Chapter 1 Nuclear magnetic resonance of organic charge-transfer complexes
About: This article is published in Progress in Nuclear Magnetic Resonance Spectroscopy.The article was published on 1969-01-01. It has received 56 citations till now. The article focuses on the topics: Nuclear magnetic resonance decoupling & Electron nuclear double resonance.
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736 citations
TL;DR: In this paper, the authors discuss the contributions provided by synthetic cyclophanes as hosts to the understanding of molecular complexation of neutral organic guest molecules in aqueous and organic solvents.
Abstract: Since the discovery of the crown ethers by Pedersen twenty years ago, the chemistry of synthetic hosts for the selective complexation of organic and inorganic guests has seen an extraordinarily rapid development. This article discusses in particular the contributions provided by synthetic cyclophanes as hosts to the understanding of molecular complexation of neutral organic guest molecules in aqueous and organic solvents. In aqueous solution, cyclophanes form stoichiometric complexes with neutral aromatic guests which can approach enzyme-substrate complexes in their stability. Efficient molecular complexation is also observed in organic environments. Here, as a result of large solvation effects, the strength of complexation is strongly dependent on the nature of the organic solvent. Electron donor-acceptor interactions can contribute significantly to the stability of complexes formed between cyclophane hosts and aromatic guests. Force-field calculations together with computer graphics are powerful tools in the design of water-soluble, optically active hosts for chiral recognition of complexed racemic guests. Simple and selective functionalization of the cyclophane framework leads to stable, bioorganic catalysts. Like enzymes, these catalysts bind their substrates in a rapid equilibrium prior to the reaction steps. As a perspective, some fascinating research objectives in the field of molecular recognition and catalysis which can be targeted with designed cyclophane hosts are shown.
353 citations
TL;DR: The chain length selectivity studies of dicarboxylic acids within the cavities of new fluorescent Troger's base molecular frameworks have been carried out with a critical examination of their role of rigidity as well as flexibility in selective binding in comparison to receptor 5.
Abstract: Artificial receptors (1-5) have been designed and synthesized from simple precursors. The chain length selectivity studies of dicarboxylic acids within the cavities of new fluorescent Troger's base molecular frameworks (1-3) have been carried out with a critical examination of their role of rigidity as well as flexibility in selective binding in comparison to receptor 5. The chiral resolution of the racemic Troger's base receptors (1 and 2) by chiral recognition with (+)- camphoric acid using hydrogen-bonding interactions has been studied.
147 citations
TL;DR: In this paper, the authors erortert, howchen Beitrag the Untersuchungen with synthetischen Cyclophanen als Wirten zum Verstandnis der Komplexierung neutraler organischer Gastmolekule in wasrigen und organischen Losungsmitteln geliefert haben.
Abstract: Die Chemie synthetischer Wirtkomponenten zur gezielten Komplexierung organischer und anorganischer Gastkomponenten hat sich seit der Entdeckung der Kronenether durch Pedersen von zwanzig Jahren sturmisch entwickelt. In diesem Aufsatz wird erortert, welchen Beitrag die Untersuchungen mit synthetischen Cyclophanen als Wirten zum Verstandnis der Komplexierung neutraler organischer Gastmolekule in wasrigen und organischen Losungsmitteln geliefert haben. Cyclophane konnen in wasriger Losung stochiometrische Komplexe mit aromatischen Neutralmolekulen bilden; die Stabilitat dieser Komplexe ist mit derjenigen von Enzym-Substrat-Komplexen vergleichbar. Wirksame Komplexierung kann auch in organischen Medien erreicht werden, wobei die sehr starke Abhangigkeit der Bindungsstarke von der Art des Losungsmittels sich im wesentlichen auf Solvatationseffekte zuruckfuhren last. Elektronen-Donor-Acceptor-Wechselwirkungen konnen fur die Stabilitat der Cyclophan-Aren-Komplexe von groser Bedeutung sein. Fur die gezielte Entwicklung optisch aktiver Wirtkomponenten zur selektiven Erkennung von Enantiomeren („chirale Erkennung”) bieten sich Kraftfeldrechnungen in Kombination mit computergraphischen Darstellungen der Wirte und ihrer Komplexe an. Die gezielte Funktionalisierung von Cyclophanen hat zu stabilen, bioorganischen Katalysatoren gefuhrt, die ihre Substrate wie Enzyme in einem vorgelagerten Gleichgewicht komplexieren. Auf dem Gebiete der molekularen Komplexierung und Katalyse hat sich die Forschung faszinierende Ziele gesteckt. Cyclophane als Wirte konnen in hohem Mase dazu beitragen, diese Ziele zu erreichen.
116 citations
TL;DR: The quantifiable relationship between the equilibrium solution composition and electrospray (ESI) mass spectral peak intensities of simple host-guest complexes was investigated in this paper.
103 citations
References
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TL;DR: In this paper, it was shown that the position of the absorption bands of iodine solutions in the visible region moved gradually from violet to brown, the shift being only small in the case of benzene in which the color of iodine is a red-violet.
Abstract: Introduction The various colors of iodine in different solvents have attracted the attention of investigators for over half a century. Beckmann’ showed that the differences are not due to any change in the molecular weight of iodine and suggested solvation as the cause of brown color. Lachman2 supported this explanation, pointing out that the solvents which give a violet color are saturated, those which give brown colors are unsaturated. He stated that small additions of alcohol to a solution of iodine in chloroform suffice to shift the color stepwise from violet to brown. Hildebrand and Glascock3 found that when iodine and an alcohol were both added to a “violet” solvent, bromoform or ethylene bromide, the molal lowering of the freezing point was considerably less than additive, indicating combination of iodine with alcohol. Furthermore, they investigated colorimetrically the equilibria of iodine with ethyl alcohol, ethyl acetate and nitrobenzene dissolved together in a “violet” solvent such as carbon tetrachloride, chloroform or carbon disulfide, and found in each case an equilibrium constant corresponding to a 1 : 1 compound. They calculated the heat of combination in one case. There followed a long series of investigations of the general problem, well summarized by Gmelin4 and in a recent review by Kleinberg and Davidson.6 But the original division of iodine solutions into two or more distinct color classes became untenable with the work of Waentig, Groh’ and \\X7alkers which showed that the position of the absorption bands of iodine solutions in the visible region moved gradually from violet to brown, the shift being only small in the case of benzene in which the color of iodine is a red-violet. We have long been curious about the chemical basis for iodine solvates. It is not a question of dipole moment, as Walkers concluded, for we have recently shown rather conclusively that the iodine solubility, which would be altered by solvation effects, bears little or no relation to the dipole moment of the solvent mole~ule .~ The alcohols are extraordinary dipoles, and their effect is readily understandable, but the case of benzene seemed
6,904 citations
2,105 citations
TL;DR: In this paper, the modified Bloch equations for nuclear magnetic resonance are modified to describe the magnetic resonance of a single nuclear species X which is transferred back and forth between two (or more) magnetic environments (A,B) by kinetic molecular processes.
Abstract: The Bloch equations for nuclear magnetic resonance are modified to describe the magnetic resonance of a single nuclear species X which is transferred back and forth between two (or more) magnetic environments (A,B) by kinetic molecular processes. The modified Bloch equations involve the usual assumptions of the Bloch theory and, in addition, require (a) that the X nuclear relaxation times be independent of the molecular exchange rates, and (b) that the X nuclear magnetization in A relax independently of the X magnetization in B, and vice versa. The modified Bloch equations are easily solved in the slow passage case, with arbitrary rf saturation. Earlier relations between reaction rates, and resonance line shapes, which were developed by Gutowsky, McCall, and Slichter, and extended by a number of other investigators, are easily derived using the modified Bloch equations. In the present work the modified equations are used to show how rapid exchange rates can sometimes be measured in solutions where the X r...
1,453 citations
TL;DR: In this article, the free electron model of Pauling was used to calculate the magnetic field around a benzene ring which is rotating rapidly about all axes in an external magnetic field.
Abstract: The free electron model of Pauling is used to calculate the magnetic field around a benzene ring which is rotating rapidly about all axes in an external magnetic field. It is assumed that the π electrons precess in two circular paths, one on each side of the ring, equal in radius to the C–C distance in the benzene ring. The separation of these rings is taken as 1.28 A, which gives a calculated value for the nuclear magnetic resonance shielding value for benzene protons equal to the observed value. The field thus calculated is employed to predict shielding values for other aromatic compounds. Agreement with experiment is in general good, but there are some exceptions.
1,124 citations
TL;DR: In this paper, a model for calculating rate constants of processes which narrow nuclear magnetic resonance absorption lines having discrete components is presented for calculating the free energy of activation required for reorientations about the C-N amide bond.
Abstract: Mathematical methods are presented for calculating rate constants of processes which narrow nuclear magnetic resonance absorption lines having discrete components. High resolution proton spectra show that the R1CO‐NR2R3 skeletons of N,N‐dimethylformamide (DMF) and N,N‐dimethylacetamide (DMA) are planar and suggest that N‐methylformamide, N‐methylacetamide, N‐methylformanilide and N‐methylacetanilide exist predominantly in one configuration. The presence of a significant amount of double bond character in the C–N amide bond is proved by the temperature dependent coalescence observed for the chemically shifted proton doublet of the N(CH3)2 groups in DMF and DMA, which gives values of about 22 and 19 kcal respectively for the free energy of activation required for reorientations about the bond.
1,040 citations