15N NMR shielding tensors in bent nitrosyl complexes of cobalt

Ab initio calculations of nitrogen NMR chemical shielding tensors are presented for square–pyramidal cobalt model complexes with an apical nitrosyl ligand as a function of the geometry of the CoNO grouping. It is shown that the isotropic nitrogen shielding decreases with increasing N–O distance (in the range 101–116 pm) and with increasing CoN distance (in the range 173–193 pm). With increasing CoNO angle, in the models studied, the isotropic shielding initially decreases up to about 140° and then increases. For complexes with the same CoNO geometry and S-, N- or O-ligating coligands, the shielding increases in the order S<N,O. The variation of the shielding with CoNO geometry is mainly due to variations in the shielding tensor component parallel to the NO bond which mixes n(N) and π¹(NO) orbitals. The calculations give no evidence for differing CoNO geometries producing similar isotropic shieldings but very different spans and skews of the shielding tensor, as given by the experiment in some series of compounds. It is likely that there is CoNO motional averaging (libration or spinning) in the solid state, as described for [Co(NO)(TPP)], in complexes for which anomalously small spans are observed.     

A 15N NMR investigation of some azolopyridines

¹⁵N NMR chemical shift data are presented for 14 azolopyridines, together with the results of INDO/S-SOS calculations of nitrogen shieldings. Previous ¹⁴N NMR results for some of these compounds are reinterpreted. The ¹⁴N data and their assignments are shown to be reliable for the indolizine nitrogen atom from arguments based on relative line widths. The pyridine-type nitrogens are more reliably assigned from the ¹⁵N spectra combined with the results of the INDO/S-SOS calculations for individual molecules. A combination of ¹⁴N and ¹⁵N NMR spectra, together with the shielding calculations, provides a basis for unambiguous assignments of all the various nitrogen environments considered.     

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.     

Structural and solvent effects on the 13C and 15N NMR chemical shifts of indoloquinoline alkaloids: experimental and DFT study

Indoloquinoline alkaloids represent an important class of antimalarial, antibacterial and antiviral compounds. They have been shown to bind to DNA via intercalation preferentially at GC-rich sequences containing nonalternating CC sites. The stability of complexes formed with biological macromolecules depends on noncovalent binding. In the present study, the ability of indoloquinolines to form intermolecular interactions with solvents was investigated by using NMR spectroscopy and density functional theory (DFT) (B3LYP/6-31G**) calculations. NMR data measured for indoloquinoline bases and the corresponding hydrochlorides are discussed in relation to the structure. DFT calculations of shielding constants in vacuo and in solution allowed the investigation of the influence of the environment on the NMR parameters. Calculations incorporating solvent effects indicated significant changes in the anisotropy of the electron distribution, reflected in the span of the chemical shielding tensor (Omega = sigma11 - sigma33). Solvent effects on the span of the 13C and 15N shielding tensor depended on the type of atom and the data indicated a significant influence of solute-solvent interactions.     

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.     

Substituent effects on 15N and 13C NMR chemical shifts of 3-phenylisoxazoles: a theoretical and spectroscopic study

The synthesis and assignment of 15N and 13C NMR signals of the isoxazole ring in a series of para-substituted 3-phenyl derivatives are reported. DFT calculations of 15N and 13C chemical shifts are presented and compared to observed values. Substituent effects are interpreted in terms of the Hammett correlation and calculated bond orders.     

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.     

Deuterium isotope effects on 13C and 15N nuclear shielding in o-hydroxyazo dyes

Deuterium isotope effects on the ¹³C and ¹⁵N nuclear shieldings of o-hydroxyazo compounds are described. Both the direct and the equilibrium contributions were determined. Large direct deuterium isotope effects on ¹⁵N nuclear shieldings for ¹Δ, ²Δ and ⁴Δ and negative Δ¹J(¹⁵NH) [D] values were observed. Isotope effects on ¹⁵N nuclear shieldings, because of their magnitudes, are shown to be very useful in determining changes in the azo–hydrazone equilibrium. Isotope effects on ¹³C nuclear shieldings are smaller, but the effects observed at the carbon resonances of the N-phenyl ring are likewise very useful in determining the shift in the equilibrium. Deuterium substitution leads in all cases to a shift in the equilibrium so that the content of the predominant form of the protio compound is increased.     

A theoretical investigation of hydrogen bonding effects on oxygen and hydrogen chemical shielding tensors of aspirin

A computational investigation was carried out to characterize the ¹⁷O and ¹H chemical shielding (CS) tensors in crystalline aspirin. It was found that O–H⋯O and C–H⋯O hydrogen bonds around the aspirin molecule in the crystal lattice have a different influence on the calculated ¹⁷O and ¹H CS eigenvalues and their orientations in the molecular frame of axes. The calculations were performed with the BLYP, B3LYP, and M06 functionals employing 6-311++G(d,p) standard basis set. Calculated CS tensors were used to evaluate the ¹⁷O and ¹H chemical shift isotropy (δ_iso) and anisotropy (Δσ) in crystalline aspirin, which are in reasonable agreement with available experimental data. The difference between the calculated NMR parameters of the monomer and molecular clusters shows how much hydrogen-bonding interactions affect the CS tensors of each nucleus.     

An NMR and quantum mechanical investigation of solvent effects on conformational equilibria of butanedinitrile

Vicinal proton-proton NMR couplings and ab initio quantum mechanics have been used to investigate solvent effects on conformational equilibria of butanedinitrile. The trans and gauche conformations are about equally favored at room temperature in solvents of low dielectric constant while the equilibrium is essentially the statistical proportions of one-third trans and two-thirds gauche in water with a high dielectric constant. The coupling assignments were confirmed with the aid of stereospecific deuterium-labeled (R,R or S,S)-1,2-dideuteriobutanedinitrile. The calculations support the observed trends. Similar results were observed for 1,2-dibromo- and dichloroethanes.     

99Tc NMR as a promising technique for structural investigation of biomolecules: theoretical studies on the solvent and thermal effects of phenylbenzothiazole complex

The phenylbenzothiazole compounds show antitumor properties and are highly selective. In this paper, the ⁹⁹Tc chemical shifts based on the (^99mTc)(CO)₃(NNO) complex conjugated to the antitumor agent 2‐(4′‐aminophenyl)benzothiazole are reported. Thermal and solvent effects were studied computationally by quantum‐chemical methods, using the density functional theory (DFT) (DFT level BPW91/aug‐cc‐pVTZ for the Tc and BPW91/IGLO‐II for the other atoms) to compute the NMR parameters for the complex. We have calculated the ⁹⁹Tc NMR chemical shifts of the complex in gas phase and solution using different solvation models (polarizable continuum model and explicit solvation). To evaluate the thermal effect, molecular dynamics simulations were carried, using the atom‐centered density matrix propagation method at the DFT level (BP86/LanL2dz). The results highlight that the ⁹⁹Tc NMR spectroscopy can be a promising technique for structural investigation of biomolecules, at the molecular level, in different environments. Copyright © 2014 John Wiley & Sons, Ltd.     

Conformational analysis of cis-2-halocyclohexanols; solvent effects by NMR and theoretical calculations

Conformational problems often involve very small energy differences, even low as 0.5 kcal mol(-1). This accuracy can be achieved by theoretical methods in the gas phase with the appropriate accounting of electron correlation. The solution behavior, on the other hand, comprises a much greater challenge. In this study, we conduct and analysis for cis-2-fluoro-, cis-2-chloro-, and cis-2-bromocyclohexanol using low temperature NMR experiments and theoretical calculations (DFT, perturbation theory, and classical molecular dynamics simulations). In the experimental part, the conformers' populations were measured at 193 K in CD(2)Cl(2), acetone-d(6), and methanol-d(4) solutions; the preferred conformer has the hydroxyl group in the equatorial and the halogen in the axial position (ea), and its population stays at about 60-70%, no matter the solvent or the halogen. Theoretical calculations, on the other hand, put the ae conformer at a lower energy in the gas phase (MP2/6-311++G(3df,2p)). Moreover, the theoretical calculations predict a markedly increase in the conformational energy on going from fluorine to bromine, which is not observed experimentally. The solvation models IEF-PCM and C-PCM were tested with two different approaches for defining the atomic radii used to build the molecular cavity, from which it was found that only with explicit consideration of hydrogens can the conformational preference be properly described. Molecular dynamic simulations in combination with ab initio calculations showed that the ea conformer is slightly favored by hydrogen bonding.     

The 15N NMR study of silatranes

Various silatrane compounds were studied by means of ¹⁵N NMR spectroscopy. The quantum chemical calculations of some of the compounds were carried out using CNDO/2 method. The following correlations were obtained, i.e., ¹⁵N chemical shifts vary linearly with Taft's polar substituent constants (s) of the substituents R on the silicon atoms, and also with the net charge densities on the nitrogen atoms. From both experimental and theoretical aspects, it could be concluded that the SiN dative bonds in a series of silatrane compounds actually exist.     

Experimental and theoretical investigation on the relative stability of the PdS2- and pyrite-type structures of PdSe2

Under ambient condition PdSe2 has the PdS2-type structure. The crystal structure of PdSe2 under pressure (up to 30 GPa) was investigated at room temperature by X-ray diffraction in an energy-dispersive configuration using a diamond anvil cell with a mixture of water/ethanol/methanol as a pressure transmitting medium. A reversible structural transition from the PdS2-type to the pyrite-type structure occurs around 10 GPa, and the applied pressure reduces the spacing between adjacent 2/proportional to [PdSe2] layers of the PdS2-type structure to form the three-dimensional lattice of the pyrite-type structure. First principles and extended Hückel electronic band structure calculations were carried out to confirm the observed pressure-induced structural changes. We also examined why the isoelectronic analogues NiSe2 and PtSe2 adopt structures different from the PdS2-type structure on the basis of qualitative electronic structure considerations.     

A theoretical study of the relative stability of the isomeric forms of N2O3

The electronic structure, geometrical parameters and relative stability of the isomeric forms of N₂O₃ are analysed by means of ab initio calculations. Total energies of the different isomers are given. The energy difference between the most stable conformers of the symmetric N₂O₃ is 4.31 Kcal mol^–1 as provided by 6–31G basis set. The height of the rotational barrier determined by the ab initio technique is 7.12 kcal mol^–1.Member of the Carrera del Investigador CICPBA, R. Argentina.Member of the Carrera del Investigador CONICET, R. Argentina.Predoctoral fellow of CONICET, R. Argentina.     

Theoretical and experimental 13C and 15N NMR investigation of guanylhydrazones in solution

Experimental and theoretical ¹⁵N and ¹³C NMR data for the three nitrobenzaldehyde guanylhydrazones are reported. The theoretical data were obtained using sequential molecular dynamics/quantum mechanics methodology for the calculation of flexible molecules in a condensed phase, followed by the use of the GIAO/DFT method with the 6–311G** basis set. The experimental ¹⁵N chemical shifts for the guanylhydrazones are compared with the calculated shifts. Copyright © 2003 John Wiley & Sons, Ltd.     

15N Chemical shielding in glycyl tripeptides: measurement by solid-state NMR and correlation with X-ray structure

15N chemical shielding parameters are reported for central glycyl residues in crystallographically characterized tripeptides with alpha-helix, beta-strand, polyglycine II (3(1)-helix), and extended structures. Accurate values of the shielding components (2-5 ppm) are determined from MAS and stationary spectra of peptides containing [2-(13)C,(15)N]Gly. Two dipolar couplings, (1)H-(15)N and (13)C(alpha)-(15)N, are used to examine (15)N shielding tensor orientations in the molecular frame and the results indicate that the delta(11), delta(33) plane of the shielding tensor is not coincident with the peptide plane. The observed isotropic shifts, which vary over a range of 13 ppm, depend on hydrogen bonding (direct and indirect) and local conformation. Tensor spans, delta(span) = delta(11) - delta(33), and their deviations from axial symmetry, delta(dev) = delta(22) - delta(33), vary over a larger range and are grouped according to 2 degrees structure. Augmented by previously reported (13)C(alpha) shielding parameters, a prediction scheme for the 2 degrees structure of glycyl residues in proteins based on shielding parameters is proposed.