The conformational dependence of the nuclear spin-spin coupling from methyl protons to ring protons, to the fluorine nucleus, and to protons of other methyl groups in toluene, p-fluorotoluene, and the xylenes is computed by the finite perturbation technique in the INDO approximation of molecular orbital theory. The calculated coupling over six bonds to the proton in the para position agrees quantitatively with experiment and its predicted dependence on the rotational angle of the methyl group supports a commonly assumed x electron mechanism for the transmission of spin information between the nuclei. Similar remarks apply to the fluorine nucleus inp-fluorotoluene. The couplings over five and four bonds to the protons in the meta and ortho positions display a more complex angular dependence and the former can be interpreted in terms of a dominant u electron mechanism. The coupling between protons in different methyl groups in the ortho and meta xylenes is calculated as rather larger than the values observed in their derivatives and in the main shows the behavior expected from a x electron mechanism. Those conformations of ortho xylene in which the coupled protons are in close proximity yield computed values plausibly attributable to "direct" and/or "through-space" mechanisms. The preferred conformation of the methyl group in toluene is predicted to have a C-H bond eclipsing the plane of the aromatic ring and the calculated barriers to rotation of 0.013 kcal/mol in toluene and of 0.014 kcal/mol in p-fluorotoluene are in quantitative accord with microwave data.Nous avons compile la dependance conformationnelle du couplage nucleaire spin-spin des protons methyle aux protons du cycle, aux noyaux de fluor, et aux protons des autres groupes mithyle dans le toluene, lep-fluorotolutne, et les xylenes, par la technique de perturbation finie dans I'approximation INDO de la thtorie des orbitales moltculaires. Le couplage calcule de six liens au proton en position para concorde quantitativement avec I'experience et sa dtpendance predite sur I'angle de rotation du groupe mtthyle supporte un mtcanisme d'electron x qu'on suppose communtment pour la transmission d'information de spin entre les noyaux. On peut appliquer les m&mes remarques dans le cas du noyau de fluor dans le p-fluorotoluene. Les couplages au travers de cinq et quatre liaisons a partir des protons en position mtta et ortho occasionnent une dipendance angulaire plus complexe et le premier cas peut dtre interpret6 en termes d'un mecanisme a electron sigma (u) dominant. Nous avons calcule que le couplage entre les protons des differents groupes mCthyle des xyltnes ortho et mtta est plus grand que les valeurs observees chez leurs derives et le principal d'entre eux possede le comportement qui laisserait soupGonner un mecanisme d'tlectron x. Les conformations de I'ortho-xylene pour lequel les protons couples sont rapproches nous amenent h des valeurs compilees qu'on pourrait vraisemblablement attribuer a des mecanismes "directs" et/ou "h travers I'espace". Nous prtdi...
Precise analyses of the proton and some fluorine magnetic resonance spectra in acetone solution are reported for the three monofluorobenzaldehydes as well as for 2-chloro-6-fluorobenzaldehyde and for 4-fluoro-2-nitrobenzaldehyde. The conformational dependence of the coupling parameters allows the measurement of energy differences between the 0-cis and 0-trans conformations. The energy differences are in better agreement with the INDO predictions than they are with energies derived from i.r. data. Dipole moments are computed reliably and their measurement is suggested as a good guide to conformational preferences for molecules of this kind. The spin-spin coupling constants between the aldehyde proton and the ring protons and fluorine nuclei are computed for benzaldehyde and the three monofluorobenzaldehydes by the INDO and CNDO molecular orbital approximations. In many instances the agreement between calculated and observed couplings is quantitative. Les analyses detaillkes des spectres de rksonnance magnetique du proton et du fluor en solution dans l'acttone sont rapportees pour les trois monofluorobenzaldthydes ainsi que pour le chloro-2 fluoro-6 benzaldehyde et le fluoro-4 nitro-2 benzaldthyde. La relation entre les parametres de couplage et la conformation permet de mesurer la difference d'energie entre les conformations 0-cis et 0-trans. Les differences energktiques sont en meilleur accord avec les previsions donnees par la methode INDO qu'avec celles obtenues 2 partir des donnees i.r. Les moments dipolaires sont calculCs avec certitude et leur mesure constitue un bon exemple de prtference conformationnelle pour des moltcules de ce type. Les constantes de couplage spin-spin entre le proton aldthydique et les protons du cycle et le noyau du fluor sont calculCes pour le benzaldehyde et les trois monofluorobenzaldthydes par les approximations INDO et CNDO d'orbitales moleculaires. Dans plusieurs cas, I'accord entre les couplages calcults et observes est quantitatif.
Molecular orbital theory at the INDO level of approximation is used to calculate the Fermi contact contribution to three-bond carbon–proton coupling constants in propane. The calculations predict a dihedral angle dependence of 3J(13C,H) in the 13C—C—C—H fragment similar to that observed for 3J(H,H), 3J(19F,H), and for 3J(31P,H) in the saturated X—C—C—H fragments.
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