Thermochemical Electronegativities

Allred, A.L. Electronegativity Values from Thermochemical Data J. Inorg. Nucl. Chem. 1961, 17, 215-221. 

On the basis of this equation, systematically considering all the available thermochemical data and arbitrarily assigning to the most electronegative element, fluorine, a value of about 4, Pauling was able to prepare a complete electronegativity scale. This is shown in Table 2.1. 

From gas-phase ion-molecule reactions we obtain54 the necessary enthalpy of formation of 34, 44 8 kJmol-1, and so derive A//f(33, g) as equal to ca —46 kJmol-1. Is this number plausible We start with the trimerization enthalpy. Intuitively, this number for C=N bonded species should interpolate the trimerization enthalpy of species with C=C and C=0 bonds. More precisely, since it is N and not C that is methylated, the trimerization enthalpy of 34 should be close to that of CH2NH. As such, this reaction enthalpy should interpolate that of CH20 and CH2CH2. Our thermochemical archive includes the enthalpies of formation of these latter two substances and of their trimers, 1,3,5-trioxane (33, X = O), and cyclohexane (alternatively identified as 33, X = CH2 and 2, n = 6), from which derive the desired reaction enthalpies —140.1 and —276.0 kJmol-1 respectively. The reaction enthalpy for the X = NMe case interpolates that of X = CH2 and X = O as expected from size and electronegativity reasoning. 

Pauling based his scale on thermochemical data. We shall examine his methods shortly, but we may note that his scale is an arbitrary one chosen so that hydrogen is given a value of about 2 and the most electronegative element, fiuorine, has a value of about 4 ... 

Hence, some hetero-atomic bonds must be considerably stronger than the homo-atomic bonds in the elements. Linus Pauling considered this tendency to be so fundamental that the defined the thermochemical electronegativity x from the relation of bond strengths (AH0 values)/ for dissociation to gaseous atoms... 

The first known attempt to quantify electronegativity was due to Pauling, who devised an empirical scale, on the basis of thermochemical data. It... 

All possible diatomic combinations, AB, correspond to situations between the two extremes. The larger the difference in electronegativity, xa %b, the larger is the electrovalent component that stabilizes the linkage, and the smaller the covalent contribution. In thermochemical terms [68] - page 88 ... 

Nitrogen, sulfur and bromine have a diagonal relation in the periodic table and so have comparable electronegativity values. This suggests that amines, thiols and bromides are good thermochemical mimics. Do thiols and bromides so qualify From the enthalpies of formation in our archives we find the differences S23 (g Br, SH R) (equation 23) ... 

For the iodides, we know of no established thermochemical mimicry, although the comparative closeness of the electronegativities of carbon and iodine is suggestive. More precisely, the value for the enthalpies of zz-propyl to isopropyl iodide rearrangement, 10.0 ... 

Having a method to determine relative electronegativities for the elements using molecular thermochemical data required the choice of an origin in order to obtain... 

Allred, A. L. (1961). Electronegativity values from thermochemical data. J. In-org. Nucl. Chem. 17, 215-21. 

Electronegativities, Xp. shown in bold text and rounded to one decimal place, are taken from Allred or from Pauling. The values are based on thermochemical data analysed using Pauling s approach for the elements in their normal oxidation state, viz. for Sc, Y and La, M for Cu. Ag and Au, for the other d-block metals, Ln for the lanthanides and An for the actinides. 

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