13C substituent effects in monosubstituted benzenes
TL;DR: In this paper, a brief analysis of the current understanding of substituent perturbations in monosubstituted benzenes as determined by substituents induced carbon chemical shifts is presented.
Abstract: A brief analysis is presnted of the current understanding of substituent perturbations in monosubstituted benzenes as determined by substituent induced carbon chemical shifts. A critical tabulation of the four substituent chemical shifts is given for c. 700 monosubstituted benzenes.
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TL;DR: Applications of quantum chemical descriptors in the development of QSAR/QSPR dealing with the chemical, physical, biochemical, and pharmacological properties of compounds are reviewed.
Abstract: Quantitative structure-activity and structureproperty relationship (QSAR/QSPR) studies are unquestionably of great importance in modern chemistry and biochemistry. The concept of QSAR/QSPR is to transform searches for compounds with desired properties using chemical intuition and experience into a mathematically quantified and computerized form. Once a correlation between structure and activity/property is found, any number of compounds, including those not yet synthesized, can be readily screened on the computer in order to select structures with the properties desired. It is then possible to select the most promising compounds to synthesize and test in the laboratory. Thus, the QSAR/QSPR approach conserves resources and accelerates the process of development of new molecules for use as drugs, materials, additives, or for any other purpose. While it is not easy to find successful structureactivity/property correlations, the recent exponential growth in the number of papers dealing with QSAR/ QSPR studies clearly demonstrates the rapid progress in this area. To obtain a significant correlation, it is crucial that appropriate descriptors be employed, whether they are theoretical, empirical, or derived from readily available experimental characteristics of the structures. Many descriptors reflect simple molecular properties and thus can provide insight into the physicochemical nature of the activity/ property under consideration. Recent progress in computational hardware and the development of efficient algorithms has assisted the routine development of molecular quantummechanical calculations. New semiempirical methods supply realistic quantum-chemical molecular quantities in a relatively short computational time frame. Quantum chemical calculations are thus an attractive source of new molecular descriptors, which can, in principle, express all of the electronic and geometric properties of molecules and their interactions. Indeed, many recent QSAR/QSPR studies have employed quantum chemical descriptors alone or in combination with conventional descriptors. Quantum chemistry provides a more accurate and detailed description of electronic effects than empirical methods.1 Quantum chemical methods can be applied to quantitative structure-activity relationships by direct derivation of electronic descriptors from the molecular wave function. In many cases it has been established that errors due to the approximate nature of quantum-chemical methods and the neglect of the solvation effects are largely transferable within structurally related series; thus, relative values of calculated descriptors can be meaningful even though their absolute values are not directly applicable.2 Moreover, electronic descriptors derived from the molecular wave function can be also partitioned on the basis of atoms or groups, allowing the description of various molecular regions separately. Most work employing quantum chemical descriptors has been carried out in the field of QSAR rather than QSPR, i.e. the descriptors have been correlated with biological activities such as enzyme inhibition activity, hallucinogenic activity, etc.3-6 In part this has been because, historically, the search for quantitative relationships with chemical structure started with the development of theoretical drug design methods. Quantum-chemical descriptors have also been reported to correlate the reactivity of organic compounds, octanol/water partition coefficients, chromatographic retention indices, and various physical properties of molecules.7-11 The present article reviews applications of quantum chemical descriptors in the development of QSAR/QSPR dealing with the chemical, physical, biochemical, and pharmacological properties of compounds.
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TL;DR: In this paper, the authors describe the formation, structure (stereochimie and stereodynamique), and donnees de spectroscopie RMN des composes du titre.
Abstract: Formation, structure (stereochimie et stereodynamique), et donnees de spectroscopie RMN des composes du titre
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TL;DR: Control experiments involving vegetative a/alpha cells and nonsporulating alpha/ alpha cells under sporulation conditions showed that dityrosine is indeed sporulation-specific, and the identity of this hitherto unknown component of the yeast ascospore wall with standard dityosine was proven by 1H NMR and by mass spectrometry.
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TL;DR: The first helical ladder polymers with unbroken pathways of conjugation that extend not only through each monomeric unit, but between them as well are described in this paper, where the key step is the condensation of 1,2-phenylenediamine and a transition metal salt with a helicene having salicylaldehyde's functionality at both ends.
Abstract: The synthesis is described of the first helical ladder polymers with unbroken pathways of conjugation that extend not only through each monomeric unit, but between them as well. The key step is the condensation of 1,2-phenylenediamine and a transition metal salt with a helicene having salicylaldehyde's functionality at both ends. This gives rise to “metal salophen” units that bind adjacent helicenes, provide conjugated links from one ring system to the next, and constrain the p-orbitals of the rings they unite to be nearly parallel. Because the helicene monomers are enantiopure, so too are the polymeric structures to which they give rise. One of the polymers (6) winds continuously in only one direction. Another (4a) winds in one direction through the helicene moieties and in the other direction through the metal−salophens. The circular dichroisms of the former at wavelengths near 600 nm are notably large. The corresponding circular dichroisms of the latter are much smaller. MALDI-TOF mass spectra provide ...
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TL;DR: In this paper, it was shown that the electric field at a particular nucleus arising from polar groups in other parts of a molecule can lead to chemical shifts proportional to the first power of the field strength.
Abstract: It is shown that the electric field at a particular nucleus arising from polar groups in other parts of a molecule can lead to chemical shifts proportional to the first power of the field strength....
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TL;DR: The origin of the relative nuclear resonance shifts in monosubstituted benzenes has been investigated in this article, where the largest resonance shifts were observed for the carbon atom directly bonded to X.
Abstract: The origin of the relative nuclear resonance shifts in monosubstituted benzenes has been investigated. In order to obtain more complete experimental information both C13 and H1 resonance shifts in a variety of aryl‐X compounds were measured. The H1 resonances were measured on 5 mole % solutions in cyclohexane to minimize solvent effects; the carbon shifts were obtained from natural abundance C13 resonance measurements in the neat liquid. Unambiguous assignments of both H1 and C13 resonance spectra were made possible with the aid of deuterated compounds. The largest resonance shifts were observed for the carbon atom directly bonded to X. As in the corresponding CH3X compounds, these shifts arise primarily from the inductive and magnetic anisotropy effects of X. Magnetic anisotropy effects of X are also observable in both the C13 and H1 resonances at the ortho position. A very close correspondence between C13 and H1 resonances is observed at the para position, where the primary contribution to the relative shifts arises from resonance effects of X. This implies that the proton resonance responds to the π‐electron density on the carbon to which it is bonded, and that under favorable conditions, both H1 and C13 resonance shifts might be employed to obtain information about the π‐electron density distribution in aromatic systems. At the meta position the C13 resonance shifts are surprisingly small and uniform, indicating small or negligible inductive effects due to X, and there is no evident correlation with the meta‐proton shifts. Both the C13 and H1 shifts at the para position show an approximate correlation with chemical reactivity parameters (Hammett σ constants) but no such correlation exists for the meta‐carbon or meta‐hydrogen shifts.
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366 citations