Effective consideration of ring structures in CAST/CNMR for highly accurate C NMR chemical shift prediction

A new function that effectively takes into account ring structural environments achieves extensive highly accurate prediction of ¹³C NMR chemical shift in the CAST/CNMR system. The approach adapts a fast and flexible ring perception algorithm and a new CAST coding method for the ring information. ¹³C NMR chemical shift prediction is performed for complicated polycyclic natural products and their synthetic intermediates as the demonstration, which shows the reliability of the function in extending the scope of the practically accurate ¹³C NMR prediction for wide range of organic compounds.     

Use of the 13C-NMR chemical shift/charge density linear relationship for recognition and ranking of chemical structures

A novel scheme for computer-aided recognition of chemical structures based on the ¹³C-NMR chemical shift/charge density linear relationship is described. The spectral signals are uniquely assigned to the carbon atoms of each of the compared structures via an automatic assignment procedure. On the basis of this assignment, a factor reflecting the agreement between the spectrum and the molecular structure is defined. This factor has different values for different structures, similar values for similar structures, equal values for equivalent (isomorphic in the graph-theoretical sense) structures, and the lowest value for the structure best corresponding to the ¹³C-NMR spectrum. The potential of this approach for retrieval and ranking of chemical structures is discussed with examples.     

Use of an internal reference in 13C chemical shift measurements

The use of the nominal value of the chemical shift of the solvent to calibrate the spectrometer in ¹³C NMR spectroscopy was found to introduce errors due to the effect which the solute has on the solvent. In addition, hexamethyldisiloxane (HM) is proposed as an internal standard; owing to its high boiling point it is easier to manipulate than tetramethylsilane (TMS) and it is therefore possible to prepare solutions of known concentrations. In order to convert the data obtained with this standard to the TMS scale, the chemical shift of HM was determined in 16 solvents using cyclohexane as external standard in a spherical cell (5% w/w concentration of HM) as a function of the solvent factor g². Comparing these results with a similar plot obtained previously for TMS by other workers, it is possible to convert one type of data to the other by a simple linear expression.     

Counterintuitive deshielding on the 13C NMR chemical shift for the trifluoromethyl anion

The trifluoromethyl anion (CF₃⁻) displays ¹³C NMR chemical shift (175.0 ppm) surprisingly larger than neutral (CHF₃, 122.2 ppm) and cation (CF₃⁺, 150.7 ppm) compounds. This unexpected deshielding effect for a carbanion is investigated by density functional theory calculations and decomposition analyses of the ¹³C shielding tensor into localized molecular orbital contributions. The present work determines the shielding mechanisms involved in the observed behaviour of the fluorinated anion species, shedding light on the experimental NMR data and demystify the classical correlation between electron density and NMR chemical shift. The presence of fluorine atoms induces the carbon lone pair to create a paramagnetic shielding on the carbon nucleus.     

The13C NMR spectra of some polychlorinated dibenzo-p-dioxins. A calculation of13C chemical shifts

The¹³C NMR spectra of a number of polychiorinated dibenzo-p-dioxins (PCDD) were measured. These and previously known spectra were used for the development of a method for calculation of¹³C NMR spectra of chloroaromatics in the framework of a two-particle increment scheme for carbon chemical shifts. The scheme one allows to calculate¹³C chemical shifts for all 75 PCDD.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 4, pp. 760–761, April, 1994.     

13 C NMR spectra of some indole derivatives

The effect of acyl and carboxyl groups in position 2 or 3 of the indole ring on the¹³C chemical shifts of the ring was studied, -, -, and -Increments of the indole and isatin rings for¹³ C chemical shifts of the substituants at the ring N-atom were determined.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 1, pp. 66–69, January, 1994.     

Novel methods of automated structure elucidation based on 13C NMR spectroscopy

Three new approaches for automated structure elucidations of organic molecules using NMR spectroscopic data were introduced recently. These approaches apply a neural network 13C NMR chemical shift prediction method to rank the results of structure generators by their agreement of the predicted and experimental chemical shifts. These three existing implementations are compared using realistic example molecules. The applicability and reliability of such approaches is addressed.     

The application of empirical methods of 13C NMR chemical shift prediction as a filter for determining possible relative stereochemistry

The reliable determination of stereocenters contained within chemical structures usually requires utilization of NMR data, chemical derivatization, molecular modeling, quantum‐mechanical (QM) calculations and, if available, X‐ray analysis. In this article, we show that the number of stereoisomers which need to be thoroughly verified, can be significantly reduced by the application of NMR chemical shift calculation to the full stereoisomer set of possibilities using a fragmental approach based on HOSE codes. The applicability of this suggested method is illustrated using experimental data published for a series of complex chemical structures. Copyright © 2009 John Wiley & Sons, Ltd.     

致癌性胺离子反应过程中电荷和化学位移的计算

应用原子-键电负性均衡方法中的σπ模型(ABEEMσπ)计算了由致癌性胺离子所参与的反应过程中的电荷分布,所计算出的电荷分布可以和从头算很好的相关联,并且所需要的时间也大大的缩短;同时应用从头计算程序计算致癌性胺离子反应过程中的NMR化学位移.结果表明,在反应过程中电荷的变化和NMR化学位移的变化有很好的对应关系.     

萜类化合物13C NMR化学位移的定量构谱相关研究

利用原子电性作用矢量(Atomic electro-negativity interaction vector,AEIV)和原子杂化状态指数(Atomic hybridization state index,AHSI)对萜类化合物中的C原子进行结构表征并与其核磁共振碳谱(13C NMR)建立了优良的定量构谱相关(QSSR)模型.其中29个单萜类化合物中的290个C原子建模的计算值经留一法(Leave-one-out,LOO)交互校验(Cross-validation,CV)预测值的复相关系数(R)分别为0.9900和0.9867,进一步使用倍半萜、二萜、三萜化合物分子中65个C原子的13C NMR化学位移值来检测该模型的稳定性,模型预测值和观测值间复相关系数(R)为0.9777,取得了令人满意的结果.     

Increments for 1H and 13C NMR chemical shifts in areneboronic acids

A series of areneboronic acids were studied by NMR spectroscopy. Increments for the ¹H and ¹³C chemical shifts caused by the boronic acid substituent B(OH)₂ in areneboronic acids were determined. Copyright © 2003 John Wiley & Sons, Ltd.     

13C NMR chemical shifts of carbonyl groups in substituted benzaldehydes and acetophenones: substituent chemical shift increments

13C NMR Substituent chemical shift (SCS) increments have been determined for the carbonyl carbon of a variety of substituted benzaldehydes and acetophenones. The 13C NMR chemical shift of the carbonyl carbon can be predicted for many di- and trisubstituted benzaldehydes and acetophenones through simple additivity of the SCS increments. The magnitude and sign of the SCS increments have been explored using Hartree-Fock 6-31G* calculations to determine the natural atomic charges of the carbonyl carbon. When a substituent capable of intermolecular hydrogen bonding is present, deviations from additivity on the order of 2 ppm are observed in dilution experiments; deviations of up to 6 ppm can result from intramolecular hydrogen bonding.     

A general chemical shift decomposition method for hyperpolarized 13C metabolite magnetic resonance imaging

Metabolic imaging with hyperpolarized carbon‐13 allows sequential steps of metabolism to be detected in vivo. Potential applications in cancer, brain, muscular, myocardial, and hepatic metabolism suggest that clinical applications could be readily developed. A primary concern in imaging hyperpolarized nuclei is the irreversible decay of the enhanced magnetization back to thermal equilibrium. Multiple methods for rapid imaging of hyperpolarized substrates and their products have been proposed with a multi‐point Dixon method distinguishing itself as a robust protocol for imaging [1‐¹³C]pyruvate. We describe here a generalized chemical shift decomposition method that incorporates a single‐shot spiral imaging sequence plus a spectroscopic sequence to retain as much spin polarization as possible while allowing detection of metabolites that have a wide range of chemical shift values. The new method is demonstrated for hyperpolarized [1‐¹³C]pyruvate, [1‐¹³C]acetoacetate, and [2‐¹³C]dihydroxyacetone. Copyright © 2016 John Wiley & Sons, Ltd.     

Characterization of cyclotriveratrylene inclusion compounds by means of solid state13C NMR

Solid inclusion compounds of cyclotriveratrylene (CTV) with benzene, toluene, acetone, tetrahydrofuran, acetic acid and chloroform, as well as two hydrated forms, were prepared and characterized by solid state¹³C NMR. The inclusion process for CTV appears to be quite selective, and in some instance depends critically on the presence or absence of water. A number of different structural types are indicated by the solid state¹³C NMR splitting paterns. For the guests listed above,¹H NMR in solution indicated a guest to host ratio close to 0.5, except for chloroform, for which the ratio is closer to 2.     

Theoretical calculations of hypersurfaces of the C chemical shift anisotropy in the COHN hydrogen bond and the benefit for the ab initio structure determination

We investigated the influence of structural changes on the anisotropic part of the carbonyl ¹³C chemical shift tensor in a model complex containing hydrogen bonded cyanuric acid and pyrrole. The model was chosen for its chemical resemblance to cyameluric acid. In the solid state this compound comprises three different hydrogen bonds which are well distinguishable based on the anisotropy parameters δ_aniso and η of the carbonyl ¹³C atoms. The variation of six relevant structural variables in the model system produced hypersurfaces for the isotropic shift, δ_aniso and η. Our goal was to investigate whether such surfaces can be used for the ab initio structure determination of hydrogen bonds. With a medium size basis set it could be shown that although the absolute values differ DFT describes the relative change in δ_aniso and η close to the quality of MP2 calculations. Due to the high dimensionality of the hypersurface we had to reduce the number of variables in our study. We systematically created subsurfaces each described by three of the six variables and investigated their isolated influence on the NMR observables. We identified the most important structure parameters and on this base built a minimal model. For a fixed NO distance the hydrogen bond arrangement was altered by two angular variations and one dihedral distortion. In this model evidence was found that the η surfaces for different NO distances exhibit a uniform shape and can be transformed into one another by a simple shift and multiplication by a mean factor. Furthermore, the experimental parameters δ_aniso and η of cyameluric acid were taken as a base for the extraction of structures from the hypersurfaces. δ_aniso and η unequivocally selected ensembles of similar structure and the COHN arrangement in two of the three cyameluric hydrogen bonds could be predicted with good quality from the theoretical model. Our results show that it is possible to predict the distance and at least qualitatively the orientation in a hydrogen bond environment from an analysis of the anisotropic part of the ¹³C chemical shift tensor.     

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.     

Structure-NMR chemical shift relationships for novel functionalized derivatives of quinoxalines

(1)H, (13)C and (15)N NMR chemical shifts for a variety of novel quinoxalines were determined by different 2D methods and were calculated using the GIAO DFT approach. Comparison with experimental data shows good correlations in the case of (1)H, (13)C and (15)N chemical shifts. Different combinations of basis sets were tested. In non-polar solvents quinoxalines exist as dimers owing to strong hydrogen bonding. Calculations for dimers improve the correlation between experiment and theory. Additive empirical methods for estimating chemical shifts have drawbacks and have to be used with a great care for this type of compound.     

The increment scheme for the prediction of13C NMR chemical shifts in polychlorinated dibenzo-p-dioxins

A topological method for the calculation of¹³C NMR chemical shifts was developed for polychlorinated dibenzo-p-dioxins (PCDD). Based on previous results for polychlorinated benzenes and polyhydroxybenzenes, the collective influence of the substituents on carbon chemical shifts is presented as the sum of two-particle increments. The increments only of two new monosubstituted graphs have to be added to those known for PCDD spectra: 1-Cl-DD and 2-Cl-DD. All structural situations in the¹³C NMR chemical shifts of the whole class of 75 PCDD can be covered with a few model compounds. The coefficients of the increment scheme are independent of the change of CDCl₃ for acetone-d₆, so it may be a new reliable criterion for recognizing PCDD by¹³C NMR, in spite of the close resemblance of NMR spectra of aromatic compounds.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 2, pp. 280–284, February, 1995.     

1H and 13C NMR spectral assignments of isoquinuclidine-3,5-dione derivatives

The 1H and 13C NMR spectra of six 5-substituted 2-azabicyclo[2.2.2]octane derivatives were fully assigned by COSY and HSQC experiments.