15N NMR of 1,4‐dihydropyridine derivatives

In this article, we describe the characteristic ¹⁵N and ¹H_N NMR chemical shifts and ¹J(¹⁵N–¹H) coupling constants of various symmetrically and unsymmetrically substituted 1,4‐dihydropyridine derivatives. The NMR chemical shifts and coupling constants are discussed in terms of their relationship to structural features such as character and position of the substituent in heterocycle, N‐alkyl substitution, nitrogen lone pair delocalization within the conjugated system, and steric effects. Copyright © 2013 John Wiley & Sons, Ltd.     

15N NMR spectroscopy 24—chemical shifts and coupling constants of α-amino acid N-carboxyanhydrides and related heterocycles

The chemical shifts of amino acid N-carboxyanhydrides (NCAs) and cyclic or linear urethanes are less sensitive to solvent effects than those of amides and lactams. The values of the one-bond ¹⁵N? ¹H coupling constants depend on the solvent and are 5-8 Hz larger than those of ureas and amides. The ¹⁵N? ¹³C coupling constant of the N? CO group is also unusually high, while that of the N—CH group lies within the range known for N-acylated aliphatic amines. The one-bond ¹⁵N? ¹³C coupling constant was found to be insensitive to conformational changes.     

Group IV organometal derivatives of pyridine. I—A 1H and 13C NMR investigation of trialkylmetal derivatives of pyridine

The proton and carbon-13 NMR spectra of thirteen trialkylmetal derivatives of pyridine, several of which were previously unknown, have been recorded and analysed. The proton NMR spectra show variations in proton chemical shifts but not in proton-proton coupling constants when the metal substituent is changed; the ring proton-metal coupling constants ⁿJ(M? H) in the tin and lead derivatives correspond closely with the corresponding proton-proton couplings ⁿJ(H? H) in pyridine. The carbon-13 chemical shifts of the carbons bound to the metal can apparently be correlated with the electron-donating ability of the trialkylmetal group. In the trimethylstannylpyridines the value of ¹J(Sn? C_ring) varies greatly with the position of the Me₃Sn group.     

Nitrogen-15 NMR evidence for the structures of N-9-H-Xanthen-9-yl- and N,N′-Di-9 H-xanthen-9-yl-ureas

A new monoxanthen‒9‒yl derivative of urea has been synthesized and the structure of this product (N‒9 H‒xanthen‒9‒ylurea) and that of the previously known N,N′‒di‒9 H‒xanthen‒9‒ylurea have been proved by ¹⁵N NMR and other spectroscopic techniques. A series of ¹³C and ¹⁵N labeled urea derivatives has been prepared and the utility of their ¹³C and ¹⁵N chemical shifts and coupling constants in the structural analysis of urea derivatives has been investigated.     

Proton polarization transfer in natural abundance 15N NMR studies of E- and Z-isomers of N-alkylformamides

Proton polarization transfer has been used for the convenient observation of proton coupled natural abundance ¹⁵N NMR spectra of the E- and Z-isomers of N-methylformamide at 10.14 MHz. Second-order spectral analysis is required to determine the ¹⁵N? H coupling constants, at least for the E-isomer of secondary formamides. An useful stereospecificity is observed for both the one- and two-bond ¹⁵N? H coupling constants.     

High‐resolution 15N solid‐state NMR investigations on borazine‐based precursors

Reference compounds based on borazine units and polyborylborazines have been characterized by ¹⁵N solid‐state NMR. The various nitrogen sites (B₃N, B₂NH, B₂NX (X = H, Me, ⁱPr), BN(H)X and BNX₂ (X = Me, ⁱPr) have been discriminated according to their cross‐polarization behaviour and chemical shift values, which range from ?265 to ?350 ppm. This has permitted the elucidation of the polymerization mechanism associated with the polycondensation of two borazine‐based derivatives. In particular, this technique appears to be a powerful investigation tool for finding whether the B₃N₃ rings are linked through three‐atom N? B? N aminoboryl bridges or connected by direct B? N bonds. Copyright © 2004 John Wiley & Sons, Ltd.     

15N-NMR study of activated enamines. Structural dependence of δ (15N) and nJ(N,H) in primary,secondary and tertiary enamino-ketones,esters and amides

The pulse sequence INEPT was used to obtain proton-coupled ¹⁵N-NMR spectra in natural isotope abundance for enamines substituted in 2-position with electron-with-drawing groups. The chemical shifts and coupling constants are discussed in terms of their relationship to structural features such as multiple N-alkyl substitution, double-bond configuration, H-bonding, N-lone-pair delocalization within the conjugated system, and steric effects. It is concluded that ¹⁵N chemical shifts are a sensitive probe for local structural modifications at the N-atom and conformational changes in a remote part of a conjugated molecule, while one-bond N,H-coupling essentially reflects N-hybridization and subtle local geometric distortions. Stereospecific three-bond N,H spin coupling to olefinic protons (4.0 ± 0.2 Hz) has been found a characteristic feature of (Z)-isomers in all investigated compounds, whereas two-bond coupling to olefinic protons (²J(N,H) = 0.5 to 5 Hz) is observed in (E)-isomers. The sensitivity to solvents and steric properties of remote substituents renders geminal coupling a useful probe for studying electronic effects in the C? N bond.     

1H‐, 13C‐, and 15N‐NMR chemical shifts for selected glucosides and ribosides of aromatic cytokinins

The ¹H and ¹³C NMR resonances of 16 purine glucosides were assigned by a combination of one‐ and two‐dimensional NMR experiments, including gs‐COSY, gs‐HSQC, and gs‐HMBC, in order to characterize the effect of substituent and the position of glucose unit on the NMR chemical shifts. In addition, ¹⁵N NMR chemical shifts for selected derivatives were investigated by using ¹H? ¹⁵N chemical shift correlation techniques. To map the influence of sugar moiety on the directly bonded nitrogen atom, selected N⁹‐glucosides and their ribose analogs were compared. Characteristic long‐range ¹H? ¹⁵N coupling constants, measured by using ¹H? ¹⁵N gradient‐selected single‐quantum multiple bond correlation (GSQMBC), are also reported and discussed. All compounds investigated here belong to cytokinins, an important group of plant hormones. Copyright © 2010 John Wiley & Sons, Ltd.     

NMR studies in the heterocyclic series XXIV—1H, 13C and 15N study of 15N labelled indazoles

The ¹⁵N chemical shifts and ¹H? ¹⁵N and ¹³C? ¹⁵N coupling constants of nine monolabelled indazoles were measured and assigned. The experimental values are discussed in terms of the indazolic and iso-indazolic structures, and compared with literature data for other related heterocycles. All the results are consistent with an N-1(H) tautomeric structure for indazole in DMSO-d₆.     

A natural abundance 15N NMR investigation of bilirubin IX-α

Natural abundance ¹⁵N NMR spectra have been obtained for bilirubin IX-α using polarization transfer via the INEPT and SINEPT-2 techniques. The resonances for all four nitrogens are clearly resolved. ¹⁵N chemical shifts and ¹⁵N? ¹H coupling constants over one and three bonds are reported. Heteronuclear chemical shift correlation between ¹⁵N and ¹H for the four NH groups has been established by means of the SINEPT-2 pulse sequence.     

NMR investigations on alanyl-[15% 13C, 95% 15N]-proline: 15N chemical shifts and 13C?15N coupling constants

The dipeptide alanylproline has been prepared with the proline residue both ¹³C (15%) and ¹⁵N (95%) enriched. ¹⁵N NMR spectra of alanylproline reveal signals for both possible conformations—cis and trans—of the dipeptide backbone in solution. Different pK values for both conformers are obtained from the pH dependence of the ¹⁵N chemical shifts using a least square programme based on the Henderson–Hasselbach equation. These different values are discussed in terms of interaction between the α-amino group and the carboxylate group and between the carboxylate oxygen and the carbonyl oxygen of the dipeptide via hydrogen bonding. Further evidence for these interactions is obtained from the pH dependence of the ratio of the ¹⁵N NMR signal intensities of the two conformers. One, two or three bonded ¹³C? ¹⁵N coupling constants measured in the ¹³C NMR high resolution spectra have different values in the cis and trans isomers of alanylproline and thus indicate different geometry in the pyrrolidine ring.     

Constantes de couplage vicinales à travers deux hétéroatomes: III—Calculs théoriques et mesures expérimentales dans les hydrazides enrichies en 15N

Different calculations, among them those utilizing the finite perturbation theory with INDO wave functions, have been effected to calculate the value of the ³J(¹H ? N? N? ¹H ) coupling constant in hydrazides as a function of the dihedral angle. Experimental coupling constants have been compared with calculated ones in order to determine the conformation around the N? N bond. The first example of a ²J(¹H ? N? ¹⁵N ) coupling is described.     

Application of 13C and 15N NMR spectroscopy to structural studies on nitramines

¹⁵N and ¹³C chemical shifts and the ¹J(¹⁵N¹⁵N), ¹J(¹⁵N¹³C) and ¹J(¹³CH) coupling constants have been determined for a number of ¹⁵N-enriched cyclic and acyclic secondary nitramines. The results are interpreted in terms of both electronegativity effects and conformational factors.     

Nitrogen-15 nuclear magnetic resonance spectroscopic study of compounds with nitrogen?nitrogen bonds

The ¹⁵N NMR spectra of several substances with nitrogen—nitrogen bonds have been obtained at the natural-abundance level by high-resolution NMR spectroscopy. Azo (? N?N? ) nitrogens are 300–500 ppm deshielded compared with hydrazo (? NH? NH? ) nitrogens. The sensitivity of the ¹⁵N shifts in these types of substances to substituent changes are reported, along with ¹⁵N–¹H spin–spin couplings for some hydrazo compounds. Hydrogen-bonding effects arising from solvent changes on the ¹⁵N shifts of azoxybenzene are different for its two kinds of nitrogen. The ¹⁵N NMR spectrum of N,N'-dinitrosopiperazine in dimethyl sulfoxide at room temperature is consistent with the presence of two different conformations about the N? NO bonds.     

Benzo- and indolo-quinolizine derivatives XI—The configurational and conformational study of 1,2,3,4,4a,6,7,8,9,13b-decahydro-9aH-pyrido[1,2-f]phenanthridine isomers by 270 MHz 1H n.m.r.

The preferred conformations of the four isomers of 1,2,3,4,4a,6,7,8,9,13b-decahydro-9aH-pyrido[1,2-f] phenanthridine have been determined by 270 MHz ¹H n.m.r. and i.r. spectroscopy. N.m.r. assignments are based on the specific chemical shifts of the protons adjacent to the nitrogen atom, their geminal coupling constants and on the shifts induced by the C? C bonds and by the benzene nucleus. Specifically deuterated derivatives and double resonance experiments confirm the assignments unequivocally. These data are completed by solvent and temperature studies.     

1H, 1 3C,and 1 5N Chemical shifts and 1H-1 5N and 1 3C-1 5N heteronuclear spin-spin coupling constants in the NMR spectra of 5-substituted furfural oximes

The ¹H, ¹³C, and ¹⁵N NMR spectra of ¹⁵N-enriched 5-substituted furfural oximes were investigated. It was shown that the chemical shifts of the ring atoms and the oxime group correlate satisfactorily with the F and R substituent constants, whereas their sensitivity to the effect of the substituents is lower than in monosubstituted furan derivatives. The constants of spin-spin coupling between the ring protons and the oxime group were determined. An analysis of the ¹H-¹H spin-spin coupling constants (SSCC) on the basis of their stereospecificity indicates that the E isomers have primarily an s-trans conformation in polar dimethyl sulfoxide, whereas the Z isomers, on the other hand, have an s-cis conformation. The signs of the direct and geminal ¹³C-¹⁵N SSCC were determined for 5-trimethylsilylfurfural oxime.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 9, pp. 1172–1177, September, 1985.The authors thank N. O. Saldabol, L. M. Ignatovich, and N. P. Erchak for providing us with the investigated compounds.     

Linear solvation energy relationships VIII—solvent effects on NMR spectral shifts and coupling constants

The solvatochromic comparison method is used to unravel solvent polarity and hydrogen bonding effects on a variety of NMR spectral shifts and coupling constants. Solvent effects are rationalized in terms of the solvatochromic parameters π*, δ, α and β. Properties analyzed include ¹⁹F shifts of 5-fluoroindole, ¹H shifts of fluorodinitromethane, tert-butanol, phenol, 2-methylbut-1-en-3-yne, and thioacetamide, ¹H and ¹³C shifts and J(¹³C¹H) coupling constants of chloroform, ¹³C shifts of acetone, ¹⁵N shifts of pyridine, ¹⁵N and ²⁹Si shifts of 1-methylsilatrane, and some J(¹¹⁹Sn,C,¹⁹F) coupling constants of polyalkyltin compounds.     

An ab initio study of 15N-11B spin-spin coupling constants for borazine and selected derivatives

Ab initio equation-of-motion coupled-cluster singles and doubles (EOM-CCSD) calculations have been performed to investigate substituent effects on coupling constants for borazine and selected substituted borazines. For molecules in which F atoms are not bonded to adjacent atoms in the ring, F substitution increases the one-bond (11)B-(15)N coupling constants involving the atom at which substitution occurs but leaves the remaining one-bond B-N coupling constants essentially unchanged. For these molecules, the magnitudes of one-bond B-N coupling constants are only slightly dependent on the number of F atoms present. Fluorine substitution at adjacent B and N atoms in the borazine ring further increases the one-bond B-N coupling constant involving the substituted atoms and has the same effect on the other one-bond coupling constants as observed for corresponding molecules in which substitution occurs at alternate sites. In contrast to the effect of F substitution, substitution of Li at either N or B decreases one-bond B-N coupling constants relative to borazine. The effects of F and Li substitution on one-bond B-N coupling constants for borazine are similar to F and Li substitution effects on (13)C-(13)C coupling constants for benzene.     

Non-empirical calculations of the nuclear magnetic resonance spectra of imidazole and hydrated imidazole

The magnetic shielding constants of the ¹H, ¹³C and ¹⁵N nuclei of imidazole are calculated for the isolated and hydrated molecules. The results show that the hydrogen bonds produce not only large variations of the chemical shifts for the nitrogen nuclei and the NH proton which are directly involved in the intermolecular bonding, but also measurable shifts for the carbon nuclei. The calculated shielding constants and their variation with hydration are discussed in relation to experimental results concerning imidazole, the 5-membered ring of the purine bases and the imidazole ring of histidine. The calculated values of the spin-spin coupling constants confirm that it is possible to study the tautomeric equilibrium of the imidazole ring from the measurement of these coupling constants and that spin-spin coupling constants are not very sensitive to solvent effects.     

Heterocycles Contenant du Phosphore—V. Analyse des Spectres de Resonance Magnetique Protonique des Oxo-2 Methoxy ou Phénoxy-2, Methyl-3, Oxazaphospholanes-1,3,2: Exemples de Spectres du type ABXY

The ³¹P decoupled PMR spectra of the title oxazaphospholanes give rise to well resolved ABXY patterns. Among the eight possible solutions, six can be readily eliminated from chemical shifts considerations and the choice between the remaining two is based on tickling experiments. Good fits are observed between experimental and calculated spectra. The relative signs of the ³J(P? O? C? H) and ³J(P? N? C? H) coupling constants are given by tickling and Indor experiments. The ring conformation is discussed.