Long-Range Spin-Spin Coupling in Alkylfluorobenzenes. The Stereochemical Requirements for Coupling of Fluorine and Hydrogen Separated by Five Bonds<sup>1</sup>

The through-space or proximate nuclear spin-spin coupling constant, 'J(F,CH3) = 'J, between methyl protons and ring fluorine nuclei in alkylfluorobenzenes is postulated as 'J/HZ = A(cos4 0) + sin^ (0/2)), 0 being the torsional angle for the C, 2-C, , bond. A and B are obtained from the known internal rotational behaviour in 2,6-difluoroethylbenzene and the corresponding cumene derivative. The parameterization is tested on the observed ' 5 in derivatives of 2,4,6-tri-tert-butyl-and 2,4,6-tri-isopropyl-fluorobenzene, in 2-chloro-6-fluoroisopropylbenzene, 2,6-difluoro-a-methylstyrene, and N-methyl-8-fluoroquinolinium halides. A prediction is made for ' J in 2,6-difluoro-tert-butylbenzene. It appears that the present parameterization allows the derivation of approximate torsional potentials from proximate couplings, for example in a , adimethyl-2,6-difluorobenzyl alcohol.

supporting

confidence: 80%

mentioning

confidence: 90%

See 1 more Smart Citation

The proximate spin–spin coupling, ⁵J(F,CH₃), as a quantitative conformational indicator in alkylfluorobenzenes and related compounds

Schaefer¹,

Sebastian²,

Penner³

et al. 1986

“…CAN J CHEM VOL 51,1973 ly and a through-space (51,52) 8, 14, 47, 55). Furthermore, agreement is emphasized by the very small the difference between coupling calculated at the CND0/2 estimates for 7J37, ,J38* and ,J47: an INDO and CNDO/2 levels can be an approxi-all-trans configuration, as for H, and H, must mate measure of the role of the n-electrons in exist for a G electron i n v o~~e m e n t in 6~.…”

Section: 58mentioning

confidence: 99%

Experimental and Theoretical Estimates of Sigma and Pi Electron Contributions to Long-range Spin–Spin Coupling Constants in Coumarin and its Methyl Derivatives

Rowbotham¹,

Schaefer²

1973

A full analysis of the p.m.r. spectra of coumarin and five of its methyl derivatives allows the assignment of o and n electron components to the inter-ring proton-proton spin-spin coupling constants. Magnitudes of up to 0.3 Hz are observed over a coupling path of eight bonds. The o and n components can be combined on the basis of the appropriate valence bond structures to predict closely the observed parameters in naphthalene, suggesting only weak participation of the ether oxygen atom in the 7c electron system of coumarin. Calculations of the inter-ring coupling constants at the INDO and CNDO/2 ievels of molecular orbital theory are helpful in the interpretation of the observed data, the agreement with experiment being quantitative in many instances. Comparison of the observed coupling constants in coumarin and styrene with the ZNDO results for an assumed planar form of the latter suggests that styrene exists in a nonplanar conformation.Une Ctude complete des spectres r.m.n, du coumarin et de cinq de ses derives methyles permet I'attribution des composantes o et .rr aux constantes de couplages deduites spin-spin des interactions protonproton. Des amplitudes allant jusqu'a 0.3 Hz sont observees pour une zone de couplage inciuant huit liaisons. Les composantes o et n peuvent ktre combinCes en se basant sur les structures appropriees (obtenues par la theorie du lien de valence) pour prCdire avec precision les parametres observes pour le naphtaline suggerant une faibie participation seulement de I'atome d'oxygene du groupe ether dans le systemc d'Clectrons n du coumarin. Les calculs des constantes de couplage deduites aux niveaux INDO et CNDO/2 de la theorie des orbitales moleculaires sont utiles dans l'interpretation des donnees obtenues, l'accord avec les donnees experimentales etant tres souvent quantitatif. La comparaison des constantes de couplage observees pour le coumarin et le styrene avec les rPsultats TNDO pour une structure supposee planaire pour ce dernier suggere que le styrene existe dans une conformation non-planaire.[Traduit par Ie journal]Can. J . Chem.. 51. 953 (1973) The dependence on conformation and on bonding situation of the spin-spin coupling constants between protons on a side-chain and ring protons in benzene and pyridine derivatives has been extensively studied by experimental and theoretical methods (1)(2)(3)(4)(5)(6)(7)(8)(9)(10)(11)(12)(13)(14)(15)(16)(17)(18)(19)(20). Less extensive investigations of inter-ring long-range proton coupling constants in fused aromatic systems (21, 22) have been reported for naphthalene (23), acridine (24), isoquinoline (25), and for several coumarin derivatives (26). The use of the 'methyl group replacement technique' (27-29) and of approximate molecular orbital theory at the INDO and CND8/2 levels (30) is enlightening in the elucidation of the coupling mechanisms operating in the single ring compounds (7, 8, 14). In this paper we apply these methods to the interring coupling constants in coumarin (I). Com- 'Postdoctoral Fellow, 1970 parisons are made...

“…The observed 5-bond F-H coupling is reminiscent of the stereospecific 5-bond F-F coupling reported by Weigert and Roberts (17), which was ascribed to "through space" or "direct" coupling, a mechanism which has substantial experimental support (28,29). Although Weigert has urged restraint in the correlation of structure with "through space" coupling constants (19), the observed couplings 'JFBeH = 4.3 Hz in 6, 5JFB-H = 4.7 Hz in 7 are very nearly those expected for r(FB-H) = 2.5A using Myhre's curve which relates JHF to r(F-H) (29).…”

Section: Stereochemistry Of the Irldiuitl~raljuorilzementioning

confidence: 52%

Magnetic non-equivalence of fluorine atoms of a trifluoromethyl group

Khan¹,

Tavares²,

Rauk³

1982

The three fluorine atoms of N,N-dialkyl amide and chloride derivatives of 2-methoxy-2-phenyl-3,3,3-trifluoropropanoic acid are magnetically nonequivalent below –60 °C and exhibit a clear ABC coupling pattern from which chemical shifts and geminal coupling constants are readily derived. The chemical shifts span a range of about 10 ppm and the geminal coupling constants average about 110 Hz. A five bond coupling of about 5 Hz is observed between the hydrogens of the methoxy group and one of the nonequivalent fluorine atoms of the trifluoromethyl group. Barriers to the hindered rotation about the single bond to the trifluoromethyl group are in the range 36–46 kJ/mol.