Search results for "Chemical shift"
showing 10 items of 205 documents
NMR Study of the (Z)-Phenylhydrazones of 5-Alkyl- and 5-Aryl-3-benzoyl-1,2,4-oxadiazoles: Support for the Interpretation of Kinetic Results on the Re…
2005
An analysis of the 1H, 13C, and 15N NMR substituent chemical shifts (SCSs) of the title compounds in CDCl3 solution has been carried out by using the SCSs of other classes of compounds (arenes, methyl esters or amides), Hammett substituent constants or kinetic data. The results obtained provide information concerning the ground-state electronic distribution of the compounds examined. The results relevant to the carbon and nitrogen atoms of the 1,2,4-oxadiazole ring can be considered of special interest, as the effects of the substit- Introduction For several years our research group has been involved in the use of NMR spectroscopic data (1H, 13C, 15N, and 17O) to obtain information about th…
Configurational effects in17O NMR chemical shifts of hydroxyl-substituted, bicyclo [2.2.1]heptenes and -heptanes, their methylated derivatives and so…
1993
Well resolved natural abundance 17O NMR spectra of 27 mono-, di-, tri- and tetrahydroxybicyclo[2.2.1]heptenes and -heptanes (norbornenes or norbornanes), their methylated derivatives and some related compounds were measured for 0.5 M or saturated solutions in acetonitrile at 65°C. In addition, the 17O NMR chemical shifts of 14 aliphatic and alicyclic alcohols were also determined for reference purposes. The 17O NMR chemical shifts of all compounds were assigned. Both methyl- and hydroxyl-induced deshielding β-effects were consistent with those reported in the literature. Similarly, observed shielding γ-effects were in agreement with the earlier observations and characteristic especially for…
Predominance of 2-arylhydrazones of 1,3-diphenylpropane-1,2,3-trione over its proton-transfer products
2001
2-Phenylhydrazones of 1,3-diphenyl-1,2,3-trione are the dominant tautomeric form detected in chloroform solution by 15N NMR chemical shifts. The substituent in the phenylhydrazone moiety does not affect this tautomeric preference. The substituent effect is transmitted effectively only to the hydrazone nitrogen and hydrogen atoms. Ab initio calculations show that the ketohydrazone tautomer is really very much favoured over its proton-transfer products in chloroform solution. The same tautomer was also detected in the crystal state by X-ray crystallography. Copyright © 2001 John Wiley & Sons, Ltd.
1H,13C and17O NMR study of chlorovanillins and some related compounds
1992
1H, 13C and 17O NMR chemical shifts and nJ(H,H), 1J(C,H) and 3J(C-6, H-formyl) spin—spin coupling constants of chlorinated vanillins (3-methoxy-4-hydroxybenzaldehydes) were determined. The variation in the long-range 4J(H,H) value between the formyl proton and an ortho-proton suggests that the ortho-substituent strongly forces the formyl group out of the aromatic plane. This can also be observed by considering the 3J(C-6,H-formyl) values and the 13C and 17O NMR chemical shifts. The 17O NMR chemical shifts show a linear correlation with the torsion angle of the formyl group calculated by molecular mechanics. The 13C and 17O NMR chemical shifts of the methoxyl group also possess strong predic…
Dimethoxy aromatic compounds. IV.—determination of stereochemistry of 2,3,6,7-tetraalkoxy-9,10-dihalomethyl-9,10-dihydroanthracenes by13C NMR chemica…
1987
The analysis of the 13C NMR spectra of several 2,3,6,7-tetraalkoxy-9,10-dihalomethyl-9,10-dihydroanthracenes showed a strong dependence of the chemical shift values on the orientation of the halomethyl groups. On this basis it was possible to determine both the configuration (cis or trans) and conformation of the isomers, even if only one isomer was available.
17O NMR study of isomeric monochloro- and monohydroxy-benzaldehydes and chlorinated salicylaldehydes
1991
The 17O NMR chemical shifts of isomeric monochloro- and monohydroxy-benzaldehydes and chlorinated salicylaldehydes were measured at 40°C for 0.25 M CDCI3 solutions. The 17O NMR chemical shift of the carbonyl oxygen of the compounds studied varies from 506 to 573 ppm measured from external D2O. The observed variation is probably mainly due to the intramolecular hydrogen bonding between the adjacent carbonyl and hydroxyl groups. The 17O NMR chemical shift range of the hydroxyl oxygen is from 80 to 98 ppm.
Multinuclear NMR study of 1,3,3-trimethylbicyclo[2.2.1]heptan-2-one (fenchone) and its six monochlorinated derivatives
1990
1H, 13C and 17O NMR spectra of fenchone and six monochlorofenchones have been recorded. The second-order 1H NMR spectra were analysed by an iterative computer program. The 1H and 13C chlorine-induced substituent chemical shifts (SCS) were calculated. The vicinal coupling constants proved to be essential in the assignment of the chemical shifts of the geminal methyls. No clear correlation was observed between the chemical shifts of the carbonyl oxygen and the carbonyl carbon. All monochlorofenchones were previously unknown compounds and were synthesized for this work.
Five-to-Six Membered Ring-rearrangements in the Reaction of 5-Perfluoroalkyl-1,2,4-oxadiazoles with Hydrazine and Methylhydrazine
2006
The hydrazinolysis reaction of 5-perfluoroalkyl-1,2,4-oxadiazoles with hydrazine or methylhydrazine as bidentate nucleophiles has been investigated. The reaction occurred through the addition of the bidentate nucleophile to the C(5)-N(4) double bond of the 1,2,4-oxadiazole followed by ring-opening and ring-closure (ANRORC) involving the second nucleophilic site of the reagent. This ring-closure step could involve either the original C(3) of the 1,2,4-oxadiazole (giving a five-to-five membered ring rearrangement) or an additional electrophilic center linked to it (exploiting a five-to-six membered ring rearrangement). An alternative initial nucleophilic attack may involve the additional elec…
On the aromaticity of uracil and its 5-halogeno derivatives as revealed by theoretically derived geometric and magnetic indexes
2020
AbstractThe problem of aromaticity in heterocyclic rings of uracil and its 5-halogenoderivatives (5XU) was analyzed theoretically by calculating modified harmonic oscillator model of aromaticity (HOMA) for Heterocycle Electron Delocalization (HOMHED), nucleus-independent chemical shift parameters (NICS) and the so-called scan experiments, using helium-3 atom as a magnetic probe. The impact of halogen electronegativity on C5 atom’s NBO charges was also investigated. Water, as a polar environment, has a negligible impact on 5XU aromaticity. The most stable diketo tautomer shows a very low aromaticity while the “rare” dihydroxy form (tautomer No 6) is aromatic and resembles benzene. This is in…
Discrete bond model (DBM) of sodium silicate glasses derived from XPS, Raman and NMR measurements
1993
In sodium silicate glasses, the fraction of differently bound Si species Q[i] (i = 0−4), depending on the number i of bridging oxygens bound to the quarternary silicon, is a function of stoichiometry and the Na/Si ratio. Sodium silicate glasses were investigated by high resolution X-ray photoelectron spectroscopy. To explain the differences in chemical shifts and linewidths of the O 1s signal of the bridging and the non-bridging oxygen as a function of alkali concentration, and extended glass model was developed. This new model takes into account the influence of the alkali concentration on the Q[i] distribution and on the appearance and concentrations of differently bound bridging oxygens …