A Three-Dimensional Energy Surface for the Conformational Inversion of Cyclohexane

Leventis, Nicholas; Hanna, Samir B.; Sotiriou‐Leventis, Chariklia

doi:10.1021/ed074p813

Cited by 37 publications

(24 citation statements)

References 1 publication

Supporting

Mentioning

Contrasting

Order By: Relevance

“…This ratio changes from 1:10 to 10:1 within a temperature interval of ~50K, which gives an estimate for the enthalpy of the transition 10–11 Kcal/mol. Similar values were observed for a number of cases of conformational interconversion of simple organic molecules, see, for example 18,27. HF ESR helps to understand the nature of the transition.…”

Section: Resultssupporting

confidence: 74%

Multifrequency ESR study of spin-labeled molecules in inclusion compounds with cyclodextrins

Dzikovski

Tipikin

Лившиц

et al. 2009

Phys. Chem. Chem. Phys.

View full text Add to dashboard Cite

The molecular dynamics of spin labeled compounds included into the solid phase of cyclodextrins (CD) has been studied using conventional (X-Band) ESR at 9 GHz and high-field high-frequency (HFHF) ESR at 240 and 170GHz. The patterns of axial rotation at these higher frequencies are clear just by inspection of the spectrum, unlike the case for 9 GHz spectra. That is HFHF ESR is sensitive to molecular motion about the diffusion axis collinear with the X, Y or Z-direction of the magnetic g-and A-tensors of the nitroxide moiety (referred to respectively as X, Y or Z-rotation). For doxyl stearic acids (Z-rotation) and TEMPOyl caprylate (X-rotation) included in β-and γ-CD's we were able to determine the rate of molecular motion and the corresponding potential barriers. We emphasize that determining the rate of Z-rotation by ESR is feasible only using HFHF ESR. For the X-rotation case we suggest that the motion of the nitroxide moiety consists of fast small-angle librations about the magnetic X-axis superimposed by rotational diffusion about the same axis. The potential barrier of 1.7 Kcal/mol for this rotational diffusion is unusually low. A fascinating feature of TEMPO derivatives included in β-CD is the detectable molecular motion at temperatures below 77K. For the other CD -spin probe systems, we used multifrequency analysis to assign the conformations of spin-labeled molecules. A dramatic spectral change for 16-sasl in β-and γ-CDs at 260K corresponds to a tilting of the position of the nitroxide moiety on the rotating molecule relative to the long diffusion axis, while for TEMPO derivatives in γ cyclodextrin below 200K, we observe a rapid transition from fast to very slow rotational motion. More complex features are best studied by means of multifrequency ESR experiments. The visual clarity and the simplicity of analysis of the ESR spectra shown in this work should provide a benchmark for future studies of molecular motion by HFHF ESR.

show abstract

Section: Resultssupporting

confidence: 74%

Multifrequency ESR study of spin-labeled molecules in inclusion compounds with cyclodextrins

Dzikovski

Tipikin

Лившиц

et al. 2009

Phys. Chem. Chem. Phys.

View full text Add to dashboard Cite

show abstract

“…A number of transition structures were also located connecting the respective minima. For the chair‐to‐chair interconversion of cyclohexane two reaction pathways are mentioned in the literature 27. Both chairs convert to the same twist conformer via a half‐chair or a sofa transition structure.…”

Section: Resultsmentioning

confidence: 99%

Chair, Boat and Twist Conformation of Dodecamethylcyclohexasilane and Undecamethylcyclohexasilane: A Combined DFT and Raman Spectroscopic Study

et al. 2006

View full text Add to dashboard Cite

The conformations of dodecamethylcyclohexasilane Si6Me12 and undecamethylcyclohexasilane Si6Me11H have been investigated by ab initio calculations employing the B3LYP density functional with a 6-31+G(d) basis set. Local minima as well as transition structures were calculated with imposed symmetry constraints. For Si6Me12, three unique minima, which correspond to the chair, twist and boat conformations were located with relative zero-point-vibration-corrected energies of 0.0, 7.8 and 11.4 kJ mol(-1). A half-chair conformation with four coplanar silicon atoms connects the chair and twisted minima via an energy barrier of 16.0 and 8.2 kJ mol(-1), respectively. A second transition structure with a barrier of 3.9/0.3 kJ mol(-1) connects the twist with the boat structure. Solution Raman spectra of Si6(CH3)12 and Si6(CD3)12 fully corroborate these results. Below -40 degrees C, the symmetric SiSi ring breathing vibration is a single line, which develops a shoulder (originating from the twist conformer) at longer wavelengths whose intensity increases with increasing temperature. From a Van't Hoff plot, the chair/twist enthalpy difference is 6.6+/-1.5 kJ mol(-1) for Si6(CH3)12 and 6.0+/-1.5 kJ mol(-1) for Si6(CD3)12, which is in reasonable agreement with the ab initio results. Due to the low barrier, the boat conformation cannot be observed, because either the lowest torsional vibration level lies above it or a rapid interconversion between the twist and boat conformations occurs, resulting in averaged Raman spectra. For Si6Me11H, six local minima were located. The chair with the hydrogen atom in the axial position (axial chair) is the global minimum, followed by the equatorial chair (+1.9 kJ mol(-1)) and the three twist conformers (+5.3, +8.0 and +8.1 kJ mol(-1)). The highest local minimum (+11.9 kJ mol(-1)) is a C(s) symmetric boat with the hydrogen atom in the equatorial position. Two possible pathways for the chair-to-chair interconversion with barriers of 13.9 and 14.5 kJ mol(-1) have been investigated. The solution Raman spectra in the SiSi ring breathing region clearly show that below -50 degrees C only the axial and equatorial chairs are present, with an experimental deltaH-value of 0.46 kJ mol(-1). With increasing temperature a shoulder develops which is attributed to the combined twist conformers. The experimental deltaH-value is 6.9 kJ mol(-1), in good agreement with the ab initio results. Due to the low interconversion barriers, the various twist conformers cannot be detected separately.

show abstract

“…24) [11]. However, for the chair flip to occur, cyclohexane does not have to pass through a boat conformation.…”

Section: Interconversions Of Cyclohexane Conformationsmentioning

confidence: 99%

Conformational analysis of cycloalkanes

Dragojlović

2015

ChemTexts

View full text Add to dashboard Cite

Conformational analysis is a comparatively new area of organic chemistry that has been developed well after the theories of organic reactions, bonding in organic compounds and stereochemistry. It was only in the second half of twentieth century that its importance was fully recognized and its central role with respect to bonding, reactivity and stability of organic compounds was appreciated. Fundamental concepts of conformational analysis, a deeper discussion of the conformational analysis of small and common rings and an introduction into the conformational analysis of medium and large rings are presented at a level suitable for an introductory organic chemistry course. This discussion aims to provide a better understanding of the complex relationship among different types of strain, while also discussing the factors that determine stability of a particular conformation. Finally, the unique nature of each class of cycloalkanes is explored.

show abstract

A Three-Dimensional Energy Surface for the Conformational Inversion of Cyclohexane

Cited by 37 publications

References 1 publication

Multifrequency ESR study of spin-labeled molecules in inclusion compounds with cyclodextrins

Multifrequency ESR study of spin-labeled molecules in inclusion compounds with cyclodextrins

Chair, Boat and Twist Conformation of Dodecamethylcyclohexasilane and Undecamethylcyclohexasilane: A Combined DFT and Raman Spectroscopic Study

Conformational analysis of cycloalkanes

Contact Info

Product

Resources

About