Bond dissociation enthalpy additivity, 44

There is an additional advantage in using relative solution phase bond dissociation enthalpies. In most cases, solution phase bond dissociation enthalpies do not refer to standard states (see section 2.3), and the required corrections are hard to predict. When we consider relative bond dissociation enthalpies in a series of similar molecules in solution it is likely that the unknown corrections to standard states are nearly constant. 

The value of 169.9 kJ mol" (1.75 eV) corresponds to AE(ch3+) = 14.35 eV which is in good agreement with published values of about 14.3 eV. [28,39] In addition, this is only 40 % of the homolytic C-H bond dissociation enthalpy of the neutral methane molecule, thereby indicating the weaker bonding in the molecular ion. 

In addition to this type of empirical approach, there are several other approaches that are related more directly to specific properties of the organic, such as the C-H bond dissociation enthalpies (Heicklen, 1981 Jolly et a.L, 1985), ionization energy (Gaffney and Levine, 1979), or NMR shifts (Hodson, 1988). In addition, molecular orbital calculations (Klamt, 1993) and transition state theory (Cohen and Benson, 1987) have been applied. 

In addition, if n is not equal to 1, the total bond dissociation enthalpy. E"BDE(E ), required to convert E into E g), will have to be included. The atoms of will then need to be ionized to absorbing the... 

Gryzbowska et al. [106] compared the reaction products formed when pulses of allyl iodide or propene were passed over bismuth oxide or molybdenum oxide. A clear limitation of these experiments is that even the simplest bismuth molybdate catalysts contain neither bismuth oxide nor molybdenum oxide, but instead are made up of a binary oxide of bismuth and molybdenum, whose structure is different to that of bismuth oxide and molybdenum oxide. Gryzbowska et al. selected allyl iodide because of the very low bond dissociation enthalpy associated with the C-I bond, implying that a surface allyl species would readily form from this starting material. In addition, a lower reaction temperature was required for the reaction of allyl iodide than for propene reflecting the greater inherent reactivity of the former. 

The addition reactions involve cleavage of the H-H bond and formation of two (equivalent in trans-2) M-H bonds. The experimental H2 bond dissociation enthalpy is 104.2 kcal/mol (55). The computed values from the BLYP and B3LYP methods nicely bracket this value at 103.9 kcal/mol and 104.5 kcal/mol, respectively. The MP2 value for the H-H bond dissociation enthalpy is only 95.2 kcal/mol the calculated value improves somewhat at MP4(SDTQ) (99.9 kcal/ mol) and CCSD(T) (100.3 kcal/mol). From the computed exothermicities of reaction (5), we can estimate the apparent M-H bond energies in the TBP product. For trans-la, we find apparent Rh-H bond energies (kcal/mol) of 59.6 (BLYP),... 

Given that Z)(H-H) and Z)(F-F) in H2 and F2 are 436 and ISSkJmol, estimate the bond dissociation enthalpy of HF nsing a simple additivity rnle. Compare the answer with the experimental valne of 570 kJ mol. ... 

Additional data see Appendix 10 the bond dissociation enthalpy of HCl is 432 kJ moU. Comment on the results. 

Chapter 11 deals with free radicals and their reactions. Fundamental structural concepts such as substituent effects on bond dissociation enthalpies (BDE) and radical stability are key to understanding the mechanisms of radical reactions. The patterns of stability and reactivity are illustrated by discussion of some of the absolute rate data that are available for free radical reactions. The reaction types that are discussed include halogenation and oxygenation, as well as addition reactions of hydrogen halides, carbon radicals, and thiols. Group transfer reactions, rearrangements, and fragmentations are also discussed. 

The molybdenum-alkyl mean bond dissociation enthalpies are in the expected order and in good agreement e.g. with the trend reported for Th(Cp ) R complexes (Cp = rj -C Me ) (9a). On the other hand, it is interesting to note that the group additivity rule seems to apply to the Mo-alkyl family, as shown by the excellent linear relationships 7 and 8, where N is the number of carbon atoms in the alkyl chain. 

Vertical exceptions within a group also occur. The E.A. for F is less than that for Cl, for example. The smaller-than-expected E.A. for fluorine can be rationalized because of fluorine s extremely small radius. The addition of an electron to its valence shell would therefore increase the magnitude of the electron-electron repulsions. Consequently, the E.A. for fluorine is somewhat less than that for chlorine. Additionally, the bond dissociation enthalpy for Fj (155 kj/mol) is considerably less than that expected based on the other members of its group. For comparison, Clj, Br2, and I2 have bond dissociation enthalpies of 242, 193, and 151 kj/mol. Other anomalies occur for N and O, whose electron affinities are also less than the group trend would have predicted. By analogy with the F-F bond strength, the N-N and 0-0 bonds are likewise weaker than those for P-P or S-S. In fact, both the hydrazine (N-N) and peroxide (0-0) classes of compounds are particularly reactive. Hydrazine, N2H4, was once used as a rocket fuel, and many peroxides are potentially explosive. 

Usefid reviews address alkyl zirconocene catalysts for the pol)mierisation of silanes to poly silanes by a a-bond metathesis mechanism, chiral titanates as promoters in aldol reactions, and MeTiCl3 as a reagent for chelate-controlled carbonyl addition reactions. 5 xhe reactions of terminally functionalized alkenes with zirconocene hydrides are reviewed. Thermochemical studies show that while the bond dissociation enthalpies of Zr—C6H13 and Zr— CgHjj in zirconocene systems are comparable, the insertion of cyclohexene into the Zr—bond is more exothermic than the insertion of hexene. 

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... 

The decomposition of tri- and tetrasulfane in CCI4 solution (0.2 mol 1 ) at 70 °C and in the absence of oxygen has been studied by H NMR spectroscopy [64]. Initially, tetrasulfane decomposes to a mixture of tri- and pentasul-fane but slowly and after an induction period hydrogen sulfide and disulfane are formed in addition. These results have been interpreted in terms of a radical-chain reaction. The initial step is assumed to be the homolytic cleavage of the central SS bond which has by far the lowest dissociation enthalpy of the molecule ... 

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