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.     

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     

Calculation of chemical shifts in the13C NMR spectra of uncharged molecules and their π-dianions

Physics Institute, Academy of Sciences of the Belorussian SSR. Translated from Zhurnal Strukturnoi Khimii, Vol. 31, No. 2, pp. 70–76, March–April, 1990.     

Long range substituent effects relfected in the 13C NMR spectra of allyl alcohols and their derivatives

Acylation of allyl alcohols induces strong carbon shifts, shielding γ and deshielding δ effects. These shifts are a consequence of through-bond polarization of the olefinic carbons. Allyl ethers show similar, but milder perturbations. The olefinic carbon shifts reveal a strong concentration dependence. This effect, in greatly reduced form, is also observed in non-oxygenated alkenes. Hydrogen bonding by alcohols enhances the chemical shift difference in olefinic carbon pairs by increased polarization.     

13C NMR spectra of polyiodosubstituted compounds and structural increments in the chemical shifts for carbon in polyiodoalkanes

Conclusions We propose a topological scheme for empirical estimates of ¹³C in polyiodoalkanes containing both individual substituent increments and also structural increments characterizing two-particle steric interactions of substituents, and also including terms taking into account the asymmetry of the total heterograph.Deceased.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 12, pp. 2719–2724, December, 1987.     

Quantum-chemical calculations of NMR chemical shifts of organic molecules: XIV. Solvation effects in calculations of chemical shifts in 13C NMR spectra of chlorine-containing compounds

Analysis of precision factors in calculations of ¹³C NMR chemical shifts in the series of saturated and unsaturated organochlorine compounds was performed in the framework of the method of electron density functional theory GIAO-DFT-KT3/pcS-2 in the gas phase and with accounting for solvent effect by the polarized continuum model IEF-PCM. The accounting for solvation effects in calculations of ¹³C NMR chemical shifts within the framework of the IEF-PCM model is not fundamental for organochlorine compounds, yet it considerably improves the precision of calculations up to 2.5 ppm.     

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.     

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.     

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.     

13C NMR chemical shifts of some highly-branched acyclic compounds

A study has been made of the ¹³C chemical shifts of a number of acyclic alkanes, alkenes, nitriles and ketones which contain quaternary carbon atoms. Similar data have also been obtained for the series of compounds involved in the synthesis of triisopropylacetic acid. Substituent effects and steric factors in these highly substituted compounds are discussed in relation to the chemical shifts.     

Principal component analysis of carbon-13 substituent-induced chemical shifts of some unsaturated compounds

Principal component analysis (PCA) is applied to 32 disubstituted unsaturated compounds (Y–CH₂–X): cyanides, oximes and propenes; bearing 12 -substituents: F, Cl, Br, I, OMe, OEt, SMe, SEt, NMe₂, NEt₂, Me, and Et. The experimental ¹³C chemical shifts for the -carbon and functional carbon atoms are correlated with theoretically derived molecular properties, i.e. partial charges, electronegativity, hardness, dipole moments and the nuclear repulsion energies. In the first PCA, the clustering of these three classes of organic compounds occurred mostly because of the chemical shifts and partial charges, and also of the dipole moments, hardness and electronegativity parameters as confirmed by loading graph. A strong grouping is observed in the second PCA, showing the chemical shift dependence on the type of heteroatom substituents. Therefore, sulfur, nitrogen, oxygen and neutral groups exhibit four types of C-13 SCS influences, indicating that the heteroatom (Y) properties play a significant role on the effects on chemical shifts. The -halogenated compounds represent a very heterogeneous group due to possible orbital interactions between the functional group and the substituent. The third PCA shows the grouping of F, Cl, Br and I derivatives, confirming the second PCA results that same halogen presents the same or very similar effects on the chemical shifts.     

13C and 15N NMR chemical shifts of alkylsubstituted benzonitriles

¹³C NMR data of 14 and ¹⁵N NMR data of four alkylsubstituted benzonitriles are reported and discussed in relation to substituent effects and stereochemistry.     

Structure revision of hassananes with use of quantum mechanical 13C NMR chemical shifts and UV-vis absorption spectra

Geometry optimization and gauge including atomic orbitals (GIAO) (13)C NMR chemical shifts in chloroform solvent calculated at the level of MPW1PW91/6-31G(d,p) were applied to przewalskins A (3) and B (4), which are novel diterpenoids with a 6/6/7 carbon ring skeleton. The good linear correlations between the calculated and experimental (13)C NMR chemical shifts indicated the reliability of the computational method. This method was employed to the structural revision of natural product hassananes (1), which was reassigned to 2 with a similar skeleton as przewalskins A (3) and B (4). Furthermore, the UV-vis absorption spectra in gas phase and solvents were also predicted in order to further support our structure revision of hassananes.     

Topological increment scheme for13C NMR chemical shifts in aliphatic alcohols

We propose an additive empirical scheme for calculating the¹³C NMR chemical shifts in acyclic aliphatic alcohols. We show that the electron-acceptor effect of the OH group does not damp out at the -carbon and is apparent in the shielding of the - and -carbon atoms. The effect of the OH group has two components: a constant component determined by the electron-acceptor properties of the substituent (the true a effect of the OH group, equal to the change in the chemical shift of the methane carbon upon substitution of a hydrogen atom by a hydroxyl), and a variable component of steric origin, depending on some topological characteristics of the given molecule in the immediate vicinity of the carbon atom under consideration. Analogous relations are obtained for the and effects of the OH group.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 12, pp. 2742–2746, December, 1990.     

丁烷衍生物类木脂素13C NMR化学位移预测

借助原子电性作用矢量(AEIV)和原子杂化状态指数(AHSI),对39种丁烷衍生物类木脂素共计854个等价C原子进行表征,并建立用于模拟该类分子13C NMR化学位移的多元线性回归方程.所得定量结构波谱关系(QSSR)模型及留一法交互检验相关系数分别为r=0.981和q=0.962.进一步用从马尾松松针中分离所得新木脂素中20个13C NMR化学位移对模型进行外部验证,预测结果与实验值较接近.表明所建模型有良好稳定性和泛化力,可对丁烷衍生物类木脂素13C NMR谱学数据准确模拟.