Computational Study of the One‐ and Two‐Dimensional Infrared Spectra of a Proton‐Transfer Mode in a Hydrogen‐Bonded Complex Dissolved in a Polar Nanocluster

The signatures of nanosolvation on the one‐ and two‐dimensional (1D and 2D) IR spectra of a proton‐transfer mode in a hydrogen‐bonded complex dissolved in polar solvent molecule nanoclusters of varying size are elucidated by using mixed quantum–classical molecular dynamics simulations. For this particular system, increasing the number of solvent molecules successively from N=7 to N=9 initiates the transition of the system from a cluster state to a bulk‐like state. Both the 1D and 2D IR spectra reflect this transition through pronounced changes in their peak intensities and numbers, but the time‐resolved 2D IR spectra also manifest spectral features that uniquely identify the onset of the cluster‐to‐bulk transition. In particular, it is observed that in the 1D IR spectra, the relative intensities of the peaks change such that the number of peaks decreases from three to two as the size of the cluster increases from N=7 to N=9. In the 2D IR spectra, off‐diagonal peaks are observed in the N=7 and N=8 cases at zero waiting time, but not in the N=9 case. It is known that there are no off‐diagonal peaks in the 2D IR spectrum of the bulk version of this system at zero waiting time, so the disappearance of these peaks is a unique signature of the onset of bulk‐like behavior. Through an examination of the trajectories of various properties of the complex and solvent, it is possible to relate the emergence of these off‐diagonal peaks to an interplay between the vibrations of the complex and the solvent polarization dynamics.     

Molecular Reorientational Dynamics of the Neat Ionic Liquid 1‐Butyl‐3‐methylimidazolium Hexafluorophosphate by Measurement of 13C Nuclear Magnetic Relaxation Data

The reorientational dynamics of the ionic liquid 1butyl‐3‐methylimidazolium hexafluorophosphate ([BMIM]PF₆) were studied over a wide range of temperatures by measurement of ¹³C spin–lattice relaxation rates and NOE factors. The reorientational dynamics were evaluated by performing fits to the experimental relaxation data. Thus, the overall reorientational motion was described by a Cole–Davidson spectral density with a Vogel–Fulcher–Tammann temperature dependence of the correlation times. The reorientational motion of the butyl chain was modelled by a combination of the latter model for the overall motion with a Bloembergen–Purcell–Pound spectral density and an Arrhenius temperature dependence for the internal motion. Except for C2 in the aromatic ring, an additional reduction of the spectral density by the Lipari–Szabo model had to be employed. This reduction is a consequence of fast molecular motions before the rotational diffusion process becomes effective. The C2 atom did not exhibit this reduction, because the librational motion of the corresponding C2? H vector is severely hindered due to hydrogen bonding with the hexafluorophosphate anion. The observed dynamic features of the [BMIM]⁺ cation confirm quantum‐chemical structures obtained in a former study.     

On the Origin of Water Flow through Carbon Nanotubes

The transport of water molecules through carbon nanotubes (CNTs) is of primary importance for understanding water‐mediated biological activities as well as for the design of novel nanoporous materials. Herein, we analyze the water flow through CNTs by using molecular dynamics simulations with the hope of finding basic parameters determining the flow value. Of particular interest is that a simple equation as a function of water diffusion, occupancy and CNT size, can well describe the water flow through CNTs with different sizes. Specifically, both the simulation and equation flow exhibit power law relations with the CNT diameter and length, where the two exponents are close to each other. The water occupancy and translocation time also demonstrate interesting relations with the CNT size. The water dipole orientations and density profiles are also sensitive to the change of CNT size. These results greatly enhance our knowledge on the nature of water flow through CNTs and are helpful in predicting the water flow of CNTs up to the experimental length scale.     

Size‐Dependent Water Structures in Carbon Nanotubes

Water surrounded by hydrophobic interfaces affects a variety of chemical reactions and biological activities. Carbon nanotubes (CNTs) can be used to investigate the behavior of water at hydrophobic interfaces. Here, we determined the fundamental unit of water by evaluating the ice‐like cluster formation of water in the limited hydrophobic nanospaces of CNTs, using X‐ray diffraction and molecular simulation analysis. The water in CNTs with a diameter of 1 nm had fewer hydrogen bonds than bulk water under ambient conditions. In CNTs with diameters of 2 and 3 nm, water formed nanoclusters even under ambient conditions, because of prolific hydrogen bonding; predominant ice‐like cluster formation was induced in the 2–3 nm nanospaces. The results confirming the cluster formation in the CNTs also demonstrated that the critical cluster size was 0.8–3.4 nm. The fundamental cluster size was 0.8 nm; these results indicated that 0.8 nm clusters are the fundamental units of water assemblies.     

Hydrogen Adsorption and Diffusion in p‐tert‐Butylcalix[4]arene: An Experimental and Molecular Simulation Study

Experimental adsorption isotherms were measured and computer simulations were performed to determine the nature of the H₂ gas uptake in the low‐density p‐tert‐butylcalix[4]arene (tBC) phase. ¹H NMR peak intensity measurements for pressures up to 175 bar were used to determine the H₂ adsorption isotherm. Weak surface adsorption (up to ≈2 mass % H₂) and stronger adsorption (not exceeding 0.25 mass % or one H₂ per calixarene bowl) inside the calixarene phase were detected. The latter type of adsorbed H₂ molecule has restricted motion and shows a reversible gas adsorption/desorption cycle. Pulsed field gradient (PFG) NMR pressurization/depressurization measurements were performed to study the diffusion of H₂ in the calixarene phases. Direct adsorption isotherms by exposure of the calixarene phase to pressures of H₂ gas to ≈60 bar are also presented, and show a maximum H₂ adsorption of 0.4 H₂ per calixarene bowl. Adsorption isotherms of H₂ in bulk tBC have been simulated using grand canonical Monte Carlo calculations in a rigid tBC framework, and yield adsorptions of ≈1 H₂ per calixarene bowl at saturation. Classical molecular dynamics simulations with a fully flexible calixarene molecular force field are used to determine the guest distribution and inclusion energy of the H₂ in the solid with different loadings.     

Multifunctional Tricarbazolo Triazolophane Macrocycles: One‐Pot Preparation,Anion Binding,and Hierarchical Self‐Organization of Multilayers

Programming the synthesis and self‐assembly of molecules is a compelling strategy for the bottom‐up fabrication of ordered materials. To this end, shape‐persistent macrocycles were designed with alternating carbazoles and triazoles to program a one‐pot synthesis and to bind large anions. The macrocycles bind anions that were once considered too weak to be coordinated, such as PF₆^?, with surprisingly high affinities (β₂=10¹¹ M ^?2 in 80:20 chloroform/methanol) and positive cooperativity, α=(4 K₂/K₁)=1200. We also discovered that the macrocycles assemble into ultrathin films of hierarchically ordered tubes on graphite surfaces. The remarkable surface‐templated self‐assembly properties, as was observed by using scanning tunneling microscopy, are attributed to the complementary pairing of alternating triazoles and carbazoles inscribed into both the co‐facial and edge‐sharing seams that exist between shape‐persistent macrocycles. The multilayer assembly is also consistent with the high degree of molecular self‐association observed in solution, with self‐association constants of K=300 000 M ^?1 (chloroform/methanol 80:20). Scanning tunneling microscopy data also showed that surface assemblies readily sequester iodide anions from solution, modulating their assembly. This multifunctional macrocycle provides a foundation for materials composed of hierarchically organized and nanotubular self‐assemblies.     

Improving MCM‐41 as a Nitrosamines Trap through a One‐Pot Synthesis

Copper oxide was incorporated into MCM‐41 by a one‐pot synthesis under acidic conditions to prepare a new mesoporous nitrosamines trap for protection of the environment. The resulting composites were characterized by XRD, N₂ adsorption–desorption, and H₂ temperature‐programmed reduction techniques, and their adsorption capabilities were assessed in the gaseous adsorption of N‐nitrosopyrrolidine (NPYR). The adsorption isotherms were consistent with the Freundlich equation. The copper salt was deposited onto MCM‐41 during the evaporation stage and was fixed on the host in the calcination process that followed. MCM‐41 was able to capture NPYR in air below 373 K but not at 453 K. Loading of copper oxide on MCM‐41 greatly improved its adsorption capability at elevated temperatures. The influence of the incorporation of copper into MCM‐41 samples and the adsorption behavior of these samples are discussed in detail.     

Surface Charges at the CaF2/Water Interface Allow Very Fast Intermolecular Vibrational‐Energy Transfer

We investigate the dynamics of water in contact with solid calcium fluoride, where at low pH, localized charges can develop upon fluorite dissolution. We use 2D surface‐specific vibrational spectroscopy to quantify the heterogeneity of the interfacial water (D₂O) molecules and provide information about the sub‐picosecond vibrational‐energy‐relaxation dynamics at the buried solid/liquid interface. We find that strongly H‐bonded OD groups, with a vibrational frequency below 2500 cm^?1, display very rapid spectral diffusion and vibrational relaxation; for weakly H‐bonded OD groups, above 2500 cm^?1, the dynamics slows down substantially. Atomistic simulations based on electronic‐structure theory reveal the molecular origin of energy transport through the local H‐bond network. We conclude that strongly oriented H‐bonded water molecules in the adsorbed layer, whose orientation is pinned by the localized charge defects, can exchange vibrational energy very rapidly due to the strong collective dipole, compensating for a partially missing solvation shell.     

Slow Molecular Motions in Ionic Liquids Probed by Cross‐Relaxation of Nuclear Spins During Overhauser Dynamic Nuclear Polarization

Solution‐state Overhauser dynamic nuclear polarization (ODNP) at moderate fields, performed by saturating the electron spin resonance (ESR) of a free radical added to the sample of interest, is well known to lead to significant NMR signal enhancements in the steady state, owing to electron–nuclear cross‐relaxation. Here it is shown that under conditions which limit radical access to the molecules of interest, the time course of establishment of ODNP can provide a unique window into internuclear cross‐relaxation, and reflects relatively slow molecular motions. This behavior, modeled mathematically by a three‐spin version of the Solomon equations (one unpaired electron and two nuclear spins), is demonstrated experimentally on the ¹⁹F/¹H system in ionic liquids. Bulky radicals in these viscous environments turn out to be just the right setting to exploit these effects. Compared to standard nuclear Overhauser effect (NOE) work, the present experiment offers significant improvement in dynamic range and sensitivity, retains usable chemical shift information, and reports on molecular motions in the sub‐megahertz (MHz) to tens of MHz range—motions which are not accessed at high fields.     

Photo‐ and Redox‐Active Dendritic Molecules with Soft,Layered Nanostructures

Molecules with one photoactive group (porphyrin) and multiple redox‐active groups (ferrocenes) are described. The molecules are based on dendritic frameworks, with the ferrocenyl groups attached at the “internal” positions and the porphyrin attached at the focal point, leading to a characteristic layer architecture. Molecules of up to the third generation were synthesized and examined. The results of ¹H NMR spectroscopy and fluorescence quenching indicated that the ferrocenyl groups at the second layer approach the core porphyrin most closely, which is consistent with the results of molecular‐dynamics simulations. The electrochemistry of the molecules was also examined in detail, and a new formula is proposed for the analysis of multiple‐electron transfer in these “redox‐pool” molecules.     

Understanding the Role of Water in Aqueous Ruthenium‐Catalyzed Transfer Hydrogenation of Ketones.

We report an accurate computational study of the role of water in transfer hydrogenation of formaldehyde with a ruthenium‐based catalyst using a water‐specific model. Our results suggest that the reaction mechanism in aqueous solution is significantly different from that in the gas phase or in methanol solution. Previous theoretical studies have shown a concerted hydride and proton transfer in the gas phase (M. Yamakawa, H. Ito, R. Noyori, J. Am. Chem. Soc. 2000 , 122, 1466–1478;J.‐W. Handgraaf, J. N. H. Reek, E. J. Meijer, Organometallics 2003 , 22, 3150–3157; D. A. Alonso, P. Brandt, S. J. M. Nordin, P. G. Andersson, J. Am. Chem. Soc. 1999 , 121, 9580–9588; D. G. I. Petra, J. N. H. Reek, J.‐W. Handgraaf, E. J. Meijer, P. Dierkes, P. C. J. Kamer, J. Brussee, H. E. Schoemaker, P. W. N. M. van Leeuwen, Chem. Eur. J. 2000 , 6, 2818–2829), whereas a delayed, solvent‐mediated proton transfer has been observed in methanol solution (J.‐W. Handgraaf, E. J. Meijer, J. Am. Chem. Soc. 2007 , 129, 3099–3103). In aqueous solution, a concerted transition state is observed, as in the previous studies. However, only the hydride is transferred at that point, whereas the proton is transferred later by a water molecule instead of the catalyst.     

Rapid Determination of Fast Protein Dynamics from NMR Chemical Exchange Saturation Transfer Data

Functional motions of ¹⁵N‐labeled proteins can be monitored by solution NMR spin relaxation experiments over a broad range of timescales. These experiments however typically take of the order of several days to a week per protein. Recently, NMR chemical exchange saturation transfer (CEST) experiments have emerged to probe slow millisecond motions complementing R_1ρ and CPMG‐type experiments. CEST also simultaneously reports on site‐specific R₁ and R₂ parameters. It is shown here how CEST‐derived R₁ and R₂ relaxation parameters can be measured within a few hours at an accuracy comparable to traditional relaxation experiments. Using a “lean” version of the model‐free approach S² order parameters can be determined that match those from the standard model‐free approach applied to ¹⁵N R₁, R₂, and {¹H}‐¹⁵N NOE data. The new methodology, which is demonstrated for ubiquitin and arginine kinase (42 kDa), should serve as an effective screening tool of protein dynamics from picosecond‐to‐millisecond timescales.