Multiple bond, contributions

TABLE 22.5. Multiple Bond Contributions Replacing Single Bonds... 

In the NDDO Hamiltonian the exchange interaction between electrons of two gemi-nals involved in one multiple bond contributes to the electronic energy ... 

The paramagnetic term depends on the number of electrons in the 2p orbital and multiple bond contributions as well as the average excitation energy A . Therefore, this term will have a significant deshielding effect on n systems. For further discussion the reader is referred to Pople s reference [63]. 

Partial Wiener indices are other multiple bond descriptors derived by a splitting of the Wiener index into different multiple bond contributions. 

In SiF4 n bonding, using F 2p orbitals and Si 3d orbitals, could contribute to the shortening. It is probable that both ionic-covalent resonance and multiple bonding contribute significantly. 

The thermodynamic bond strength of the B—N framework in borazines is, in part, a consequence of multiple-bond contributions from the unshared pair of electrons on nitrogen with the vacant p-orbital of boron, as represented by the resonance forms ... 

Formation and Elimination of Multiple Bond Functionalities. Reactions that involve the formation and elimination of multiple bond functional groups may significantly effect the color of residual lignin in bleached and unbleached pulps. The ethylenic and carbonyl groups conjugated with phenoHc or quinoid stmctures are possible components of chromophore or leucochromophore systems that contribute to the color of lignin. 

Each atom makes a characteristic contribution, called its covalent radius, to the length of a bond (Fig. 2.21). A bond length is approximately the sum of the covalent radii of the two atoms (36). The O—H bond length in ethanol, for example, is the sum of the covalent radii of H and O, 37 + 74 pm = 111 pm. We also see from Fig. 2.21 that the covalent radius of an atom taking part in a multiple bond is smaller than that for a single bond of the same atom. 

Double bonds in conjugation with the carbon-hetero multiple bond also lower addition rates, for similar reasons but, more important, may provide competition from 1,4 addition (p. 977). Steric factors are also quite important and contribute to the decreased reactivity of ketones compared with aldehydes. Highly hindered ketones like hexamethylacetone and dineopentyl ketone either do not undergo many of these reactions or require extreme conditions. 

A range of symbolic conventions is used in representing atomic and molecular stractures at the electronic level. So for example double and triple lines are used for multiple bonds. This seems a clear convention, which helps keep check of valency rales. However the symbol = for a double bond is not intended to imply two equal bonds (which the symmetry of the symbol could seem to suggest) as u and TT components have different geometries, contributions to bond strength , and consequences for chemical properties. The novice learner may well find interpreting such representations a considerable challenge. 

The fourth chapter gives a comprehensive review about catalyzed hydroamina-tions of carbon carbon multiple bond systems from the beginning of this century to the state-of-the-art today. As was mentioned above, the direct - and whenever possible stereoselective - addition of amines to unsaturated hydrocarbons is one of the shortest routes to produce (chiral) amines. Provided that a catalyst of sufficient activity and stabihty can be found, this heterofunctionalization reaction could compete with classical substitution chemistry and is of high industrial interest. As the authors J. J. Bmnet and D. Neibecker show in their contribution, almost any transition metal salt has been subjected to this reaction and numerous reaction conditions were tested. However, although considerable progress has been made and enantios-electivites of 95% could be reached, all catalytic systems known to date suffer from low activity (TOP < 500 h ) or/and low stability. The most effective systems are represented by some iridium phosphine or cyclopentadienyl samarium complexes. 

This structure is isoelectronic with (Si03)36 and (P03)33-. Gaseous S03 has a trigonal planar structure that has several contributing resonance structures. When the structure is drawn with only one double bond, the sulfur atom has a +2 formal charge that is relieved by stmctures having two double bonds. Therefore, multiple bonding is extensive. 

We started by generating a data base of inner-shell correlation contributions for some 130 molecules that cover the first two rows of the periodic table. In order to reduce the number of parameters in the model to be fitted, we introduced a Mulliken-type approximation for the parameters Dab (Da+Db)/2. Furthermore we did retain different parameters for single and multiple bonds, but assumed Da=b (3/2)Da=b-... 

Classical shielding arguments indicate an electron-rich phosphorus atom, or equally, an increase in coordination number. The silicon atom seems also to be electron-rich, while the carbon has a chemical shift in the range expected for a multiply bonded species. The coupling constant data are difficult to rationalize, as it is not possible to predict the influence of orbital, spin-dipolar, Fermi contact, or higher-order quantum mechanical contributions to the magnitude of the coupling constants. However, classical interpretation of the NMR data indicates that the (phosphino)(silyl)carbenes have a P-C multiple bond character. 

The effect of introducing multiple bonds in a molecule is treated separately. The appropriate corrections have been assembled in Table 22.5 and require no special comments, except perhaps to emphasize the additional contribution that must be introduced every time a pair of conjugated double bonds is formed by any of the preceding substimtions in this table. 

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