Orbital effect

FIGURE 8. Hyperconjugation between the lone pairs at O and the t x-o and t y-o orbitals  

TABLE 3. Geometric parameters of substituted peroxides X—O—O—Y (electron diffraction) 

FIGURE 9. The effect of jr-type interactions on C—O and 0—0 distances in alkynyl, vinyl and phenyl hydroperoxide 14-17.  

A common structural motif in peroxides of the type R3C—O —0°—Y (R = alkyl, aryl, fluorine, Y = R3C or H) is a marked distortion of the R3C fragment from tetrahedral geometry. The R—C bond, which is located in the C—O —plane, exhibits the smallest of the three R—C—O angles (Table 4) °. This tetrahedral distortion has been interpreted, on the basis of an electron population analysis, with a ctr c r o-o and/or cro-o a R c hyperconjugation. 

Acetyl peroxynitrate (18) and perfluoroacetyl peroxynitrate (19), two important atmospheric oxidation products of hydrocarbons (formation of 18) or chlorofluorocarbon replacements, such as CF3CH3 (formation of 19), preferentially adopt a gauche conformation (C—O—O—N = 84.7° for 18 and 85.8° for 19 electron diffraction). The two peroxides are characterized by comparatively short 0—0 bonds on one side and long 0°—N connectivities (Table 5) on the other. The observed O —N distances may be explained on the basis of an no ct od-n orbital overlap. This type of interaction lowers the 0°—N bond order and could explain the low bond dissociation energies of this connectivity in peroxides 18 and 19 (118 4 klmol for both compounds). It should be noted that this interpretation does not reflect a possible r-type interaction between a lone pair at 0° and virtual orbitals of the nitro group and therefore requires future investigation. 

It is useful to think of this situation as a donor-acceptor interaction. The high-lying filled orbital donates electrons to the low-lying empty orbital, producing a stabilizing interaction. However, this interaction is not electron transfer nor the kind of donor-acceptor interactions often discussed in excited-state phenomena (Section 3.2.4). It is simply orbital 

Within this framework, then, it is useful to classify the donor and acceptor capabilities of certain kinds of groups. Useful sequences are shown in the margin. 

The trends are fairly standard. Lone pairs are better donors than bonding pairs because they are at higher energy. Amongst lone pairs two effects dominate. First, donor ability increases as electronegativity decreases and second, donor ability increases as you move down a column of the periodic table. These trends are consistent with the bonding models we developed in Chapter 1. 

Preferred geometry for the interaction of a donor (shown as a lone pair) with an acceptor r orbital. The a orbital is modeled after the LUMO of CH3CI, shown at the bottom. 

We begin with a simple system that very nicely illustrates the key principles. Consider (fluoromethyl)amine, FCH2NH2. As shown in Eq. 2.35, this system is perfectly set up for a donor-acceptor interaction. The preferred conformation puts the nitrogen lone pair (donor) anti to the C-F bond, optimizing the donor-acceptor interaction. This is really an optimal case, and the conformational preference is substantial. 

TABLE 2. Geometric parameters of selected peroxides R1 —O—O—R2 (H5C6)3COOH (H5C6)3COOC(C6H5)3 

TABLE 4. Tetrahedral distortion in the peroxide bound alkyl substituent9 

If we compare the p/ Q values of alkanes (typically 50), alkenes (typically 40-45), and alkynes (typically 25), we can attribute the differences to the greater stability of the lone pair in an 

TABLE 8.1 pK Values (in Water) for Compounds Important in Organic Chemistry Where the Proton Is Attached to a Heteroatom 

Acid Conjugate Base PK. Acid Conjugate Base PK. 

CF3SO3H [CF3SO3]- -14 [CH3C(=0H)0CH3]+ CH3C(=0)0CH3 -6.5 

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Electronic Hamiltonia for Atoms Without Spin-Orbit Effects... 

During the cationic homopolymerization, orbital effects as well as charge effects are essential in contrast to the EDA complex formation where apparently orbital effects dominate. The polymerizations are also aided by appearence of negative partial charges at the p-C-atom. 

Although there have been many experimental and theoretical studies on the behavior of facially perturbed dienes (see below), only a few systematic experiments have been carried out to characterize facially perturbed dienophiles. Dienophiles embedded in the norbomane or norbomene motif have been rather intensively studied [146-150]. In most cases, steric effect controls selectivity, but in some cases the reactions are considered to be free from steric bias, and the selectivity has been explained in terms of other factors, such as orbital effects [151, 152]. 

CCSD(T) calculations predict a relativistic increase in the dissociation energy of 78kJmol [155]. In comparison, electron correlation increases the dissociation energy by 134kJ mol . Spin-orbit effects increase the dissociation energy further by about 3kJmol [158]. Thus we predict a ArDe value of about SOkJmol. A comparison of calculated force constants reveals a similar picture, see Ref. [131]. 

Rusakov, A.A., Rykova, E., Scuseria, G.E. and Zaitsevskii, A. (2007) Importance of spin-orbit effects on the isomerism profile of AU3 An ah initio study. Journal of Chemical Physics, 127, 164322-1-164322-5. 

An alternative interpretation is that the carbonyl group rr-antibonding orbital, which acts as the LUMO in the reaction, has a greater density on the axial face.118 At the present time the importance of such orbital effects is not entirely clear. Most of the stereoselectivities that have been reported can be reconciled with torsional and steric effects being dominant.119... 

The spin-orbit interaction is also called spin-orbit effect or spin-orbit coupling, which is one cause of magnetocrystalline anisotropy. SOC, the intrinsic interaction between a particle spin and its motion, is responsible for various important phenomena, ranging from atomic fine structure to topological condensed matter physics. SOC plays a major role in many important condensed matter phenomena and applications, including spin and anomalous Hall effects, topological insulators, spintronics, spin quantum computation, and so on. 

Wan and coworkers48 studied the magnetic moments of Ni clusters, taking into account both spin and orbital effects. Wan et al.48 used the following TB Hamiltonian... 

Figure 1. The relation between central density and the mass of various degenerate star models. Chandrasekhar s curve is for white dwarfs with a mean molecular weight 2 of atomic mass units. Rudkjobing s curve is the same except for inclusion of the relativistic spin-orbit effects Rudkjobing (1952). The curve labeled Oppenheimer and Volkoff is for a set of neutron star models. The solid line marked Wheeler is a set of models computed with a generalized equation of state, from Cameron (1959).

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