The 13C NMR resonances corresponding to the C-Me group of C-6 and/or C-8 C-methylated-flavonoids absorb between 6.7–10.0 ppm and typically between 6.7–8.7 ppm. A comparative 13C NMR study reflects that the 13C NMR chemical shifts reported for 6-hydroxy-5-methyl-3′,4′,5′-trimethoxyaurone-4-O-α-L-rhamnoside from Pterocarpus santalinus and 8-C-methyl-5,7,2′,4′- tetramethoxyflavanone from Terminalia alata are inconsistent with the assigned structures, and therefore need reconsideration.
The hitherto elusive mono-O-protonated deltic acid C3O3H3+ was prepared by protolysis of di-tert-butoxy deltate in FSO3H-SO2ClF and in FSO3H:SbF5 (Magic Acid; 1:1 molar solution) in SO2ClF as solvent at -78°C and was characterized by 1H and 13C NMR spectroscopy. The structure and NMR chemical shifts were also calculated by the ab initio/IGLO method. No NMR evidence was found for persistent di-O-protonated deltic acid under these conditions, although a limited equilibrium with the mono-O-protonated species can be involved. Di-, tri-, and tetra-O-protonated deltic acids were also studied by ab initio/IGLO method.Key words: protonated deltic acid, aromaticity, superacids, NMR spectroscopy, ab initio and IGLO calculations.
The origin in deshielding of 29Si NMR chemical shifts in R3Si–X, where X = H, OMe, Cl, OTf, [CH6B11X6], toluene, and OX (OX = surface oxygen), as well as iPr3Si+ and Mes3Si+ were studied using DFT methods.
The 13C NMR chemical shifts of fifteen 6 substituted 2,3-dihydro-1,4-diazepinium salts (cis trimethincyanines) (1) and twelve 2 substituted bis(dimethylamino)trimethinium salts (trans trimethincyanines) (2) have been determined. A comparison of the substituentinduced shifts (13C SCS) of 1 and 2 allows no distinction between steric and electronic effects. In the three 6 п-electron systems 1, 2 and monosubstituted benzenes the 13C SCS are similar for the substituent bearing carbon atoms. A surprisingly large 4JFCCNC coupling constant has been observed.
The 13C NMR spectra of a series of 5,6-epoxides in decalinic systems were studied. The interpretation of the chemical shifts allowed us to formulate an empirical rule to predict the epoxide stereochemistry. A discussion of the scope and limitations of this method and its extension to larger carbon skeletons is also presented.Key words: epoxide stereochemistry, 13C NMR, NMR, decalinic systems, oxiranes.
Abstract
The linear correlation of the pKa to the σ* constants of the substituted phenylthio group indicates the validity of the σ* being equal to the sum of the Hammett's constant of these substituent groups and the σ* of the phenyl group (0.6). The inhibitory activity of the compound LSR depends on the reactivity of BtSSR with BtSH and the general stability of the mixed disulfides which form during the curing process. For BtSSR as accelerators, the Hammett's rule applies to scorch delay time versus σ* constant as the variable with negative slope in the LFER. In benzothiazole-2-sulfenamides, the 13C NMR chemical shift of the carbon at the 2-position of the benzothiazolyl group (C8) is consistently greater than the chemical shift at the carbon adjacent to the amino nitrogen (CN) in the same molecule. It indicates that the electronic distribution at the S-N bond is consistently more positive at the S atom and more negative at the amino nitrogen. The 13C NMR chemical shift at C8 is inversely proportional to the σ* constant of the amino substituents. A wider range of amino substituents is applicable for the correlation analysis with 13C chemical shifts than σ* constants as the variable. When benzothiazole-2-sulfenamides are used as accelerators, two linear relationships with slopes of opposite signs are obtained for the N-substituted phenyl and the N-alkyl sulfenamides, respectively, in the relationship of the scorch delay to the 13C8 chemical shift or the σ* constant. Longer scorch delay was obtained with the pertinent electron-withdrawing phenyl or the sterically hindered alkyl substituents. The more basic amino derivatives give a faster acceleration rate and a higher crosslink efficiency. A significant linear correlation was obtained for scorch delay versus Hammett's σ constant of the phenyl substituents of the N,N′-dithiobisformanilides as vulcanizing agents.
13C nmr chemical shifts are reported for some aromatic and aliphatic tellurium compounds. For a given organic group, the shift of the C1 atom varies in the order [Formula: see text], as expected from electronegative considerations. The C2 atom experiences an opposite trend while the C3 and C4 atoms of the ring experience smaller changes. The chemical shifts of para-substituted aromatic tellurium compounds do not show additivity of contributions from the substituents.
1H and 13C NMR spectra of twelve 1,4-dihydropyridines I and twelve corresponding pyridine derivatives II have been measured in hexadeuteriodimethyl sulphoxide and interpreted. Correlation equations are given for the chemical shifts of the atoms in the heterocyclic rings of the two series of compounds.
We have developed a two-layer Molecules-in-Molecules (MIM2) fragmentation-based quantum chemical method including an efficient solvation model for the prediction of NMR chemical shifts with a target accuracy of ∼0.30 ppm for 1H and ∼2–3 ppm for 13C.
The unexpected roles of σ and π orbitals in electron donor and acceptor group effects on the 13C NMR chemical shifts in substituted benzenes