Proton Magnetic Resonance Studies of Rotational Isomerism in Halotoluene Derivatives. IX. Rotational Barriers and Conformational Energy Differences in α,α,α′,α′,α″,α″, 2,4,6-Nonachloromesitylene

1973 ◽  
Vol 51 (13) ◽  
pp. 2110-2117 ◽  
Author(s):  
J. Peeling ◽  
B. W. Goodwin ◽  
T. Schaefer ◽  
J. B. Rowbotham
Keyword(s):  
Magnetic Resonance ◽  
Carbon Disulfide ◽  
Proton Magnetic Resonance ◽  
Methylene Chloride ◽  
Activation Parameters ◽  
Rotational Isomerism ◽  
Conformational Energy ◽  
The Other ◽  
Free Energies ◽  
Magnetic Resonance Studies

The activation parameters for the rotation of the dichloromethyl groups in the two conformations of α,α,α′,α′,α″,α″,2,4,6-nonachloromesitylene are reported for solutions in toluene-d8 and in methylene chloride. In addition, free energies of activation are given for solutions in bromochloromethane, tri-chloroethylene, and in carbon disulfide. The free energies of activation are lower in the toluene solution than in the other solutions. The entropies of activation are near zero, perhaps slightly negative. The symmetrical conformation is more stable than the unsymmetrical one in all the solvents.

Ribo-2′-deoxyribo hybrid dinucleoside monophosphates. Proton magnetic resonance studies of 3′,5′- and 2′,5′-uridylyl-2′-deoxythymidine

1978 ◽  
Vol 56 (4) ◽  
pp. 522-529 ◽  
Author(s):  
James Peeling ◽  
Frank E. Hruska ◽  
Peter C. Loewen
Keyword(s):  
Aqueous Solution ◽  
Magnetic Resonance ◽  
Proton Magnetic Resonance ◽  
Molecular Conformation ◽  
The Other ◽  
Magnetic Resonance Studies ◽  
Dinucleoside Monophosphates ◽  
Standard Techniques

This 1Hmr study initiates our examination of the conformations of dinucleoside monophosphates possessing ribo 2′- or 3′-nucleotidyl units linked to 5′-nucleotidyl units possessing the 2′-deoxyribo sugar. The syntheses of uridylyl-3′,5′-2′-deoxythymidine (3′,5′-UpdT) and its 2′,5′-isomer, 2′,5′-UpdT, were carried out with standard techniques. The 1Hmr data were obtained at frequencies up to 270 MHz and used to derive the dominant conformation of the dimers in aqueous solution. Comparison with data for the component mononucleotides reveals that dimerization does not lead to drastic changes in the molecular conformation. Literature data for dimers possessing only the ribo sugar (3′,5′-UpU) and the 2′-deoxyribo sugar (3′,5′-d(TpT)) are also presented. The results indicate that, at least for our dipyrimidine dimers, the molecular conformation of a particular fragment is not critically dependent on the nature (ribo or 2′-deoxyribo) of the other nucleotide unit.

Proton Magnetic Resonance Studies of Rotational Isomerism in Halotoluene Derivatives. XI. Experimental and Theoretical Barriers to Rotation in α,α,2,4,6-Pentabromo-, α,α-Dibromo-2,4,6-trichloro-, and α,α-Dibromo-2,6-dichlorotoluene

1974 ◽  
Vol 52 (5) ◽  
pp. 849-854 ◽  
Author(s):  
James Peeling ◽  
Ludger Ernst ◽  
Ted Schaefer
Keyword(s):  
Energy Barrier ◽  
Proton Magnetic Resonance ◽  
Geometry Optimization ◽  
Activation Parameters ◽  
Rotational Isomerism ◽  
Partial Geometry ◽  
Single Bond ◽  
Hindered Rotation ◽  
Magnetic Resonance Studies ◽  
Potential Energy Barrier

Using rate constants determined from an analysis of the p.m.r. lineshape of the ring protons as a function of temperature, the activation parameters for the hindered rotation about the sp2–sp3 carbon–carbon single bond in α,α,2,4,6-pentabromotoluene dissolved in perchlorobutadiene are determined. Values of ΔG≠ for the analogous hindered rotations in α,α-dibromo-2,6-dichlorotoluene in toluene-d8, and for α,α-dibromo-2,4,6-trichlorotoluene in solutions of toluene-d8 and methylcyclohexane are also given. The results are compared with semiempirical potential energy barrier calculations employing partial geometry optimization. The agreement is satisfactory.

scholarly journals Proton Magnetic Resonance Studies of Rotational Isomerism in Halotoluene Derivatives. VI. Rotational Barrier and Conformational Energy Differences in α,α,2,3,6-Pentachlorotoluene

1971 ◽  
Vol 49 (7) ◽  
pp. 1085-1091 ◽  
Author(s):  
M. A. H. Stewart ◽  
T. Schaefer ◽  
H. M. Hutton ◽  
C. M. Wong
Keyword(s):  
Free Energy ◽  
Magnetic Resonance ◽  
Proton Magnetic Resonance ◽  
Energy Difference ◽  
Rotational Isomerism ◽  
Free Energy Difference ◽  
Magnetic Resonance Studies ◽  
Shape Fitting ◽  
Carbon Carbon Bond ◽  
Entropy Of Activation

Rotation by π radians about the sp2–sp3 carbon–carbon bond of the dichloromethyl group in α,α,2,3,6-pentachlorotoluene interconverts the two conformations which are characterized by coplanarity of the aromatic ring with the C—H bond of the sidechain. The ring protons undergo effective nonmutual exchange during this rotation. The rate of rotation as a function of temperature is extracted by line-shape fitting of the ring p.m.r. spectra. The rotation is characterized by a negative entropy of activation. The free energy difference between the two conformations is 50 ± 20 cal/mol at 240 °K.

Application of Proton Magnetic Resonance to Rotational Isomerism in Halotoluenes. VII. Barriers to Rotation and Conformational Preferences in α,α,α′,α′,2,4,5,6-Octachloro-m-xylene

1971 ◽  
Vol 49 (9) ◽  
pp. 1489-1496 ◽  
Author(s):  
J. Peeling ◽  
B. W. Goodwin ◽  
T. Schaefer ◽  
C. Wong
Keyword(s):  
Magnetic Resonance ◽  
Low Temperatures ◽  
Proton Magnetic Resonance ◽  
Resonance Spectrum ◽  
Activation Parameters ◽  
Rotational Isomerism ◽  
Proton Resonance ◽  
Temperature Dependent ◽  
Conformational Preferences ◽  
Carbon Carbon Bonds

At low temperatures α,α,α′,α′,2,4,5,6-octachloro-m-xylene exists as three conformers in solution and the proton resonance spectrum can be assigned by comparison to the spectrum of α,α,α′,α′,2,4,6-heptachloro-m-xylene. The temperature dependent spectra of the former in toluene-d8 solution are fitted to computed spectra based on a set of coupled Bloch equations. The activation parameters for the rotation about the sp2–sp3 carbon–carbon bonds are Ea = 14.93 ± 0.13 kcal/mol, log A = 12.45 ± 0.09, ΔH≠ = 14.30 ± 0.14 kcal/mol, ΔS≠ = −2.0 ± 0.5 e.u., ΔG≠ = 14.9 ± 0.05 kcal/mol at 298 °K. The quoted errors are standard deviations.

Proton magnetic resonance studies of substituted 3,5-dimethylbenzenes: Influence of substituents on the methyl proton chemical shifts

1985 ◽  
Vol 23 (7) ◽  
pp. 521-523 ◽  
Author(s):  
Fadhil S. Kamounah ◽  
Ikbal. S. Al-Sheibani ◽  
Nazar L. Shibaldain ◽  
Salman R. Salman
Keyword(s):  
Magnetic Resonance ◽  
Proton Magnetic Resonance ◽  
Chemical Shifts ◽  
Methyl Proton ◽  
Magnetic Resonance Studies ◽  
Proton Chemical Shifts ◽  
Influence Of Substituents
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