17O NMR Chemical Shifts Research Articles

Magnetic resonance tensors in Uracil: Calculation of 13C, 15N, 17O NMR chemical shifts, 17O and 14N electric field gradients and measurement of 13C and 15N chemical shifts

The experimental 13C NMR chemical shift components of uracil in the solid state are reported for the first time (to our knowledge), as well as newer data for the 15N nuclei. These experimental values are supported by extensive calculated data of the 13C, 15N and 17O chemical shielding and 17O and 14N electric field gradient (EFG) tensors. In the crystal, uracil forms a number of strong and weak hydrogen bonds, and the effect of these on the 13C and 15N chemical shift tensors is studied. This powerful combination of the structural methods and theoretical calculations gives a very detailed view of the strong and weak hydrogen bond formation by this molecule. Good calculated results for the optimized cluster in most cases (except for the EFG values of the 14N3 and 17O4 nuclei) certify the accuracy of our optimized coordinates for the hydrogen nuclei. Our reported RMSD values for the calculated chemical shielding and EFG tensors are smaller than those reported previously. In the optimized cluster the 6-311+G** basis set is the optimal one in the chemical shielding and EFG calculations, except for the EFG calculations of the oxygen nuclei, in which the 6-31+G** basis set is the optimal one. The optimal method for the chemical shielding and EFG calculations of the oxygen and nitrogen nuclei is the PW91PW91 method, while for the chemical shielding calculations of the 13C nuclei the B3LYP method gives the best results.

Neptunium(VII) in high-ionic-strength alkaline solutions — [NpO2(OH)4]1– or [NpO4(OH)2]3–?

Relativistic density functional theory (ZORA-PBE, COSMO solvation) is used to address the title question, based on comparison with recent experimental data ( 1 ). Structural data (bond lengths), vibrational frequencies, and 17O NMR chemical shifts are used to prove that [NpO4(OH)2]3– is the predominant species in high-ionic-strength alkaline solutions of NpVII. Neptunium(VII) complexes have stronger bonds than their formally isoelectronic uranium(VI) analogues. The experimentally observed 300 ppm shift in 17O chemical shifts between the known [UO2(OH)4]2– and NpVII solution is shown to be partly a function of the central metal (NpVII vs. UVI) and not of the coordination environment (tetraoxo vs. dioxo). Comparing, for a given An (UVI or NpVII), actinyl complexes [AnO2X4]2–/1–, X = Cl, F, OH, a decreasing strength of the axial actinyl bond is observed that is traced to electronic factors (equatorial π-competition).

On resonance interactions in methyl 4-X-benzoates and the effect of 2,6-dimethyl substitution. 13C or 17O NMR chemical shifts as effective indicators of π-electron-density distribution?

On resonance interactions in methyl 4-X-benzoates and the effect of 2,6-dimethyl substitution. 13C or 17O NMR chemical shifts as effective indicators of π-electron-density distribution?

Oxoperoxo Vanadium(V) Complexes of l-Lactic Acid: Density Functional Theory Study of Structure and NMR Chemical Shifts

Various combinations of density functionals and pseudopotentials with associated valence basis-sets are compared for reproducing the known solid-state structure of [V 2O 2(OO) 2 l-lact 2] (2-) cis . Gas-phase optimizations at the B3LYP/SBKJC level have been found to provide a structure that is close to that seen in the solid state by X-ray diffraction. Although this may result in part from error compensation, this optimized structure allowed satisfactory reproduction of solution multinuclear NMR chemical shifts of the complex in all-electron DFT-IGLO calculations (UDFT-IGLO-PW91 level), suggesting that it is probably close to that found in solution. This combination of approaches has subsequently been used to optimize the structures of the vanadium oxoperoxo complexes [V 2O 3(OO) l-lact 2] (2-) cis , [V 2O 3(OO) l-lact 2] (2-) trans , and [VO(OO)( l-lact)(H 2O)] (-) cis . The (1)H, (13)C, (51)V, and (17)O NMR chemical shifts for these complexes have been calculated and compared with the experimental solution chemical shifts. Excellent agreement is seen with the (13)C chemical shifts, while somewhat inferior agreement is found for (1)H shifts. The (51)V and (17)O chemical shifts of the dioxo vanadium centers are well reproduced, with differences between theoretical and experimental shifts ranging from 22.9 to 35.6 ppm and from 25.1 to 43.7 ppm, respectively. Inferior agreement is found for oxoperoxo vanadium centers, with differences varying from 137.3 to 175.0 ppm for (51)V shifts and from 148.7 to 167.0 ppm for (17)O(oxo) shifts. The larger errors are likely to be due to overestimated peroxo O-O distances. The chosen methodology is able to predict and analyze a number of interesting structural features for vanadium(V) oxoperoxocomplexes of alpha-hydroxycarboxylic acids.

Matrix-Isolation Study and Ab Initio Calculations of the Structure and Spectra of Hydroxyacetone

The structure of hydroxyacetone (HA) isolated in an argon matrix (at 12 K) and in a neat solid phase (at 12-175 K) was characterized by using infrared (IR) spectroscopy. The interpretation of the experimental results was supported by high-level quantum chemical calculations, undertaken by using both ab initio (MP2) and density functional theory methods. A potential-energy surface scan, carried out at the MP2/6-311++G(d,p) level of theory, predicted four nonequivalent minima, Cc, Tt, Tg, and Ct, all of them doubly degenerate by symmetry. The energy barriers for conversion between most of the symmetrically related structures and also between some of the nonequivalent minima (e.g., Tg --> Tt and Ct --> Tt) are very small and stay below the zero-point vibrational level associated with the isomerization coordinate in the higher-energy form in each pair. Therefore, only Cc and Tt conformers have physical significance, with populations of 99 and 1%, respectively, in gas phase at room temperature. For the matrix-isolated compound, only the most stable Cc conformer was observed. On the other hand, the polarizable continuum model calculations indicated that in water solution, the population of Tt and Ct conformers might be high enough (ca. 6 and 11%, respectively) to enable their experimental detection, thus supporting the conclusions of a previous IR spectroscopy study [ Spectrochim. Acta A 2005, 61, 477] in which the presence of more than one HA conformer in aqueous solution was postulated. The signatures of these minor conformers, however, do not appear in the spectra of the neat HA crystal, and the crystal structure was rationalized in terms of centrosymmetric hydrogen-bonded dimers consisting of two Cc-like units. Finally, we calculated (1)H, (13)C, and (17)O NMR chemical shifts at different levels of theory and found them to agree with available experimental data.

93Nb and 17O NMR chemical shifts of niobiophosphate compounds

Niobiophosphate compounds with a large range of niobium and oxygen environments were studied with 93Nb and 17O solid-state NMR. 93Nb isotropic chemical shift of pure niobate Nb(ONb) 6, pure phosphate Nb(OP) 6 and mixed phosphate–niobate Nb(OP) x (ONb) (6− x ) (1⩽ x⩽5) sites were measured at a high magnetic field (18.8 T). 93Nb chemical shifts were found to be sensitive to the variation of local charge on Nb, but not to the local bond geometry (i.e. crystallographic site and edge or corner connectivity). A systematic shift to high field of the 93Nb chemical shift is measured when x increases. Then, 17O NMR spectra of a series of enriched samples provided the chemical shift and quadrupolar parameters for 4 types of oxygen environment (P–O–P, P–O–Na, P–O–Nb and Nb–O–Nb). Finally, Nb–O–Nb sites were characterized by a large 17O chemical shift anisotropy.

Influence of N–H…O and O–H…O hydrogen bonds on the 17O, 15N and 13C chemical shielding tensors in crystalline acetaminophen: A density functional theory study

A computational investigation was carried out to characterize the 17O, 15N and 13C chemical shielding tensors in crystalline acetaminophen. We found that N–H…O and O–H…O hydrogen bonds around the acetaminophen molecule in the crystal lattice have different influences on the calculated 17O, 15N and 13C chemical shielding eigenvalues and their orientations in the molecular frame of axes. The calculations were performed with the B3LYP method and 6–311++G(d, p) and 6–311+G(d) standard basis sets using the Gaussian 98 suite of programs. Calculated chemical shielding tensors were used to evaluate the 17O, 15N, and 13C NMR chemical shift tensors in crystalline acetaminophen, 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 chemical shielding tensors of each nucleus. The computed 17O chemical shielding tensor on O(1), which is involved in two intermolecular hydrogen bonds, shows remarkable sensitivity toward the choice of the cluster model, whereas the 17O chemical shielding tensor on O(2) involved in one N–H…O hydrogen bond, shows smaller improvement toward the hydrogen-bonding interactions. Also, a reasonably good agreement between the experimentally obtained solid-state 15N and 13C NMR chemical shifts and B3LYP/6–311++G(d, p) calculations is achievable only in molecular cluster model where a complete hydrogen-bonding network is considered. Moreover, at the B3LYP/6–311++G(d, p) level of theory, the calculated 17O, 15N and 13C chemical shielding tensor orientations are able to reproduce the experimental values to a reasonably good degree of accuracy.

Correlation between 13C and 17O chemical shifts and torsional strain in spiroacetals

The relationship between the 13C and 17O NMR chemical shifts and the dihedral energies (non-bonding interactions) of 1,4-dioxaspiro[4.4]nonane, 1,4-dioxa- and 6,10-dioxaspiro[4.5]decane, 1,4-dioxa- and 6,11-dioxaspiro[4.6]undecane, 1,5-dioxaspiro[5.5]undecane, 1,5-dioxa and 7,12-dioxaspiro[5.6]dodecane and 1,6-dioxaspiro[6.6]tridecane were analyzed. These data showed correlation of the non-bonding interactions with the chemical shift of the spiranic carbon, as well as a linear relationship between 13C and 17O.

Density‐Functional Calculation of the 183W and 17O NMR Chemical Shifts for Large Polyoxotungstates

AbstractA phosphane sulfate relativistic DFT method (ZORA)has been used to calculate the 183W and 17O NMR chemicalshifts for large polyoxotungstates, including W6O192–,CH3OTiW5O183–, W5O18WIINO3–, W10O324–, α‐δ‐γ‐XW12O40n–, β‐PW9O28Br63–, P2W18O626–, PW2O143–, and W7O246–, based on their optimized molecular structures. Despite sizeable deviations between the calculated and experimental values, the calculations correctly reproduce the trends in the change of the chemical shift for both nuclei. As expected, the diamagnetic term is almost constant throughout the whole series. The change in the chemical shifts is shown to be determined by the paramagnetic term, which depends on the electronic structure of the whole anion under study and, in particular, on the local geometry around a given tungsten atom. On the other hand, there is no correlation between the chemical shift and HOMO–LUMO gap, showing that deeper occupied levels and several unoccupied orbitals play an important role in the paramagnetic term. However, analysis of the components of the paramagnetic shielding has shown that the most significant transitions determining the change of both 183W and 17O NMR chemical shifts for anions consisting of tungsten and oxygen atoms are the occupied–occupied and not the occupied–virtual ones.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)

DFT Calculations of the 183W NMR Chemical Shifts in Reduced Polyoxotungstates

AbstractA relativistic DFT method (ZORA) is used to calculate the 183W and 17O NMR chemical shifts for the reduced polyoxotungstates W5O18WIINO3–, γ‐SiW12O406–, P2W18O628–, and W10O326– with different degrees of (de)localization of the electrons introduced. Despite some discrepancies in the calculated and experimental data, the calculations adequately describe the changes in the chemical shifts upon reduction. The reduction of a fully oxidized anion is accompanied by an expansion of the geometry. The effect of the charge localization and change in geometry on the chemical shift is evaluated. The shift of the resonance of the tungsten atoms largely depends on the reduced charge acquired upon reduction. On the other hand, those tungsten atoms that do not increase the electron density upon reduction have a negative shift. The calculations also reproduce the negative shifts of all the 17O NMR resonances upon reduction of the polyoxotungstates. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)

17O NMR study of oxo metalloporphyrin complexes: Correlation with electronic structure of M [dbnd]O moiety

17O NMR spectroscopy of oxo ligand of oxo metalloporphyrin can be considered as an excellent means to derive information about structure, electronic state, and reactivity of the metal bound oxo ligand. To show the utility of 17O NMR spectroscopy of oxo ligand of oxo metalloporphyrin, 17O NMR spectra of oxo ligands of dioxo ruthenium(VI), oxo chromium(IV), and oxo titanium(IV) porphyrins are measured. For all oxo metalloporphyrins, well-resolved 17O NMR signals are detected in far high frequency region. The 17O NMR signal of the metal bound oxo ligand shifts high frequency in order of Ru(VI) < Ti(IV) < Cr(IV), thus the 17O NMR chemical shift does not directly correlate with the oxo-transfer reactivity, Ti(IV) < Cr(IV) < Ru(VI). On the other hand, the 17O NMR shift of oxo ligand correlates with the bond strength of metal–oxo bond. This suggests that the 17O NMR signal of metal bound oxo ligand is a sensitive probe to study the nature of metal–oxo bond in oxo metalloporphyrin. The effect of the electron-withdrawing meso-substituent on the 17O NMR shift of the oxo ligand is also investigated. With increase in the electron-withdrawing effect of the meso-substituent, the 17O NMR signal of the oxo ligand of oxo chromium(IV) porphyrin shifts high frequency while that of dioxo ruthenium(VI) porphyrin hardly change resonance position. The changes in metal–oxo bonds induced by the electron-withdrawing meso-substituent are discussed on the basis of the 17O NMR shifts, the strengths of the metal–oxo bonds, and the oxo-transfer reaction rates.

Effects of ions in natural water on the 17O NMR chemical shift of water and their relationship to water cluster

Effects of temperature and some aqueous solutions of electrolytes consisting of ions in natural water on the chemical shifts of NMR of water ( 17O NMR) have been measured. Combining the cluster model of liquid water structure and the dependence of δ( 17OH 2) on temperature a relationship deduced of δ( 17OH 2) to median water cluster size effects of ions on median water cluster and the size are analyzed. The relationships between 17O NMR and concentration of ions are linear. Cations increase the median water cluster size and anions decrease it. The bigger the radius of ions with equal ionic charge, the stronger the effect of the ion. The extent of the ionic charge effect is much bigger than that of the ionic radius. The order of the effect of cations and anions is Na +<K +<Mg 2+<Ca 2+<Al 3+<Fe 3+, and OH −<HCO 3 −<Cl −, and CO 3 2−<SO 4 2−, respectively.

Synthesis, 17O NMR spectroscopy and structure of 2-trifluoroacetyl-1-methoxycycloalkenes

Among the synthesis of a series of five well-known 2-trifluoroacetyl-1-methoxycycloalkenes derived from cyclopentanone and substituted cyclohexanones, this paper describes the synthesis of three new 2-trifluoroacetyl-1-methoxycycloalkenes derived from cycloheptanone, cyclooctanone and cyclododecanone in 60–68% yield. Subsequently, the 17O NMR chemical shift analysis of the carbonyl and the methoxy groups for these cyclic molecules clearly showed the electron push–pull phenomenon and revealed large and irregular variations of 17O NMR chemical shifts with the ring size. Finally, a more stable conformation of these trifluoroacetyl-containing cycloalkenes was determined by energy minimization calculations using Austin Model 1 (AM1) semi-empirical method and correlations between 17O NMR data and torsion angles or oxygen net charge calculated by AM1 semi-empirical method were performed.

First-Principles Calculation of 17O and 25Mg NMR Shieldings in MgO at Finite Temperature: Rovibrational Effect in Solids

The temperature dependence of (17)O and (25)Mg NMR chemical shifts in solid MgO have been calculated using a first-principles approach. Density functional theory, pseudopotentials, a plane-wave basis set, and periodic boundary conditions were used both to describe the motion of the nuclei and to compute the NMR chemical shifts. The chemical shifts were obtained using the gauge-including projector augmented wave method. In a crystalline solid, the temperature dependence is due to both (i) the variation of the averaged equilibrium structure and (ii) the fluctuation of the atoms around this structure. In MgO, the equilibrium structure at each temperature is uniquely defined by the cubic lattice parameters, which we take from experiment. We evaluate the effect of the fluctuations within a quasiharmonic approximation. In particular, the dynamical matrix, defining the harmonic Hamiltonian, has been computed for each equilibrium volume. This harmonic Hamiltonian was used to generate nuclear configurations that obey quantum statistical mechanics. The chemical shifts were averaged over these nuclear configurations. The results reproduce the previously published experimental NMR data measured on MgO between room temperature and 1000 degrees C. It is shown that the chemical shift behavior with temperature cannot be explained by thermal expansion alone. Vibrational corrections due to the fluctuations of atoms around their equilibrium position are crucial to reproduce the experimental results.

Synthesis and Characterization of the Novel Monoamide Derivatives of Gd−TTDA

For this study, the N'-monoamide derivatives of TTDA (3,6,10-tri(carboxymethyl)-3,6,10-triazadodecanedioic acid), N'-methylamide (TTDA-MA), N'-benzylamide (TTDA-BA), and N'-2-methoxybenzylamide (TTDA-MOBA), were synthesized. Their protonation constants and stability constants (log K(ML)'s) formed with Ca(2+), Zn(2+), Cu(2+), and Gd(3+) were determined by potentiometric titration in 0.10 M Me(4)NCl at 25.0 +/- 0.1 degrees C. The relaxivity values of [Gd(TTDA-MA)](-), [Gd(TTDA-BA)](-), and [Gd(TTDA-MOBA)](-) remained constant with respect to pH changes over the range 4.5-12.0. The (17)O NMR chemical shift of H(2)O induced by [Dy(TTDA-MA)(H(2)O)](-) at pH 6.80 showed 0.9 inner-sphere water molecules. Water proton relaxivity values for [Gd(TTDA-MA)(H(2)O)](-), [Gd(TTDA-BA)(H(2)O)](-), and [Gd(TTDA-MOBA)(H(2)O)](-) at 37.0 +/- 0.1 degrees C and 20 MHz are 3.89, 4.21, and 4.25, respectively. The water-exchange lifetime (tau(M)) and rotational correlation time (tau(R)) of [Gd(TTDA-MA)(H(2)O)](-), [Gd(TTDA-BA)(H(2)O)](-), and [Gd(TTDA-MOBA)(H(2)O)](-) are obtained from reduced the (17)O relaxation rate and chemical shifts of H(2)(17)O. The (2)H NMR longitudinal relaxation rates of the deuterated diamagnetic lanthanum complexes for the rotational correlation time were also thoroughly investigated. The water-exchange rates (K(298)(ex) for [Gd(TTDA-MA)(H(2)O)](-), [Gd(TTDA-BA)(H(2)O)](-), and [Gd(TTDA-MOBA)(H(2)O)](-) are lower than that of [Gd(TTDA)(H(2)O)](2)(-) but significantly higher than those of [Gd(DTPA)(H(2)O)](2)(-) and [Gd(DTPA-BMA)(H(2)O)]. The rotational correlation times for [Gd(TTDA-BA)(H(2)O)](-) and [Gd(TTDA-MOBA)(H(2)O)](-) are significantly longer than those of [Gd(TTDA)(H(2)O)](2)(-) and [Gd(DTPA)(H(2)O)](2)(-) complexes. The marked increase of the relaxivity of [Gd(TTDA-BA)(H(2)O)](-) and [Gd(TTDA-MOBA)(H(2)O)](-) results mainly from their longer rotational correlation time. The noncovalent interaction between human serum albumin (HSA) and [Gd(TTDA-BA)(H(2)O)](-) and [Gd(TTDA-MOBA)(H(2)O)](-) complexes containing a hydrophobic substituent was investigated by measuring the water proton relaxation rate of the aqueous solutions. The binding association constant (K(A)) values are 1.0 +/- 0.2 x 10(3) and 1.3 +/- 0.2 x 10(3) M(-1) for [Gd(TTDA-BA)(H(2)O)](-) and [Gd(TTDA-MOBA)(H(2)O)](-), which indicates a stronger interaction of [Gd(TTDA-BA)(H(2)O)](-) and [Gd(TTDA-MOBA)(H(2)O)](-) with HSA.

Synthesis and spectroscopic properties of Schiff bases derived from 3-hydroxy-4-pyridinecarboxaldehyde

A series of six new Schiff bases has been prepared by reacting aniline and 4-R-substituted anilines (R=CH 3, OCH 3, Br, Cl, NO 2) with 3-hydroxy-4-pyridinecarboxaldehyde. The 1H, 13C, 15N and 17O NMR data of these compounds are used to discuss the tautomerism. 15N NMR and 17O NMR chemical shifts established the tautomer existing in solution as the hydroxy/imino. 13C CPMAS NMR confirms that the same tautomer is found in the solid state. The stabilities of the tautomeric forms have been approached using density functional calculations (B3LYP/6-31G**) in the gas phase. In all cases the neutral hydroxy/imino with E configuration is more stable than the oxo/enamino form (by ∼22 kJ mol −1) and significantly more stable than the betaine (by ∼75 kJ mol −1).

17O NMR investigation of rigid polycyclic systems: experimental and calculated chemical shifts

In this work we describe a systematic and comparative study of the 17O NMR and related parameters of 17 polycyclic compounds: bicyclic, tricyclic, tetracyclic ( endo–endo and endo–exo fusion), pentacyclic compounds (ketone; endo and exo-alcohols), hexacyclic alcohol. Concentration, temperature and solvent effects on the 17O NMR chemical shifts and the relaxation rate are also described. We compare all experimental 17O NMR chemical shifts with chemical shifts calculated from gauge-independent atomic orbitals within density functional theory (GIAO-DFT). Correlations between calculated and experimental 17O NMR chemical shifts for respective functional groups were carried out and linear equations for chemical shift predictions were generated.

17O and 33S NMR studies of sulfur dioxide and sulfur trioxide

17O and 33S NMR chemical shifts of sulfur dioxide (SO 2) were measured for the range of density (0.05–0.3 mol l −1) in the gas phase at 300 and 333 K. The chemical shifts for both nuclei are linearly dependent on density showing an increase of 17O magnetic shielding and a decrease of 33S shielding due to intermolecular interactions. The present measurements have permitted the quantitative separation of the shielding contributions due to intermolecular interactions ( σ 1) and the shielding parameters of isolated molecules ( σ 0). Analogous 17O NMR measurements were also carried out for gaseous sulfur trioxide (SO 3) at 333 K. A decrease of oxygen shielding was observed here for larger densities, but the latter compound could be examined only in the very small range of density (∼0.02–0.08 mol l −1) and the shielding parameters were determined with lower accuracy. It was confirmed by the 17O and 33S NMR spectra of liquid SO 2 and SO 3 that intermolecular interactions in gases and liquids change the shielding constants in the same directions.

17O and 33S NMR studies of sulfur dioxide and sulfur trioxide

17O and 33S NMR chemical shifts of sulfur dioxide (SO2) were measured for the range of density (0.05–0.3 mol l−1) in the gas phase at 300 and 333 K. The chemical shifts for both nuclei are linearly dependent on density showing an increase of 17O magnetic shielding and a decrease of 33S shielding due to intermolecular interactions. The present measurements have permitted the quantitative separation of the shielding contributions due to intermolecular interactions (σ1) and the shielding parameters of isolated molecules (σ0). Analogous 17O NMR measurements were also carried out for gaseous sulfur trioxide (SO3) at 333 K. A decrease of oxygen shielding was observed here for larger densities, but the latter compound could be examined only in the very small range of density (∼0.02–0.08 mol l−1) and the shielding parameters were determined with lower accuracy. It was confirmed by the 17O and 33S NMR spectra of liquid SO2 and SO3 that intermolecular interactions in gases and liquids change the shielding constants in the same directions.

Theoretical Investigation of Superelectrophilic Fluorooxonium Dications FOH32+ and F2OH22+: Comparison with Parent H4O2+ Dication

Structures of the superelectrophilic fluorooxonium dications FOH32+ and F2OH22+ were calculated at the QCISD(T)/6-311G** level. Proton affinities of their precursor monocationic species were estimated by using G2 theory. For comparison, the parent oxonium dication H4O2+ was also calculated at the same level. The O,O- and O,F-diprotonated HFO structures 2c and 2d, respectively, were found to be energy minima. However, the oxonium dication 2c is less stable than the oxonium−fluoronium dication 2d by 14.9 kcal/mol. On the other hand, O,O-diprotonated F2O 3c was found to be the only minimum for the analogous system. All of these superelectrophilic, dicationic species have substantial kinetic barriers for deprotonations. The possible existence of these dicationic species in either superacidic media or the gas phase is implicated from these studies. The 17O and 19F NMR chemical shifts of the mono- and dications were also computed by the GIAO-MP2 method.