Two-dimensional Double-quantum Spectroscopy Research Articles

Two-dimensional double-quantum spectroscopy: peak shapes as a sensitive probe of carrier interactions in quantum wells

We identify carrier scattering at densities below which it has previously been observed in semiconductor quantum wells. These effects are evident in the peakshapes of 2D double-quantum spectra, which change as a function of excitation density. At high excitation densities ($\geq 10^{9}$ carriers/,cm$^{-2}$) we observe untilted peaks similar to those reported in previous experiments. At low excitation densities (<$10^{8}$ carriers cm$^{-2}$) we observe narrower, tilted peaks. Using a simple simulation, we show that tilted peak-shapes are expected in double-quantum spectra when inhomogeneous broadening is much larger than homogeneous broadening, and that fast pure-decoherence of the double-quantum coherence can obscure this peak tilt. These results show that carrier interactions are important at lower densities than previously expected, and that the `natural' double-quantum peakshapes are hidden by carrier interactions at the excitation densities typically used. Furthermore, these results demonstrate that analysis of 2D peak-shapes in double-quantum spectroscopy provides an incisive tool for identifying interactions at low excitation density.

Characterization of Long-Chain Aliphatic Polyesters: Crystalline and Supramolecular Structure of PE22,4 Elucidated by X-ray Scattering and Nuclear Magnetic Resonance

The crystalline packing and the phase structure of a long-chain aliphatic polyester, (O(CH2)22OOCCH2CH2CO)n, PE22,4, have been studied in molecular detail by small-angle X-ray scattering (SAXS) and wide-angle X-ray diffraction (WAXD) experiments as well as various solid-state nuclear magnetic resonance (NMR) methods, in particular two-dimensional double-quantum spectroscopy (DOQSY) NMR. The volume crystallinity is 73 ± 3% according to quantitative 13C NMR and SAXS analyses. The DOQSY NMR spectra show signal characteristic of trans ester groups incorporated into straight poly(methylene) chains, as expected on the basis of WAXD. DOQSY spectra of singly 13COO-labeled diesters prove close proximity of ester groups in neighboring chains, confirming the ester layering deduced from SAXS, with three diester layers per crystallite. SAXS shows a 37° chain tilt with respect to the diester layers and crystallite surface, and the DOQSY NMR spectra confirm the resulting significant displacement of ester groups along neighboring chains. The data suggest a αβ22 tilting of the chain axes. NMR detects no significant disorder along the chain axes; this suggests that the disappearance of (h, k, l ≠ 0) reflections in WAXD is due to the small crystallite thickness, which is 5.6 ± 0.5 nm according to SAXS. The DOQSY NMR patterns show that the planes of the chains are far from the perpendicular relative orientation found in orthorhombic polyethylene, constraining the angle between the (normals to the) O−CO ester planes to 55° ± 20°. DOQSY NMR also indicates that ∼1/3 of the COO groups directly at the crystalline−amorphous interface are disordered. The chain loops in the amorphous phase contain only 6% of the esters and thus mostly consist of the C22 polymethylene section of one C26 repeat unit. The C22 loops connect ∼71% of the ends of crystalline stems, while 9% are terminated by chain ends and 20% are connected to a loose loop or tie molecule. NMR relaxation measurements confirm that, in spite of the relatively small fraction of ester groups among the poly(methylene) chains, they strongly suppress the fast 180° chain flips observed in polyethylene crystallites.

Multiplex phase cycling

We discuss a new class of phase cycling procedures, in which a set of individual phase-shifted transients are stored separately in the computer and processed afterwards to yield the separated NMR signals from two or more coherence transfer pathways. In the case of two-dimensional double-quantum spectroscopy, this multiplex acquisition procedure allows the acquisition of pure-absorption spectra in only 62.5% of the time needed by previous methods.

High resolution solution state double quantum spectroscopy of two-spin-1 AX systems and mimics

Double quantum spectroscopy (DQS) of the two-spin-1 AX system in the liquid state is investigated theoretically. The two kinds of double quantum coherence (DQC) that arise are studied and expressions are given for their evolution under weak scalar coupling and chemical shifts. A one-dimensional double quantum J (DQJ) filter sequence for the separation of the two kinds of DQC is introduced. Analytical expressions are given for the spin response to the general two-dimensional DQS sequence, for an arbitrary reconversion pulse flip angle β. A detailed investigation of the coherence transfer (CT) amplitudes has been carried out by computer simulation of CT amplitudes as a function of β. The differences in the behaviour of CT amplitudes of one-spin DQC, ‘outer’ and ‘central’ components of two-spin DQCs are discussed. Optimum flip angles for maximizing sensitivity of N-type, P-type and pure phase double quantum spectra of this system are deduced. The detailed predictions are borne out by recently published experimental work on two-spin-1 AX systems, as well as our studies on a mimic arrangement, viz., a spin-1/2 A2X2 system.

Application of two-dimensional double-quantum spectroscopy for spin-system identification in oligonucleotides

Application of two-dimensional double-quantum spectroscopy for spin-system identification in oligonucleotides

Correlation of connected double-quantum and single-quantum transitions. Two-dimensional double-quantum spectroscopy with simplified in-phase direct connectivity multiplets

The purpose of this Communication is to present a new two-dimensional double-quantum experiment which yields with simplified in-phase direct connectivity multiplets and antiphase remote connectivity multiplets

Uniform excitation of double-quantum coherence in two-dimensional correlation spectroscopy

Two-dimensional double-quantum spectroscopy can be of great assistance for the assignment of complicated high-resolution spectra and for the characterization of networks of scalar-coupled spins ( 1-9). Double-quantum spectroscopy is an attractive alternative to two-dimensional correlation spectroscopy (COSY), for the spectra are not obscured by a dominant diagonal ridge, and the fine structure of the multiplets is less complicated. In a double-quantum spectrum, pairs of multiplets that appear symmetrically disposed with respect to the skew diagonal have distinct structures, so that unfortunate signal cancellations are unlikely to occur on both sides of the spectrum ( 9). Furthermore, double-quantum spectra actually provide a great deal more information than COSY spectra: the so-called remote connectivity peaks allow one to identify three-spin subunits, in a manner that is reminiscent of the information derived from two-dimensional spectra obtained by relayed coherence transfer. Unfortunately, the most widely used pulse sequences for double-quantum spectroscopy suffer from a major drawback: the efficiency of the excitation of the various double-quantum coherences is not uniform, since it depends on the magnitude of the coupling constants and on the complexity of the coupling network. The following experiment, henceforth called the “conventional” sequence, contains a constant preparation interval rP:

The application of a simple analog phase-shifting device in multiple-quantum NMR

The indirect observation of multiple-quantum coherences offers a useful aid to the established techniques of one-dimensional double-resonance, two-dimensional correlated (COSY), and relayed coherence transfer spectroscopy for the identification of spin systems in large molecules (1-4). Two-dimensional doubleand zeroquantum spectroscopy have been used to obtain chemical-shift correlations in both 13C and ‘H NMR spectra (5-11). It has also been shown that spectral simplification can be achieved using multiple-quantum filtration in conjunction with one or twodimensional experiments (12-16). The order-selective excitation method for the separation of multiple-quantum coherences of order N requires a linear combination of 2N experiments where the excitation pulses are phase shifted in increments of r/N radians (I 7). Recently we have shown that chemical-shift correlations and spectral simplification in the aromatic and methyl regions of protein spectra can be achieved through the use of two-dimensional double-quantum spectroscopy (9, 10). For the selection of doublequantum coherences no modifications to the spectrometer are required when the transmitter offset is placed at the downfield edge of the spectrum because only x/2 phase shifts of the excitation pulses are required. Extension of two-dimensional multiple-quantum spectroscopy to higher order coherences can be achieved with hardware modifications to obtain the required phase shifts. An alternative approach involves the use of composite z pulses (8) but on our spectrometer this has been found to give less satisfactory results because of the incomplete suppression of unwanted coherences. Several digital devices which will produce the necessary radiofrequency phase shifts have been described recently (19-22). Here we describe an analog device which can be used to generate the desired phase shifts. The unit has the advantages of simplicity, low cost, and the ease of interfacing to standard NMR computers (in our case a Nicolet Instruments 1180). The device described relies on a simple pulsedelay technique to provide a continuously adjustable phase shift under the control of a d/a converter. The main disadvantages are relatively slow switching speed and the need for careful calibration to enable accurate phase shifts to be obtained. In practice we find that the unit switches between 0 and 180” in 5 PS. We have not found this to be a disadvantage for the experiments described below. The reproduc-

Uniform excitation of multiple-quantum coherence. Application to two-dimensional double-quantum spectroscopy

An improved technique for two-dimensional multiple-quantum spectroscopy is described. It employs a symmetrical excitation/detection scheme for the uniform excitation of multiple-quantum transitions which is largely insensitive to the magnitudes of the individual J coupling constants. Compared to conventional multiple-quantum spectroscopy it is of advantage that the resulting spectrum is in pure adsorption, with positive peaks for direct connectivities and negative peaks for remote connectivities between the nuclear spins, permitting spectral editing. Experimental results are presented for the dinucleotide 2′-deoxythymidyly-(3′, 5′)-2′-deoxy-3′-thymidylate, d(TpTp), and the small globular protein basic pancreatic trypsin inhibitor.

Two-dimensional double-quantum spectroscopy for the hydrogen NMR assignment of oligodeoxyribonucleotides

Application de la technique INADEQUATE, associee a des mesures de difference de l'effet Overhauser a une dimension, pour attribuer, sans equivoque, les resonances de 1 H des oligodesoxy-ribonucleotides, d(ApGpCpT) et d (ApCpGpT), et montrer les avantages de cette methode

Correlation of proton chemical shifts in proteins using two-dimensional double-quantum spectroscopy

La spectroscopie a 2 quanta et 2 dimensions correles offre des avantages nombreux sur la spectroscopie a 2 dimensions correlees: 1) la multiplicite des pics est tres facile a distinguer; 2) les experiences a 2 quanta permettent l'identification de pics couples avec des differences de deplacements chimiques extremement faibles