Carbon-13 spin-lattice relaxation times and nuclear overhauser enhancements in some simple fluorocarbons

Carbon-13 spin-lattice relaxation times (T₁) and ¹³C-{¹⁹F} nuclear Overhauser enhancement (NOE) factors have been measured for some simple fluorocarbons by pulse-Fourier transform methods—‘progressive saturation’ and ‘dynamic Overhauser enhancement’. The NOE factors are shown to be large for ¹³C with directly bonded fluorines, and the values for T₁ and the NOE factors are similar to those obtained elsewhere on the corresponding hydrocarbons.     

Nuclear magnetic resonance spectroscopy of petroporphyrins : Self-aggregation effects, nuclear overhauser enhancements, and spin-lattice relaxation used in structural elucidation

The substitution pattern of the two major petroporphyrins of Marl Slate, the ETIO III and the C₃₂ DPEP, is determined by NMR spectroscopy alone, using self-aggregation effects, nuclear Overhauser enhancements and spin-lattice relaxation times. Protoporphyrin IX dimethyl ester was used as a model compound.     

A 31P-NMR. Study of Some Phosphite Complexes of Platinum (0)

³¹P-NMR. spectra for a series of phosphite complexes of platinum (0) have been measured. Low temperature studies have revealed the existence in solution of the mixed complexes Pt[PPh₃]₂[P(OPh₃)₃]₂ and Pt[PPh₃][P(OPh)₃]₃ of which the former is isolable.     

Analysis of carbon-13 longitudinal relaxation times and nuclear overhauser enhancements in alkyl chains

A simple procedure is proposed which yields, from carbon-13 longitudinal relaxation times and nuclear Overhauser enhancements, information concerning the local mobility of a given CH vector in an alkyi chain. This method is based on the definition of an effective rotation axis related to internal motions and makes use of Woessner's equations. It yields a correlation time τ_R characteristic of the overall reorientation. In addition, for each carbon C_i a correlation time τ_Gi, associated with the fast motion of the C_iH vector, and an angle θ_i, between C_iH and an effective rotation axis, are determined. The effect of cross-correlation spectral densities is discussed. This model is used for analysing relaxation data of decylammonium chloride micelles. It is further checked by interpreting experimental results at two different observation frequencies.     

Acceleration of carbon-13 spin-lattice relaxation times in amino acids by electrolytes

A series of amino acids and carboxylic acids were determined by 13C NMR spectroscopy.The results showed that addition of 3M MgCl2 led to the 13C NMR integral area of samples being well proportional to number of carbon atoms that produce the particular signal with reliability over 95%. Measurements of 13C spin-lattice relaxation times (T1's) are reported for a number of amino acids. T1's of all the carbons in amino acids generally tend to decrease with the increase of the concentration of electrolytes, and the presence of magnesium slats is of significant. Carboxylic carbons in amino acids are the most sensitive "acceptor" of the 13C spin-lattice relaxation accelerating effects in electrolytes, and the 13C spin-lattice relaxation accelerating ability of electrolytes is Mg(ClO4)2 ＞MgCl2 ＞CaCl2 ＞NaCl ＞KCl ＞LiClO4 ＞NaOH. In general, T1's of C1 carbons in nonpolar a-amino acids are higher than those in polar and basic a-amino acids both in aqueous and 3M MgCl2 medium. In aliphatic straight-chain amino acids, a-, a-, a-, ai- and a- amino acids, T1's of C1 carbons tend to reduce with the increase of inserted carbon numbers between amino and carboxylic groups compared with Gly. T1's can be decreased even more when amino acids are mixed in 3M MgCl2, but T1's of carbons in amino acids decrease slightly with increase of the concentration of amino acids in 3M MgCl2. The mechanisms of the observed phenomena are discussed in terms of intermolecular interaction and paramagnetic impurity in electrolytes, large contributions of intermolecular interaction which is enhanced in electrolytes concentrate on the incoming "unsaturation" of the primary solvation shell of cations with the increase of electrolytes concentration and complexes formation of amino acids with metal ions. In electrolytes, amino acids are "anchored" to cations and molecule tumbling is slowed down, molecular rigidity is increased and molecular size is "enlarged", all of these are helpful to accelerate the 13C spin-lattice relaxation. Atlast, MgCl2 is proposed as an efficient relaxation agent for analysis of amino acids and some carboxylic acids.Samples were dissolved with the aid of supersonic which has the effect of degassing, and they were degassed again with supersonic for 30 seconds right before determination. All of the 13C NMR was obtained with a Bruker DPX-300 NMR instrument, using NOE-suppressed inverse gated decoupling with a recycle delay 8.00s and a sweep width 30120.48Hz, experiment temperature is integral of the carbon with the smallest chemical shift is calibrated as 10.00. Spin-lattice relaxation times (T1's) were determined by using inversion recovery according to Bruker avance user's guide.The pulse sequence is (T-90.°-T-180°-o-90t°) n.     

Acceleration of carbon-13 spin-lattice relaxation times in amino acids by electrolytes

Measurements of the enhancement, by various electrolytes, of the spin-lattice relaxation time of carbon-13 at different locations in a number of amino acids are reported. Spin-lattice relaxation times T1 of all the carbons in amino acids generally tend to decrease with increase in the concentration of electrolytes, the largest effects often being observed for the charged carboxylate groups of the amino acids. Carboxylic carbons in amino acids are the sensitive 'acceptor' of the 13C spin-lattice relaxation accelerating effects offered by electrolytes, and the 13C spin-lattice relaxation accelerating ability of electrolytes decreases in the order Mg(ClO4)2 > MgCl2 > CaCl2 > NaCl > KCl > LiClO4 > NaOH. The mechanisms of the observed phenomena are discussed in terms of intermolecular interaction, paramagnetic impurities in electrolytes and other mechanisms; large contributions of intermolecular interactions with electrolytes are present on complex formation between amino acids and metal ions and the incoming 'unsaturation' of the primary solvation shell of cations with the increase in electrolyte concentration.     

Effects of uniaxial stretching and shrinkage on proton spin-lattice and spin-spin relaxation times of isotactic polypropylene

An isotactic polypropylene film was stretched at 120 °C in poly(ethylene glycol) and thermally shrunk at various temperatures. Proton spin-lattice,T ₁, and spin-spin,T ₂, relaxation times were measured using a broad line pulse spectrometer operating at 19.8 MHz in the temperature range 40 °C–100 °C. The temperature ofT ₁ minimum shifts to higher temperatures and the value ofT ₁ minimum increases in magnitude as the stretching ratio is increased. In contrast the temperature ofT ₁ minimum shifts to lower temperatures as shrinkage is increased, whereas the value ofT ₁ minimum increases in magnitude because of the increase in crystallimty during shrinkage. T_2a, the longestT ₁ associated with the mobile amorphous regions, increases during shrinkage, indicating that chain mobility in the amorphous regions increases substantially during shrinkage. It was found that an orientation function of the amorphous regions,f _a, correlates well withT _2a .Presented in part at the 52nd Annual Meeting of the Japan Chemical Society, Kyoto, April 1986.     

Carbon-13 NMR spin-lattice relaxation times of isotactic and syndiotactic sequences in amorphous polypropylene

Spin-lattice relaxation times (T₁) for methyl, methylene, and methine carbons in an amorphous polypropylene have been measured as a function of temperature from 46 to 138°C. The carbons from isotactic sequences characteristically exhibited the longest T₁'s of those observed. The T₁ differences increased with temperature with the largest difference occuring for methine carbons where a 32% difference was observed. Activation energies were determined for the motional processes affecting T₁'s for isotactic and syndiotactic sequences with essentially no dependence upon configuration noted.     

Nuclear spin relaxation times dispersion in disordered polymer systems

Nuclear spin relaxation reveals a wealth of information concerning motional dynamics and morphology within a polymer system. The conventional breakdown of relaxation into discrete decay components is usually arbitrary and often ambiguous due to strong coupling and spin diffusion between domains. To avoid being limited by pre-determined models, we have explored the utility of maximum entropy regularization scheme to reconstruct the complete NMR spin relaxation distribution continuum. Case studies showed that the three characteristics of the distribution correlate with various aspects of polymer morphology and with physical properties. These positive results suggested that this approach has the potential of yielding profiles on polymer dynamics and information concerning phonon coupling between domains in complex polymer systems which are not previously accessible. Furthermore, the nondestructive nature of NMR gives this approach an extra advantage over conventional tests for “in-situ” studies of polymers.     

Dynamics of cellulose-water interfaces: NMR spin-lattice relaxation times calculated from atomistic computer simulations

Solid-state nuclear magnetic resonance (CP/MAS 13C NMR) spectroscopy has often been used to study cellulose structure, but some features of the cellulose NMR spectrum are not yet fully understood. One such feature is a doublet around 84 ppm, a signal that has been proposed to originate from C4 atoms at cellulose fibril surfaces. The two peaks yield different T1, differing by approximately a factor of 2 at 75 MHz. In this study, we calculate T1 from C4-H4 vector dynamics obtained from molecular dynamics computer simulations of cellulose I beta-water interfacial systems. Calculated and experimentally obtained T1 values for C4 atoms in surface chains fell within the same order of magnitude, 3-20 s. This means that the applied force field reproduces relevant surface dynamics for the cellulose-water interface sufficiently well. Furthermore, a difference in T1 of about a factor of 2 in the range of Larmor frequencies 25-150 MHz was found for C4 atoms in chains located on top of two different crystallographic planes, namely, (110) and (10). A previously proposed explanation that the C4 peak doublet could derive from surfaces parallel to different crystallographic planes is herewith strengthened by computationally obtained evidence. Another suggested basis for this difference is that the doublet originates from C4 atoms located in surface anhydro-glucose units with hydroxymethyl groups pointing either inward or outward. This was also tested within this study but was found to yield no difference in calculated T1.     

Proton spin-lattice relaxation studies. N-aryl-1-isoindolinones

Proton spin-lattice relaxation rates (R₁ values) have been measured, at 270 MHz, for a series of N-aryl isoindolinones. A normalization procedure has been used to enable comparison of R₁ values in different compounds by minimizing the effects on relaxation rates of changes in motional correlation times accompanying changes in substitution patterns. A substantial (4.3-fold) dynamic range of R₁ values has been observed, and individual values have been correlated with the molecular environments of the nuclei. There is evidence for an interring relaxation process.     

The conformational analysis of aromatic methoxyl groups from carbon-13 chemical shifts and spin-lattice relaxation times

In a series of polymethoxy benzaldehydes and acetophenones it was found that diortho-substitution distorts the ArOCH₃ conformation. This distortion is accompanied by a downfield shift in the corresponding methoxyl ¹³C frequencies and an increase in their spin-lattice relaxation times.     

15N-NMR. and 31P-NMR. Studies of palladium and platinum complexes

¹⁵N-NMR. parameters for the complexes trans-[MCl₂ (¹⁵NH₂ (CH₂)₅CH₃)L] are reported; M = Pt, Pd, L = PBu, PMePh₂, P (p-CH₃? C₆H₄)₃, AsBuⁿ₃, AsMePh₂, As (p-CH₃C₆H₄)₃, NH₂ (CH₂)₅CH₃ and (for Pt) C₂H₄. For both metals, the NMR. parameters depend on the trans-influence of the ligand L. The values ¹J (¹⁹⁵Pt, ¹⁵N) vary from 138 to 336 Hz and can be shown to correlate with the values ¹J (¹⁹⁵Pt, ³¹P) in the complexes trans-[PtCl₂ (PBu)L]. There is a linear relation between the ¹⁵N chemical shifts in the complexes of the two metals. The reactions of the complexes sym-trans-[M₂Cl₄L₂], M = Pd, Pt, L = a tertiary phosphine or arsine, with neutral ligands are described. ¹⁹⁵Pt-, ³¹P- and ¹³C-NMR. data are reported.     

Probing protein hydration and aging of food materials by the magnetic field dependence of proton spin-lattice relaxation times

Most cheeses can be considered as solid emulsions of milk fat in a matrix of water and proteins. Regions of each of the phases can be liquid during processing and maturation. Identifying these regions and monitoring changes in them is important as a prelude to controlling the structure of the final cheese. We concentrate on the behavior of water in the vicinity of proteins as a function of cheese aging. Our method utilizes nuclear magnetic relaxation dispersion (NMRD) associated with the frequency dependence of water spin-lattice relaxation rates using the field cycling NMR technique. This method provides insight into the dynamical behavior of water molecules on a very large time scale. Moreover, we can distinguish between molecular motion in bulk and motion in the vicinity of a source of relaxation, such as proteins. A fit of our dispersion data using a theory developed by J.-P. Korb and R.G. Bryant (J. Chem. Phys. 115 (2001) 23) allowed us to determine the degree of hydration of proteins as a function of aging. In particular, we find that protein hydration increases with ripening.     

Theory of paramagnetic spin-lattice relaxation in solids. Part I

A method is proposed for calculating T₁ for an isotropic solid described by the Debye model; the formulas reduce the computations of T₁ to determination of the form of the spin-lattice hamiltonian, which is governed by the spin of the paramagnetic particle and by the atomic coordinates. This provides an explanation of the behavior of T₁ over the entire temperature range from a single point of view.The Kronig-van Vleck theory disagrees with experiment at low temperatures; the causes of this are demonstrated, together with the changes in the formulas that are needed to eliminate it. The T₁ for radicals with appreciable anisotropy in g is calculated to illustrate the argument.     

Effects of lattice anharmonicity on spin-lattice relaxation in solids

A derivation is given for the formulas defining the operator for the spin-lattice interaction as averaged over the orbital motion of the unpaired electron in the solid. Simple formulas are derived for the constants of the spin-lattice interaction. It is shown that the relaxation time T₁ of this interaction may be substantially dependent on the concentration of lattice defects at helium temperatures; approximate relations between T₁ and the phonon mean free path are derived.     

Relation between 35Cl NQR frequencies and spin-lattice relaxation times in HGeCl3

An equation relating ³⁵Cl NQR frequencies toinverse spin-lattice relaxation times was found for HGeCl₃ crystalline phases. The equation was used to identify shortand long-range order contributions to the NQR frequency in the environment of the resonance chlorine atom. Changes in the strength of local electric fields caused by the crystal field were determined.