A study of the observable axial- and equatorial-2,4-dimethyl-2-adamantyl cations. Further examples of equilibrating nonclassical structures in "ordinary" tertiary carbocations

1990 ◽  
Vol 68 (10) ◽  
pp. 1889-1893 ◽  
Author(s):  
D. J. Buffam ◽  
T. S. Sorensen ◽  
S. M. Whitworth
Keyword(s):  
Chemical Shifts ◽  
The Other ◽  
Temperature Dependent ◽  
Lower Field ◽  
Tertiary Alcohols ◽  
Cation System ◽  
Different Temperatures ◽  
The Face ◽  
Opposite Face ◽  
C Nmr

The title cations have been prepared insitu from the corresponding tertiary alcohols, using three different superacid systems. The 13C NMR spectra were measured at a number of different temperatures, using the structurally related 13C peaks of internal 2-methyl-2-adamantyl cation as a reference. Using this protocol, one finds that the 13C chemical shifts of the four β carbons are quite temperature dependent, two of them shifting to higher field and the other two on the opposite face shifting to lower field, or vice versa, as the temperature is increased or decreased. This behavior is uniquely ascribed to a rapid equilibration between two "structures" for each cation system. The two structures in question involve selective C–C hyperconjugative delocalization on one face or the other of the 2-adamantyl skeleton. In the case of the 4-equatorial-methyl isomer, a structure with C–C hyperconjugation on the methyl-substituted face is most stable, while for the 4-axial-methyl cation, the structure with delocalization from the face opposite the substituent is favored. The former preference is regarded as stereoelectronic in origin while the latter is ascribed to steric effects. Keywords: non-planar carbocations, temperature dependent NMR shifts, stereoelectronic effects.

17O and 13C NMR spectra of some geminal diacetates

1988 ◽  
Vol 53 (3) ◽  
pp. 588-592 ◽  
Author(s):  
Antonín Lyčka ◽  
Josef Jirman ◽  
Jaroslav Holeček
Keyword(s):  
13C Nmr ◽  
Nmr Spectra ◽  
Chemical Shifts ◽  
The Other ◽  
13C Nmr Spectra ◽  
Other Hand ◽  
Carboxylic Esters ◽  
C Nmr ◽  
Oxygen Atoms

The 17O and 13C NMR spectra of eight geminal diacetates RCH(O(CO)CH3)2 derived from simple aldehydes have been measured. In contrast to the dicarboxylates R1R2E(O(CO)R3)2, where E = Si, Ge, or Sn, whose 17O NMR spectra only contain a single signal, and, on the other hand, in accordance with organic carboxylic esters, the 17O NMR spectra of the compound group studied always exhibit two well-resolved signals with the chemical shifts δ(17O) in the regions of 183-219 ppm and 369-381 ppm for the oxygen atoms in the groups C-O and C=O, respectively.

scholarly journals HR-DOSY and sulfoxide enantiomeric discrimination by cyclodextrin

2000 ◽  
Vol 14 (4) ◽  
pp. 203-213 ◽  
Author(s):  
Ricardo Batista Borges ◽  
Antonio Laverde Jr. ◽  
André Luiz Meleiro Porto ◽  
Anita Jocelyne Marsaioli
Keyword(s):  
Chiral Recognition ◽  
Chemical Shifts ◽  
Binding Constants ◽  
Chiral Selector ◽  
Point Of View ◽  
The Other ◽  
Stoichiometric Ratio ◽  
Enantiomeric Resolution ◽  
Rp Hplc ◽  
C Nmr

Racemic and chiral ethyl-phenylsulfoxide (solute) andβ-cyclodextrin (chiral selector) were used to compare two NMR methodologies to predict RP-HPLC enantiomeric resolution efficiency. One of them based on the classical approach involving apparent binding constants and complexation‒induced chemical shifts at saturation and the other based on13C NMR signal splittings (solute and chiral selector in stoichiometric ratio) and HR-DOSY of the same solution. We have concluded that the latter methodology is rather efficient and though more elaborate from the NMR point of view, the results are promising and constitute an alternative method to investigate chiral recognition and other supramolecular phenomena.

scholarly journals Influence of urea on methyl $$\upbeta$$-D-glucopyranoside in alkali at different temperatures

Cellulose ◽  
2019 ◽  
Vol 26 (18) ◽  
pp. 9413-9422 ◽  
Author(s):  
Maria Gunnarsson ◽  
Merima Hasani ◽  
Diana Bernin
Keyword(s):  
Conformational Changes ◽  
Chemical Shifts ◽  
Regenerated Cellulose ◽  
Temperature Dependent ◽  
Hydroxymethyl Group ◽  
Overhauser Effect ◽  
Specific Manner ◽  
Acting Mechanism ◽  
Different Temperatures ◽  
Dissolution Efficiency

Abstract The dissolution efficiency plays an important role on the properties of regenerated cellulose-based products. Urea is known to be one of the additives aiding to improve cellulose dissolution in the NaOH(aq) system. The acting mechanism caused by urea has been debated and one of the hypothesis is that urea could induce a conformational change on cellulose, which promotes dissolution. Here we used NMR spectroscopy on a model system for cellulose, namely, methyl $$\upbeta$$β-D-glucopyranoside ($$\upbeta$$β-MeO-Glcp) and compared chemical shifts and J couplings, which both are indicators for conformational changes, as a function of temperature and upon the addition of urea. We found that in NaOH(aq), the hydroxymethyl group changes its conformation in favour of the population of the gt rotamer, while the presence of urea induced temperature dependent conformational changes. Heteronuclear Overhauser effect experiments showed that urea associates with cellulose but in a non-specific manner. This suggests that urea rather than binding to the carbohydrate, changes the chemical environment inducing a change in conformation of $$\upbeta$$β-MeO-Glcp and likely also for cellulose when dissolved in NaOH(aq) with urea.

The structures and reactions of stannylene acetals from carbohydrate-derived trans-diols. Part I. In the absence of added nucleophiles

1990 ◽  
Vol 68 (7) ◽  
pp. 1007-1019 ◽  
Author(s):  
T. Bruce Grindley ◽  
Rasiah Thangarasa
Keyword(s):  
Nuclear Magnetic Resonance ◽  
Nmr Spectroscopy ◽  
Magnetic Resonance ◽  
Magnetic Resonance Spectroscopy ◽  
Chemical Shifts ◽  
The Other ◽  
Resonance Spectroscopy ◽  
Pyranose Ring ◽  
Regioselective Reactions ◽  
C Nmr

Di-n-butylstannylene acetals of benzyl 4,6-O-benzylidene-α- and -β-D-glucopyranoside and galactopyranoside have been prepared and studied in solution by 1H, 13C, and 119Sn nuclear magnetic resonance spectroscopy. The species present in solution have been identified from the 119Sn nmr spectral data, by comparison of the 13C nmr chemical shifts of the stannylene acetals and their precursor diols and also by analysis of the products of reactions performed without added nucleophiles. The orientations of the two substituents on the carbons in the pyranose ring attached to the carbons in the stannylene ring determine the structures adopted by the stannylene acetal in solution. If one substituent is axial and the other equatorial, the stannylene acetal exists as a single symmetrical dimer in which the two oxygen atoms in the two 1,3,2-dioxastannolane rings adjacent to the axial substituents are dicoordinate. A stannylene acetal with two adjacent equatorial substituents exists as a non-interconverting mixture of dimers; one with two adjacent axial substituents is present as a rapidly interconverting mixture of dimers, trimers, and tetramers. Benzoylation and benzylation of the latter two types of stannylene acetals have been performed and have been shown to be only slightly regioselective in contrast to the known highly regioselective reactions of the first type. Only when single dimers are present are regiospecific or highly regioselective reactions obtained. The causes of the variation in the species present and of the reaction regioselectivity for different stannylene acetals are discussed. Keywords: stannylene acetals, 1,3,2-dioxastannolanes, 119Sn NMR spectroscopy, regioselective reactions, carbohydrates.

Chemometrical analysis of substituent effects on 13C and 15N NMR chemical shifts in 1-aroyl-3-substituted thioureas

1989 ◽  
Vol 54 (9) ◽  
pp. 2399-2407 ◽  
Author(s):  
Oldřich Pytela ◽  
Josef Jirman ◽  
Antonín Lyčka
Keyword(s):  
Latent Variables ◽  
Chemical Shifts ◽  
Substituent Effects ◽  
The Other ◽  
15N Nmr ◽  
Nmr Chemical Shifts ◽  
Additional Constant ◽  
Substituent Constants ◽  
Derivatives Of ◽  
C Nmr

The methods of conjugated deviations and regression analysis have been used to study the substituent effects on 13C and 15N NMR chemical shifts of 12 derivatives of 1-aroyl-3-phenylthiourea and 1-aroyl-3-methylthiourea. The 13C NMR chemical shifts can be described by two latent variables, one univocally correlated with the Hammett substituent constants (r = 0.993) and the other reflecting the increased shielding of the nuclei due to overlap of the adjacent bond electrons as a consequence of electron-donor or electron-acceptor character of the substituents.This effect is less pronounced with the 15N nuclei. Application of dual substituent constants σR, σF with the additional constant σα describing the polarization has failed in giving sufficiently close correlations and explanation of the substituent effect found.

PD Fluids Contain High Concentrations of Cytotoxic GDPs Directly after Sterilization

2004 ◽  
Vol 24 (4) ◽  
pp. 392-398 ◽  
Author(s):  
Martin Erixon ◽  
Torbjörn Lindén ◽  
Per Kjellstrand ◽  
Ola Carlsson ◽  
Malin Ernebrant ◽  
...  
Keyword(s):  
Peritoneal Dialysis ◽  
Main Part ◽  
Room Temperature ◽  
Degradation Products ◽  
The Other ◽  
Dependent Manner ◽  
Peritoneal Membrane ◽  
Temperature Dependent ◽  
Different Temperatures ◽  
High Concentrations

Objective Glucose degradation products (GDPs) in peritoneal dialysis (PD) fluids are cytotoxic and affect the survival of the peritoneal membrane. One of the most reactive GDPs in PD fluids is 3,4-dideoxyglucosone-3-ene (3,4-DGE). 3,4-DGE has been reported as an intermediate between 3-deoxyglucosone (3-DG) and 5-hydroxymethyl furaldehyde (5-HMF) during degradation of glucose. In PD fluids, 3,4-DGE exists in a temperature-dependent equilibrium with a pool of unidentified substances. The aim of this study was to explore this equilibrium and its temperature dependence during the first months of storage after the sterilization procedure. Methods GDPs and inhibition of cell growth (ICG) were measured directly after sterilization of the PD fluid and during storage at different temperatures for 60 days. The following GDPs were analyzed: 3-DG, 3,4-DGE, 5-HMF, formaldehyde, acetaldehyde, glyoxal, and methylglyoxal. Results Immediately after sterilization, the concentration of 3,4-DGE was 125 μmol/L. During the first weeks of storage, it decreased by about 80%. At the same time, the 3-DG concentration increased. None of the other GDPs were significantly affected. Cytotoxicity correlated well with the concentration of 3,4-DGE. When pure 3,4-DGE was substituted for the lost amount of 3,4-DGE after 30 days of storage, the initial ICG was almost completely regained. Conclusions Heat sterilization of PD fluids promotes the formation of large quantities of 3,4-DGE, rendering the fluid highly cytotoxic. During storage, the main part of 3,4-DGE is reversibly converted in a temperature-dependent manner to a less cytotoxic pool, consisting mainly of 3-DG. Cytotoxicity seems to be dependent exclusively on 3,4-DGE. In order to avoid higher levels of 3,4-DGE concentrations, PD fluids should not be used too soon after sterilization and should not be stored at temperatures above room temperature.

The 7-Cycloheptatrienylmethyl/7-Norcaradienylmethyl Cation System

1975 ◽  
Vol 53 (12) ◽  
pp. 1818-1828 ◽  
Author(s):  
Ivan Pikulik ◽  
Ronald F. Childs
Keyword(s):  
Lewis Acid ◽  
Chemical Shifts ◽  
Product Distribution ◽  
Carbonyl Oxygen ◽  
Lower Field ◽  
Model Compounds ◽  
Contrast Reaction ◽  
Cation System ◽  
Proton Resonances ◽  
Methyl Phenylacetate

Reaction of 7-carbomethoxycycloheptatriene with either BCl3 or BBr3 gave a 1:1 complex in which the Lewis acid was complexed to the carbonyl oxygen. The low temperature p.m.r. and u.v. spectra of these zwitterions indicated that they existed almost entirely in the norcaradiene forms 10 and 14, respectively. In contrast, reaction of N,N-dimethyl-1,3,5-cycloheptatriene-7-carboxamide with BCl3 was found to result in the formation of a mixture of the cycloheptatriene 12 (65%) and norcaradiene 13 (35%). Comparison of the chemical shifts of the cyclopropyl proton resonances of 10 and 14 with model compounds showed that their positions were anomolous, the C7 protons resonating at much higher and the C1 and C6 protons resonating at much lower field than expected and to account for this it is suggested that these norcaradienes are aromatic systems. Both 10 and 14 were thermally unstable and underwent an irreversible rearrangement to form the complex of methyl phenylacetate, the phenylmethylacylium cation and the appropriate counterion, [Formula: see text] respectively. The rate of these rearrangements and the product distribution was found to be dependent upon both the ratio of Lewis acid to ester used and the nature of the Lewis acid.

Mimetic Monsters

2010 ◽  
Vol 4 (1) ◽  
pp. 85-93
Author(s):  
Timothy Beal
Keyword(s):  
The Other ◽  
Jewish Tradition ◽  
Book Of Job ◽  
Edge Of Chaos ◽  
The Face ◽  
The Relationship

This article reads between two recent explorations of the relationship between religion, chaos, and the monstrous: Catherine Keller’s Face of the Deep and Author's Religion and Its Monsters. Both are oriented toward the edge of chaos and order; both see the primordial and chaotic as generative; both pursue monstrous mythological figures as divine personifications of primordial chaos; both find a deep theological ambivalences in Christian and Jewish tradition with regard to the monstrous, chaotic divine; both are critical of theological and cultural tendencies to demonize chaos and the monstrous; and finally, both read the divine speech from the whirlwind in the book of Job as a revelation of divine chaos. But whereas one sees it as a call for laughter, a chaotic life-affirming laughter with Leviathan in the face of the deep, the other sees it as an incarnation of theological horror, leaving Job and the reader overwhelmed and out-monstered by God. Must it be one way or the other? Can laughter and horror coincide in the face of the deep?

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