Bond energy reference values

As for bond stretching, the simplest description of the energy necessary for a bond angle to deviate firom the reference value is a harmonic potential following Hooke s law, as shown in Eq. (22). 

The energy parameters used for the reference polyene by Hess and Schaad were developed on a strictly empirical basis. Subsequendy, Moyano and Paniagua developed an alternative set of reference bond energies on a theoretical basis. These values are shown... 

Reaction enthalpies can be estimated by using mean bond enthalpies to determine the total energy required to break the reactant bonds and form the product bonds. In practice, only the bonds that change are treated. Because bond enthalpies refer to gaseous substances, to use the tabulated values, all substances must be gases or converted into the gas phase. 

The results of the various semi-empirical calculations on the reference structures contained within the JSCH-2005 database (134 complexes 31 hydrogen-bonded base-pairs, 32 interstrand base pairs, 54 stacked base pairs and 17 amino acid base pairs) are summarised in Table 5-10. The deviations of the various interaction energies from the reference values are displayed in Figure 5-5. As with the S22 training set, the AMI and PM3 methods generally underestimate the interactions whereas the dispersion corrected method (PM3-D) mostly over-estimates the interactions a little. Overall the PM3-D results are particularly impressive given that the method has only... 

The bond dissociation energies that follow are taken from the review of McMillan and Golden [Ann. Rev. Phys. Chem. 33, 493 (1982)]. The reader should refer to this publication for the methods of determining the values presented, their uncertainty, and the original sources. In the tables presented, all bond energies and heats of formation are in kJ/mol. The values listed in the first column are the heats of formation at 298 K for the reference radical and those above the column heading for the associated radical. Thus, the tables presented are not only a source of bond energies, but also of heats of formation of radicals. 

LCA toward amino acids and nucleic bases has also been measured. Wesdemiotis and Cerda measured the alkali metal ion affinities of nucleobases in the gas phase from the dissociation of metal ion-bound heterodimers [nucleobase + B]M+, in which B represents a reference base of known affinity and M is an alkali metal. By assessing the dimer decomposition for two different internal energies, entropy is deconvoluted from enthalpy and LCA values are obtained. For guanine, cytosine, adenine, thymine and uracil, the corresponding Li+-nucleobase bond energies are as follows 57.2, 55.5, 54.1,... 

The perturbation A(T f + 2T ) describes the replacement of model densities and inter-nuclear distances by the values that are appropriate for the molecule under scrutiny. Similarly, appropriate reference atomic energies must be used in the atomic-like formula (4.15) to get A °. Ingeniously selected references require small corrections. Nature helps a lot in that matter by keeping the changes of p(r) as small as possible. The bond energy theory is rooted in Eq. (4.47). 

For the C—N bonds of amines, the analysis presented in Section 15.1 validates this result with the usual reference values =35.1 me and 5 = 5.8 ppm from TMS, which are exactly those of the paraffins. The presence of nitrogen next to carbon does not seem to require any ad hoc revision this is certainly an acceptable approximation [139], at least within the limits of the energy calculations involved in this evaluation. 

Despite the marked differences in both geometric parameters and the SCF Ar values between the molecules involved in this comparison, there are striking regularities the F value calculated for propene is, for all practical reasons, th that of ethylene (which takes care of 3 CH bonds) plus th that of tetramethylethylene (for the CC bond). Capitalizing on this idea, we may well consider transferable bond contributions modeled after Eq. (11.12) and use them to generate new reference bond energies satisfying Eq. (10.36). 

Similarly, we can calculate bond energies for any type of bond we wish to create. Refer to Appendix 1 for bond energy values. 

The values given in Table 3-5 were used for the enthalpies of the gases of atoms in their normal states (the reference states for the bond energies) relative to the standard states of the elements, to which the enthalpies of formation given in the Bureau of Standards compilation refer. Most of the values in Table 3-5 are taken from the Bureau of Standards compilation an important exception is the value for nitrogen, which has been shown by recent spectroscopic and thermochemical... 

There is at present no convenient, self-consistent source of all bond energies. The standard work is Cottrell. T. L. The Strengths of Chemical Bonds, 2nd ed. Butter-worths London, 1958, but it suffers from a lack of recent data. Darwent (National Bureau of Standards publication NSRDS-NBS 31, 1970) has summarized recent data on dissociation energies but did not include some earlier work or values known only for total energies of atomization rather than for stepwise dissociation. Three useful references of the latter type are Brewer, L. Brackett, E. Chem. Rev. 1961,61,425 Brewer, L. et al. Chem. Rev. 1963, 63, 111 Feber, R. C. Los Alamos Report LA-3164, 1965. The book by Darwent mentioned above also lists bond energy values for some common bonds. 

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