31P NMR Chemical Shifts Research Articles

NMR spectroscopic study of organic phosphate esters coprecipitated with calcite

Organic phosphorus incorporated in calcite during laboratory precipitation experiments and in natural cave deposits was investigated by solid-state NMR spectroscopy. For calcite precipitated in the presence of organic phosphoesters of varying size and functionality, solid-state 31P{1H} CP/MAS NMR shows that the phosphoesters were incorporated intact into the solid. Systematic changes in the 31P NMR chemical shift of the phosphate group were observed between the solid phosphoester and that incorporated in the solid precipitate, yielding 31P NMR chemical shifts of the coprecipitates in the range of +1.8 to −2.2ppm. These chemical shifts are distinct from that of similarly prepared calcite coprecipitated with inorganic phosphate, 3.5ppm. Only minor changes were noted in the phosphoester 31P chemical shift anisotropy (CSA) which suggests no significant change in the local structure of the phosphate group, which is dominated by C–O–P bonding. Close spatial proximity of the organic phosphate group to calcite structural components was revealed by 31P/13C rotational echo double resonance (REDOR) experiments for coprecipitates prepared with 13C-labeled carbonate. All coprecipitates showed significant 31P dephasing effects upon 13C-irradiation, signaling atomic-scale proximity to carbonate carbon. The dephasing rate for smaller organophosphate molecules is similar to that observed for inorganic phosphate, whereas much slower dephasing was observed for larger molecules having long and/or bulky side-chains. This result suggests that small organic molecules can be tightly enclosed within the calcite structure, whereas significant structural disruption required to accommodate the larger organic molecules leads to longer phosphate–carbonate distances. Comparison of 31P NMR spectroscopic data from the synthetic coprecipitates with those from calcite moonmilk speleothems indicates that phosphorus occurs mainly as inorganic orthophosphate in the natural deposits, although small signals occur with characteristics consistent with phosphate monoesters. The results of this study indicate that trace- to minor concentrations of dissolved organic molecules can be effectively taken up during calcite precipitation and incorporated in the structure, leaving a resilient record of materials present during crystallization.

Assessing Phosphine–Chalcogen Bond Energetics from Calculations

Phosphine chalcogenides are useful reagents in chalcogen atom transfer reactions and nanocrystal syntheses. Understanding the strength and electronic structure of these bonds is key to optimizing their use, but a limited number of experimental and computational studies probe these issues. Using density functional theory (DFT), we computationally screen multiple series of trisubstituted phosphine chalcogenide molecules with a variety of phosphorus substituents and examine how these affect the strength of the phosphorus–chalcogen bond. DFT provides valuable data on these compounds including P–E bond dissociation energies, P–E bond order, Lowdin charge on phosphorus and chalcogen atoms, and molecular geometries. Experimentally monitoring the 31P and 77Se NMR chemical shifts and published Hammett constants provides good estimates and confirmation of the relative magnitude of electronic shielding around these nuclei and confirms the predictive value of the computational results.

Biotransformations of 2-hydroxy-2-(ethoxyphenylphosphinyl)acetic acid and the determination of the absolute configuration of all isomers

2-Hydroxy-2-(ethoxyphenylphosphinyl)acetic acid, a new type of organophosphorus compound possessing two stereogenic centers, was investigated. Racemic 2-butyryloxy-2-(ethoxyphenylphosphinyl)acetic acid was synthesized and hydrolyzed using four bacterial species as biocatalysts. In all cases the reaction was more or less stereoselective and isomers bearing a phosphorus atom with an (SP)-configuration were hydrolyzed preferentially. The observed 1H and 31P NMR chemical shifts of Mosher esters of 2-hydroxy-2-(ethoxyphenylphosphinyl)acetic acid were correlated with the configurations of both stereogenic centers of all four stereoisomers.

ChemInform Abstract: Structural Characterization of Frustrated Lewis Pairs and Their Reaction Products Using Modern Solid‐State NMR Spectroscopy Techniques

AbstractReview: [45 refs.

Frontispiece: Unusual Bonding and Properties in Main Group Element Chemistry: Rational Synthesis, Characterization, and Experimental Electron Density Determination of Mixed‐Valent Tetraphosphetes

Phosphorus HeterocyclesExceptional bonding and molecular properties are observed in mixed‐valent tetraphosphetes now accessible by a simplified synthetic protocol. Besides anisotropic absorption of visible light and remarkable 31P NMR chemical shifts, the dispirocyclic compounds combine bis(ylidic) properties with elements of non‐classical π‐bonding. For more details, see the Full Paper by V. Breuers, W. Frank and C. W. Lehmann on page 4596 ff.

Structural Characterization of Frustrated Lewis Pairs and Their Reaction Products Using Modern Solid‐State NMR Spectroscopy Techniques

AbstractFrustrated Lewis pair (FLP) chemistry has provided a new strategy for small‐molecule binding and/or catalytic activation. The most prominent FLPs are based on intramolecular phosphaneborane adducts, the catalytic properties of which can be tailored over wide ranges of reactivity and selectivity. Advanced solid‐state NMR spectroscopic techniques, together with DFT calculations, can provide new structural insights in these systems. This review illustrates the utility of 31P and 11B NMR chemical shifts, 11B electric field gradient tensors, and 31P11B indirect and direct dipoledipole interactions for characterizing intramolecular boranephosphane FLPs. We demonstrate the potential of this method to 1) quantify the extent of boronphosphorus bonding interactions (and hence, the “degree of frustration”); 2) reveal specific structural details (i.e., boronphosphorus distances and other local geometric aspects) related to their catalytic activities; and (3) characterize products of FLP reactions with regard to molecular structure, stereochemistry, and aggregation properties in terms of internuclear distances, bonding connectivities, and orientational parameters.

Synthesis and Characterization of New Pyrospirophosphoranes Containing a P-O-P Bond by the Atherton-Todd Reaction

The classical Atherton–Todd reaction has been successfully applied to the synthesis of pentacoordinate pyrospirophosphoranes containing a P-O-P bond under mild conditions. The absolute configurations of the products were confirmed by X-ray diffraction analysis and spectroscopic identification. And 31P NMR chemical shifts of the products were correlated with stereochemistry. A possible mechanism for explaining the stereochemistry of the reaction was proposed, and it could well explain the results of the reactions of different pentacoordinate hydrospirophosphoranes.

N‐Heterocyclic Carbene–Phosphinidyne Transition Metal Complexes

AbstractThe N‐heterocyclic carbene–phosphinidene adduct IPrPSiMe3 is introduced as a synthon for the preparation of terminal carbene–phosphinidyne transition metal complexes of the type [(IPrP)MLn] (MLn=(η6‐p‐cymene)RuCl) and (η5‐C5Me5)RhCl). Their spectroscopic and structural characteristics, namely low‐field 31P NMR chemical shifts and short metal–phosphorus bonds, show their similarity with arylphosphinidene complexes. The formally mononegative IPrP ligand is also capable of bridging two or three metal atoms as demonstrated by the preparation of bi‐ and trimetallic RuAu, RhAu, Rh2, and Rh2Au complexes.

31P NMR Spectroscopically Quantified Hydrogen‐Bonding Strength of Thioureas and Their Catalytic Activity in Diels–Alder Reactions

AbstractThe hydrogen‐bonding strength of a variety of commonly employed thiourea catalysts was quantified by using a trialkylphosphine oxide as a 31P NMR probe. Simple diarylthioureas and more complex bifunctional amine‐ and hydroxy‐substituted thiourea derivatives were examined. Their catalytic activity was determined in a Diels–Alder reaction, and the obtained pseudo‐first‐order rate constants were correlated with the 31P NMR chemical shifts. A linear correlation between both variables was observed throughout the functionalized thioureas. The 31P NMR probe correlation fared better in comparison to a pKa correlation. Accordingly, the quantification presented herein by using a 31P NMR probe offers an elegant way to estimate the catalytic activity of thiourea catalysts in hydrogen‐bond‐activated reactions such as the Diels–Alder reaction.

Towards the versatile DFT and MP2 computational schemes for 31P NMR chemical shifts taking into account relativistic corrections.

The main factors affecting the accuracy and computational cost of the calculation of (31)P NMR chemical shifts in the representative series of organophosphorous compounds are examined at the density functional theory (DFT) and second-order Møller-Plesset perturbation theory (MP2) levels. At the DFT level, the best functionals for the calculation of (31)P NMR chemical shifts are those of Keal and Tozer, KT2 and KT3. Both at the DFT and MP2 levels, the most reliable basis sets are those of Jensen, pcS-2 or larger, and those of Pople, 6-311G(d,p) or larger. The reliable basis sets of Dunning's family are those of at least penta-zeta quality that precludes their practical consideration. An encouraging finding is that basically, the locally dense basis set approach resulting in a dramatic decrease in computational cost is justified in the calculation of (31)P NMR chemical shifts within the 1-2-ppm error. Relativistic corrections to (31)P NMR absolute shielding constants are of major importance reaching about 20-30 ppm (ca 7%) improving (not worsening!) the agreement of calculation with experiment. Further better agreement with the experiment by 1-2 ppm can be obtained by taking into account solvent effects within the integral equation formalism polarizable continuum model solvation scheme. We recommend the GIAO-DFT-KT2/pcS-3//pcS-2 scheme with relativistic corrections and solvent effects taken into account as the most versatile computational scheme for the calculation of (31)P NMR chemical shifts characterized by a mean absolute error of ca 9 ppm in the range of 550 ppm.

Nuclear magnetic resonance study of the stoichiometry and stability of several HgX2–acetylemethylentriparatolylphosphorane complexes in various acetone–dimethylformamide mixtures

The reaction of phosphorus ylide, acetylmethylenetriparatolylphosphorane (L) with HgX2 (X = Cl, Br, I and NO3) in equimolar ratios using acetone and dimethylformamide as solvents leads to binuclear products. 31P NMR spectroscopy was used to investigate the stoichiometry and stability of a HgX2 complex with CH3COCHP(p-tolyl)3 (L) in binary acetone–dimethylformamide mixtures of varying composition. In all cases studied, the variation of 31P NMR chemical shift with the [HgX2]/[L] mole ratio indicated the formation of 1:1 complexes. The formation constants of the resulting complexes were evaluated from computer fitting of the mole ratio data to an equation that relates the observed chemical shifts to the formation constant. In all mercuric salts used, the stabilities of the resulting 1:1 complexes varied in the order Hg(NO3)2 > HgCl2 > HgBr2 > HgI2. It was found that, in the case of all complexes, an increase in the percentage of dimethylformamide in the solvent mixtures significantly decreases the stability of the complexes.

Complexation of the [Rh(H2O)6]3+ ion with phosphoric acid as probed by 31P NMR

The kinetics of the reaction [Rh(H2O)6]3+ + H3PO4 ↔ [Rh(H2O)5H2PO4]2+ + H3O+ has been studied by 31P NMR; Ea = 142 ± 12 kJ/mol, logA = 17 ± 2. An empirical dependence of the 31P NMR chemical shift on the equilibrium pH value has been found. The acid dissociation constant of the coordinated ion has been evaluated: pK = 1.5. The 31P NMR chemical shifts of individual [Rh(H2O)5H2PO4]2+ and [Rh(H2O)5HPO4]+ complex ions are, respectively, 14.5 and 15.8 ppm at 323 K.

Crystal structure, spectroscopic investigations and quantum chemical calculational studies of N-diphenylphosphino-4-methylpiperidine sulfide

The title molecule, N-diphenylphosphino-4-methylpiperidine sulfide (I), has been synthesized and characterized by elemental analysis, 1H NMR, 31P NMR, IR and X-ray single-crystal determination. The molecular geometry from X-ray determination, vibrational frequencies and gauge, including atomic orbital (GIAO) 1H NMR and 31P NMR chemical shift values of the title compound (I) in the ground state have been calculated using the density functional theory with the 6-31G(d), 6-31G(d,p) and 6-311G(d,p) basis sets. The calculated results show that the optimized geometry can well reproduce the crystal structure, and theoretical vibrational frequencies and chemical shift values show good agreement with experimental values. The predicted nonlinear optical properties of the title compound are greater than those of urea. In addition, DFT calculations of the molecular electrostatic potentials, frontier molecular orbitals of the title compound were carried out at the B3LYP/6-31G(d) level of theory.

Mechanistic Insights into the Role of Alkylamine in the Synthesis of CdSe Nanocrystals

This paper reports a detailed mechanistic study of the effect of alkylamine on the synthesis of CdSe nanocrystals. Alkylamines are one of the most important additives for the synthesis of colloidal semiconductor nanocrystals. However, their effect on the monomer production as well as nanocrystal nucleation and growth are not well understood, as indicted by inconsistent and contradictory conclusions in the literature. We found that alkylamines slow down the reaction between cadmium oleate and trialkyl phosphine selenide by binding to cadmium and preventing the activation of trialkyl phosphine selenide. A linear correlation was observed between the observed reaction rate constant and the (31)P NMR chemical shift or (1)J(P-Se) of phosphine selenide. In the presence of alkylamine, an alkylaminophosphonium intermediate was observed. Mechanistic study suggests that the cleavage of P═Se bond is through nucleophilic attack by carboxylate instead of alkylamine. Interestingly, although alkylamines decrease the rate of monomer production, it increases the rate of CdSe nanocrystal growth. Although seemingly contradictory, this is due to a drastic decrease in the nanocrystal nucleation events in the presence of alkylamines. As a result, each nucleus is fed with more monomers and grows faster in the presence of alkylamine than in its absence.

NMR spectroscopy on flavins and flavoproteins.

(1)H-, (11)B-, (13)C-, (15)N-, (17)O-, (19)F-, and (31)P-NMR chemical shifts of flavocoenzymes and derivatives of it, as well as of alloxazines and isoalloxazinium salts, from NMR experiments performed under various experimental conditions (e.g., dependence of the chemical shifts on temperature, concentration, solvent polarity, and pH) are reported. Also solid-state (13)C- and (15)N-NMR experiments are described revealing the anisotropic values of corresponding chemical shifts. These data, in combination with a number of coupling constants, led to a detailed description of the electronic structure of oxidized and reduced flavins. The data also demonstrate that the structure of oxidized flavin can assume a configuration deviating from coplanarity, depending on substitutions in the isoalloxazine ring, while that of reduced flavin exhibits several configurations, from almost planar to quite bended. The complexes formed between oxidized flavin and metal ions or organic molecules revealed three coordination sites with metal ions (depending on the chemical nature of the ion), and specific interactions between the pyrimidine moiety of flavin and organic molecules, mimicking specific interactions between apoflavoproteins and their coenzymes. Most NMR studies on flavoproteins were performed using (13)C- and (15)N-substituted coenzymes, either specifically enriched in the pterin moiety of flavin or uniformly labeled flavins. The chemical shifts of free flavins are used as a guide in the interpretation of the chemical shifts observed in flavoproteins. Although the hydrogen-bonding pattern in oxidized and reduced flavoproteins varies considerably, no correlation is obvious between these patterns and the corresponding redox potentials. In all reduced flavoproteins the N(1)H group of the flavocoenzyme is deprotonated, an exception is thioredoxin reductase. Three-dimensional structures of only a few flavoproteins, mostly belonging to the family of flavodoxins, have been solved. Also the kinetics of unfolding and refolding of flavodoxins has been investigated by NMR techniques. In addition, (31)P-NMR data of all so far studied flavoproteins and some (19)F-NMR spectra are discussed.

⁹Be and ³¹P NMR analyses on the influence of imino groups on Be²⁺ complex stabilities of a series of cyclo-μ-imido triphosphate anions.

The complexation behaviors of Be²⁺ with cyclo-μ-imido triphosphate anions, cP₃O₉-n(NH)n(3-)n= 1, 2),have been investigated by both ⁹Be and ³¹P NMR techniques at -2.3 °C in order to clarify the coordination structures of the complexes. The spectra showed that cP₃O₉n(NH)n (n = 1, 2) ligands form ML, ML₂, and M₂L complexes with Be²⁺ ions, and the formation of complexes coordinating with nitrogen atoms of the cyclic framework in the ligand molecule has been excluded. These complexation trends are very similar to those of Be²⁺-cP₃O₆(NH)⁻³₃system, which has been reported by us. The peak deconvolution of ⁹BeNMR spectra made these beryllium complexes amenable to stability constant determinations. The stability constants of the complexes increase with an increase in the protonation constants of the ligands as the number of imino groups, which constitute the ligand molecules, is ascended. This increase is primarily attributable to the lower electronegativity of nitrogen atoms than oxygen atoms, which are directly bonded to central phosphorus atoms; moreover, tautomerism equilibrium in the entire of the imidopolyphosphate molecule is also responsible to the higher basicity. ³¹P NMR spectra measured concurrently have verified the formation of the complexes estimated by the ⁹Be NMR measurement. Intrinsic ³¹P NMR chemical shift values of the phosphorus atoms belonging to ligand molecules complexed with Be²⁺ cations have been determined. Not only the protonation constants but also the stability constants of all Be²⁺ complexes increase approximately linearly with an increase in the number of imino groups.

Effect of Noncovalent Interactions on the 31P Chemical Shift Tensor of Phosphine Oxides, Phosphinic, Phosphonic, and Phosphoric Acids, and Their Complexes with Lead(II)

The effect of noncovalent interactions such as C–H···O and O–H···O hydrogen bonds and coordination to Pb2+ on the 31P NMR chemical shift tensor in the titled compounds has been studied experimentally and simulated theoretically using the density functional theory gauge-invariant atomic orbital (DFT-GIAO) approach. It has been shown that only in few cases the most suitable measure of these interactions is the isotropic 31P NMR chemical shift. In contrast, the analysis of the anisotropy and the principal components of the 31P NMR chemical shift tensor can be very useful to discriminate between different interactions and to characterize their properties. A great advantage is that these NMR parameters can be correctly simulated at relatively inexpensive levels of theory, namely, B3LYP/cc-pVDZ and B3LYP/6-311G**. Thus, a combination of 31P NMR and time-efficient calculations can be used to study the structural pattern in polycrystalline and noncrystalline materials. An illustrative result is the demonstration of the similarity of the unknown crystal structures of methylphosphonic acid, diphenyl hydrogen phosphate, and lead(II) methylphosphinate to the known crystal structures of tert-butylphosphonic acid, bis(4-chlorophenyl) hydrogen phosphate, and lead(II) vinylphosphonate, respectively. The scope of this approach is wide and covers the study of the local chemical structures in amorphous materials, at surfaces and interfaces.

Hydrogen Bond and Proton Transport in Acid–Base Complexes and Amphoteric Molecules by Density Functional Theory Calculations and 1H and 31P Nuclear Magnetic Resonance Spectroscopy

Intermolecular and intramolecular hydrogen bond (H-bond) and proton transport in acid-base complexes and amphoteric molecules consisting of phosphonic acid groups and nitrogenous heterocyclic rings are investigated by density functional theory calculations and (1)H NMR and (31)P NMR spectroscopy. It is concluded that a phosphonic acid group can act both as H-bond donor and H-bond acceptor, while an imine nitrogen atom can only act as H-bond acceptor and an amine group as H-bond donor. And the intramolecular H-bond is weaker than the intermolecular H-bond attributing to configurational restriction. In addition, the strongest H-bond interaction is observed between a phosphonic acid and a 1H-indazole because of the formation of double H-bonds. The (1)H NMR and (31)P NMR chemical shifts for the acid-base complexes are consistent with the density functional theory calculations. From the (1)H NMR chemical shifts, fast proton exchange is observed between a phosphonic acid and 1H-benzimidazole or 1H-indazole. Finally, it is proposed that polymeric material tethered with 1H-benzimidazole or 1H-indazole rings is a favorable component for high-temperature proton exchange membranes based on acid-base complexes or acid-base amphoteric molecules.

Lewis acid properties of some metal salts for lactic acid formation in water: 31P NMR spectroscopy with trimethylphosphine oxide as a molecular probe

31P NMR spectroscopy was applied to the evaluation of water-tolerant Lewis acid catalysis of various metal triflates and chlorides in water using trimethylphosphine oxide (TMPO) as a probe molecule. Direct interaction of the TMPO molecule with the Lewis acid in water changes the original 31P NMR chemical shift and line width. ScCl3, Sc(OTf)3 and In(OTf)3 exhibit larger changes in the 31P chemical shift by the formation of a Lewis acid–TMPO complex than other triflates and chlorides. This is due to strong interaction between the Lewis acid and TMPO, which leads to the slow exchange between TMPO and H2O in ScCl3, Sc(OTf)3 and In(OTf)3. Lewis acids with large change in the 31P chemical shift exhibit high catalytic activities for lactic acid formation from 1,3-dihydroxyacetone (1.3-DHA) and pyruvic aldehyde in water: Strong interaction with TMPO on the Lewis acid is a dominant factor for lactic acid formation.

Speciation of VX in Aqueous Solutions

31P and 13C NMR studies of the toxic chemical warfare agent VX [O-ethyl S-(2-N,N′-(diisopropylamino)ethyl) methylphosphonothioate] have been used to understand the reactivity in a wide variety of matrices. The presence of protonated VX has been invoked in many mechanisms of VX degradation and decontamination. A change of speciation of VX has been justified by a relatively large shift in the 31P NMR chemical shift from about 55 ppm (in neat material or organic solvents) to >60 ppm (in aqueous solutions). This study shows that in acetonitrile/water mixtures and in aqueous solutions at a range of pH values, the 31P NMR resonance was sensitive to the percentage water in the mixture, but was not sensitive to the pH of the solution. This result indicates that the 31P chemical shift to >60 ppm is not associated with protonated VX, which is only present at pH < 8. However, the 13C NMR resonances did respond to changes in the pH of the solution, indicating that these shifts are associated with protonation of the V...