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GIAO Calculations (giao + calculation)

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Chemistry100%

Selected Abstracts

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]

1H chemical shifts in NMR.

MAGNETIC RESONANCE IN CHEMISTRY, Issue 1 2005
Part 2, Prediction of the 1H chemical shifts of molecules containing the ester group: a modelling, ab initio investigation
Abstract The 1H NMR spectra of 24 compounds containing the ester group are given and assigned. These data were used to investigate the effect of the ester group on the 1H chemical shifts in these molecules. These effects were analysed using the CHARGE model, which incorporates the electric field, magnetic anisotropy and steric effects of the functional group for long-range protons together with functions for the calculation of the two- and three-bond effects. The effect of the ester electric field was given by considering the partial atomic charges on the three atoms of the ester group. The anisotropy of the carbonyl group was reproduced with an asymmetric magnetic anisotropy acting at the midpoint of the carbonyl bond with values of ,,parl and ,,perp of 10.1 × 10,30 and ,17.1 × 10,30 cm3 molecule,1. An aromatic ring current (=0.3 times the benzene ring current) was found to be necessary for pyrone but none for maleic anhydride. This result was confirmed by GIAO calculations. The observed 1H chemical shifts in the above compounds were compared with those calculated by CHARGE and the ab initio GIAO method (B3LYP/6,31G**). For the 24 compounds investigated with 150 1H chemical shifts spanning a range of ca 10 ppm, the CHARGE model gave an excellent r.m.s. error (obs , calc) of <0.1 ppm. The GIAO calculations gave a very reasonable r.m.s. error of ca 0.2 ppm although larger deviations of ca 0.5 ppm were observed for protons near to the electronegative atoms. The accurate predictions of the 1H chemical shifts given by the CHARGE model were used in the conformational analysis of the vinyl esters methyl acrylate and methyl crotonate. An illustration of the use of the CHARGE model in the prediction of the 1H spectrum of a complex organic molecule (benzochromen-6-one) is also given. Copyright © 2004 John Wiley & Sons, Ltd. [source]

Quantum Mechanical Calculations of Conformationally Relevant 1H and 13C,NMR Chemical Shifts of N-, O-, and S-Substituted Calixarene Systems

CHEMISTRY - A EUROPEAN JOURNAL, Issue 25 2007
Giuseppe Bifulco Prof.
Abstract QM GIAO calculations of 13C and 1H chemical shift values of the ArCH2Ar group in N-, O-, and S-substituted calixarene systems were performed with a hybrid DFT functional MPW1PW91 and 6-31G(d,p) basis set. A good reproduction of experimental data was obtained for some representative calixarenes and for a series of simplified calixarene models. This allowed the derivation of chemical shift surfaces versus , and , dihedral angles. The applicability of chemical shift surfaces in the study of calixarene conformational features is illustrated. [source]