Charge distributions and chemical effects. XXVI. Relationships between nuclear magnetic resonance shifts and atomic charges for <sup>17</sup>O nuclei in ethers and carbonyl compounds

. The electronic effects of a series of 18 heterocyclic carboxaldehydes (furans, thiophenes, pyrroles, and pyridines) have been studied by means of the correlation existing between I3C chemical shifts of the carbonylic carbon and calculated total and T charges (AMI). The implications of this theoretical model to explain polar and resonance contributions to the total electronic effect are discussed.

Section: Resultsmentioning

confidence: 99%

“…Furans and pyrroles (5)(6)(7)(8)(9)(10)(11)(12) show the most marked differences in shielding when comparing positions 2 and 3. Thiophenes and pyridines (1)(2)(3)(4)(13)(14)(15)(16)(17)(18) show comparatively fewer important differences in this respect.…”

Section: Resultsmentioning

confidence: 99%

Electronic effects of heterocyclic substituents. Spectroscopical and theoretical (AM1) study in a series of heterocyclic carboxaldehydes

Cativiela¹,

Garcı́a²

1990

“…Changes in 0, and thus in both Sij and H i j , lead to a change in the electron density on the oxygen atoms q5. This change has a dramatic effect on the chemical shift of the 1 7 0 nucleus (15). An increase of the electron density on oxygen results in a spatial expansion of the 2p orbitals and this to a reduction of the term ( r -3 ) Z p N in eq.…”

Section: Resultsmentioning

confidence: 99%

Determination of the intercarbonyl dihedral angle of 1,2-diketones by ¹⁷O nmr

Cerfontain¹,

Kruk²,

Rexwinkel³

et al. 1987

HANS CERFONTAIN, COR KRUK, ROEL REXWINKEL, and FRANK STUNNENBERG. Can. J. Chem. 65, 2234 (1987). This study describes a method using 170 nmr to determine the intercarbonyl dihedral angle in a homologous series of seven 1,2-diketones. It is shown that the 170 nmr shifts of the 1,2-diketones are linearly dependent on both the electron density at the oxygen atoms and the n-n * excitation energy of the 1,2-diketo chromophore. Reduction of the overlap in the .rr system of the 1,2-diketo moiety leads to a gradual upfield shift of the 170 nmr signal, due to the increase in both the T-electron density at the oxygen atoms q6 and the mean transition energy AE.HANS CERFONTAIN, COR KRUK, ROEL REXWINKEL et FRANK STUNNENBERG. Can. J. Chem. 65, 2234 (1987). Cette Ctude dCcrit une mkthode qui fait appel B la rmn du 170 pour dkterminer l'angle dikdre intercarbonyles dans un sCrie homologue de sept dicttones-1,2. On dCmontre que les deplacements des dicktones-1,2 en rmn du 170 dipendent d'une fa~on IinCaire A la fois sur la densit6 Clectronique des atomes d'oxygkne que sur l'energie d'excitation n-n* du chromophore dictto-1,2. La rtduction dans le recouvrement du systkme-.rr de la portion dicCto-!,2 conduit a un dCplacement graduel vers les hauts champs du spectre rmn du 170; ce rksultat provient d'une augmentation tant de la densit6 des Clectrons-.rr au niveau des atomes d'oxygkne (96) que de l'inergie de transition moyenne (AE).[Traduit par la revue]

“…[7] is limited by the accuracy of the experimental data used in this type of verification. Furthermore, ab initio calculations on ether oxygen atoms indicate that the AqdtiO slope is -1.8 times that of the Aq,/6, slope determined under identical conditions, giving (14) [8]…”

Section: Approximations Formentioning

confidence: 99%

“…[7]- [9], and with the evaluation of vibrational and thermodynamic data used in this analysis. Clearly, the difficulties are now in the evaluation of fraction of kcal/mol uncertainties, 2Ab initio calculations (14) reveal that carbonyl oxygen atoms behave quite like ethylenic s p Z carbons, in that any increase of electronic charge is reflected in an upfield shift. For the latter, the chargelshift slope is --11.7 times that of alkane carbon atoms (15) giving Aq, = I .7&, a result which is of the same order as that expressed by eq.…”

Section: Approximations Formentioning

confidence: 99%

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

Fliszár¹,

Béraldin²

1982

. 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...