Analysis of fine splittings in the fourier transform 1H magnetic resonance spectra of some 15N labeled borazine derivatives

Proton NMR spectra are reported for ¹⁵N enriched borazine and a series of ¹⁵N enriched derivatives: N-methyl-borazine, N,N′-dimethylborazine and a new photochemical product, 1-methyl-2-aminoborazine. Chemical shifts for the ring (¹⁵N? H) protons have been measured. Using a Fourier transform spectrometer, fine structure in the ¹⁵N? H doublet is resolved. Ortho and meta ring proton and three-bond ¹⁵N to H coupling constants have been determined. Substituent effects on chemical shifts and coupling constants for borazine derivatives are compared with those for analogous benzene derivatives.     

Study of conformations and hydrogen bonds in the configurational isomers of pyrrole‐2‐carbaldehyde oxime by 1H, 13C and 15N NMR spectroscopy combined with MP2 and DFT calculations and NBO analysis

The ¹H, ¹³C and ¹⁵N NMR studies have shown that the E and Z isomers of pyrrole‐2‐carbaldehyde oxime adopt preferable conformation with the syn orientation of the oxime group with respect to the pyrrole ring. The syn conformation of E and Z isomers of pyrrole‐2‐carbaldehyde oxime is stabilized by the N? H···N and N? H···O intramolecular hydrogen bonds, respectively. The N? H···N hydrogen bond in the E isomer causes the high‐frequency shift of the bridge proton signal by about 1 ppm and increase the ¹J(N, H) coupling by ～3 Hz. The bridge proton shows further deshielding and higher increase of the ¹J(N, H) coupling constant due to the strengthening of the N? H···O hydrogen bond in the Z isomer. The MP2 calculations indicate that the syn conformation of E and Z isomers is by ～3.5 kcal/mol energetically less favorable than the anti conformation. The calculations of ¹H shielding and ¹J(N, H) coupling in the syn and anti conformations allow the contribution to these constants from the N? H···N and N? H···O hydrogen bondings to be estimated. The NBO analysis suggests that the N? H···N hydrogen bond in the E isomer is a pure electrostatic interaction while the charge transfer from the oxygen lone pair to the antibonding orbital of the N? H bond through the N? H···O hydrogen bond occurs in the Z isomer. Copyright © 2010 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.     

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.     

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.     

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.     

Etude des Signes des Constantes de Couplage 3J(P?X?C?H) et 4J(F?P?X?C?H) dans les Diaza-, Dioxa- et Dithiaphospholanes Fluores

The NMR spectra of the trivalent fluorophospholanes ( 1, 2, 3 ) have been analysed at length. The absolute signs of the ³J(P? H) and ⁴J(F? H) coupling constants have been referred to the known negative sign of the ¹J(P? F) coupling constant from selective heteronuclear double resonance experiments. The ³J(P? O? C? H) and ³J(P? N? C? H) coupling are positive. The weak values observed for ³J(P? S? C? H) have opposite signs, the larger being positive. All the ⁴J(F? P? X? C? H) coupling constants are positive showing a lack of stereospecificity.     

One-bond 1J(15N,H) coupling constants at sp2-hybridized nitrogen of Schiff bases,enaminones and similar compounds: A theoretical study

¹J(¹⁵N,H) coupling constants for enaminones and NH-forms of intramolecularly hydrogen-bonded Schiff bases as model compounds for sp²-hybridized nitrogen atoms are evaluated using density functional theory (DFT) to find the optimal functionals and basis sets. Ammonia is used as a test molecule and its one-bond coupling constant is compared with experiment. A methylamine Schiff base of a truncated molecule of gossypol is used for checking the performance of selected B3LYP, O3LYP, PBE, BHandH, and APFD density functionals and standard, modified, and dedicated basis sets for coupling constants. Both in vacuum and in chloroform, modeled by the simple continuum model of solvent, the modified basis sets predict significantly better the ¹J(¹⁵N,H) value in ammonia and in the methylamine Schiff base of a truncated molecule of gossypol than the standard basis sets. This procure is then used on a broad set of intramolecularly hydrogen-bonded molecules, and a good correlation between calculated and experimental one-bond NH coupling constants is obtained. The ¹J(¹⁵N,H) couplings are slightly overestimated. The calculated data show for hydrogen-bonded NH interatomic distances that the calculated values depend on the NH bond lengths. The shorter the bond lengths, the larger the ¹J(¹⁵N,H). A useful correlation between ¹J(¹⁵N,H) and NH bond length is derived that enables realistic predictions of one-bond NH coupling constants. The calculations reproduce experimentally observed trends for the studied molecules.     

GIAO,DFT, AIM and NBO analysis of the N?H···O intramolecular hydrogen‐bond influence on the 1J(N,H) coupling constant in push–pull diaminoenones

In the series of diaminoenones, large high‐frequency shifts of the ¹H NMR of the N? H group in the cis‐position relative to the carbonyl group suggests strong N? H···O intramolecular hydrogen bonding comprising a six‐membered chelate ring. The N? H···O hydrogen bond causes an increase of the ¹J(N,H) coupling constant by 2–4 Hz and high‐frequency shift of the ¹⁵N signal by 9–10 ppm despite of the lengthening of the relevant N? H bond. These experimental trends are substantiated by gauge‐independent atomic orbital and density functional theory calculations of the shielding and coupling constants in the 3,3‐bis(isopropylamino)‐1‐(aryl)prop‐2‐en‐1‐one (12) for conformations with the Z‐ and E‐orientations of the carbonyl group relative to the N? H group. The effects of the N? H···O hydrogen‐bond on the NMR parameters are analyzed with the atoms‐in‐molecules (AIM) and natural bond orbital (NBO) methods. The AIM method indicates a weakening of the N? H···O hydrogen bond as compared with that of 1,1‐di(pyrrol‐2‐yl)‐2‐formylethene (13) where N? H···O hydrogen bridge establishes a seven‐membered chelate ring, and the corresponding ¹J(N,H) coupling constant decreases. The NBO method reveals that the LP(O) →σ*_{N? H} hyperconjugative interaction is weakened on going from the six‐membered chelate ring to the seven‐membered one due to a more bent hydrogen bond in the former case. A dominating effect of the N? H bond rehybridization, owing to an electrostatic term in the hydrogen bonding, seems to provide an increase of the ¹J(N,H) value as a consequence of the N? H···O hydrogen bonding in the studied diaminoenones. Copyright © 2010 John Wiley & Sons, Ltd.     

Geminal C,H spin coupling in substituted alkenes

Two-bond C,H coupling constants for the coupling pathways C_?C? H_ (type I) and (type II) have been studied in a variety of mono-, di- and trisubstituted ethylenes. For ²J(CH) of type I, additive increments have been determined for 20 substituents, which allow a satisfactory calculation of coupling constants, including relative signs, in di? and trisubstituted ethylenes. Several applications for E/Z isomers are presented. In cyclic compounds a positive coupling increment, δJ, is observed in going from 6? to 5?membered ring compounds.     

Simultaneous measurement of N?H and Cα?Hα coupling constants in proteins

We present a pulse sequence for the simultaneous measurement of N? H and Cα? Hα couplings in double‐labeled proteins from 2D spectra. The proposed sequence, a modification of the HN(CO)CA experiment, combines the J‐modulation method and the IPAP scheme. The couplings can be readily retrieved from a series of 2D ¹⁵N? ¹H correlation spectra, differing in the time point at which a ¹H 180° pulse is applied. This induces an intensity modulation of the ¹⁵N? ¹H correlation peaks with the Cα? Hα coupling. The Cα? Hα coupling is then obtained by fitting the observed intensities to the modulation equation. The N? H coupling is measured in each member of the set from peak‐to‐peak separations in the IPAP subspectra. The pulse sequence is experimentally verified with a sample of ¹⁵N/¹³C‐enriched ubiquitin. Copyright © 2009 John Wiley & Sons, Ltd.     

Heterocycles Contenant du Phosphore—XII: Etude par Resonance Magnetique Nucleaire des Oxo- et Thio-2 Phenoxy-2 Trimethyl-3,5,5 Oxazaphosphorinanes-1,3,2. Un Cas Particulier de Systeme ABXY

After elimination of the P? H spin coupling, the PMR spectra of the title 1,3,2-oxazaphosphorinanes give rise to an ABXY pattern where J_AX = J_AY = J_BX = 0. The particular ABXY system where J_AY = J_BX = 0 is studied to calculate the expression of the E₇, E₈, E₉ and E₁₀ energy-levels and the frequency of the transitions connected to these energy-levels. The relative sign of the ³J_P? O? C? H and ³J_P? N? C? H coupling constants is given by INDOR experiments. The ring conformation is discussed.     

Assignment of oxime and hydrazone configuration using 1H‐15N and 13C‐15N coupling measurements at natural abundance

Measurement of ¹H‐¹⁵N and ¹³C‐¹⁵N coupling constants at natural abundance is demonstrated to be a reliable and generic method to determine the configuration of oximes, hydrazines, and related systems. Data on ¹H‐¹⁵N and ¹³C‐¹⁵N coupling constants on a variety of systems obtained at natural abundance confirm the geometric dependence of the measured ¹H‐¹⁵N and ¹³C‐¹⁵N coupling constants. In addition, we summarize a simple “decision‐tree” for determining configuration based on practical considerations of sample quantity, solubility, and complexity. Copyright © 2016 John Wiley & Sons, Ltd.     

15N chemical shifts as a conformational probe in enaminones A variable temperature study at natural isotope abundance

The high sensitivity of ¹⁵N shielding to the displacement of the lone pair electrons makes it a useful conformational probe for remote parts of a conjugated molecule. Thus, the chemical shifts are observed for different rotamers of enaminones in the slow exchange limit. The interpretation of the ¹⁵N chemical shifts in terms of the non-planarity of the E, s-E rotamers is in accord with ¹³C chemical shifts and ¹J(CH) coupling constants.     

13C NMR: The stereochemical dependence of 15N?13C coupling constants in pyrazoles

Geminal ¹⁵N? ¹³C coupling constants have been measured in a series of ¹⁵N-enriched 1?phenyl?3,5?dialkyl-substituted pyrazoles. The importance of the orientation of the nitrogen lone-pair in determining the magnitude of ²J(¹⁵N¹³C) values is reflected in the enhanced coupling observed for those carbon atoms lying close in space to the lone-pair electrons.     

A detailed evaluation of the dependence of 3J(H?H) on bond angle in alkenes and cycloalkenes

Newly determined and accurate data for the magnitudes of cis vinyl proton-proton spin-spin coupling constants in cis-dialkylethylenes and cycloalkenes have been obtained. With these new data and also values taken from the recent literature, it has proved possible to make a critical determination of the correlation between ³J(H? H) and C?C? H bond angles in ethylenic systems. It is suggested that it is possible to obtain accurate estimates of bond angles using NMR coupling constants, even though much more data will be required to fully substantiate this proposal. Whereas cis-³J(H? H) decreases rapidly with increasing C?C? H bond angles, evidence is presented that the opposite is the case for trans-³J(H? H). A brief theoretical discussion of these trends in coupling constants is given.     

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.     

Pronounced stereospecificity of 1H, 13C, 15N and 77Se shielding constants in the selenophenyl oximes as shown by NMR spectroscopy and GIAO calculations

In the ¹H and ¹³C NMR spectra of 1‐(2‐selenophenyl)‐1‐alkanone oximes, the ¹H, the ¹³C‐3 and ¹³C‐5 signals of the selenophene ring are shifted by 0.1–0.4, 2.5–3.0 and 5.5–6.0 ppm, respectively, to higher frequencies, whereas those of the ¹³C‐1, ¹³C‐2 and ¹³C‐4 carbons are shifted by 4–5, ～11 and ～1.7 ppm to lower frequencies on going from the E to Z isomer. The ¹⁵N chemical shift of the oximic nitrogen is larger by 13–16 ppm in the E isomer relative to the Z isomer. An extraordinarily large difference (above 90 ppm) between the ⁷⁷Se resonance positions is revealed in the studied oxime isomers, the ⁷⁷Se peak being shifted to higher frequencies in the Z isomer. The trends in the changes of the measured chemical shifts are well reproduced by the GIAO calculations of the ¹H, ¹³C, ¹⁵N and ⁷⁷Se shielding constants in the energy‐favorable conformation with the syn orientation of the? C?N? O? H group relative to the selenophene ring. Copyright © 2009 John Wiley & Sons, Ltd.     

Theoretical study of bifurcated hydrogen bonding effects on the 1J(N,H), 1hJ(N,H), 2hJ(N,N) couplings and 1H, 15N shieldings in model pyrroles

According to the density functional theory calculations, the X···H···N (X?N, O) intramolecular bifurcated (three‐centered) hydrogen bond with one hydrogen donor and two hydrogen acceptors causes a significant decrease of the ^1hJ(N,H) and ^2hJ(N,N) coupling constants across the N? H···N hydrogen bond and an increase of the ¹J(N,H) coupling constant across the N? H covalent bond in the 2,5‐disubsituted pyrroles. This occurs due to a weakening of the N? H···N hydrogen bridge resulting in a lengthening of the N···H distance and a decrease of the hydrogen bond angle at the bifurcated hydrogen bond formation. The gauge‐independent atomic orbital calculations of the shielding constants suggest that a weakening of the N? H···N hydrogen bridge in case of the three‐centered hydrogen bond yields a shielding of the bridge proton and deshielding of the acceptor nitrogen atom. The atoms‐in‐molecules analysis shows that an attenuation of the ^1hJ(N,H) and ^2hJ(N,N) couplings in the compounds with bifurcated hydrogen bond is connected with a decrease of the electron density ρ_H···N at the hydrogen bond critical point and Laplacian of this electron density ?²ρ_H···N. The natural bond orbital analysis suggests that the additional N? H···X interaction partly inhibits the charge transfer from the nitrogen lone pair to the σ*_{N? H} antibonding orbital across hydrogen bond weakening of the ^1hJ(N,H) and ^2hJ(N,N) trans‐hydrogen bond couplings through Fermi‐contact mechanism. An increase of the nitrogen s‐character percentage of the N? H bond in consequence of the bifurcated hydrogen bonding leads to an increase of the ¹J(N,H) coupling constant across the N? H covalent bond and deshielding of the hydrogen donor nitrogen atom. Copyright © 2010 John Wiley & Sons, Ltd.