Calculated NMR Parameters Research Articles

ChemInform Abstract: Solid‐State NMR in the Analysis of Drugs and Naturally Occurring Materials

AbstractReview: 99 refs.

ChemInform Abstract: Development of 43Ca Solid State NMR Spectroscopy as a Probe of Local Structure in Inorganic and Molecular Materials

AbstractReview: 156 refs.

NMR Chemical Shifts of11B in Metal Borohydrides from First-Principle Calculations

Lightweight complex metal hydrides pose a challenge for experimental insight because of the lack of long-range order in the nanoconfined state or in intermediate phases. Understanding of the atomic scale properties might be the key to the utilization of complex hydrides in applications for versatile hydrogen storage. The first-principle calculations support of nuclear magnetic resonance methods applied to metal borohydrides is presented. We show that boron NMR chemical shifts can be accurately calculated within a density functional theory approach for a broad class of crystalline metal borohydrides. The calculated NMR parameters together with electronic and structural data provide detailed insight, on the atomic scale, into the properties of bulk and in particular of nanosized structures. It is explained that for small nanoclusters of LiBH4, the boron chemical shift is low-frequency shifted because of lower coordination of ions compared to that of the bulk. The relation between Pauling electronegativity a...

1,3,2-Dithiaphospholanes with an annelated 1,2-dicarba-closo-dodecaborane(12) unit: formation and reactivity

1,3,2-Dithiaphospholanes were prepared, containing an annelated 1,2-dicarba-closo-dodecaborane(12) unit, bearing halogens (F, Cl, Br, and I), H, NEt2, OEt groups, or organo substituents (R = i-Pr, t-Bu, Cy, 3,5-Me2-C6H3-CH2, Ph) at phosphorus. The t-Bu derivative was structurally characterised as its sulfide. The organo-substituted derivatives escape the ring strain by irreversible (R = t-Bu; X-ray analysis) or reversible dimerisation, by which 10-membered rings are formed. Using 1,1-bis(dichlorophosphino)methane, another dimer containing a 14-membered ring could be isolated and structurally characterised. The preferred conformation of the 1,3,2-dithiaphospholanes depends on the nature of the substituent at phosphorus. For instance, the substituents R = t-Bu, NEt2 prefer the equatorial position, as indicated by diagnostic NMR data, supported by DFT calculations [B3LYP/6-311+G(d,p) level of theory]. The calculations also predict the tendency for dimerisation, and calculated NMR parameters are in good agreement, in most cases, with experimental data.

First-principles electronic structure study of rhizoferrin and its Fe(III) complexes

We have determined the structure and coordination chemistry of rhizoferrin (Rf), which is a particular type of siderophore, and its Fe(III) complexes using density functional theory calculations. Our results show that the Fe(III) ion binds in an octahedral coordination, with a low-spin (S = 1/2) charge-neutral chiral complex having the largest binding energy of the investigated complexes. We have also calculated nuclear magnetic resonance parameters, such as chemical shifts for (1)H and (13)C, and indirect nuclear spin-spin couplings for (1)H-(1)H and (13)C-(1)H in free Rf and in a low-spin neutral Rf metal complex, as well as nuclear quadrupole interaction parameters, such as asymmetry parameter and nuclear quadrupole coupling constants for (14)N. Our calculated values for the chemical shifts for free Rf are in excellent agreement with experimental data while the calculated NMR parameters for Fe(III) complexes are predictions for future experimental work.

ChemInform Abstract: First‐Principles Calculation of NMR Parameters Using the Gauge Including Projector Augmented Wave Method: A Chemist′s Point of View

AbstractReview: 439 refs.

Application of NMR crystallography to the determination of the mechanism of charge-balancing in organocation-templated AlPO STA-2

The combination of solid-state MAS NMR spectroscopy and first-principles calculations is used to elucidate the structure of an as-prepared microporous AlPO (STA-2), in which the organocation template (bis-diazabicyclooctane-butane) is charge balanced by hydroxyl groups coordinated to framework aluminium species. 27Al MAS NMR spectra show Al exists in both tetrahedral and five-fold coordination, with the latter directly coordinated to hydroxyl O atoms as well as framework O atoms. The relative intensities of these resonances can be used to determine that the hydroxyls bridge between the two types of Al. Calculation of NMR parameters using a periodic density functional theory approach are able to suggest assignments for the resonances in both 27Al and 31P NMR spectra. 31P chemical shifts are shown to depend not only on the topologically-distinct sites in the SAT framework but also on whether or not the P atoms form part of a six-membered P(OAl)2OH ring, where OH is a bridging hydroxyl. By diffraction six possible sites for the charge-balancing hydroxyls have been identified, but all are refined with an average occupancy of 0.33. However, predicted peak positions in two-dimensional J-HETCOR NMR spectra indicate that only one hydroxyl is found in each cancrinite cage, and suggest that the most likely arrangements are those that avoid the close approach of bridging hydroxyl groups in adjacent cages. We show that the use of a combination of NMR spectroscopy and calculation to elucidate structure, often referred to as “NMR Crystallography”, offers a promising route for the future study of as-made and substituted microporous materials.

Structural assessment of anhydrous sulfates with high field 33S solid state NMR and first principles calculations

A combination of solid state NMR, first principles calculations and single crystal XRD was applied to relate solid state 33S NMR parameters obtained from a series of anhydrous sulfates of the elements of groups I–III. Operating at high magnetic field of 21.14 T provides a dramatic improvement in the quality of spectra due to significantly enhanced sensitivity and a reduction of second order quadrupolar effects. Experimental 33S NMR spectra for most studied sulfates are dominated by quadrupolar interactions with the quadrupolar parameters unique for each compound. Magnetic shielding constants and quadrupolar parameters for sulfur were calculated using plane wave pseudo-potential density functional theory as implemented in the CASTEP computational package. The calculated NMR parameters are in very good agreement with the experimental results and help in assignment of the stationary spectra. The results demonstrate that such a combined computational – experimental solid state NMR approach can aid in an assessment and improved interpretation of the crystallographic data.

Theoretical Studies of Solvent Effects on Binding of Sn (CH3)2(N-acetyl-L-cysteinate) with Single-walled Carbon Nanotube

The potential of nanotechnology to revolutionise medicine in particular seems endless, and one significant application of this technology is the use of carbon nanotubes for the targeted delivery of drug molecules. Sn(CH3)2(N-acetyl-L-cysteinate) is a drug that has been proposed as a therapeutic against the human hepatocarcinoma HCC Hep G2 cells. In our model, (N-acetyl-Lcysteinato-O,S) dimethyl tin(IV) (Sn(CH3)2(N-acetyl-L-cysteinate)) was binded to carbon nanotube. Quantum chemical ab initio calculations at HF/ (LanL2DZ+STO-3G), and HF/ (LanL2DZ+6-31G) levels in gas phase and solution have been carried out to calculate the optimized geometry, energies, dipole moments and thermochemical analysis of Sn(CH3)2(N-acetyl-L-cysteinate)-CNT complex. Results from frequency calculation (large negative values of the ΔG and high positive values of ΔS) confirmed the structural stability of the Sn(CH3)2(NAC) – CNT in both gas phase and in solution. NMR parameters such as isotropic magnetic shielding constants (σiso), anisotropic magnetic shielding tensors (σaniso), Chemical shifts (δ) and total atomic charges were also calculated using the GIAO and CSGT approaches in both gas phase and in solution to determine the structural characterization of the complex. The results in solution show that the δ, σiso, and σaniso are sensitive to hydrogen-bonding interactions. A different influence of various hydrogen bond types, N5-H·····O, C-O1·····H, and C=O24·····H was observed on the calculated NMR parameters. Finally the NMR parameters obtained from GIAO method are in good agreement with the CSGT results.

Water in the Earth's mantle: a solid-state NMR study of hydrous wadsleyite

Wadsleyite, β-(Mg,Fe)2SiO4, is the main component of the transition zone in the Earth's mantle, at depths of 410–530 km below the surface. This mineral has received considerable interest as a potential reservoir for the vast amount of hydrogen, as hydroxyl, referred to as water, that is thought to be contained within the mantle. However, the exact way in which water is incorporated into the structure of wadsleyite is not fully understood and has been the subject of considerable debate. In this work, 17O, 25Mg, 29Si, 1H and 2H solid-state NMR spectra were obtained from isotopically enriched samples of anhydrous and hydrous β-Mg2SiO4. First-principles DFT calculations were also carried out for a range of model structures to aid interpretation of the experimental data. The results are consistent with a model for hydrous wadsleyite whereby hydrogen bonds to the O1 site to form hydroxyl groups that are charge balanced by cation vacancies on the Mg3 site. Structural models containing cation vacancies on the Mg2 site are found to be energetically less favourable and calculated NMR parameters show poor agreement with the experimental data. Disorder was also observed in the hydrous wadsleyite samples, and 1H and 2H NMR are consistent with not only Mg–O1–H but also more strongly hydrogen-bonded Si–O–H environments. These silanol protons can be incorporated into the structure with only a small increase in energy. Two-dimensional 1H–29Si and 1H–17O NMR correlation experiments confirm that the additional resonances do not correspond to Mg–OH protons and enable the identification of 29Si and 17O species within the Si–OH groups. This assignment is also confirmed by first-principles DFT calculations of NMR parameters. Silanol protons within Mg3 vacancies could account for up to 20% of the protons in the structure.

Differentiation between 6- and 7-Membered Rings Based on Theoretical Calculation of NMR Parameters

The determination of ring size can vary from simple to complex, but the process in difficult cases can be advantageously augmented by DFT calculation of NMR parameters such as the chemical shifts of (), (), and other nuclei as well as pertinent spin-spin (scalar) coupling constants, for example, those between protons (). Differentiation between 6- and 7-membered ring formation in the case of 3,4-dihydro-2H-3-hydroxymethyl-1,4-benzoxazine and 2,3,4,5-tetrahydro-1,5-benzoxazepine-3-ol was evaluated with a view to not only affecting 6- versus 7-membered ring differentiation generally for cases on hand, but also in the case of literature reports where the assigned structures may be in doubt. Thus, the main focus was on the usually reported NMR parameters of , , and and wherein the analysis was found to be highly successful, particularly for , and thus potentially amenable for broad application.

ChemInform Abstract: Theoretical Modeling of 13C NMR Chemical Shifts — How to Use the Calculation Results

AbstractReview: 74 refs.

ChemInform Abstract: Applications of High‐Resolution 1H Solid‐State NMR

AbstractReview: 310 refs.

SMARTER crystallography of the fluorinated inorganic–organic compound Zn3Al2F12·[HAmTAZ

We present in this paper the structure resolution of a fluorinated inorganic-organic compound--Zn(3)Al(2)F(12)·[HAmTAZ](6)--by SMARTER crystallography, i.e. by combining powder X-ray diffraction crystallography, NMR crystallography and chemical modelling of crystal (structure optimization and NMR parameter calculations). Such an approach is of particular interest for this class of fluorinated inorganic-organic compound materials since all the atoms have NMR accessible isotopes ((1)H, (13)C, (15)N, (19)F, (27)Al, (67)Zn). In Zn(3)Al(2)F(12)·[HAmTAZ](6), (27)Al and high-field (19)F and (67)Zn NMR give access to the inorganic framework while (1)H, (13)C and (15)N NMR yield insights into the organic linkers. From these NMR experiments, parts of the integrant unit are determined and used as input data for the search of a structural model from the powder diffraction data. The optimization of the atomic positions and the calculations of NMR parameters ((27)Al and (67)Zn quadrupolar parameters and (19)F, (1)H, (13)C and (15)N isotropic chemical shifts) are then performed using a density functional theory (DFT) based code. The good agreement between experimental and DFT-calculated NMR parameters validates the proposed optimized structure. The example of Zn(3)Al(2)F(12)·[HAmTAZ](6) shows that structural models can be obtained in fluorinated hybrids by SMARTER crystallography on a polycrystalline powder with an accuracy similar to those obtained from single-crystal X-ray diffraction data.

Theoretical study of fMet-tRNA and fAla-tRNA structures by using quantum calculation

In the prokaryotes, protein synthesis always starts with N-formylmethionine amino acid. Comparison of this amino acid with other amino acids is attempted and that is why formylmethionine is always the first amino acid to begin protein synthesis, in this paper we added a formyl group to alanine amino acid and then studied it when attached to the tRNA molecule and compared this structure with formylmethionine-tRNA structure. The quantum chemical calculations have performed using Gaussian 03 suite of programs. The fAla-tRNA and fMet-tRNA structures have fully optimized at the HF and B3LYP levels with 3–21G∗ and 6–31G∗ basis sets as well as MP2/3–21G∗ level and theoretically solvent effects on the structures were investigated. Then we studied electronic structures of the compounds using Natural Bond Orbital (NBO) analysis and calculated NMR parameters at the gas-phase. Frequency analysis was also calculated at the HF and B3LYP/3-21G∗ levels in the different solvents in 298.15K, 310.15K temperatures and 1.00 atmosphere pressure.

Theoretical modeling of 13C NMR chemical shifts—How to use the calculation results

AbstractOwing to wide accessibility of the general utility quantum chemistry programs, theoretical calculations of spectral parameters defining NMR spectra have become relatively easy for medium size molecules. This new and powerful tool allows investigators to perform much deeper interpretation of the NMR data and gain new information valuable for structural chemistry studies. It is usually realized and well understood that theoretical calculations of NMR parameters, as for any quantum mechanical calculations, have several limitations, difficult to overcome, and that the accuracy of calculation results is strongly dependent on the theoretical method applied. On the other hand, it is less commonly recognized that the effectiveness of all the combined calculational/spectroscopic approach depends meaningfully on the method of comparison of the experimental and theoretical data as well. This article is addressed primarily to experimental chemists who use in their everyday work 13C NMR spectroscopy to solve various structural and physicochemical problems. After recalling some basic concepts concerning NMR parameters and their calculations, some suggestions concerning the rational choice of the calculational method and the experiment/theory comparison method are formulated. To support these recommendations and convince the reader of their utility, several practical examples are presented in the text. All these concepts reflect the author's beliefs which, albeit well‐grounded, by no means should be treated as indispensable truths; they are rather worth considering as hints about how to solve various practical dilemmas. © 2011 Wiley Periodicals, Inc. Concepts Magn Reson Part A 38: 289–307, 2011.

Structure and NMR Spectra of Some [2.2]Paracyclophanes. The Dilemma of [2.2]Paracyclophane Symmetry

Density functional theory (DFT) quantum chemical calculations of the structure and NMR parameters for highly strained hydrocarbon [2.2]paracyclophane 1 and its three derivatives are presented. The calculated NMR parameters are compared with the experimental ones. By least-squares fitting of the (1)H spectra, almost all J(HH) coupling constants could be obtained with high accuracy. Theoretical vicinal J(HH) couplings in the aliphatic bridges, calculated using different basis sets (6-311G(d,p), and Huz-IV) reproduce the experimental values with essentially the same root-mean-square (rms) error of about 1.3 Hz, regardless of the basis set used. These discrepancies could be in part due to a considerable impact of rovibrational effects on the observed J(HH) couplings, since the latter show a measurable dependence on temperature. Because of the lasting literature controversies concerning the symmetry of parent compound 1, D(2h) versus D(2), a critical analysis of the relevant literature data is carried out. The symmetry issue is prone to confusion because, according to some literature claims, the two hypothetical enantiomeric D(2) structures of 1 could be separated by a very low energy barrier that would explain the occurrence of rovibrational effects on the observed vicinal J(HH) couplings. However, the D(2h) symmetry of 1 with a flat energy minimum could also account for these effects.

Characterization of Noninnocent Metal Complexes Using Solid-State NMR Spectroscopy: o-Dioxolene Vanadium Complexes

(51)V solid-state NMR (SSNMR) studies of a series of noninnocent vanadium(V) catechol complexes have been conducted to evaluate the possibility that (51)V NMR observables, quadrupolar and chemical shift anisotropies, and electronic structures of such compounds can be used to characterize these compounds. The vanadium(V) catechol complexes described in these studies have relatively small quadrupolar coupling constants, which cover a surprisingly small range from 3.4 to 4.2 MHz. On the other hand, isotropic (51)V NMR chemical shifts cover a wide range from -200 to 400 ppm in solution and from -219 to 530 ppm in the solid state. A linear correlation of (51)V NMR isotropic solution and solid-state chemical shifts of complexes containing noninnocent ligands is observed. These experimental results provide the information needed for the application of (51)V SSNMR spectroscopy in characterizing the electronic properties of a wide variety of vanadium-containing systems and, in particular, those containing noninnocent ligands and that have chemical shifts outside the populated range of -300 to -700 ppm. The studies presented in this report demonstrate that the small quadrupolar couplings covering a narrow range of values reflect the symmetric electronic charge distribution, which is also similar across these complexes. These quadrupolar interaction parameters alone are not sufficient to capture the rich electronic structure of these complexes. In contrast, the chemical shift anisotropy tensor elements accessible from (51)V SSNMR experiments are a highly sensitive probe of subtle differences in electronic distribution and orbital occupancy in these compounds. Quantum chemical (density functional theory) calculations of NMR parameters for [VO(hshed)(Cat)] yield a (51)V chemical shift anisotropy tensor in reasonable agreement with the experimental results, but surprisingly the calculated quadrupolar coupling constant is significantly greater than the experimental value. The studies demonstrate that substitution of the catechol ligand with electron-donating groups results in an increase in the HOMO-LUMO gap and can be directly followed by an upfield shift for the vanadium catechol complex. In contrast, substitution of the catechol ligand with electron-withdrawing groups results in a decrease in the HOMO-LUMO gap and can directly be followed by a downfield shift for the complex. The vanadium catechol complexes were used in this work because (51)V is a half-integer quadrupolar nucleus whose NMR observables are highly sensitive to the local environment. However, the results are general and could be extended to other redox-active complexes that exhibit coordination chemistry similar to that of the vanadium catechol complexes.

Synthesis and Characterization of Crystalline Structures Based on Phenylboronate Ligands Bound to Alkaline Earth Cations

We describe the preparation of the first crystalline compounds based on arylboronate ligands PhB(OH)(3)(-) coordinated to metal cations: [Ca(PhB(OH)(3))(2)], [Sr(PhB(OH)(3))(2)]·H(2)O, and [Ba(PhB(OH)(3))(2)]. The calcium and strontium structures were solved using powder and single-crystal X-ray diffraction, respectively. In both cases, the structures are composed of chains of cations connected through phenylboronate ligands, which interact one with each other to form a 2D lamellar structure. The temperature and pH conditions necessary for the formation of phase-pure compounds were investigated: changes in temperature were found to mainly affect the morphology of the crystallites, whereas strong variations in pH were found to affect the formation of pure phases. All three compounds were characterized using a wide range of analytical techniques (TGA, IR, Raman, XRD, and high resolution (1)H, (11)B, and (13)C solid-state NMR), and the different coordination modes of phenylboronate ligands were analyzed. Two different kinds of hydroxyl groups were identified in the structures: those involved in hydrogen bonds, and those that are effectively "free" and not involved in hydrogen bonds of any significant strength. To position precisely the OH protons within the structures, an NMR-crystallography approach was used: the comparison of experimental and calculated NMR parameters (determined using the Gauge Including Projector Augmented Wave method, GIPAW) allowed the most accurate positions to be identified. In the case of the calcium compound, it was found that it is the (43)Ca NMR data that are critical to help identify the best model of the structure.

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 (δiso) 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.