Oxygen bond enthalpy terms

Table 15.2 Some covalent bond enthalpy terms (kJmol ) for bonds involving oxygen, sulfur, selenium and tellurium.

The favourable intra-molecular hydrogen-bonding between the oxime proton and the phenol oxygen atom will reduce or overcome liganddigand repulsion enthalpy terms normally involved in the formation of bis-chelate complexes. The integrity of this head-to-tail macrocyclic assembly is preserved in c/.v-octahcdral complexes which are formed in the presence of a, co-diamines. [66, 67]... 

The decomposition of tri- and tetrasulfane in CCI4 solution (0.2 mol 1 ) at 70 °C and in the absence of oxygen has been studied by H NMR spectroscopy [64]. Initially, tetrasulfane decomposes to a mixture of tri- and pentasul-fane but slowly and after an induction period hydrogen sulfide and disulfane are formed in addition. These results have been interpreted in terms of a radical-chain reaction. The initial step is assumed to be the homolytic cleavage of the central SS bond which has by far the lowest dissociation enthalpy of the molecule ... 

There is also a relation between the amount of ionic character of a single bond and the enthalpy of formation of the bond. The amount of ionic character in percentage is roughly equal numerically to the heat of formation in kcaJ/mole. In applying this rule one must, of course, correct the heat of formation for the special stability of oxygen and nitrogen in their standard states, as expressed in Equation 3-13. This relation may be derived by expanding the exponential-function in Equation 3-15. The first term in the expansion, i(xA — zb)2, may be compared with the term 23 (xa — xb)2 of Equation 3-13. 

We note that the cyclopropanation of ketene to form cyclopropanone, isoelectronically and otherwise formally related to the cyclopropanation of allene to form methylenecyclopropane, has an altogether different enthalpy of reaction. This value is (63 4) kJmoT favoring ketene and is credible in terms of considerable resonance stabilization of the type CH2=C=0 CH2—C=0 and of a 3-atom n system that includes the double bond and the appropriate, antisymmetric (i.e. difference) combination of the oxygen lone pairs. Further documentation of the stabilization of ketene is seen by comparing the energetics of the hydrogenation reaction... 

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