. Can. J. Chem. 60, 106 (1982). The I7O nuclear magnetic resonance shifts of dialkylethers are linearly related to the electron populations on the oxygen atoms, in a range covering L -130 ppm, showing that any increase of electronic charge at the oxygen atom is accompanied by a downfield nmr shift. The opposite trend is observed for the oxygen atoms of ketones and aldehydes.MARIE-THERESE BERALDIN. EDOUARD VAUTHIER et SANDOR FLISZLR. Can. J. Chem. 60, 106 (1982). Les deplacements chimiques des noyaux I7O dans les ethers aliphatiques sont lineairement relies aux populations electroniques des atomes d'oxygene, dans un domaine couvrant L -130 ppm, toute augmentation de population electronique se manifestant par un deplacement vers les champs faibles. L'effet oppose est observe pour l'oxygene des cetones et des aldehydes.

. Can. J. Chem. 59, 1074 (1981). With the definition of a "screening factor" o, transforming the nuclear charge Z, of an atom k into an "effective" nuclear charge Zk* = Z,ok, the total energy E = E(Z,*, Zl*, ...) of a molecule at equilibrium can be expressed as a homogeneous function of degree yla in Z,*, Zl*, ..., i.e. E = -C,Z,'~/~, wherea is theaverage ofthe a,'s (a, = a InZ,a,la In Z,) and y = (Vne + 2Vnn)/E isafunction of the nuclear-electronic, nuclear-nuclear, and total energies. Molecular energies calculated in this manner agree within 0.045% (average deviation) with their SCF counterparts. The sums of the orbital energies are correctly predicted from the equation xvigurb = (3 -y)E. A separation of the individual nuclear-electronic and nuclear-nuclear contributions to the total potential energy V(k, mol)of eachnucleus k indicates that the K , m ' l L f a~t~r~ of theenergy formula E = &KkmU1V(k, mol) are constant for each typeof atom, thus stressing the basic homogeneity of the total energy rather than of the electronic energy only. They function, which is the inverse of the molecular average of the Kkm""s, can be estimated in a simple way from atomic data. = (3 -y ) E La separation des contributions noyau-electron et noyau-noyau I'knergie potentielle totale V(k, mol) de chaque noyau k revele que les facteurs Kkm"' de la formule E = 1 , KkmU1V(k, mol) sont des constantes pour chaqueespece atornique, en confirmant ainsi I'hommogeneite de I'energie totale plut8t que celle de 1'6nergie electronique seulement. La fonction y, qui est I'inverse de la moyenne moleculaire des facteurs Kkm0', peut Ctre facilement deduite a partir de donnkes atomiques.

The energy formula describing bond contributions in terms of the charges carried by the bond-forming atoms is applied to ethylenic compounds. It is shown in what manner σ and π electrons can be treated within the framework of the bond energy theory giving the atomization energy of the vibrationless molecule at 0 K. Proper consideration of zero-point and thermal vibrational energies leads to standard enthalpies of formation. These calculations, which are carried out on the basis of, 13C nuclear magnetic resonance spectra, agree with their experimental counterparts, within experimental uncertainties (~0.3 kcal mol−1 average deviation).

Articles you may be interested inCharge distributions and chemical effects. XXI. A new energy formula for molecules J. Chem. Phys. 71, 700 (1979); 10.1063/1.438356Charge distributions and chemical effects. XVIII. On the relationship between total valenceelectron energies and nuclear-electronic interaction energies in atoms and ionsThe exact quantum mechanical formulation of atomic and molecular energies and the postulate that "chemical bonds" exist combine to show that the nuclear-electronic and nuclear-nuclear potential energy V(k,mol) involving the kth atom in a molecule is for its largest part determined by "local effects" related to the number and type of bonds formed by k. These local effects are measured by the derivatives O€k/aZk of the bond energies €kj involving k, with respect to its nuclear charge Zk' To a good approximation, neglecting the small nonbonded contributions, V(k ,mol) = V n , (free atom k) -ZkiL o€kjlaZ k • Applications to saturated hydrocarbons at the level of experimental accuracy indicate that the Emol/(V .. + 2V nn ) ratios derived in this manner are close to their ab initio counterparts.These ratios are averages K ~~' = IK ~OIV(k,mol)lIV{k,mol) of the ratios K ~Ol = Ek(mol) IV(k,mol) defining the individual total energies Ek(mol) of atoms being part of a molecule, whereby Emol = LEk(mol). The K ~OI'S, in tum, are shown to be constants in saturated hydrocarbons, i.e., 1/2.3329 for C and 1/2 for H, and are instrumental in accurate calculations of molecular energies, Emol = !;K ~ol V(k,mol), and for deriving energy differences, Ek(free atom) -Ek(mol) = .IlEk =K~oZkIa€kilaZk + (K k tom -K~OI)Vn,(free atom k), between free and bonded atoms. Positive .:1Ek values indicate that both the carbon and the hydrogen atoms are more stable in their bonded states than the free atoms, which is now understood in terms of dominating local binding properties, rather than in terms of local electron populations which play only a minor, though chemically significant, part on the scale of molecular energies. II where H:t and fiMol are the appropriate Hamiltonians and J. Chern. Phys. 12(2), 15 Jan. 1980 0021·9606/80/021 013.()5$01 ,00

. Can. J. Chem. 60,792 (1982).Energy calculations are presented for carbonyl compounds and ethers by applying the equation E~~ = E~O + aijAqi + ajiAqj which describes individual bond contributions in terms of electronic charge increments Aq at the bond-forming atoms i and j , relative to atomic charges in selected reference bonds with energies E,O. At a molecular level, the sum CiZjaijAqi accounts for virtually the entire energy variations due to charge redistributions accompanying isodesmic structural changes. Coulomb-type interactions between nonbonded atoms play only a minor role in that respect. Using charges derived from 13C and 1 7 0 nmr shift-charge correlations, calculated and experimental energies agree within 0.16 kcal/mol (average deviation).S . FLISZAR et M. T. BERALDIN. Can. 3. Chem. 60, 792 (1982). IntroductionFascinating new information can be extracted in The simplest "theoretical" description o f a mol-this way. For example, in the study (2) of linear ecule, its structural formula featuring the chemical and branched paraffins and in com~ounds containbonds, still represents the single most valuable ing chair orboat c~clohexane rings it appeared that "material" support for the discussion of a number 0.001 electron (= 1 me) added to hydrogen stabiof problems regarding chemical transformations lized a CH bond by 0.632 kcallmol whereas 1 me and molecular properties. Because of the undis-added to carbon had a stabilizing effect of 0.247 on puted qualities of this type of description, much can a CH bond and of 0.488 kcallmol on a CC bond. It be gained (namely, in simplicity) with the use of a ~O~~O W S therefrom that any electron enrichment on theoretical approach retracing the essential fea-carbon atoms at the expense of electron populatures linked to the concept of chemical bonds.tions at the hydrogen atoms results in a gain in While, of course, a molecule is a distinct entity molecular stability, which is well reflected in comwhich can be characterized in a number of man-parisons between isomers Or conformers. In the ners, the constituent atoms retain much of their Present study, we examine carbon-oxygen bonds In fact, a molecule can be in ethers and carbonyl compounds, in order to learn considered as a coljection of with ener.. how these and the remaining parts of the molecules gies differing from their free state values (1-3). are affected by intramolecular electron redistribuThese energy differences are, for each individual tions accompanying structural changes. While conatom, a measure for the process of becoming part of sideration is given to the theoretical interpretation the molecule and contain a portion of the molecular of the energetic aspects related to the individual binding energy. Alternatively, in an entirely equiv-bonds, we also offer comparisons with ex~erimen-alent description, molecules can be regarded as tal results, namely, with enthalpies of formation. assemblies of "chemical bonds" whose energies depend in a subtle way on the bond-forming atoms, namely, on their electron po...