Electron addition enthalpy

The addition of an electron to an uncharged atom or negatively charged ion is referred to as the electron addition enthalpy. The energy associated with removal of an electron from a negatively charged species (the atom that has gained an electron) is the electron affinity. 

D is the dissociation enthalpy of Cl2,1 is the ionization potential of Na, E is the electron addition enthalpy of Cl (which is the negative of the electron affinity), and U is the lattice energy. The Born-Haber cycle shows that the lattice energy corresponds to the energy required to separate a mole of crystal into the gaseous ions, and forming the crystal from the ions represents -U. 

Negative ions also have two unique thennodynainic quantities associated with them the electron affinity, EA, defined as the negative of the enthalpy change for addition of an electron to a molecule at 0 K [117. 121. 122]... 

To date there is no evidence that sodium forms any chloride other than NaCl indeed the electronic theory of valency predicts that Na" and CU, with their noble gas configurations, are likely to be the most stable ionic species. However, since some noble gas atoms can lose electrons to form cations (p. 354) we cannot rely fully on this theory. We therefore need to examine the evidence provided by energetic data. Let us consider the formation of a number of possible ionic compounds and first, the formation of sodium dichloride , NaCl2. The energy diagram for the formation of this hypothetical compound follows the pattern of that for NaCl but an additional endothermic step is added for the second ionisation energy of sodium. The lattice energy is calculated on the assumption that the compound is ionic and that Na is comparable in size with Mg ". The data are summarised below (standard enthalpies in kJ) ... 

Several portions of Section 4, Properties of Atoms, Radicals, and Bonds, have been significantly enlarged. For example, the entries under Ionization Energy of Molecular and Radical Species now number 740 and have an additional column with the enthalpy of formation of the ions. Likewise, the table on Electron Affinities of the Elements, Molecules, and Radicals now contains about 225 entries. The Table of Nuclides has material on additional radionuclides, their radiations, and the neutron capture cross sections. 

The meaning of the word aromaticity has evolved as understanding of the special properties of benzene and other aromatic molecules has deepened. Originally, aromaticity was associated with a special chemical reactivity. The aromatic hydrocarbons were considered to be those unsaturated systems that underwent substitution reactions in preference to addition. Later, the idea of special stability became more important. Benzene can be shown to be much lower in enthalpy than predicted by summation of the normal bond energies for the C=C, C—C, and C—H bonds in the Kekule representation of benzene. Aromaticity is now generally associated with this property of special stability of certain completely conjugated cyclic molecules. A major contribution to the stability of aromatic systems results from the delocalization of electrons in these molecules. 

Table 2 gives our calculated results for the equilibrium volume Vq, bulk modulus Bq, and enthalpy of formation AH. Theoretical results refer to T=0, uncorrected for zero point motion, whereas experimental values refer to room temperature. Note that the extensive quantities AH and Vq arc reported per atom in the present paper, i.e., divided by the total number of atoms. As well known the LDA underestimates the volume. Comparing the bulk modulus for T3 and D8s we see that the addition of Si to pure Ti has a large (26 %) effect on the bulk modulus, indicating that p electrons of Si have a strong effect on the bonding in this system. 

MnAs exhibits this behavior. It has the NiAs structure at temperatures exceeding 125 °C. When cooled, a second-order phase transition takes place at 125 °C, resulting in the MnP type (cf. Fig. 18.4, p. 218). This is a normal behavior, as shown by many other substances. Unusual, however, is the reappearance of the higher symmetrical NiAs structure at lower temperatures after a second phase transition has taken place at 45 °C. This second transformation is of first order, with a discontinuous volume change AV and with enthalpy of transformation AH. In addition, a reorientation of the electronic spins occurs from a low-spin to a high-spin state. The high-spin structure (< 45°C) is ferromagnetic,... 

In addition to the enthalpy and polar interaction, the following factors influence the activation energy of these reactions triplet repulsion, electron affinity of atoms C and O in the TS, radii of C and O atoms, and n-bonds in the vicinity of the reaction center. These factors were discussed in Chapter 6 in application to the reaction of peroxyl radicals with hydrocarbons. 

Carbonyl group of the aldehyde decreases the BDE of the adjacent C—H bond. This is due to the stabilization of the formed acyl radical, resulting from the interaction of the formed free valence with Tr-electrons of the carbonyl group. For example, DC—H = 422kJmol 1 in ethane and D( n 373.8 kJ mol 1 in acetaldehyde. The values of Dc H in aldehydes of different structures are presented in Table 8.1. In addition, the values of the enthalpies of acylperoxyl radical reactions with aldehydes were calculated (D0 H= 387.1 kJ mol-1 in RC(0)00 H). 

Table 6.25 Comparison of experimental reaction enthalpies at 0 K (kJ/mol) for the addition of methyl radical to alkenes CH2=CXY with those calculated3 with the wavefunction-based electronic structure methods.

Table 3 presents the experimental enthalpies of formation of polynitrobenzenes and Table 4 presents the calculated additivity values and DSEs for these same compounds. Enthalpy-of-formation values have been determined experimentally for all three dinitrobenzene isomers in the gaseous state. The enthalpy-of-formation difference between the meta and para isomers is indistinguishable from 0. Conventional wisdom suggests that the para isomer should be destabilized relative to the meta because of adjacent positive charges in key ionic or polar resonance structures. Thus it seems that electronic effects due to meta/para dinitro substituent position are small. This small enthalpy-of-formation difference is similar to that for the meta and para dicyano, difluoro and dichloro benzenes, but does not mimic the ca 22 kJ mol 1 difference for the phthalic acids with which the... 

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