Substitution effects in the 15N NMR chemical shifts of heterocyclic azines evaluated at the GIAO‐DFT level

A systematic study of the accuracy factors for the computation of ¹⁵N NMR chemical shifts in comparison with available experiment in the series of 72 diverse heterocyclic azines substituted with a classical series of substituents (CH₃, F, Cl, Br, NH₂, OCH₃, SCH₃, COCH₃, CONH₂, COOH, and CN) providing marked electronic σ‐ and π‐electronic effects and strongly affecting ¹⁵N NMR chemical shifts is performed. The best computational scheme for heterocyclic azines at the DFT level was found to be KT3/pcS‐3//pc‐2 (IEF‐PCM). A vast amount of unknown ¹⁵N NMR chemical shifts was predicted using the best computational protocol for substituted heterocyclic azines, especially for trizine, tetrazine, and pentazine where experimental ¹⁵N NMR chemical shifts are almost totally unknown throughout the series. It was found that substitution effects in the classical series of substituents providing typical σ‐ and π‐electronic effects followed the expected trends, as derived from the correlations of experimental and calculated ¹⁵N NMR chemical shifts with Swain–Lupton's F and R constants.     

n-π-Interaction in Enamines and Enaminoketones. A 15N-NMR. Study

¹⁵N-Chemical shifts of 32 enamines, 11 enaminoketones and 28 closely related amines have been determined with the isotope in natural abundance. In order to eliminate substituent effects, differential chemical shifts Δδ(N) are defined as δ_N(amine)-δ_N(enamine). This parameter is shown to correlate well with the free enthalpy of activation ΔG# for restricted rotation about the N? C(α) bond in enamines with extended conjugation. Δδ(N) values of substituted anilinostyrenes correlate also with ¹³C-chemical shifts of the β-carbon in the enamine system and with Hammett σ-constants of the aniline substituents. The experimental results suggest that differential ¹⁵N shifts are a useful probe to study n, π-interaction in enamines.     

Multinuclear NMR and vibrational spectroscopy studies of the substituent effects in benzensulphonamide inhibitors of the enzyme carbonic anhydrase

Substituent effects on the electronic structure of sixteen biologically active benzensulphonamide derivatives were investigated by means of ¹⁵N, ¹³C, ¹H NMR, and IR spectroscopy, as well as by quantum chemical calculations. Good correlations were found between the spectroscopic data and both substituent constant and computed electronic charges. On this basis the substituent effects are interpreted in terms of electronic charge perturbation, which is linearly transmitted from the substituent to the biofunctional −SO₂NH₂ group. The resonance nature of the substituent seems most important in determining the ¹⁵N chemical shifts, which follow a “reverse” trend; i.e., electron-donor substituents induce downfield ¹⁵N shifts.     

15N NMR studies of amino acids and their reaction products with formaldehyde

Natural abundance ¹⁵N NMR spectroscopy has been used to investigate the effect of pH on the ¹⁵N chemical shifts of lysine and of ε-hydroxymethyllysine. A computer calcualtion which fits the chemical shifts of both α-and ε-nitrogen atoms versus pH has been used to predict the pK_a values. ¹⁵N chemical shifts and some ¹J(¹⁵NH) values of some other amino acids and of their reaction products with formaldehyde are also reported.     

15N NMR substituent effects in pyridines and pyrimidines

¹⁵N chemical shifts of 32 substituted pyridines and 19 substituted pyrimidines, together with additional data from the literature, are used to evalute substituent increments, A_i and A_ik, in the respective series. Differential chemical shifts, Δδ(N), correlate with corresponding Δδ(C) values whereby, on the ppm scale, nitrogen shifts are approximately three times more sensitive towards substituents than carbon shifts. The ¹⁵N increments have proven additive and useful for assignment purposes.     

15N NMR study of azo-hydrazone tautomerism of 15N-labelled azo dyestuffs

¹⁵N chemical shifts of 3-methyl-1-phenylpyrazole-4,5-dione 4-phenylhydrazone (1), 4-hydroxyazobenzene (2), 2-hydroxy-5-tert-butylazobenzene (3) and 1-phenylazo-2-naphthol (4), monolabelled with ¹⁵N at α-(compounds prepared from ¹⁵N-aniline) and β-positions (compounds prepared from Na¹⁵NO₂), have been measured and the temperature dependence of these chemical shifts followed between 240 and 360 K. For 4, representing a mixture of the azo and hydrazone forms, the hydrazone content has been calculated from the ¹⁵N chemical shifts of both nitrogen atoms at various temperatures. The two calculations gave identical results.     

Crown Ethers Analogues and their Complexes with NaSCN: Stoichiometry and Stability Constants Determined by 13C NMR Spectroscopy

¹³C NMR spectroscopy was used to study thecomplexation reaction between sodium ion and12-crown-4, 15-crown-5, methylurazolyl-12-crown-4 andmethylurazolyl-15-crown-5 in CD₃OD solutions. Thetype of complexes, the stability constants and therelative chemical shifts have been established byfitting experimental titration curves with theoreticalfunctions of the observed chemical shifts. Thesingle-crystal X-ray diffraction measurements for anew crystalline complex confirmed the stoichiometry ofthe complex.     

Isotope Shifts of Nitrogen around 800 nm

The Doppler-limited absorption spectra of ¹⁴N and ¹⁵N atoms were measured around 800 nm using concentration modulation spectroscopy to study their isotope shifts. The nitrogen atoms were generated by discharging molecular nitrogen buffered with helium in a homemade discharge tube. The isotope shifts of four multiplets (3s⁴P_J→3p⁴D_J^o, 3s⁴P_J→3p⁴P_J^o, 3s²D_J→5s²P_J^o, and 3p²P_J^o→5s²D_J^o) were measured and their J-dependent specific mass shifts were observed and discussed.     

Solvent-Induced Deuterium Isotope Effects in 13C- and 15N-NMR. Spectra of Enaminones

The effect of deuterium on the ¹³C and ¹⁵N chemical shifts of enaminones has been investigated. D/H isotope shifts are reported for neutral and protonated species, i.e., when the isotope is exchanged on the C(2)-, N-, or O-atoms. In cases of slow exchange the isotope shifts were obtained from solutions containing both isotopomers, whereas for fast exchange (acidic solutions) either separate NMR. sample tubes (¹⁵N-NMR.) or coaxial tubes (¹³C-NMR.) were used. In neutral molecules the isotope effects δ_C(D, H) are intrinsic in nature. In acidic solutions, the enaminocarbonyl cations formed exhibit δ_C(D, H)- and δ_N(D, H)-values which are discussed in terms of the proton transfer. The mesomeric character of the cations is reflected by characteristic features in the δ_C(D, H)- and δ_N(D, H)-values, which can be ascribed to isotopic perturbation of resonance. O-Protonation shifts in the ¹⁵N-resonance, observed for the first time, are large and positive (+60 to +76 ppm), in contrast to amides, where the effects are of the same sign but an order of magnitude smaller. Both protonation shifts and solvent-induced isotope effects are discussed in connection with the nucleophilic character of the reactive centers in the enaminone synthon.     

15N, 31P,and 13C NMR Spectroscopy of N-Arylsulfonyl-and N-Arylcarbonyl-P,P,P-triphenylphospha-λ-Azenes. Substituent Effects and Electronic Structure

Abstract

The ¹⁵N, ³¹P and ³¹C NMR spectra of several series of phospha-λ⁵-azenes are reported. For the N-arylsulfonyl-P,P,P-triphenylphospha-δ⁵-azene series (R-C₆H₄N-SO₂-PPh₃), the ³¹P chemical shifts, various ¹³C chemical shifts and ¹J_PN were observed to correlate linearly with the Hammett σ constants. Interestingly, the ¹⁵N chemical shifts did not correlate acceptably with any σ or with the Taft dual substituent parameter equation, and 1J_PC was invariant with substituent. For the N-arylcarbonyl-P,P,P-triphenylphospha-λ⁵-azene series (R-C₆H₄-CO-N=PPh₃), δ_31P and various δ_13C's were observed to linearly correlate with the δ constants, while δ_15N, ¹J_PN and ¹J_PC correlated with both the σ and σ constants. For the N-phenyl-P,P,P-triarylphospha-λ⁵-azene series [Ph-N=P(C₆H₄-R)₃] the best correlations were observed between ³¹P, ¹⁵N and several ¹³C chemical shifts and the σ constants.     

A direct hydrogen-1, carbon-13, and nitrogen-15 NMR study of lutetium(III)-isothiocyanate complexation in aqueous solvent mixtures

A direct, low-temperature hydrogen-1, carbon-13, and nitrogen-15 nuclear magnetic resonance study of lutetium(III)-isothiocyanate complex formation in aqueous solvent mixtures has been completed. At –100°C to –120°C in water-acetone-Freon mixtures, ligand exchange is slowed sufficiently to permit the observation of separate¹H,¹³C, and¹⁵N NMR signals for coordinated and free water and isothiocyanate ions. In the¹³C and¹⁵N spectra of NCS^–, resonance signals for five complexes are observed over the range of concentrations studied. The¹³C chemical shifts of complexed NCS^– varied from –0.5 ppm to –3 ppm from that of free anion. For the same complexes, the¹⁵N chemical shifts from free anion were about –11 ppm to –15 ppm. The magnitude and sign of the¹⁵N chemical shifts identified the nitrogen atom as the binding site in NCS^–. The concentration dependence of the¹³C and¹⁵N signal areas, and estimates of the fraction of anion bound at each NCS^–:Lu³⁺ mole ratio, were consistent with the formation of [(H₂O)₅Lu(NCS)]²⁺ through [(H₂O)Lu(NCS)₅]^2–. Although proton and/or ligand exchange and the resulting bulk-coordinated signal overlap prevented accurate hydration number measurements, a good qualitative correlation of the water¹H NMR spectral results with those of¹³C and¹⁵N was possible.     

15N-, 1H-, and 13C-NMR chemical shifts and electronic properties of aromatic diamines and dianhydrides

We measured the ¹⁵N-, ¹H-, and ¹³C-NMR chemical shifts for a series of aromatic diamines and aromatic tetracarboxylic dianhydrides dissolved in DMSO-d₆, and discuss the relationships between these chemical shifts and the rate constants of acylation (k) as well as such electronic-property-related parameters such as ionization potential (IP), electronic affinity (EA), and the energy ε of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO). The ¹⁵N chemical shifts of the amino group of diamines (δ_N) depend monotonically on the logarithm of k (log k) and on IP. We inferred the reactivities of diamines whose acylation rates have not been measured from their δ_N, and we propose an arrangement of diamines in the order of their reactivity. The ¹H chemical shift of amino hydrogens (δ_H) and the ¹³C chemical shift of carbons bonded to nitrogen (δ_C) are roughly proportional to δ_N, but these shifts are not as closely correlated with log k and IP. Although the ¹³C chemical shifts of the carbonyl carbon of dianhydrides (δ_C,) varies much less than the δ_C and δ_N of diamines, δ_C, can be an index of acylation reactivity for dianhydrides because it is closely correlated with ε_LUMO. These facts indicate that the chemical shifts of diamines and dianhydrides are displaced according to their electron-donor and electron-acceptor properties, and that these chemical shifts can be used as indices of the electronic properties of monomers. Changes in reactivity caused by the introduction of trifluoromethyl groups into diamines and dianhydrides are inferred from the displacements of δ_N and δ_C © 1992 John Wiley & Sons, Inc.     

15N NMR Study of Coordinated Amine,Aminocarbyne, and Carboxamido Groups in Triosmium Clusters

The ¹⁵N NMR spectra of the complexes Os₃(CO)₁₀(μ₂-CONHPrⁱ)(μ₂-C? NHR) (1a, R = Pr; 1b, R = CH₂Ph) and Os₃(CO)₉(NH₂Prⁱ)(μ₂-CONHPrⁱ)(μ₂-C? NHR) (2a, R = Pr; 2b, R = CH₂Ph) are studied by using the ¹H detected (inverse) ¹H-¹⁵N correlated spectroscopy. The ¹⁵N chemical shifts and the ¹H-¹⁵N coupling constants fall in characteristic regions for each of the coordinated amine, aminocarbyne, and carboxamido ligands and these values are related to their bonding types. The NMR data are discussed in terms of the influence of the paramagnetic term which is the major factor determining the chemical shifts. A comparison is made to understand the ¹⁵N chemical-shift differences between the coordinated nitrogen-containing ligands and the corresponding free organic molecules.     

Computational NMR Spectroscopy of Transition‐Metal/Nitroimidazole Complexes: Theoretical Investigation of Potential Radiosensitizers

The computed chemical shifts of transition‐metal complexes with dimetridazole (= 1,2‐dimethyl‐5‐nitro‐1H‐imidazole; 1 ), a prototypical nitro‐imidazole‐based radiosensitizer, are reported at the GIAO‐BP86 and ‐B3LYP levels for BP86/ECP1‐optimized geometries. These complexes comprise [MCl₂( 1 )₂] (M = Zn, Pd, Pt), [RuCl₂(DMSO)₂( 1 )₂], and [Rh₂(O₂CMe)₄( 1 )₂]. Available δ(¹H) and δ(¹⁵N) values, and Δδ(¹H) and Δδ(¹⁵N) coordination shifts are well‐reproduced theoretically, provided solvation and relativistic effects are taken into account by means of a polarizable continuum model and suitable methods including spin–orbit (SO) coupling, respectively. These effects are particularly important for the metal‐coordinated N‐atom, where the contributions from solvation and relativity can affect δ(¹⁵N) and Δδ(¹⁵N) values up to 10–20 ppm. The ¹⁹⁵Pt chemical shifts of cis‐ and trans‐[PtCl₂( 1 )₂] are well‐reproduced using the zero‐order regular approximation including SO coupling (ZORA‐SO). Predictions are reported for ⁹⁹Ru and ¹⁰³Rh chemical shifts, which suggest that these metal centers could be used as additional, sensitive NMR probes in their complexes with nitro‐imidazoles.     

Solid‐state 13C, 15N and 29Si NMR characterization of block copolymers with CO2 capture properties

Natural abundance solid‐state multinuclear (¹³C, ¹⁵N and ²⁹Si) cross‐polarization magic‐angle‐spinning NMR was used to study structures of three block copolymers based on polyamide and dimethylsiloxane and two polyamides, one of which including ferrocene in its structure. Assignment of most of the resonance lines in ¹³C, ¹⁵N and ²⁹Si cross‐polarization magic‐angle‐spinning NMR spectra were suggested. A comparative analysis of ¹³C isotropic chemical shifts of polyamides with and without ferrocene has revealed a systematic shift towards higher δ ‐values (de‐shielding) explained as the incorporation of paramagnetic ferrocene into the polyamide backbone. In addition, the ¹³C NMR resonance lines for ferrocene‐based polyamide were significantly broadened, because of paramagnetic effects from ferrocene incorporated in the structure of this polyamide polymer. Single resonance lines with chemical shifts ranging from 88.1 to 91.5 ppm were observed for ¹⁵N sites in all of studied polyamide samples. ²⁹Si chemical shifts were found to be around ?22.4 ppm in polydimethylsiloxane samples that falls in the range of chemical shifts for alkylsiloxane compounds. The CO₂ capture performance of polyamide‐dimethylsiloxane‐based block copolymers was measured as a function of temperature and pressure. The data revealed that these polymeric materials have potential to uptake CO₂ (up to 9.6 cm³ g^?1) at ambient pressures and in the temperature interval 30–40 °C. Copyright © 2016 John Wiley & Sons, Ltd.     

NMR “Crystallography” for Uniformly (13C, 15N)-Labeled Oriented Membrane Proteins

In oriented-sample (OS) solid-state NMR of membrane proteins, the angular-dependent dipolar couplings and chemical shifts provide a direct input for structure calculations. However, so far only ¹H–¹⁵N dipolar couplings and ¹⁵N chemical shifts have been routinely assessed in oriented ¹⁵N-labeled samples. The main obstacle for extending this technique to membrane proteins of arbitrary topology has remained in the lack of additional experimental restraints. We have developed a new experimental triple-resonance NMR technique, which was applied to uniformly doubly (¹⁵N, ¹³C)-labeled Pf1 coat protein in magnetically aligned DMPC/DHPC bicelles. The previously inaccessible ¹H_α–¹³C_α dipolar couplings have been measured, which make it possible to determine the torsion angles between the peptide planes without assuming α-helical structure a priori. The fitting of three angular restraints per peptide plane and filtering by Rosetta scoring functions has yielded a consensus α-helical transmembrane structure for Pf1 protein.     

Observation of Diastereotopic Signals in 15N NMR Spectroscopy

The first example in the literature of a compound showing anisochronous ¹⁵N atoms resulting from diastereotopicity is described. Racemic 1,3‐dimethyl‐2‐phenyloctahydro‐1H‐benzimidazole was prepared and studied by ¹H, ¹³C and ¹⁵N NMR spectroscopy. If convenient conditions were used (monitored by theoretical calculations of ²J_N‐H spin–spin coupling constants), two ¹⁵N NMR signals were observed and corresponded to the diastereotopic atoms. GIAO/density‐functional calculations of chemical shifts were not only in good agreement with the experimental values but also served as prediction tools. This study suggests that ¹⁵N NMR spectroscopy could be used to probe chirality.     

A C and N experimental NMR and theoretical study of the structure of linear primary aliphatic amines and ammonium salts: from C1 to C18

Eighteen aliphatic linear amines, from methylamine to stearylamine, have been experimentally studied by NMR and theoretically calculated at the GIAO/B3LYP/6-311++G(d,p) level. A partial exploration of their conformation has been carried out, mainly to determine the effect on the chemical shifts. In solution and for neutral amines, ¹⁵N chemical shifts indicate a mixture of two conformations. In the solid state (CPMAS NMR) only the subset of solid amines has been studied (from C14 to C18). The ¹⁵N signals of the corresponding ammonium salts in the solid state depend on the counteranions, Cl⁻ and CF₃CO₂⁻, a result that is theoretically proven.     

NMR-spektroskopische Untersuchungen an 15N-markierten geminal disubstituierten Cyclotriphosphazenen

NMR Spectroscopic Studies of ¹⁵N Labelled Geminally Disubstituted Cyclotriphosphazenes It is demonstrated by means of some selected ¹⁵N labelled geminally disubstituted cyclotriphosphazenes, ¹⁵N₃P₃X₄Y₂ (X = Cl; Y = F, NH₂, or SEt), as an example, that the coupling constants ¹J_PN may be of different signs. The absolute value of ¹J_PN is significantly influenced only by those substituents, which are bonded to the phosphorus nucleus directly concerned in the coupling. Also the ¹⁵N chemical shifts are only changed by substituents on directly bonded phosphorus atoms.     

1H, 13C and 15N NMR spectra of [1,2-15N2]pyrazole derivatives

The chemical shifts and coupling constants of [1,2-¹⁵N₂]pyrazole, 2-(1-[1,2- ¹⁵N₂]pyrazolyl)-2-[l,3-²H₆]propanol, 1-nitro[1,2¹⁵N₂] and 3-nitro[1,2-¹⁵N₂]pyrazole are reported.