¹H nuclear magnetic resonance and molecular orbital studies of 2-formylstyrene. Three significant nonplanar conformers at 300 K

Abstract

The 'H nuclear magnetic resonance spectra of 2-forrnylstyrene, from dilute solutions in CS2-C,D,, and acetone-d6, are analyzed to yield precise chemical shifts and spin-spin coupling constants. The long-range coupling constants imply aconformational distribution in which theO-trans conformer is 55% abundant in both polar and nonpolar environments. They also imply that the vinyl group, on average, is twisted out of the aromatic plane to a much larger extent than in styrene. The 6-3 lG* basis set gives an ab initio potential for the torsion of the vinyl moiety with a relatively deep minimum at 38" out-of-plane, for the 0-cis conformer. For the 0-trans conformer, two minima are found, one at 45' and another at 129.6'. Essentially the same potential is obtained with the 6-31G** basis. The latter corresponds to a close approach of the hydrogen atom of the formyl group and a orbitals or the P-carbon atom of the olefinic side chain. This local minimum is interesting in terms of a hypothesis used to explain the photochemistry of the molecule. The long-range coupling constants are consistent with the conformational properties calculated for the free molecule; they also indicate no significant difference between the conformational behaviour of the molecule in the two solvents. A proximate coupling constant of -0.16 Hz exists between the formyl and methine (a) protons. The latter is strongly deshielded in the presence of the formyl group, so that it becomes even less shielded than some of the aromatic protons.Key words: 'H NMR, 2-formylstyrene (0-vinylbenzaldehyde); long-range spin-spin coupling constants, 2-formylstyrene; conformations, three nonplanar of 2-formylstyrene; molecular orbital calculations, conformations of 2-formylstyrene.RCsumC : Dans le but d'obtenir des dtplacements chimiques et des constantes de couplage spin-spin prCcis, on a analysC les spectres RMN du 'H de solutions dilutes du 2-formylstyrkne dans du CS2-C6DI2 et de l'acttoned6. Les constantes de couplage a longue distance impliquent l'existence d'une distribution conformationnelle dans laquelle l'abondance du conformkre 0-trans est Cgale a 55% dans les environnements tant polaires que non polaires. Elles impliquent aussi que, en moyenne, le dCplacement du groupe vinyle, par rapport au plan aromatique, est beaucoup plus prononcC que dans le styrkne. Des calculs ab initio a l'aide d'un ensemble de base 6-3 lG* permettent dlCvaluer le potentiel de torsion de la portion vinylique et d'Ctablir que, pour le conformkre 0-cis, il existe un puits de potentiel relativement profond pour un dCcalage de 38" par rapport au plan aromatique. Pour le conformkre 0-trans, on a trouvt deux minima, un h 45' et l'autre a 129,6'. On obtient essentiellement le m&me potentiel avec un ensemble 6-3 1G**. Ce dernier correspond a un grand rapprochement de l'atome d'hydrogkne du groupe formyle et des orbitales a de l'atome de carbone P de la chaine 1atCrale olkfinique. Ce minimum local est inttressant en termes d'une hypothkse utilistepour expliquer la photochimie de l...

Natural bond orbital dissection of fluorine–fluorine through-space NMR coupling (JF,F) in polycyclic organic molecules

“…In agreement, several experimental reports on J(H,H) and J(F,H) couplings offer evidence that through-space transmission as a result of direct overlap between two bonds elicits a negative contribution. 3,18 With respect to different lone pairs within the HF dimer, the J LP (F,F) breakdown suggests that (a) the lone pairs of both fluorine atoms whose NLMOs are of p-symmetry do not participate in through-space transmission; (b) the NLMOs of lowest energy yield the largest and positive contribution to J FC (F,F); and (c) the J-contribution of the lone pairs of highest energy are negative but of notably smaller absolute value than that described in (b). However, actual values of each component depend strongly on a for a given d(F…F) distance.…”