Line-broadening Effects Research Articles

Effects of dissipation and mode broadening in emission spectra of optical cavities and semiconductor microcavities

Line-broadening effects in the coupled mode spectrum of optical cavities and microcavities are included explicitly by (i) the interaction of the electromagnetic mode with a continuum of electromagnetic modes and (ii) the coupling of the electronic excitations to a dissipative source. The effects of interaction with an electromagnetic continuum are treated within a Fano model, and the electronic excitations are represented by harmonic oscillators. The coupled mode frequencies and their emission spectra are given explicitly in terms of the parameters of these line-broadening mechanisms, with particular attention given to expressions for semiconductor microcavities.

Determination of the solution conformation of d- gluco-dihydroacarbose, a high-affinity inhibitor bound to glucoamylase by transferred NOE NMR spectroscopy

The determination of the bound solution conformation of d- gluco-dihydroacarbose (GAC), a tight-binding inhibitor of several glycosidase and amylase enzymes, by glucoamylase is described. Transferred NOE NMR experiments and line-broadening effects indicate that GAC is bound in a conformation resembling that observed in the crystal structure. This contrasts with the predominant conformation of GAC when free in solution. The NMR results also suggest regions on the carbohydrate that are in close contact with the protein. The determination of the bound solution conformation of GAC by glucoamylase using transferred NOE (trNOE) measurements is a significant achievement given the high affinity constant ( K a=3×10 7 M −1) for this receptor–ligand pair. It is striking that the off-rate for complexation is still sufficiently high to permit observation of trNOEs.

NMR line shape of rotating 3He-B at large counterflow velocity

We have investigated the NMR line shapes of superfluid 3He-B in a rotating cylinder. In the vortex-free state at sufficiently large angular velocity of rotation, Ω≳1 rad/s, the main feature of the absorption spectrum is a large frequency-shifted peak above the Larmor frequency. The shape of this counterflow peak is both measured and calculated as a function of external magnetic field strength, angular velocity, temperature, and number of vortex lines. The NMR spectrum is derived from the calculated order-parameter texture. Reasonable agreement with the measured line shape is obtained by including line-broadening effects due to the external field inhomogeneity and Leggett–Takagi relaxation.

Solvent effects on intramolecular electron exchange in the 1,4-dinitrobenzene radical anion

The rate constants of the intramolecular electron exchange reaction in the 1,4-dinitrobenzene radical anion in linear alcohols were determined from alternating line-broadening effects in EPR spectra. Rate constants at 293 K range from 2.9×109 s-1 in methanol to 2.7×108 s-1 in octan-1-ol. The rate constant was found to depend on the reverse of the longitudinal correlation time of the solvent, τL, as expected for a solvent-controlled adiabatic reaction. Applying Marcus theory and the ellipsoidal cavity model for evaluating λ0, the amount of transferred charge between nitro groups, z, and the outer-sphere reorganisation energy were determined. z changes from 0.56 at 273 K to 0.52 at 313 K. λ0 at 293 K changes from 61 kJ mol-1 in octan-1-ol to 80 kJ mol-1 in methanol. The activation energies were obtained from the temperature dependence of the rate constants, changing from 26 kJ mol-1 in methanol to 36 kJ mol-1 in octan-1-ol. The variation of this energy clearly reflects the increase in the exponential term of τL for the more viscous alcohols. The results for the 1,4-dinitrobenzene radical anion were compared with those previously reported for the radical anion of 1,3-dinitrobenzene. Rate constants are smaller in the former, by three orders of magnitude, due to conjugation of the nitro groups.

Intermolecular electron-self exchange kinetics measured by electron paramagnetic resonance-linebroadening effects: useful rate constants for the application of Marcus theory

The different methods to obtain rate constants of homogeneous intermolecular electron-self exchange reactions in solutions from dynamic linebroadening effects in CW-electron paramagnetic resonance (EPR) spectra are discussed in details. The limitations of the slow and fast exchange region are explained. The medium exchange region is also discussed. Experimental details are given. The exchange rates of the 2-methyl-1,4-naphtoquinone/2-methyl-1,4-napthoquinone radical anion and the 1-nitronaphthalene/1-nitronaphthalene radical anion redox couples are reported to 9.2 · 10 8 M −1 s −1 (EtOH, 263 K) and 1.8 · 10 8 M −1 s -1 (DMF, 266 K), respectively.

Microturbulence in O supergiants

ABSTRA C T We investigate the role of classical microturbulence in the non-LTE He i/He ii line formation problem for luminous O-type stars. We find that the shapes and strengths of certain saturated He i lines, in particular the triplets ll4471, 4713 and the singlet l4921, are sensitive to microturbulent velocities in excess of 5 km s π1 . Weaker lines, including most of the He i singlets, are effectively independent of this parameter, as are the Fowler series He ii lines ll4199, 4541, 5411. We show that this behaviour is due to interaction between direct linebroadening effects in the radiative transfer, and indirect changes in the atmospheric structure and the populations of absorbing states. Using an analysis of high-resolution, high signal-tonoise ratio observations of the O9.7 supergiant HD 152003 as an illustrative example, we show how the introduction of microturbulence in non-LTE models allows consistent fits to be obtained for all blue-region He i lines ‐ including the strong triplets ll4026, 4713, 4471 ‐ at an assumed solar helium abundance, thereby offering a resolution to the problem of the ‘generalized dilution effect’ described by Voels et al. We argue that by extension this result may also have implications for the so-called ‘helium discrepancy’ identified in OB-type stars by Herrero et al.

Direct Detection of Miscibility in Saturated Polymer Blends

We have used variable-temperature solid-state NMR methods to determine that amorphous polypropylene (a-PP) is miscible with blends of poly(ethylene)ethylethylene (PEE) and poly(ethylene-co-octadecene) (PE-OD). Rothwell−Waugh line-broadening effects and selective 1H T1ρ measurements indicate that the frequency of reorientational chain dynamics increases for a-PP upon blending with PEE and PE-OD but remains unchanged for blends with poly(ethylene-co-dodecene) (PE-DD). We infer partial miscibility on the basis of converged but incompletely averaged relaxation curves for the a-PP/PEE and a-PP/PE-OD blends, while the a-PP/PE-DD blends were found to be immiscible. Dipolar filter experiments in which spin-diffusion was used to monitor polarization transfer provided direct evidence for miscibility in the a-PP/PE-OD blends and immiscibility in the a-PP/PE-DD blends. From these data, domain sizes were calculated to be 5.1 nm (i.e., on the order of chain dimensions) for the miscible regions of the PP/PE-OD blend.

Radical-anions of dinitrophenols in aqueous solution: intramolecular electron exchange and acid–base equilibria

The radical-anions of three symmetrical dinitrophenols (3,5-dinitrophenol, 2,6-dinitrophenol and 4,6-dinitrobenzene-1,3-diol) in aqueous solution have been studied by EPR spectroscopy in the range of pH values from 6.5 to 13. EPR spectra of radical-anions of 3,5- and 2,6-dinitrophenol show line-broadening effects due to intramolecular electron exchange induced by asymmetric solvation of the two nitro groups. The position of the OH group is a decisive factor in the variation of the rate constant with the deprotonation of the radical-anion. This variation is larger when the OH group is in the ortho position to one nitro group. The activation parameters for the intramolecular electron exchange reaction have been determined by using the Marcus theory and the Rips–Jortner equation for the rate constant of a uniform and adiabatic reaction. ΔG* values range from 12.6 kJ mol–1 for radical-dianion of 2,6-dinitrophenol to 46.0 kJ mol–1 for radical-dianion of 3,5-dinitrophenol. The EPR spectra of the radicals derived from 4,6-dinitrobenzene-1,3-diol show no line-broadening effects in the experimental range of temperatures. In the radical-anion and radical-dianion, the electron is localised mainly in one nitro group, while in the radical-trianion the spin density is evenly distributed over the two nitro groups. The pKa values of the radical-anions are in the range 8–11.

Transferred nuclear Overhauser effect spectroscopy study of a peptide from the PapG pilus subunit bound by the Escherichia coli PapD chaperone

Interaction of the Escherichia coli PapD chaperone with the synthetic peptide PapG308-314 (Thr–Met–Val–Leu–Ser–Phe–Pro), corresponding to the seven C-terminal residues of the PapG pilus subunit, was studied by transferred nuclear Overhauser effect (TRNOE) spectroscopy. The observation of cross-peaks corresponding to either intraresidue or sequential C αH/NH and C βH/NH TRNOEs and the absence of sequential NH i/NH i+1 TRNOEs indicate that the peptide binds to PapD in an extended conformation. In addition, line-broadening effects gave information of the peptide's mode of interaction with PapD. These observations were in excellent agreement with a recent crystal structure of a PapG peptide complexed with PapD.

Nitroxide reduction with ascorbic acid in spin labeled human plasma LDL and VLDL

The LDL and VLDL were spin labeled with Tempo which partitions both in the aqueous and lipid phase. The ESR spectra were measured in the equilibrium state as well as during the reduction of the spin label with ascorbic acid. The kinetics of the concentration decay curves was parametrized with two exponentials. The theoretical simulation of the experimental spectra revealed a drastic linewidth narrowing in the VLDL samples exposed to the ascorbic acid. Since the transport properties of the specific monolayer are reflected in the observed reaction rates, the analysis of the fatty acid composition of phospholipids, triglycerides and cholesterol esters in LDL and VLDL was performed. It is concluded that different lipid packing at the surface of LDL and VLDL might be the consequence of different intermolecular forces between phospholipids and cholesterol. This finding was connected to the experimentally detected different reaction kinetics in LDL and VLDL as well as their different susceptibility to the ESR linebroadening effects during the nonequilibrium conditions of the spin label reduction with ascorbic acid.

Complexation of aluminium(III), gallium(III) and indium(III) ions withD-gluconic and lactobionic acids. A potentiometric and nuclear magnetic resonance spectroscopic study

The interaction of the trivalent metal ions aluminium(III), gallium(III) and indium(III) with D-gluconic and lactobionic (4-O-β-D-galactopyranosylgluconic) acids has been studied by means of potentiometry and NMR spectroscopy. Potentiometric measurements gave the stoichiometry, species distribution and equilibrium constants of the complexes in aqueous solution at 20.0 °C and /= 0.10 mol dm–3(NaNO3). The 13C NMR spectroscopic studies revealed line-broadening effects upon complexation, which selectively affected those carbons involved in the complex formation. This enabled the co-ordination sites at both acids to be elucidated. It seems that whereas aluminium(III) prefers to displace the OH protons from the sugar acids even in alkaline media, gallium(III) and indium(III) are able to form hydroxo complexes on increasing the pH.

Type 1 blue copper protein amicyanin from Thiobacillus versutus: line-broadening effects of chromium(III) complexes and related studies

The effect of redox-inactive cationic and anionic paramagnetic chromium(III) complexes on the 1H NMR spectrum of the reduced type 1 blue copper protein amicyanin, AmCuI, from Thiobacillus versutus has been studied as a means of defining sites for association at the protein surface. With [Cr(CN)6]3– two sites are detected, one at the adjacent hydrophobic patch close to the exposed imidazole of the co-ordinated His-96, and the other at Phe-92 which has Lys-59, Lys-60, Arg-69 and Arg-100 in close proximity and is adjacent to the active site-co-ordinated Cys-93. In contrast, the cationic complexes [Cr(NH3)6]3+ and [Cr(en)3]3+(en = ethane-1,2-diamine) cause no significant line broadening and no preferred sites for association are detected. Kinetic stopped-flow studies on the competitive inhibition by [Cr(CN)6]3– of the [Fe(CN)6]3– oxidation of AmCuI indicate that [Fe(CN)6]3– reacts at two sites, one of which is inhibited by [Cr(CN)6]3– and the other is unaffected by [Cr(CN)6]3–. It is suggested that the first of these corresponds to reaction at the Phe-92 site, contributing 25% to the reaction, and the second to reaction at His-96. Therefore, in its reaction with [Fe(CN)6]3– amicyanin has adjacent and remote binding sites.

Pure phase-encoded MRI and classification of solids

Here, the authors combine a pure phase-encoded magnetic resonance imaging (MRI) method with a new tissue-classification technique to make geometric models of a human tooth. They demonstrate the feasibility of three-dimensional imaging of solids using a conventional 11.7-T NMR spectrometer. In solid-state imaging, confounding line-broadening effects are typically eliminated using coherent averaging methods. Instead, the authors circumvent them by detecting the proton signal at a fixed phase-encode time following the radio-frequency excitation. By a judicious choice of the phase-encode time in the MRI protocol, the authors differentiate enamel and dentine sufficiently to successfully apply a new classification algorithm. This tissue-classification algorithm identifies the distribution of different material types, such as enamel and dentine, in volumetric data. In this algorithm, the authors treat a voxel as a volume, not as a single point, and assume that each voxel may contain more than one material. They use the distribution of MR image intensities within each voxel-sized volume to estimate the relative proportion of each material using a probabilistic approach. This combined approach, involving MRI and data classification, is directly applicable to bone imaging and hard-tissue contrast-based modeling of biological solids.

Nature of 9,9′-bianthryl and 10,10′-dimethoxy-9,9′-bianthryl radical anions

Electrochemically generated 9,9′-bianthryl (BA) and 10,10′-dimethoxy-9,9′-bianthryl (DA) radical anions have been studied by means of UV-VIS and ESR spectroscopy. The electronic absorption spectra as well as ESR studies indicate that within the electrochemically generated radical anions the unpaired electron is localized on one of both anthracene subunits of the A-A parent molecule. The rate constants for the intramolecular electron exchange reactions within the radical anions A-A −. were measured by ESR line-broadening effects. In dimethylformamide rate constants of (4.2±0.3) × 10 7 s −1 and (2.2±0.3) × 10 7 s −1 were found for Ba −. and Da −. at 298 K, respectively. The rates have been interpreted in terms of Marcus electron transfer theory and it was found that the reactions are strongly nonadiabatic. Theoretical calculations verify these circumstances.

Substituent effects on the intramolecular electron exchange reaction in 5‐substituted 1,3‐dinitrobenzene radical anions

AbstractNine new radical anions of 1,3‐dinitrobenzenes with various substituents on position 5 (2, 3, 5–7, 9–11 and 13) were prepared by electrochemical reduction in acetonitrile and studied by EPR at temperatures between 240 K and 320 K. Another eight known radicals of the same type were reexamined in the same conditions in order to complete the set of data. Their EPR spectra showed alternating linebroadening effects due to intramolecular electron exchange induced by asymmetric solvation of the two nitro groups. Rate constants at 293 K range from 1.4×109s−1 (3,5‐dinitrophenolate) to 1.2×1010s−1 (3,5‐dinitrobenzonitrile). Good Hammett correlations between the rate constants of intramolecular electron exchange and the parameter σm of the substituent on position 5 were obtained. The reaction is accelerated by electron withdrawing groups. ρ values range from 0.95 at 240 K to 0.74 at 320 K. Activation parameters were estimated by using the Rips and Jortner equation for the rate constant of a uniform adiabatic reaction. ΔG* was found to increase with the electron donating ability of the substituent, mainly due to an increase of the interaction between the reactants and the solvent. The pre‐exponential factor A is constant within the experimental error, as predicted by theory.

1H CRAMPS Based on TREV

Line-narrowing effects of the 1H TREV-8 pulse sequence applied to solids rotating at the magic angle have been examined. Polycrystalline samples of adipic, malonic, and fumaric acids, adamantane, a barley seed, and a pyridine-saturated coal sample were examined. The TREV-based 1H CRAMPS technique is seen to provide reasonably effective line narrowing for both "soft" solids (adamantane) and rigid solids (carboxylic acids), enabling 1H chemical-shift fine structure to be observed. For adamantane, the 1H linewidth is reduced from about 13 kHz (static) to 40 Hz (CRAMPS) at a 840 Hz MAS rate and a pulse spacing (τ) of 7.8 μs. The line-narrowing efficiency increases for larger τ values, but does not approach the efficiency of BR-24. However, unlike the case for BR-24 or other multiple-pulse line-narrowing sequences based on the formation of solid echoes, the line-narrowing efficiency of TREV-8 is sustained for large offsets; this feature, along with a relatively large bandwidth, may be an important factor when the sequence is implemented in 19F CRAMPS and in solid-sample imaging studies. TREV-CRAMPS may also be useful for samples in which the somewhat inferior line-narrowing behavior may be masked by line-broadening effects not based on homonuclear dipolar interactions.

2H NMR lineshapes of immobilized uniaxially oriented membrane proteins

As a method for the structure determination of integral membrane proteins or other large macromolecular complexes, a solid state 2H NMR approach is presented, capable of measuring the orientations of individual chemical bond vectors. In an immobilized uniaxially oriented sample, the bond angle of a deuterium-labeled methyl group relative to the axis of ordering can be calculated from the quadrupole splitting in the “zero-tilt” spectrum where the sample normal is aligned parallel to the spectrometer field direction. However, since positive and negative values of this splitting cannot be distinguished, there may appear to be two solutions, of which only one describes the correct molecular geometry. We show that it is possible to determine the bond angle uniquely between 0° and 90°, by analysing the lineshapes of a tilt series of spectra acquired over different sample inclinations. The lineshape equation describing such oriented 2H NMR spectra will be derived (for asymmetry parameter η = 0) and discussed, with an illustration of the various linebroadening effects from which the orientational distribution function in the macroscopically ordered system can be determined. This strategy is then applied to specifically deuterium-labelled retinal in dark-adapted bacteriorhodopsin, prepared in a uniaxially oriented sample from purple membrane fragments. From the quadrupole splitting in the zero-tilt spectrum and by lineshape simulations, the deuteromethyl group at C 20 on retinal is found to make an angle of 32° ± 1° with the membrane normal, and the sample mosaic spread to be around ± 8°. The resulting orientation of retinal is in excellent agreement with its known structure in bacteriorhodopsin, and together with the results on other methyl groups it will be possible to construct a detailed picture of the chromophore in the protein binding pocket.

Multiple-pulse sequences for homonuclear decoupling. Experimental verification

The manipulation of a spin system to isolate or enhance one interaction while removing the effects of all other interactions is an ability that is used constantly in solid-state NMR (l-3). An increasing number of experimental approaches are being developed to enhance this ability and probe interactions which are present in a spin system in even greater detail. One fruitful area has been in the arena of homonuclear decoupling in the solid state. In this instance, the line-broadening effects of homonuclear dipolar couplings are removed, revealing the underlying finer details, e.g., the chemicalshift interaction. Beginning with the pioneering WAHUHA pulse sequence (4), a number of pulse sequences have been developed for the task of homonuclear decoupling (2, 5-7). Much of the recent work requiring the need for homonuclear decoupling has involved the use of MREV-8 (8) or BR-24 (9) for the observation of ‘H or 19F in hydrogenor fluorine-rich solid materials. When combined with magic-angle spinning (CRAMPS), it is possible, in some cases, to obtain highly resolved solid-state ‘H NMR spectra (6 ) . Many of these homonuclear-decoupling multiple-pulse sequences owe their improved decoupling performance to supercycling of the basic WAHUHA sequence. In the WAHUHA sequence, homonuclear decoupling is achieved by applying a train of 7r/2 pulses, phase shifted by 90” from one another, and sampling the magnetization at specific intervals. At these specific times, the spin system appears to evolve in the absence of homonuclear dipolar couplings. The improved line-narrowing performance of many of these multiple-pulse sequences has come at the expense of longer cycle times and, in the case of MREV-8 and BR-24, a greater scaling of those interactions linear in I,. What one must keep in mind is that these multiple-pulse sequences are but one family in a larger set. The WAHUHA sequence and pulse sequences built upon it have tip angles near 7r/2, identical pulse widths, constant pulse delays, and phase shifts of 90”. Other homonuclear decoupling pulse sequences might be found by removing constraints on tip angles, pulse widths, pulse delays, and phase shifts. Recently Liu et al. (IO) introduced a class of semiwindowless sequences with nonquadrature

Spatial variations of photoluminescence line broadening around oval defects in GaAs/AlGaAs multiple quantum wells

The authors have investigated spatially resolved photoluminescence spectra around several alpha - and lambda -type oval defects in GaAs/AlGaAs multiple-quantum-well structures. In their low-temperature photoluminescence set-up they combine a high spectral resolution with a 2 mu m spot size on the sample. In the alpha -type defects the authors observe a 25-45 meV low-energy tail of the main quantum well recombination peak near the core of the oval defect as well as an approximately 1 meV line broadening originating from two regions which are symmetrically located at either side of the (110) symmetry axis of the defect. These line-broadening effects are explained by vertical growth rate variations within the oval defect.

Intramolecular electron exchange in the 2,7-dinitronaphthalene radical anion : electron paramagnetic resonance and kinetic data

The rates of intramolecular electron exchange in the radical anion of 2,7-dinitronaphthalene in protic and aprotic solvents have been determined from alternating line-broadening effects in EPR spectra. Rate constants in alcohols range from 9.0 × 105 s–1(methanol, 297 K) to 1.7 × 108 s–1(propan-2-ol, 361 K), while in aprotic solvents they range from 1.2 × 108 s–1(acetonitrile, 232 K) to 3.7 × 109 s–1(dimethylformamide, 342 K). The reactions were found to be adiabatic and uniform.Applying Marcus theory and the Rips–Jortner approach to the solvent dependence of the rates, the transferred charge ze0 was determined. It was found that z decreases with increasing temperature, changing from 1.0 at 260 K to 0.98 at 320 K in protic solvents and from 0.58 at 280 K to 0.47 at 340 K in aprotic solvents. This result is explained on the basis of the effect of temperature on the asymmetric solvation of the anions.The outer-sphere reorganization energy, λ0, was obtained by applying the ellipsoidal cavity model and was found to be strongly dependent on the solvent polarity, changing at 293 K from 57.7 kJ mol–1 in hexamethylphosphoric triamide to 235.8 kJ mol–1 in methanol. Activation energies were found to be higher in protic than in aprotic solvents.Results for the 2,7-dinitronaphthalene radical anion are compared with previous data reported for the 1,3-dinitrobenzene radical anion. Rate constants are smaller in the former while outer-sphere reorganization energies are larger. Activation parameters were found of the same order of magnitude for both radical anions.