Concerning the mechanism of in toluene and its derivatives

1983 ◽  
Vol 61 (12) ◽  
pp. 2785-2789 ◽  
Author(s):  
Ted Schaefer ◽  
Reino Laatikainen
Keyword(s):  
Coupling Constants ◽  
Spin Coupling ◽  
Similar Magnitude ◽  
Conformational Preference ◽  
Conformational Studies ◽  
Methyl Group ◽  
Ring Substituent ◽  
Spin Coupling Constants ◽  
Spin Spin Coupling ◽  
Derivatives Of

On the basis of the observed five-bond spin–spin coupling constants between the α protons and the meta ring protons in the 2,6-difluoro derivatives of toluene, ethylbenzene, and cumene, it is argued that [Formula: see text] in toluene can be written as A[Formula: see text]. A and B are of the same sign and of very similar magnitude. In consequence, [Formula: see text] cannot be used to measure the conformational preference of the methyl group. However, [Formula: see text] and [Formula: see text] in α-substituted toluene derivatives will be useful in conformational studies. [Formula: see text] in toluene derivatives varies between 0.30 and 0.46 Hz and some patterns in its ring substituent dependence can be gleaned from some fifty precise values.

Sigma and Pi Electron Contributions to Long-range Spin–Spin Coupling Constants in the Methyl Derivatives of the Fluoropyridines

1972 ◽  
Vol 50 (14) ◽  
pp. 2344-2350 ◽  
Author(s):  
J. B. Rowbotham ◽  
T. Schaefer
Keyword(s):  
Nitrogen Atom ◽  
Long Range ◽  
Coupling Constants ◽  
Spin Coupling ◽  
Coupling Mechanism ◽  
Methyl Group ◽  
Spin Coupling Constants ◽  
Methyl Derivatives ◽  
Spin Spin Coupling ◽  
Derivatives Of

Seven methyl derivatives of the 3- and 4-fluoropyridines are synthesized and their p.m.r. spectra are analyzed. The nuclear spin–spin coupling constants are compared with previous results for the four methyl derivatives of 2-fluoropyridine. A model in which the nitrogen atom polarizes primarily the σ electron system of the ring, leaving the π electron contribution to the coupling mechanism relatively unaffected, qualitatively accounts for the large majority of the coupling constants. For example, the coupling over six bonds between methyl protons and a fluorine nucleus, [Formula: see text] is the same whether the fluorine atom or the methyl group is placed ortho to the nitrogen atom and is little different from its value in p-fluorotoluene. The model is consistent with significant σ electron contributions to long-range couplings over four and five bonds from methyl protons to fluorine nuclei or ring protons. Evidence is adduced for resonance structures in which fluorine conjugates with nitrogen or with ring carbon atoms. An earlier suggestion, that hyperconjugation of the methyl group with nitrogen is necessary to the interpretation of the observed couplings, is dropped. Instead, a substantial polarization of the σ electron core near C-2 and -6 is invoked but apparently does not extend appreciably beyond these atoms in the ring.

Proton spin–spin coupling constants and intramolecular hydrogen bonding in bromine derivatives of 1,3-dihydroxybenzene

1976 ◽  
Vol 54 (14) ◽  
pp. 2228-2230 ◽  
Author(s):  
Ted Schaefer ◽  
J. Brian Rowbotham
Keyword(s):  
Hydrogen Bond ◽  
Hydrogen Bonding ◽  
Proton Spin ◽  
Coupling Constants ◽  
Spin Coupling ◽  
Hydroxyl Groups ◽  
Oh Groups ◽  
Spin Coupling Constants ◽  
Spin Spin Coupling ◽  
Derivatives Of

The conformational preferences in CCl4 solution at 32 °C of the hydroxyl groups in bromine derivatives of 1,3-dihydroxybenzene are deduced from the long-range spin–spin coupling constants between hydroxyl protons and ring protons over five bonds. Two hydroxyl groups hydrogen bond to the same bromine substituent in 2-bromo-1,3-dihydroxybenzene but prefer to hydrogen bond to different bromine substituents when available, as in 2,4-dibromo-1,3-dihydroxybenzene. When the OH groups can each choose between two ortho bromine atoms, as in 2,4,6-tribromoresorcinol, they apparently do so in a very nearly statistical manner except that they avoid hydrogen bonding to the common bromine atom.

"Through-space" spin–spin coupling between thallium and fluorine in fluorophenylthallium compounds

1989 ◽  
Vol 67 (11) ◽  
pp. 1847-1850
Author(s):  
Gerald Norman Pecksen ◽  
Raymond Frederick Martin White
Keyword(s):  
Coupling Constants ◽  
Spin Coupling ◽  
Methyl Group ◽  
Fluorine Nmr ◽  
Thallium Atom ◽  
Spin Coupling Constants ◽  
Spin Spin Coupling

Thallium to fluorine spin–spin coupling constants have been measured for a number of fluoro- and trifluoro-methyl-substituted mono- and di-arylthallium derivatives. The results provide evidence of "through-space" coupling in the diaryl derivatives when the fluoro- or trifluoro-methyl group is ortho to the thallium atom. Keywords: thallium, fluorine, NMR, through-space coupling.

A proton magnetic resonance determination of the small rotational barriers about the C—Si bond in phenyl derivatives of chloro and methyl silanes

1977 ◽  
Vol 55 (3) ◽  
pp. 557-561 ◽  
Author(s):  
William J. E. Parr ◽  
Ted Schaefer
Keyword(s):  
Magnetic Resonance ◽  
Proton Magnetic Resonance ◽  
Coupling Constants ◽  
Spin Coupling ◽  
Molecular Orbital Calculations ◽  
Spin Coupling Constants ◽  
Spin Spin Coupling ◽  
Derivatives Of ◽  
Low Energy Conformations

The long-range spin–spin coupling constants between protons bonded to silicon and ring protons in C6H5SiH3, C6H5SiH2Cl, C6H5SiH2CH3, C6H5SiHCl2, and C6H5SiH(CH3)2 are determined from the proton magnetic resonance spectra of benzene solutions. A hindered rotor treatment of the barrier to internal rotation about the C—Si bond, in conjunction with the coupling constants over six bonds, allows the deduction of the low-energy conformations for C6H5SiH(CH3)2 and for C6H5SiHCl2, as well as of barriers of 1.0 ± 0.2 kcal/mol. The approach becomes less reliable for C6H5SiH2CH3 and for C6H5SiH2Cl and, particularly for the latter compound, the derived barrier is very likely an upper limit only. Ab initio molecular orbital calculations of the conformational energies are reported for C6H5SiH3, C6H5SiH2Cl, and for C6H5SiHCl2.

scholarly journals The conformational behaviour of 5Jm(F,CH) in 3,5-difluoro derivatives of toluene, ethylbenzene, and cumene. Applications

1988 ◽  
Vol 66 (6) ◽  
pp. 1490-1494 ◽  
Author(s):  
Ted Schaefer ◽  
Craig S. Takeuchi ◽  
Steven Eric Sveinson
Keyword(s):  
Large Degree ◽  
Coupling Constants ◽  
Spin Coupling ◽  
Spin Coupling Constants ◽  
Spin Spin Coupling ◽  
Derivatives Of

The CNDO/2 and INDO MO FPT values for 5Jm(F,CH3) in 3-fluoro- and 3,5-difluorotoluene are exactly reproduced by A cos2 θ + B sin2 θ + C sin2 (θ/2). Here θ is the angle by which the α C—H bond twists out of the benzene plane. Adjustment of A,B, and C to give an agreement with experiment for 3,5-difluorotoluene yields an equation, best considered empirical, which is tested by 5J(F,CH) in 3,5-difluoroethylbenzene and 3,5-difluoroisopropylbenzene. The equation reproduces 5J(F,CH) in these two compounds, the values of [Formula: see text] being derived from 6Jp(H,CH), the spin–spin coupling constants over six bonds between the α and para ring protons. 5J(F,CH) is obtained for the asymmetrical compounds, 2,3-difluorobenzylidene diacetate and 2-bromo-5-fluorobenzylidene diacetate. It is shown how 5J(F,CH) in the latter can discriminate between two conformers, each of which, on the basis of 6J(H,CH), will be characterized by a large degree of torsion about the [Formula: see text] bond.

A proton magnetic resonance estimate of the extent of intramolucular hydrogen bonding in derivatives of 2-trifluoromethyl phenol. Solvent effects

1976 ◽  
Vol 54 (14) ◽  
pp. 2243-2248 ◽  
Author(s):  
Ted Schaefer ◽  
J. Brian Rowbotham
Keyword(s):  
Free Energy ◽  
Hydrogen Bonding ◽  
Vapor Phase ◽  
Coupling Constants ◽  
Spin Coupling ◽  
Hydroxyl Group ◽  
Spin Coupling Constants ◽  
Hydroxyl Protons ◽  
Spin Spin Coupling ◽  
Derivatives Of

The long-range spin–spin coupling constants between hydroxyl protons and ring protons or fluorine nuclei are used to establish the conformer populations in iodine and brornine derivatives of 2-trifluoromethylphenol in C6H12, CCl4, and C6D6 solutions. The sequence Cl, [Formula: see text] is established for the so-called hydrogen bonding preferences of the hydroxyl group in 2,4,6-trisubstituted phenols, the corresponding free energy sequence being −ΔG = 1690, 1690 > 1300 > 1230 > 0 ± 200 cal/mol at 32 °C in CCl4 solution. An indirect estimate of the free energy differences in the vapor phase suggests the sequence −ΔG = 2800, 2800 > 2400 > 2300 > 1100 ± 300 cal/mol; the latter value meaning that the hydroxyl group in 4-bromo-2-trifluoromethylphenol prefers the CF3 group by this amount in the vapor phase. Benzene interacts preferentially with the OH group in this compound to the extent of 1300 cal/mol (ΔG), referenced to the vapor phase.

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