Nuclear relaxation in atactic polystyrenes: T1 and T1ρ measurements

Measurements have been made on a series of linear atactic polystyrenes whose molecular weights range from 900 to 1.8 × 10⁶, where M _w/M _n ? 1.2. Spin lattice relaxation times have been measured in the laboratory frame (T₁) and in the rotating frame (T_1ρ) in the temperature range 90–500°K. Two major relaxation minima were observed in both sets of measurements. The high temperature process corresponds to the glass transition (α process), the position of the minimum depending on the chain length. The low temperature process appears to originate from the n-butyl endgroups in the polymer, its position being independent of chain length while its intensity is inversely proportional to molecular weight. No other minima were observed, in contrast to some other observations made by broadline and pulsed NMR techniques. Relaxation was exponential in all cases except in the region of the high temperature T_1ρ minimum and above. This nonexponential behavior is possibly connected with the transition at T > T_g observed by a number of other techniques and which is thought to correspond to a transition between two types of liquid state. A correlation frequency diagram has been drawn for all the processes observed in polystyrene by other techniques, (α, β, αβ, γ, and δ) which shows that the T₁ and T_1ρ minimum positions correlate well with the α process and that there is a possible contribution to the relaxation due to the γ process on the low temperature side of the α process. At these measurement frequencies the α and β processes are merged into an αβ process. There is no evidence for a contribution from the mechanical δ process. The effect of the endgroups is observed to very high molecular weights (4.98 × 10⁵), and it seems that a three-dimensional diffusion model would be more adequate than the one-dimensional model used to interpret similar behavior of paraffins and polyethylenes. Measurements of T₁ in the low-temperature region would constitute a method for a rough measurement of the molecular weight of these polymers.     

Single-shot measurement of solids and liquids T1 values by a small-angle flip-flop pulse sequence

We propose the small-angle flip-flop (SAFF) pulse sequence as an alternative procedure for the rapid measurement of the ¹H spin–lattice relaxation time in the laboratory frame (T₁) of solid and liquid substances, in a time-domain NMR experiment. Based on the original flip-flop pulse sequence, this technique allows the fast estimation of T₁ values of samples that require minutes to hours of acquisition time if traditional pulse sequences are employed. We have applied SAFF to different substances, with T₁ ranging from microseconds up to seconds, including natural clays, polymers, and organic and inorganic solvents. We also demonstrate the potential of the pulse sequence in the real-time monitoring of dynamic processes, such as the conformational changes of polymeric materials during heating. The results we obtained with SAFF are comparable with those acquired with the inversion-recovery pulse sequence, with the addition of several benefits. This pulse sequence obeys steady-state and magnetization-conserving principles, making it possible to dismiss the need for relaxation delay times of the order of 5T₁. SAFF has shown high sensitivity in the resolution of individual components of T₁ in multiexponential systems and can be easily integrated to well-established pulse sequences, such as Magic Sandwich Echo and Carr–Purcell–Meiboom–Gill, for the single-shot determination of T₁ and T₂ or T_2*.     

Synthesis and VT-NMR studies of cationic hydride platinum clusters

The synthesis of three trinuclear platinum hydrides [Pt₃(L-L)₃(H)₃]⁺ (L-L = 1,2-bis(diphenylphosphino)ethane, dppe, l; 1-diphenylphosphino-2-diphenylarsinoethane, dppae. 2; 1,2-bis(diphenylarsino)ethane, dpae, 3) is reported. The complexes were characterized by IR, FAB-MS, and NMR (¹H,³¹P and¹⁹⁵Pt) spectroscopic techniques. The fast exchange of the hydride ligands, observed at ambient temperature, is frozen out at low temperature. The low-temperature¹H and³¹P NMR spectra are consistent with an open array of Pt atoms in the clusters, in keeping with a 16-electron configuration on each platinum atom. Two of the hydride ligands are terminally bonded to two metal centers, whereas the third one is µ₃-coordinated, interacting more tightly with the unique platinum atom.     

Synthesis and structures of anionic rhenium polyhydride complexes of boron–hydride ligands and their application in catalysis

The rhenium complex, [K(DME)(18-c-6)][ReH₄(Bpin)(η²-HBpin)(κ²-H₂Bpin)] 1, comprising hydride and boron ligands only, has been synthesized by exhaustive deoxygenation of the commercially available perrhenate anion (ReO₄⁻) with pinacol borane (HBpin). The structure of 1 was analysed by X-ray crystallography, NMR spectroscopy, and DFT calculations. While no hydrides were located in the X-ray crystal structure, it revealed a trigonal arrangement of pinacol boron ligands. Variable-temperature NMR spectroscopy supported the presence of seven hydride ligands but further insight was hindered by the fluxionality of both hydride and boron ligands at low temperature. Further evaluation of the structure by Ab Initio Random Structure Searching (AIRSS) identified the presence of hydride, boryl, σ-borane, and dihydroborate ligands. This complex, either isolated or prepared in situ, is a catalyst for the 1,4-hydroboration of N-heteroaromatic substrates under simple operating procedures. It also acts as a reagent for the stoichiometric C–H borylation of toluene, displaying high meta regioselectivity in the borylated products. Reaction of 1 with 9-BBN resulted in HBpin substitution to form the new anionic tetra(dihydroborate) complex [K(DME)(18-c-6)][Re(κ²-H-9-BBN)₄] 4 for which the hydride positions were clearly identified by X-ray crystallography. The method used to generate these isolable yet reactive boron–hydride complexes is direct and straightforward and has potential utility for the exploitation of other metal oxo compounds in operationally simple catalytic reactions.

Exhaustive deoxygenation of perrhenate by pinacol borane forms a new Re anion of boron and hydride ligands only that undergoes borane ligand exchange, stoichiometric C–H boration, and catalytic pyridine hydroboration.     

Characterization of the complex formation of 1,6‐anhydro‐β‐maltotriose with potassium using 1H and 39K NMR spectroscopy

The ¹H and ³⁹K longitudinal relaxation times (T₁) and ¹H diffusion coefficients were measured to investigate the complex formation of 1,6‐anhydro‐β‐maltotriose and potassium ions. Although the ¹H‐T₁ values of H3′, H5′, H1″ and H4″ decreased in the presence of potassium ions, ¹H chemical shifts and ¹H diffusion coefficients did not show significant changes. The long‐range coupling constants of ³J_C?H around the glycosyl bonds did not show significant changes either. In the measurements of ³⁹K spectra, the ³⁹K signal obviously broadened and the ³⁹K‐T₁ values decreased in the presence of 1,6‐anhydro‐β‐maltotriose, indicating the complex formation of 1,6‐anhydro‐β‐maltotriose and potassium ions. These results indicate that the conformation and molecular volume were unaffected in the complex formation. Copyright © 2009 John Wiley & Sons, Ltd.     

Dispersion of T1 and T2 Nuclear Magnetic Resonance Relaxation in Crude Oils

Crude oils, which are complex mixtures of hydrocarbons, can be characterized by nuclear magnetic resonace diffusion and relaxation methods to yield physical properties and chemical compositions. In particular, the field dependence, or dispersion, of T₁ relaxation can be used to investigate the presence and dynamics of asphaltenes, the large molecules primarily responsible for the high viscosity in heavy crudes. However, the T₂ relaxation dispersion of crude oils, which provides additional insight when measured alongside T₁, has yet to be investigated systematically. Here we present the field dependence of T₁‐T₂ correlations of several crude oils with disparate densities. While asphaltene and resin‐containing crude oils exhibit significant T₁ dispersion, minimal T₂ dispersion is seen in all oils. This contrasting behavior between T₁ and T₂ cannot result from random molecular motions, and thus, we attribute our dispersion results to highly correlated molecular dynamics in asphaltene‐containing crude oils.     

Structure of Bischloro tris (1, 10‐phenanthroline) copper (II) Dichloromethane Solvate Nonahydrate: [Cu(phen)3]Cl2CH2Cl2·9H2O

The crystal and molecular structures of [Cu(phen)₃] Cl₂ · CH₂Cl₂^.9H₂O (PHEN= 1, 10‐pbenanthroline) have been determined by X‐ray crystallography. The complex crystallizes in triclinic system, space group P1, with lattice parameters a = 1.26000(3), b = 1.37525(4), c = 1.42750(3)nm, α = 85.2970(1),β = 66.8400(1), γ= 83.09(1)°, and Z = 2. The coordinated cations contain a six‐coordinated copper atom chelated by three PHEN ligands, and the Jahn‐Teller effect of the Cu(II) ion results in a distorted octahedral arrangement with the six Cu? N distances ranging from 0.2112(6) to 0.2265(7) nm. In addition to the copper coordinated cation, there are two chloride ions, one dichloromethane solvate and nine water molecules in its asymmetric unit. In the solid state, the title compound forms three dimensional network structures through hydrogen bonds. The intermolecular hydrogen bonds connect the [Cu(phen)₃]²⁺, chloride ion, dichloromethane solvate and H₂O moieties altogether.     

A new one‐dimensional nickel metal–organic framework: catena‐poly[[[diaquabis(4‐{[(1‐phenyl‐1H‐tetrazol‐5‐yl)sulfanyl]methyl}benzoato‐κO)nickel(II)]‐μ‐4,4′‐bipyridine‐κ2N:N′] monohydrate]

The title complex, {[Ni(C₁₅H₁₁N₄O₂S)₂(C₁₀H₈N₂)(H₂O)₂]·H₂O}_n, was synthesized by the reaction of nickel chloride, 4‐{[(1‐phenyl‐1H‐tetrazol‐5‐yl)sulfanyl]methyl}benzoic acid (HL) and 4,4′‐bipyridine (bpy) under hydrothermal conditions. The asymmetric unit contains two half Ni^II ions, each located on an inversion centre, two L⁻ ligands, one bpy ligand, two coordinated water molecules and one unligated water molecule. Each Ni^II centre is six‐coordinated by two monodentate carboxylate O atoms from two different L⁻ ligands, two pyridine N atoms from two different bpy ligands and two terminal water molecules, displaying a nearly ideal octahedral geometry. The Ni^II ions are bridged by 4,4′‐bipyridine ligands to afford a linear array, with an Ni...Ni separation of 11.361 (1) Å, which is further decorated by two monodentate L⁻ ligands trans to each other, resulting in a one‐dimensional fishbone‐like chain structure. These one‐dimensional fishbone‐like chains are further linked by O—H...O, O—H...N and C—H...O hydrogen bonds and π–π stacking interactions to form a three‐dimensional supramolecular architecture. The thermal stability of the title complex was investigated via thermogravimetric analysis.     

Ab-initio SCF potential energy surfaces for the nucleophilic attack of hydride on coordinated carbon monoxide

Ab-initio SCF calculations are reported for the nucleophilic addition of the hydride ion H⁻ on the iron pentacarbonyl Fe(CO)₅ complex. The stereochemistry of the attack has been established from the determination of two potential energy surfaces. The reaction is shown to be highly exothermic and with no activation barrier. This activation of CO towards the nucleophilic addition is rationalized in terms of molecular orbital interactions.     

Kinetics and rate constants for the reaction of tri-n-butylgermanium hydride with methyl iodide and carbon tetrachloride. The combination of methyl and trichloromethyl radicals in solution.

The kinetics of the photoinitiated reductions of methyl iodide and carbon tetrachloride by tri-n-butylgermanium hydride in cyclohexane at 25°C have been studied and absolute rate constants have been measured. Rate constants for the combination of CH₃? and CCl₃? radicals are equal within experimental error and are also equal to the values found for the self-reactions of most non-polymeric radicals in low viscosity solvents, i.e. ～1–3 × 10⁹ M^?1 sec^?1. Rate constants for hydrogen atom abstraction by CH₃? and CCl₃? radicals are both ～1?2 × 10⁵ M^?1 sec^?1. Tri-n-butyltin hydride is about 10–20 times as good a hydrogen donor to alkyl radicals as is tri-n-butylgermanium hydride. The strength of the germanium–hydrogen bond, D(n-Bu₃Ge–H) is estimated to be approximately 84 kcal/mole.     

Alignment of Axial Anisotropy in a 1D Coordination Polymer shows Improved Field Induced Single Molecule Magnet Behavior over a Mononuclear Seven Coordinated FeII Complex

Herein, we report a CN‐bridged alternating Fe^II?Ni^II 1D chain to ensure the alignment of axial anisotropy and improve the single molecule magnet (SMM) behavior in seven coordinated Fe^II compound. The chain was constructed from hepta coordinated Fe(II) complex as an anisotropic building unit and diamagnetic nickel tetra cyanate as a bridging ligand. The magnetic measurements show the easy‐axis anisotropy of the seven coordinated Fe(II) complex and field induced SMM behavior with spin reversal energy barrier U_eff=61(2) K (42 cm^?1) and pre‐exponential relaxation time τ₀=1.9×10^?8 s. The detailed analysis of the relaxation dynamics discloses that the Orbach process plays an important role in slow relaxation of magnetization for this compound. Notably, this example represents a remarkable energy barrier observed in hepta coordinated Fe(II) SMMs. The ab initio calculations estimate the magnitude of axial anisotropy and show the parallel orientation of the anisotropic axis throughout the 1D polymeric chain. In addition, it is also reported that the presence of weak π accepter ligands in the distorted axial position enhance the easy‐axis anisotropy.     

Barium oxoaluminate hydride

The crystal structure of Ba₃(AlO₄)H is isotypic with Ba₃SiS₅ and contains AlO₄^5? and H^? anions. The hydride and oxide anions are coordinated by six Ba and five Ba/one Al atoms in an octahedral geometry. The hydrogen content was ex­amined by MAS–NMR experiments of the deuterated compound.     

A copper(I) coordination polymer incorporation the corrosion inhibitor 1H‐benzotriazole: poly[μ3‐benzotriazolato‐κ3N1:N2:N3‐copper(I)]

The title complex, [Cu(C₆H₄N₃)]_n, was synthesized by the reaction of cupric nitrate, 1H‐benzotriazole (BTAH) and aqueous ammonia under hydrothermal conditions. The asymmetric unit contains three crystallographically independent Cu^I cations and two 1H‐benzotriazolate ligands. Two of the Cu^I cations, one with a linear two‐coordinated geometry and one with a four‐coordinated tetrahedral geometry, are located on sites with crystallographically imposed twofold symmetry. The third Cu^I cation, with a planar three‐coordinated geometry, is on a general position. Two Cu^I cations are doubly bridged by two BTA⁻ ligands to afford a noncentrosymmetric planar [Cu₂(BTA)₂] subunit, and two [Cu₂(BTA)₂] subunits are arranged in an antiparallel manner to form a centrosymmetric [Cu₂(BTA)₂]₂ secondary building unit (SBU). The SBUs are connected in a crosswise manner via the sharing of four‐coordinated Cu^I cations, Cu—N bonding and bridging by two‐coordinate Cu^I cations, resulting in a one‐dimensional chain along the c axis. These one‐dimensional chains are further linked by C—H...π and weak van der Waals interactions to form a three‐dimensional supramolecular architecture.     

31P-NMR. Measurements on Palladium-Phosphine Complexes. Nuclear overhauser effects and spin-lattice relaxation times

The ³¹P{¹H} nuclear Overhauser effects (NOE's) and ³¹P-spin-lattice relaxation times (T₁) for a series of trans-[PdCl₂P₂], P ? PEt₃, PPr₃ⁿ, PBu₃ⁿ, PMe₂Ph, PMePh₂, P(p-Tol)₃, P(cyclohexyl)₃ complexes are reported. Both the NOE and T₁ values depend upon the choice of solvent. The dipole-dipole mechanism dominates the spin-lattice relaxation of the coordinated phosphorus atom with the T₁ values for the PEt₃, PPr₃ⁿ, and P (cyclohexyl)₃ complexes decreasing with increasing molecular weight of the phosphine.     

Metalation of 1-R-2-benzyl-o-carboranes with lithium aluminum hydride

Carboranes with the general formula I-R-2-PhCH₂-1,2-C₂B₁₀H₁₀ (R = Me, Prⁱ, Ph, PhCH₂) are readily metalated with lithium aluminum hydride in a THF solution at the CH₂ group. In this case only one hydrogen atom in LiAlH₄ is substituted, and trihydride complexes 1-R-2-PhCH(AlH₃Li)-1,2-C₂B₁₀H₁₀, are formed, which are stable in a solutionTranslated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 5, pp. 1282–1284, May, 1996.     

Synthesis of polyacetylene with titanocene-aluminum hydride catalysts

Heterometallic hydride titanocene-aluminum complexes Cp₂Ti(-H)₂AlH(X) and (Cp₂Ti)₂AlH₄X are highly efficient homogeneous catalysts for acetylene polymerization. The binuclear complex of the composition Cp₂Ti(-H)₂AlH₂ at 2.2–3.2M concentrations in ether-toluene solutions exhibits the maximum activity in this reaction. It is believed that the mechanisms of the isomerization of olefins and the polymerization of acetylene are similar and, correspondingly, the compositions and structures of the active sites in both processes are close to each other. The polyacetylene formed with hydride catalysts (mostly thecis-isomer) after doping with iodine has an electrical conductivity of 1.5–2.0 · 10⁴ Ohm^–1 cm^–1.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 3, pp. 405–409, March, 1994.The work was carried out with the financial support of the Russian Foundation for Basic Research, project No. 93-03-5757.     

CRYSTAL STRUCTURES OF TWO 1:1 COMPLEXES OF MANGANESE(II) WITH 1,10-PHENANTHROLINE,CONTAINING COORDINATED AND UNCOORDINATED SULFATE ANIONS

Abstract

The crystal structures of fac-triaqua-(1,10-phenanthroline-N,N′)-sulfato-manganese(II) (1) and tetraaqua-(1,10-phenanthroline-N,N′)-manganese(II) sulfate dihydrate (2) are described. The coordination polyhedron around the Mn(II) ion displays a distorted octahedral shape and contains the N,N′-chelating phen ligand. In the first complex the coordination sphere is completed by three water molecules and a monodentate O-sulfato ligand (C _3v), whereas in the second one, an uncoordinated sulfate ion (T_d ) and four coordinated water molecules have been found. In both structures, the molecular units are extensively 2-D hydrogen bonded, with the 1,10-phenanthroline ligands arranged roughly perpendicular to the sheets; the bidimensional sheets are piled up giving rise to polar layers made by the hydrogen bonded species, the aqua-ions, sulfate anions, and water molecules, alternated with apolar layers formed by the 1,10-phenanthroline ligands.     

Crystal structure and DFT calculations of Cp2NbH(SiIMe2)(SiFMe2): an asymmetric bis(silyl) niobocene hydride complex

An asymmetric bis(silyl) niobocene hydride complex, namely, bis(η⁵-cyclopentadienyl)(fluorodimethylsilyl)hydrido(iododimethylsilyl)niobium, [Nb(C₅H₅)₂(C₂H₆FSi)(C₂H₆ISi)H] or Cp₂NbH(SiIMe₂)(SiFMe₂), has been studied to determine the effect of the silyl ligand on the position of the hydride attached to the Nb atom. It has been shown that when a Group 17 atom is substituted onto one of the silyl ligands, there is a greater interaction between the hydride and this ligand, as demonstrated by a shorter Si…H distance. In the present work, we have investigated the effect when the silyl ligands are substituted by different Group 17 atoms. We present here the structure and DFT calculations of Cp₂NbH(SiIMe₂)(SiFMe₂), showing that the position of the hydride is located between the two silyl ligands. The results from our investigation show that the hydride is closer to the silyl ligand that is substituted by fluorine.     

Decomposition of electron-excited molecules of molybdenum and tungsten phosphine hydride and phosphine nitrogen complexes

The kinetics and mechanism of photodehydrogenation of the phosphine hydride complexes MH₄L₄ (M = Mo, W; L are phosphine ligands) and the formation of coordinatively unsaturated species M_L4 were studied by the absorbance of long-wavelength bands with λ_max at 450–460 nm appeared in the absorption spectra of the photoproducts. The rate constants of the reactions of the coordinatively unsaturated M(DPPE)₂ species (M = Mo, W; DPPE = Ph₂PCH₂CH₂PPh₂) with molecular nitrogen in benzene were determined (k _W = 200 s⁻¹, k _Mo = 8700 s⁻¹). Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 2, pp. 282–284, February, 2008.     

pH Dependence of T1 for 13C-Labelled Small Molecules Commonly Used for Hyperpolarized Magnetic Resonance Imaging

Hyperpolarization is a method to enhance the nuclear magnetic resonance signal by up to five orders of magnitude. However, the hyperpolarized (HP) state is transient and decays with the spin-lattice relaxation time (T₁), which is on the order of a few tens of seconds. Here, we analyzed the pH-dependence of T₁ for commonly used HP ¹³C-labelled small molecules such as acetate, alanine, fumarate, lactate, pyruvate, urea and zymonic acid. For instance, the T₁ of HP pyruvate is about 2.5 fold smaller at acidic pH (25 s, pH 1.7, B₀=1 T) compared to pH close to physiological conditions (66 s, pH 7.3, B₀=1 T). Our data shows that increasing hydronium ion concentrations shorten the T₁ of protonated carboxylic acids of most of the analyzed molecules except lactate. Furthermore it suggests that intermolecular hydrogen bonding at low pH can contribute to this T₁ shortening. In addition, enhanced proton exchange and chemical reactions at the pK_a appear to be detrimental for the HP-state.