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.

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.

Nuclear Magnetic Resonance Studies of some Trimethyl Tin Carbamates

1972 ◽  
Vol 50 (9) ◽  
pp. 1386-1389 ◽  
Author(s):  
A. E. Lemire ◽  
J. C. Thompson
Keyword(s):  
Nuclear Magnetic Resonance ◽  
Magnetic Resonance ◽  
Low Temperature ◽  
Chloroform Solution ◽  
Activation Parameters ◽  
Hindered Rotation ◽  
Magnetic Resonance Studies ◽  
Hexane Solution ◽  
Nuclear Magnetic

The low temperature n.m.r. spectra of some trimethylstannyl esters of N,N-dimethylcarbamic acid, N,N-dimethyldithiocarbamic acid, and N,N-dimethylmonothiocarbamic acid have been examined. Activation parameters for hindered rotation about the C—N bond in the trimethylstannyl ester of N,N-dimethylmonothiocarbamic acid have been determined to be: in hexane solution, Ea = 17.0 + 0.5 kcal/mol, [Formula: see text][Formula: see text][Formula: see text] in chloroform solution, Ea = 19.9 ± 0.5 kcal/mol, [Formula: see text][Formula: see text][Formula: see text]not available

The kinetics of proton and deuteron transfer from 1-(4-nitrophenyl)-1-nitroethane to1,8-diazabicyclo[5.4.0]undec-7-ene in aprotic solvents

1982 ◽  
Vol 60 (13) ◽  
pp. 1692-1695 ◽  
Author(s):  
Kenneth T. Leffek ◽  
Przemyslaw Pruszynski
Keyword(s):  
Energy Barrier ◽  
Reaction Rate ◽  
Significant Contribution ◽  
Transfer Reaction ◽  
Isotope Effects ◽  
Activation Parameters ◽  
Proton Transfer Reaction ◽  
Potential Energy Barrier ◽  
Deuteron Transfer ◽  
Kinetics Of

1-(4-Nitrophenyl)-1-nitroethane reacts with the base1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) in both acetonitrile and toluene solvents in a normal second-order proton-transfer reaction, in contrast to its behaviour with the base 2,7-dimethoxy-1,8-bis(dimethylamino)naphthalene in acetonitrile.The primary isotope effect, kHlkD = 12.0 at 25° in toluene is very similar to that observed by other workers for the reaction of 4-nitrophenylnitromethane with DBU under the same conditions. In acetonitrile solvent a kHlkD ratio of 7.8 was found at 25 °C. The isotope effects on the activation parameters for the reaction in both solvents indicate that tunnelling of the proton through the potential energy barrier makes a significant contribution to the reaction rate.

Applications of Proton Magnetic Resonance to Rotational Isomerism in Halotoluene Derivatives. V. α,α,α′,α′,2,3,5,6-Octachloro-p-xylene, Semiempirical Barrier Calculations

1971 ◽  
Vol 49 (5) ◽  
pp. 789-795 ◽  
Author(s):  
B. H. Barber ◽  
T. Schaefer
Keyword(s):  
Shape Analysis ◽  
Proton Magnetic Resonance ◽  
Activation Parameters ◽  
Rotational Isomerism ◽  
Standard Errors ◽  
Energy Functions ◽  
Potential Energy Functions ◽  
Rate Dependent ◽  
Line Shape Analysis ◽  
Cis And Trans

In toluene-d8 solution the p.m.r. spectrum of α,α,α′,α′,2,3,5,6-octachloro-p-xylene at temperatures below −25 °C consists of two sharp peaks corresponding to the two conformations in which the methine protons lie cis and trans to each other in the plane of the aromatic ring. The barrier to rotation of the dichloromethyl groups is derived from a line-shape analysis of the rate-dependent spectra using the computer program ABXFIT. The activation parameters are EA = 13.6 ± 0.4 kcal/mol, log A = 11.3 ± 0.3, ΔH‡ = 13.1 ± 0.4 kcal/mol, ΔS‡ = −7.3 ± 1.3 e.u., ΔG‡ = 15.4 kcal/mol at 286 °K. The quoted errors are standard errors from least squares fits. These parameters are compared to the extensive data known for α,α,2,4,6-pentachlorotoluene. A series of barrier calculations, based on modified Buckingham and on van der Waals potential energy functions, are discussed with reference to various halotoluenes.

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.

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