A Test for the Number of Coupled Spins I=1/2 in Magic‐Angle‐Spinning Solids: Zero‐Quantum Recoupling of Multiple‐Quantum Coherences

Current methodologies for estimating the number of coupled spins I=1/2 in solids are based upon the maximum multiple‐quantum order that can be observed. This strategy establishes a clear lower bound on the number of coupled spins I=1/2. However, it is difficult to ascertain the exact number of coupled spins, since the absence of a peak could be due either to the limited size of the spin system or to the experimental difficulty of exciting high‐quantum orders and recovering those coherences into detectable signals. Herein, a supplementary test is proposed that allows one to determine whether a given coherence has the highest possible order in the spin system. The sample is subjected to magic‐angle spinning and the behaviour of the coherence under a rotor‐synchronised spin‐echo sequence is compared to its behaviour under a zero‐quantum recoupling sequence. A similar decay of the coherence in these two experiments is strong evidence for the coherence order being the maximum possible. We propose applications to biomolecular solid‐state NMR spectroscopy.     

Pulsed-laser production and detection of spin-polarized hydrogen atoms.

Spin-polarized hydrogen (SPH) atoms have traditionally been produced and detected using complex experimental methods with poor time resolution. Recently, SPH has been produced by pulsed-laser photodissociation of HCl using circularly polarized light. In combination with the proposed detection of SPH via polarized fluorescence, this approach should allow the production and spatially resolved detection of SPH with a higher sensitivity than that currently available, and with a time resolution in the nanosecond regime. This represents an improvement of several orders of magnitude over the existing methods.     

A Nuclear Singlet Lifetime of More than One Hour in Room‐Temperature Solution

Nuclear magnetic resonance (NMR) and magnetic resonance imaging (MRI) are supremely important techniques with numerous applications in almost all branches of science. However, until recently, NMR methodology was limited by the time constant T₁ for the decay of nuclear spin magnetization through contact with the thermal molecular environment. Long‐lived states, which are correlated quantum states of multiple nuclei, have decay time constants that may exceed T₁ by large factors. Here we demonstrate a nuclear long‐lived state comprising two ¹³C nuclei with a lifetime exceeding one hour in room‐temperature solution, which is around 50 times longer than T₁. This behavior is well‐predicted by a combination of quantum theory, molecular dynamics, and quantum chemistry. Such ultra‐long‐lived states are expected to be useful for the transport and application of nuclear hyperpolarization, which leads to NMR and MRI signals enhanced by up to five orders of magnitude.     

Spin‐Polarized Structures and Solid‐State NMR Spectroscopy of Paramagnetic Compounds

On an atomic scale and with high sensitivity, solid‐state NMR spectroscopy can provide information about the electronic spin density and coupling mechanisms in paramagnetic compounds. The picture shows how the hyperfine splitting collapses through relaxation. Insights into which compounds are suitable and which approximations have to be made are given.

     

用色谱联用技术分析桂腈和溴代苏合香烯顺反异构体

应用气相色谱／红外光谱（ＧＣ／ＦＴＩＲ）、核磁共振（ＮＭＲ）等技术快速分离、鉴定了桂腈和溴代苏合香烯的顺反异构体,解决了气相色谱／质谱（ＧＣ／ＭＳ）分析时出现的保留行为不同的顺反异构体因为质谱图类似而导致谱库检索鉴别困难的问题。从而,可将保留指数与质谱数据相结合,互补各自在鉴定上的局限性。     

Cold Molecules: Preparation,Applications, and Challenges

Research with cold molecules has developed rapidly in recent years. There is now a variety of established methods for cooling molecules into the millikelvin range. Nevertheless, a focal point of current research is directed toward finding new ways to bring the temperature of molecules even closer to absolute zero. Samples of cold molecules offer not only important applications for high‐resolution spectroscopy, which benefit from the increased interaction time of slow molecules with electromagnetic radiation; they also promise access to an exotic regime of chemical reactivity, in which phenomena such as quantum tunneling and quantum resonances predominate. This review begins with an introduction to the methods by which cold molecules can be prepared, with special emphasis on Stark deceleration and traps. In addition to applications of cold molecules that have already been partially achieved, an important focus of the review concentrates on possible future applications, and both aspects are illustrated with selected examples.     

The Quest for Cold and Ultracold Molecules

Cool molecules: The cooling of molecules to sub‐Kelvin temperatures promises to have a great impact in chemistry and physics. Recently, the first experimental realizations of samples of deeply bound molecules that are approaching the ultracold regime were reported. In this contribution, these interesting results are briefly discussed.     

Revisiting Prussian Blue Analogues with Solid‐State MAS NMR Spectroscopy: Spin Density and Local Structure in [Cd3{Fe(CN)6}2]⋅15 H2O

No legendary Prussian order! The distribution of vacancies in Prussian blue analogues is not random, and the spin density on the Cd²⁺ ion varies depending on the number of paramagnetic ions in its surroundings. This conclusion follows from ¹¹³Cd solid‐state magic‐angle spinning NMR studies of [Cd₃{Fe/Co(CN)₆}₂]?15 H₂O, where the presence of small but significant spin density on the observed ¹¹³Cd nucleus leads to improved spectral resolution.

     

Taking snapshots of photoexcited molecules in disordered media by using pulsed synchrotron X-rays

Photoexcited molecules are quintessential reactants in photochemistry. Structures of these photoexcited molecules in disordered media in which a majority of photochemical reactions take place remained elusive for decades owing to a lack of suitable X-ray sources, despite their importance in understanding fundamental aspects in photochemistry. As new pulsed X-ray sources become available, short-lived excited-state molecular structures in disordered media can now be captured by using laser-pulse pump, X-ray pulse-probe techniques of third-generation synchrotron sources with time resolutions of 30-100 ps, as demonstrated by examples in this review. These studies provide unprecedented information on structural origins of molecular properties in the excited states. By using other ultrafast X-ray facilities that will be completed in the near future, time-resolution for the excited-state structure measurements should reach the femtosecond time scales, which will make "molecular movies" of bond breaking or formation, and vibrational relaxation, a reality.     

SESAME-HSQC for simultaneous measurement of NH and CH scalar and residual dipolar couplings

We present a novel pulse sequence, SESAME-HSQC, for the simultaneous measurement of several NH and CH scalar and residual dipolar couplings in double labeled proteins. The proposed Spin-statE Selective All Multiplicity Edited (SESAME)-HSQC combines gradient selected and sensitivity enhanced (15)N- and constant-time (13)C-HSQC experiments with the recently introduced spin-state selective method (Nolis et al., J. Magn. Reson. 180 (2006) 39-50) for measuring couplings simultaneously at amide and aliphatic regions. Excellent resolution and high sensitivity is warranted by removing all coupling interactions during the indirectly detected t(1) period, and by employing pulsed field gradients for coherence selection and utilizing coherence order selective spin-state selection. The scalar and residual dipolar couplings can be readily measured from a two-dimensional (15)N/(13)C-HSQC spectrum without additional spectral crowding. SESAME-HSQC can be used for epitope mapping by observing chemical shift changes in both amide and aliphatic regions. Simultaneously, potential conversion in protein conformation can be probed by analyzing changes in residual dipolar couplings induced by ligand binding. The pulse sequence is experimentally verified with a sample of (15)N/(13)C enriched human ubiquitin. The internuclear vector directions determined from the residual dipolar couplings are found to be in excellent correlation with those predicted from ubiquitin's refined solution structure.     

Matrix‐Free DNP‐Enhanced NMR Spectroscopy of Liposomes Using a Lipid‐Anchored Biradical

Magic‐angle spinning dynamic nuclear polarization (MAS‐DNP) has been proven to be a powerful technique to enhance the sensitivity of solid‐state NMR (SSNMR) in a wide range of systems. Here, we show that DNP can be used to polarize lipids using a lipid‐anchored polarizing agent. More specifically, we introduce a C16‐functionalized biradical, which allows localization of the polarizing agents in the lipid bilayer and DNP experiments to be performed in the absence of excess cryo‐protectant molecules (glycerol, dimethyl sulfoxide, etc.). This constitutes another original example of the matrix‐free DNP approach that we recently introduced.     

Identification of two isomers from an organic mixture by double-stimulated-echo NMR and construction of the DOSY spectra by the regularized resolvent transform method.

DOSY-NMR experiments were used to characterize two structural isomers that have different shapes, but identical mass fragmentation patterns, from an organic mixture. It is shown that the spherical molecule diffuses faster than the ellipsoidal one. This distinct behaviour is tentatively explained in terms of microfriction effects from the solvent. Finally, to increase the resolution and efficiency of the 2D DOSY map construction, the data were treated with the regularized resolvent transform method. This method provides good separation for mildly overlapped peaks and requires fewer points than other classical DOSY processing methods.     

Dynamic Nuclear Polarization with a Rigid Biradical

A new polarizing agent with superior performance in dynamic nuclear polarization experiments is introduced, and utilizes two TEMPO (2,2,6,6‐tetramethylpiperidine‐1‐oxyl) moieties connected through a rigid spiro tether (see structure). The observed NMR signal intensities were enhanced by a factor of 1.4 compared to those of TOTAPOL, a previously described TEMPO‐based biradical with a flexible tether.