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Spin-spin Coupling Constants (spin-spin + coupling_constant)
Selected AbstractsA Pragmatic Recipe for the Treatment of Hindered Rotations in the Vibrational Averaging of Molecular PropertiesCHEMPHYSCHEM, Issue 1 2008Brendan C. Mort Dr. Abstract A computational protocol for the treatment of hindered rotations in the vibrational averaging of molecular properties is described. As examples, the specific rotations of (R) -methyloxirane, (1S) -methylnorbornanone, and (S) -epichlorohydrin and the spin-spin coupling constant for are investigated. For each of the four molecules, the relaxed and unrelaxed potential energy surfaces along the rotational coordinate of interest are used to solve the nuclear Schrödinger equation using a pseudospectral method. Analysis of the results demonstrate that the vibrational averaging for low-frequency modes that represent hindered rotations can lead to unphysical behavior at high temperatures while simply neglecting the rotational contributions is also not desirable. The method described in this work connects the (an)harmonic oscillator behavior at low temperatures with the free rotor-like behavior at high temperatures. [source] High-pressure NMR characterization of triacetyl-,-cyclodextrin in supercritical carbon dioxideMAGNETIC RESONANCE IN CHEMISTRY, Issue 2 2009G. I. Ivanova Abstract Cyclodextrins are used in many drug formulations since their cavities provide microenvironments where drug molecules can enter and form inclusion complexes for controlled drug delivery. Supercritical carbon dioxide (scCO2) is an alternative to organic solvents and a very attractive medium for the preparation of these inclusion complexes. The potential ability of triacetyl-,-cyclodextrin (TA-,-CD) to form inclusion complexes in addition to its high miscibility in liquid and scCO2 could offer a chance for an economical and environmental friendly chemical processing. In this work, high-pressure NMR studies were performed in order to obtain information on the molecular structure and dynamics of TA-,-CD in scCO2 at 313.15 K and 20 MPa and its ability to form inclusion complexes under these conditions was studied. The influence of scCO2 on a number of NMR spectral parameters, such as chemical shifts, spin-spin coupling constants, nuclear Overhauser effect (NOE) and spin-lattice relaxation (T1) has been studied. We unequivocally show for the first time structural changes of TA-,-CD in scCO2, like acetyl chain orientation and overall shape distortions that can affect its inclusion capability in this medium. The possibility of cavity self-closure is discussed and the results of two inclusion studies that support cavity self-closure, with the angiotensin-converting enzyme inhibitor, captopril, and the nonsteroid anti-inflammatory drug, flufenamic acid, are presented. Copyright © 2008 John Wiley & Sons, Ltd. [source] Predicting the NMR spectra of nucleotides by DFT calculations: cyclic uridine monophosphateMAGNETIC RESONANCE IN CHEMISTRY, Issue 6 2008Alessandro Bagno Abstract We present an experimental and quantum chemical NMR study of the mononucleotide cyclic uridine monophosphate in water. Spectral parameters (1H and 13C chemical shifts and 1H1H, 13C1H, 31P13C and 31P1H spin-spin coupling constants) have been carefully obtained experimentally and calculated using DFT methods including the solvent effect and the conformational flexibility of the solute. This study confirms that the 1H and 13C spectra of polar, flexible molecules in aqueous solution can be predicted with a high level of accuracy, comparable to that obtained for less complex systems. Copyright © 2008 John Wiley & Sons, Ltd. [source] NMR spin-spin couplings involving nuclei in the neighborhood of a carbonyl group.MAGNETIC RESONANCE IN CHEMISTRY, Issue 3 2008-substituted acetamides, 3JCH couplings in Abstract In this work 3JCH spin-spin coupling constants (SSCCs) for the cis- and trans- conformers for ,-X-acetamides (X = F, Cl, Br and CN) (1,4) were studied in detail since they were found to be notably different for both conformers. These differences are rationalized as originating in the changes of the strong negative hyperconjugative interactions that take place within the carbonyl group. Such changes are found to depend not only on conformation, but also on solvent. For the cis -conformers there is a close proximity between the X-substituent and the in-plane oxygen lone pair of pure p character, which affects notably their respective negative hyperconjugative interactions. Both the efficiency for transmitting the Fermi contact (FC) term through the coupling pathway of 3JCH SSCCs and its potential as a probe to study the stereochemical properties of the XH2C group are discussed. Copyright © 2008 John Wiley & Sons, Ltd. [source] | ||||||||||