Dative

Because it is desirable to break a peptide this way, some flexibility is required in the rigorous definition of sp -sp single bond. In particular, the dative Cqj-N bond in the backbone of a peptide is considered to be such a bond since the definition is based on the number of neighbors — four for carbon, three for nitrogen, two for Oxygen, etc. If this were not the case, you couldn t break a protein into classical and quantum regions at all. 

This reaction is catalyzed by iron, and extensive research, including surface science experiments, has led to an understanding of many of the details (72). The adsorption of H2 on iron is fast, and the adsorption of N2 is slow and characterized by a substantial activation energy. N2 and H2 are both dis so datively adsorbed. Adsorption of N2 leads to reconstmction of the iron surface and formation of stmctures called iron nitrides that have depths of several atomic layers with compositions of approximately Fe N. There is a bulk compound Fe N, but it is thermodynamically unstable when the surface stmcture is stable. Adsorbed species such as the intermediates NH and NH2 have been identified spectroscopically. 

Radicals are particularly strongly stabilized when both an electron-attracting and an electron-donating substituent are present at the radical site. This has been called mero-stabilization" or " capto-dative stabilization. This type of stabilization results from mutual reinforcement of the two substituent effects. Scheme 12.3 gives some information on the stability of this type of radical. 

A comparison of the rotational barriers in allylic radicals A-D provides evidence for the stabilizing effect of the capto-dative combination ... 

The capto-dative effect has also been demonstrated by studying the bond dissociation process in a series of 1,5-dienes substituted at C-1, C-3, C-4, and C-6. 

When the combination X,Yor X, Y is of the capto-dative type, as is the case for an alkoxy and an ester group, the enthalpy of bond dissociation is 10-15 kcal lower than when all four groups are electron-attracting. When the capto-dative combination CN/NR2 occupies both the X, Y and the X, Y positions, the enthalpy for dissociation of the C(3)—C(4) bond is less than lOkcal/mol. ... 

Scheme 12.3. Radicals with Capto-dative Stabilization...

The reaction rates and product yields of [2+2] cycloadditions are expectedly enhanced by electronic factors that favor radical formation. Olefins with geminal capto-dative substituents are especially efficient partners (equations 33 and 34) because of the synergistic effect of the electron acceptor (capto) with the electron donor (dative) substituents on radical stability [95]... 

There has been much discussion of the relative contributions of the no-bond and dative structures to the strength of the CT complex. For most CT complexes, even those exhibiting intense CT absorption bands, the dative contribution to the complex stability appears to be minor, and the interaction forces are predominantly the noncovalent ones. However, the readily observed absorption effect is an indication of the CT phenomenon. It should be noted, however, that electronic absorption shifts are possible, even likely, consequences of intermolecular interaetions of any type, and their characterization as CT bands must be based on the nature of the spectrum and the structures of the interaetants. This subject is dealt with in books on CT complexes. ... 

It should be noted that dative bonds, like metal complexes and charge transfer species, in general have RHF wave functions which dissociate correctly, and the equilibrium bond lengths in these cases are normally too long. 

The HF wave funetion eontains equal amounts of ionie and eovalent eontributions (Section 4.3), For covalently bonded systems, like H2O, the HF wave funetion is too ionie, and the effect of electron correlation is to increase the covalent contribution. Since the ionic dissociation limit is higher in energy than the covalent, the effect is that the equiUbrium bond length increases when correlation methods are used. For dative bonds, such as metal-ligand compounds, the situation is reversed. In this case the HF wave function dissociates correctly, and bond lengths are normally too long. Inclusion of... 

The other cases are either the same as the corresponding nominative or formed from it, as follows genitive singular, -en, -ens. -es, -n, -ns, or -s dative and accusative singular and dative plural, -n or -en. 

However, upon terminating chains with thiols, sulphur centered low-molecular weight radicals are formed that are able to start a polymerization of the remaining monomer B. Therefore, formation of homopolymer consisting of B is inevitable if thiols are used. A suitable alternative to the classical transfer additives are degra-dative chain transfer agents, such as allylmalonic acid... 

Polymerization in the presence of capto-dative substituted monomers has been... 

With eight outer electrons in the formally neutral atom, iron needs to acquire an electron to achieve covalence. Often the ninth electron is obtained from a carbonyl group forming a dative single covalent bond, Fe —CO+. The Fe(CO)3 group then involves five of the nine bonds formed by the iron atom, the bond number of the iron-carbonyl bonds being 1.67, and the iron atom forms four other bonds. If the added electron... 

With one dative bond from carbonyl and n = 1.67, the average bond angle for Fe(CO)3 is predicted from Eq. 1 to have the value 94.5°. Most of the numerous experimental values lie within about 2° of this value, and their average, 95.6°, with mean deviation 1.2° (Table 2), is in acceptable agreement with it. 

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