New insights into frustrated Lewis pairs: structural investigations of intramolecular phosphane-borane adducts by using modern solid-state NMR techniques and DFT calculations

Covalent bonding interactions between the Lewis acid and Lewis base functionalities have been probed in a series of "frustrated Lewis pairs" (FLPs) (mainly substituted vinylene linked intramolecular phosphane-borane adducts), using solid-state nuclear magnetic resonance techniques and accompanying DFT calculations. Both the (11)B NMR isotropic chemical shifts and nuclear electric quadrupolar coupling parameters turn out to be extremely sensitive experimental probes for such interactions, revealing linear correlations with boron-phosphorus internuclear distances. The principal component V(zz) of the (11)B electric field gradient tensor is tilted slightly away (~20°) from the boron-phosphorus internuclear vector, leading to an improved understanding of the remarkable reactivity of the FLPs. Complementary (31)P{(1)H}-CPMAS experiments reveal significant (31)P-(11)B scalar spin-spin interactions ((1)J ≈ 50 Hz), evidencing covalent bonding interactions between the reaction centers. Finally, (11)B{(31)P} rotational echo double resonance (REDOR) experiments show systematic deviations from calculated curves based on the internuclear distances from X-ray crystallography. These deviations suggest non-zero contributions from anisotropic indirect spin-spin (J anisotropy) interactions, thereby offering additional evidence for covalent bonding.     

Aluminum orthovanadate (AlVO4): synthesis and characterization by (27)Al and 51V MAS and MQMAS NMR spectroscopy

Polycrystalline samples of AlVO(4) have been prepared by two methods of synthesis and characterized by (27)Al and (51)V MAS NMR spectroscopy at 14.1 T. The MAS NMR spectra clearly reveal that essentially pure samples with minor impurities of V(2)O(5) and alumina have been obtained. From these samples, (27)Al quadrupole coupling parameters and isotropic chemical shifts as well as the magnitudes and relative orientations of the (51)V quadrupole coupling and chemical shift tensors have been determined with high precision for AlVO(4). These data have been obtained from a combined analysis of multiple-quantum (MQ) MAS NMR spectra and MAS NMR spectra of the central and satellite transitions. The (27)Al and (51)V NMR data show that the asymmetric unit for AlVO(4) contains three isolated VO(4) tetrahedra, one pentacoordinated Al site, and two AlO(6) octahedra. This is in agreement with the supposition that AlVO(4) is isostructural with FeVO(4) and with a recent structure refinement for AlVO(4) based on powder X-ray diffraction (XRD) data. The favorable agreement between the refined crystal structure from powder XRD and the NMR parameters is apparent from a convincing correlation between experimental (51)V quadrupole tensor elements and calculated (51)V electric field gradient tensor elements obtained by the point-monopole approach. An assignment of the (27)Al NMR data is obtained from similar calculations of the (27)Al electric field gradients and by estimation of the distortion of the AlO(6) octahedra.     

Ab initio study of the ground state properties of molecular oxygen

The electric and magnetic properties of the ground state of oxygen molecule are calculated by multiconfiguration self-consisted field (MCSCF) method and compared with experimental data: the quadrupole moment, polarizability, the 17O nuclear quadrupole coupling constant, magnetizability tensor, nuclear spin-rotation coupling constant and rotational g factor. The last two constants are calculated for all possible isotope modifications. The rotational, ESR and NMR spectra are discussed. Fermi-contact hyperfine coupling parameter is additionally estimated by different methods. The NMR chemical shielding tensor for 17O16O species at high temperature limit (without electron spin contribution) is predicted. Potential energy curves for 10 excited bound states and the internuclear distance dependence of the studied properties are also presented.     

Dihydrogen Splitting by Intramolecular Borane-Phosphane Frustrated Lewis Pairs: A Comprehensive Characterization Strategy Using Solid State NMR and DFT Calculations

Four hydrogenated intramolecular phosphane-borane frustrated Lewis pair (B/P FLP) compounds bearing unsaturated cyclic or aromatic carbon backbones have been synthesized and structurally characterized using ¹¹B, ³¹P, ¹H and ²H solid-state NMR spectroscopy. A comparison of the spectra with those of the corresponding free B/P FLPs shows that both ¹¹B isotropic chemical shifts as well as nuclear electric quadrupolar coupling constants decrease significantly upon FLP hydrogenation, revealing the breakage of the partial B−P bond present in the starting materials. Likewise, the ³¹P isotropic chemical shift, the chemical shift anisotropy, and the asymmetry parameter decrease significantly upon FLP hydrogenation, reflecting the formation of a more symmetric, C_3v-like local environment. ¹¹B{³¹P} rotational echo double resonance (REDOR) experiments can be used to measure the B−P internuclear distance (about 3.2 Å) of these compounds. Observation of the hydrogen atoms bound to the Lewis centers is best accomplished via ³¹P{¹H} and ¹¹B{¹H} cross-polarization-heteronuclear correlation experiments or by direct observation of the ²H MAS NMR signals on especially prepared FLP-D₂ adducts. For accurately measuring the phosphorus-deuterium distance via ³¹P{²H} rotational echo adiabatic passage double resonance (REAPDOR), it is essential to take the secondary dipolar coupling of ³¹P with the boron-bonded ²H nuclei explicitly into consideration, by simulating a ²H_P-³¹P-²H_B three-spin system based on structural input. All of the experimental NMR interaction parameters are found in excellent agreement with values calculated by DFT methods, using the geometries obtained either by energy optimization or from single-crystal structures.     

DFT calculations as a powerful technique to probe the crystal structure of Al(acac)3

(27)Al, (17)O and (13)C chemical shieldings of aluminum acetylacetonate complex, Al(acac)(3), were calculated at some Density Functional Theory (DFT) levels of theory. In these calculations the X-ray structures of its different polymorphs were used. Using these calculated data observed discrepancies between the X-ray crystallography and solid state NMR experiment were explained in terms of the quality of the NMR data. In this survey we resorted to the simulated spectra using our calculated chemical shifts. In order to confirm our conclusions, electric field gradient (EFG) tensors of the (27)Al and (17)O nuclei were calculated at the same levels of theory as used in the chemical shielding calculations. On the other hand, these calculated chemical shifts and nuclear quadrupole coupling constants (NQCCs) made a correlation between X-ray crystallography and solid state NMR experiments.     

Computational studies on aluminum nitride and aluminum phosphide nanotubes: density functional calculations of 27Al electric field gradient tensors

Abstract  

Nuclear quadrupole resonance (NQR) parameters including the nuclear quadrupole coupling constant (C _Q) and asymmetry parameter (η _Q) at the sites of various ²⁷Al nuclei on (6,0) zigzag and (4,4) armchair AlN and AlP nanotubes (NTs) were calculated by using the density functional theory (DFT) method to study the properties of the electronic structures of the nanotubes. Geometry optimizations were carried out at the B3LYP/6-31G* level of theory using the Gaussian 03 suite of programs. The calculated electric field gradient tensors were converted to the nuclear quadrupole resonance parameters, C _Q constant, and η _Q parameter. The quadrupole resonance parameters in each of the structures were divided into four layers with equivalent electric field gradient tensor eigenvalues in each layer. The results show that, in AlN and AlP nanotubes, the Al atoms at the edges of the nanotubes play dominant roles in determining the electronic behavior of the nanotubes and important roles in growth and synthesis processes of the nanotubes. Also the average values of C _Q(²⁷Al) for the AlNNT models were higher in comparison with the AlPNT models, while variations of C _Q(²⁷Al) in AlPNTs were greater in comparison with in AlNNTs.     

11B-MAS-NMR-Untersuchungen zur Anionenstruktur von Boraten

¹¹B MAS NMR Studies on the Structure of Borate Anions The results of ¹¹B MAS NMR studies on selected crystalline borates and peroxoborates show that the boron coordination number can be determined not only from the quadrupole coupling constant but also from the chemical shift values. Correspondingly, the B[3]:B[4] ratio can be determined quantitatively. In contrast to ²⁷Al, ²⁹Si, and ³¹P NMR data, however, the observed small variation of the ¹¹B chemical shifts does not allow a more detailed information on the linkage of the coordination polyhedra and their environments. The method was applied to Ag borates with unknown structures. In the case of a borate with an Ag:B ratio of 1:3 the measured B[3]:B[4] ratio suggests a triborate structure of the anion.     

27Al-NMR-Untersuchungen an Alkalifluoroaluminaten

²⁷Al NMR Studies on Alkali Fluoroaluminates The ²⁷Al NMR spectra of the alkali fluoroaluminates sensitively reflect the kind of condensation of the AlF₆ octahedra. Whereas the chemical shift of the octahedral Al is shown to be rather independent on structural details, clear differences in the electric field gradients of isolated and condensed AlF₆ octahedra were found. In contrast to the isolated octahedra which show only weak effects of quadrupole interaction, both for chains and layers of AlF₆ octahedra axial EFG tensors result with quadrupolar coupling constants of between 10 and 13 MHz; for the threefold chains in CsAlF₄ only a value of 7.5 MHz is obtained.     

Further evidence of incorporation of chromium ions into the framework of silicoaluminophosphate molecular sieves

Three different molecular sieves were synthesised and characterized using³¹P and²⁷Al magic angle spinning nuclear magnetic resonance (³¹P and²⁷Al MAS NMR) spectroscopy and acidity measurement techniques. The synthesized solids were: a silicoaluminophosphate (SAPO-11) sample, a chromium-substituted silicoaluminophosphate (CrAPSO-11) sample and a chromium-supported SAPO-11 (Cr/SAPO-11) sample. Significant differences were observed between the CrAPSO-11 MAS NMR spectra and the spectra for the other two solids. The differences can be understood in terms of a different chemical environment for the Al(III) and P(V) ions in the molecular sieve framework, as a result of a different type of interaction, probably with substituted chromium ions in the framework. The acidity measurements were in agreement with the MAS NMR spectroscopy results, providing further evidence for the incorporation of chromium ions into the molecular sieve framework.     

Dependence of the27Al and31P NMR chemical shifts in alumophosphates on the composition of the second coordination sphere

Conclusions The isotropic²⁷Al NMR chemical shifts in alumophosphates in the solid state and alumophosphate complexes in solution vary additively upon substitution of water by phosphate ions in the aluminum coordination sphere, while the³¹P NMR chemical shifts of phosphate vary additively depending on the number of coordinated aluminum atoms.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 10, pp. 2340–2342, October, 1987.     

Solid-state natural abundance 25Mg NMR studies of Na2MgEDTA x 4 H2O--a possible new reference compound for 25Mg NMR spectroscopy

Natural abundance solid-state (25)Mg NMR measurements were made of the disodium salt of magnesium ethylenediaminetetraacetate tetrahydrate (Na(2)MgEDTA x 4 H(2)O). Both magic angle spinning (MAS) and static experiments were employed to determine the quadrupole coupling constant (C(q)) and the asymmetry parameter (eta(q)) of the electric field gradient (EFG) tensor associated with (25)Mg in this compound, giving the values C(q) = 1.675(5) MHz and eta(q) = 0.15(1). The isotropic chemical shift was determined to be delta(iso) = 0.25(10) ppm (relative to 11 M MgCl(2) aqueous solution) and a small chemical shift anisotropy (CSA) contribution (approximately -13 ppm) was detected, one of the first CSA reports in (25)Mg NMR. This compound exhibited remarkably good (25)Mg NMR sensitivity, due to its fast spin-lattice relaxation and modest quadrupole coupling, which allowed its use as a secondary shift reference and as a test sample for the implementation and optimisation of signal-enhancement methods in (25)Mg NMR spectroscopy, such as double frequency sweeps (DFS) and the use of adiabatic hyperbolic secant (HS) and WURST pulses.     

Magic angle spinning29Si and27Al NMR study of mordenite dealumination

Magic angle spinning²⁹Si NMR presents a rapid qualitative method of assessing the degree of dealumination of sodium mordenite modified by acid leaching and heat treatment. A quantitative determination of the²⁹Si coordination sphere is hampered by the overlap of chemical shift ranges for Si?O?Al and Si?O?H species. MAS²⁷Al NMR indicates the presence of octahedrally coordinated Al in interstitial sites in all treated samples. On treating mordenite samples at high temperatures (～700°C) much of the Al becomes invisible to the NMR experiment because of its location in sites of low symmetry and large quadrupole coupling constant.     

Chemical shift and quadrupole coupling of the 27Al NMR spectra of LiAIO2 polymorphs

²⁷Al chemical shift data of polycrystalline LiAlO₂ polymorphs, obtained from high field measurements in combination with magic-angle spinning techniques, have been used for the determination of the Al coordination in the α-, β-, and γ-phase. The β-phase was shown to have only tetrahedrally coordinated Al atoms. In addition quadrupole coupling data have been derived from analysis of low field NMR spectra of 27A1 in order to characterize the symmetry of the Al sites. The NMR results are discussed with regard to the crystal structure of the phases.     

Mechanochemical Preparations of Anion Coordinated Architectures Based on 3-Iodoethynylpyridine and 3-Iodoethynylbenzoic Acid

The halogen bond has previously been explored as a versatile tool in crystal engineering and anion coordination chemistry, with mechanochemical synthetic techniques having been shown to provide convenient routes towards cocrystals. In an effort to expand our knowledge on the role of halogen bonding in anion coordination, here we explore a series of cocrystals formed between 3-iodoethynylpyridine and 3-iodoethynylbenzoic acid with halide salts. In total, we report the single-crystal X-ray structures of six new cocrystals prepared by mechanochemical ball milling, with all structures exhibiting C≡C−I⋅⋅⋅X⁻ (X=Cl, Br) halogen bonds. Whereas cocrystals featuring a pyridine group favoured the formation of discrete entities, cocrystals featuring a benzoic acid group yielded an alternation of halogen and hydrogen bonds. The compounds studied herein were further characterized by ¹³C and ³¹P solid-state nuclear magnetic resonance, with the chemical shifts offering a clear and convenient method of identifying the occurrence of halogen bonding, using the crude product obtained directly from the mechanochemical ball milling. Whereas the ³¹P chemical shifts were quickly able to identify the occurrence of cocrystallization, ¹³C solid-state NMR was diagnostic of both the occurrence of halogen bonding and of hydrogen bonding.     

“NMR invisible” aluminum species in dealuminated mordenite zeolite: a1H/27 AI TRAPDOR NMR study

Various kinds of aluminum species in dealuminated mordenite were investigated in detail, and the quadrupole coupling constants (QCCs) for aluminum atoms associated with these species were obtained by means of the newly introduced¹H/²⁷ AI TRAPWR method as well as²⁷Al magic angle spinning (MAS) nuclear magnetic resonance (NMR). QCC values of 11.3, 15.3, 13.3 and (14.0± 0.6) MHz were determined from the TRAPDOR profiles for Lewis acid sites, Bronsted acid sites (SiOHAl) and two kinds of non-framework aluminum species Al(OH) _n , respectively. The source of the “invisible Al” is discussed on the basis of the NMR experimental results.     

An experimental and theoretical study of the 13C and 31P chemical shielding tensors in solid O-phosphorylated amino acids

A series of l and dl forms of O-phosphorylated amino acids (serine, threonine, tyrosine) have been studied by using solid-state multinuclear NMR spectroscopy and ab initio calculations. Principal elements of the (13)C and (31)P chemical shielding tensors have been measured and discussed in relation to zwitterionic structures and intermolecular contacts. DFT calculations have been compared with experimental data showing their ability to reproduce experimentally obtained tensor values in this challenging class of compounds. The changes of orientation of (31)P chemical shielding tensor with respect to the molecular frame in the presence of hydrogen bonds have been revealed and discussed on the ground of theoretical calculations. The measurements of internuclear P...P distances, based on Zeeman magnetization exchange between (31)P spins with differing chemical shielding tensor orientations, were exploited for a clear distinction between enantiomers and racemates.     

Interactions of phosphorous molecules with the acid sites of H-Beta zeolite: Insights from solid-state NMR techniques and theoretical calculations

The local structures of various Brønsted and Lewis acid sites in H-Beta zeolite were resolved with the combined ³¹P MAS NMR, ³¹P–²⁷Al TRAPDOR NMR experiments and theoretical calculations at different levels. In addition, the interacting mechanisms of these acid sites with probe molecules such as trimethylphosphine (TMP) and trimethylphosphine oxide (TMPO) were clarified, which greatly aids the understanding of acid catalysis. Owing to the narrow chemical shift range and close Brønsted acid strengths, only an average resonance at −4.5 ppm was observed in TMP adsorbed H-Beta zeolite, consistent with the calculated data of acidities (substitution energies and proton affinities), geometries, adsorption energies as well as ³¹P chemical shifts. However, two types of Brønsted acids were distinguished by TMPO, and the HF/DZVP2 (MP2/DZVP2) chemical shifts were calculated at 68.1 (69.5) and 69.7–72.1 (71.7–74.9) ppm, respectively. Two types of Lewis acids were identified at −32.0 and −47.0 ppm with the latter exhibiting strong ³¹P–²⁷Al TRAPDOR effects. With theoretical calculations, these two peaks were attributed to the extra-lattice oxo-AlOH²⁺ species and the three-fold coordinated lattice-Al, extra-framework Al(OH)₃, oxo-AlO⁺ species, respectively. The peak at −60.0 ppm was conventionally assigned to the TMP physisorption, but the calculations indicated that the EFAL monovalent Al(OH)₂⁺ species coordinating with two lattice-O atoms near the framework Al atom can contribute to it as well.     

An Investigation of Lanthanum Coordination Compounds by Using Solid‐State 139La NMR Spectroscopy and Relativistic Density Functional Theory

Lanthanum‐139 NMR spectra of stationary samples of several solid La^III coordination compounds have been obtained at applied magnetic fields of 11.75 and 17.60 T. The breadth and shape of the ¹³⁹La NMR spectra of the central transition are dominated by the interaction between the ¹³⁹La nuclear quadrupole moment and the electric field gradient (EFG) at that nucleus; however, the influence of chemical‐shift anisotropy on the NMR spectra is non‐negligible for the majority of the compounds investigated. Analysis of the experimental NMR spectra reveals that the ¹³⁹La quadrupolar coupling constants (C_Q) range from 10.0 to 35.6 MHz, the spans of the chemical‐shift tensor (Ω) range from 50 to 260 ppm, and the isotropic chemical shifts (δ_iso) range from ?80 to 178 ppm. In general, there is a correlation between the magnitudes of C_Q and Ω, and δ_iso is shown to depend on the La coordination number. Magnetic‐shielding tensors, calculated by using relativistic zeroth‐order regular approximation density functional theory (ZORA‐DFT) and incorporating scalar only or scalar plus spin–orbit relativistic effects, qualitatively reproduce the experimental chemical‐shift tensors. In general, the inclusion of spin–orbit coupling yields results that are in better agreement with those from the experiment. The magnetic‐shielding calculations and experimentally determined Euler angles can be used to predict the orientation of the chemical‐shift and EFG tensors in the molecular frame. This study demonstrates that solid‐state ¹³⁹La NMR spectroscopy is a useful characterization method and can provide insight into the molecular structure of lanthanum coordination compounds.     

Density functional theory study of nitrogen‐14 nuclear quadrupole coupling parameters of L‐histidine: hydrogen‐bonded system

The calculations of nitrogen‐14 nuclear quadrupole parameters, nuclear quadrupole coupling constant, χ, and asymmetry parameter, η, of L‐His were done in two distinct environments: one as a free fully optimized molecule, an isolated molecule with the geometrical parameters taken from X‐ray, and the other in the orthorhombic and monoclinic solid states. The most probable interacting molecules with the central molecule in the crystalline phase were considered in the hexameric clusters to include hydrogen‐bonding effects in the calculations. The computations were performed with PW91P86/6‐31++G** and B3LYP6‐31++G** methods using the Gaussian 98 program. The good agreement between the nitrogen‐14 quadrupole parameters of the free His and imidazole molecules with their microwave available data demonstrates that the applied level of theory and the 6‐31++G** basis set are suitable to obtain reliable electric field gradient values. In the solid state, the shifts of quadrupole coupling parameters from the monomer to the solid phase are reasonably well reproduced for the amino and imino sites of imidazole ring in a hexameric cluster. That implies the fact that the hexameric cluster worked effectively to generate the results which are compatible with the experiment. The quadrupole coupling constant values of –N⁺H₃ group are in fair agreement with the experiment. This discrepancy is due to the absences of vibrational effects and the rotation of –N⁺H₃ group around N–C(α) bond. Copyright © 2012 John Wiley & Sons, Ltd.     

Calculation of binary magnetic properties and potential energy curve in xenon dimer: second virial coefficient of (129)Xe nuclear shielding

Quantum chemical calculations of the nuclear shielding tensor, the nuclear quadrupole coupling tensor, and the spin-rotation tensor are reported for the Xe dimer using ab initio quantum chemical methods. The binary chemical shift delta, the anisotropy of the shielding tensor Delta sigma, the nuclear quadrupole coupling tensor component along the internuclear axis chi( parallel ), and the spin-rotation constant C( perpendicular ) are presented as a function of internuclear distance. The basis set superposition error is approximately corrected for by using the counterpoise correction (CP) method. Electron correlation effects are systematically studied via the Hartree-Fock, complete active space self-consistent field, second-order M?ller-Plesset many-body perturbation, and coupled-cluster singles and doubles (CCSD) theories, the last one without and with noniterative triples, at the nonrelativistic all-electron level. We also report a high-quality theoretical interatomic potential for the Xe dimer, gained using the relativistic effective potential/core polarization potential scheme. These calculations used valence basis set of cc-pVQZ quality supplemented with a set of midbond functions. The second virial coefficient of Xe nuclear shielding, which is probably the experimentally best-characterized intermolecular interaction effect in nuclear magnetic resonance spectroscopy, is computed as a function of temperature, and compared to experiment and earlier theoretical results. The best results for the second virial coefficient, obtained using the CCSD(CP) binary chemical shift curve and either our best theoretical potential or the empirical potentials from the literature, are in good agreement with experiment. Zero-point vibrational corrections of delta, Delta sigma, chi (parallel), and C (perpendicular) in the nu=0, J=0 rovibrational ground state of the xenon dimer are also reported.