The Effect of α-Substituents on Benzylic Spin–Spin Coupling Constants. Derivatives of 2,6-Dichlorotoluene

1971 ◽  
Vol 49 (11) ◽  
pp. 1818-1822 ◽  
Author(s):  
A. F. Janzen ◽  
T. Schaefer
Keyword(s):  
Potential Energy ◽  
Coupling Constants ◽  
Spin Coupling ◽  
Energy Calculations ◽  
Potential Energy Calculations ◽  
Electron Contribution ◽  
Spin Coupling Constants ◽  
Spin Spin Coupling ◽  
Derivatives Of

Spin–spin coupling constants between methylene protons and ring protons over six and five bonds, [Formula: see text], are reported for α-substituted derivatives of 2,6-dichlorotoluene. Potential energy calculations suggest that the C—X bond of the CH2X group lies perpendicular to the plane of the ring. The six-bond coupling is dominated by a π electron mechanism and depends on the conformation of CH2X with respect to the aromatic plane and on the electronegativity of the substituent, X. The five-bond coupling is independent of X and it is suggested that the decrease in the π electron contribution to this coupling caused by the increasing electronegativity of X, is compensated by a corresponding increase in the sigma electron contribution.

Proton Magnetic Resonance of the Methyl Derivatives of 2-Fluoropyridine. Sigma and Pi Electron Contributions to Spin–Spin Coupling Constants

1971 ◽  
Vol 49 (11) ◽  
pp. 1799-1803 ◽  
Author(s):  
J. B. Rowbotham ◽  
R. Wasylishen ◽  
T. Schaefer
Keyword(s):  
Nitrogen Atom ◽  
Proton Magnetic Resonance ◽  
Electron System ◽  
Coupling Constants ◽  
Spin Coupling ◽  
Electron Contribution ◽  
Spin Coupling Constants ◽  
Methyl Derivatives ◽  
Spin Spin Coupling ◽  
Derivatives Of

The p.m.r. spectra of the methyl derivatives of 2-fluoropyridine are analyzed. The signs and magnitudes of the long-range spin–spin coupling constants between the methyl protons and the ring protons and between the methyl protons and fluorine are consistent with a model in which the nitrogen atom polarizes the sigma electron system but leaves the pi electron contribution to the coupling constants relatively unchanged. There are dramatic changes in the ring proton – fluorine couplings while the couplings involving the methyl protons vary little from those in the corresponding toluene derivatives. Thus the coupling over six bonds between fluorine and methyl protons is 1.25 ± 0.03 Hz in 2-fluoro-5-methyl pyridine compared to 1.15 ± 0.02 Hz in p-fluorotoluene.

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.

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.

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.

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