Chemical Shifts In Organic Compounds Research Articles

A convenient and accurate method for predicting13C chemical shifts in organic molecules

13C chemical shifts of organic compounds including 5α-androstan-17-one are predicted by the MMFF94 semi-empirical and GIAO B3LYP/6-31G(d)ab initiomethods.

Correlation Between Experimental and DFT/GIAO Computed 13C NMR Chemical Shifts of Organic Compounds. Effects of Asymmetry

The experimental 13 C chemical shifts of five different series of organic compounds are compared with pre- dicted 13 C NMR chemical shifts obtained via empirically scaled GIAO shieldings. Our results indicate that the inclusion of a scaling factor allow to obtain an excellent correlation between δcalc and δexp. Although the inclusion of asymmetry im- proves this correlation, such enhancement was not observed for all the 54 tested compounds. We found RMS, a parameter related with the structural feature of the whole molecule, which could indicate the benefits of including asymmetry in these calculations.

Redshift and Blueshift Due to Interaction between C–H Bond of Methyl Radicals and Highly Electronegative Oxygen

Conventional chemical shifts in organic compounds usually consist of a redshift; however, hybrid-type SiOC films were found to exhibit a redshit and a blueshift. These chemical shifts originate from the interaction between the C–H bond and highly electronegative oxygen. The bonding structures of SiOC films can be divided into two types depending on the observed chemical shifts and consist of a Si–O–C cross-link structure and a Si–O–C cage-link structure. The blue shift in SiOC films is associated with the porosity due to an increase in the number of electron-rich groups, such as methyl radicals, resulting in the formation of a Si–O–C cage link structure. On the other hand, the Si–O–C cross-link structure associated with a redshift is generated from the adhesion due to C–H bond elongation.

The impact of available experimental data on the prediction of 1H NMR chemical shifts by neural networks.

Two different ways were explored to incorporate new available experimental data into previously trained ensembles of feed-forward neural networks, for the structure-based prediction of (1)H NMR chemical shifts of organic compounds. One approach used the new data as the memory of an associative neural network (ASNN) system. For an independent prediction set of 952 cases, a mean average error of 0.19 ppm was achieved (0.13 ppm for 90% of the cases). This approach advantageously avoids retraining the networks, and the predictions compared favorably with those obtained by available commercial software packages. Excellent predictions could also be achieved by retraining the networks with the new data, but only if the training sets were selected so as to be balanced or if the retraining started with the weights of the previously trained networks.

ChemInform Abstract: Prediction of Nitrogen and Oxygen NMR Chemical Shifts in Organic Compounds by Density Functional Theory

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Prediction of Nitrogen and Oxygen NMR Chemical Shifts in Organic Compounds by Density Functional Theory

Sum-over-states density functional perturbation theory (SOS-DFPT) and Hartree−Fock (HF) theory based on the “individual gauge for localized orbitals” (IGLO) scheme were used to calculate 13C, 14N/15N, and 17O NMR chemical shifts of 23 molecules. Employing the (11s7p2d/6s2p)[7s6p2d/4s2p] basis set, SOS-DFPT chemical shifts were obtained that are clearly better than HF- or UDFT-based chemical shifts, as is reflected by mean absolute deviations of 2.8 (SOS-DFPT), 7.6 (UDFT), and 5.6 ppm (HF) for 13C chemical shifts, 11.8, 22.1, and 100.5 ppm for 14N/15N chemical shifts, and 36.4, 57.2, and 45.7 ppm for 17O chemical shifts with regard to experimental values. By estimating appropriate gas phase values for 14N/15N chemical shifts from measured solvent effects, average errors of SOS-DFPT chemical shifts are reduced to 3 ppm, which is close to uncertainties in experimental values. SOS-DFPT is the method of choice for predicting reasonably accurate NMR chemical shifts at relatively low computational cost even in c...

15N chemical shifts in organic compounds

Concepts in Magnetic ResonanceVolume 7, Issue 3 p. 243-243 Article 15N chemical shifts in organic compounds First published: 1995 https://doi.org/10.1002/cmr.1820070305AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat No abstract is available for this article. Volume7, Issue31995Pages 243-243 RelatedInformation

15N chemical shifts in organic compounds

¹⁵N chemical shifts in organic compounds

A computer program for the prediction of 13-C-NMR chemical shifts of organic compounds

A computer program is described for the estimation of carbon-13 chemical shifts. It automatically selects and applies the additivity rules appropriate for the individual carbon atoms of the structure entered. Besides connectivity, the configuration and conformation can be entered and evaluated for some types of structure. All rule parameters (base values and increments) can be extended or modified by the user. New rules based on the corresponding linear relationships can also be added. A test against 168 807 known chemical shifts shows that the implemented rules cover roughly 97% of all cases. The standard deviation of the predicted values relative to the experimental values is 5.5 ppm.

Tritium nuclear magnetic resonance spectroscopy. Part 11. Further consideration of referencing, isotope effects and coupling constants: Preparation of [3H]tetramethylsilane

AbstractSmall discrepancies between 3H and 1H chemical shifts in organic compounds led to the finding that the ratio of Larmor frequencies ωT/ωH depends on the carbon‐hydrogen bond hybridisation. The ‘best’ ratio for ghost referencing of 3H NMR spectra was then determined from measurements on purified partially tritiated TMS as 1.066639738±2 × 10−9. Some 3H isotope effects on chemical shifts are listed and the 3H isotope effect on the methylene geminal coupling constant in benzyl methyl sulphoxide is measured. Use of 2H in the measurement of 2J(HH) coupling constants has inherent disadvantages, overcome by use of 3H substitution.

14N chemical shifts in organic compounds

Quadrupole broadening of nitrogen nuclear resonances can be minimized by using solvents of low viscosity. An extensive survey is presented of 14N chemical shifts in organic compounds.