High-Resolution Solid-State NMR Characterization of Morphology in Annealed Polylactic Acid

Single-pulse ¹³C NMR spectra and spin-lattice relaxation times T₁(¹H), detected indirectly via ¹³C carbons, and T₁(¹³C) were measured at 31°C for virgin pelletized and annealed polylactic acid (PLA) samples using the magic-angle spinning technique. The structural relaxation resulting in more regular crystals with narrower conformation distribution and increase in the lamellae thickness and crystallinity brought about by annealing at 100°C was deduced from the narrowing of the ¹³C NMR lines and proton spin-lattice relaxation times T₁(¹H). The spin-lattice relaxation times T₁(¹³C) related to the respective carbons of the α-polymorph of PLA are also discussed in the study.     

Assignment of the 13C NMR resonances in trialkylphosphines from spin-lattice relaxation time measurements

Analysis of the ¹³C NMR chemical shift and coupling constant data for a number of straight-chain aliphatic trialkylphosphines and their transition metal carbonyl complexes suggests that complexation leads to: (1) a deshielding of C(1) and an increase in ¹J(¹³C³¹P), (2) a slight shielding of C(2) and a decrease in ²J(¹³C³¹P), and (3) little or no change in the chemical shift for C(3) and a slight increase in ³J(¹³C³¹P). Application of these rules to the assignment of the ¹³C NMR spectrum of P(butyl)₃ led to conflict with prior work. A study of segmental motion in these derivatives via spin-lattine (T₁) relaxation time measurements was therefore performed, and these data are in complete agreement with the proposed assignments. These generalizations must be applied with care, however, since the presence of either unsaturation or branching near the phosphorus can interfere with this pattern.     

A Trinuclear Radical‐Bridged Lanthanide Single‐Molecule Magnet

Assembly of the triangular, organic radical‐bridged complexes Cp*₆Ln₃(μ₃‐HAN) (Cp*=pentamethylcyclopentadienyl; Ln=Gd, Tb, Dy; HAN=hexaazatrinaphthylene) proceeds through the reaction of Cp*₂Ln(BPh₄) with HAN under strongly reducing conditions. Significantly, magnetic susceptibility measurements of these complexes support effective magnetic coupling of all three Ln^III centers through the HAN^3−. radical ligand. Thorough investigation of the Dy^III congener through both ac susceptibility and dc magnetic relaxation measurements reveals slow relaxation of the magnetization, with an effective thermal relaxation barrier of U_eff=51 cm⁻¹. Magnetic coupling in the Dy^III complex enables a large remnant magnetization at temperatures up to 3.0 K in the magnetic hysteresis measurements and hysteresis loops that are open at zero‐field up to 3.5 K.     

NMR of metal-core bonded31P in polynuclear metal cluster compounds

In this paper we present measurements of nuclear relaxation times of³¹P in the polynuclear cluster compounds Au₅₅(PPh₃)₁₂Cl₆ and Ru₅₅(P(t-Bu)₃)₁₂Cl₂₀. Above 15 K the data can be described as a Korringa process, while below 15 K the relaxation time appears to be thermally activated.     

Thallium(III) coordination compounds: chemical information from 205Tl NMR longitudinal relaxation times

²⁰⁵Tl longitudinal relaxation rate measurements were performed on several thallium(III) complexes with the composition Tl(OH)_n(H₂O)_6?n^(3?n)+ (n = 1,2), Tl(Cl)_n(H₂O)_m?n^(3?n)+, Tl(Br)_n(H₂O)_m?n^(3?n)+ (m = 6 for n = 1–2, m = 5 for n = 3, m = 4 for n = 4), Tl(CN)_n(H₂O)_m?n^(3?n)+ (m = 6 for n = 1–2, m = 4 for n = 3–4) in aqueous solution, at different magnetic fields and temperatures. ¹³C and ²D isotopic labelling and ¹H decoupling experiments showed that the contribution of the dipolar relaxation path is negligible. The less symmetric lower complexes (n < 4) had faster relaxation rate dominantly via chemical shift anisotropy contribution which depended on the applied magnetic field: T₁ values are between 20 and 100 ms at 9.4 T and the shift anisotropy is Δσ = 1000–2000 ppm. The tetrahedral complexes, n = 4, relax slower; their T₁ is longer than 1 s and the spin–rotation mechanism is probably the dominant relaxation path as showed by a temperature dependence study. In the case of the TlCl₄^? complex, presumably a trace amount of TlCl₅^2? causes a large CSA contribution, 300 ppm. Since the geometry and the bond length for the complexes in solution are known from EXAFS data, it was possible to establish a correlation between the CSA parameter and the symmetry of the complexes. The relaxation behaviour of the Tl–bromo complexes is not in accordance with any known relaxation mechanism. Copyright © 2002 John Wiley & Sons, Ltd.     

Enhancement of Spin Relaxation in an FeDy2Fe Coordination Cluster by Magnetic Fields

Two [FeLn₂Fe(μ₃‐OH)₂(teg)₂(N₃)₂(C₆H₅COO)₄] compounds (where Ln=Y^III and Dy^III; teg=triethylene glycol anion) have been synthesized and studied using SQUID and Mössbauer spectroscopy. The magnetic measurements on both compounds indicate dominant antiferromagnetic interactions between the metal centers. Analysis of the ⁵⁷Fe Mössbauer spectra complement the ac magnetic susceptibility measurements, which show how a static magnetic field can quench the slow relaxation of magnetization generated by the anisotropic Dy^III ions.     

Premicellar interaction of PEO–PPO–PEO triblock copolymers with partially hydrophobic alcohols: NMR study

The interactions of three alcohols, namely, 2‐butanol (BuOH), 3‐methyl‐2‐butanol (MeBuOH), and 3,3‐dimethyl‐2‐butanol (Me2BuOH) with propylene oxide octamer (PO8) and the copolymers (EO)₈(PO)₁₃(EO)₈ (L35) and (EO)₁₃(PO)₃₀(EO)₁₃ (L64) in D₂O were studied using ¹³C NMR spectra and relaxations and ¹H PFG NMR diffusion measurements. For L64, it was shown that the temperature at which the PO chain starts to change its conformation under dehydration decreases by 6 K for each additional methyl group in the alcohol molecule (i.e. with increasing its hydrophobicity), and the analogous conformation states are attained at temperatures approximately 10 K lower compared using ketonic analogs of the alcohols under the same conditions. Also, the first signs of L64 aggregation, according to the normalized diffusion coefficients, are at temperatures 7, 10, and 13 K lower for BuOH, MeBuOH, and Me2BuOH, respectively. These effects are much weaker for (PO)₁₃ in L35 or nonexistent for (PO)₈ in PO8, thus showing the role of cooperativity in dehydration and aggregation processes. According to diffusion measurements, the molar fraction of the alcohol hydrogen bonded to L64 increases with its hydrophobicity and, in an apparent conflict with thermodynamics, with increasing temperature at which also higher NOE can be observed. Strong hydrogen bond interaction, which is in cooperation with hydrophobic interaction, does not preclude the exchange between bound and free states of the alcohol, however. Using ¹³C transverse relaxation, its correlation time is shown to be of the order of 10 ms. Copyright © 2013 John Wiley & Sons, Ltd.     

Arsenic containing heterocycles. II. Confirmation of the structure of benzo[a]-9-chlorophenarsazine 12-chloride by 13C-NMR spectroscopy and spin-lattice relaxation time studies: Possible evidence for the contribution of an 75as-13C mechanism to the relaxation of the arsenic bearing quarternary carbons in the phenarsazine ring system

The product of the reaction of (3-chlorophenyl)-2-naphthylamine with arsenic trichloride has been confirmed to be benzo(a)-9-chlorophenarsazine 12-chloride by ¹³C-nmr spectroscopy and spin-lattice (T₁) relaxation time measurements. A considerable shortening of the spin-lattice relaxation times of the arsenic bearing quaternary carbons, relative to the calculated relaxation times, was observed. This discrepancy has been interpreted in terms of an ⁷⁵As-¹³C dipolar contribution to quaternary carbon relaxation.     

Investigations of Polyoxometalates in Aqueous Solutions. I. The Formation of Al13(OH)7+ 31 Cation

Abstract

The formation of polyhydroxo aluminum(III) complexes has been investigated at 30°C and in a 3 M (K)Cl ionic medium by p[H] measurements. The uncommon “integral titration” technique employed has enabled measurements of oversaturated solutions up to OH^? to Al(III) ratios as large as 2.65. This has allowed the detection of the undescribed species Al₁₃(OH)⁴⁺ ₃₅. The data can very satisfactorily be explained by assuming the species Al₂(OH)⁴⁺ ₂, Al₃(OH)³⁺ ₆, Al₁₃(OH)⁷⁺ ₃₂, and Al₁₃(OH)⁴⁺ ₃₅. The Al(III) concentration has been changed from ≈0.0025 to ≈ 0.040 M and the spacings of the titration curves at different aluminum levels are a clear and direct evidence for the formation of Al₁₃(OH)⁷⁺ ₃₂, which dominates the hydrolysis products. The data presented in this paper are best accounted for if the trimer Al₃(OH)³⁺ ₆ is substituted for Al₃(OH)⁵⁺ ₄ which is frequently reported. The formation of the “13” cations may result from the reaction of four Al₃(OH)³⁺ ₆ with a transient Al(OH)^? ₄ species which is formed, upon addition of a rather concentrated basic solution, owing to a local excess of OH^?.     

Magnetic Properties of Two New Fe4 Single‐Molecule Magnets in the Solid State and in Frozen Solution

Two novel tetranuclear, star‐shaped iron(III) clusters, [Fe₄(acac)₆(Br‐mp)₂] and [Fe^III₄(acac)₆(tmp)₂], are described. Both have S=5 ground states resulting from antiferromagnetic nearest‐neighbour superexchange interactions, with J=?8.2 cm^?1 and J=?8.5 cm^?1 for 1 and 2 , respectively. Energy barriers for the relaxation of the magnetisation of approximately 12 cm^?1 were derived from AC susceptibility measurements. Magnetic resonance measurements revealed a zero‐field splitting parameter D=?0.34 cm^?1 for both complexes. AC susceptibility measurements in solution demonstrated that the complexes are reasonably stable in solution. Interestingly, the magnetisation relaxation slows down significantly in frozen solution, in contrast to what is generally observed for single‐molecule magnets. This was shown to result from a large increase in τ₀, the prefactor in the Arrhenius equation, with the energy barrier remaining unchanged.     

NMR investigation of bis(2,3-alkanedione dioximato)-bis(pyridine)cobalt(III) iodide complexes: 1H and 13C NMR spectra and 13C spin-lattice relaxation time measurements

The complexes of trans-[Co(III)(R,CH₃-dioxH)₂(py)₂]I2 (R = CH₃, C₂H₅, n-C₃H₇ and n-C₄H₉) were investigated in solution by ¹H and ¹³C NMR spectra and ¹³C spin-lattice relaxation time measurements. The ¹H and ¹³C-resonances of the R = C₂H₅, n-C₃H₇ and n-C₄H₉) groups were shifted to higher field than those of the free ligands by the complexation; it was attributable to the ring current shielding due to the axial pyridine ligands of the complexes. ¹³C spin-lattice relaxation times were interpreted as due to movement of the axial pyridine ligands as if they twist around the CoN (pyridine nitrogen) bond axis and the above R groups were moving segmentally. These segmental movements allowed the R groups to approach closely toward the axial pyridine ring plane to experience the ring current shielding.     

A study of C–35Cl coupling constants and 35Cl relaxation times through 13C relaxation in the rotating frame

Values of one-bond J(C³⁵ Cl) and T₁(³⁵Cl) were determined for small chlorine-containing molecules through ¹³C scalar relaxation in the rotating frame. A method is suggested for evaluating the contribution of scalar relaxation which eliminates experimental imperfections. The magnitude of J(C³⁵Cl) seems to be influenced by the same factors as J(CF) values. The differences in ³⁵Cl relaxation times are rationalized in terms of the molecular reorientation time.     

Etude RMN du 13C de composes fluoroaliphatiques et de tensioactifs non ioniques perfluoroalkyles

Carbon-13 chemical shifts, spin-lattice relaxation times and nuclear Overhauser enhancement factors at 28°C are reported for a series of polyfluoroaliphatic compounds :

, and perfluoroalkyl nonionic surfactants C_mF_2m+1CH₂(OC₂H₄)_nOH with m = 6, 7 and n = 3, 4, 5, 6 and C₆F₁₃CH₂CH₂CONH(C₂H₄O)_nH with n = 3, 4. The influence of the perfluoroalkyl group on the ¹³C chemtcal shifts of the neighbouring hydrogenated carbons is discussed in terms of hyperconjugative type interactions between lone electron pairs on fluorine and the neighbouring CC or CO bond. Relaxation data show similar flexibilities of the fluorinated chains in the different molecules investigated. Nonionic surfactants exhibit segmental motions in both the hydrophobic perfluoroalkyl and the hydrophilic polyoxyethylene chains ; these motions appear to be similar to those of the analogous hydrogenated surfactants.     

Single‐Molecule Magnet Behavior in Two Tetranuclear Cyanide‐bridged FeIII2NiII2 Compounds

Two tetranuclear cyanide‐bridged Fe^III₂Ni^II₂ compounds [Ni₂(L1)₄Fe₂(μ‐CN)₄(CN)₂(L2)₂] · 2ClO₄ · CH₃OH · 4H₂O ( 1 ) and [Ni₂(L1)₄Fe₂(μ‐CN)₄(CN)₂(L3)₂] · 2ClO₄ ( 2 ) [L1 = 4,4′‐dichloro‐2,2′‐bipyridine; L2 = hydrotris(pyrazolyl)borate; L3 = tetrakis(pyrazolyl)borate] were synthesized. Magnetic measurements indicated that both two compounds showed single‐molecule magnet (SMM) behaviors with the relaxation energy barrier of Δ/k_B = 68.9(8) K for 1 and 12.6(1) K for 2 . Magneto‐structural analysis indicated that the intermolecular interactions played an important part in the slow magnetic relaxation behaviors.     

Utilisation de l'effet overhauser,des mesures de relaxation en RMN 1H et des deplacements chimiques en RMN 13C dans l'etude de la conformation des amides—II: Application a l'etude conformationnelle d'amides alicycliques: acyl-prolines

¹H and ¹³C chemical shifts in the two decoalesced rotamers of the deuteromethyl ester of N-acetyl-l-proline are established by use of NOE and relaxation measurements. This requires the previous determination of the rotational barrier ΔG_c^≠ and the measurement of the relaxation times T₁ of the α-proton in the two conformers. The observed results reinforce the previous structural conclusions inferred from ¹³C studies on several acyl l-prolines and hydroxy l-prolines. The introduction of an acyl group in the prolines does not affect the different carbon atoms of the cycle in the two rotamers in the same way. These results can be interpreted in terms of electric field effects by the examination of X-Pro dipeptides with the N-terminal amino group shifted from α (COCH₂NH₂) to δ position (CO(CH₂)₄NH₂).     

Etude infrarouge des effets de structure et de solvant sur la frequence itvC=O de cetones aliphatiques saturees. Relation frequence- topologie-milieu

The sensitivity of saturated aliphatic ketone stretching frequencies, vCO to structure and solvent effects is expressed by a four-term equationv_co = 1740 + I(XXX) -0.24 G + 2.10^?3G ·I(XXX)The four terms represent: the frequency of the base element (acetone) calculated in the absence of intermolecular interactions; the contribution of intramolecular environment E given by structure parameter I(itXXX) previously established in gas phase; that of the intermolecular environment related to the Allerhand and Schleyer G parameter; that of solvent-solute interactions. This equation covers an experimental range of 78 cm^?1 for 192 measurements in four highly diverse solvents (C₆H₁₄, CC1₄, CH₃CN, CHBr₃) with a standard deviation of 1.6 cm^?1; it expresses an overall statistical behavior, but masks individual behaviors. The latter are determined by comparing two characteristic parameters of ketones, a topological parameter I(XXX), and p expressing solvent sensitivity [slope of straight lines ν = f(G)]; they are Interpreted in terms of geometrical effect, variation of valency angles and of conformations.     

Raman bandshape analysis of the symmetric bending vibration in liquid chloroform

In order to determine whether accurate rotational diffusion coefficients in liquids may be determined from the bandshapes of isotopically broadened vibrational peaks, we have investigated the isotropic and anisotropic Raman spectra of the ν₃(A₁), CCl₃ symmetric bending, vibration in CHCl₃ as a function of temperature in the liquid phase. The spectral lineshapes were fitted by a model containing four Lorentzian/Gaussian summation bands with relative peak intensities equal to the relative abundances of the four isotopic combinations and frequency displacements constrained to values measured in the matrix infrared spectrum. The calculated room temperature perpendicular diffusion coefficient, D_⊥ (25°C) = 8.310¹⁰ s⁻¹, was within the range of values reported from Raman measurements on the ν₁, symmetric carbon-hydrogen stretching, vibration, but was somewhat lower than published results from NMR relaxation time measurements, T₁(²D), on CDCl₃, and from dielectric relaxation. The activation energy, E_a(D_⊥), determined from the ν₃ bandshape measurements was 30% higher than the average value from the NMR and dielectric studies. The deviation is believed to result from the sensitivity of this quantity to the fractional Lorentzian character of the fitting functions.     

Intramolecular Dynamics of Lanthanide(III) Tetraoxadiaza Macrocycle Complexes in Solution as Studied by NMR

¹H and ¹³C NMR and ¹H NMR relaxation spectroscopy (RS)measurements are reported for the CDCl₃ and CD₂Cl₂ solutions of [La(NO₃)₃(diaza-18-crown-6)] ({bf I}), [Pr(NO₃)₃(diaza-18-crown-6)] ({bf II}) and [Nd(NO₃)₃(diaza-18-crown-6)] ({bf III}) complexes. Temperature dependencies of the ¹H NMR spectra of II have been analyzed using the dynamic NMR methods for multi-site exchange. Enantiomeric isomer interconversion in II is characterized by H = 21.5 ± 4 kJ mol^-1. Studies of the values of the lanthanide-induced shifts and the longitudinal relaxation rate enhancement revealed that the structure of complexes in solution is similar to that reported for the [La(NO₃)₃(18-crown-6)] complex in the crystal state. Nevertheless, it appears that the principal values of the molar paramagnetic susceptibility tensor (_i) significantly differ in complexes II and III. The possible reasons for the different characteristics of these complexes are discussed.     

Enthalpies de formation des hydroxynitrates de zinc

The standard enthalpies of reaction of four zinc hydroxide nitrates Zn(OH)(NO₃)-H₂O, Zn₃(OH)₄(NO₃)₂, Zn₅(OH)₈(NO₃)₂·2H₂O et Zn₅(OH)₈(NO₃)₂ and zinc oxide with a solution of nitric acid (2N) were measured in a solution calorimeter. These results, combined with auxiliary thermochemical values from the literature, yielded values of ?429.34, ?442.41, ?897.41 and ?750.70 kcal mol^?1 respectively, for the molar enthalpies of formation of these zinc hydroxide nitrates.