And s character

In p-block elements, a central-atom lone pair is visualized as the limit of bonding to a ligand of lowest possible electronegativity, and thus acquires high s character. In d-block elements, a central-atom lone pair is of essentially pure d character (except for the special n(h) hybrid of ML2 coordination) and s character is essentially reserved for the covalent sigma skeletal bonds. 

In these structures the formation of a double covalent bond with one oxygen atom may be correlated with the ionic aspect of the bond to another atom, so that the bond orbitals used by the sulfur atom have the normal p nature (with only the usual small amount of d and s character—Chap. 4) come use may also be made of the 3d orbitals, In sp d or sp

[Pg.321]

The spin-spin coupling data indicate that the platinum s character tends to be concentrated in the bonds to the more polarizable phosphine ligands. Pidcock et al. (46) adopted the term "trans influence to describe the trans bond weakening effect observed with ligands such as phosphines. In the cis complex, the metal d and s character can be maximized in the bonds to the two phosphines, leaving the trans bonds with predominantly 6p character consistent with their ease of substitution and long bond lengths (56). 

The important fact is that the magnitude of Vab (e.g., between Ha and Hb in Figure 9.5) is (in addition to its dependence on y values and s-character effects) a function of the dihedral angle (6) between the nuclei. The Karplus equation (named for the discoverer of the relationship) for vicinal H-H coupling has the form... 

It may seem somewhat anomalous that the value of 37HH in the linear molecule acetylene (H-C=C-H) is only 9 Hz, because both the presence of the triple bond (cr + 2 n) and s-character effects might have led us to predict a larger value. The problem here is that the two C-H molecular orbitals are collinear (and face opposite directions), so there is no dihedral angle. 

Electron pairs involved in the n component of multiple (i.e double or triple) bonds behave somewhat differently from those in single (a) bonds (Section 7.4.1). Not only are there differences in geometry, hybridization, and s character (Section 9.3), but 7i-bond electrons interact with (delocalize into) neighboring a molecular orbitals in the molecule (hyperconjugation). Because of this delocalization, 71-bond electrons can communicate nuclear spin information over distances further than three bonds. Several examples of this type of long-range coupling are listed in Table 9.5. 

Fig. 12.12 JcH for hydrocarbons and s-character of C hybrid orbitals defined by the maximum overlap criterion (see also ref. 156).

Table 5.1 The NBO s-character, hybridization, and energy of all lone pairs (X=N, O, S) in selected saturated heterocycles at the B3LYP/6-31C level, the NBO plots of the lone pairs and s-character in C-X bonds. The axial and equatorial lone pairs are drawn as dissected by H -C3-Xi or H -C3-XI planes, respectively. If two lone pairs are present at an atom, data for equatorial lone pair are given in parenthesis.

The coefficients Z , etc. determine the direction and s-character of the hybrids. The optimal calculation of these parameters is a difficult task and several procedures are known in the literature for this purpose [221, 222]. For the present purposes it is sufficient to invoke the chemical intuition and start with a set of hybrids with standard s-characters and directed along the bonds. Since such hybrids are not orthogonal on each atomic center (except in the case of some special bond angles), a consecutive Lowdin-orthogonalization allows one to readjust their 5-characters and bond directions. 

In the same way. the ""Au Mfissbauer spectrum of CsAu is very similar to that of [Cs(C222) Au (Chapter 12). indicating that the. Au anion is present in both (Fig. Ifl.V). Since the M0s.sbauer effect is a nuclear one. it is very sensitive to electron density at the nucleus and ihcrcforc lo both atomic charge and s character. ... 

Basicity and s character, (a) In what kind of hybrid orbital do we find the unshared pair of electrons of nitrogen in each of the following compounds, given in order of decreasing basicity ... 

A mechanism assuming the existence of two groups of enantio-morphic sites having more or less R and S character was proposed in order to explain the formation of polymers of different... 

With our standard initiator at room temperature the percentage of crystalline isotactic fraction was 20 % for methyloxirane (25), (25), 35 % for methyl thiirane ( and practically 100 % for t-blTEyl thiirane (27). The isotactic fraction comes from sites of almost pure R and S character and therefore the distribution between different types of sites for one typical initiator is again depending on the nature of the monomer. 

In a first step the monomer reacts with the initiator to form a full spectrum of sites having different R and S character. Some of these formed species have a complete selectivity and produce crystalline isotactic polymers. The proportion of such selective species for a given initiator is depending on the nature of the monomer. We have seen that for monomers with bulky substituents like t-butyl thiirane almost all the sites are purely selective, while for other monomers like methyl oxirane only 20 % of the active species are selective. If the initiator is optically active there is an unbalanced amount of R type and S type species and therefore stereoelection will occur when polymerizing a racemic monomer mixture. 

Tsuruta (2) had proposed that the presence of chains of different tacticities is due to active sites of various stereospecificity. There is probably a full spectra of sites in the initiator with a more or less pronounced R and S character. The sites with pure R and S character produce isotactic fraction. This hypothesis which satisfactoraly explains most of the results was substantiated by optical resolution of racemic polymers. Thus, fractions of one sign were isolated from polymethyl-oxirane by preferential complexation (28) and fractions of both sign of very low optical activity were obtained using sucrose (29). 

Due to the often significant contributions from the noncontact terms and the variability of the contribution of the nonbonding electrons to the contact term, a simple empirical relationship, such as eq. 18, between coupling and s character... 

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