N-Oxides of polychloropyridines.13C NMR study

The effect ofN-oxidation of a series of polychloropyridines on¹³C NMR parameters has been studied. It has been established that inN-oxides of polychloropyridines an electric field through-space effect of theN-oxide group predominates in the shielding of the -carbon atom compared to the other carbon atoms. A linear correlation between¹³C NMR chemical shifts and total charge densities calculated by the MNDO method for the carbon atoms ofN-oxides of polychloropyridines has been found.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 12, pp. 2425–2428, December, 1995.The authors are grateful to V. V. Kolchanov for help in the synthesis of compounds under study.     

The use of MM/QM calculations of 13C and 15N chemical shifts in the conformational analysis of alkyl substituted anilines

The calculation of the ¹³C and ¹⁵N NMR chemical shifts by a combined molecular mechanics (Pcmodel 9.1/MMFF94) and ab initio (GIAO (B3LYP/DFT, 6-31 + G(d)) procedure is used to investigate the conformations of a variety of alkyl substituted anilines. The ¹³C shifts are obtained from the GIAO isotropic shielding (Ciso) with separate references for sp³ and sp² carbons (δc = δref − Ciso). The ¹⁵N shifts are obtained similarly from the GIAO isotropic shielding (Niso) with reference to the ¹⁵N chemical shift of aniline. Comparison of the observed and calculated shifts provides information on the molecular conformations. Aniline and the 2,6-dialkylanilines exist with a rapidly inverting symmetric pyramidal nitrogen atom. The 2-alkylanilines have similar conformations with the NH₂ group tilted away from the 2-alkyl substituent. The N,N-dialkylanilines show more varied conformations. N,N-dimethylaniline has a similar structure to aniline, but N-ethyl, N-methylaniline, N,N-diethylaniline, and N,N-diisopropylaniline are conformationally mobile with two rapidly interconverting conformers. In contrast, the anilines substituted at C₂ and the nitrogen atom exist as one conformer where the steric interaction between the C₂ substituent and the N substituent determines the conformation. In 2-methyl-N-methylaniline, the nitrogen atom is pyramidal as usual with the N-methyl opposite to the 2-methyl, but in 2-methyl-N,N-dimethyl aniline, the NMe₂ group is now almost orthogonal to the phenyl plane. This is also the case with 2-methyl-N,N-diethylaniline and 2,6-diisopropyl-N,N-dimethylaniline. The comparison of the observed and calculated ¹⁵N chemical shifts confirms the above findings, in particular the pyramidal conformation of aniline and the above observations with respect to the conformations of the N,N-dialkylanilines.     

13C n.m.r. spectra of some thiobenzamides. Correlation between 13C and 1H n.m.r. spectra and signal assignments of syn and anti CH2 groups

¹³C n.m.r. spectra of a series of N,N-disubstituted thioamides have been recorded and signal assignments were performed. Separate signals are observed for methylene groups fixed on the nitrogen atom. Since the carbon atom syn to the thiocarbonyl sulfur resonates at higher field than the anti carbon, the syn-anti assignment in ¹H n.m.r. is easily obtained by selective double irradiation. This method, which is rapid and reliable, affords a rather general solution to the interesting problem of resonance assignments in tertiary amides and thioamides (and in analogous molecules such as oximes and nitrosamines).     

13C NMR spectral data for some N,N-dialkylbenzylamines and their cyclopalladated complexes

¹³C NMR data for some N,N-dialkylbenzylamines and their chloro-(N,N-dialkylbenzylamine-6,C,N)-tri-phenylphosphinepalladium(II) complexes are presented and simple shift parameters defined.     

Saturated amine oxides: part 10. hydroacridines: part 32. linear 17O/13C and 13C/13C chemical shift correlations between saturated azaheterocyclic N‐methylamine N‐oxides,and the methiodides of their parent amines

Two kinds of good linear correlations were found between the chemical shifts of saturated six‐membered azaheterocyclic N‐methylamine N‐oxides and the chemical shifts of the methiodides of their parent amines. One of the correlations occurs between the ¹⁷O chemical shift of the N⁺―O^– oxygen in the N‐oxides and the ¹³C chemical shift of the N⁺―CH₃ methyl group analogously situated in the appropriate methiodide (r = 0.9778). This correlation enables unambiguous configuration assignment of the N⁺―O^– bond, even if the experimentally observed ¹⁷O chemical shift of only one N‐epimer is available, provided the ¹³C chemical shifts of both N⁺―CH₃ groups in the methiodide are known and assigned; furthermore, it can be used also for the estimation of ¹⁷O chemical shifts of the N⁺―O^– oxygens in N‐epimeric pairs of N‐oxides, for which observed ¹⁷O data hardly become available. The second correlation is observed between the ¹³C chemical shift of the N⁺―CH₃ methyl group in the N‐oxides and the ¹³C chemical shift of the N⁺―CH₃ methyl group analogously situated in the appropriate methiodide (r = 0.9785). It can be used for safe configuration assignment of the N⁺―CH₃ group and, indirectly, also of the N⁺―O^– bond in an amine N‐oxide, even if no ¹⁷O NMR data, and the ¹³C chemical shift of only one N‐epimer is available. Copyright © 2015 John Wiley & Sons, Ltd.     

The effect of solvents on the 13C NMR chemical shifts of the carbonyl carbon and the rotational barriers of N,N-dimethylbenzamide

The ¹³C solvent induced chemical shifts (SICS) of the carbonyl carbon and the thermodynamic barriers to rotation about the C? N bond of N,N-dimethylbenzamide are linearly related to the solvent parameter, E_T(30). A multi-parametric solvent parameter approach indicates that the SICS are influenced equally by polar effects and hydrogen-bond donor effects. Rotational barriers for N,N-dimethylbenzamide may, in principle, be determined by measurement of the ¹³C chemical shift of the carbonyl carbon in a particular solvent.     

The 13C chemical shift and the anisotropy effect of the carbene electron-deficient centre: Simple means to characterize the electron distribution of carbenes

Both the ¹³C chemical shift and the calculated anisotropy effect (spatial magnetic properties) of the electron-deficient centre of stable, crystalline, and structurally characterized carbenes have been employed to unequivocally characterize potential resonance contributors to the present mesomerism (carbene, ylide, betaine, and zwitter ion) and to determine quantitatively the electron deficiency of the corresponding carbene carbon atom. Prior to that, both structures and ¹³C chemical shifts were calculated and compared with the experimental δ(¹³C)/ppm values and geometry parameters (as a quality criterion for obtained structures).     

13C NMR studies of conformational isomerism in thioureas: Signal assignments via chemical shift and population trends

The 22.63 MHz ¹³C NMR spectra of a series of alkylated thioureas are reported. Characteristic Z and E spectral regions were found for the ¹³C ? S resonances. The two regions were generally found to be non-overlapping for the series, with the region of the Z, Z resonances occurring more downfield than those of either the Z, E or E, Z conformers in the cases of 1,3-disubstitution. The Z, Z configuration became favored and the relative chemical shift difference (Rδ) increased linearly with increasing substituent size. At 217 K, hindered internal rotation caused a multiplicity of resonances which were normally single peaks in the broad band ¹H decoupled 62.86 MHz ¹³C spectrum of CH₃NHCSNH(CH₂)₂NHCSNHCH₃ (2MTE) at room temperature. The trends in chemical shifts and populations were employed to assign tentatively the resonances of five of the six possible configurational isomers contributing to the 2MTE spectra at 217 K. The isomer populations are given. The ¹³C NMR spectra reported here led to signal assignments of Z and E isomers which supported prior ¹H NMR results and contradicted more recent results of another ¹³C NMR study of N-methylthiourea. The major peak of the exchange doublet occurs at relatively high field strengths in both methanol-d₅.     

13C NMR of methyl N-methylthiobenzimidates. A novel approach to the interpretation of the relative 13C-E/Z shifts in amides and related compounds

The ¹³C NMR shifts of the E/Z mixtures of twelve, mainly ortho substituted, methyl N-methylthio-benzimidates have been completely assigned. The angles of twist about the aryl-imino-carbon bond were determined by means of the C-1 aryl shifts. Depending on the bulk of the ortho substituents the values vary from 18° in methyl 2-fluoro-N-methylthiobenzibenzimidate (Z) to 73° in methyl 4-bromo-N,2,6-trimethylthio-benzimidate (E). An explanation for the ¹³C-E/Z-N-alkyl and -S-alkyl resonances is facilitated by a new model which is initially based on the influence of electric fields. As this model is derived from amides, thioamides etc. it is possible to interpret the ¹³C-E/Z shifts in all related classes of compounds. The role of steric compression does not seem to be as important as was previously assumed.     

Application of 13C and 15N spectroscopy to the study of electronic delocalization in N?N bonds: Nitrosamines,hydrazones, triazenes and related protonated species

¹³C and ¹⁵N spectroscopies at natural abundance have been applied to the study of nitrogen lone-pair delocalization in N? N containing compounds: nitrosamines, nitramines, hydrazines, hydrazones and triazenes. Structure-chemical shift correlations have been derived for nitrosamines; the ¹³C upfield effect of a γ substituent has been used for assigning the configuration of both diastereoisomers in N,N-unsymmetrically substituted nitrosamines. Equations have been computed which permit the prediction of the electronic delocalization, expressed in terms of free enthalpy of activation ΔG, as a function of δ¹⁵N and of the length of the N? N bond. ¹⁵N spectroscopy has also been applied to the study of the protonated species of nitrosamines and of acceptor-donor complexes of nitrosamines with Lewis acids. The behaviour of such N? N containing compounds is compared to that of amides.     

Identification of N-methylpyrazole analog isomers by 13C NMR

Using several N-methyl pyrazole analogs the utility of ¹³C nmr in determining isomeric structures was examined. The chemical shift assignments of the pyrazole carbons and thus the isomeric structures were determined using a combination of proton coupled and proton decoupled ¹³C nmr spectra.     

Konfigurationsbestimmung von geometrischen Isomeren mittels selektiver 13C{1H}-NOE-Differenzspektroskopie

Application of Selective ¹³C{¹H}-NOE Difference Spectroscopy in Determination of the Configuration of (Z)/(E)-Isomers Heteronuclear ¹³C{¹H}-NOE studies have been performed to elucidate the configuration at the imino double bond in N,N′-Dicyanoquinonediimines. Selective low-power irradiation of the neighbouring ring protons in compounds 1 – 5 increases the signal intensities of CN as well as of the imino-C-atoms in the ¹³C-NMR spectra. The increase of signal intensity can easily be detected by the difference spectroscopy. The results of this investigation are in good agreement with a recently published study on the stereochemistry of N,N′-Dicyanoquinonediimines, based on the interpretation of ¹³C-chemical shifts.     

NMR chemical shifts and coupling constants of various nuclei of N,N-diethylbenzenesulfenamide,N,N-diethylbenzeneseleneamide and some related sulfur amides

The ⁷⁷Se, ¹⁵N, ¹³C and ¹H NMR parameters have been determined for N,N-diethylbenzenesulfen- and N,N-diethylbenzeneseleneamide and the ¹H¹³C coupling constants additionally for N,N-dimethylmethanesulfinamide, N,N-dimethylbenzenesulfinamide, N,N-dimethylmethanesulfonamide, N,N-dimethylbenzenesulfonamide and diphenylsulfone. The nitrogen-15 nucleus is slightly more shielded in the sulfenamide than in the seleneamide. Compared to sulfin- and sulfonamides the nitrogen nuclei of these amides are more shielded. The solvent effects on the chemical shift of both selenium-77 and nitrogen-15 nuclei are observable. The nitrogen-15 chemical shifts of sulfur and seleneamides are less sensitive to the solvents than those of carboxamides. This change in the nitrogen-15 chemical shift in sulfen- and seleneamide is the same order of magnitude, but opposite in direction compared with that in sulfon- and sulfinamides. The ¹H¹³C direct aromatic coupling constants increase along with the increasing oxidation state of sulfur in sulfur amides, being greatest in sulfonamides.     

Deuterium isotope effects on 13C chemical shifts of negatively charged NH…N systems

Deuterium isotope effects on ¹³C chemical shifts are investigated in anions of 1,8‐bis(4‐toluenesulphonamido)naphthalenes together with N,N‐(naphthalene‐1,8‐diyl)bis(2,2,2‐trifluoracetamide) all with bis(1,8‐dimethylamino)napthaleneH⁺ as counter ion. These compounds represent both “static” and equilibrium cases. NMR assignments of the former have been revised. The NH proton is deuteriated. The isotope effects on ¹³C chemical shifts are rather unusual in these strongly hydrogen bonded systems between a NH and a negatively charged nitrogen atom. The formal four‐bond effects are found to be negative indicating transmission via the hydrogen bond. In addition, unusual long range effects are seen. Structures, ¹H and ¹³C NMR chemical shifts and changes in nuclear shieldings upon deuteriation are calculated using density functional theory methods. Copyright © 2013 John Wiley & Sons, Ltd.     

The conformational dependence of vicinal 13C13C spin–spin coupling constants in alicyclic compounds

A series of alicyclic compounds with dihedral angles of 0°, 60°, 90°, 120° and 180° between a ¹³C-labelled carbon atom and a carbon atom separated by three bonds from the label has been synthesized. The vicinal ¹³C¹³C spin coupling constants were measured, and from the results a Karplus-type relationship between ¹³C¹³C spin coupling and dihedral angle is proposed.     

Stereospecificity of <Superscript>13</Superscript>C shielding constants in acetone azine as a tool for configurational assignment of ketone azines

According to the ¹³C NMR data, the chemical shift of the methyl carbon atom in acetone azine in the trans position with respect to the lone electron pair on the neighboring nitrogen atom is lower by 7 ppm than that of the methyl carbon atom in the cis position. The corresponding direct ¹³C-¹³C coupling constant for the trans-methyl groups is lower by 10 Hz as compared to the cis-methyl groups. The experimental spectral data are reproduced well by nonempirical quantum-chemical calculations. The observed stereospecificity of ¹³C chemical shifts and ¹³C-¹³C coupling constants may be used as an effective tool in configurational analysis of various ketone azines.     

13C nuclear magnetic resonance of organophosphorus compounds XI—aminophosphines

Natural abundance ¹³C NMR studies have been carried out on a series of organophosphorus compounds possessing P? N bonds. For the first time a one-bond temperature-dependent ¹³C—³¹P nuclear spin coupling was observed for the P-phenyl carbons in bis(N,N-dibenzylamino)phenylphosphine (0-9 Hz) and bis(N,N-diethylamino)phenylphosphine (0–2 Hz). This temperature-dependent behavior can be rationalized in terms of free rotation about the phenyl phosphorus bond with concomitant hindered rotation about the P? N bonds. A conformational preference for the nitrogen and phosphorus lone pairs to exist in the trans orientation is indicated. In the similarly substituted 5-membered heterocyclic ring compound, 1,3-dimethyl-2-phenyl-1,3-diazaphospholidine, the phenyl one-bond coupling increases to (?) 42.1 Hz and becomes temperature independent. These data suggest that ¹J(PC) is very responsive to electronic effects.     

13C n.m.r. investigation on the nitrogen methylation of some azabenzenes

The ¹H and ¹³C n.m.r. spectra of N-methylated pyridine, pyridazine, pyrimidine and pyrazine and N,N-dimethylated pyrimidine and pyrazine have been recorded and analysed. The change in the ¹³C chemical shifts under the influence of N-methylation (Δδ) in the diazabenzenes could be predicted by the Δδ values of pyridine. A comparison of the Δδ values of N-methylation with those of N-protonation showed that both reactions have a similar effect.     

13C and 1H NMR spectral studies of N-alkylmethylquinolinium salts

¹³C and ¹H chemical shifts of fourteen N-alkylmethylquinolinium salts in DMSO-d₆ are reported, and compared with those of the eleven corresponding methylquinoline bases. The influence of ring substitution by methyl groups in the salts and substitution at the nitrogen atom and the effect of the anion are discussed.     

Effects of molecular asymmetry on 13C chemical shifts in some O- and N-glycidyl compounds

In the ¹³C? {¹H} n.m.r. spectra of certain organic compounds containing two or three glycidyl (2,3-epoxyprop-1-yl) groups, chemical shift nonequivalence has been observed, between diastereoisomers, for chemically similar carbon atoms in the glycidyl groups. Nonequivalence was only apparent when the glycidyl groups were linked through a single atom. In three derivatives of diglycidylaminobenzene, ¹³C chemical shift nonequivalence was also observed for aromatic carbon atoms ortho to the nitrogen substituent.