Color‐Discriminating Retinal Configurations of Sensory Rhodopsin I by Photo‐Irradiation Solid‐State NMR Spectroscopy

SRI (sensory rhodopsin I) can discriminate multiple colors for the attractant and repellent phototaxis. Studies aimed at revealing the color‐dependent mechanism show that SRI is a challenging system not only in photobiology but also in photochemistry. During the photoreaction of SRI, an M‐intermediate (attractant) transforms into a P‐intermediate (repellent) by absorbing blue light. Consequently, SRI then cycles back to the G‐state. The photoreactions were monitored with the ¹³C NMR signals of [20‐¹³C]retnal‐SrSRI using in situ photo‐irradiation solid‐state NMR spectroscopy. The M‐intermediate was trapped at ?40 °C by illumination at 520 nm. It was transformed into the P‐intermediate by subsequent illumination at 365 nm. These results reveal that the G‐state could be directly transformed to the P‐intermediate by illumination at 365 nm. Thus, the stationary trapped M‐ and P‐intermediates are responsible for positive and negative phototaxis, respectively.     

Lab‐on‐a‐Chip Reactor Imaging with Unprecedented Chemical Resolution by Hadamard‐Encoded Remote Detection NMR

The development of microfluidic processes requires information‐rich detection methods. Here we introduce the concept of remote detection exchange NMR spectroscopy (RD‐EXSY), and show that, along with indirect spatial information extracted from time‐of‐flight data, it provides unique information about the active regions, reaction pathways, and intermediate products in a lab‐on‐a‐chip reactor. Furthermore, we demonstrate that direct spatial resolution can be added to RD‐EXSY efficiently by applying the principles of Hadamard spectroscopy.     

Long‐Lived Trifluoromethanide Anion: A Key Intermediate in Nucleophilic Trifluoromethylations

The trifluoromethanide anion is the postulated key intermediate in nucleophilic trifluoromethylation reactions. However, for more than six decades, the trifluoromethanide anion was widely believed to exist only as a short‐lived transient species in the condensed phase. It has now been prepared in bulk for the first time in THF solution. The trifluoromethanide anion with the [K(18‐crown‐6)]⁺ cation was unequivocally characterized by low‐temperature ¹⁹F and ¹³C NMR spectroscopy. Its intermediacy in nucleophilic trifluoromethylation reactions was directly evident by its reaction chemistry with various electrophilic substrates. Variable‐temperature NMR spectroscopy, along with quantum mechanical calculations, support the persistence of the trifluoromethanide anion.     

Chemical Reaction Monitoring using Zero‐Field Nuclear Magnetic Resonance Enables Study of Heterogeneous Samples in Metal Containers

We demonstrate that heterogeneous/biphasic chemical reactions can be monitored with high spectroscopic resolution using zero‐field nuclear magnetic resonance spectroscopy. This is possible because magnetic susceptibility broadening is negligible at ultralow magnetic fields. We show the two‐step hydrogenation of dimethyl acetylenedicarboxylate with para‐enriched hydrogen gas in conventional glass NMR tubes, as well as in a titanium tube. The low frequency zero‐field NMR signals ensure that there is no significant signal attenuation arising from shielding by the electrically conductive sample container. This method paves the way for in situ monitoring of reactions in complex heterogeneous multiphase systems and in reactors made of conductive materials while maintaining resolution and chemical specificity.     

Two‐dimensional Fourier transform infrared correlation spectroscopy and evolving factor analysis in the study of cationic curing of DGEBA and γ‐valerolactone mixtures

Generalized two‐dimensional (2D) correlation spectroscopy and evolving factor analysis (EFA) have been applied to the study of cationic curing reaction of mixtures of diglycidyl ether of bisphenol A (DGEBA) and γ‐valerolactone (γ‐VL). The reaction has been monitored by Fourier transform infrared spectroscopy. The reaction periods in which a chemical change takes place are identified using EFA and then the sequence of changes are established by means of the interpretation of the synchronous and asynchronous spectra obtained with 2D correlation spectroscopy. By combining this information, in the curing process, four reactions have been detected: (1) DGEBA and γ‐VL reaction to obtain a spiroorthoester intermediate, (2) homopolymerization of DGEBA, (3) copolymerization of DGEBA with the spiroorthoester intermediate, and (4) homopolymerization of the spiroorthoester. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 3886–3899, 2008     

Sequence analysis of glycolide and p‐dioxanone copolymers

Low and medium molecular weight copolymers constituted by glycolide and p‐dioxanone units have been synthesized by a ring‐opening polymerization. The p‐dioxanone monomer was obtained from (2‐hydroxyethoxy)acetate or by thermal depolymerization of poly(p‐dioxanone). ¹H and ¹³C NMR spectra were highly sensitive to the chemical sequences, which were effectively assigned by considering the data from samples with different compositions, and the acquisition of heteronuclear ¹H and ¹³C NMR‐correlated spectra. End groups were also identified, allowing methylene protons of sequences involving up to two glycolide units to be distinguished. These data seem basic to analyze degradation products or the influence of thermal treatments in chain microstructure. Glycolide/p‐dioxanone copolymers are an interesting system because changes on chemical sequences can easily occur due to a depolymerization reaction that eliminates p‐dioxanone residues. Furthermore, depending on the polymerization conditions, the occurrence of transesterification reactions may be highly significant. These reactions have a great impact in properties such as the melting temperature and can be easily quantified by NMR spectroscopy because of the occurrence of a new chemical sequence. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2009     

Transition‐state spectroscopy using ultrashort laser pulses

To gain a complete understanding of a chemical reaction, it is necessary to determine the structural changes that occur to the reacting molecules during the reaction. Chemists have long dreamed of being able to determine the molecular structure changes that occur during a chemical reaction, including the structures of transition states (TSs). The use of ultrafast spectroscopy to gain a detailed knowledge of chemical reactions (including their TSs) promises to be a revolutionary way to increase reaction efficiencies and enhance the reaction products, which is difficult to do using conventional methods that are based on trial and error. To confirm the molecular structures of TSs predicted by theoretical analysis, chemists have long desired to directly observe the TSs of chemical reactions. Direct observations have been realized by ultrafast spectroscopy using ultrashort laser pulses. Our group has been able to stably generate visible to near‐infrared sub‐5‐fs laser pulses using a noncollinear optical parametric amplifier (NOPA). We used these sub‐5‐fs pulses to study reaction processes (including their TSs) by detecting structural changes. We determine reaction mechanisms by observing the TSs in a chemical reaction and by performing density‐functional theory calculations. DOI 10.1002/tcr.201000018     

Solution‐State 17O Quadrupole Central‐Transition NMR Spectroscopy in the Active Site of Tryptophan Synthase

Oxygen is an essential participant in the acid–base chemistry that takes place within many enzyme active sites, yet has remained virtually silent as a probe in NMR spectroscopy. Here, we demonstrate the first use of solution‐state ¹⁷O quadrupole central‐transition NMR spectroscopy to characterize enzymatic intermediates under conditions of active catalysis. In the 143 kDa pyridoxal‐5′‐phosphate‐dependent enzyme tryptophan synthase, reactions of the α‐aminoacrylate intermediate with the nucleophiles indoline and 2‐aminophenol correlate with an upfield shift of the substrate carboxylate oxygen resonances. First principles calculations suggest that the increased shieldings for these quinonoid intermediates result from the net increase in the charge density of the substrate–cofactor π‐bonding network, particularly at the adjacent α‐carbon site.     

Recovering Invisible Signals by Two‐Field NMR Spectroscopy

Nuclear magnetic resonance (NMR) studies have benefited tremendously from the steady increase in the strength of magnetic fields. Spectacular improvements in both sensitivity and resolution have enabled the investigation of molecular systems of rising complexity. At very high fields, this progress may be jeopardized by line broadening, which is due to chemical exchange or relaxation by chemical shift anisotropy. In this work, we introduce a two‐field NMR spectrometer designed for both excitation and observation of nuclear spins in two distinct magnetic fields in a single experiment. NMR spectra of several small molecules as well as a protein were obtained, with two dimensions acquired at vastly different magnetic fields. Resonances of exchanging groups that are broadened beyond recognition at high field can be sharpened to narrow peaks in the low‐field dimension. Two‐field NMR spectroscopy enables the measurement of chemical shifts at optimal fields and the study of molecular systems that suffer from internal dynamics, and opens new avenues for NMR spectroscopy at very high magnetic fields.     

Identification of tert‐Butyl Cations in Zeolite H‐ZSM‐5: Evidence from NMR Spectroscopy and DFT Calculations

Experimental evidence for the presence of tert‐butyl cations, which are important intermediates in acid‐catalyzed heterogeneous reactions, on solid acids has still not been provided to date. By combining density functional theory (DFT) calculations with ¹H/¹³C magic‐angle‐spinning NMR spectroscopy, the tert‐butyl cation was successfully identified on zeolite H‐ZSM‐5 upon conversion of isobutene by capturing this intermediate with ammonia.     

Exploring Between the Extremes: Conversion‐Dependent Kinetics of Phosphite‐Modified Hydroformylation Catalysis

The kinetics of the hydroformylation of 3,3‐dimethyl‐1‐butene with a rhodium monophosphite catalyst has been studied in detail. Time‐dependent concentration profiles covering the entire olefin conversion range were derived from in situ high‐pressure FTIR spectroscopic data for both, pure organic components and catalytic intermediates. These profiles fit to Michaelis–Menten‐type kinetics with competitive and uncompetitive side reactions involved. The characteristics found for the influence of the hydrogen concentration verify that the pre‐equilibrium towards the catalyst substrate complex is not established. It has been proven experimentally that the hydrogenolysis of the intermediate acyl complex remains rate limiting even at high conversions when the rhodium hydride is the predominant resting state and the reaction is nearly of first order with respect to the olefin. Results from in situ FTIR and high‐pressure (HP) NMR spectroscopy and from DFT calculations support the coordination of only one phosphite ligand in the dominating intermediates and a preferred axial position of the phosphite in the electronically saturated, trigonal bipyramidal (tbp)‐structured acyl rhodium complex.     

Partial‐Homogeneity‐Based Two‐Dimensional High‐Resolution Nuclear Magnetic Resonance Spectroscopy under Inhomogeneous Magnetic Fields

High‐resolution multidimensional nuclear magnetic resonance (NMR) spectroscopy serves as an irreplaceable and versatile tool in various chemical investigations. In this study, a method based on the concept of partial homogeneity is developed to offer two‐dimensional (2D) high‐resolution NMR spectra under inhomogeneous fields. Oscillating gradients are exerted to encode the high‐resolution information, and a field‐inhomogeneity correction algorithm based on pattern recognition is designed to recover high‐resolution spectra. Under fields where inhomogeneity primarily distributes along a single orientation, the proposed method will improve performances of 2D NMR spectroscopy without increasing the experimental duration or significant loss in sensitivity, and thus may open important perspectives for studies of inhomogeneous chemical systems.     

Spatiotemporal Kinetics in Solution Studied by Time‐Resolved X‐Ray Liquidography (Solution Scattering)

Information about temporally varying molecular structure during chemical processes is crucial for understanding the mechanism and function of a chemical reaction. Using ultrashort optical pulses to trigger a reaction in solution and using time‐resolved X‐ray diffraction (scattering) to interrogate the structural changes in the molecules, time‐resolved X‐ray liquidography (TRXL) is a direct tool for probing structural dynamics for chemical reactions in solution. TRXL can provide direct structural information that is difficult to extract from ultrafast optical spectroscopy, such as the time dependence of bond lengths and angles of all molecular species including short‐lived intermediates over a wide range of times, from picoseconds to milliseconds. TRXL elegantly complements ultrafast optical spectroscopy because the diffraction signals are sensitive to all chemical species simultaneously and the diffraction signal from each chemical species can be quantitatively calculated from its three‐dimensional atomic coordinates and compared with experimental TRXL data. Since X‐rays scatter from all the atoms in the solution sample, solutes as well as the solvent, the analysis of TRXL data can provide the temporal behavior of the solvent as well as the structural progression of all the solute molecules in all the reaction pathways, thus providing a global picture of the reactions and accurate branching ratios between multiple reaction pathways. The arrangement of the solvent around the solute molecule can also be extracted. This review summarizes recent developments in TRXL, including technical innovations in synchrotron beamlines and theoretical analysis of TRXL data, as well as several examples from simple molecules to an organometallic complex, nanoparticles, and proteins in solution. Future potential applications of TRXL in femtosecond studies and biologically relevant molecules are also briefly mentioned.     

Chemical/Light‐Powered Hybrid Micromotors with “On‐the‐Fly” Optical Brakes

Hybrid micromotors capable of both chemically powered propulsion and fuel‐free light‐driven actuation and offering built‐in optical brakes for chemical propulsion are described. The new hybrid micromotors are designed by combining photocatalytic TiO₂ and catalytic Pt surfaces into a Janus microparticle. The chemical reactions on the different surfaces of the Janus particle hybrid micromotor can be tailored by using chemical or light stimuli that generate counteracting propulsion forces on the catalytic Pt and photocatalytic TiO₂ sides. Such modulation of the surface chemistry on a single micromotor leads to switchable propulsion modes and reversal of the direction of motion that reflect the tuning of the local ion concentration and hence the dominant propulsion force. An intermediate Au layer (under the Pt surface) plays an important role in determining the propulsion mechanism and operation of the hybrid motor. The built‐in optical braking system allows “on‐the‐fly” control of the chemical propulsion through a photocatalytic reaction on the TiO₂ side to counterbalance the chemical propulsion force generated on the Pt side. The adaptive dual operation of these chemical/light hybrid micromotors, associated with such control of the surface chemistry, holds considerable promise for designing smart nanomachines that autonomously reconfigure their propulsion mode for various on‐demand operations.     

Nanoscale Control Over Interfacial Properties in Mixed Reverse Micelles Formulated by Using Sodium 1,4‐bis‐2‐ethylhexylsulfosuccinate and Tri‐n‐octyl Phosphine Oxide Surfactants

The interfacial properties of pure reverse micelles (RMs) are a consequence of the magnitude and nature of noncovalent interactions between confined water and the surfactant polar head. Addition of a second surfactant to form mixed RMs is expected to influence these interactions and thus affect these properties at the nanoscale level. Herein, pure and mixed RMs stabilized by sodium 1,4‐bis‐2‐ethylhexylsulfosuccinate and tri‐n‐octyl phosphine oxide (TOPO) surfactants in n‐heptane were formulated and studied by varying both the water content and the TOPO mole fraction. The microenvironment generated was sensed by following the solvatochromic behavior of the 1‐methyl‐8‐oxyquinolinium betaine probe and ³¹P NMR spectroscopy. The results reveal unique properties of mixed RMs and we give experimental evidence that free water can be detected in the polar core of the mixed RMs at very low water content. We anticipate that these findings will have an impact on the use of such media as nanoreactors for many types of chemical reactions, such as enzymatic reactions and nanoparticle synthesis.     

A Photophysical Study of Sensitization‐Initiated Electron Transfer: Insights into the Mechanism of Photoredox Activity

The development of photocatalytic reactions has provided many novel opportunities to expand the scope of synthetic organic chemistry. In parallel with progress towards uncovering new reactivity, there is consensus that efforts focused on providing detailed mechanistic insight in order to uncover underlying excited‐state reactions are essential to maximise formation of desired products. With this in mind, we have investigated the recently reported sensitization‐initiated electron transfer (SenI‐ET) reaction for the C?H arylation of activated aryl halides. Using a variety of techniques, and in particular nanosecond transient absorption spectroscopy, we are able to distinguish several characteristic signals from the excited‐state species involved in the reaction, and subsequent kinetic analysis under various conditions has facilitated a detailed insight into the likely reaction mechanism.     

Visible-Light-Mediated Liberation and In Situ Conversion of Fluorophosgene

The first example for the photocatalytic generation of a highly electrophilic intermediate that is not based on radical reactivity is reported. The single-electron reduction of bench-stable and commercially available 4-(trifluoromethoxy)benzonitrile by an organic photosensitizer leads to its fragmentation into fluorophosgene and benzonitrile. The in situ generated fluorophosgene was used for the preparation of carbonates, carbamates, and urea derivatives in moderate to excellent yields via an intramolecular cyclization reaction. Transient spectroscopic investigations suggest the formation of a catalyst charge-transfer complex-dimer as the catalytic active species. Fluorophosgene as a highly reactive intermediate, was indirectly detected via its next downstream carbonyl fluoride intermediate by NMR. Furthermore, detailed NMR analyses provided a comprehensive reaction mechanism including a water dependent off-cycle equilibrium.     

Accelerated NMR Spectroscopy with Low‐Rank Reconstruction

Accelerated multi‐dimensional NMR spectroscopy is a prerequisite for high‐throughput applications, studying short‐lived molecular systems and monitoring chemical reactions in real time. Non‐uniform sampling is a common approach to reduce the measurement time. Here, a new method for high‐quality spectra reconstruction from non‐uniformly sampled data is introduced, which is based on recent developments in the field of signal processing theory and uses the so far unexploited general property of the NMR signal, its low rank. Using experimental and simulated data, we demonstrate that the low‐rank reconstruction is a viable alternative to the current state‐of‐the‐art technique compressed sensing. In particular, the low‐rank approach is good in preserving of low‐intensity broad peaks, and thus increases the effective sensitivity in the reconstructed spectra.