The Conformations of the Diastereomeric 2,5-Dimethyl-2,5-Dimethoxy-3,4-Diphenylhexanes

The relative free energies of the various confor~llations for the diastereomeric 2,5-dimethyl-2,5-din1ethoxp-3,4-diphenylhexanes have been determined. Hindered and free rotation about the various bonds has been observed and assignments of the proton resonance signals to specihc conformations have been made.Proton resonance spectroscopy has been found well suited to the study of hindered rotation in substituted ethanes (1, 2). Hindered rotation in the 2,5-dimethyl-2,5-dimethoxy-3,4-diphenylhexanes has been demonstrated by dipole moment studies (3). The present iilvestigation extends the previous work and provides some quantitative data.For a substituted ethane with the symnletry CHXY-CHXY there are two 11011-equivalent conforinations for the meso diastereoiner and three non-equivalent conformations for the d,l diastereomer. In one of the conformations of the ineso diastereoiner the two hydrogeils will be non-equivalent. In the other they will both be equivalent, but will not be in the same environment as either of the hydrogens which are non-equivalent. Therefore if rotation about the central carbon-carbon bond is sufficieiltly hindered one could theoretically observe one signal from one conformation and a pair of peaks from the other conforn~ation. This assumes that there is a sufficient population of molecules in each of the conformations. However, with rapid rotation, only a single signal will be observed, whose position is the weighted average of the individual signals. All three conforinations of the d,l diastereonier have the hydrogens of a given conformation equivalent; but each conformation has them in a unique environment. Therefore with hindered rotation one signal should be observed for each confornlation. The same conclusions apply to the groups X and Y if they also contain protons.The relative populations in any two conforrllations of a given diastereomer will be given by where A E is the free energy difference between the two conformations. Therefore if rotation is sufficiently hindered so that protoil resonance signals are observed for the individual conformations, but sufficiently rapid so that therinal equilibrium of the populations is obtained in a reasoilable time, then the variation in relative intensities of the signals as the temperature is varied will yield the free energy differences between the conformations.The rotatioils and conformations will be discussed with respect to the nun~bering shown in the following formula.

supporting

confidence: 81%

The Proton Resonance Spectra of the 2,5-Dimethyl-2,5-Dimethoxy-3,4-Diphenylhexanes

Brownstein¹

1961

“…We have not been able to make a direct comparison for dicyanobenzene, but the data obtained for dd,ll-2,5-dimethoxy-2,Sdimethyl-3,4-diphenylhexane is informative. The earlier studies of this substance (12) showed that it exhibited a temperature-variant dipole moment. The Newman stereochemical diagram for one conformation of this substance indicates that the observed moment may be attributed to the pivotal bond (1 3).…”

Section: Introductionmentioning

confidence: 88%

“…227-225". The 2,s-dimethoxy-2,s-dimethyl-3,4-diphenylhexane has been purified according to procedures previously described (12). Benzene (previously freed from toluene by partial freezing) and dioxane were purified by techniques already described (8).…”

Section: Equigmentmentioning

confidence: 99%

The Electrical Polarization of 1,4-Dicyanobenzene

Huber¹,

Wright²

1973

Self Cite

The anomalous total electric polarization of 1,4-dicyanobenzene has been shown not to be due to high atom polarization by dielectric constant measurement of the solid substance and by its spectral absorption in the far i.r. However the evaluation of atom polarization from the spectral absorption is questioned because these absorptions have a large temperature coefficient in the 50-150 cm-' region. The apparent dipole moment of 1,4-dicyanobenzene in solution has been measured and found to be ternperaturedependent.On a montrC que l'anomalie dans la polarisation Clectrique totale du dicyano-l,4 benzene n'etait pas caus6 par la polarisation des atomes appartenant a des niveaux Cleves. Ce fait fut dtmontre par la mesure de la constante diilectrique de la substance solide et par son spectre d'absorption dans le lointain i.r. Cependant, 1'6valuation de la polarisation des atomes B partir du spectre d'absorption est remise en question a cause de la presence d'un coefficient Cleve d e temperature pour ces absorptions dans la r6gion de 50 150 cm-I. Le moment dipolaire apparent du dicyano-1,4 benzene en solution a CtC mesurt et est relit ala temperature.[Traduit par l e journal]Can. J. Chem., 51, 2234 (1973)

“…T h e behavior is remindful of certain sterically hindered diastereomers, the moments of which have been studied recently with respect t o tenlperati~re (28).…”

Section: Polarizatioys O F Substances To Which Centrosymmetrymentioning

confidence: 99%

The Electric Moments of Supposedly Symmetrical Cyclic Substances

Meredith¹,

Wright²

1960

Self Cite

The distortion polarizations of polysubstituted benzenes and cyclobutanes which have been examined by the randomly oriented solid pellet technique seem to be lower but never higher than the electronic polarization values derived from bond refractions, and are considered to be more reliable than the latter. In almost all of the supposedly symmetrical substances which have been examined the total solute polarization al\vaysexceeds the measured distortion polarization and usually exceeds the calculated MRD. These substances must then be polarized, although the moments which have been determined are not numerically reliable because of solvent effects. Whenever possible the temperature coefficients of these apparent moments have been determined and are usually but not always positive. From these temperature coefficients it is presumed that some substances, like naphthalene, are rigidly dipolar whereas others, like hexachlorobenzene, have activation states which are unsymn~etrical and therefore contributory to the moment which actually is measured.The graphic formula of a single molecule is patently a cursory description of a chemical substance, useful chiefly for illustrative and bibliotechnical communication. Nevertheless there seems to be a tendency to ascribe to the graphic formula a verity which it does not deserve. Especially is this the case when molecular symmetry is concerned. Indeed the simple diagram may become an unexcited state of reference in ailalysis of "loss" determinations such as those of absorptioil spectroscopy, or it inay be used as the basis for trial stuctures during diffraction studies. There is some danger that the inadequate graphic formula thus becomes "built into" the physical interpretation.Although a reactance determination, of which dielectric constailt is an example, has the obvious disadvantage of an integrated, 11011-detailed result in which the entire system participates it does not require the same degree of preconception that is coillr~~oil to seine other physical methods. No irreversible energy transfer occurs, so that the illeasure~nent is that of the actual system if sufficiently weak electromagnetic fields are applied. If environmental influences can be evaluated, say by variation of solvent, temperature, and concentration, the modulated value of electrical polarizatioil sets a definition to \vl~ich the molecular construct of the substance 111ust conform.This definition is especially applicable to substances for which the graphic forillula \vould predict, for reasons of symmetry, a zero dipole moment. Several examples in which the inadequacy of the ordinary graphical construct becomes apparent, have recently been reported (1-4).In the present report this list has been extended to include aromatic hydrocarbons with aliphatic and halogeno substituents. These substances would not be expected to display any orientation polarization if the frequently quoted symmetries and the graphic formulae specifying them were valid.I n some instances zero moments have been assigned here...