Charge distributions and chemical effects. XXVII. Molecular energies of carbonyl compounds and ethers, a calculation involving the charges of the bond-forming atoms

Abstract

. 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...

Overtone spectral investigation of substituent-induced bond-length changes in gas-phase fluorinated benzenes and their correlation with ab initio STO-3G and 4-21G calculations

“…1981, 103, 5061-5063. (8) Henry, B. R.; Gough, K. M. "Photochemistry and Photobiology: Proceedings of the International Conference, Jan [5][6][7][8][9][10]1983, University of Alexandria, Egypt"; Zewail, A. H., Ed. ; Harwood Academic Publishers: Chur, Switzerland, 1983; Vols.…”

Thermochemical parameters for organic radicals and radical ions. Part 3. The relationship between bond dissociation enthalpy and radical stability in alkyl systems

“…The significatzce of eqs. [15]- [19] is that the individltal bond contributions to the differences itz the relative stabitit?, of two species catz be determined, if the required ail and Aq values are known (14)(15)(16)(17)(18)(19)(20)(21).…”

“…(SEO(C'"H*)), from changes in the charges (in the units of me) on the C* and H''' atoms relative to the carbon and hydrogen atoms of ethane. Similar equations can be developed to assess SEO(C:"H*) values in carbonyl compounds (19), ethers (19), and ethylenes (20).…”

Charge distributions and chemical effects. XXXII. Energies of multiple carbon–carbon bonds

“…Note that the sum of all the E~' S yields the bonded part of AE. : , i.e., Equation [12] offers a detailed insight into charge effects governing bond energies and has been applied to a body of over 120 molecules, with an accuracy which is well within experimental uncertainties (3,11,12). The point made here is that only a few E"S need now be calculated by means of eq.…”

“…For these compounds, the expansion of eq. [12] yields (3) where the Aqc's are the increments in net charge at the carbon atoms, relative to that of ethane, whose net charge is q: (= 0.035 1 e), with q: = -q;/3 = hydrogen net charge. X I = acc -acH, k, = 4ncH -a,,; Ncc = number of CC bonds a C atom is engaged in.…”