Bond dissociation energies molecular species

The predominant species observed in SIMS spectra are singly charged atomic and molecular ions [51], However, inorganic and organic cluster ions can also be formed. If the sample consists of a simple single-component metal, then clusters such as M, M, etc., are observed in addition to M+ [52], Oxidation of the metal results in formation of MO ", MO/, M Oll", etc. The relative yield of MO+ to M+ depends on the bond dissociation energy of the oxide [52], For a two-component, oxidized metal, clusters of the type M/", M N, MjO, and M N O/ are observed [51]. 

The standard heats of formation AH of gaseous HX diminish rapidly with increase in molecular weight and HI is endothermic. The very small (and positive) value for the standard free energy of formation AGj of HI indicates that (under equilibrium conditions) this species is substantially dissociated at room temperature and pressure. However, dissociation is slow in the absence of a catalyst. The bond dissociation energies of HX show a similar trend from the very large value of 574kJmol for HF to little more than half this (295kJmol ) for HI. 

A relative scale of PA(B) has been established by examining the reaction shown in equation 23 for a large number of organic and inorganic bases. These data can be calibrated by reference to a variety of species for which absolute values of PA(B) may be derived from appearance potential measurements. The molecular ionization potentials IP(B) and the homo lytic bond dissociation energies D(B+—H) are linear functions of the proton affinity PA(B) for a homologous series of amines117-121. 

The appearance potentials for molecular ions (ionization potentials) and for fragment ions formed in the mass spectra of metallocenes and related compounds are listed in Table XIII. These appearance potentials have been used to calculate bond dissociation energies and heats of formation of organometallic compounds, but the results obtained must be treated cautiously because the appearance potentials of fragment ions include excess energy due to excited species. The values obtained for the heats of formation are best considered as upper limits, rather than precise determinations. The extent to which energy due to excited states can contribute... 

Figure 4.10 BCD data plotted as In Kp versus 1.000/7. Chlorobenzene and chloro-naphthalene dissociate via an intermediate molecular ion. They are designated DEC(2) for dissociative electron capture by a two-step process. The slope in the high-temperature region multiplied by R is equal to the EDEA. Given the electron affinity of the dissociating species, in this case Cl(—), the C—Cl bond dissociation energy can be measured. Data from [17-19].

Table II. Calculated bond dissociation energies for various nitro, nitroso, nitrite, nitrate, and azido molecular species. BDE s are determined from BAC-MP4 heats of formation at 298 K. (Energies in kcal-mol-i.)...

Knowledge of thermodynamic properties of even small elementary actinide molecular ions (and neutral species) are severely limited, particularly for the actinides other than Th and U. Some reported bond dissociation energies have been noted in the preceding sections the focus here is on the few systematic studies of actinide molecular ion thermochemistry. The most comprehensive set of experimental results are available for actinide monoxides and dioxides for Th through Cm, with little additional data available since the 2009 evaluation by Marqalo and Gibson (2009) the derived bond dissociation enthalpies... 

Note that Equation 7.8 uses bond-dissociation energies of neutral species (Table A.3), to which the organic chemist s intuition and knowledge of mechanistic principles should be directly applicable. For two competing reactions of the same molecular ion (and thus the same E M) values). Equation 7.8 predicts that AEq is determined by Alt of the respective radicals (Stevenson s Rule) and AD of the respective bonds. Further, a structural change usually affects / (AB ) much more than 0(AB—CD) or /E(ABCD), which is why product ion stability is usually the most influential factor determining product ion abundance (Section 8.2). 

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