The conformational dependence of in some 4-fluorophenyl derivatives of methane, ethene, and cyclohexane

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. Can. J. Chem. 66, 1641 (1988). The long-range spin-spin coupling constant over six bonds between the "F nucleus and the I3C nucleus in the side chain, 6~( C ,~) , is reported for 4--F--C6H4-X-R, where X = 0 , S and R = CH3, CH2CH3, CH(CH3)? and C(CH3),. 6~(~,~) depends on sin2 0, where 0 is the angle by which the side chain twists out-of-plane about the C, -X bond. Expectation values of sin2 0 are obtained from 6~(C,F), yielding magnitudes of the apparent twofoldbarrier to rotation about the CW2-X bond. In these terms, the most stable conformation is that for 0 = 0" for all compounds, with the exception of R = C(CH3)3 and of X = S, R = CH(CH3)'; there is effectively free rotation about the C,r--S bond in isopropyl4-fluorophenyl sulfide in acetone-d6 solution. Good correlations exist between 6~( C ,~) and a number of other molecular properties, including certain differences of ionization potentials of the molecular orbitals in the ethers. In particular, the chemical shifts of C-4 are correlated with 6~( C ,~) . Because J(C,C), the coupling constant involving C-4, also depends on sin' 0, it is measured for the methyl and ethyl selenides and tellurides, as are other l3C,I3C couplings involving a I3C nucleus in the side chain. The literature values for the I3C nuclear magnetic resonance chemical shifts in alkyl phenyl selenides and tellurides can be related to 0 preferences and also allow estimates of the extrema in 'J(C,C). The resultant values of arcsin (sin' 0)"' for R = CH3 are in good agreement with estimates of 0 obtained from electron diffraction patterns, photoelectron spectra, and nuclear magnetic resonance in the nematic phase.

“…Although they are almost certain to be positive (10,(36)(37)(38), their sign has previously not been determined. Negative numbers indicate the perpendicular conformer as most stable.…”

Section: Resultsmentioning

confidence: 96%

Long-range coupling constants for α-¹³C nuclei in phenyl-X-R (X = O,S,Se,Te) derivatives. Internal rotational information

Schaefer¹,

Penner²

1988

Self Cite

“…The proportionality constant is 6~9 0 , the value (sin2 0) is derivable from the observed 6~ and this expectation value is related to the preferred conformation and the barrier to rotation about the C-X bond (2). It also appears that 6~( '~, 1 3~) and ' J ( '~c , '~F ) are a-TT couplings, the 13C nucleus being situated in the sidechain, when X = CH2, CH, or C with sp2 hybridizatlon (4,5) or when X = 0 (6). In such compounds 6~ ( 1 3~, 1 9~) is particularly easy to measure under conditions of 'H decoupling and is therefore a convenient conformational indicator.…”

Section: H(f)mentioning

confidence: 86%

The carbonyl group as a reluctant transmitter of hyperconjugative or σ–π spin–spin coupling interactions in derivatives of benzaldehyde, acetophenone, and benzophenone

Schaefer¹,

Peeling²,

Penner³

et al. 1986

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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]

“…Similar behaviour was found here for 4-fluorophenyl formate. The previous work also implied (14) that 6~ is not very sensitive to the hybridization state of the carbon atom containing the coupled 13C nucleus.…”

Section: Sto-3g Mo Structure Of Phenyl Formatementioning

confidence: 92%

“…In previous work on 4-fluorophenyl derivatives of methane, ethene, and cyclohexane, 6~('3C, 1 9~) = 6~ was established as a a-IT coupling. Such a coupling constant is proportional to sin2 8 and is therefore a valuable indicator of the conformational behaviour of the side chain (14). Again, in anisole derivatives (15), 6~ also appears as a-IT coupling constant.…”

Section: Sto-3g Mo Structure Of Phenyl Formatementioning

confidence: 99%

“…Because 'J is proportional to the mesomeric properties of the para substituent in fluorobenzene derivatives (17,18), it is therefore reasonable that 'J in 4-fluorotoluene is -241.4 Hz (14) and is -241.9Hz in 4-fluorophenyl acetate (Table I), both measured in acetone solution ('J ranges from -233 Hz in the 4-amino compound to -257 Hz in the 4-nitro derivative (18)). The 'J values in Table 1 appear consistent with only minor changes in the side-chain conformation from compound to compound.…”

Section: Sto-3g Mo Structure Of Phenyl Formatementioning

confidence: 99%

See 1 more Smart Citation

The conformational properties of some phenyl esters. Molecular orbital and nuclear magnetic resonance studies

Schaefer¹,

Penner²

1987

Self Cite

. Can. J. Chem. 65, 2175 (1987). Extensive, geometry-optimized, STO-3G MO computations on phenyl formate imply a strongly nonplanar Z conformer (C=O bond cis to the phenyl group) at ambient temperatures. The internal barrier to rotation about the C(1)-0 bond in this conformer is computed as v/k~mol-' = (-5.17 * 0.27) sin2 0 -(2.42 k 0.27) sin2 20, 0 being zero for the planar conformer; the twofold is nearly twice as large as the fourfold component. The expectation value of 0 is 58" at 300 K. The spin-spin coupling constants over six bonds between I3C and I9F nuclei in 4-fluorophenyl formate, acetate, propionate, and isobutyrate, as well as in the 2,6-dichloro-4-fluorophenyl acetate, are adduced as evidence for nonplanar conformers of these molecules. The magnitudes of these six-bond coupling constants are consistent with internal barriers to rotation about the C(1)-0 bonds, which are similar in magnitude to those given by the computations on the Z conformer of phenyl formate. The energies of the planar and nonplanar E conformers, as well as the interconversion energies for E = Z isomerization, are computed. Small amounts of the nonplanar E conformer are predicted at ambient temperatures. The I3C chemical shifts and the one-bond 13C, I9F coupling constants are consistent, respectively, with only minor variations in the conformational behavior of the ester moieties caused by the fluorine substituent and by changes in the structures of these moieties themselves.TED SCHAEFER et GLENN H. PENNER. Can. J. Chem. 65, 2175 (1987). Des calculs ktendus d'OM effectuks au niveau STO-3G sur du formate de phknyle optimisC pour la gkomktrie laissent fortement supposer que le conformkre Z non-plan (la liaison C=O est cis par rapport au groupement phknyle) est privilCgiC B la tempkrature ambiante. On a calculC que la barrikre interne B la rotation autour de la liaison C(1)-0 de ce conformtre peut &tre exprimCe par 1'Cquation V/kJ mol-I = (-5,17 * 0,27) sin2 0 -(2,42 ? 0,27) sin2 20 dans laquelle 0 : st Cgal B zCro pour le conformtre plan; la composante binaire est deux fois plus importante que la composante quaternaire. A 300 K , la valeur attendue pour 0 est de 58". On a aussi utilisC les constantes de couplage spin-spin a travers six liaisons entre les noyaux de I3C et de I9F des formate, acetate, propionate et isobutyrate de fluoro-4 phCnyle de mCme que de 1'acCtate du dichloro-2,6 fluoro-4 phtnyle comme preuve de l'existence des conformkres non-plans de ces molCcules. Les amplitudes de ces constantes de couplage B travers six liaisons sont en accord avec des barritres 1 la rotation autour des liaisons C(1)-0 qui sont semblables en amplitude B celles obtenues par calculs sur la conformkre Z du formate de phCnyle. On a aussi calculC. les Cnergies des conformtres E plan et non-plan ainsi que les Cnergies d'interconversion pour 1'isomCrisation E = Z. A la tempkrature arnbiante, on prCdit que de faibles proportions du conformkre E non-lan existeront. Le dCplacements chimiques en rksonance ! magnktique nuclkaire du I3C et les consta...