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Shielding Constants (shielding + constant)

Distribution by Scientific Domains

Chemistry	69%

Kinds of Shielding Constants

magnetic shielding constant

Selected Abstracts

Nuclear magnetic moments from NMR spectra,Experimental gas phase studies and nuclear shielding calculations

CONCEPTS IN MAGNETIC RESONANCE, Issue 5 2007
Karol Jackowski
Abstract NMR spectra of gaseous compounds and quantum chemical calculations are combined to determine new accurate values of magnetic dipole moments for a series of nuclei. We have analyzed shielding constants, resonance frequencies, and nuclear magnetic moments for a group of simple molecules. The chemical shifts and resonance frequencies are measured at 300 K and extrapolated to the zero-density limit in order to remove all the intermolecular effects from the experimental parameters. The absolute shielding constants in the studied molecules are obtained from ab initio calculations. Assuming the proton magnetic moment as the reference, we determine the nuclear magnetic moments of 13C, 14N, 15N, 17O, 19F, 29Si, 31P, 33S, and 73Ge. The new nuclear magnetic moments are consistent with the experimental NMR parameters, and using these new values one can for the first time predict successfully the shielding constant of a nucleus in a molecule when the corresponding resonance frequency is known. © 2007 Wiley Periodicals, Inc. Concepts Magn Reson Part A 30A: 246,260, 2007. [source]

Prediction of gas-phase 13C nuclear magnetic shielding constants using ONIOM and optimally selected basis functions

CONCEPTS IN MAGNETIC RESONANCE, Issue 6 2008
M. Tafazzoli
Abstract The wave functions for calculating gas-phase 13C nuclear magnetic shielding constants of 22 molecules have been optimally selected using factorial design as a multivariate technique. GIAO and CSGT methods were used for computation of shielding constants. Different wave functions for different types of carbons were recommended. A wave function as the best level of the theory is proposed for almost similar carbons. ONIOM approach for molecules with different types of carbons is applied. The results of GIAO method using the proposed wave function are in very good agreement with the experimental values. An additional series (21 carbons) were used as test sets and their results confirmed the validity of the approaches. © 2008 Wiley Periodicals, Inc.Concepts Magn Reson Part A 32A: 449,461, 2008. [source]

Nuclear magnetic moments from NMR spectra,Experimental gas phase studies and nuclear shielding calculations

Theoretical study of NMR chemical shift induced by H/D isotope effect

INTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 15 2010
Kimikazu Sugimori
Abstract The isotope effect induced by deuterium substituted species is observed in molecular properties, such as geometry, kinetics, and electronic state, of the molecules through nuclear-electron interaction. Theoretical considerations and experimental alignments have been studied by ab initio molecular orbital, density functional theory, and other empirical strategies. The Born-Oppenheimer approximation with nuclear vibrational wavefunction can treat isotope effect because nuclear mass effect account for the average distance of vibrational motion. In this study, we introduce Morse anharmonic oscillator model to calculate average internuclear distance of diatomic molecules having X-H bonding and X-D bonding. Morse parameters are determined by fitting to potential energy surface of molecular orbital and/or density functional calculations, and then the average distance are obtained as the expectation value of the analytical Morse vibrational wavefunction. Nuclear magnetic resonance shielding constants of the H/D isotopomer are calculated again on the average distance by using GIAO with B3LYP and CCSD calculation. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2010 [source]

Pronounced stereospecificity of 1H, 13C, 15N and 77Se shielding constants in the selenophenyl oximes as shown by NMR spectroscopy and GIAO calculations

MAGNETIC RESONANCE IN CHEMISTRY, Issue 10 2009
Andrei V. Afonin
Abstract In the 1H and 13C NMR spectra of 1-(2-selenophenyl)-1-alkanone oximes, the 1H, the 13C-3 and 13C-5 signals of the selenophene ring are shifted by 0.1,0.4, 2.5,3.0 and 5.5,6.0 ppm, respectively, to higher frequencies, whereas those of the 13C-1, 13C-2 and 13C-4 carbons are shifted by 4,5, ,11 and ,1.7 ppm to lower frequencies on going from the E to Z isomer. The 15N chemical shift of the oximic nitrogen is larger by 13,16 ppm in the E isomer relative to the Z isomer. An extraordinarily large difference (above 90 ppm) between the 77Se resonance positions is revealed in the studied oxime isomers, the 77Se peak being shifted to higher frequencies in the Z isomer. The trends in the changes of the measured chemical shifts are well reproduced by the GIAO calculations of the 1H, 13C, 15N and 77Se shielding constants in the energy-favorable conformation with the syn orientation of theCNOH group relative to the selenophene ring. Copyright © 2009 John Wiley & Sons, Ltd. [source]

Conformation analysis and molecular mobility of ethylene and tetrafluoroethylene copolymer using solid-state 19F MAS and 1H , 19F CP/MAS NMR spectroscopy

MAGNETIC RESONANCE IN CHEMISTRY, Issue 7 2004
Keitaro Aimi
Abstract The changes in the conformation and molecular mobility accompanied by a phase transition in the crystalline domain were analyzed for ethylene (E) and tetrafluoroethylene (TFE) copolymer, ETFE, using variable-temperature (VT) solid-state 19F magic angle spinning (MAS) and 1H , 19F cross-polarization (CP)/MAS NMR spectroscopy. The shifts of the signals for fluorines in TFE units to higher frequency and the continuing decrease and increase in the T1,F values suggest that conformational exchange motions exist in the crystalline domain between 42 and 145 °C. Quantum chemical calculations of magnetic shielding constants showed that the high-frequency shift of TFE units should be induced by trans to gauche conformational changes at the CH2CF2 linkage in the E,TFE unit. Although the 19F signals of the crystalline domain are substantially overlapped with those of the amorphous domain at ambient probe temperature (68 °C), they were successfully distinguished by using the dipolar filter and spin-lock pulse sequences at 145 °C. The dipolar coupling constants for the crystalline domain, which can be estimated by fitting the dipolar oscillation behaviors in the 1H , 19F CP curve, showed a significant decrease with increasing temperature from 42 to 145 °C. This is due to the averaging of 1H19F dipolar interactions originating from the molecular motion in the crystalline domain. The increase in molecular mobility in the crystalline domain was clearly shown by VT T1,F and 1H , 19F CP measurements in the phase transition temperature range. Copyright © 2004 John Wiley & Sons, Ltd. [source]

Experimental and theoretical NMR study of selected oxocarboxylic acid oximes

MAGNETIC RESONANCE IN CHEMISTRY, Issue 1 2004
Kamilla Malek
Abstract 1H and 13C NMR spectra of the oxocarboxylic acid oximes 2-hydroxyiminopropanoic acid (1), 2-(4-methylthiazol-2-yl)-2-(hydroxyimino)acetic acid (2) and 2-cyano-2-(hydroxyimino)acetic acid (3) were measured in DMSO- d6, D2O and acetone- d6 solutions. The data indicate the presence of hydrogen bonding in 1 and 2 and a strong electron-withdrawing effect due to the cyano group in 3. The effect of intra- and intermolecular hydrogen bonding on the hydrogen and carbon chemical shifts in these molecules was studied theoretically. Total energy calculations of the stability of various hydrogen-bonded species, in addition to equilibrium parameters and chemical shifts, were calculated using ab initio methods (RHF, MP2) and density functional theory (B3LYP), implemented in the Gaussian 98 software package. The gauge-including atomic orbital (GIAO) method was used to predict magnetic shielding constants. Chemical shift calculations for the most stable species agree fairly well with the observed data, especially for the hydroxyl protons. Substituents adjacent to the ,-carbon show some influence of the oximic and carboxyl groups on the 13C chemical shifts, as expected for groups with different polar and anisotropic character. Copyright © 2003 John Wiley & Sons, Ltd. [source]

Theoretical study on 19F magnetic shielding constants of some metal fluorides

MAGNETIC RESONANCE IN CHEMISTRY, Issue 11 2003
Shu-Hui Cai
Abstract A study is presented of 19F NMR magnetic shielding constants of MF2 (M = Zn, Cd), ,-PbF2, MF3 (M = Al, Ga, In), AMF3 (A = K, Rb, Cs, Ba; M = Mg, Ca, Li, Zn, Cd), BaMgF4, BaZnF4 and Ba2ZnF6 using the DFT/GIAO method. The aug-cc-pVTZ basis set was used for the fluorine atom under investigation and LanL2DZ for the remaining fluorine atoms. The 3,21G(2d) basis set was used for the aluminum atom and CRENBL for the other metal atoms. When appropriate cluster models were employed, the theoretical results obtained from the B3LYP/GIAO calculations are in good agreement with experimental measurements and may be better than those obtained from empirical calculations. The correlation coefficient and the slope of the fitting line between our theoretical predictions and the experimental observations are close to unity. Copyright © 2003 John Wiley & Sons, Ltd. [source]

Thermal Effects and Vibrational Corrections to Transition Metal NMR Chemical Shifts

CHEMISTRY - A EUROPEAN JOURNAL, Issue 21 2004
Sonja Grigoleit Dr.
Abstract Both zero-point and classical thermal effects on the chemical shift of transition metals have been calculated at appropriate levels of density functional theory for a number of complexes of titanium, vanadium, manganese and iron. The zero-point effects were computed by applying a perturbational approach, whereas classical thermal effects were probed by Car,Parrinello molecular dynamics simulations. The systematic investigation shows that both procedures lead to a deshielding of the magnetic shielding constants evaluated at the GIAO-B3,LYP level, which in general also leads to a downfield shift in the relative chemical shifts, ,. The effect is small for the titanium and vanadium complexes, where it is typically on the order of a few dozen ppm, and is larger for the manganese and iron complexes, where it can amount to several hundred ppm. Zero-point corrections are usually smaller than the classical thermal effect. The pronounced downfield shift is due to the sensitivity of the shielding of the metal centre with regard to the metal,ligand bond length, which increase upon vibrational averaging. Both applied methods improve the accuracy of the chemical shifts in some cases, but not in general. [source]

103Rh NMR Chemical Shifts in Organometallic Complexes: A Combined Experimental and Density Functional Study

CHEMISTRY - A EUROPEAN JOURNAL, Issue 16 2004
Laura Orian Dr.
Abstract Experimental 103Rh NMR chemical shifts of mono- and binuclear rhodium(I) complexes containing s - or as -hydroindacenide and indacenediide bridging ligands with different ancillary ligands (1,5-cyclooctadiene, ethylene, carbonyl) are presented. A protocol, based on density functional theory calculations, was established to determine 103Rh NMR shielding constants in order to rationalise the effects of electronic and structural variations on the spectroscopic signal, and to gain insight into the efficiency of this computational method when applied to organometallic systems. Scalar and spin,orbit relativistic effects based on the ZORA (zeroth order regular approximation) level have been taken into account and discussed. A good agreement was found for model compounds over a wide range of chemical shifts of rhodium (,10,000 ppm). This allowed us to discuss the experimental and calculated ,(103Rh) in larger complexes and to relate it to their electronic structure. [source]