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The energy of atomization, ma*, of a hypothetical vibrationless molecule is conveniently partitioned into bonded and nonbonded contributions. The portion of Ma* associated with the bond formed by atoms i and j (Le., the "bond energy", t i j ) can be deduced from the derivatives of tij with respect to the nuclear charges of i and j . Bond energies derived for one molecule are not transferable to other molecules because, as a rule, the simple transfer of selected tij's would not satisfy the requirement for molecular electroneutrality. The appropriate charge renormalization accompanying the use of selected reference bond energies toij leads to a description of the "bonded part" of ma*, = xtoij + xiX,aijAqi, featuring the effects due to changes in net atomic charges (Aqi) with respect to the charges of the atoms forming the reference bonds. Theoretical expressions are given for the aii coefficients. Applications to saturated hydrocarbons indicate that the charge-dependent Xix,aiiAqi part is by far the leading term accounting for the energetic differences between isomers or conformers and that nonbonded coulomb interactions play an almost negligible role in that respect. The structural effects in linear and branched paraffins and in compounds containing chair or boat cyclohexane rings are deduced from the theoretical ai:s, indicating that 1 me e) of electronic charge added to hydrogen stabilizes a CH bond by 0.632 kcal/mol, whereas 1 me added to carbon has a stabilizing effect of 0.247 on a CH bond and of 0.488 kcal/mol on a CC bond. The calculated molecular atomization energies agree with their experimental counterparts within 0.16 kcal/mol (average deviation). IntroductionSeveral fundamental aspects of chemical binding are discussed in this paper, namely, the view that molecules can be regarded as assemblies of "chemical bonds" with energies of their own or, else, as collections of "atoms in the molecule" with energies differing from their free-state values. Various facets of energy partitioning are reviewed and expanded. The roles of electronic charge distributions and finally that of "steric effects" are analyzed in detail. The whole adds up to give a novel view of the important physical features governing molecular stabilities and casts a new light on relevant aspects facilitating the interpretation of organic chemistry. Numerical examples are worked out for saturated hydrocarbons in order to illustrate in a comprehensive way the detailed features of this unified picture of molecular energies. A qualitative description of the relevant results and implications in organic chemistry is offered in the Conclusions.

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In a series of closely related compounds, it may occur that the change in total (diaplus para-) magnetic shielding of atomic nuclei is nearly that of the local paramagnetic term, because of cancellation effects involving nonlocal dia-and paramagnetic contributions. This offers a justification for relationships between nuclear magnetic resonance shifts and local atomic populations, which are occasionally observed. It is important, however, to consider the type (2s, 2p, , or x) of electrons which are responsible for the variations in atomic charges. Correlations between UC NMR shifts and atomic populations of aromatic compounds, for example, should not be interpreted in terms of x electrons only, because the slope of shift vs. x charge (i.e., the ~160-ppm/electron value which is usually invoked) does not describe an intrinsic effect of x charges on magnetic shielding but accounts for the fact that a and x charges vary in opposite directions in this class of compounds. The explicit consideration of the inverse variations of a and x charges, where appropriate, offers an explanation for the observation that charge-shift correlations can have positive or negative slopes. It appears, indeed, that an increase in total electronic population is accompanied (i) by a high-field shift when the electron enrichment results from a gain in x charge prevailing over the concurrent loss in electrons (aromatic and vinyl C, carbonyl O atoms) or (ii) by a downfield shift when the increase in charge is dictated by that of the population (sp1 23 C, carbonyl C, and dialkyl ether O atoms).