Influence of molecular motions on multipulse NMR spectra

Spin-lattice relaxation in multipulse experiments is considered. Experimental values of T₁₁ (WHH-4) as a function of temperature are presented.     

Thermoactivated molecular motions in solids and determination of the energy of their activation by NQR spectroscopy methods

The activation energy of thermoactivated molecular motions in solids is determined by examining the influence of these motions on the temperature dependence of the nuclear quadrupole spin-lattice relaxation rate and quadrupole resonance (NQR) signal intensity for chlorine-35. In the latter case, the heating of the crystals is accompanied by the fading of resonance signals, which is analyzed using a linear correlation between the activation energy of this motion and the fade-out temperature. The correlation parameters are demonstrated to be dependant on the type of molecular motion. The relaxation method is shown to be more effective in studying molecular motion and evaluating its activation energy as compared to the NQR signal fading technique.     

Pulsed ESR and molecular motions

The possibilities of applying three different pulsed ESR techniques have been considered: 1. Phase relaxation measurements by electron spin echo (ESE) affords the estimation of the correlation time of the motion in the region up to 10^?5 s. The results of theoretical and experimental analysis of the effect of methyl group rotation in nitroxide radicals have been proposed. 2. The method of pulsed saturation involving detection of ESE signal allows the range of the measured times to be extended up to the values of about 10^?2 s. The rotation of CH₂ group in the CH₂CO₂ ^? radical and that of the CH₃ group in the CH₃CHCO₂ ^? radical have been investigated. 3. The method of pulsed saturation combined with pulsed scanning of H₀ allows the analysis of the rotationally induced redistribution of the pulsed saturation throughout the ESR spectra of the radicals. This version of pulsed ESR has been used to study the mobility of nitroxide spin labels.     

Quench echoes in molecular dynamics--A new phonon spectroscopy

We demonstrate a new echo phenomena that appears in molecular dynamics simulations of a solid. This echo occurs in the behavior of the temperature after the system recovers from two abrupt quenches. We give a simple explanation of this effect and show how it can be used to determine the density of states and the coupling between normal modes. We have found considerable anharmonicities even at low temperature in a Lennard-Jones glass.     

Fluorescence methods for studying molecular motions

The characteristics of photoluminescence are affected by the molecular motions in the local environment of the excited molecule. Hence, by knowing the lifetime of the excited state, various relaxation times of the molecular motions can be estimated from the emission characteristics. In the present paper the effect of the molecular motions manifested in the fluorescence emission, such as the rotational depolarization of fluorescence, the internal quenching of fluorescence due to internal rotation, and the inter-and intramolecular excimer formation, are reviewed and the possible applications of such phenomena to the study of molecular motions in polymer systems are discussed. The application of fluorescence methods provide unique information about the molecular motions in the nanosecond range in polymer solutions, melts, and solids.     

Application of photon echoes to electronic relaxation in molecular crystals

The possibility of using photon echoes to elucidate the pathways of intramolecular electronic relaxation in molecular crystals is discussed.     

Influence of the source bandwidth on the observation of microwave photon echoes

The fluctuations of the source frequency are shown to be responsible of an extra damping of the optical precession and of the photon echoes observed in the experiments involving many shots and an averaging of the detected signals. Assuming a phase diffusing model and various profiles for the frequency power spectrum, the r.m.s. accumulated phase error is calculated and two asymptotic behaviours are pointed out according to the relative values of the sequence duration and of the frequency correlation time. The calculation is well supported by a photon echo experiment at a 3-mm wavelength.     

Nuclear relaxation,molecular motions and interactions

The ¹⁴N quadrupolar relaxation time for pyrrole as the pure liquid and in solution in 1,4-dioxan, pyridine, 3,5-lutidine and 2-fluoropyridine has been determined from the line-shape analysis of the imido proton magnetic resonance signal. The dependence of the line shape on solute concentration and temperature was studied and the activation parameters for molecular reorientation determined.     

Raman and Rayleigh spectroscopy and molecular motions

Raman and Rayleigh scattering experiments on HCl, DCl, HBr and DBr molecules (pure liquid or isotopically diluted) have been carried out at room temperature. Results are discussed in the light of the existing theories on molecular motions in the liquid phase. The comparison of Rayleigh and Raman scattering results shows that reorientational processes are not sufficient to explain completely the profile of anisotropic Raman scattering. According to a recent theory due to Bratos, the rotational correlation function can be obtained only after elimination of the vibrational correlation function determined from the isotropic Raman scattering. In this last case broadening may arise from three causes: translational motion, resonance broadening and vibration-rotation coupling. A comparative study of the band profile of anisotropic Rayleigh and Raman scattering shows that the wings in both cases come from essentially the same origin while a band moment analysis is consistent with Gordon's theory according to which they are of reorientational origin.     

NMR investigations of molecular motions inp-n-hexyloxybenzylidene-p′-n-propylaniline

Proton spin-lattice relaxation times,T ₁, have been measured in the smectic phases, S _G ² , S _G ¹ and S_A, and in the nematic phase of HxBPA, in the temperature range, 220K<T<360 K. In the S _G ¹ and S _G ² phases,T ₁ has been measured at 15 and 40 MHz. The (S _G ¹ →S _G ² ) and (S _G ² →S _G ¹ ) transitions are clearly seen as discontinuities inT ₁. The former transition is seen to be due to possible freezing or change of hydrocarbon chain motions of the molecule. Our data in the S _G ¹ phase have been fitted to a model in which anisotropic rotational diffusion of the end hydrocarbon chains as also that of the rigid part of the molecule are considered. In the nematic phase, at 351 K, the observed behaviour ofT ₁, measured in the frequency range, 5 to 40 MHz, agrees well with the theoretical predictions of Uklejaet al and Freed, who take into account long range collective order fluctuations and local reorientations. Using these theories, the correlation time for short range reorientations has been calculated from our results to be 4.3 × 10⁻¹⁰ and 1.1 × 10⁻⁹ s respectively.     

NQR application to the study of hydrogen dynamics in hydrogen-bonded molecular dimers

The temperature dependences of ¹H NMR as well as ³⁵Cl NQR spin-lattice relaxation times T ₁ were investigated in order to study the hydrogen transfer dynamics in carboxylic acid dimers in 3,5-dichloro- and 2,6-dichlorobenzoic acids. The asymmetry energy A/ k _B and the activation energy V/ k _B for the hydrogen transfer were estimated to be 240 K and 900 K, and 840 K and 2500 K, respectively, for these compounds. In spite of a large asymmetric potential the quantum nature of hydrogen transfer is recognized in the slope of the temperature dependence of T ₁ on the low-temperature side of the T ₁ minimum. The NQR T ₁ measurements was revealed to be a good probe for the hydrogen transfer dynamics.     

Studies of molecular motions and interactions in acetonitrile

Microwave and far-infrared absorption coefficients are reported for acetonitrile over the whole concentration range in carbon tetrachloride solution. The data have been used to compute dipole reorientational correlation functions and relaxation times as a function of concentration. These show that τ_1R ^T increases non-linearly as the acetonitrile concentration increases (in contrast to the values of τ_2R ^T obtained from light scattering measurements) and as the viscosity decreases. This behaviour is examined in terms of intermolecular correlations between rotating dipoles. Intensity data are used to provide a check on the importance of induced dipole absorption in the far-infrared spectra and to calculate ‘static’ correlation factors g ⁽¹⁾. Both g ⁽¹⁾ and the dynamic correlation factor f ⁽¹⁾ reflect the effects of strong molecular interactions in these solutions.     

NMR relaxation study of molecular motions in liquid chlorobenzotrifluorides

The temperature dependences of the ¹H and ¹⁹F nuclear spin-lattice relaxation times T₁ in liquid o-, m-, and p-chlorobenzotrifluorides have been measured. The analysis of the temperature dependences of the ¹H spin-lattice relaxation times leads to the conclusion that the overall molecular reorientational motion in o-, m-, and p-benzotrifluorides is nearly the same. Data for ¹H and ¹⁹F spin-lattice relaxation times of o-chlorobenzotrifluoride jointly lead to the determination of the individual contributions to relaxation rate in the entire temperature range studied. A knowledge of these contributions for o-chlorobenzotrifluoride, together with the assumption of equal correlation times for overall molecular reorientation in o- and p-chlorobenzotrifluorides, leads to the determination of the spin-internal-rotation interaction contribution to relaxation for p-chlorobenzotrifluoride in the same range of temperature.     

Regeneration of photon echoes with amplified photon echoes

Photon-echo-based devices have been proposed for many applications in data storage, image processing, and optical communications. Many of these applications would benefit if the output from the photon-echo process could be used as input in a second photon-echo process. We demonstrate the generation of such secondary echoes, using the amplified output from an initial photon-echo process. The amplification is performed with a Pr:ZBLAN fiber amplifier, which gives a gain of more than 300 at 606 nm when pumped with 320 mW of power at 476 nm.     

NMR spin echoes for molecular rotators with C(3) symmetry

NMR spin echoes are calculated for a rotator consisting of three spin-(1/2) nuclei whose three-particle wave function obeys C(3) symmetry. On decomposing the nuclear dipole-dipole interactions in terms of irreducible operators of the representations of the group C(3) it is found that for spins belonging to the A-representation, the echo amplitude is maximized for a RF pulse sequence. The dependences of the echo formation on the orientation of the rotor and on the strengths of the dipolar interactions and the magnetic field inhomogeneities are discussed. Copyright 2000 Academic Press.