The spin-spin coupling over six bonds between '"F and "C nuclei on the sidechain in thirteen 4-fluorophenyl derivatives appears to be mediated by a a-.rr mechanism. Its magnitude depends somewhat on the hybridization state of the carbon atom canying the coupled nucleus, as well as on the electronegativity of substituents attached to this carbon atom. A consistent behaviour of this coupling is observed if its value is assumed to be proportional to sin2 0, where 0 is the angle by which the bond canying the coupled carbon nucleus twists out of the ring plane. However, in 4-fluorostyrene 'J%" is a -rr electron coupling in the planar form, so that its magnitude decreases as the vinyl group twists out of the benzene plane. The a--rr contribution to this coupling is smaller than the -rr component.
IntroductionIn the study of long-range spin-spin coupling constants, their mechanisms, and their application to conformational problems (1-16), it has been shown ( 5 ) that 6~:.CH varies as sin2 0 in toluene derivatives. Here 0 is the angle by which the a C-H bond twists out of the benzene plane. The coupling proceeds via a u-.rr mechanism and involves the sidechain proton and the para ring proton. Similarly, S~;,CH, the coupling between the sidechain proton and the para carbon-13 nucleus also varies as sin2 0 (17,18), implying that this parameter allows an extension (8) of the J method (I) as a conformational
The kinetic parameters for hindered rotation about the C-N bond have been determined by total nuclear magnetic resonance line shape analysis for the methyl esters of N,N-dimethyldithiocarbamic acid, and N,N-dimethylmonothiocarbamic acid. A qualitative picture of the effect of substituents on the potential barrier in carbamate and thiocarbamate esters is presented.
Unlike their counterparts in anisole or toluene derivatives, the six-bond spin-spin coupling constants between para ring protons or 19F nuclei and protons or 13c nuclei in the sidechain of derivatives of benzaldehyde, acetophenone, and benzophenone can apparently contain components of opposite sign, at least for the fluorine derivatives. The u-7i components are much smaller in magnitude than in toluene derivatives, leading to very small or unobservable coupling constants. Consequently they are of limited use in conformational analysis. INDO MO FPT computations and their modifications are examined as to the reasons for the small o-TI magnitudes. Although the spin polarizability of the 2pz orbital on oxygen appears to play an important role in the transmission of nuclear spin state information, the computations do not account for a "F coupling mechanism that appears to be significant for planar conformations. On the other hand, spin-spin coupling constants over five formal bonds to meta protons are sizeable and stereospecific.TED SCHAEFER, JAMES PEELING, GLENN H. PENNER et ALBERTA LEMIRE. Can. J. Chem. 64, 1859 (1986).Contrairement leurs contreparties dans les dCrivCs de l'anisole ou du toluene, les constantes de couplage spin-spin a travers six liaisons, entre les protons du cycle en position para ou les noyaux de I9F et les protons ou les noyaux de I3C de la chaine laterale des dCrivCs du benzaldehyde, de I'acCtophCnone et de la benzophCnone peuvent apparernment contenir des composants de signes opposes, au moins dans les derivCs fluores. Les constituants a-7i ont une intensit6 infkrieure a celle des dCriv6s du toluene conduisant ainsi a des constantes de couplage tr&s faibles qu'on ne peut observer. En consequence, ces constantes sont trbs peu utiles dans l'analyse conformationnelle. On a utiIisC des calculs INDO OM FPT ainsi que leurs modifications pour dCterminer les causes des faibles intensitks des constituants cr-TI. Bien que la polarisabilitk du spin de l'orbitale 2pz de I'oxygkne semble jouer un r6le important dans la transmission de I'information sur 1'Ctat du spin nuclkaire, les calculs ne tiennent pas compte du mCcanisme de couplage du 19F qui semble Ctre important dans les conformations planes. Par ailleurs, les constantes de couplage spin-spin a travers cinq liaisons formelles par rapport aux protons en position meta sont quantifiables et stCrCospCcifiques.[Traduit par la revue]
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.