Shielding Anisotropy Research Articles

Separation of 47Ti and 49Ti solid-state NMR lineshapes by static QCPMG experiments at multiple fields

Experimental procedures are proposed and demonstrated that separate the spectroscopic contribution from both 47Ti and 49Ti in solid-state nuclear magnetic resonance spectra. These take advantage of the different nuclear spin quantum numbers of these isotopes that lead to different ‘effective’ radiofrequency fields for the central transition nutation frequencies when these nuclei occur in sites with a significant electric field gradient. Numerical simulations and solid-state NMR experiments were performed on the TiO 2 polymorphs anatase and rutile. For anatase, the separation of the two isotopes at high field (21.1 T) facilitated accurate determination of the electric field gradient (EFG) and chemical shift anisotropy (CSA) tensors. This was accomplished by taking advantage of the quadrupolar interaction between the EFG at the titanium site and the different magnitudes of the nuclear quadrupole moments ( Q) of the two isotopes. Rutile, having a larger quadrupolar coupling constant ( C Q), was examined by 49Ti-selective experiments at different magnetic fields to obtain spectra with different scalings of the two anisotropic tensors. A small chemical shielding anisotropy (CSA) of −30 ppm was determined.

Calculation of nuclear magnetic shieldings using an analytically differentiated relativistic shielding formula

Two expressions for nuclear-magnetic-shielding tensor components based on analytically differentiating the electronic energy of a system are presented. The first is based on a second-order Douglas-Kroll-Hess approach, in which the off-diagonal block terms of the transformed Dirac Hamiltonian are diminished to second order with respect to both the electrostatic nuclear attraction potential V and the magnetic vector potential A. The second expression is based on the method of Barysz-Sadlej-Snijders, in which the off-diagonal block terms in the transformed Dirac Hamiltonian are completely eliminated with respect to purely V terms, while they are diminished to second order with respect to terms including A. The two approaches are applied to the calculation of nuclear magnetic shieldings of HX (X=F, Cl, Br, I), H2X (X=O, S, Se, Te), and noble gas X (X =He,Ne,Ar,Kr,Xe) systems with common gauge origins. The results show that relativistic corrections of higher than second order are negligibly small, except for the paramagnetic parts of I, Te, and Xe shieldings. The present calculations yield very large positive values for the anisotropy of proton shielding, deltasigma(H) = sigmaparallel(H)-sigmaperpendicular(H), of HI compared to previous reports. Unfortunately, no experimental values for the anisotropy of proton shielding in HI are available for verification.

Solid-State93Nb and13C NMR Investigations of Half-Sandwich Niobium(I) and Niobium(V) Cyclopentadienyl Complexes

Solid-state 93Nb and 13C NMR experiments, in combination with theoretical calculations of NMR tensors, and single-crystal and powder X-ray diffraction experiments, are applied for the comprehensive characterization of structure and dynamics in a series of organometallic niobium complexes. Half-sandwich niobium metallocenes of the forms Cp'Nb(I)(CO)4 and CpNb(V)Cl4 are investigated, where Cp = C5H5- and Cp' = C5H4R- with R = COMe, CO2Me, CO2Et, and COCH2Ph. Anisotropic quadrupolar and chemical shielding (CS) parameters are extracted from 93Nb MAS and static NMR spectra for seven different complexes. It is demonstrated that 93Nb NMR parameters are sensitive to changes in temperature and Cp' ring substitution in the Cp'Nb(I)(CO)4 complexes. There are dramatic differences in the 93Nb quadrupolar coupling constants (C(Q)) between the Nb(I) and Nb(V) complexes, with C(Q) between 1.0 and 12.0 MHz for Cp'Nb(CO)4 and C(Q) = 54.5 MHz for CpNbCl4. The quadrupolar Carr-Purcell Meiboom-Gill (QCPMG) pulse sequence is applied to rapidly acquire, in a piecewise fashion, a high signal-to-noise ultra-wide-line 93Nb NMR spectrum of CpNbCl4, which has a breadth of ca. 400 kHz. Solid-state 93Nb and 13C NMR spectra and powder XRD data are used to identify a new metallocene adduct coordinated at the axial position of the metal site by a THF molecule: CpNb(V)Cl4.THF. 13C MAS and CP/MAS NMR experiments are used to assess the purity of samples, as well as for measuring carbon CS tensors and the rare instance of one-bond 93Nb, 13C J-coupling, 1J(93Nb,13C). Theoretically calculated CS and electric field gradient (EFG) tensors are utilized to determine relationships between tensor orientations, the principal components, and molecular structures.

A Solid-State 39K and 13C NMR Study of Polymeric Potassium Metallocenes

The quadrupolar Carr-Purcell Meiboom-Gill (QCPMG) and double frequency sweep (DFS)/QCPMG pulse sequences are applied in order to acquire the first solid-state 39K NMR spectra of organometallic complexes, the polymeric main group metallocenes cyclopentadienyl potassium (CpK) and pentamethylcyclopentadienyl potassium (Cp*K). Piecewise QCPMG NMR techniques are used to acquire a high S/N 39K spectrum of the broad central transition of Cp*K, which is ca. 200 kHz in breadth. Analytical and numerical simulations indicate that there is a significant quadrupolar interaction present at both potassium nuclei (C(Q)(39K) = 2.55(6)/2.67(8) MHz and 4.69(8) MHz for CpK (static/MAS) and Cp*K, respectively). Experimental quadrupolar asymmetry parameters suggest that both structures are bent about the potassium atoms (eta(Q)(39K) = 0.28(3)/0.29(3) for CpK (static/MAS) and eta(Q)(39K) = 0.30(3) for Cp*K). Variable-temperature (VT) 39K NMR experiments on CpK elucidate temperature-dependent changes in quadrupolar parameters which can be rationalized in terms of alterations of bond distances and angles with temperature. 13C CP/MAS NMR experiments are conducted upon both samples to quantify the carbon chemical shielding anisotropy (CSA) at the Cp' ring carbon atoms. Ab initio carbon CSA and 39K electric-field gradient (EFG) and CSA calculations are conducted and discussed for the CpK complex, in order to correlate the experimental NMR parameters with molecular structure in CpK and Cp*K. 39K DFS/QCPMG and 13C CP/MAS experiments prove invaluable for probing molecular structure, temperature-dependent structural changes, and the presence of impurities in these systems.

Synthesis and 1H NMR structural analysis of 11‐aryl/heteroarylnaphtha[2,1‐b]furans: X‐ray crystal structure of 11‐(4′‐pyridyl)naphtho[2,1 ‐b]furan

AbstractSynthesis of biaryl type systems, 11‐aryl/heteroarylnaphtho[2,1‐b]furans 8‐11 has been described with a view to studying the conformational orientation of C‐11 aryl/heteroaryl groups. Synthesis of 8‐11 was accomplished by a two‐step sequence involving O‐alkylation of 2‐naphthol with appropriate halo‐ketones 2‐4, followed by cyclization of the resulting naphthoxy‐ketones 5‐7 with methanesulphonic acid. The structures of 8‐11 are based on detailed 2D NMR spectral analysis. The H8 in these compounds is not subject to significant anisotropic upfield shielding effects. The slightly upfield chemical shift of H8 in molecules 9‐11, relative to 8 has been correlated with the electron density at C17 and C8 positions. While molecular modeling indicated dihedral angle (φ) between the C11 aryl/heteroaryl groups and the naphthofuran plane to be in the range of 70–60° in 8‐10, a single X‐ray crystal structural analysis of 11‐(4′‐pyridyl)naphtho[2,1‐b]furan 10 indicated (φ) of 64.36°. In view of significant deviations from the orthogonal oreintation, the absence of any significant anisotropic shielding in 8‐11 is not entirely unexpected.

Interaction of three-finger toxins with phospholipid membranes: comparison of S- and P-type cytotoxins.

The CTs (cytotoxins) I and II are positively charged three-finger folded proteins from venom of Naja oxiana (the Central Asian cobra). They belong to S- and P-type respectively based on Ser-28 and Pro-30 residues within a putative phospholipid bilayer binding site. Previously, we investigated the interaction of CTII with multilamellar liposomes of dipalmitoylphosphatidylglycerol by wide-line (31)P-NMR spectroscopy. To compare interactions of these proteins with phospholipids, we investigated the interaction of CTI with the multilamellar liposomes of dipalmitoylphosphatidylglycerol analogously. The effect of CTI on the chemical shielding anisotropy and deformation of the liposomes in the magnetic field was determined at different temperatures and lipid/protein ratios. It was found that both the proteins do not affect lipid organization in the gel state. In the liquid crystalline state of the bilayer they disturb lipid packing. To get insight into the interactions of the toxins with membranes, Monte Carlo simulations of CTI and CTII in the presence of the bilayer membrane were performed. It was found that both the toxins penetrate into the bilayer with the tips of all the three loops. However, the free-energy gain on membrane insertion of CTI is smaller (by approximately 7 kcal/mol; 1 kcal identical with 4.184 kJ) when compared with CTII, because of the lower hydrophobicity of the membrane-binding site of CTI. These results clearly demonstrate that the P-type cytotoxins interact with membranes stronger than those of the S-type, although the mode of the membrane insertion is similar for both the types.

Theoretical Studies of Nuclear Magnetic Resonance Parameters for the Proton-Exchange Pathways in Porphyrin and Porphycene

The nuclear magnetic resonance (NMR) parameters in porphyrin and porphycene have been calculated to investigate their changes during the process of proton exchange, using density-functional theory (DFT) for both the spin-spin coupling constants and the shielding constants. In addition, in calculations on the smaller 1,3-bis(arylimino)isoindoline molecule, we have tested the performance of our computational approach against experimental data. The calculated nuclear spin-spin coupling constants and shielding constants have been analyzed as functions of the progress of the proton transfer between two nitrogen atoms. The one-bond couplings between proton and nitrogen, dominated by the Fermi-contact term, decay steeply as the internuclear distance increases. The small changes in the intramolecular J(HH) coupling between two inner protons are mainly determined by the sum of relatively large spin-orbit terms. The isotropic shielding constant shows a strong deshielding of the nitrogen nuclei as the proton migrates away. Both the isotropic shielding of the exchanged protons and the shielding anisotropy exhibit a minimum close to the transition states.

Molecular Insight into the Electrostatic Membrane Surface Potential by14N/31P MAS NMR Spectroscopy: Nociceptin−Lipid Association

Exploiting naturally abundant (14)N and (31)P nuclei by high-resolution MAS NMR (magic angle spinning nuclear magnetic resonance) provides a molecular view of the electrostatic potential present at the surface of biological model membranes, the electrostatic charge distribution across the membrane interface, and changes that occur upon peptide association. The spectral resolution in (31)P and (14)N MAS NMR spectra is sufficient to probe directly the negatively charged phosphate and positively charged choline segment of the electrostatic P(-)-O-CH(2)-CH(2)-N(+)(CH(3))(3) headgroup dipole of zwitterionic DMPC (dimyristoylphosphatidylcholine) in mixed-lipid systems. The isotropic shifts report on the size of the potential existing at the phosphate and ammonium group within the lipid headgroup while the chemical shielding anisotropy ((31)P) and anisotropic quadrupolar interaction ((14)N) characterize changes in headgroup orientation in response to surface potential. The (31)P/(14)N isotropic chemical shifts for DMPC show opposing systematic changes in response to changing membrane potential, reflecting the size of the electrostatic potential at opposing ends of the P(-)-N(+) dipole. The orientational response of the DMPC lipid headgroup to electrostatic surface variations is visible in the anisotropic features of (14)N and (31)P NMR spectra. These features are analyzed in terms of a modified "molecular voltmeter" model, with changes in dynamic averaging reflecting the tilt of the C(beta)-N(+)(CH)(3) choline and PO(4)(-) segment. These properties have been exploited to characterize the changes in surface potential upon the binding of nociceptin to negatively charged membranes, a process assumed to proceed its agonistic binding to its opoid G-protein coupled receptor.

Selenium chemistry with DFT: molecular structures and 77Se NMR shielding constants

Optimized geometries of fourteen selenium-containing molecules are determined using density functional theory, with the KT3, HCTH and OLYP generalized gradient approximations and the B3LYP and B97-2 hybrid functionals. Bond length quality can be summarized as B97-2 > KT3 > B3LYP > HCTH > OLYP. For a subset of seven molecules, the same functionals are used to determine isotropic and anisotropic shielding constants. The shielding quality can be summarized as KT3 > B97-2 > HCTH ≈ OLYP > B3LYP. The study supports earlier observations that, by DFT standards, KT3 can simultaneously provide good quality geometries and shielding constants, without the inclusion of exact exchange. The study also highlights the relatively poor results from the widely-used B3LYP functional.

1H and 13C Nuclear Magnetic Resonance Studies of the Hindered Phencyclone Adducts of Some Smaller Branched N-Alkyl Maleimides: Rigorous Aryl Proton Assignments with High-Resolution Two-Dimensional (COSY45) Spectroscopy, and Anisotropic Shielding Effects and Ab Initio Geometry Optimizations

Phencyclone, 1, a potent Diels-Alder diene, reacts with a series of N-alkylmaleimides, 2, to form hindered adducts, 3. The 300 MHz 1H and 75 MHz 13C NMR studies of these adducts at ambient temperatures have demonstrated slow rotations on the nuclear magnetic resonance (NMR) timescales for the unsubstituted bridgehead phenyl groups, and have revealed substantial magnetic anisotropic shielding effects in the 1H spectra of the N-alkyl groups of the adducts. The selected N-alkyl groups for the target compounds emphasized smaller branched alkyls, including C3 (isopropyl, a); C4 (isobutyl, b; and t-butyl, c); C5 (n-pentyl, d; isopentyl [isoamyl], e; 1-ethylpropyl, f; t-amyl, g;) and a related C8 isomer (1,1,3,3-tetramethylbutyl ["t-octyl"], h). The straight-chain n-pentyl analog was included as a reference. This present work on the branched N-alkylmaleimide adducts appreciably extends our earlier compilation on the N-n-alkylmaleimide adducts. Key methods for proton assignments included "high-resolution" 1H-1H chemical shift correlation spectroscopy, COSY45. 13C NMR of the adducts, 3, verified the expected number of aryl carbons for slow exchange limit (SEL) spectra of the bridgehead phenyl groups. The synthetic routes involved reaction of the corresponding amines, 4, with maleic anhydride to give the N-alkylmaleamic acids, 5, which underwent cyclodehydration to form the maleimides, 2. Magnetic anisotropic shielding magnitudes for alkyl group protons in the adducts were calculated relative to corresponding proton chemical shifts in the maleimides. Geometry optimizations for the above adducts (and for the N-n-butylmaleimide adduct) were performed at the Hartree-Fock level with the 6-31G* basis set. The existence of different contributing conformers for the adducts is discussed with respect to their calculated energies and implications regarding experimentally observed anisotropic shielding magnitudes.

Diels-Alder Adducts of 3,6-Dibromophencyclone with Symmetrical 1,4-Disubstituted-Cis-2-Butenes: Comparisons with the Adduct of Phencyclone and N-Benzylmaleimide, and One-Dimensional and Two-Dimensional Nuclear Magnetic Resonance Studies and Ab Initio Structure Calculations

Hindered Diels-Alder adducts have been prepared from 3,6-dibromophencyclone, 2, with cis-1,4-diacetoxy-2-butene, 3; cis-2-butene-1,4-diol, 4; and N-benzylmaleimide, 5. The adduct from the parent phencyclone, 1, with N-benzylmaleimide was prepared for comparison. One- and two-dimensional (1D and 2D) proton and carbon-13 NMR studies (at 7.05 tesla, ambient temperatures), including high-resolution COSY45 and HETCOR (XHCORR) chemical shift correlation spectra, were performed, allowing extensive rigorous assignments for protons and protonated carbons. Substantial anisotropic shielding was seen for the ortho protons of the N-benzyl group in the adducts of 5 with 1 or 2, with these aryl protons resonating at 6.25 ppm (CDCl3) for each adduct. The unsubstituted bridgehead phenyls of all four adducts showed slow exchange limit (SEL) 1H and 13C spectra. Greater shift dispersions for the bridge-head phenyl protons in the adducts from 5 relative to those from 3 or 4 suggested the role of the imide carbonyls for anisotropic contributions or for influences on adduct geometry. Ab initio geometry optimizations were performed at the Hartree-Fock level with the 6-31G* basis set (or the LACVP* basis set for the bromine-containing compounds) for each of the adducts. For the two adducts from benzylmaleimide, separate minima were located corresponding to conformers in which the benzyl group was directed into the adduct cavity (syn) or out of the adduct cavity (anti). Calculated energies and geometric parameters for the adducts are presented, and these suggested a significantly different structure for the dibromo diacetate adduct, in terms of general symmetry and bridgehead phenyl geometries, compared to the other adducts.

Diels-Alder Adducts of 3,6-Dibromophencyclone with Short-Chain N-n-Alkylmaleimides: 1H and 13C Nuclear Magnetic Resonance Studies of Hindered Rotations and Magnetic Anisotropy, and Ab Initio Calculations for Optimized Structures

3,6-Dibromophencyclone, 2, reacted with N-ethylmaleimide, 3a; N-n-propylmaleimide, 3b; and N-n-butylmaleimide, 3c; to form the corresponding Diels-Alder adducts, 4a, 4b, and 4c. The nuclear magnetic resonance (NMR) spectra of the adducts were studied at ambient temperatures at 300 MHz for proton and 75 MHz for carbon-13. Full proton assignments were achieved by high-resolution COSY45 spectra for the aryl proton regions. Rigorous assignments for protonated carbons were obtained with the heteronuclear chemical shift correlation spectra (HETCOR). Slow exchange limit (SEL) spectra were observed for both proton and carbon-13 NMR for each adduct, with slow rotation on the NMR timescales for the unsubstituted bridgehead phenyl groups. Endo Diels-Alder adduct stereochemistry was supported by substantial magnetic anisotropic shielding effects in the 1H NMR spectra of the alkyl groups. Proton NMR shifts are compared with those previously reported for the corresponding adducts, 5b and 5c, obtained from 3b and 3c, respectively, with the parent compound, phencyclone, 1. Results of ab initio molecular modeling calculations at the Hartree-Fock level using the LACVP* basis set for conformers of the dibrominated adducts, 4a-4c, are presented, together with HF/6-31G* results for the non-brominated adducts, 5a, 5b, and 5c. Novel aspects of this present work include: (a) attempts to quantitatively evaluate alkyl proton NMR shielding magnitudes in the adducts, relative to maleimide precursors, and (b) use of ab initio Hartree-Fock level calculations to try to reconcile adduct geometries with the observed shielding magnitudes. Our results here complement and extend studies of: (a) adducts of the parent phencyclone with straight-chain N-n-alkylmaleimides, and (b) adducts of 3,6-dibromophencyclone with other symmetrical dienophiles.

Orientation of single crystals using linear approximations to NMR transits

We have derived analytical expressions for determining the orientation of high-symmetry single crystals from line-crossings in a single rotation plot. We demonstrate the utility of the method using the strontium-87 resonance in strontium nitrate. Employing our new method, which we call orientation of single crystals using linear approximations to NMR transits (OSCULANT), in combination with fourth-order perturbation theory, we obtain a highly accurate value for the quadrupole coupling constant, and an estimate for the chemical shielding anisotropy.

MAS NMR with and without double-quantum filtration at and near the n = 0 rotational resonance condition

Spectral lineshapes of MAS NMR spectra of dipolar (re)coupled spin pairs exhibiting considerable chemical shielding anisotropies at and near the so-called n = 0 rotational resonance ( R 2) condition are considered. The n = 0 R 2 condition is found to be not extremely sharp. Anisotropic interaction parameters such as chemical shielding tensor orientations and the magnitude of the dipolar coupling constant remain sensitively encoded in such lineshapes even when differences in isotropic chemical shielding values of up to 400 Hz (corresponding to ca. half the size of the dipolar coupling constant) are present. Additional double-quantum filtration (DQF) may enhance the sensitivity of spectral lineshapes to anisotropic interaction parameters for even larger differences in isotropic chemical shielding values. The dependence of the DQF efficiency on spin-system parameters as well as on external parameters (Larmor and MAS frequencies) is investigated. Away from R 2 conditions a trend to lower DQF efficiencies is found whereas some spin-system parameters are more sensitively encoded in the corresponding spectral lineshapes. Our study is based on numerical simulations, with the known parameters of the 31P spin pair in Na 4P 2O 7 · 10H 2O representing our model case.

The solvation of the mercury(II) ion—a199Hg NMR study

The solvation of the mercury(II) ion in solvents with different solvation properties, water, dimethylsulfoxide, N,N-dimethylthioformamide, and liquid ammonia, has been studied by means of (199)Hg NMR. The (199)Hg chemical shift shows a pronounced dependence on the coordination number of the mercury(II) ion in the solvates resulting in a difference of over 1200 ppm between basically tetrahedral and octahedral complexes. The chemical shifts can furthermore be associated with electron-pair donor properties of the solvents. The spin-lattice relaxation times of the (199)Hg nucleus in the solvates have been measured at different applied magnetic fields, concentrations, temperatures, and isotope substitutions. Possible mechanisms for the (199)Hg relaxation were proposed and the chemical shielding anisotropy in the solvates has been estimated. The (199)Hg relaxation rates and the anisotropy are correlated with the structure of the solvate complexes in solution obtained from recent LAXS and EXAFS studies.

Ab initio NMR study of the isomeric hydrogen‐bonded methanol–water complexes

AbstractIsotropic and anisotropic chemical shifts for all atoms of complexes CH3HO ˙˙˙ H2O and CH3OH ˙˙˙ OH2 have been calculated at the Hartree–Fock, second‐order Møller‐Plesset (MP2) and density functional (B3LYP) theoretical levels using the 6‐311++G(2d,2p) basis set. The influence of the hydrogen bond formation on the nuclear magnetic resonance chemical shifts in all atoms is analyzed. The basis set superposition error was taken into account, and its effects were more significant for the anisotropic shieldings on the oxygen atoms of the proton donor CH3OH ˙˙˙ OH2 and proton acceptor methanol CH3HO ˙˙˙ H2O. Using counterpoise correction to the MP2 results, our best estimate for the calculated isotropic and anisotropic shifts of the H atom involved in the hydrogen bond are −2.98 and 11.95 ppm for CH3HO ˙˙˙ H2O and −2.91 and 11.48 ppm for the CH3OH ˙˙˙ OH2 isomer, respectively. For the O atom of the OH hydrogen bonded, these calculated shifts are −5.21 and −5.35 ppm for CH3HO ˙˙˙ H2O and −6.32 and −8.75 ppm for CH3OH ˙˙˙ OH2. The effect of monomer relaxation was also considered and was found to be appreciable only for the oxygen atom of the proton donor OH due to the distance increase after complex formation. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2005

Proton and Carbon‐13 NMR Studies of the Phencyclone Adducts of Medium Alkyl Chain‐Length N‐n‐Alkylmaleimides. Hindered Rotations and Magnetic Anisotropic Effects. Ab initio Calculations for Optimized Structures

The Diels–Alder adducts, 3a–e, of phencyclone, 1, have been prepared from a series of N‐n‐alkylmaleimides, 2, with medium chain‐length n‐alkyl groups. The maleimides were obtained by cyclodehydration of the N‐n‐alkylmaleamic acids, 4, formed from reaction of maleic anhydride with the corresponding n‐alkylamines. The five adducts prepared included derivatives from n‐heptyl, 3a; n‐octyl, 3b; n‐nonyl, 3c; n‐decyl, 3d; and n‐dodecyl, 3e. The NMR spectra of the adducts were studied in CDCl3 at ambient temperatures at 300 MHz for proton and 75 MHz for carbon‐13, with full proton assignments achieved by high‐resolution COSY45 spectra for the aryl and the alkyl regions. Slow exchange limit (SEL) spectra were observed for both 1H and 13C spectra showing slow rotation on the NMR timescales of the unsubstituted bridgehead phenyl groups. Endo Diels–Alder adduct stereochemistry was supported by striking magnetic anisotropic shielding effects in the 1H spectra of the alkyl groups, with the NCH2 CH 2 signal of each adduct appearing upfield of tetramethylsilane (TMS) at ca. −0.32 ppm. Proton NMR spectra for precursor maleamic acids and maleimides are reported, with some solvent effects found (CDCl3 vs. d 6‐acetone) for the carbon‐bound HC˭CH protons of 4. Ab initio molecular modeling calculations at the Hartree‐Fock level using the 6‐31G* basis set have been performed for two key conformers of the phencyclone adduct of N‐n‐octylmaleimide, as a representative structure for these hindered adducts, to estimate geometric parameters for the adduct. A syn conformer, with the alkyl chain directed into the adduct cavity, was found to be ca. 0.23 kcal/mol lower energy than an anti conformer (in which the alkyl chain was directed away from the phenanthrenoid moiety).

The NMR response of boroxol rings: a density functional theory study

Cluster models of boron oxide glasses are studied computationally using density functional theory. It is shown that the isotropic chemical shielding of boron in boroxol rings is about 5 ppm less than for boron in non-ring BO 3 / 2 units, and that the quadrupole coupling in ring sites is about 0.1 MHz larger than in non-ring sites, confirming assignments made in glasses and crystalline model compounds. The chemical shielding anisotropy of these sites is computed and shown to be in agreement with recent experimental measurements. Furthermore, it is shown that the reason for the different responses is not the co-planarity of BO 3 / 2 groups bound in rings, but rather the contraction in the B–O–B bond angle from about 134 ∘ in relaxed structures to 120 ∘ as found in rings.

Solid-State 31P NMR Chemical Shielding Tensors in Phosphonates and Bisphosphonates: A Quantum Chemical Investigation

We report the results of a quantum chemical investigation of the 31P nuclear magnetic resonance (NMR) isotropic chemical shieldings (σiso) and the 31P NMR chemical shielding tensor principal values (σii, i = 1−3) in a series of eight different phosphonates, including the bisphosphonates pamidronate and risedronate currently in use in bone resorption therapy. We used primarily Hartree−Fock methods with a 6-311++G(2d,2p) basis set and the CSGT (continuous set of gauge transformations) formalism to predict the experimental observables, using various approaches to incorporating intermolecular, crystal lattice effects. Good predictions of the 31P NMR isotropic chemical shielding, shielding tensor principal values, and tensor orientations were obtained, with R2 = 0.95 and ∼7% root-mean-square error from experiment. In the zwitterionic aminophosphonates, electrostatic (lattice) interactions were found to be strong but could be well accounted for by incorporating charge lattice effects into the calculations. The ability to now predict both isotropic and anisotropic shielding (shift) tensors in phosphonates and bisphosphonates should open the way to the determination of their protonation states when bound to proteins, information which is not accessible from crystallographic studies.

Modulation of the distance dependence of paramagnetic relaxation enhancements by CSA × DSA cross-correlation

Paramagnetic metal ions with fast-relaxing electronic spin and anisotropic susceptibility tensor provide a rich source of structural information that can be derived from pseudo-contact shifts, residual dipolar couplings, dipole–dipole Curie spin cross-correlation, and paramagnetic relaxation enhancements. The present study draws attention to a cross-correlation effect between nuclear relaxation due to anisotropic chemical shielding (CSA) and due to the anisotropic dipolar shielding (DSA) caused by the electronic Curie spin. This CSA × DSA cross-correlation contribution seems to have been overlooked in previous interpretations of paramagnetic relaxation enhancements. It is shown to be sufficiently large to compromise the 1/ r 6 distance dependence usually assumed. The effect cannot experimentally be separated from auto-correlated DSA relaxation. It can increase or decrease the observed paramagnetic relaxation enhancement. Under certain conditions, the effect can dominate the entire paramagnetic relaxation, resulting in nuclear resonances narrower than in the absence of the paramagnetic center. CSA × DSA cross-correlation becomes important when paramagnetic relaxation is predominantly due to the Curie rather than the Solomon mechanism. Therefore the effect is most pronounced for relaxation by metal ions with large magnetic susceptibility and fast-relaxing electron spin. It most strongly affects paramagnetic enhancements of transverse relaxation in macromolecules and of longitudinal relaxation in small molecules.