Activation energy for methoxy group relaxation

1978 ◽  
Vol 56 (13) ◽  
pp. 1800-1803 ◽  
Author(s):  
M. A. Mazid ◽  
J. P. Shukla ◽  
S. Walker
Keyword(s):  
Activation Energy ◽  
Aromatic Ring ◽  
Energy Barrier ◽  
Methoxy Group ◽  
Steric Effects ◽  
Molecular Process ◽  
Dielectric Absorption ◽  
Polystyrene Matrix ◽  
Absorption Studies ◽  
Methoxy Groups

Dielectric absorption studies hove been made on seven compounds which hove methoxy groups attached to an aromatic ring and on some related rigid molecules. The solutes hove been dispersed in a polystyrene matrix. In some cases, the absorptions due to the group and the molecular process have been separated completely, and this has permitted the estimation of more accurate Eyring enthalpies of activation for methoxy group relaxation. These values hove been compared with those in the literature and, on the whole, it would seem likely that the energy barrier to group relaxation is small and of the order of 10 kJ mol−1 for cases where there is no mutual conjugation or steric effects.

Acetyl group relaxation in four aromatic ketones

1977 ◽  
Vol 55 (4) ◽  
pp. 583-587 ◽  
Author(s):  
C. K. McLellan ◽  
S. Walker
Keyword(s):  
Energy Barrier ◽  
Benzene Solution ◽  
Far Infrared ◽  
Aromatic Ketones ◽  
Dielectric Absorption ◽  
Nmr Data ◽  
Polystyrene Matrix ◽  
Absorption Studies ◽  
Acetyl Group ◽  
Experimental Errors

Dielectric absorption studies have been made of group relaxation of acetophenone, 1,4-diacetylbenzene,4-acetylbiphenyl, and 2-acetylfluorene in a polystyrene matrix, and two rigid molecules have also been examined. For acetyl group relaxation in acetophenone, ΔHE = 29.6 ± 0.6 kJ mol−1 and ΔSE = 26 ± 3 JK−1 mol−1 (95% confidence intervals). The energy barrier for acetyl group relaxation is of the same order as that obtained from nmr data. Our results for acetophenone differ appreciably from the values reported for the gaseous phase by the far-infrared approach and from those by the dielectric absorption of aromatic ketones in benzene solution. The enthalpies of activation for acetyl group relaxation in 1,4-diacetylbenzene, 4-acetylbiphenyl, and 2-acetylfluorene are 29, 30, and 32 kJ mol−1, respectively, which values are identical within their experimental errors. These three substances have also been examined as pure solids, yielding enthalpies of activation for acetyl group relaxation of a similar magnitude.

Dielectric absorption of a few aromatic amines in a polystyrene matrix

1981 ◽  
Vol 59 (2) ◽  
pp. 232-237 ◽  
Author(s):  
S. P. Tay ◽  
S. Walker
Keyword(s):  
Aromatic Amines ◽  
Amino Group ◽  
Torsional Barrier ◽  
Dielectric Absorption ◽  
Enthalpy Of Activation ◽  
Polystyrene Matrix ◽  
Absorption Studies ◽  
Intramolecular Process ◽  
Nitrogen Bond

Dielectric absorption studies in the ranges 77–373 K and 102–107 Hz have been made on aniline, poly(4-aminostyrene), p-toluidine, 4-chloroaniline, 1-aminopyrene, and 4-aminobiphenyl in a polystyrene matrix. An intramolecular process has been observed in poly(4-aminostyrene) and 1-aminopyrene with enthalpies of activation of 25 and 26 kJ mol−1 respectively. The enthalpy of activation of the intramolecular process is of the same order as that found for the torsional barrier in aniline and 4-fluoroaniline. The most likely candidate for the intramolecular process appears to be rotation of the amino group about the carbon–nitrogen bond.

Molecular and group relaxation studies in parasubstituted benzenes in various media

1979 ◽  
Vol 57 (21) ◽  
pp. 2843-2847 ◽  
Author(s):  
J. Crossley ◽  
J. P. Shukla ◽  
S. P. Tay ◽  
M. S. Walker ◽  
S. Walker
Keyword(s):  
Group Process ◽  
Activation Enthalpy ◽  
Small Contribution ◽  
Molecular Processes ◽  
Dielectric Absorption ◽  
Polystyrene Matrix ◽  
Absorption Studies ◽  
The Matrix ◽  
Intramolecular Motion ◽  
Relaxation Studies

Dielectric absorption studies have been made on molecules of the type [Formula: see text][Formula: see text] where X—Y is —OCH3, —COCH3, —CH2Cl, and —CH2CN. The molecules have been dissolved in (a) a polystyrene matrix and (b) in the viscous liquid o-terphenyl. For 1,4-dimethoxybenzene the group process dominates in both media, and estimates have been made of the activation enthalpy for group relaxation. For 1,4-dimethoxybenzene in benzene, decalin, Nujol, and o-terphenyl solutions, however, there appears to be a small contribution from the molecular process. No such contribution was detected in the matrix studies on this solute molecule. For 1,4-diacetylbenzene, 1,4-bis(chloromethyl)benzene, and 1,4-bis(cyanomethyl)benzene in o-terphenyl solution it again seems that the group and molecular processes overlap. However, in polystyrene the enthalpies of activation which emerge are characteristic of the group relaxation process as determined by the nmr approach. Altogether, the work illustrates that o-terphenyl may be a useful solvent in helping to decide whether a solute has contributions from an intramolecular motion.

scholarly journals The role of methoxy substituents in regulating the activity of selenides that serve as spirodioxyselenurane precursors and glutathione peroxidase mimetics

2016 ◽  
Vol 94 (4) ◽  
pp. 305-311 ◽  
Author(s):  
David J. Press ◽  
Thomas G. Back
Keyword(s):  
Hydrogen Peroxide ◽  
Catalytic Activity ◽  
Glutathione Peroxidase ◽  
Methoxy Group ◽  
Steric Effects ◽  
Disulfide Formation ◽  
Excess Hydrogen ◽  
Meta Position ◽  
Methoxy Groups

A series of o-(hydroxymethyl)phenyl selenides containing single or multiple methoxy substituents was synthesized, and the rate at which each compound catalyzed the oxidation of benzyl thiol to its disulfide with excess hydrogen peroxide was measured. This assay provided the means for comparing the relative abilities of the selenides to mimic the antioxidant selenoenzyme glutathione peroxidase. The mechanism for catalytic activity involves oxidation of the selenides to their corresponding selenoxides with hydrogen peroxide, cyclization to spirodioxyselenuranes, followed by reduction with two equivalents of thiol to regenerate the original selenide with concomitant disulfide formation. A single p-methoxy group on each aryl moiety afforded the highest catalytic activity, while methoxy groups in the meta position had little effect compared to the unsubstituted selenide, and o-methoxy groups suppressed activity. The installation of multiple methoxy groups on each aryl moiety provided no improvement. These results can be rationalized on the basis of dominating mesomeric and steric effects of the p- and o-substituents, respectively.

Proximity and conformation effects in 29Si and 13C NMR spectra of di-ortho substituted trimethylsiloxybenzenes

1990 ◽  
Vol 55 (8) ◽  
pp. 2027-2032 ◽  
Author(s):  
Jan Schraml ◽  
Robert Brežný ◽  
Jan Čermák
Keyword(s):  
Benzene Ring ◽  
13C Nmr ◽  
Proximity Effect ◽  
Nmr Spectra ◽  
Methoxy Group ◽  
Chemical Shifts ◽  
Proximity Effects ◽  
13C Nmr Spectra ◽  
Methoxy Groups ◽  
C Nmr

29Si and 13C NMR spectra of five 4-substituted 2,6-dimethoxytrimethylsiloxybenzenes were studied with the aim to elucidate the nature of the deshielding proximity effects observed in the spectra of ortho substituted trimethylsiloxybenzenes. The sensitivity of 29Si chemical shifts to para substitution is in the studied compounds essentially the same as in mono ortho methoxytrimethylsiloxybenzenes. The deshielding proximity effect of the ìsecondî methoxy group is somewhat smaller than that of the ìfirstî group. The present results indicate that the two methoxy groups assume coplanar conformations with the benzene ring and are turned away from the trimethylsiloxy group which is not in the benzene plane. It is argued that in mono ortho methoxytrimethylsiloxybenzenes the two substituent groups adopt the same conformations as in the compounds studied here.

scholarly journals The Activating Effect of Strong Acid for Pd-Catalyzed Directed C–H Activation by Concerted Metalation-Deprotonation Mechanism

Molecules ◽  
2021 ◽  
Vol 26 (13) ◽  
pp. 4083
Author(s):  
Heming Jiang ◽  
Tian-Yu Sun
Keyword(s):  
Activation Energy ◽  
Dft Calculations ◽  
Energy Barrier ◽  
Computational Study ◽  
Strong Acid ◽  
Activation Energy Barrier ◽  
Pd Catalysts ◽  
Acid Ligand ◽  
Strong Acids

A computational study on the origin of the activating effect for Pd-catalyzed directed C–H activation by the concerted metalation-deprotonation (CMD) mechanism is conducted. DFT calculations indicate that strong acids can make Pd catalysts coordinate with directing groups (DGs) of the substrates more strongly and lower the C–H activation energy barrier. For the CMD mechanism, the electrophilicity of the Pd center and the basicity of the corresponding acid ligand for deprotonating the C–H bond are vital to the overall C–H activation energy barrier. Furthermore, this rule might disclose the role of some additives for C–H activation.

The molten helix: low activation energy barrier for helix-helix transitions

Peptides ◽  
1994 ◽  
pp. 896-898
Author(s):  
G. R. Marshall ◽  
M. L. Smythe ◽  
S. E. Huston ◽  
R. D. Bindal
Keyword(s):  
Activation Energy ◽  
Energy Barrier ◽  
Activation Energy Barrier

scholarly journals Effect of Two-Step Austempering Process on Transformation Kinetics of Nanostructured Bainitic Steel

Materials ◽  
2019 ◽  
Vol 12 (1) ◽  
pp. 166 ◽  
Author(s):  
Chunhe Chu ◽  
Yuman Qin ◽  
Xuemei Li ◽  
Zhinan Yang ◽  
Fucheng Zhang ◽  
...  
Keyword(s):  
Activation Energy ◽  
Energy Barrier ◽  
Bainitic Ferrite ◽  
Transformation Process ◽  
Transformation Rate ◽  
Bainitic Steel ◽  
Transformation Kinetics ◽  
Activation Energy Barrier ◽  
Quenching Process ◽  
Kinetics Of

The two-step austempering process has been reported to be an effective method to accelerate the bainitic transformation process by introducing martensite (Q-M-B). However, in this study, it was found that the Q-M-B process reduced the incubation time, but the transformation duration remained nearly unchanged. The notably reduced activation energy barrier for nucleation of bainitic ferrite on the preformed martensite should be responsible for the reduced duration time of the Q-M-B process. A process that both of the two steps were above, Ms (Q-B-B), has been demonstrated to increase transformation rate and improve the amount of bainitic ferrite, which probably results from the additional hysteresis free energy provided by the first quenching process.

Sign in / Sign up

Export Citation Format

Share Document