Assignment of the carbon-13 chemical shifts of quinacrine,chloroquine and related compounds in D2O solution

The carbon resonances of quinacrine, chloroquine, acranil, 4-aminopyridine and 9-aminoacridine in D₂O solution have been assigned. Resonance assignments were made using empirical shift parameters, partial proton decoupling, selective proton decoupling and by interpretation of the fully coupled spectra. The effect of pD on the carbon chemical shifts for quinacrine and chloroquine over the range of about 4.5 to 8.5 was observed. Characteristic chemical shifts for the aromatic ring carbons for deprotonation of the heteroaromatic nitrogen were observed.     

13C chemical shifts of monosubstituted adamantanes

¹³C chemical shifts are reported for adamantane, nine 1-substituted adamantanes and nine 2-substituted adamantanes. The substituents are F, Cl, Br, I, NH₂, OH, CH², CN and CO₂H. The assignments and results are discussed in terms of chemical shift patterns.     

13C chemical shifts of some model olefins

An extensive carbon-13 nuclear magnetic resonance study of selected model olefins dissolved in deuteriochloroform has been carried out under standardized conditions. Assignments of the chemical shifts have been made. The influence of the nature of the solvent and the effect of changing the concentration of the solute have been investigated. The results are intended to provide a practical aid for the analysis of olefinic materials.     

Correlations of13C chemical shifts and geometry modifications

The effect of geometry modifications of¹³C chemical shifts has been investigated in a small subset of molecules using both LO-INDO and Gaussian 70 (4–31) calculations. The Gaussian calculations, while known to give poor absolute shifts, compare well to the reparameterized semi-empirical INDO determinations in calculated shift changes. In virtually all cases the signs of the shift changes were found to be opposite to that of the changes in the calculated electronic energy.     

Prediction of 13C NMR chemical shifts in substituted naphthalenes

The prediction of the ¹³C NMR signals for derivatives of naphthalene has been investigated using mathematical modeling techniques. Two empirical multiple regression models which utilize the field, resonance, and Charton's steric parameters together with molar refractivity were developed, one for α- and the other for β-substituted naphthalene derivatives. In the α case the model had a correlation coefficient of observed versus predicted line positions of r = 0.973 with a standard deviation of 2.2 ppm while in the β case r = 0.979 with the standard deviation being 2.3 ppm. The database consisted of 3152 signals from 394 naphthalene derivatives. We also report the use of the Taft steric parameter in place of the Charton steric parameter in the above- mentioned prediction equations. Received: 19 June 1998 / Accepted: 20 October 1998 / Published online: 16 March 1999     

13C chemical shifts and conformational analysis of S-methylthiolanium cations

The ¹³C NMR spectra of all cations obtained by methylation at sulphur of the mono-and dimethylthiolanes are reported. The methyl substituent on sulphur affects the shieldings of the adjacent carbons in a manner which allows easy identification of the cis and trans isomers. For most compounds the ¹³C pattern is consistent with a half-chair ring conformation with maximum staggering at C-3, C-4. Only with methyl groups at the 1,2-or 1,2,3-positions is the half-chair appreciably deformed. It is suggested that in these cases the preferred conformation is a quasi-envelope with C-3 at the top.     

13C NMR chemical shifts of N-unsubstituted- and N-methyl-pyrazole derivatives

¹³C shielding data for 100 derivatives of pyrazole are reported. These include methyl, ethyl, n-propyl, tert-butyl, phenyl, hydroxymethyl, carboxyl, ethoxycarbonyl, cyano, amino, hydrazino, nitro, azido, chloro, bromo and iodo groups as substituents on the ring carbon atoms.     

Remarks on GIAO-DFT predictions of 13C chemical shifts

Predicting (13)C chemical shifts by GIAO-DFT calculations appears to be more accurate than frequently expected provided that: (a) the comparison between experimental and theoretical data is performed using the linear regression method, (b) a sufficiently high level of theory [e.g. B3LYP/6-311 + + G(2d,p)//B3LYP/6-311 + + G(2d,p) or PBE1PBE/6-311 + G(2df,p)//B3LYP/6-311 + + G(2d,p)] is used, (c) the experimental data originate from the measurements performed in one solvent whose influence is taken into account at the molecular geometry optimization step and, first of all, during the shielding calculation, (d) the experimental data are free of heavy atom effects or such effects are appropriately treated in calculations, and finally (e) the conformational compositions of the investigated objects are known.     

35Cl/37Cl isotope effects on 13C chemical shifts. Assignment of carbon-13 NMR lines of polychlorinated hydrocarbons

An isotope effect of about 0·1 Hz at 25 MHz (0·004 ppm) on the ¹³C chemical shifts due to directly bonded chlorine has been observed at ?50°C for hexachloropropene, hexachlorobutadiene and tetrachlorobutenyne. This effect yields an unambiguous assignment of ¹³C NMR lines to specific carbon atoms in these polychlorinated hydrocarbons.     

Substituent effects in the 13C NMR chemical shifts of alpha-mono-substituted acetonitriles

13C chemical shifts empirical calculations, through a very simple additivity relationship, for the alpha-methylene carbon of some alpha-mono-substituted acetonitriles, Y-CH(2)-CN (Y=H, F, Cl, Br, I, OMe, OEt, SMe, SEt, NMe(2), NEt(2), Me and Et), lead to similar, or even better, results in comparison to the reported values obtained through Quantum Mechanics methods. The observed deviations, for some substituents, are very similar for both approaches. This divergence between experimental and calculated, either empirically or theoretically, values are smaller than for the corresponding acetones, amides, acetic acids and methyl esters, which had been named non-additivity effects (or intramolecular interaction chemical shifts, ICS) and attributed to some orbital interactions. Here, these orbital interactions do not seem to be the main reason for the non-additivity effects in the empirical calculations, which must be due solely to the magnetic anisotropy of the heavy atom present in the substituent. These deviations, which were also observed in the theoretical calculations, were attributed in that case to the non-inclusion of relativistic effects and spin-orbit coupling in the Hamiltonian. Some divergence is also observed for the cyano carbon chemical shifts, probably due to the same reasons.     

Computational studies of 13C NMR chemical shifts of saccharides

The (13)C NMR chemical shifts for alpha-D-lyxofuranose, alpha-D-lyxopyranose (1)C(4), alpha-D-lyxopyranose (4)C(1), alpha-D-glucopyranose (4)C(1), and alpha-D-glucofuranose have been studied at ab initio and density-functional theory levels using TZVP quality basis set. The methods were tested by calculating the nuclear magnetic shieldings for tetramethylsilane (TMS) at different levels of theory using large basis sets. Test calculations on the monosaccharides showed B3LYP(TZVP) and BP86(TZVP) to be cost-efficient levels of theory for calculation of NMR chemical shifts of carbohydrates. The accuracy of the molecular structures and chemical shifts calculated at the B3LYP(TZVP) level is comparable to those obtained at the MP2(TZVP) level. Solvent effects were considered by surrounding the saccharides by water molecules and also by employing a continuum solvent model. None of the applied methods to consider solvent effects was successful. The B3LYP(TZVP) and MP2(TZVP)(13)C NMR chemical shift calculations yielded without solvent and rovibrational corrections an average deviation of 5.4 ppm and 5.0 ppm between calculated and measured shifts. A closer agreement between calculated and measured chemical shifts can be obtained by using a reference compound that is structurally reminiscent of saccharides such as neat methanol. An accurate shielding reference for carbohydrates can be constructed by adding an empirical constant shift to the calculated chemical shifts, deduced from comparisons of B3LYP(TZVP) or BP86(TZVP) and measured chemical shifts of monosaccharides. The systematic deviation of about 3 ppm for O(1)H chemical shifts can be designed to hydrogen bonding, whereas solvent effects on the (1)H NMR chemical shifts of C(1)H were found to be small. At the B3LYP(TZVP) level, the barrier for the torsional motion of the hydroxyl group at C(6) in alpha-D-glucofuranose was calculated to 7.5 kcal mol(-1). The torsional displacement was found to introduce large changes of up to 10 ppm to the (13)C NMR chemical shifts yielding uncertainties of about +/-2 ppm in the chemical shifts.     

Deuterium isotope effects on 13C chemical shifts of nitromalonamide

The OH chemical shift of the enol form of nitromalonamide is found at 18.9 ppm both in DMSO-d(6) and in DMF-d(7) indicating a very strong hydrogen bond. The OH chemical shift is insensitive to temperature changes. Contrary to the large OH chemical shift, a small two-bond deuterium isotope effect of 0.135 ppm due to deuteration at the OH position is found at the enolic carbon. This is confirmed by density functional theory calculations. The observed effects are interpreted as due to an equilibrium between identical enolic forms. These show a strong OH...O hydrogen bond as well as a NH...O-N=O hydrogen bond.     

核磁共振化学位移的MNDO/GIAO微扰法计算

本文首次应用MNDO近似和规范不变的原子轨道(GIAO)计算了化学位移,提出了MNDO/GIAO微扰计算法及单电子算符1/r_m,L_m及L_m/r_m~3的简单有效积分法.通过优化选择MNDO的β_μ参数,对一组典型有机分子~(13)C化学位移的计算结果表明,理论值与实验数据符合得很好,并克服了SOS微扰法对坐标原点的依赖性.     

13C chemical shifts of Δ1-pyrrolines

¹³C chemical shifts are reported for the ring carbons of several substituted Δ¹-pyrrolines. Average values for methine and methylene ring carbons facilitate structure elucidation of substituted δ¹-pyrrolines by ¹³C NMR spectroscopy.     

13C and 19F chemical shifts of bridgehead halogenated adamantanes

The ¹³C chemical shifts of the sixteen bridgehead substituted mono-, di-, tri- and tetrahaloadamantanes (halo = F, Cl, Br, I) and four mixed 1,3-dihaloadamantanes are reported. The effect of bridgehead halogens on the shift values of carbons in β and δ positions is well correlated by the simple additivity relationship based on substituent shifts of 1-monohaloadamantanes. A substituted α-carbon is shifted upfield with an increase in the number of halogens at other bridgehead positions and this shift is relatively greater in the order F < Cl < Br < I. This observed upfield shift of α-carbons is opposite to the downfield shift expected from additivity. An unsubstituted bridgehead γ-carbon is moved to lower fields by one, two and three bromines (or iodines) at other bridge-heads while, in contrast, a third fluorine weakly shields the remaining unsubstituted γ-carbon. Some special noncumulative effects of halogens operating across the 1,3-bridgehead positions of adamantane are indicated by the data. The ¹⁹F chemical shifts of 1-fluoro-, 1,3-difluoro-, 1,3,5-trifluoro- and 1,3,5,7-tetrafluoroadamantanes are contrary to expectations based on inductive effects in that they move progressively upfield. Other 1-fluoroadamantanes with chloro, bromo, or methyl groups present also show substituent-induced chemical shifts which shield the fluorine.