Relaxation Of Transitions Research Articles

Optimal design of adaptively sampled NMR experiments for measurement of methyl group dynamics with application to a ribosome-nascent chain complex

NMR measurements of cross-correlated nuclear spin relaxation provide powerful probes of polypeptide dynamics and rotational diffusion, free from contributions due to chemical exchange or interactions with external spins. Here, we report on the development of a sensitivity-optimized pulse sequence for the analysis of the differential relaxation of transitions within isolated 13CH3 spin systems, in order to characterise rotational diffusion and side chain order through the product S2τc. We describe the application of optimal design theory to implement a real-time ‘on-the-fly’ adaptive sampling scheme that maximizes the accuracy of the measured parameters. The increase in sensitivity obtained using this approach enables quantitative measurements of rotational diffusion within folded states of translationally-arrested ribosome–nascent chain complexes of the FLN5 filamin domain, and can be used to place strong limits on interactions between the domain and the ribosome surface.

Scalar relaxation of NMR transitions at ultralow magnetic field.

Nuclear magnetic resonance signals for 1H in simple chlorinated, brominated and deuterated liquids were detected at field strengths between 1 nT and a few μT to investigate the influence of scalar relaxation of the second kind (SR2K). SR2K describes the acceleration in magnetization decay rate for a spin-1/2 nucleus that is scalar coupled to a fast-relaxing quadrupolar nucleus. In agreement with simple theoretical models, the experimental data show that couplings to nuclei with small, nonzero quadrupole moments (2H) give rise to higher transverse relaxation rates at ultralow field than rapidly relaxing quadrupolar nuclei (Cl and Br). This behavior is opposite to the case normally encountered in high-field NMR, and demonstrates that certain nuclei in the spin system may be "weakly coupled" or even decoupled when the applied magnetic field is zero. The results show that the capability for precision determination of NMR frequencies and molecular structural information depends strongly on the composition and topology of the nuclear spin system.

Carbon Dioxide Reduction: Visible and Near‐Infrared Photothermal Catalyzed Hydrogenation of Gaseous CO2 over Nanostructured Pd@Nb2O5 (Adv. Sci. 10/2016)

Some Like It Hot: Geoffry A. Ozin and co-workers demonstrate that the reverse water gas shift (RWGS) reaction, driven by a visible and near infrared responsive Pd@Nb2O5 is thermally activated with measured conversion rates as high as 1.8 mmol gcat−1 h−1, in article 1600189. Specifically, the RWGS reaction is enabled by heat generated from thermalization of photoexcited charge carriers in the Pd nanocrystals that function as “nanoheaters” for the entire Pd@Nb2O5 assembly, through non-radiative relaxation of inter-band and intra-band conduction electron transitions. Overall this study advances our understanding of the underlying mechanism of photothermally driven CO2 reduction. Cover illustration by Chenxi Qian.

The energetic of (CH2F2)2 investigated by TDL IR spectroscopy and DFT computations: From collision induced relaxation of ro-vibrational transitions to non-covalent interactions.

Difluoromethane (CH2F2) is an atmospheric pollutant presenting strong absorptions within the 8-12 μm atmospheric window, hence it can contribute to global warming. Its dimer, (CH2F2)2, is bound through weak hydrogen bonds (wHBs). Theoretically, wHBs are of paramount importance in biological systems, though their modeling at density functional theory (DFT) level requires dispersion correlations to be accounted for. In this work, the binding energy (3.1 ± 0.5 kcal mol(-1)) of (CH2F2)2 is experimentally derived from the foreign broadening coefficients of the monomer compound, collisionally perturbed by a range of damping gases. Measurements are carried out on CH2F2 ro-vibrational transitions by means of tunable diode laser spectroscopy. Six stationary points on the potential energy surface (PES) of the dimer are investigated at DFT level by using some of the last generation density functionals (DFs). The Minnesota M06 suite of functionals as well as range separated DFs and DFs augmented by the non-local (NL) van der Waals (vdW) dispersion corrections are considered. DFT results are compared to reference values at the estimated complete basis set (CBS) limit of CCSD(T) theory (coupled cluster with singles and doubles augmented by a perturbational estimate of connected triples) and to the experimental binding energy. The M06-2X, M06-HF, VV10, BLYP-NL, and B3LYP-NL DFs reproduce CCSD(T)/CBS binding energies with a mean absolute deviation <0.4 kcal mol(-1) and about the same deviation from the experimental value. The present results are of twofold relevance: (i) they show that binding energy of homodimers can be conveniently obtained from the monomer's foreign broadening coefficients and that the correct simulation of hydrogen bonds involved in (CH2F2)2 needs non-covalent interactions to be included into DFT; (ii) O2- and N2-pressure broadening parameters represent fundamental data for exploiting the efficacy of remote sensing measurements employed to retrieve temperature and concentration profiles of our atmosphere.

Relaxation and modulation interference effects in two-pulse electron spin echo envelope modulation (ESEEM)

Two-pulse electron spin echo envelope modulation (ESEEM) line widths are influenced by transverse electron spin relaxation, which is in turn induced by local field fluctuations. Simultaneous analysis of the decays of the unmodulated and modulated parts of the ESEEM signal provides deeper insight into the relaxation of a spin system consisting of an electron spin 1/2 coupled to NI nuclei with spin 1/2. Standard two-pulse ESEEM formulas either do not account for relaxation or assume uniform relaxation for all lines. In general, the relaxation rates on allowed and forbidden transitions may not be the same. Experimental results obtained on a single crystal of Cu(II)-doped l-histidine suggest that such a difference exists. Theoretical considerations show that in such a case the product rule for two-pulse ESEEM does not extend to expressions including relaxation. Product rules in general do not properly account for relaxation in three-pulse ESEEM and HYSCORE experiments. Decay of the apparently non-oscillatory part of the two-pulse echo may be strongly affected by modulation interference. Such interference of difference frequencies of matrix nuclei may cause a rather flat initial feature, which was previously attributed solely to non-exponential phase relaxation of electron spin transitions due to spin diffusion of the matrix nuclei. In addition, the sometimes observed drastic initial decay of the time domain signal is related to modulation interference of multiple-quantum coherences that arise from a strong cross-suppression effect.

Improving Signal-To-Interference Ratio in Rich Hydrocarbon—Air Flames Using Picosecond Coherent Anti-Stokes Raman Scattering

There is growing interest in the use of short-pulse lasers for coherent anti-Stokes Raman scattering (CARS) to minimize non-resonant background (NRB) contributions in a variety of applications. Using time-coincident picosecond (ps) pump and Stokes beams and a time-delayed ps probe beam, we show that a three orders of magnitude reduction in NRB interference can be achieved in rich hydrocarbon-air flames while preserving 60% to 80% of the CARS signal. This represents a significant improvement in signal-to-interference ratio compared with previous measurements in room temperature air and is attributable to reduced rates of collisional dephasing and relaxation at flame temperatures. Measurements within the flame zone of a laminar flat-flame burner are used to investigate the characteristics of time-coincident and probe-delayed broadband ps N(2)-CARS spectra for C(2)H(4)-air equivalence ratios of 0.5 to 1.2. Up to three ro-vibrational bands of N(2) are excited with each laser shot using 135 ps pump and 106 ps Stokes beams, and the CARS signal is generated using a 135 ps probe beam delayed by 165 ps. The enhanced signal-to-interference ratio achieved in the current work is one to two orders of magnitude higher than that previously achieved using polarization-selection techniques without sensitivity to the effects of birefringence caused by density gradients or test cell windows. Moreover, the use of a 135 ps laser source in this study enables frequency domain "broadband" CARS with sufficient resolution to extract ro-vibrational spectral features under various flame conditions. The effect of probe delay and NRB suppression on characteristics of these broadband CARS spectra are investigated, and evidence of preferential collisional dephasing and relaxation of different ro-vibrational transitions is not detected. This is a promising but preliminary result to be investigated further in future work.

Relaxation and dephasing of the intersubband transitions in n-type InAs∕AlSb multi quantum wells

Intersubband electron relaxation and dephasing has been studied in InAs∕AlSb multi quantum wells using time resolved femtosecond spectroscopy. The authors have measured a relaxation time T1 of 3ps at T=300K (and 4.6ps at T=10K) for a transition energy of 260meV, which is in good agreement with our calculations. A dephasing time T2 of ∼320fs for optically excited electrons in the second subband was measured which determines the homogeneous broadening (∼4.1meV) of the absorption line.

Constitutive equations for polymer viscoelasticity derived from hierarchical models in cases of failure of time–temperature superposition

Hierarchical viscoelastic elements whose behaviour is intermediate between linear elasticity and Newtonian viscosity (springpots) have been introduced previously into classical analog models describing linear viscoelastic behaviour. This approach allows a concise description of typical polymer behaviour, including non-exponential relaxation, memory effects and hierarchy of viscoelastic transitions. This approach is extended in the present work to describe complex behaviour, including failure of time–temperature superposition in a semi-crystalline polymer, and the terminal transition from self-similar viscoelasticity to pure flow in a thermoplastic elastomer. Tschoegl's formulation of a finite Gross Marvin ladder model is generalized and applied to other models, and this approach is compared with Friedrich's method based on application of an exponential cutoff to the relaxation function. The method is illustrated using dynamic mechanical measurements on a triblock adhesive in isothermal frequency sweeps. This material displays thermorheological complexity precluding application of time–temperature superposition. Tschoegl's formulation affords a better description of this material than Friedrich's approach. The method described here is a useful alternative to time–temperature superposition requiring a limited number of adjustable parameters.

Sub-picosecond electron relaxation of near-infrared intersubband transitions in n-doped (CdS/ZnSe)/BeTe quantum wells

We report on intersubband (ISB) absorption and ultrafast ISB energy relaxation of carriers in n-type doped (CdS/ZnSe)/BeTe quantum wells (QWs), grown by molecular-beam epitaxy. The highly n-type doped QW samples were obtained by introducing a few monolayer CdS into a ZnSe/BeTe QW, and ISB absorption with a peak wavelength as short as 1.62 μm, covering 1.55 μm within its absorption bandwidth, was achieved. The ISB carrier relaxation was investigated by means of femtosecond (∼150 fs) one-color pump and probe technique at the ISB absorption peak. The ISB carrier relaxation time of 270 fs was observed in the sample with the absorption peak at 1.82 μm. The slow decay component with a time constant of a few ps, which has been observed in ZnSe/BeTe QWs, was not observed in the (CdS/ZnSe)/BeTe QWs, indicating that the Γ(ZnSe)–X(BeTe) electron transfer is suppressed, as expected from the band alignment.

Multiphonon relaxation of mid-IR transitions of rare-earth ions in the crystals with fluorite structure

Fluorescence kinetics decay of the 4I 9/2 initial level for the 4.6 μm laser transition of erbium in the fluoride crystals with fluorite structure (CaF 2, SrF 2, CdF 2, BaF 2, and PbF 2) with short phonon spectra was measured at room temperature and 77 K. It was found that in the row of these crystals doped with erbium, the governing factor that influenced multiphonon relaxation (MR) rates is the number of phonons n, which is determined by the frequency of effective longitudinal optical phonon. A decrease of effective phonon frequency raises the number of phonons n. This decreases the MR rate and increases the fluorescence lifetimes for PbF 2 and BaF 2. The latter fact could be favorable in laser generation in the 4–5 μm spectral region where multiphonon relaxation usually dominates. Influence of the unit dimension of crystal lattice (rare-earth ion to ligand distance R 0) on the MR rate is discussed.

Conformational dynamics in polyethylene under isochoric conditions: A molecular dynamics simulation study

Conformational dynamics in polyethylene (PE) have been studied via molecular dynamics simulations under isochoric conditions at four specific volumes (1.26, 1.20, 1.15 and 1.10 cm3 g−1) as a function of temperature (300–500 K). The autocorrelation function (ACF) decay for virtual perpendicular dipolar relaxation and rates of conformational transitions were monitored. The isochoric Kohlrausch–Williams–Watts (KWW) relaxation times determined from the ACFs show Vogel–Fulcher (V-F) behavior with temperature. From the pressures determined at each T, V point, constant pressure relaxation times vs temperature could also be constructed along several isobars by interpolation. The resulting temperature dependent activation energies at constant volume and at constant pressure are compared. At temperatures where the isochoric and isobaric P, V values coincide, i.e., where the ischoric and isobaric V-F curves intersect, the ratios of the constant volume activation energies to those at constant pressure are found to be ∼0.8. This high ratio can be considered as an indicator of activated barrier crossing dynamics dominating in comparison to the role of free volume. The dipolar ACF relaxation times diverge from the conformational transition rates as the temperature is lowered (under isochoric conditions) and analogously with increasing pressure (under isothermal conditions). The spatial heterogeneity of the conformational transitions increases with decreasing temperature and an increase in self-correlation of the conformational transitions ensues. These signatures of approaching vitrification are consistent with those previously found for PE under isobaric conditions. Qualitatively, decreasing the temperature at constant volume, increasing the pressure at constant temperature, and decreasing the temperature at constant pressure result in very similar behavior in the conformational dynamics.

NMR relaxation of spin‐3/2 nuclei: Effects of structure, order, and dynamics in aqueous heterogeneous systems

AbstractThe mathematical expressions for interpreting the results of spin‐lattice relaxation, transverse relaxation, spin‐lattice relaxation in the rotating frame, and Ostroff–Waugh multiple‐pulse sequence relaxation experiments on spin‐3/2 nuclei with electric quadrupole interaction in an isotropic environment are biexponential. A protocol for applying these expressions to investigate quadrupole interactions and ionic motions in complex aqueous heterogeneous systems such as biopolymer gels and biological tissue is described. Experimental evidence for at least two different environments for hydrated sodium ions, including an isotropic liquid aqueous phase and an anisotropic interfacial phase at the macromolecules, is reviewed. The use of the basic equations and experimental protocol are illustrated for systems of increasing complexity, ranging from simple NaCl aqueous solutions, highly concentrated agarose gels, gelatin gels, and gellan gels to xanthan gels that contain ordered macromolecules. Many different experiments on various biopolymer samples indicate a correlation time of approximately 1 ns for ions at the interface that dominates in determining spin‐lattice relaxation and also the long component of the transverse relaxation that arises from the central transitions. The transverse relaxation of the satellite transitions is dominated by much longer correlation times that may be related to the geometry of the macromolecular matrix in which the hydrated aqueous sodium ions diffuse. In the case of xanthan, the macromolecular ordering causes residual quadrupolar splitting and enables the measurement of several very long correlation times. One of these appears to agree with the Kuhn length of xanthan that was determined by light scattering experiments on dilute solutions. © 2001 John Wiley &amp; Sons, Inc. Concepts Magn Reson 13: 294–325, 2001

Cross relaxation and upconversion coefficients of the mid-infrared transitions of Pr3+:LaCl3

Short pulse laser-induced fluorescence experiments on the lowest-energy electronic states (υ&amp;lt;10−4 cm−1) of trivalent praseodymium ions are reported. Crystals of lanthanum trichloride with a range of Pr3+ concentrations were examined. We compute cross relaxation and upconversion coefficients for the metastable energy levels using a standard rate equation analysis. The limitations of the rate equation approach are examined with a focus on the underlying dynamics of this rare-earth system. The implications of these results for laser emission in the 1.3–7.2 μm spectral range are discussed.

Radiative and multiphonon relaxation of the mid-IR transitions of Pr/sup 3+/ in LaCl/sub 3/

Electric and magnetic dipole transition probabilities, fluorescence branching ratios, integrated emission coefficients, fluorescence efficiencies, and fluorescence lifetimes are determined for the lower lying states of Pr:LaCl/sub 3/ using the Judd-Ofelt theory and calculated multiphonon emission probabilities. The results are used to characterize demonstrated mid-IR laser transitions and predict new potential mid-IR laser transitions in Pr:LaCl/sub 3/.

Cross-correlation effects in the transverse relaxation of multiple-quantum transitions of heteronuclear spin systems

The relaxation of multiple quantum transitions in a heteronuclear three-spin system is described in terms of Redfield theory including intramolecular dipolar and chemical shift anisotropy (CSA) mechanisms. CSA-dipolar cross-correlations, interference terms between CSA terms at different nuclear sites and cross-correlations between different dipolar interactions are illustrated and discussed. It is shown that cross-correlations between various relaxation mechanisms lead to non-exponential relaxation behaviour of multiple-quantum transitions, thus causing differential line broadening.

Optical Nuclear Spin Polarization (ONP) Studies of Short‐Lived Excited Triplet States in Anthracene Crystals Doped with Phenazine. Structure and Mechanism of Reversible Photochemical H‐Transfer

AbstractIn anthracene crystals doped with phenazine two types of ONP active triplet states are identified: delocalized triplet excitons of anthracene and localized triplet states with small zero‐field splitting and characteristic hyperfine interaction (hfi). All three basic ONP mechanisms can be demonstrated in one system with transfer of the photoinduced electron polarization (OEP) to the nuclear spins by (a) static hfi in the level crossing region (LAC‐ONP), (b) time modulated hfi inducing electron‐nuclear cross relaxation (Overhauser effect) and (c) rf saturation of forbidden transitions (RF‐ONP, solid effect). The localized triplet states are identified as a radical pair product of a H‐transfer photoreaction, in which an H atom is abstracted from an anthracene X‐trap to the neighboring phenazine guest molecule, between two translationally equivalent lattice sites along the crystalline b‐axis. Kinetic rates determined with RF‐ONP techniques indicate a cyclic reaction with slow radical pair product formation and faster, spin selective, triplet product decay. Analogies are discussed with similar H‐transfer reactions in doped fluorene. Whereas anthracene acts as H‐acceptor when doped in fluorene, it becomes H‐donor in the anthracene crystal doped with phenazine.

Relaxation-induced violations of coherence transfer selection rules in nuclear magnetic resonance

The effect of multiexponential transverse relaxation of degenerate transitions on coherence transfer phenomena in high-resolution nuclear magnetic resonance is discussed. Dipolar relaxation of X 3 and AX 3 systems is considered, and the implications for multiple-quantum-filtered two-dimensional correlation spectra of large molecules are analyzed in detail. An expansion of the density operator in terms of products of irreducible tensor operators is used for tracing the pathways of coherence transfer.

Relaxation of individual transitions in an AX spectrum. Use of interference terms to separate the dipolar and chemical-shift anisotropy contributions to the relaxation of 31P and 19F nuclei in macromolecules

Relaxation of individual transitions in an AX spectrum. Use of interference terms to separate the dipolar and chemical-shift anisotropy contributions to the relaxation of 31P and 19F nuclei in macromolecules

Computer molecular dynamics of 2-d water (ice) structures under shock by

Abstract Computer molecular dynamics has been used to study the shock-front transition region for a series of two-dimensional hydrogen-bonded structures composed of water-vapor molecules. Two different structures were examined and different intermolecular potentials were considered. Sample sizes and running times were chosen to correspond to the predicted shock-front rise-time in real water. In this way, the effect of different potentials and initial structures on the equilibration associated with the shock-front transition in water could be investigated. In addition to studying the development and incipient relaxation of shock-polarization states, we have also considered the propagation and possible incipient relaxation of structural phase transitions occurring in two-dimensional structures. The results of these molecular dynamics calculations are compared with the experimental shock behavior of real water; in particular, comparisons are made with respect to intermolecular hydrogen-bond breaking, dissoci...

Coherent pumping of microwave transitions between highly excited states

The problem of describing the absorption of microwave radiation between pairs of multi-degenerate molecular rotational energy levels, and collisional relaxation of the irradiated transitions, is approached generally by deriving a system of coupled tensorial Bloch equations. These are solved directly for the case of highly excited states, where a number of simplifying assumptions are valid.