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1.
The calculation of 15N NMR chemical shifts of 27 azoles and azines in 10 different solvents each has been carried out at the gauge including atomic orbitals density functional theory level in gas phase and applying the integral equation formalism polarizable continuum model (IEF‐PCM) and supermolecule solvation models to account for solvent effects. In the calculation of 15N NMR, chemical shifts of the nitrogen‐containing heterocycles dissolved in nonpolar and polar aprotic solvents, taking into account solvent effect is sufficient within the IEF‐PCM scheme, whereas for polar protic solvents with large dielectric constants, the use of supermolecule solvation model is recommended. A good agreement between calculated 460 values of 15N NMR chemical shifts and experiment is found with the IEF‐PCM scheme characterized by MAE of 7.1 ppm in the range of more than 300 ppm (about 2%). The best result is achieved with the supermolecule solvation model performing slightly better (MAE 6.5 ppm). Copyright © 2014 John Wiley & Sons, Ltd.  相似文献   

2.
15N chemical shifts of twenty-four substituted indoles have been determined in natural abundance (in organic solvents) using Fourier transform NMR. The overall chemical shift range is 27 ppm, with groups in the 2-, 3- and 5-ring positions showing the largest substituent effects. Substituents capable of resonance interaction with the indole nitrogen give shifts in the expected directions but they cannot be correlated with known substituent parameters. Compounds measured in DMSO give 0·2 to 10·2 ppm downfield shifts with respect to the same compound measured in CDCl3. 13C NMR data for previously unreported compounds are also reported.  相似文献   

3.
The NMR chemical shifts of alkali and thallium(I) salts with various monovalent anions have been measured in N-methylformamide solution. Lithium-7 chemical shifts are virtually concentration and counter-ion independent, presumably due to an absence of direct cation-anion interactions. The sodium-23, potassium-39 and cesium-133 chemical shifts of the salts studied depend on the anion and vary linearly with the concentration. The observed behavior can be accounted for by the formation of collisional ion pairs. On the other hand, the thallium-205 chemical shifts of thallium(I) nitrate and perchlorate were anion-dependent and varied non-linearly with the salt concentration. These results are indicative of contact ion pair formation; formation constants were calculated to be 2.6±0.4 M –1 for TlNO 3 and 1.7±0.5 M –1 for TlClO 4 . The cesium-133 NMR spectra of several mixed electrolyte systems also have been measured in N-methylformamide solution. The133Cs chemical shifts also change linearly with the concentrations of the salts added to 0.10 M CsI/NMF solutions. The influence of the anions on the chemical shifts is the same as that observed for cesium salts alone.  相似文献   

4.
Fluorine-19 and sodium-23 NMR measurements were carried out on sodium hexafluorophosphate solutions in a number of solvents. In solvents of medium polarity and donicity (e.g., propylene carbonate, acetone, acetonitrile) the 19 F chemical shift moved upfield with increasing concentration of the salt. This behavior is indicative of anion-cation interactions which may be of long-range type, i.e., formation of solvent-separated ion pairs; the possibility of contact ion pair formation, however, cannot be excluded. In solvents of low polarity and donicity (acetic acid, tetrahydrofuran), the salt is essentially completely associated in the 0.1–1.0M concentration range. On the other hand, in solvating solvents with high dielectric constants, such as dimethyl-formamide, dimethylsulfoxide, and formamide, there is very little ionic association in the same concentration range. The above conclusions are supported by 23 Na chemical shift measurements. Potassium hexafluorophosphate solutions do not show any concentration dependence of the 19 F chemical shifts, while for tetra-n-butylammonium solutions the 19 F resonance moves downfield with increasing concentration of the salt.To whom correspondence should be addressed.  相似文献   

5.
Conformational and relativistic effects on the 31P and 77Se chemical shifts of phosphine selenides were analyzed in terms of the ZORA-GIAO-B1PW91/TZP approach. The effect of conformation of phosphine selenides related to internal rotation about the single P-C bonds was found to be insignificant, while the contribution of relativistic spin-orbit interaction to the calculated values of 77Se chemical shifts did not exceed 10 ppm. On the other hand, relativistic effects arising from magnetic shielding of the phosphorus nucleus in the P=Se fragment by selenium are fairly strong (25–30 ppm), which indicates the necessity of including the contribution of relativistic spin-orbit interaction in the calculation of 31P chemical shifts in phosphine selenides.  相似文献   

6.
Derivatives of 3-imidazoline 3-oxide have been studied by 14N and 17O NMR methods. Regularities of the influence of substituents and of a hydrogen bond on chemical shifts have been made apparent. The range of changes of the chemical shifts of the nitrogen and oxygen nuclei of the nitrone group has been determined. Both in the 17O and in the 14N NMR spectra the signals of the amino derivatives are the highest field signals for the nitrone group, and the lowest field signals are the signals of the cyano derivatives in the series of derivatives investigated. Depending on the substituent (from amino to cyano group) the 17O chemical shifts varied over a range ∼155 ppm, but the interval of change of the 14N chemical shifts for the same substituents was ∼110 ppm. __________ Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 9, pp. 1336–1341. September, 2005.  相似文献   

7.
Ten new pyrazoles have been prepared and their 13C nmr chemical shifts compared with those of twelve other pyrazoles, some of them prepared purposely for this study. The chemical shifts are discussed statistically assuming that they are additive. A formyl group in the position 4 of the pyrazole ring produces a large effect on carbon C4 (SCS = 17.3 ppm) and medium effects on carbons C3 (SCS = 1.9 ppm) and C5 (SCS = 3.8 ppm). The azines derived from pyrazole-4-carboxaldehydes are of the E,E-configuration.  相似文献   

8.
The 17O chemical shift data for a series of azine N-oxides, diazine N-oxides and di-N-oxides at natural abundance are reported. Isomeric methyl substituted quinoline N-oxides exhibited chemical shifts which are interpreted in terms of electronic and compressional effects. The 17O chemical shift for 8-methylquinoline N-oxide (370 ppm) is deshielded by 25 ppm more than predicted, based upon electronic considerations. The 17O chemical shift for the N-oxide of 8-hydroxyquinoline (289 ppm) is substantially shielded as a result of intramolecular hydrogen bonding. The relative 17O chemical shifts for diazine N-oxides of pyrazine, pyridazine and pyrimidine follow predictions based on back donation considerations. Because of solubility limitations, spectra of only two N,N′-dioxides were obtained. The chemical shift of benzopyrazine di N-oxide in acetonitrile was shielded by 18 ppm compared to that of its mono N-oxide.  相似文献   

9.
A number of computational schemes based on different Density Functional Theory (DFT) functionals in combination with a number of basis sets were tested in the calculation of 1H and 13C NMR chemical shifts of strychnine, as a typical representative of the vitally important natural products, and used as a challenging benchmark and a rigorous test for such calculations. It was found that the most accurate computational scheme, as compared with experiment, was PBE0/pcSseg-4//pcseg-3 characterized by a mean absolute error of 0.07 ppm for the range of about 7 ppm for 1H NMR chemical shifts and that of only 1.13 ppm for 13C NMR chemical shifts spread over the range of about 150 ppm. For more practical purposes, including investigation of larger molecules from this series, a much more economical computational scheme, PBE0/pcSseg-2//pcseg-2, characterized by almost the same accuracy and much less computational demand, was recommended.  相似文献   

10.
Calculations of 29Si NMR chemical shifts of 68 silanes possessing various substituents, in particular, with heavy halogens attached to silicon atom, were carried out applying an efficient calculation scheme of locally dense basis set in the framework of the electron density functional theory utilizing the Keal–Tozer functional combined with relativistic Dyall basis sets on a four-component relativistic level. The main factors of calculation accuracy of silicon chemical shifts were analyzed including the relativistic effects, environmental impact, and vibrational corrections. The mean absolute calculation error for the studied compounds series accounting for all mentioned factors was 14.0 ppm for the nonrelativistic calculation and 6.7 ppm for the four-component relativistic calculation at the range of silicon chemical shifts variation of ~250 ppm.  相似文献   

11.
The main factors affecting the accuracy and computational cost of the Second‐order Möller‐Plesset perturbation theory (MP2) calculation of 77Se NMR chemical shifts (methods and basis sets, relativistic corrections, and solvent effects) are addressed with a special emphasis on relativistic effects. For the latter, paramagnetic contribution (390–466 ppm) dominates over diamagnetic term (192–198 ppm) resulting in a total shielding relativistic correction of about 230–260 ppm (some 15% of the total values of selenium absolute shielding constants). Diamagnetic term is practically constant, while paramagnetic contribution spans over 70–80 ppm. In the 77Se NMR chemical shifts scale, relativistic corrections are about 20–30 ppm (some 5% of the total values of selenium chemical shifts). Solvent effects evaluated within the polarizable continuum solvation model are of the same order of magnitude as relativistic corrections (about 5%). For the practical calculations of 77Se NMR chemical shifts of the medium‐sized organoselenium compounds, the most efficient computational protocols employing relativistic Dyall's basis sets and taking into account relativistic and solvent corrections are suggested. The best result is characterized by a mean absolute error of 17 ppm for the span of 77Se NMR chemical shifts reaching 2500 ppm resulting in a mean absolute percentage error of 0.7%. Copyright © 2015 John Wiley & Sons, Ltd.  相似文献   

12.
Chemical shifts of 1H and 17O nuclei in DMF, acetic acid, and ethanol dissolved in CH3COOH-DMF and C2H5OH-DMF mixtures were evaluated in all the range of the solvents compositions at 298.15 K. Analysis of concentration dependences of the shift values obtained from the point of view of complex formation and association was carried out. Differences observed in the behavior of acid and alcohol are connected with higher ability of the acid to associalion as compared to ethanol. On the basis of the obtained values of 17O chemical shifts the values of the extra chemical shifts were calculated and described by means of the Redlich-Kister equation.  相似文献   

13.
The chemical shifts and the direct and indirect spin–spin coupling constants of HCN have been measured in various liquid crystal solvents. The sign of the indirect coupling constant J(15NH) is found to be negative. The 13C shift anisotropy is 334±20 ppm. The molecular structure apparently varies considerably with the solvent used. These solvent effects, which can be attributed to a correlation between vibration and rotation of the molecule, are corrected.  相似文献   

14.
The 1H spectra of 37 amides in CDCl3 solvent were analysed and the chemical shifts obtained. The molecular geometries and conformational analysis of these amides were considered in detail. The NMR spectral assignments are of interest, e.g. the assignments of the formamide NH2 protons reverse in going from CDCl3 to more polar solvents. The substituent chemical shifts of the amide group in both aliphatic and aromatic amides were analysed using an approach based on neural network data for near (≤3 bonds removed) protons and the electric field, magnetic anisotropy, steric and for aromatic systems π effects of the amide group for more distant protons. The electric field is calculated from the partial atomic charges on the N.C═O atoms of the amide group. The magnetic anisotropy of the carbonyl group was reproduced with the asymmetric magnetic anisotropy acting at the midpoint of the carbonyl bond. The values of the anisotropies Δχparl and Δχperp were for the aliphatic amides 10.53 and ?23.67 (×10?6 Å3/molecule) and for the aromatic amides 2.12 and ?10.43 (×10?6 Å3/molecule). The nitrogen anisotropy was 7.62 (×10?6 Å3/molecule). These values are compared with previous literature values. The 1H chemical shifts were calculated from the semi‐empirical approach and also by gauge‐independent atomic orbital calculations with the density functional theory method and B3LYP/6–31G++ (d,p) basis set. The semi‐empirical approach gave good agreement with root mean square error of 0.081 ppm for the data set of 280 entries. The gauge‐independent atomic orbital approach was generally acceptable, but significant errors (ca. 1 ppm) were found for the NH and CHO protons and also for some other protons. Copyright © 2013 John Wiley & Sons, Ltd.  相似文献   

15.
The PBE0/pcSseg-2//pcseg-2 calculations of 1H and 13C NMR chemical shifts were performed for a classical series of 12 Strychnos alkaloids (except for the earlier studied parent strychnine), namely akuammicine, isostrychnine, rosibiline, tsilanine, spermostrychnine, diaboline, cyclostrychnine, henningsamide, strychnosilidine, strychnobrasiline, holstiine, and icajine. It was found that the calculated 1H and 13C NMR chemical shifts show markedly good correlations with available experimental data, as characterized by a mean absolute error of 0.22 ppm for the range of 8 ppm for protons and 1.97 ppm for the range of 180 ppm for carbons. Complementarily, the present results provide essential NMR update and fill a gap in the NMR data of this distinguished group of vitally important natural products.  相似文献   

16.
In this article, we describe the characteristic 15N chemical shifts of isatin oxime ethers and their isomer nitrone. These oxime ethers and nitrones are the alkylation reaction products of isatin oximes. In our study, the 15N chemical shifts observed in these oxime ethers were in the 402–408 (or 22–28) ppm range, although those for their corresponding nitrone series were in the 280–320 (or ?100 to ?60) ppm range. This remarkable difference in 15N NMR chemical shift values could potentially be used to determine the Oversus N‐alkylation of oximes, even when only one isomer is available. In this paper, the differences in 15N NMR chemical shifts serve as the basis for a discussion about how to distinguish both regioisomers derived from the oximes alkylation. Copyright © 2010 John Wiley & Sons, Ltd.  相似文献   

17.
The 1H chemical shifts of 48 amides in DMSO solvent are assigned and presented. The solvent shifts Δδ (DMSO‐CDCl3) are large (1–2 ppm) for the NH protons but smaller and negative (?0.1 to ?0.2 ppm) for close range protons. A selection of the observed solvent shifts is compared with calculated shifts from the present model and from GIAO calculations. Those for the NH protons agree with both calculations, but other solvent shifts such as Δδ(CHO) are not well reproduced by the GIAO calculations. The 1H chemical shifts of the amides in DMSO were analysed using a functional approach for near ( ≤ 3 bonds removed) protons and the electric field, magnetic anisotropy and steric effect of the amide group for more distant protons. The chemical shifts of the NH protons of acetanilide and benzamide vary linearly with the π density on the αN and βC atoms, respectively. The C=O anisotropy and steric effect are in general little changed from the values in CDCl3. The effects of substituents F, Cl, Me on the NH proton shifts are reproduced. The electric field coefficient for the protons in DMSO is 90% of that in CDCl3. There is no steric effect of the C=O oxygen on the NH proton in an NH…O=C hydrogen bond. The observed deshielding is due to the electric field effect. The calculated chemical shifts agree well with the observed shifts (RMS error of 0.106 ppm for the data set of 257 entries). Copyright © 2014 John Wiley & Sons, Ltd.  相似文献   

18.
The highly toxic plant alkaloid strychnine is often isolated in the form of the anion salt of its protonated tertiary amine. Here, we characterize the relative influence of different counterions on 1H and 13C chemical shifts in several strychnine salts in D2O, methanol‐d4 (CD3OD), and chloroform‐d (CDCl3) solvents. In organic solvents but not in water, substantial variation in chemical shifts of protons near the tertiary amine was observed among different salts. These secondary shifts reveal differences in the way each anion influences electronic structure within the protonated amine. The distributions of secondary shifts allow salts to be easily distinguished from each other as well as from the free base form. Slight concentration dependence in chemical shifts of some protons near the amine was observed for two salts in CDCl3, but this effect is small compared with the influence of the counterion. Distinct chemical shifts in different salt forms of the same compound may be useful as chemical forensic signatures for source attribution and sample matching of alkaloids such as strychnine and possibly other organic acid and base salts. Copyright © 2013 John Wiley & Sons, Ltd.  相似文献   

19.
We propose a new method for calculating 1H and 19F chemical shifts in saturated organic compounds. Results show that the shifts are a balance of several contributions and allow an evaluation of these effects: a diamagnetic contribution d(—?) proportional to the net charge ? of the electronic environment of the nucleus considered and paramagnetic contributions pX of each of substituents, constant attached to the substituent X in any molecule. For both nuclei, d values were deduced from a theoretical calculation of the screen in the free atom. This method, using a restricted number of parameters obtained from experiment, lead to the observed chemical shifts with a mean accuracy of ± 0,32 ppm for 1H and ± 11 ppm for 19F. Therefore this method gives a simple quantitative relation between chemical shift and structure from which precise indications about the origin of the contributions from neighbouring electronic surroundings can be obtained.  相似文献   

20.
The density functional theory calculation of 1H and 13C NMR chemical shifts in a series of ten 10 classically known Strychnos alkaloids with a strychnine skeleton was performed at the PBE0/pcSseg-2//pcseg-2 level. It was found that calculated 1H and 13C NMR chemical shifts provided a markedly good correlation with experiment characterized by a mean absolute error of 0.08 ppm in the range of 7 ppm for protons and 1.67 ppm in the range of 150 ppm for carbons, so that a mean absolute percentage error was as small as ~1% in both cases.  相似文献   

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