Multinuclear magnetic resonance studies of 2‐aryl‐1,3,4‐thiadiazoles

The ¹H, ¹³C and ¹⁵N spectra of aryl‐substituted 1,3,4‐thiadiazoles were recorded. The results obtained were correlated with Hammett coefficients. The experimental results were compared with DFT‐calculated chemical shifts. The results obtained were compared with those for 1,3,4‐oxadiazoles and 1,3,4‐selenadiazoles. Copyright © 2012 John Wiley & Sons, Ltd.     

Multinuclear magnetic resonance studies of fluoronitroanilines

Ten fluoronitroanilines have been synthesized and the ¹H, ¹³C, ¹⁵N and, ¹⁹F NMR spectra of these compounds have been recorded and fully assigned. Density functional theory(DFT) calculations have been performed for all compounds studied. Experimental and theoretical results are compared and the structure and atom character influence on the accuracy of the calculation discussed. Copyright © 2009 John Wiley & Sons, Ltd.     

Characterization of 2,5‐diaryl‐1,3,4‐oxadiazolines by multinuclear magnetic resonance and density functional theory calculations. Investigation on a case of very remote Hammett correlation

Two series of 2,5‐diaryl‐1,3,4‐oxadiazolines have been studied by multinuclear magnetic resonance and density functional theory calculations. A full NMR spectroscopic characterization has been performed and excellent remote Hammett correlations (σ_p or ) have been found for para substitution in the two aryl rings through at least 11 bonds, notwithstanding the presence in the path of atoms that should act as insulators and a lack of correlation for some of the intermediate atoms. The computational investigation on the electronic delocalization, performed with the ACID (anisotropy of the induced current density) method, reveals indeed that electrons are delocalized in almost the entire molecule despite the presence of the insulators. Copyright © 2009 John Wiley & Sons, Ltd.     

Characterization of 2‐aryl‐1,3,4‐oxadiazoles by 15N and 13C NMR spectroscopy

¹⁵N NMR chemical shifts of 2‐aryl‐1,3,4‐oxadiazoles were assigned on the basis of the ¹H–¹⁵N HMBC experiment. Chemical shifts of the nitrogen and carbon atoms in the oxadiazole ring correlate with the Hammett σ‐constants of substituents in the aryl ring (r² ≥ 0.966 for N atoms). ¹⁵N NMR data are a suitable and sensitive means for characterizing long‐range electronic substituent effects. Additionally, ¹³C NMR data for these compounds are presented. Copyright © 2003 John Wiley & Sons, Ltd.     

15N NMR study of substituted 2-(phenylamino)-5-phenyl-1,3,4-oxadiazoles

Substituted 2-(phenylamino)-5-phenyl-1,3,4-oxadiazoles were studied by 15N NMR spectroscopy. All signals were assigned on the basis of HMQC and HMBC experiments. Chemical shifts values were correlated with empirical Hammett parameters as well as with calculated electron densities and chemical shieldings.     

1H and 13C NMR analysis of 2‐acetamido‐3‐mercapto‐3‐methyl‐N‐aryl‐butanamides and 2‐acetamido‐3‐methyl‐3‐nitrososulfanyl‐N‐aryl‐butanamide derivatives

The complete assignment of the ¹H and ¹³C NMR spectra of various 2‐acetamido‐3‐mercapto‐3‐methyl‐N‐aryl‐butanamides and 2‐acetamide‐3‐methyl‐3‐nitrososulfanyl‐N‐aryl‐butanamides with p‐methoxy, o‐chloro and m‐chloro substituents is reported. Copyright © 2013 John Wiley & Sons, Ltd.     

15N NMR spectroscopy of 3‐substituted 5‐trichloromethyl‐1,2‐dimethyl‐1H‐pyrazolium chlorides

¹⁵N NMR data of a series of 3‐alkyl[aryl] substituted 5‐trichloromethyl‐1,2‐dimethyl‐1H‐pyrazolium chlorides (where the 3‐substituents are H, Me, Et, n‐Pr, n‐Bu, n‐Pe, n‐Hex, (CH₂)₅CO₂Et, CH₂Br, Ph and 4‐Br‐C₆H₄), are reported. The ¹⁵N substituent chemical shifts (SCS) parameters are determined and these data are compared with the ¹³C SCS values and data obtained by MO calculations. Copyright © 2002 John Wiley & Sons, Ltd.     

Multinuclear magnetic resonance analysis of 2-(trifluoromethyl)-2-oxazoline

(1)H, (19)F, (13)C, (15)N, and (17)O NMR chemical shifts and (1)H-(1)H, (1)H-(19)F, (1)H-(13)C, (19)F-(13)C, and (19)F-(15)N coupling constants are reported for 2-(trifluoromethyl)-2-oxazoline.     

NMR spectroscopic characterization of new 2,3‐dihydro‐1,3,4‐thiadiazole derivatives

The ¹H and ¹³C NMR resonances of twenty‐seven 2,2‐dimethyl‐5‐(2‐nitrophenyl‐5‐substituted)‐2,3‐dihydro‐1,3,4‐thiadiazoles, and twenty‐seven 3‐acyl‐5‐(2‐amino‐5‐substituted)‐2,2‐dimethyl‐2,3‐dihydro‐1,3,4‐thiadiazoles were assigned completely using the concerted application of one‐dimensional and two‐dimensional experiments (DEPT, HMQC and HMBC). NOESY experiments, X‐ray crystallography and conformational analysis confirm the preferred conformation of these compounds. Copyright © 2012 John Wiley & Sons, Ltd.     

Multinuclear magnetic resonance study of the structure and tautomerism of azide and iminophosphorane derivatives of chloropyridazines

Some azido‐ and iminophosphorane derivatives of 3,6‐dichloro‐ and 3,4,5,6‐tetrachloropyridazine were synthesized and studied by means of NMR measurements. Based on multinuclear data (chemical shifts, coupling constants) for compounds containing the azide group, no potentially possible tetrazole–azide equilibrium can be observed, even under acidic conditions. An unusual substitution of a chlorine atom (in position 4) of tetrachloropyridazine in the reaction with hydrazine was demonstrated by NMR measurements of two newly synthesized compounds containing azido‐ and iminophosphorane groups. Using multinuclear magnetic resonance data, the sites of ethylation and protonation of azido‐ and iminophosphorane derivatives of chloropyridazines were established. In the case of the tetrazolopyridazines, ethylation occurs at the N1′ and N2′ atoms, whereas for monocyclic compounds it takes place at the N1 and/or N2 atoms of the pyridazine ring. Preferred sites of protonation are the N1′ atom of the tetrazole ring and the N1 atom of the pyridazine ring. Moreover, the structures of potassium salts of 6‐(3‐cyano‐1‐triazeno)tetrazolo[1,5‐b] pyridazine and its amido derivative were established using NMR data, especially ¹⁵N NMR chemical shifts. Copyright © 2002 John Wiley & Sons, Ltd.     

Identification of 2‐chloropyrazine oxidation products and several derivatives by multinuclear magnetic resonance

¹H, ¹³C, ¹⁴N and ¹⁵N NMR chemical shifts were used to prove the structures of the products of 2‐chloropyrazine oxidation. It was shown that oxidation by hydrogen peroxide in acetic acid or m‐chloroperbenzoic acid leads to the N4‐oxide, whereas potassium persulfate in sulfuric acid gives the N1‐oxide as the main product. Additionally, the results of NMR measurements of products from the nucleophilic substitution of the chlorine atom by azide anion, yielding the respective azides, and ethylation reactions of both 2‐chloropyrazine N‐oxides leading to the N‐ethyl salts confirm the structures of both isomeric N‐oxides. Protonation studies of the compounds obtained are also reported. The favoured protonation site is found to be the N atom that is not hindered by any substituents, and in some cases probably the oxygen atom of the N‐oxide function. Copyright © 2003 John Wiley & Sons, Ltd.     

Theoretical and experimental 1H, 13C and 15N NMR studies of N‐alkylation of substituted tetrazolo[1,5‐a]pyridines

The ¹⁵N as well as ¹H and ¹³C chemical shifts of nine substituted tetrazolopyridines and their corresponding tetrazolopyridinium salts have been determined by using NMR spectroscopy at the natural abundance level of all nuclei in CD₃CN. In this paper, we report, for the first time, the N‐alkylation reaction of electron deficient tetrazolopyridines. The treatment of tetrazolopyridines 5–13 with one equivalent of trialkyloxonium tetrafluoroborate leads to a mixture of two isomers, i.e. N3‐ and N2‐alkyl tetrazolo[1,5‐a]pyridinium salts. It has been observed that the N3‐isomer is always the major isomer, except in the case of the CF₃ substituent, where the two isomers are obtained in the same amount. The quaternary tetrazolopyridinium nitrogen N3 is shielded by around 100 ppm (parts per million) with respect to the parent tetrazolopyridine. Experimental data are interpreted by means of density functional theory (DFT) calculations, including solvent‐induced effects, within the conductor‐like polarizable continuum model (CPCM). Good agreements between theoretical and experimental ¹H, ¹³C and ¹⁵N NMR were found. The combination of multinuclear magnetic resonance spectroscopy with gauge including atomic orbital (GIAO) DFT calculations is a powerful tool in the structural elucidation for both neutral and cationic heterocycles and in the determination of the orientation of N‐alkylation of tetrazolopyridines. Copyright © 2009 John Wiley & Sons, Ltd.     

Structural peculiarities of configurational isomers of 1‐styrylpyrroles according to 1Н, 13С and 15N NMR spectroscopy and density functional theory calculations: electronic and steric hindrance for planar structure

Comparative analysis of the ¹Н and ¹³С NMR data for a series of the E and Z‐1‐styrylpyrroles, E and Z‐1‐(1‐propenyl)pyrroles, 1‐vinylpyrroles and styrene suggests that the conjugation between the unsaturated fragments in the former compounds is reduced. This is the result of the mutual influence of the donor p–π and π–π conjugation having opposite directions. According to the NMR data combined with the density functional theory calculations, the Z isomer of 1‐styrylpyrrole has essentially a nonplanar structure because of the steric hindrance. However, the E isomer of 1‐styrylpyrrole is also an out‐of‐plane structure despite the absence of a sterical barrier for the planar one. Deviation of the E isomer from the planar structure seems to be caused by an electronic hindrance produced by a mutual influence of the p–π and π–π conjugation. The structure of the E isomer of the 2‐substituted 1‐styrylpyrroles is similar to that of the 2‐substituted 1‐vinylpyrroles. The steric effects in the Z isomer of the 2‐substituted 1‐styrylpyrroles result in the large increase of the dihedral angle between planes of the pyrrole ring and double bond. Copyright © 2013 John Wiley & Sons, Ltd.     

17O NMR studies of substituted 1,3,4-oxadiazoles

Three series of substituted 1,3,4-oxadiazoles were studied by (17)O NMR spectroscopy. Chemical shifts values were correlated with empirical Hammett parameters as well as calculated bond lengths and chemical shielding values.     

DFT and multinuclear magnetic resonance studies of diazenedicarboxylates and related compounds

A number of diazenedicarboxylates have been studied by multinuclear magnetic resonance ((17)O, (15)N, (13)C) and compared with analogous fumaric, maleic, and phthalic diesters; the investigation of selected compounds of these classes was complemented by density functional theory (DFT) calculations using a polarizable continuum model (PCM) for the solvent, employing the PBE0 functional together with the 6-311G(d,p) basis set for geometry optimization, and the 6-311 + G(2d,p) basis set for calculating the NMR shielding using the gauge-including atomic orbital (GIAO) method. This combined approach provided important information about the preferred conformations in chloroform and their influence on the NMR parameters.     

A comparative study between para‐aminophenyl and ortho‐aminophenyl benzothiazoles using NMR and DFT calculations

Ortho‐substituted and para‐substituted aminophenyl benzothiazoles were synthesised and characterised using NMR spectroscopy. A comparison of the proton chemical shift values reveals significant differences in the observed chemical shift values for the NH protons indicating the presence of a hydrogen bond in all ortho‐substituted compounds as compared to the para compounds. The presence of intramolecular hydrogen bond in the ortho amino substituted aminophenyl benzothiazole forces the molecule to be planar which may be an additional advantage in developing these compounds as Alzheimer's imaging agent because the binding to amyloid fibrils prefers planar compounds. The splitting pattern of the methylene proton next to the amino group also showed significant coupling to the amino proton consistent with the notion of the existence of slow exchange and hydrogen bond in the ortho‐substituted compounds. This is further verified by density functional theory calculations which yielded a near planar low energy conformer for all the o‐aminophenyl benzothiazoles and displayed a hydrogen bond from the amine proton to the nitrogen of the thiazole ring. A detailed analysis of the ¹H, ¹³C and ¹⁵N NMR chemical shifts and density functional theory calculated structures of the compounds are described. Copyright © 2014 John Wiley & Sons, Ltd.     

Solid‐state NMR studies of 1,3‐imidazolidine‐2‐selenone and some related compounds

Solid‐state cross‐polarization magic angle spinning ¹³C, ⁷⁷Se and ¹⁵N NMR spectra were recorded for 1,3‐imidazolidine‐2‐selenone, its N‐substituted derivatives and some related compounds. The spinning sideband manifold intensities were used to obtain principal values of ¹³C and ⁷⁷Se chemical shift tensors. Large selenium chemical shift anisotropies were observed for these selenones. Copyright © 2003 John Wiley & Sons, Ltd.     

Complete assignment of 1H and 13C NMR spectra of α‐phenylsulfinyl‐N‐methoxy‐ N‐methylpropionamide and some p‐substituted derivatives

The complete assignment of the ¹H and ¹³C NMR spectra of the diastereomeric pairs of some α‐arylsulfinyl‐substituted N‐methoxy‐N‐methylpropionamides with the substituents methoxy, methyl, chloro, nitro is reported. Copyright © 2008 John Wiley & Sons, Ltd.     

Substituent effects on (15)N and (13)C NMR chemical shifts of 5-phenyl-1,3,4-oxathiazol-2-ones: a theoretical and spectroscopic study

The synthesis and assignment of (15)N and (13)C NMR signals of the 1,3,4-oxathiazol-2-one ring in a series of para-substituted 5-phenyl derivatives are reported. DFT calculations of (15)N and (13)C chemical shifts correspond closely to observed values. Substituent effects are interpreted in terms of the Hammett correlation and calculated bond orders.