NMR Spin-lattice Relaxation Times Research Articles

Relaxation mechanisms, bonding, and molecular reorientation in CpRe(CO) 3

Carbonyl 13C and 17O and cyclopentadienyl 13C NMR spin-lattice relaxation times of CpRe(CO) 3 were measured as a function of temperature in the solvent CDCl 3. Analysis of T 1( 13CO) revealed that, unlike in most metal carbonyls, where chemical shift anisotropy is the dominant mechanism, the major contribution to 13C relaxation is via scalar coupling to the directly bonded rhenium. The oxygen-17 quadrupole coupling constant [determined from T,( 17O)], and theirπ★ antibonding orbital population was very close to values reported in several (arene)Mo(CO) 3 complexes, indicating similar pi bond strengths. The C-K force constants ( k co), on the other hand, were quite dissimilar, indicating that k co is not a pure measure of π bonding in metal carbonyls. Comparison of rotational correlation times of the cyclopentadienyl group and M(CO) 3 unit revealed substantial internal rotation of the Cp ring. The rate, however, is slower than expected for afree rotor, indicating the prese

High Pressure NMR Studies of Confined Liquids

AbstractThe main goal of our novel NMR experiments on confined liquids was to determine the effects of pressure on the dynamics of liquids in the surface layer by using the twostate, fast exchange model and compare them to the pressure effects observed for bulk liquids. With this goal in mind, the deuteron NMR spin-lattice relaxation times, T1, in liquid pyridine-d5, nitrobenzene-d5, and methylcyclohexape-d confined to sol-gel porous silica glasses with pore radii in the range from 18Å to 49˚A were measured as a function of pressure up to 5 kbar at 300 K. In another set of high resolution natural abundance 13C NMR experiments, the 13C relaxation behavior of each carbon in 2- ethylhexyl benzoate model lubricant was measured as a function of pressure in porous silica glasses. In fact, the described experimental approach which allows investigation of the effects of pressure on the dynamic behavior of surface-layer liquids, may provide a new tool in studies of model liquid lubricants at extreme conditions of pressure and temperature. In addition selected results of a recent study of acetonitrile-d3 are reviewed. The 2H and 14N spin-lattice relaxation times of acetonitrile-d3 in porous silica glasses were measured in order to study the confinement effects on the anisotropic reorientation characterized by rotational diffusion constants, D⊥ and D‖, of this symmetric-top molecule.

Spin-lattice relaxation in murine tumors afterin vivo treatment with interferon α/β or tumor necrosis factor α

1H NMR spin-lattice relaxation times (T1) were measured in vitro and in vivo in Friend leukemia cell tumors during subcutaneous tumor growth in syngeneic mice and after in vivo administration of either purified murine interferon alpha/beta (IFN) or recombinant tumor necrosis factor alpha (TNF). Untreated tumors exhibited monoexponential T1 relaxation independently of tumor age at least until Day 16 after implantation. Histological examinations showed that under these conditions tumors were highly homogeneous and substantially free of necrotic areas. Peritumoral administrations of either IFN or TNF did not significantly alter the tumor relaxation properties at early stages of inhibition of tumor growth. The longitudinal relaxation decay became instead clearly biexponential at later stages (more than 7 days of IFN treatment or 2 days after TNF administration). While the T1 relaxation behavior could be unequivocally correlated with the presence of necrotic areas in these tumors, it could not be considered as an early marker of the altered growth capability, induced by administration of either IFN or TNF.

Multi‐frequency, variable‐temperature and high‐pressure study of carbon‐13 spin—lattice relaxation in tetraoctyltin

AbstractCarbon‐13 NMR spin–lattice relaxation times, T1, and nuclear Overhauser effect data were obtained for neat tetra‐n‐octyltin at 22.5 and 100.6 MHz over a wide temperature range. T1 values were also measured at 25.16 MHz at pressures up to 211 MPa. The ambient‐pressure data were fitted to the ‘model‐free’ two‐correlation time spectral density with a temperature‐dependent order parameter, S2. The pressure data were interpreted by assuming that internal motions of the octyl chain are independent of pressure while the overall (tumbling) motion and S2 are pressure dependent. The values of the motional parameters for tetra‐n‐octyltin lie in the ranges expected for compounds containing long n‐alkyl chains. The results show that this approach is applicable to complex liquids such as synthetic lubricants of the polydecene type.

Conformational Stability of V-Amyloses and Their Hydration-Induced Conversion to B-Type Form as Studied by High-Resolution Solid-State 13C NMR Spectroscopy

Abstract We have recorded 13C NMR spectra of several preparations of amorphous samples, V-amyloses and their hydrates to gain insight into their relative stability and subsequent hydration-induced conformational change. It was found that the 13C NMR line width of the iodine complex prepared as a precipitate in aqueous solution was the narrowest among these preparations, including the iodine complex formed from cast film. It is noteworthy that these V-amyloses are not always stable when they are equilibrated in an atmosphere of 96% R. H. in a desiccator and converted to B-type form, consistent with previous observation by Senti and Witnauer (J. Am. Chem. Soc., 70, 1438 (1948)). 2H NMR line shape and spin-lattice relaxation times were measured for partially deuterated V and B-type amylose samples containing DMSO-d6 and deuterium oxide, respectively, to explore the dynamics of the DMSO and water molecules bound to the polymer chains. It is unlikely that any drastic conformational change such as unfolding followed by refolding and plausible change of handedness is caused by such mild physical treatment as humidification. Therefore, it appears that the resulting B-type form should be ascribed to a left-handed single helix as a consequence of a slight modification of V-amylose, in contrast to the previous interpretation based on the data of X-ray diffraction and NMR studies. Gelation mechanism of amylose gel based on this view is also discussed.

Mobility of the terminal regions of flagellin in solution.

The mobility of the disordered terminal regions of flagellin was examined in detail based on 1H NMR chemical shifts and spin-lattice relaxation times in the rotating frame. Proteolytic fragments of flagellin with terminal deletions of different sizes were used to compare the dynamical properties of various N- and C-terminal segments. We found that dynamic properties of different terminal segments were similar to each other and were close to those of the heat-denatured state of flagellin. The main chain of these terminal segments undergoes rapid motions with effective correlation times of 1.3-4.1 x 10(-9) s. The terminal regions contain no large segments with well-defined structure. However, comparison with the random-coiled state of poly-L-lysine suggests significant structural constraints in the terminal regions (as well as in the heat-denatured flagellin) which may reflect the existence of some highly fluctuating secondary structure, as suggested by earlier CD studies.

Multinuclear solid state NMR investigation of titanium-containing silicate glasses

The bonding and local coordination geometry of aluminium and silicon atoms in a series of potassium aluminosilicate glasses containing 7 mol% TiO 2 have been studied by 29Si and 27Al NMR. Although the K/Al concentration ratio is varied between 0.76 and 1.40, no simple trends are observed in either the 29Si and 27Al MAS-NMR spectra. Both Si and Al atoms are found to be four-fold coordinated by oxygen atoms; thus, Al atoms occupy network sites in these glasses. In addition to MAS, non-spinning NMR spectra and spin-lattice relaxation times were measured. The 29Si T 1 data confirm that titanium is present as Ti 4+; the 29Si MAS data conclude that Ti cations are located in extra-framework sites. Thus Ti 4+ must undergo the rearrangements necessary to provide for charge balancing the aluminosilicate framework in glasses with K/Al ratios < 1. From chemical shift distributions it is known that the mean 〈 Si O T〉 (T = Si or Al) and 〈AlOSi〉 bond angles differ. A hypothesis of different local structures for silicate and aluminate tetrahedrons and different medium range ordering are in agreement with the bicomponent Si T 1 measurements.

Longitudinal NMR relaxation of degenerate two-dimensional 3He in 4He films.

The NMR spin-lattice relaxation time ${\mathit{T}}_{1}$ of a degenerate submonolayer coverage of $^{3}\mathrm{He}$ in a thin adsorbed $^{4}\mathrm{He}$ film has been measured. As a function of $^{4}\mathrm{He}$ coverage, ${\mathit{T}}_{1}$ exhibits a maximum and undergoes a change in temperature dependence at a coverage near that of the superfluid transition in the underlying $^{4}\mathrm{He}$ film. Two relaxation mechanisms contribute to the observed ${\mathit{T}}_{1}$.

Confined geometry effects on reorientational dynamics of molecular liquids in porous silica glasses

This work investigates the relative role of the pure geometrical confinement and the strength of the surface effect on the dynamics of liquids in porous silica glasses prepared by the sol-gel process. The deuteron NMR spin-lattice relaxation times T1 of several molecular liquids in porous silica glasses are reported as function of pore size in the range from 18 to 143 Å over the temperature range from 260 to 310 K. Molecular liquids studied include strongly interacting polar liquids such as pyridine-d5, aniline-d5, and nitrobenzene-d5, whereas the saturated cyclic hydrocarbon liquids of cyclohexane-d12 and cis-decalin-d18 represent the weakly interacting liquids. In a first approximation, toluene-d1 and dioxane-d18 are chosen as examples of liquids with intermediate interactions with the silica surface. The experimental relaxation data are analyzed by using the two-state, fast-exchange model which is found to be valid for the strongly interacting liquids and liquids with intermediate interactions. In terms of this model, the viscosity of the surface layer for pyridine-d5 is about 30 times higher than that for bulk liquid pyridine. The importance of the two-dimensional approach to describe motional dynamics of liquids confined to pores smaller than 30 Å is illustrated in the case of weakly interacting liquid of cyclohexane-d12. Additional information on the relative role of surface interactions and the pure topological effects on the dynamics of liquids in confined geometries was obtained by using surface-modified glasses in which the surface hydroxyl groups were replaced by OSi(CH3)2OC2H5 groups. Indeed, the effects of surface modifications on the 2H T−11 are most pronounced for strongly interacting liquids whereas they are absent for cyclohexane. In agreement with the concept of two-dimensional behavior of liquids in small pores, one finds that the low-frequency relaxation times, namely, the spin–spin relaxation time T2, and the spin-lattice relaxation time in the rotating coordinate frame, T1ρ, remain unchanged by surface modification. In fact, this is a consequence of logarithmic enhancement of the spectral density at low frequencies so that the effect of pure geometrical confinement on the T−12 and T−11ρ relaxation rates is much larger than any relaxation rate changes arising from surface modification. Several selected NMR T1 experiments on pyridine-d5 confined to anopore and zeolites are also presented.

Energetics of 3He states in 4He films.

We report measurements of the NMR spin-lattice relaxation time, ${\mathit{T}}_{1}$, of $^{3}\mathrm{He}$ in $^{4}\mathrm{He}$ films for 0.03T0.60 K as a function of $^{4}\mathrm{He}$ coverage for 0.1 layer of adsorbed $^{3}\mathrm{He}$. The results show thermally activated behavior for T&gt;0.25 K and yield the energy difference, \ensuremath{\delta}\ensuremath{\epsilon}, between the $^{3}\mathrm{He}$ ground state and the first excited state; \ensuremath{\delta}\ensuremath{\epsilon} shows substantial structure as a function of $^{4}\mathrm{He}$ coverage.

Reorientational dynmics and internal rotation in W(PMe 3)(CO) 5

Oxygen-17 and methyl group carbon-13 NMR spin-lattice (T 1) relaxation times were measured in the metal carbonyl, W(PMe 3)(CO) 5, as a function of temperature in the solvent chloroform. Perpendicular and parallel rotational diffusion coefficients (D 1 and D d), calculated from T 1( 17O) of the axial and radial carbonyls were compared with the predictions of various a priori theories of molecular reorientation. The Nu-Zwanzig ‘slip’ model yielded values more than an order of magnitude greater than experiment. Calculated diffusion constants from the Perrin ‘stick’ theory and the newer Hynes-Kapral-Weinberg model were approximately a factor of three times smaller or greater, respectively, than the measured values. Rotational correlation times, τ c(CH), were calculated from methyl 13C relaxation times and nuclear Overhauser enhancements. These values were used, together with the rotational barrier in free trimethylphosphine, to calculate the rates of internal rotation about the PW and PC bonds. It was observed that there is a significant degree of rotation about both of these bonds in W(PMe 3)(CO) 5. However, in neither case were the rates as rapid as would be expected in the absence of a barrier to internal rotation.

Cationic Self-Diffusion in Ionic Plastic Phases of Thallium Nitrite and Nitrate and in Thallium Thiocyanate

Abstract Cationic self-diffusion in the ionic plastic phases of TlNO2 and TlNO3 was investigated by measuring temperature dependences of the 203,205Tl NMR spin-lattice relaxation time and the electrical conductivity. The NMR results snowed that the Tl+ ions in these phases undergo rapid translational diffusion with the activation energy (Ea) of 48 and 77 kJ mol−1 for nitrite and nitrate, respectively. The Ea values agreed well with those obtained from the electrical conductivity, indicating that the cation is the electric carrier. The diffusion constants (D) estimated from the relaxation time and conductivity data were ca. 10−12 m2 s−1 at the respective melting point (Tm) of plastic phase. The tendency that D at Tm has the characteristic value given above is recognized for other ionic plastic phases so far reported. The Tl+ ionic conductivity in TlSCN, which makes no plastic phase, was reported in comparison.

Rotational Excitations and Tunneling of Nonequivalent Methyl Groups in Tetramethylstibonium Iodide as Studied by Nuclear Magnetic Resonance and Inelastic Neutron Scattering

Abstract Nuclear magnetic resonance (NMR) and inelastic neutron scattering techniques (INS) have been applied to study the rotational motions and methyl group tunneling in tetramethylstibonium iodide, [Sb(CH3)4 ] I, over er a wide temperature range. Parameters describing the [Sb(CH3)4]+ cation tumbling and the methyl group reorientation at high temperatures and quantum mechanical tunneling of the methyl groups at low temperatures were determined. The results for INS and NMR experiments at low temperatures can be explained in terms of two crystallographically inequivalent methyl groups CH3(1) and CH3(2), which were established earlier by the crystal structure determination. In the INS spectra two tunneling lines at 22.0 μeV for CH3(1) and 1.05 μeV for CH3(2) with inelastic intensities in the ratio 3:1 were observed at T = 4 K. The activation energies derived from proton NMR spin-lattice relaxation time measurements for the thermally activated methyl group rotation are 1.50 kJ/mol for CH3(1) and 3.81 kJ/mol for CH3(2). They are in accordance with the activation energies obtained from neutron fixed-window measurements. The activation energy for [Sb(CH3)4]+ cation tumbling is 50.9 kJ/mol as determined from the high temperature spin-lattice relaxation behaviour. Rotational potentials for the methyl groups are derived. For both kinds of inequivalent methyl groups the threefold potential terms dominate; three- and sixfold potential contributions are shifted by 180°

Molecular Motion in Solid Tetrabutylammonium Tetrafluoroborate

Abstract Proton and fluorine NMR second moments and spin-lattice relaxation times for polycrystalline tetrabutylammonium tetrafluoroborate have been measured over a wide range of temperatures at several Larmor frequencies. An analysis of cross-relaxation effects results in a determination of activation parameters for anion and cation reorientations. Three solid phases characterized by different ion dynamics are evidence

Competition between Li + and Mg 2+ for ATP and ADP in aqueous solution: A multinuclear NMR study

We used 7Li NMR spin-lattice relaxation times and 31P NMR chemical shifts to study the binding of Li + and Mg 2+ to the phosphate moieties of ATP and ADP. To examine the binding of Li + and Mg 2+ to the base and ribose moieties, we used 1H and 13C NMR chemical shifts. The 7Li NMR relaxation times of Li +/Mg 3+ mixtures of ATP or ADP increased with increasing concentrations of Mg 2+, suggesting competition between the two ions for adenine nucleotides. No significant binding of Li + and Mg 2+ to the base and ribose moieties occurred. At the pH and ionic strength used, 2:1 and 1:1 species of the Li +-ATP and Li +-ADP complexes were present, with the 2:1 species predominating. In contrast, 1:1 species predominated for the Mg 2+-ADP and Mg 2+-ATP complexes. We calculated the Li +-nucleotide binding constants in the presence and absence of Mg 2+ and found them to be somewhat greater in the presence of Mg 2+. Although competition between Li + and Mg 2+ for ATP and ADP phosphate binding sites in solution is consistent with the 31P chemical shift data, the possibility that the Li + and Mg 2+ form mixed complexes with the phosphate groups of ATP or ADP cannot be ruled out.

Structure and molecular dynamics of the cuticular wax from leaves of Citrus aurantium L

Cuticles and wax from the leaves of Citrus aurantium L. have been investigated by using NMR, DSC, X-ray diffraction and gas chromatographic methods. The wax, consisting mainly of 1-alkanols and n-alkyl esters, has chain lengths ranging from 25 to 53 carbon atoms with an average chain length of 34 carbon atoms. The X-ray results show that the wax is to a large extent ( approximately 80%) amorphous and that the crystalline fraction consists mainly of the n-alkyl esters with an average chain length of 43.5 carbon atoms. The activation energies for the different molecular motions in the wax have been extracted from the NMR spin-lattice relaxation time measurements. The activation energy associated with the reorientation of the chains in the amorphous fraction is much lower than that in other natural and Fischer-Tropsch waxes. The molecular reorientations in the cuticle membranes can best be described in terms of a model in which the correlation time has a lognormal distribution. No major difference could be detected between the DSC thermograms of the sample of cuticular membranes and the sample of matrix membranes from which the wax was extracted. Similarly, the temperature and frequency dependence of the spin-lattice relaxation times of these samples are equal. It is concluded that the presence of wax molecules does not influence the molecular dynamics of the matrix membranes noticeably.

Study of molecular motions in the orientationally disordered organic solids 1-bromoadamantane and 1-adamantanecarboxylic acid by carbon-13 NMR spin-lattice relaxation and dipolar dephasing time measurements

Study of molecular motions in the orientationally disordered organic solids 1-bromoadamantane and 1-adamantanecarboxylic acid by carbon-13 NMR spin-lattice relaxation and dipolar dephasing time measurements

Temperature dependence of the NMR relaxation time, T 1, of 3He in thin 3He- 4He films at various 4He coverages

The NMR spin-lattice relaxation time, T 1, of a degenerate submonolayer coverage of 3He on thin 4He films has been measured. As a function of the 4He coverage, T 1 exhibits a maximum, and crosses over to a region with an anomolous temperature dependence at higher coverages.

Solid-state deuteron and proton NMR of guest and host motions in the Hofmann-type and related benzene clathrates

The motional behavior of the guest benzene molecule in the Hofmann-type benzene clathrate Cd(NH 3 ) 2 Ni(CN) 4 •2C 6 D 6 and related benzene clathrates Cd(en)Ni(CN) 4 •2C 6 D 6 (en=ethylenediamine) and Cd(dma) 2 Ni(CN) 4 •(1/2)C 6 D 6 (dma=dimethylamine) were investigated by 2 H NMR line shape and spin-lattice relaxation time studies

Reorientational dynamics and the barrier to internal rotation in Cp2Co3(.mu.2-CO)(CO)3(.mu.3-CPh)

NMR spin-lattice relaxation times of the phenyl, cyclopentadienyl, and terminal carbonyl 13 C nuclei in the title compound were measured as a function of temperature in the solvent chloroform. The phenyl carbon T 1 's were used to calculate the diffusion coefficient characterizing the ring's net spinning rate. Internal rotation rates of the cyclopentadienyl groups, R(Cp), were found to be far greater than R(Ph) at all temperatures, indicating that the barrier to phenyl group rotation is not due to inter-ring steric repulsions. Rather, it can be attributed to an electronic interaction of the bridging carbonyl's π* orbital with the phenyl π system through the metal 2e and carbynyl 2p orbitals. This is only the second system in which a bridging carbonyl/capping group orbital interaction has been observed and the first for which the barrier range to internal rotation has been experimentally determined