Ionic heats of formation

Tossing, F.P. Semeluk, G.P. Free Radicals by Mass Spectrometry. XLII. Ionization Potentials and Ionic Heats of Formation for C1-C4 Alkyl Radicals. Can. J. Chem. 1970, 48, 955-965. 

These ionic heats of formation can now be combined with the thresholds measured for reactions (19) and (20) to provide neutral heats of formation. In... 

E.s.r. spectra of this cation indicated that it has a triplet ground state [261,263]. Electrochemical determination of the pKp+ value were hampered by a number of uncertainties but indicate that it is exceedingly low, and much lower than the value for the allyl cation [264]. Calculations of the ionic heats of formation of the cyclopentyl, cyclopentenyl and cyclopentadienyl cations from the measured ionisation potentials of the corresponding radicals and the heats of formation of these radicals show that whereas introduction of one double bond stabilises the cation, introduction of the second double bond destabilises the system [265]. E.s.r. spectra of the pentaphenyl [258,259] and pentachloro [259,266] derivatives show that the former has a ground state singlet structure but with a triplet state less than 1 kcal.mol" higher, while the latter has a triplet ground state [252,259]. 

F. P. Lossing and G. P. Semeluk, Free radicals by mass spectrometry. XLII. Ionization potentials and ionic heats of formation for C1-C4 alkyl radicals, Can. J. Chem. 48, 955 (1970)... 

The heat of formation CH5 + is taken as 229 =fc 3 kcal./mole as determined recently in these laboratories (9). In all these energetic considerations we assume that all the energy of the reaction is concentrated in the ionic product species. Thus, the energies written are upper limits to the energies the ionic species actually contain. 

Methods can be based on some preconceived concept of bonding, with ionic and covalent extremes, or on pattern recognition based on the periodic table. Miscellaneous methods of limited applicability link bond strength with other physical properties. The a priori calculation of heats of formation by wave mechanics is possible in theory. In practice, the most widely applied method incorporates experimental data to derive atom or bond parameters which can then be used for calculations on closely related compounds. 

Comment on heats of formation of fluoro-anions, and electron and fluoride-ion affinities of neutral fluorides, measured mass spectropho-tometrically (57,185,216,222) or derived from salt values obained by conventional calorimetry (32, 45, 46, 105) needs to be reserved until better agreement is reached between methods. However, from measurements on heats of formation of the predominantly ionic xenon fluoride adducts it has been possible to show the trend to increasing ionic-ity with pentafluoride partners Nb < Ta < Sb, which parallels the increasing Lewis acidity of these fluorides found by independent methods (44). 

The heat of formation of organic radicals and positive ions decreases with their size and even more important with their degree of branching at the radical or ionic site. A lower heat of formation is equivalent to a higher thermodynamic stability... 

Fig. 2.13. Definition of Eoi and origin of KER. The excess energy of the decomposing ion in the transition state refative to the sum of the heats of formation of the ionic and neutral product is partitioned into vibrational excitation of the products plus KER.

Astonishingly, the study of the mechanism of formaldehyde loss from anisole revealed two different pathways for this process, one involving a four- and one a five-membered cyclic transition state (Fig. 6.37). [129] The four-membered transition state conserves aromaticity in the ionic product, which therefore has the lower heat of formation. Prompted by the observation of a composite metastable peak, this rather unusual behavior could be uncovered by deconvolution of two different values of kinetic energy release with the help of metastable peak shape analysis (Chap. 2.8). 

Where do the thermochemical data that are used to determine the energetics of a reaction come from For closed-shell species that can be generated chemically via proton transfer, gas phase acidities (reaction [2]) and basicities (reaction [3]) are the principal sources. If the acidity or basicity for a reaction leading to a given ion is known, then the heat of formation for that ion can be calculated via Equations (4) and (5). This latter point is important, because this is the source for much of the ionic thermochemical data that are used for application of the no endothermic reactions tool. 

We see that the simple rectangular d band model reproduces the behaviour found by experiment and predicted by Miedema s semi-empirical scheme. However, we must stress that the model does not give credence to any theory that bases the heat of formation of transition-metal alloys on ionic Madelung contributions that arise from electronegativity differences between the constituent atoms because in the metallic state the atoms are perfectly screened and, hence, locally charge neutral. Instead, the model supports... 

Lattice energies can be related to the heats of formation AH of ionic solids through the Bom-Haber cycle, which is the counterpart of the thermochemical cycle for covalent compounds given in Section 2.7. 

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