Molecular Reorientational Time Research Articles

Time-resolved infrared spectroscopy of ethanol monomers in liquid solution: molecular reorientation and energy relaxation times

Two-color pump-probe spectroscopy has been performed with subpicosecond time resolution in the CH- and OH-stretching regions of ethanol monomers dissolved in carbon tetrachloride in the temperature range from 259 to 333 K. From the isotropic component of the probe transmission signal we deduce the lifetime of the OH-stretching vibration to be T 1 = 8 ± 1 ps while the molecular reorientation is directly observed from the anisotropic signal to occur with τ or = 2 ± 0.5 ps at room temperature. Additional information on the relaxation pathway of the excited OH-strecthing and on the asymmetric CH 3-stretching vibration of ethanol is presented as well as evidence for an inhomogeneous broadening of the monomeric OH-vibration.

Comparison of ROESY and EXSY Methods Using Bloch Equations

A comparison of longitudinal and transverse frame magnetization transfer methods is provided using modified Bloch equations for a two-spin system representative of a methylene group exchanging between two chemically distinct sites. This simple formalism incorporates chemical shift, auto- and cross-relaxation terms, chemical exchange first-order rate constants and r.f. field terms for ROESY experiments. Overhauser and exchange magnetization transfer is emulated at three limits of molecular reorientation time (τc). The calculated build up curves suggest a parasitic effect of ROE on chemical exchange magnetization transfer. In addition, for molecules harboring spin systems exchanging between sites such that the exchange rate approximates the transverse cross-relaxation rate, and reorienting in solution at rates slower than that defined by the CAMELSPIN limit (ω0τc≈1.118), the calculations suggest observation of exchange or Overhauser cross peaks may be compromised. Finally, the effect of off-resonance spin locking r.f. power level is explored upon exchange or Overhauser build-up curve profiles. The results suggest off-resonance approaches that at low r.f. spin locking power may adversely affect ROE or exchange build-up rates and may compromise either exchange rate determination or internuclear distance estimation.

Relaxometric, Structural, and Dynamic NMR Studies of DOTA-like Ln(III) Complexes (Ln = La, Gd, Ho, Yb) Containing a p-Nitrophenyl Substituent

We report here the synthesis of the ligand 1-((p-nitrophenyl)carboxymethyl)-4,7,10-tris(carboxymethyl)-1,4,7,10-tetraazacyclododecane (3) and its La(III), Gd(III), Ho(III), and Yb(III) complexes. The introduction of the p-nitrophenyl substituent on the methylenic carbon of one acetate group does not alter the overall chelating ability of 3 with respect to the parent DOTA ligand. The [Gd(3)]- complex displays a slightly higher relaxivity than that of [Gd(DOTA)]-, mainly as a consequence of a longer molecular reorientational correlation time (τR), due to the increased molecular dimension of the complex, and of limited changes to the other relaxation parameters. A further increase of relaxivity has been observed upon formation of an inclusion compound with β-cyclodextrin. The solution structure and dynamics were thoroughly investigated by high-resolution NMR spectroscopy. Of the four possible enantiomeric pairs that could be present in the solutions of monosubstituted derivatives of [Ln(DOTA)]- complexes, the proton spectra of Ho and Yb derivatives are consistent with the occurrence of only two isomeric species whose structures have been elucidated through analysis of dipolar shifts and 2D-EXSY data. In both species the bulky aromatic group has replaced the acetate proton pointing outward from the coordination cage. Unlike in the DOTA case, in [Ln(3)]- complexes the isomerization process involves the inversion of the ethylenic groups of the macrocycle rather than the motion of the acetate arms. This behavior is rationalized in terms of steric crowding at the substituent site: minimization of the steric interactions between the aromatic group and the macrocyclic ring protons results in both structural and dynamic selectivity. Interestingly, in the case of the diamagnetic La(III) complex the variable temperature behavior of the 13C NMR spectra is consistent with an exchange process involving one major species and at least one minor isomer of very low concentration, whose relative population increases with temperature. This causes a persistent exchange broadening of the resonances over a wide range of temperatures.

Molecular reorientation dynamics and microscopic friction in liquids

Molecular rotation reorientation times are investigated using time resolved fluorescence depolarization studies of three solutes of similar size and shape (nile red, neutral nile blue and cationic nile blue) dissolved in alcohol and alkane solvents as well as an extensive compilation of previous results for neautral and charged solutes dissolved in non-polar, polar and associated solvents. A universal correlation is foung between reorientation time, solvent viscosity, and solute volume for solutes dissolved in alkanes, while strongly interacting solutes experience relatively enhanced friction, and non-polar solutes dissolved in alcohols experience reduced friction. The results are compared and contrasted with slip and stick hydrodynamic predictions, and used to develop empirical correlations, which can be used to predict molecular reorientation times with an uncetainty on the order of a factor of two in virtually any solute-solvent system.

Intermolecular interactions and the nature of orientational ordering in the solid fullerenes C 60 and C 70

We have proposed an intermolecular potential for C 60 molecules that not only reproduces the correct low-temperature structure, but also correlates a wide range of experimental properties, including the molecular reorientational time in the room-temperature rotator phase, the volume change at the orientational ordering transition, and the librational frequencies in the low-temperature phase. The low- pressure phases in solid C 70 have been explored using constant-pressure molecular dynamics and an intermolecular potential derived from one that gives an excellent account of the properties of solid C 60 . The molecular dynamics calculations predict three low-pressure phases: a high-temperature rotator phase, a partly ordered phase with trigonal symmetry, and an ordered monoclinic phase. The calculations on C 70 were carried out on a cluster of IBM RS/6000s, operating in parallel.

C60 solid state rotational dynamics and production and EPR spectroscopy of fullerenes containing metal atoms

The narrow single-line NMR spectrum of solid C60 at ambient temperature demonstrates that the molecules rapidly reorient within the crystal lattice. The rate of this motion can be quantitatively probed using solid-state NMR by measuring the relaxation rate, 1/T1. The dominant contribution to this rate at ambient temperature arises from the combination of rotation and chemical shift anisotropy (CSA). The contribution, 1/T1CSA, is isolated by measuring the magnetic field dependence of T1, allowing the molecular reorientational correlation time, tau , to be determined from T1CSA and the CSA tensor components. At 283 K tau =9.1 ps, only three times longer than the calculated tau for free rotation, and shorter than the value measured for C60 in solution (15.5 ps). This tau corresponds to a rotational diffusion constant D=1.8*1010 s-1. Below 260 K a second phase with a much longer reorientation time is observed, consistent with reports of an orientational phase transition in solid C60. In both phases tau shows Arrhenius behaviour with apparent activation energies of 1.4 and 4.2 kcal mol-1 for the high-temperature (rotator) and low-temperature (ratchet) phases, respectively. The authors succeeded in producing quantities of fullerene cages containing metal atoms sufficient to allow the first characterization of such a species by electron spin resonance spectroscopy. The EPR spectrum of La@C82 shows an octet of lines centered in the region characteristic of fullerene anion radicals, with a small hyperfine splitting which indicates that the spin is predominantly associated with the carbon shell, leaving the lanthanum atom in its preferred +3 oxidation state.

Solid C70: A molecular-dynamics study of the structure and orientational ordering.

Classical constant-pressure molecular-dynamics simulations have been performed for solid ${\mathrm{C}}_{70}$ using a pairwise-additive atom-atom Lennard-Jones potential that gave a good account of the high-temperature rotator phase of solid ${\mathrm{C}}_{60}$. The simulation results indicate that freezing of the rotational motion of ${\mathrm{C}}_{70}$ molecules about the long and short axes occurs at different temperatures. At high temperatures, an orientationally disordered fcc phase is preferred over a hcp lattice. At low temperature, a triclinic phase seems to be favored by the present model, but other structures (space group Pa3, and hcp) also have very competitive potential energies. Estimates of the molecular-reorientation times are given. The librational phonon density of states has been studied. As expected, libration frequencies for rotation about the long axis are lower than that about the short axis.

C 60 Rotation in the Solid State: Dynamics of a Faceted Spherical Top

The rotational dynamics of C(60) in the solid state have been investigated with carbon-13 nuclear magnetic resonance ((13)C NMR). The relaxation rate due to chemical shift anisotropy (1/9T1(CSA)(1)) was precisely measured from the magnetic field dependence of T(1), allowing the molecular reorientational correlation time, tau, to be determined. At 283 kelvin, tau = 9.1 picoseconds; with the assumption of diffusional reorientation this implies a rotational diffusion constant D = 1.8 x 10(10) per second. This reorientation time is only three times as long as the calculated tau for free rotation and is shorter than the value measured for C(60) in solution (15.5 picoseconds). Below 260 kelvin a second phase with a much longer reorientation time was observed, consistent with recent reports of an orientational phase transition in solid C(60). In both phases tau showed Arrhenius behavior, with apparent activation energies of 1.4 and 4.2 kilocalories per mole for the high-temperature (rotator) and low-temperature (ratchet) phases, respectively. The results parallel those found for adamantane.

Modeling the orientational ordering transition in solid fullerene (C60)

The authors propose a new intermolecular potential for C{sub 60} molecules that not only reproduces the correct low-temperature structure but also correlates a wide range of experimental properties. These properties include the molecular reorientational time in the room temperature rotator phase, the volume change at the orientational ordering transition, and the librational frequencies in the low-temperature phase. 19 refs., 2 figs.

Determination of metal–proton distances and electronic relaxation times in lanthanide complexes by nuclear magnetic resonance spectroscopy

The longitudinal and transverse relaxation times of the proton resonances of three lanthanide(III) complexes of 1,4,7,10-tetraazacyclododecane-N,N′,N″,N′″-tetraacetate have been measured. This allows determination of the metal–proton distances and the electronic relaxation times of the paramagnetic metal ions by exploiting the Curie-spin mechanism contribution to the transverse relaxation rates, once the molecular reorientational time τr is known by an independent experiment. Values of the parameters calculated from data obtained at a single magnetic field (9.4 T) were in good agreement with those found from the evaluation of the magnetic field-induced line broadening of the proton resonances as measured at 2.1, 4.7 and 9.4 T. A chemical exchange contribution to the linewidths is excluded on the basis of experiments performed at different temperatures.

NMR Studies of Melanins: Characterization of a Soluble Melanin Free Acid From Sepia Ink

This paper deals with the nuclear magnetic resonance characterization of a soluble derivative (melanin free acid) of Sepia melanin obtained by a peroxidative treatment of the parent (insoluble) species. High resolution 13C and 15N solid state NMR spectroscopies allow the assessment of the chemical changes occurring in the macromolecule upon solubilization. 1H and 13C NMR solution spectra are discussed in light of the results obtained from the solid state spectra. Furthermore, the coordination properties of melanin have been investigated through 27Al NMR spectroscopy and proton relaxation enhancement studies of the paramagnetic gadolinium complex of melanin free acid. Through these experiments it has been possible to evaluate the molecular reorientational time tau R (and from it an estimated molecular weight close to 20 KDa) and the strength of the metal-macromolecule interaction.

Inclusion complexes between β‐cyclodextrin and β‐benzyloxy‐α‐propionic derivatives of paramagnetic DOTA‐ and DPTA‐Gd(III) complexes

AbstractThe addition of β‐cyclodextrin to a solution containing β‐benzyloxy‐α‐propionic‐substituted DOTA‐ and DTPA‐Gd(III) complexes leads to the formation of inclusion compounds whose equilibrium constants have been determined by NMR titrations. The main effect associated with the formation of this ‘host‐uest’ interaction is a marked elongation of the molecular reorientational time, τr, which in turn results in increased solvent proton relaxation rates.

Theoretical study of ion solvation at the water liquid–vapor interface

Molecular dynamics calculations are reported for several ions at the liquid–vapor interface of water. The transition from the bulk to the interface region is investigated from structural, energetic, and dynamical points of view by calculating ion–water geometries, radial distribution functions, solvent molecular reorientation times, solvent polarization fluctuations, and solvation free energy as a function of distance from the interface. It is shown that ions tend to keep most of the structural and dynamical properties of their first solvation shell intact as they are moved into the interface, and that the tendency for negative adsorption (positive free energy of adsorption) is associated with weaker and fewer long range interactions. A comparison of some of the molecular dynamics results to predictions of simple continuum models is discussed, showing generally poor quantitative agreement.

Trends in NMR studies of paramagnetic Gd(III) complexes as potential contrast agents in MRI

The paramagnetic Gd(III) complexes with polyaminocarboxylate ligands are intensively studied as possible Contrast Agents for Magnetic Resonance Imaging (M.R.I.). Their ability to enhance the solvent proton relaxation rate is mainly determined by the molecular reorientational time (τ R). In order to increase τ R we studied the formation of non-covalent interactions between functionalized Gd(III)-complexes and micelles. The N.M.R.D. (Nuclear Magnetic Relaxation Dispersion) profiles of aqueous solutions of these paramagnetic complexes with hexadecyltrimethylammonium bromide (CTABr) were measured and the results could be accounted for by difference in negative charge and number of hydrophobic aromatic residues among the different complexes.

Relaxations Studies of 2IZSZSpin Order with Signal Enhancement by Coherence Transfer

The pulse sequence for generating coherence transfer (or polarization transfer) is invoked to enhance the signal of heteronuclear two spin order (e.g. 2IZSZ) in a spin system with CH moiety. This allows the observation of selective conversion of Zeeman order, in a sample with natural abundant 13C nuclei, into two spin order for measuring cross-correlation of chemical shift anisotropy and dipole-dipole interactions. The molecular reorientational correlation time and the orientation of the C–H axis with respect to the principal axes of carboxyl CSA tensor may be determined simultaneously in the relaxation profile of two spin order.

Structure and dynamics of a lipoic acid-arsenical adduct

The lipoic acid-phenyldichloroarsine adduct was prepared in methanol, and the structure and molecular motions of this adduct were studied. The results showed that a six-membered heteroatom adduct was formed. One-dimensional and two-dimensional NMR spectroscopy was used to confirm the structure and assign some of the resonances in the proton and carbon spectra. Spin-lattice relaxation times of the various carbon atoms indicated that the overall molecular reorientation time (tau R) of the molecule is 0.02 ns at 30 degrees C. An Arrhenius plot of the data showed that the activation energy (Ea) for molecular tumbling is 13.4 kJ/mol.

Collective behavior of supercooled liquids in porous media.

The study of fluids in porous media has clearly demonstrated that their fundamental properties are strongly influenced by confinement in small structures. Previous microscopic studies of the molecular reorientation time of liquid oxygen filling porous glasses1 has given clear proof that the freezing point of a confined fluid in a pore of radius R may be depressed by an amount ΔT which is inversely proportional to the pore radius. A phenomenological model based only on the effects of geometrical constraint was consistent with the observed supercooling and picosecond molecular realxation. In contrast to a rapidly developing microscopic picture, little is known about confined fluids over longer length scales where the interconnecting network of pores allows collective fluid excitations. To study possible collective behavior over macroscopic length scales, we have conducted an ultrasonic investigation of supercooled ethanol in porous glass for sound wavelengths much larger than the size of the confining pores.

The effects of structure and intermolecular hydrogen bonding on molecular reorientation times of the series of amines and hydrazines

The method of calculating the high resolution NMR lineshape of multispin systems involving intramolecular exchange containing a group of strongly coupled spin- 1 2 nuclei scalar coupled to a group of nuclei having spin I> 1 2 which are relaxing via the quadrupolar mechanism is described. On the basis of the programs realizing this method the kinetics of the Z,E-isomerization of phenylhydrazine acyl derivatives and 14N relaxation times in the series of anilines, amides, and phenylhydrazine acyl derivatives have been studied using the NMR 1H and 19F complete lineshape analysis. The effects of structure and hydrogen bonding with a solvent on molecular reorientation times of the compounds have been discussed.

Comparative study of the gain dynamics of XeCl and KrF with subpicosecond resolution

We have studied the subpicosecond gain dynamics of XeCl and KrF with the following results for XeCl (KrF): saturation energy density 0.85 (2.0) mJ/cm2, small-signal-gain coefficient 0.12–0.15 (0.20–0.22) cm−1, gain recovery of 62% (25%) with a time constant of 52 (54) psec. A molecular reorientation time of ≈1 psec was observed in both excimers.

Quantum beats observed in stimulated emission in XeCl

Quantum beats are observed in stimulated emission oftheB( v' = 0) → X( v″ = 1, v'' = 2) transition of XeCl with a period of 1.28 ps and a damping time of 12 ps. A molecular reorientation time of 1 ps is also clearly seen. These findings are in good agreement with known molecular data.