinal transition state (81), the carbonyl group of the unsaturated ketone was assumed to occupy an endo orientation in relation to the allylsilane142. The transition state was thought to be favorable due to minimization of the charge separation and to possible secondary orbital stabilization. 

If two octahedral-site cations share a common face, as in the corundum or NiAs structures, the cation--cation interactions may be particularly important since the cation separations are relatively small and the Ug orbital stabilized by the resulting trigonal field is directed through the common face ( o of equation 68), as shown in Figure 43(b). 

As you will probably realize, it s not only in six-membered rings that stereoelectronic interactions between filled and unfilled orbitals stabilize some conformations more than others. Stereoelectronic effects control the conformations of many types of molecules. We shall look at three common compounds that.are stabilized by stereoelectronic effects in two cases, the stabilization is specific to one conformation, and we can use stereoele ronics to explain what would otherwise be an unexpected result. 

The carbon-silicon bond has two important effects on the adjacent alkenc. The presence of a high-energy filled CT orbital of the correct symmetry to interact with the n system produces an alkene that is more reactive with electrophiles, due to the higher-energy HOMO, and the same ff orbital stabilizes the carbocation if attack occurs at the remote end of the alkene. This lowers the transition state for electrophilic addition and makes allyl silanes much more reactive than isolated alkenes. 

Fig. 7. Frontier orbital stabilization in the bending of a main group metallocene.

The p orbital stabilizing interaction appears to be quite common. It also lowers the activation energy for the O and CO recombination reaction to form linear CO2. On a surface that interacts strongly with CO, e.g. a transition metal with partially filled d valence electron orbitals, the reaction proceeds as sketched in Fig. 4.47. 

As already indicated in the discussion on formyl formation, the transition state on a metal surface may be closely related to that found in the organometallic complexes used in homogeneous catalysis. The p orbital stabilizing interactions have also been shown to play an important role in insertion reactions occurring in organic-metallic complexes [84,85]. It explains, for instance, the higher activa-... 

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