Energy, bond hydrocarbons

Table I.IS gives total bonding energies in kilocalories per mole for some simple molecules. The B3iyP results are comparable in accuracy to G1 and G2 results. Another comparison was done with a series of cyclic hydrocarbons as the test case. The calculations were done using an isodesmic reaction scheme. The results are given in Table 1.19. Density functional calculations have also been successfully extended to functionalized molecules. ...

Certain C—H bonds have significantly lower bond dissociation energies than do the normal C—H bonds in saturated hydrocarbons. Offer a structural rationalization of the lowered bond energy in each of the following compounds, relative to the saturated... 

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. 

Data are given in Table IV for heterocyclic compounds. For piperidine there is no difference between E and E, showing that the bond energies used are applicable to saturated heterocyclic molecules. Pyridine and quinoline differ from benzene and naphthalene only by the presence of one N in place of CH and, as expected, the values 1.87 v.e. and 3.01 v.e., respectively, of the resonance energy are equal to within 10 percent to the values for the corresponding hydrocarbons. 

For nonaltemant hydrocarbons the energies of the bonding and antibonding orbitals are not equal and opposite and charge distributions are not the same in... 

The addition of metal hydrides to C—C or C—O multiple bonds is a fundamental step in the transition metal catalyzed reactions of many substrates. Both kinetic and thermodynamic effects are important in the success of these reactions, and the rhodium porphyrin chemistry has been important in understanding the thermochemical aspects of these processes, particularly in terms of bond energies. For example, for first-row elements. M—C bond energies arc typically in the range of 2, i-. i() kcal mol. M—H bond energies are usually 25-30 kcal mol. stronger, and as a result, addition of M—CH bonds to CO or simple hydrocarbons is thermodynamically unfavorable. 

C06-0100. Use average bond energies (Table 6-2) to compare the combustion energies of ethane, ethylene, and acetylene. Calculate which of these hydrocarbons releases the most energy per gram. 

Bartell and coworkers investigated the structures of a series of noncyclic alkanes by means of gas electron diffraction (14, 44, 45) and invoked for the interpretation of their results a simple force field which contained to a high extent vibrational spectroscopic constants of Snyder and Schachtschneider. This force field reproduces bond lengths and bond angles of acyclic hydrocarbons well, energies of isomerisation satisfactorily. As an example, Fig. 8 shows geometry parameters of tri-t-butylmethane as observed by electron diffraction and calculated with this force field (14). 

Each of these bond enthalpies is an average enthalpy, measured from a series of similar molecules. Values of AH, . for, say, C-H bonds in hydrocarbons are likely to be fairly similar, as shown by the values in Table 3.3. The bond energies of C-H bonds will differ (sometimes quite markedly) in more exceptional molecules, such as those bearing ionic charges, e.g. carbocations. AH, . values differ for the OH bond in an alcohol, in a carboxylic acid and in a phenol. 

Thermochemistry of cluster compounds. In this short summary of cluster structures and their bonding, a few remarks on their thermochemical behaviour are given, in view of a possible relationship with the intermetallic alloy properties. To this end we remember that for molecular compounds, as for several organic compounds, concepts such as bond energies and their relation to atomization energies and thermodynamic formation functions play an important role in the description of these compounds and their properties. A classical example is given by some binary hydrocarbon compounds. 

Hence follows a linear relationship between pA b for aromatic hydrocarbons and the localization energies (the changes in the a-bond energies are the same in the protonation of any aromatic hydrocarbon). The first calculations of this kind are due to Gold and Tye (1952) and the correlations have been demonstrated by Dallinga et al. (1957) and by Mackor et al. (1958). From such theoretical calculations on aromatic hydrocarbons, it is possible to predict the preferred site of protonation, i.e., the most stable proton addition complex. 

The transferability of several molecular properties may lead to advantages for the aliphatic hydrocarbons, e.g., the bond energies can be approximated in advance as they are known to be transferable to a good accuracy (Allen, 1959). 

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