NMR study of molecular reorientations in NH4H2AsO4

Zeeman (T_1Z) and dipolar (T_1D) spin-lattice relaxation times of protons in NH₄H₂AsO₄ were measured as a function of temperature. The existence of a slow motion (τ ≈ 10^?3 see) is established, which is most probably a low frequency hindered reorientation of H₂AsO₄ groups. This motion is slowed down below the Curie point T_c. A sharp increase of the dipolar relaxation rate above T = 314°K indicates the possibility of a new high temperature phase transition in this compound.     

Conformations and resulting hydrogen‐bonded networks of hydrogen {phosphono[(pyridin‐1‐ium‐3‐yl)amino]methyl}phosphonate and related 2‐chloro and 6‐chloro derivatives

In the crystal structures of the conformational isomers hydrogen {phosphono[(pyridin‐1‐ium‐3‐yl)amino]methyl}phosphonate monohydrate (pro‐E), C₆H₁₀N₂O₆P₂·H₂O, (Ia), and hydrogen {phosphono[(pyridin‐1‐ium‐3‐yl)amino]methyl}phosphonate (pro‐Z), C₆H₁₀N₂O₆P₂, (Ib), the related hydrogen {[(2‐chloropyridin‐1‐ium‐3‐yl)amino](phosphono)methyl}phosphonate (pro‐E), C₆H₉ClN₂O₆P₂, (II), and the salt bis(6‐chloropyridin‐3‐aminium) [hydrogen bis({[2‐chloropyridin‐1‐ium‐3‐yl(0.5+)]amino}methylenediphosphonate)] (pro‐Z), 2C₅H₆ClN₂⁺·C₁₂H₁₆Cl₂N₄O₁₂P₄²⁻, (III), chain–chain interactions involving phosphono (–PO₃H₂) and phosphonate (–PO₃H⁻) groups are dominant in determining the crystal packing. The crystals of (Ia) and (III) comprise similar ribbons, which are held together by N—H...O interactions, by water‐ or cation‐mediated contacts, and by π–π interactions between the aromatic rings of adjacent zwitterions in (Ia), and those of the cations and anions in (III). The crystals of (Ib) and (II) have a layered architecture: the former exhibits highly corrugated monolayers perpendicular to the [100] direction, while in the latter, flat bilayers parallel to the (001) plane are formed. In both (Ib) and (II), the interlayer contacts are realised through N—H...O hydrogen bonds and weak C—H...O interactions involving aromatic C atoms.     

Sterically constrained tricyclic phosphine: redox behaviour,reductive and oxidative cleavage of P–C bonds,generation of a dilithium phosphaindole as a promising synthon in phosphine chemistry

The redox behaviour of sterically constrained tricyclic phosphine 3a was investigated by spectroelectrochemistry. The data suggested a highly negative reduction potential with the reversible formation of a dianionic species. Accordingly, 3a reacted with two equivalents of Li/naphthalene by reductive cleavage of a P–C bond of one of the PC₄ heterocycles. The resulting dilithium compound 5 represents a phosphaindole derivative with annulated aromatic C₆ and PC₄ rings. It is an interesting starting material for the synthesis of new heterocyclic molecules, as was shown by treatment with Me₂SiCl₂ and PhPCl₂. The structures of the products (6 and 7) formally reflect ring expansion by insertion of silylen or phosphinidene fragments into a P–C bond of 3a. Treatment of 3a with H₂O₂ did not result in the usually observed transfer of a single O atom to phosphorus, but oxidative cleavage of a strained PC₄ ring afforded a bicyclic phosphinic acid, R₂PO₂H.

Sterically constrained tricyclic phosphines with annulated five- and six-membered rings show fascinating chemical and redox reactivity as indicated by sophisticated in situ UV-vis CV and multi-pulse chronoamperometry.     

A ladder coordination polymer based on Ca2+ and (4,5‐dicyano‐1,2‐phenylene)bis(phosphonic acid): crystal structure and solution‐state NMR study

The preparation of coordination polymers (CPs) based on either transition metal centres or rare‐earth cations has grown considerably in recent decades. The different coordination chemistry of these metals allied to the use of a large variety of organic linkers has led to an amazing structural diversity. Most of these compounds are based on carboxylic acids or nitrogen‐containing ligands. More recently, a wide range of molecules containing phosphonic acid groups have been reported. For the particular case of Ca²⁺‐based CPs, some interesting functional materials have been reported. A novel one‐dimensional Ca²⁺‐based coordination polymer with a new organic linker, namely poly[[diaqua[μ₄‐(4,5‐dicyano‐1,2‐phenylene)bis(phosphonato)][μ₃‐(4,5‐dicyano‐1,2‐phenylene)bis(phosphonato)]dicalcium(II)] tetrahydrate], {[Ca₂(C₈H₄N₂O₆P₂)₂(H₂O)₂]·4H₂O}_n, has been prepared at ambient temperature. The crystal structure features one‐dimensional ladder‐like _∞¹[Ca₂(H₂cpp)₂(H₂O)₂] polymers [H₂cpp is (4,5‐dicyano‐1,2‐phenylene)bis(phosphonate)], which are created by two distinct coordination modes of the anionic H₂cpp²⁻ cyanophosphonate organic linkers: while one molecule is only bound to Ca²⁺ cations via the phosphonate groups, the other establishes an extra single connection via a cyano group. Ladders close pack with water molecules through an extensive network of strong and highly directional O—H…O and O—H…N hydrogen bonds; the observed donor–acceptor distances range from 2.499 (5) to 3.004 (6) Å and the interaction angles were found in the range 135–178°. One water molecule was found to be disordered over three distinct crystallographic positions. A detailed solution‐state NMR study of the organic linker is also provided.     

A pH‐responsive cyclopolymer having phospho‐ and sulfopropyl pendents in the same repeating unit: Synthesis,characterization, and its application as an antiscalant

A new zwitterionic monomer 3‐[diallyl{3‐(diethoxyphosphoryl)propyl}ammonio]propane‐1‐sulfonate has been synthesized and cyclopolymerized to give the corresponding polyzwitterion (±) (PZ) bearing both phosphonate and sulfonate functionalities on each repeating unit. phosphonate ester hydrolysis in PZ gave a pH‐responsive dibasic polyzwitterionic acid (±) (PZA) bearing ? PO₃H₂ units. The PZA under pH‐induced transformation was converted into polyzwitterion/anion (± ?) (PZAN) and polyzwitterion/dianion (± =) (PZDAN) having respective ? PO₃H^? and ? PO₃^2? units. The polymers′ interesting solubility and viscosity behaviors have been investigated in detail. The apparent protonation constants in salt‐free water and 0.1 M NaCl of the ? PO₃^2? in (± =) (PZDAN) and ? PO₃H^– in (± ?) (PZAN) as well as in their corresponding monomeric units have been determined. Evaluation of antiscaling properties of the PZA using supersaturated solution of CaSO₄ revealed ≈100% scale inhibition efficiency at a meager concentration of 20 ppm for a duration of 45 h at 40 °C. The PZA has the potential to be used effectively as an antiscalant in reverse osmosis plant. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013 , 51, 5130–5142     

Investigation at the chain segmental level of the miscibility of poly(vinyl chloride)/atactic poly(methyl methacrylate) blends

We employed high‐resolution ¹³C cross‐polarization/magic‐angle‐spinning/dipolar‐decoupling NMR spectroscopy to investigate the miscibility and phase behavior of poly(vinyl chloride) (PVC)/poly(methyl methacrylate) (PMMA) blends. The spin–lattice relaxation times of protons in both the laboratory and rotating frames [T₁(H) and T_1ρ(H), respectively] were indirectly measured through ¹³C resonances. The T₁(H) results indicate that the blends are homogeneous, at least on a scale of 200–300 Å, confirming the miscibility of the system from a differential scanning calorimetry study in terms of the replacement of the glass‐transition‐temperature feature. The single decay and composition‐dependent T_1ρ(H) values for each blend further demonstrate that the spin diffusion among all protons in the blends averages out the whole relaxation process; therefore, the blends are homogeneous on a scale of 18–20 Å. The microcrystallinity of PVC disappears upon blending with PMMA, indicating intimate mixing of the two polymers. © 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 39: 2390–2396, 2001     

Disorder in the lactato group of (lactato‐O,O′)bis(triphenylphosphine‐P)silver(I) corroborated by 31P two‐dimensional CPCOSY NMR

The Ag atom in the title compound, [Ag(C₃H₅O₃)(C₁₈H₁₅P)₂], is bonded to the P atoms of two triphenylphosphine ligands and to the two O atoms of the carboxyl unit of the lactato group in a distorted tetrahedral environment. The lactato anion is disordered in the methyl and hydroxyl groups; the 1:1 disorder is corroborated by two‐dimensional ³¹P CPCOSY (cross‐polarization, correlation spectroscopy) NMR.     

Accessibility and Solid State NMR Studies on Sol‐Gel Processed Diphosphine Ligands

Novel xerogels X1 a–d were obtained by sol‐gel processing of the monomeric T‐functionalized diphosphine ligand (MeO)₃Si(CH₂)₆CH[CH₂PPh₂]₂ [1(T⁰)] with various amounts of the co‐condensing agents MeSi(OMe)₂(CH₂)₆(OMe)₂SiMe (D⁰–C₆–D⁰) and MeSi(OMe)₂(CH₂)₃(C₆H₄)(CH₂)₃(OMe)₂SiMe [Ph(1,4‐C₃D⁰)₂] . ²⁹Si CP/MAS NMR spectroscopic investigations were applied to probe the matrices and their degree of condensation. The integrity of the hydrocarbon backbone and diphosphine moiety was examined by means of solid state NMR spectroscopy (¹³C, ³¹P). To study the dynamics of the matrices and the phosphorus centers detailed measurements of relaxation time (T_1ρH) and cross polarization constants (T_SiH, T_PH) were carried out. The accessibility of the polysiloxane‐supported diphosphines was scrutinized by some typical phosphine reactions. It was found that reagents such as H₂O₂, MeI as well as bulky molecules like (NBD)Mo(CO)₄ or (COD)PdCl₂ are able to reach all phosphorus centers independent on the kind of the backbone of the matrix. SEM micrographs show the morphology of the hybrid materials and energy dispersive X‐ray spectroscopy (EDX) suggest that the distribution of the elements agree with the applied composition.     

Solid‐state NMR study on changes of phosphate and proton species in metal pyrophosphate composite (MP2O7–MO2) ceramics

In order to investigate the effects of composite production and the role of the counter cation for metal phosphate conductors, changes in the solid‐state nuclear magnetic resonance (NMR) spectra and magnetic relaxation times caused by the removal of volatile products and water washing were examined for various metal pyrophosphate (MP₂O₇–MO₂; M = Sn, Si, Ti, and Zr). Acidic species could be detected by ¹H MAS NMR spectra for all the composites except ZrP₂O₇–ZrO₂ that had the lowest conductivity. The ³¹P DD‐MAS NMR spectra for MP₂O₇–MO₂ composites showed different signal patterns depending on the counter cations participating in the ion exchange as a result of different microstructures. Combinational analysis of ³¹P DD‐MAS and ³¹P CP‐MAS NMR spectra of the composites indicated that protonic bulk phosphates were observed at slightly lower fields than non‐protonic bulk phosphates in all of the MP₂O₇–MO₂ composites. After water washing, the acidic species and the protonic bulk phosphates of MP₂O₇–MO₂ composites disappeared or were reduced to trace amounts. The T₁H values of the water‐washed composites lengthened because of removal of orthophosphoric acid, although the T₁P values remained almost unchanged. The results of the solid‐state NMR studies suggest that the protonic bulk phosphates of MP₂O₇–MO₂ composites do not generally distribute in the bulk but exist in the interface between excess H₃PO₄ and the bulk. Copyright © 2016 John Wiley & Sons, Ltd.     

On phosphorus chemical shift anisotropy in metal (IV) phosphates and phosphonates: A 31P NMR experimental study

The phosphorus chemical shift anisotropies, ³¹PΔcs, and asymmetry parameters η were measured by the ³¹P{¹H} NMR experiments in static and low-frequency spinning samples of the zirconium phosphates and phosphonates and also in the mixed Zr (IV)/Sn (IV) phosphate/phosphonate material. The data obtained have shown a 111 connectivity in the HPO₄ and PO₃ groups, which does not change at modification and intercalation of the materials. The ³¹PΔcs values of the phosphonate groups (43–49 ppm) significantly surpass the values characterizing the HPO₄ groups (23–37 ppm). The ³¹P Δcs values obtained for the metal (IV) phosphates were discussed in terms of P-O distances. The ³¹P chemical shift anisotropy parameters can help at elucidation of local structures in phosphate and phosphonate materials.     

A Soluble Phosphorus‐Centered Keggin Polyoxoniobate with Bicapping Vanadyl Groups

A water‐soluble tetramethylammonium (TMA) salt of a novel Keggin‐type polyoxoniobate has been isolated as TMA₉[PV₂Nb₁₂O₄₂]?19H₂O ( 1 ). This species contains a central phosphorus site and two capping vanadyl sites. Previously only a single example of a phosphorus‐containing polyoxoniobate, [(PO₂)₃PNb₉O₃₄]^15?, was known, which is a lacunary Keggin ion decorated with three PO₂ units. However, that cluster was isolated as an insoluble structure consisting of chains linked by sodium counterions. In contrast, the [PV₂Nb₁₂O₄₂]^9? cluster in 1 is stable over a wide pH range, as evident by ³¹P and ⁵¹V NMR, UV/Vis spectroscopy, and ESI‐MS spectrometry. The ease of substitution of phosphate into the central tetrahedral position suggests that other oxoanions can be similarly substituted, promising a richer set of structures in this class.     

Synthesis and Characterization of the Mixed‐Linker Copper(II) Coordination Polymer [Cu(HO3PC6H4SO3)(C10N2H8)]·H2O

A new copper(II) phosphonatobenzenesulfonate incorporating 4,4′‐bipyridine (4,4′‐bipy) as auxiliary ligand has been discovered through systematic high‐throughput (HT) screening of the system Cu(NO₃)₂·3H₂O/H₂O₃PC₆H₄SO₃H/4,4′‐bipy using different solvents. The hydrothermal synthesis of [Cu(HO₃PC₆H₄SO₃)(C₁₀H₈N₂)]·H₂O ( 1 ) was further optimized by screening various copper(II) salts. The crystal structure of 1 was determined by single‐crystal X‐ray diffraction and unveiled the presence of isolated sixfold coordinated Jahn–Teller‐distorted Cu²⁺ ions. The isolated CuN₂O₄ octahedra are interconnected by phosphonate and sulfonate groups to form chains along the c‐axis. The organic groups, namely phenyl rings and 4,4′‐bipy molecules cross‐link the chains into a three‐dimensional framework. Water molecules are found in the narrow voids in the structure which are held by weak hydrogen bonds. Upon dehydration, the structure of 1 undergoes a phase transition, which was confirmed by TG measurements and temperature dependent X‐ray powder diffraction. The new structure of 1‐h was refined with Rietveld methods. Detailed inspection of the structure revealed the directional switching of the Jahn–Teller distortion upon de/rehydration. Weak ferro‐/ferrimagnetic interactions were observed by magnetic investigations of 1 , which switch to antiferromagnetic below 3.5 K. Compounds 1 and 1‐h are further characterized by thermogravimetric and elemental analysis as well as IR spectroscopy.     

The effects of molecular orientation on the physical aging and mobility of polycarbonate—solid state NMR and dynamic mechanical analysis

Solid‐state NMR and dynamic mechanical (DMA) measurements were performed on a series of uniaxially hot‐drawn bisphenol‐A polycarbonate samples in order to determine the effects of stretching on the structure, mobility, and local orientation environment. Proton spin‐lattice relaxation times, ¹H T_1ρ, for the phenylene carbon protons were fitted to a biexponential decay function, and both the long and short relaxation times initially increased with stretching. Intensity data indicated an increase in the number of short relaxation time protons and a decrease in the number of long relaxation protons with orientation. Similarly, DMA spectra showed that the β‐relaxation strength also increased with drawing, which implied an increase in the number of localized segmental relaxations. It is theorized that the long and short ¹H T_1ρ relate to protons within tightly packed “cooperative domains,” and to those with greater localized free‐volume, respectively. Stretching is known to distort the free‐volume distribution, causing a decrease in the mean free‐volume but an increase in the number of larger, more elliptical holes. This is expected to cause a decrease in the α‐transition mobility (due to larger cooperative domains) and an increase in the β‐mobility (due to the increase in the number of β‐relaxing segments associated with the larger free‐volume holes). These predictions are consistent with results recently reported by Shelby and Wilkes on the physical aging and creep behavior of these samples (M. D. Shelby & G. L. Wilkes, Polymer 1998, 39, 6767; M. D. Shelby & G. L. Wilkes, J Polym Sci Part B: Polym Phys 1998, 36, 2111). © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 39: 32–46, 2001     

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; E _a = 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.     

1H, 13C, 31P NMR and conformational study of 7-membered ring organophosphorus compounds. 1,3,2-dioxaphosphepanes

The ¹H, ¹³C and ³¹P NMR data of several 2-R-2-thiono-1,3-dioxa organophosphorus molecules with 7-membered rings [R = Cl, OC₆H₅, C₆H₅, CH₃, N(CH₃)₂] are reported. The conformation of the 7-membered ring is discussed by reference to the ³J(POCH) coupling constants which are compared with those observed in 6-membered 1,3,2-dioxaphosphorinanes. It is shown that caution must be exercised in using the ³J(POCH) angular dependence as a stereochemical tool. The ³¹P spin lattice relaxation times of some of these 7-membered rings have been measured and the values are discussed.     

Two phosphonodehydrotripeptides: Boc0–Gly1–Δ(Z)Phe2–α‐Abu3PO3Me2 and Boc0–Gly1–Δ(Z)Phe2–α‐Nva3PO3Et2

The present paper reports the crystal structures of two short phosphonotripeptides (one in two crystal forms) containing one ΔPhe (dehydrophenylalanine) residue, namely dimethyl (3‐{[tert‐butoxycarbonylglycyl‐α,β‐(Z)‐dehydrophenylalanyl]amino}propyl)phosphonate, Boc⁰–Gly¹–Δ(Z)Phe²–α‐Abu³PO₃Me₂, C₂₁H₃₂N₃O₇P, (I), and diethyl (4‐{[tert‐butoxycarbonylglycyl‐α,β‐(Z)‐dehydrophenylalanyl]amino}butyl)phosphonate, Boc⁰–Gly¹–Δ(Z)Phe²–α‐Nva³PO₃Et₂, as the propan‐2‐ol monosolvate 0.122‐hydrate, C₂₄H₃₈N₃O₇P·C₃H₈O·0.122H₂O, (II), and the ethanol monosolvate 0.076‐hydrate, C₂₄H₃₈N₃O₇P·C₂H₆O·0.076H₂O, (III). The crystals of (II) and (III) are isomorphous but differ in the type of solvent. The phosphono group is linked directly to the last C_α atom in the main chain for all three peptides. All the amino acids are trans linked in the main chains. The crystal structures exhibit no intramolecular hydrogen bonds and are stabilized by intermolecular hydrogen bonds only.     

2H NMR characterization of phenylene ring flip motion in poly(p-phenylene vinylene) films

Solid-state ²H quadrupole echo nuclear magnetic resonance (NMR) spectra and measurements of ²H spin lattice relaxation times have been obtained for films of poly(p-phenylene vinylene) deuterated in phenylene ring positions (PPV-d₄). NMR line shapes show that all the phenylene rings of PPV undergo 180° rotational jumps about the 1,4 ring axis (“ring flips”) at 225°C. The temperature dependence of the ²H line shapes show that the jump motion is thermally activated, with a median activation energy, E_a = 15 kcal/mol, and a distribution of activation energies of less than ±2 kcal/mol. The jump rate was also determined from the magnitude of the anisotropic T₂ relaxation associated with ²H line shapes and from the curvature of inversion recovery intensity data. The experimental activation energy for jumps is comparable to the intramolecular potential barrier for rotation about phenylene vinylene bonds. ²H NMR provides a method for determining the phenylene-vinylene rotational barrier in pristine PPV, and may potentially be used to study conjugation in conducting films.     

Solid‐state NMR study of dopant effects on the chemical properties of Mg‐, In‐, and Al‐doped SnP2O7

The effects of doped low‐valence cations on the properties of the SnP₂O₇ proton conductor at ambient temperature are investigated from changes in solid‐state NMR spectra and nuclear magnetic relaxation times. Although the T₁H values increased with decreasing acidity as a result of cation exchange, the ¹H chemical shifts moved to lower field in Al‐ and In‐doped materials compared with undoped ones. Furthermore, the shifts changed to higher field in Mg‐doped materials, suggesting the existence of different protonic species in those materials. The bulk phosphate chemical shifts in the ³¹P dipolar‐decoupling MAS NMR spectra were very similar, regardless of the nature and amount of the doping species. On the other hand, by ¹H/³¹P cross‐polarization MAS NMR, P₂O₇ signals interacting with an interstitial proton [Q¹(proton)] were observed in all the undoped and doped SnP₂O₇, while acidic P–OH‐type phosphate signals [Q¹(acid)] were additionally observed in the Mg‐doped conductor. The different affinity of the proton with the dopants and phosphates caused lower conductivity and larger activation energy in the Mg‐doped materials, compared with those in the In‐ and Al‐doped materials. Copyright © 2014 John Wiley & Sons, Ltd.     

How to measure absolute P3HT crystallinity via 13C CPMAS NMR

We outline the details of acquiring quantitative ¹³C cross‐polarization magic angle spinning (CPMAS) nuclear magnetic resonance on the most ubiquitous polymer for organic electronic applications, poly(3‐hexylthiophene) (P3HT), despite other groups' claims that CPMAS of P3HT is strictly nonquantitative. We lay out the optimal experimental conditions for measuring crystallinity in P3HT, which is a parameter that has proven to be critical in the electrical performance of P3HT‐containing organic photovoltaics but remains difficult to measure by scattering/diffraction and optical methods despite considerable efforts. Herein, we overview the spectral acquisition conditions of the two P3HT films with different crystallinities (0.47 and 0.55) and point out that because of the chemical similarity of P3HT to other alkyl side chain, highly conjugated main chain polymers, our protocol could straightforwardly be extended to other organic electronic materials. Variable temperature ¹H NMR results are shown as well, which (i) yield insight into the molecular dynamics of P3HT, (ii) add context for spectral editing techniques as applied to quantifying crystallinity, and (iii) show why T_1ρ^H, the ¹H spin–lattice relaxation time in the rotating frame, is a more optimal relaxation filter for distinguishing between crystalline and noncrystalline phases of highly conjugated alkyl side‐chain polymers than other relaxation times such as the ¹H spin–spin relaxation time, T₂^H, and the spin–lattice relaxation time in the toggling frame, T_1xz^H. A 7 ms T_1ρ^H spin lock filter, prior to CPMAS, allows for spectroscopic separation of crystalline and noncrystalline ¹³C nuclear magnetic resonance signals. Published 2016. This article is a U.S. Government work and is in the public domain in the USA.     

New Insights into Keggin‐Type 12‐Tungstophosphoric Acid from 31P MAS NMR Analysis of Absorbed Trimethylphosphine Oxide and DFT Calculations

The acid and transport properties of the anhydrous Keggin‐type 12‐tungstophosphoric acid (H₃PW₁₂O₄₀; HPW) have been studied by solid‐state ³¹P magic‐angle spinning NMR of absorbed trimethylphosphine oxide (TMPO) in conjunction with DFT calculations. Accordingly, ³¹P NMR resonances arising from various protonated complexes, such as TMPOH⁺ and (TMPO)₂H⁺ adducts, could be unambiguously identified. It was found that thermal pretreatment of the sample at elevated temperatures (≥423 K) is a prerequisite for ensuring complete penetration of the TMPO guest probe molecule into HPW particles. Transport of the TMPO absorbate into the matrix of the HPW adsorbent was found to invoke a desorption/absorption process associated with the (TMPO)₂H⁺ adducts. Consequently, three types of protonic acid sites with distinct superacid strengths, which correspond to ³¹P chemical shifts of 92.1, 89.4, and 87.7 ppm, were observed for HPW samples loaded with less than three molecules of TMPO per Keggin unit. Together with detailed DFT calculations, these results support the scenario that the TMPOH⁺ complexes are associated with protons located at three different terminal oxygen (O_d) sites of the PW₁₂O₄₀³⁻ polyanions. Upon increasing the TMPO loading to >3.0 molecules per Keggin unit, abrupt decreases in acid strength and the corresponding structural variations were attributed to the change in secondary structure of the pseudoliquid phase of HPW in the presence of excessive guest absorbate.