Group bond enthalpies, 496

At the top of File Segment 5-1 is a heat of fomiation information block. Two sums are listed One is a sum of nomial bond enthalpies for ethylene, and the other is a sum selected from a parameter set of stiainless bonds. Both sets of bond enthalpies have been empirically chosen. A group of molecules selected as nomial generates one parameter set, and a group supposed to be strainless is selected to generate a second set of str ainless bond enthalpies designated SBE in Eile Segment 5-1. The subject of parameterization has been treated in detail in Chapter 4. See Computer Projects 3-6 and 3-7 for the specific problem of bond enthalpies. 

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. ... 

NMR line broadening is a suitable kinetic method for determining activation parameters for Co—C bond homolysis, and gave A//- values in the range 18-22.5 keal mol for a selection of Co(Por)R complexes containing secondary or tertiary alkyl groups.Bond dissociation enthalpies and entropies for several... 

Admittedly, PI13C—CPI13 (literally, hexaphenylethane as drawn) has largely uncharacterized features as well. For example, the considerable weakness of the central C—C bond is not paralleled by the central C—C bond in tetraphenylmethane and 1,1,1,2-tetraphenylethane, the sole thermo-chemically characterized species in which there is a C—(Cb)3 (C ) structural group. [The enthalpy of formation of the latter species is from H.-D. Beckhaus, B. Dogan, J. Schaetzer, S. Hellmann and C. Riichardt, Chem. Ber., 133, 137 (1990).]... 

The group 14 M—M bond enthalpy may also be derived as one-half the enthalpy of the atomization process6... 

On the assumption that the bond enthalpies M M and M C0 increase systematically down the triad Fe, Ru, and Os, it may be taken that the tendency to form M(CO)s remains more or less constant. Part of the kinetic stability will then be due to the availability of a bridged form of Os3(CO)12. This does not necessarily mean that the bridged form has to be of the same structure as Fe3(CO)12, but merely one in which there are five carbonyl groups in association with the osmium atom. Thus, an alternative might be... 

These data for D (M-L) offer some basis for making predictions about the enthalpies of metal-carbon bonds involving other metals in these groups. It is important to bear in mind that all of the data in Table 6 concern the metals in their highest formal oxidation state. It is usually true that the mean bond enthalpy increases as the formal oxidation state of the metal decreases. This is exemplified by the values of >(M-Cl) (M = Nb, Ta, Mo, W) in various oxidation states of M (Table 7a), and... 

Since the size of halogen atom Increases as we go down the group in the periodic table, fluorine atom is the smallest and iodine atom, the largest. Consequently the carbon-halogen bond length also Increases from C—to C— I. Some typical bond lengths, bond enthalpies and dipole moments are given in Table 10.2. 

A simple method for estimating enthalpies of vaporization is the CHLP protocol . The quantities and req in equation 1 refer respectively to the number of quaternary carbon atoms and non-quaternary carbon atoms in the compound, and is a value that is characteristic of the functional group bonded to the hydrocarbon parent. 

The lower the bond enthalpy of the bond between the C atom and the leaving group, the better the leaving group. This again follows from the Hammond postulate. For this reason as well, the suitability of the halide ions as leaving groups is predicted as I- > Br > Cl > F . 

The relationships between bond enthalpy, bond length and bond order which appear relatively simple in the case of a main group element such as carbon and its compounds, are more difficult to establish when the d-transition metal elements and their compounds are considered. Progress in establishing these relationships for metals is severely hindered by a lack of relevant thermochemical data. This paper reviews some of the more useful information that is available for diatomic molecules, for polynuclear binary carbonyls and for binuclear complexes of the d-transition elements. 

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