Determination of the orientations for the 17O NMR tensors in a polycrystalline l-alanine hydrochloride

We report a solid-state 17O NMR study of the 17O electric-field-gradient (EFG) and chemical shielding (CS) tensors for the carboxyl oxygen in an l-alanine hydrochloride. Using [17O]- and [13C,17O]-L-alanine hydrochlorides, both the magnitudes and the orientations in the molecular frame of the 17O EFG and CS tensors could be determined by the analysis of the 17O magic-angle spinning (MAS) and stationary NMR spectra. For the carbonyl oxygen, the smallest EFG tensor component, V(XX), and the largest EFG component, V(ZZ), roughly lies in the carboxyl molecular plane and the direction of V(XX) is parallel to the dipolar vector between 13C and 17O, that is, the direction of CO bond. The angles between the intermediate EFG component, VYY, and delta33 component, and between delta22 component and VZZ are found to be approximately 10 degrees and 35 degrees , respectively. We also present the results of the quantum chemical calculations for a theoretical hydrogen-bonding model, indicating that hydrogen-bonding strengths make it possible to vary both magnitudes and orientations of the carbonyl 17O EFG tensors in amino acid hydrochlorides.     

Determination of the 17O NMR tensors in potassium hydrogen dibenzoate: a salt containing a short O...H...O hydrogen bond

Solid-state 17O NMR spectra were obtained at 4.70, 11.75 and 19.60T for potassium hydrogen [17O(4)]dibenzoate (PHB) under both magic-angle spinning and stationary conditions. Spectral analyses yielded both the magnitude and orientation of the 17O chemical shift (CS) tensor and the electric field gradient (EFG) tensor for each of the two chemically distinct oxygen sites in PHB. For the oxygen site that is not involved in hydrogen bonding, the experimental 17O NMR tensors are: delta(iso)=287+/-2 ppm, delta(11)=470+/-5 ppm, delta(22)=380+/-5 ppm, delta(33)=10+/-5 ppm, C(Q)=8.30+/-0.02 MHz, eta(Q)=0.23+/-0.05, alpha=0+/-5 degrees, beta=90+/-5 degrees, and gamma=30+/-5 degrees. For the oxygen site in the short O...H...O hydrogen bond, the experimental 17O NMR tensors are: delta(iso)=213+/-2 ppm, delta(11)=370+/-5 ppm, delta(22)=190+/-5 ppm, delta(33)=80+/-5 ppm, C(Q)=5.90+/-0.02 MHz, eta(Q)=0.55+/-0.05, alpha=5+/-5 degrees, beta=90+/-5 degrees, and gamma=90+/-5 degrees. Extensive quantum mechanical calculations at both restricted Hartree-Fock and density functional theory levels were performed to investigate the effects of an effectively symmetrical O...H...O hydrogen bond on 17O CS and EFG tensors.     

Solid-state 17O NMR study of the electric-field-gradient and chemical shielding tensors in polycrystalline γ-glycine

We report the first experimental determination of the carboxylate oxygen electric-field-gradient (EFG) and chemical shielding (CS) tensors in polycrystalline γ-glycine. Analysis of magic-angle spinning (MAS) and stationary ¹⁷O NMR spectra of [¹⁷O]-γ-glycine obtained at 9.4, 14.1, 16.4, and 18.8 T yields the magnitudes of the ¹⁷O EFG and CS tensors and the relative orientations between the two tensors. Extensive quantum chemical calculations at both the restricted Hartree–Fock and density functional levels have been performed to present the absolute tensor orientations in term of the molecular frame. We have demonstrated that ¹⁷O NMR tensor information could be unambiguously derived by the multiple field analyses of stationary ¹⁷O NMR spectra.     

Solid-state 17O NMR study of the electric-field-gradient and chemical shielding tensors in polycrystalline gamma-glycine

We report the first experimental determination of the carboxylate oxygen electric-field-gradient (EFG) and chemical shielding (CS) tensors in polycrystalline γ-glycine. Analysis of magic-angle spinning (MAS) and stationary ¹⁷O NMR spectra of [¹⁷O]-γ-glycine obtained at 9.4, 14.1, 16.4, and 18.8 T yields the magnitudes of the ¹⁷O EFG and CS tensors and the relative orientations between the two tensors. Extensive quantum chemical calculations at both the restricted Hartree–Fock and density functional levels have been performed to present the absolute tensor orientations in term of the molecular frame. We have demonstrated that ¹⁷O NMR tensor information could be unambiguously derived by the multiple field analyses of stationary ¹⁷O NMR spectra.     

Determination of NMR interaction parameters from double rotation NMR

It is shown that the anisotropic NMR parameters for half-integer quadrupolar nuclei can be determined using double rotation (DOR) NMR at a single magnetic field with comparable accuracy to multi-field static and MAS experiments. The (17)O nuclei in isotopically enriched l-alanine and OPPh(3) are used as illustrations. The anisotropic NMR parameters are obtained from spectral simulation of the DOR spinning sideband intensities using a computer program written with the GAMMA spin-simulation libraries. Contributions due to the quadrupolar interaction, chemical shift anisotropy, dipolar coupling and J coupling are included in the simulations. In l-alanine the oxygen chemical shift span is 455 +/- 20 ppm and 350 +/- 20 ppm for the O1 and O2 sites, respectively, and the Euler angles are determined to an accuracy of +/- 5-10 degrees . For cases where effects due to heteronuclear J and dipolar coupling are observed, it is possible to determine the angle between the internuclear vector and the principal axis of the electric field gradient (EFG). Thus, the orientation of the major components of both the EFG and chemical shift tensors (i.e., V(33) and delta(33)) in the molecular frame may be obtained from the relative intensity of the split DOR peaks. For OPPh(3) the principal axis of the (17)O EFG is found to be close to the O-P bond, and the (17)O-(31)P one-bond J coupling ((1)J(OP)=161 +/- 2 Hz) is determined to a much higher accuracy than previously.     

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.     

Phosphorus-31 NMR studies of stabilized phosphorus ylids in the solid state.

Phosphorus-31 powder NMR spectra and high-resolution MAS spectra have been obtained for a number of stabilized phosphorus ylids under conditions of high-power proton decoupling and cross-polarization. The 31P CP/MAS spectra are compared to those obtained from isotropic solutions. The variation of chemical shift anisotropy and of the principal components of the 31P chemical shift tensor determined from 31P powder NMR line shapes are discussed in terms of the relative importance of accepted valence bond resonance structures. The results indicate that the invariance of the isotropic chemical shift, delta iso, observed in previous 31P NMR investigations of phosphorus ylids in solution is due to fortuitous cancellation of opposing changes in the principal components, delta 11 and delta 33, of the 31P chemical shift tensor. The 31P dipolar NMR powder spectrum of a typical stabilized ylid, (C6H5)3(31)P-13CHC(O)OCH2CH3, is analyzed in order to obtain the orientation of the 31P chemical shift tensor with respect to the 31P-13C alpha dipolar vector.     

Obtaining molecular and structural information from 13C-14N systems with 13C FIREMAT experiments

The effect of dipolar coupling to 14N on 13C FIREMAT (five pi replicated magic angle turning) experiments is investigated. A method is developed for fitting the 13C FIREMAT FID employing the full theory to extract the 13C-14N dipolar and 13C chemical shift tensor information. The analysis requires prior knowledge of the electric field gradient (EFG) tensor at the 14N nucleus. In order to validate the method the analysis is done for the amino acids alpha-glycine, gamma-glycine, l-alanine, l-asparagine, and l-histidine on FIREMAT FIDs recorded at 13C frequencies of 50 and 100 MHz. The dipolar and chemical shift data obtained with this analysis are in very good agreement with the previous single-crystal 13C NMR results and neutron diffraction data on alpha-glycine, l-alanine, and l-asparagine. The values for gamma-glycine and l-histidine obtained with this new method are reported for the first time. The uncertainties in the EFG tensor on the resultant 13C chemical shift and dipolar tensor values are assessed.     

Both experimental and theoretical investigations of solid-state 17O NMR for L-valine and L-isoleucine

We have presented an experimental investigation of the carboxyl oxygen NMR parameters for four distinct sites in l-valine and l-isoleucine. The carboxyl (17)O quadrupolar coupling constant, C(Q), and isotropic chemical shift, delta(iso), for these compounds are obtained by analyzing two-dimensional (17)O multiple-quantum magic-angle spinning (MQMAS) and/or 1D MAS spectra. The values of C(Q) and delta(iso) found to be in the range of 7.00-7.85 MHz, and 264-314 ppm, respectively. Extensive quantum chemical calculations at the density functional levels have been performed for a full cluster of l-valine molecules and a few theoretical models. The calculated results indicated that there was a correlation between the (17)O NMR parameters and C-O bond lengths, which was helpful for the spectral assignment. They also demonstrated that the torsion angle of l-valine plays an important role in determining the magnitudes of (17)O NMR parameters.     

15N chemical shift tensor magnitude and orientation in the molecular frame of uracil determined via MAS NMR

The potential of heteronuclear MAS NMR spectroscopy for the characterization of (15)N chemical shift (CS) tensors in multiply labeled systems has been illustrated, in one of the first studies of this type, by a measurement of the chemical shift tensor magnitude and orientation in the molecular frame for the two (15)N sites of uracil. Employing polycrystalline samples of (15)N(2) and 2-(13)C, (15)N(2)-labeled uracil, we have measured, via (15)N-(13)C REDOR and (15)N-(1)H dipolar-shift experiments, the polar and azimuthal angles (θ, psi) of orientation of the (15)N-(13)C and (15)N-(1)H dipolar vectors in the (15)N CS tensor frame. The (θ(NC), psi(NC)) angles are determined to be (92 +/- 10 degrees, 100 +/- 5 degrees ) and (132 +/- 3 degrees, 88 +/- 10 degrees ) for the N1 and N3 sites, respectively. Similarly, (θ(NH), psi(NH)) are found to be (15 +/- 5 degrees, -80 +/- 10 degrees ) and (15 +/- 5 degrees, 90 +/- 10 degrees ) for the N1 and N3 sites, respectively. These results obtained based only on MAS NMR measurements have been compared with the data reported in the literature.     

Solid-state 25Mg QCPMG NMR of bis(cyclopentadienyl)magnesium

Application of the "quadrupolar Carr-Purcell Meiboom-Gill" (QCPMG) sequence permits the first natural abundance solid-state 25Mg NMR study of an organometallic magnesium compound, bis(cyclopentadienyl)magnesium. Analytical and numerical simulations of both static and magic-angle spinning QCPMG NMR spectra beget an axially symmetric 25Mg electric field gradient (EFG) tensor (quadrupolar asymmetry parameter, eta(Q)=0.01(1)) with a nuclear quadrupole coupling constant of C(Q)=5.80(5)MHz. Restricted Hartree-Fock and hybrid density functional theory (B3LYP) calculations are in good agreement with experimental EFG values and predict a chemical shielding anisotropy of about 40-50 ppm, which we attempt to elucidate by numerical simulations. The parameters and orientation of the 25Mg EFG tensor are rationalized from examination of the crystal structure and molecular symmetry. The NMR properties of the cyclopentadienyl rings are examined by 13C[1H] CPMAS NMR, RHF and hybrid-DFT (B3LYP) calculations, and simulations of the effects of chemical exchange on the 13C powder pattern.     

A 13C solid-state NMR investigation of the alkynyl carbon chemical shift tensors for 2-butyne-1,4-diol

The alkynyl carbon chemical shift (CS) tensors for 2-butyne-1,4-diol are reported, based on analyses of the carbon-13 NMR spectra of stationary-powder and slow magic-angle spinning (MAS) samples for which the alkynyl carbon nuclei are enriched in 13C. NMR spectra of slow MAS samples exhibit spinning-frequency-dependent fine structure typical of crystallographically equivalent but magnetically distinct nuclei. Simulated spectra of slow MAS samples of this two-spin system are particularly sensitive to the relative orientations of the CS tensors. In addition, the value of 1J(13C, 13C), +175 +/- 10 Hz, is determined by examination of the total NMR lineshape of slow MAS samples. The CS tensors are almost axially symmetric, delta11 = 158.9 +/- 1.0 ppm and delta22 = 155.7 +/- 1.0 ppm; the direction of greatest shielding is approximately along the alkynyl C-C bond, delta33 = -57.8 +/- 2.0 ppm. Both the magnitudes of the principal components of the CS tensors and their orientations are in agreement with those predicted from first-principles calculations at the HF and MP2 levels of theory. This study demonstrates the importance of examining the NMR spectra of homonuclear two-spin systems with and without MAS under a variety of conditions (e.g., two or more applied magnetic fields and slow MAS).     

A Theoretical Study of <Superscript>51</Superscript>V Electric Field Gradient Tensors in Pyrovanadates and Metavanadates

A computational study of the ⁵¹V electric field gradient (EFG) tensors in pyrovanadates, α-Zn₂V₂O₇, Cd₂V₂O₇, β-Mg₂V₂O₇ and BaCaV₂O₇, and the metavanadates, LiVO₃, α-NaVO₃, KVO₃, ZnV₂O₆ and MgV₂O₆, is presented. Restricted Hartree–Fock and hybrid density functional theory calculations have been used to investigate the effects of the size of vanadium-oxygen clusters, basis set size, proton-termination and embedded cluster techniques on the accuracy of the calculated EFG tensors. Good agreement between theory and experiment is obtained for most of the vanadates. A sound methodology is suggested for calculating the EFG tensor in pyrovanadates which contain isolated V₂O₇ ⁴⁻ clusters. For metavanadates, the charges of the bridging oxygen atoms can be differentiated from those of terminal oxygen atoms by terminating the former with hydrogen atoms, and embedded cluster molecular orbital calculations are useful in accounting for the long-range electrostatic interactions which influence the EFG tensor components. EFG tensor orientations vary for different pyrovanadate structural types, and individual components are confined by symmetry elements in the metavanadates. A preliminary comparison is made between ⁵¹V EFG tensors calculated with ab initio and plane wave methods. Theoretical EFG tensor components and orientations, in combination with experimental ⁵¹V solid-state nuclear magnetic resonance data, are demonstrated to be useful tools for prediction of molecular structure. Authors' address: Robert W. Schurko, Department of Chemistry and Biochemistry, University of Windsor, Windsor, Ontario N9B 3P4, Canada     

The proton nuclear magnetic shielding tensors in biphenyl: experiment and theory

Line-narrowing multiple pulse techniques are applied to a spherical sample crystal of biphenyl. The 10 different proton shielding tensors in this compound are determined. The accuracy level for the tensor components is 0.3 ppm. The assignment of the measured tensors to the corresponding proton sites is given careful attention. Intermolecular shielding contributions are calculated by the induced magnetic point dipole model with empirical atom and bond susceptibilities (distant neighbours) and by a new quantum chemical method (near neighbours). Subtracting the intermolecular contributions from the (correctly assigned) measured shielding tensors leads to isolated-molecule shielding tensors for which there are symmetry relations. Compliance to these relations is the criterion for the correct assignment. The success of this program indicates that intermolecular proton shielding contributions can be calculated to better than 0.5 ppm. The isolated-molecule shielding tensors obtained from experiment and calculated intermolecular contributions are compared with isolated-molecule quantum chemical results. Expressed in the icosahedral tensor representation, the rms differences of the respective tensor components are below 0.5 ppm for all proton sites in biphenyl. In the isolated molecule, the least shielded direction of all protons is the perpendicular to the molecular plane. For the para proton, the intermediate principal direction is along the C-H bond. It is argued that these relations also hold for the protons in the isolated benzene molecule.     

Orientational information from TEDOR spectral sidebands.

In a dipolar-coupled spin-1/2 network of the type 15N1-(13)C-15N2, an assessment of the sensitivity of the N --> C and C --> N TEDOR sideband intensities to the Euler angles defining the orientation of the two heteronuclear dipolar vectors in the 13C and 15N chemical shift (CS) tensor principal axes system has been carried out via numerical calculations. The results clearly indicate the potential of TEDOR MAS NMR spectroscopy for the characterization of the CS tensor orientation in the molecular frame. The efficacy of the method has been experimentally illustrated by TEDOR studies on a polycrystalline sample of [1, 3-(15)N2, 2-(13)C]uracil, which is one of the four bases in RNA.     

EPR and HYSCORE investigation of the electronic structure of the model complex Mn(imidazole)6: exploring Mn(II)-imidazole binding using single crystals

The electronic structure of the Mn(II)-imidazole binding was studied by EPR spectroscopy using the model complex Mn(Im)(6) diluted in a single crystal of Zn(Im)(6)Cl(2).4(H(2)O). The second rank zero-field splitting (ZFS) tensor (D tensor) of the two sites, a and b, present in the crystal was determined by measuring the orientation patterns of the echo-detected EPR spectra in three different planes at 10K (D(a)=-106, D(b)=-118, E(a)=-17, E(b)=-22x10(-4)cm(-1). Euler angles with respect to the crystal habitus: alpha(a)=13 degrees , beta(a)=76 degrees , gamma(a)=108.5 degrees , alpha(b)=14 degrees , beta(b)=73.5 degrees , gamma(b)=103.5 degrees ). The contribution of cubic ZFS terms to the spectrum allowed us to determine the orientation of the N-Mn-N directions of the complex as well (Euler angles in the D tensor reference frame alpha=100 degrees , beta=23 degrees , gamma=0 degrees , both centers having the same orientation). The hyperfine interactions with (14)N were explored by HYSCORE spectroscopy. The correlation patterns and modulation amplitudes in the 2D experiments were studied for different electron spin transitions and orientations of the crystal. Signals of three different pairs of nitrogens were found. The results were analyzed considering that the N-Mn binding directions are principal directions of the hyperfine and nuclear quadrupole tensor of (14)N. All three pairs of nitrogens were found to be almost equivalent with an isotropic contribution of A(iso) approximately 3.2MHz and an almost axial anisotropic coupling of 2T approximately 1.1MHz along the N-Mn bonding direction. The nuclear quadrupole principal values are 1.5MHz along the bonding direction, -0.6MHz in the direction perpendicular to the imidazole plane, and -0.9MHz in the direction perpendicular to both.     

Electronic and magnetic properties of the paramagnetic Twenty electron Fe(O) sandwich [C6(CH3)6]2 Fe from Mössbauer measurements and molecular orbital calculations

A Mössbauer study of the title paramagnetic Fe(O) complex was carried out from 4.2 K to 250 K. The electric field gradient (EFG) tensor was found to be temperature independent in accordance with the well isolated nature of state³A_2g. Measurements in an applied magnetic field of 6 T confirmed the paramagnetic nature (S=1) of the complex and showed the EFG tensor to be negative. From the thermal variation of the hyperfine field we determined the spin hamiltonian constant D=14 K and find the g-value to be close to the free electron value. Semi-empirical molecular orbital (MO) calculations were carried out with a modelized molecule of D_6h symmetry; results are in good agreement with the experimental values (for both the electronic and magnetic properties).     

Diurnal variations in the photosynthesis-respiration activity of a cyanobacterial bloom in a freshwater dam reservoir: an isotopic study

The stable isotopic analyses of molecular oxygen dissolved in water (delta18O(DO)) and dissolved inorganic carbon (delta13C(DIC)), supplemented with basic chemical measurements, have been carried out on a diurnal basis to better understand the dynamics of photosynthesis and respiration in freshwater systems. Our observations have been carried out in a lowland dam reservoir, the Sulejow Lake (central Poland), during the summer cyanobacterial bloom. All data obtained, isotopic, hydrochemical, and biological, show a high mutual consistency. Namely, the lowest delta18O(DO) values, obtained at 10:00 and 14:00 (16.0 and 15.5 per thousand, respectively), correspond to the highest amount of cyanobacterial cells observed (66 and 63 mg dm(-3), respectively), whereas the minimum delta13C(DIC) (-10.6 per thousand) obtained at 22:00 corresponds to the maximum content of organic matter (110 mg dm(-3)). This evidence suggests that isotopic assays of delta18O(DO) and delta13C(DIC) are a reliable tool for the quantitative study of biochemical processes in freshwater systems.     

Solid-state NMR/NQR and first-principles study of two niobium halide cluster compounds

Two hexanuclear niobium halide cluster compounds with a [Nb₆X₁₂]²⁺ (X=Cl, Br) diamagnetic cluster core, have been studied by a combination of experimental solid-state NMR/NQR techniques and PAW/GIPAW calculations. For niobium sites the NMR parameters were determined by using variable B_o field static broadband NMR measurements and additional NQR measurements. It was found that they possess large positive chemical shifts, contrary to majority of niobium compounds studied so far by solid-state NMR, but in accordance with chemical shifts of ⁹⁵Mo nuclei in structurally related compounds containing [Mo₆Br₈]⁴⁺ cluster cores. Experimentally determined δ_iso(⁹³Nb) values are in the range from 2400 to 3000 ppm. A detailed analysis of geometrical relations between computed electric field gradient (EFG) and chemical shift (CS) tensors with respect to structural features of cluster units was carried out. These tensors on niobium sites are almost axially symmetric with parallel orientation of the largest EFG and the smallest CS principal axes (V_zz and δ₃₃) coinciding with the molecular four-fold axis of the [Nb₆X₁₂]²⁺ unit. Bridging halogen sites are characterized by large asymmetry of EFG and CS tensors, the largest EFG principal axis (V_zz) is perpendicular to the X-Nb bonds, while intermediate EFG principal axis (V_yy) and the largest CS principal axis (δ₁₁) are oriented in the radial direction with respect to the center of the cluster unit. For more symmetrical bromide compound the PAW predictions for EFG parameters are in better correspondence with the NMR/NQR measurements than in the less symmetrical chlorine compound. Theoretically predicted NMR parameters of bridging halogen sites were checked by ^79/81Br NQR and ³⁵Cl solid-state NMR measurements.     

Solid state 31P and 207Pb MAS NMR studies on polycrystalline O,O'-dialkyldithiophosphate lead(II) complexes

A number of lead(II) O,O'-dialkyldithiophosphate complexes were studied by (13)C, (31)P, and (207)Pb MAS NMR. Simulations of (31)P chemical shift anisotropy using spinning sideband analysis reveal a linear relationship between the SPS bond angle and the principal values delta(22) and delta(33) of the (31)P chemical shift tensor. The (31)P CSA data were used to assign ligands with different structural functions. In the cases of diethyldithiophosphate and di-iso-butyldithiophosphate lead(II) complexes, (2)J((31)P, (207)Pb)-couplings were resolved and used to confirm the suggested assignment of the ligands. The SIMPSON computer program was used to calculate (31)P and (207)Pb spectral sideband patterns.