CH Moieties Research Articles

Enhancing solution structural analysis of large molecular proteins through optimal stereo array isotope labeling of aromatic amino acids

The observation of side-chain peaks of aromatic amino acids is the prerequisite for a high-resolution three-dimensional structure determination of proteins by NMR. However, it becomes difficult with increasing molecular size due to an increased transverse relaxation and the control of the relaxation pathway is needed to achieve the observation. We demonstrated that even for the large molecular size of 82 kDa Malate synthase G (MSG), the aromatic 13C-1H (CH) peaks of Tryptophan (Trp) and Phenylalanine (Phe) residues can be observed with high quality using a systematic stable isotope labeling scheme, Stereo-Array Isotope Labeling (SAIL) method. However, the sequence specific assignments of these peaks relied on the use of amino acid substitutions, employing an inefficient method that required many isotopes labeled samples. In this study, we developed novel SAIL amino acids that allow for the observation of the aromatic ring δ,ζ and the aliphatic β position peak of Phe residues. The application of TROSY-based experiment to the isolated CH moieties resulted in the successful observation of discernible and resolved CH peaks in Phe residues in MSG. In MSG, the sequence-specific assignments of the backbone and Cβ positions have already been confirmed. Therefore, using this labeling method, the δ and β position peaks of Phe residues can be clearly assigned in a sequence-specific and stereospecific manner through experiments based on intra-residue NOE. Furthermore, the NOESY experiment also allows for the acquisition of information pertaining to the conformation of Phe residues, such as the χ1 dihedral angle, providing valuable insights for the determination of accurate protein structures and in dynamic analysis. This new SAIL amino acids open an avenue to achieve a variety of NMR analysis of large molecular proteins, including a high-resolution structure determination and dynamics and interaction analysis.

Comparative studies on the nature and kinetics of phase transitions in plastic crystals: Hydrogen-bonded adamantane derivatives

In this paper, the results of thermal, structural, and spectroscopic studies on 3-amino-1-adamantanol (3-NH2-1-OH-ADM) were discussed with those recently published for 1-hydroxyl and 1-amine derivatives of adamantane (1-OH-ADM, 1-NH2-ADM). Calorimetric measurements showed that the examined compound, like 1-NH2-ADM, is characterized by multiple thermal events in the thermogram measured on heating and cooling. Further X-ray diffraction, Fourier-transform infrared, and Raman spectroscopy investigations suggested that two of them correspond to the transition from the ordinary crystal (OC) to the plastic crystal (PC) phase I and the transition between two PC phases (PC(I) and PC(II)), while the third subtle one is rather not related to a phase transition. Interestingly, ambient pressure dielectric studies performed during heating and cooling cycles revealed changes in the imaginary and real parts of the complex dielectric permittivity at temperatures close to the temperatures of phase transitions detected by the calorimetry. Such behavior resembled that reported for 1-NH2-ADM. Finally, isothermal time-dependent dielectric and infrared studies, supported by the analysis of asynchronous 2D-IR correlation spectra, indicated that the first endothermic peak, visible in the thermogram of 3-NH2-1-OH-ADM over an enormously large temperature interval and related to the OC-PC(I) phase transition, similarly to the one, assigned to the PC(I)-PC(II) transition in 1-NH2-ADM, has a character of the kinetic process. Importantly, during the annealing of 3-NH2-1-OH-ADM at T = 328–343 K, we observed a clear change in the dc-conductivity as well as intramolecular dynamics related to the vibrations of CH, NH2, and OH moieties that occur at different rates.

Structure and intermolecular interactions in ionic liquid 1-ethyl-3-methylimidazolium bromide and its aqueous solutions investigated by vibrational spectroscopy and quantum chemical computations.

The recently developed efficient protocol to explicit quantum mechanical modeling of structure and IR spectra of liquids and solutions (S. A. Katsyuba, S. Spicher, T. P. Gerasimova, S. Grimme, J. Phys. Chem. B 2020, 124, 6664) is applied to ionic liquid (IL) 1-ethyl-3-methylimidazolium bromide (EmimBr), its C2-deuterated analog [Emim-d]Br and its aqueous solutions. It is shown that the solvation strongly modifies frequencies and IR intensities of the CH/CD stretching vibrations (νCH/νCD) of the imidazolium ring. The main vibrational spectroscopic features of the neat IL are reproduced by the simulations for a cluster (EmimBr)9, in which all three imidazolium CH moieties of the solvated cation form short contacts with three Br- anions, and another two Br- anions are located on top and bottom of imidazolium ring. Cluster models of aqueous solutions reproduce the experimental vibrational frequencies of actual solutions, provided that the Br- anion of solvated contact ion pair (CIP) is situated on top of imidazolium ring, and CH/CD moieties of the latter participate in short contacts with surrounding water molecules. Both structural and spectroscopic analysis allow to interpret the short contacts CH/CD⋯Br- and CH/CD⋯OH2 as hydrogen bonds of approximately equal strength. Enthalpies of bonding of these liquid-state H-bonds, estimated with the use of empirical correlations, amount to ca. 1.4 kcal⋅mol-1, while the analogous estimates obtained for the gas-phase charged species [Emim]2Br+ increase to 5.6 kcal⋅mol-1. It is shown that formation of solvent-shared ion pair (SIP) in aqueous solution, where the counterions of IL are separated by two water molecules H-bonded to a Br- anion, produces frequency shifts ΔνCH/CD, strongly different from the case of CIP formation. This difference can be used for IR/Raman spectroscopic differentiation of the type of solvated ion pairs of EmimBr or other related ILs.

Electron Nuclear Dynamics of H+ + C2H2 at ELab = 30, 200, and 450 eV.

We present a complete simplest-level electron nuclear dynamics (SLEND) investigation of H+ + C2H2 at collision energies ELab = 30, 200, and 450 eV. This reaction is relevant in astrophysics and provides a computationally feasible prototype for proton cancer therapy reactions. SLEND is a time-dependent, variational, direct, and nonadiabatic method that adopts a classical-mechanics description for the nuclei and a Thouless single-determinantal wave function for the electrons. We perform this study with our code PACE, which incorporates the One Electron Direct/Electron Repulsion Direct (OED/ERD) atomic integrals package developed by the Bartlett group. Current SLEND simulations with the 6-31G** basis set involves 2,646 trajectory calculations from 9 nonredundant, symmetry-inequivalent projectile-target orientations. For H+ + C2H2 at ELab = 30 eV, SLEND/6-31G** simulations predict one simple scattering process, and three reactive ones: C2H2 hydrogen substitution, C2H2 fragmentation into two CH moieties, and C2H2 fragmentation into CHC and H moieties, respectively. We reveal and analyze the mechanisms of these processes through computer animations; this valuable chemical information is inaccessible by experiments. The SLEND/6-31G** scattering angle functions exhibit primary and secondary rainbow scattering features that vary with the projectile-target orientations and collision energies. SLEND/6-31G** predicts 1-electron-transfer (1-ET) integral cross sections at ELab = 30, 200, and 450 eV in good agreement with their experimental counterparts. SLEND/6-31-G** predicts 1-ET differential cross sections (DCSs) at ELab = 30 eV that agree well with their experimental counterparts over all the measured scattering angles. In addition, SLEND/6-31G** predicts 0-ET DCSs at ELab = 30 eV that agree well with their experimental counterparts at low scattering angles, but less satisfactorily at higher ones. Remarkably, both the 0- and 1-ET DCSs from SLEND/6-31G** exhibit distinct primary rainbow scattering signatures in excellent agreement with their experimentally inferred counterparts. Furthermore, both SLEND/6-31G** and the experiment indicate that the primary rainbow scattering angles from the 0- and 1-ET DCSs are identical (an unusual fact in proton-molecule collisions). Through these rainbow scattering predictions, SLEND has also validated a procedure to extract primary rainbow angles from structureless DCSs. We analyze the obtained theoretical results in comparison with available experimental data and discuss forthcoming developments in the SLEND method.

Protic Processes in an Extended Pyrazinacene: The Case of Dihydrotetradecaazaheptacene.

Pyrazinacenes are linearly fused heteroaromatic rings, with N atoms replacing all apical CH moieties. Component rings may exist in a reduced state, having NH groups instead of N, causing cross-conjugation. These compounds have interesting optical and electronic properties, including strong fluorescence in the near-infrared region and photocatalytic properties, leading to diverse possible applications in bio-imaging and organic synthesis, as well as obvious molecular electronic uses. In this study, we investigated the behavior of seven-ring pyrazinacene 2,3,11,12-tetraphenyl-7,16-dihydro-1,4,5,6,7,8,9,12,13,14,15,16,17,18-tetradecaazaheptacene (Ph4H2N14HEPT), with an emphasis on protic processes, including oxidation, tautomerism, deprotonation, and protonation, and the species resulting from those processes. We used computational methods to optimize the structures of the different species and generate/compare molecular orbital structures. The aromaticity of the species generated by the different processes was assessed using the nucleus-independent chemical shifts, and trends in the values were associated with the different transformations of the pyrazinacene core. The computational data were compared with experimental data obtained from synthetic samples of the molecule tBu8Ph4H2N14HEPT.

NMR-monitoring of H/D exchange reaction of ketones in solutions of imidazolium ionic liquids

NMR spectroscopy was used to study hydrogen–deuterium exchange in CH3, CH2 and CH moieties of ketones dissolved in mixtures of solvents containing exchangeable deuteriums and imidazolium ionic liquids (IL) acting as catalysts. Factors affecting the efficiency of the exchange, as well as the role of the deuterated solvent, temperature and concentration, were investigated. Depending on the sample composition and temperature, the degree of deuteration can reach different values at equilibrium state, and the exchange rate can vary from several minutes to several months. ILs with OAc anions and with ethyl chain cations exhibit significant catalytic properties and high degrees of deuteration (up to 98%), which can be achieved by consecutive deuteration cycles. A convenient practical protocol for monitoring the exchange process by NMR spectroscopy and calculating the degree of deuteration was developed and can be used for various molecular systems.

What quantum chemical simulations tell us about the infrared spectra, structure and interionic interactions of a bulk ionic liquid.

The recently developed efficient protocol to explicit quantum mechanical modeling of the structure and IR spectra of liquids and solutions [Katsyuba et al., J. Phys. Chem. B, 2020, 124, 6664-6670] is applied to ionic liquid 1-ethyl-3-methyl-imidazolium tetrafluoroborate [Emim][BF4], and its C2-deuterated analog [Emim-d][BF4]. It is shown that the solvation strongly modifies the frequencies and IR intensities of both cationic and anionic components of the ionic liquids. The main features of the bulk spectra are reproduced by the simulations for cluster ([Emim][BF4])8, representing an ion pair solvated by the first solvation shell. The geometry of the cluster closely resembles the solid-state structure of the actual ionic liquid and is characterized by short contacts of all CH moieties of the imidazolium ring with [BF4]- anions. Both structural and spectroscopic analyses allow the contacts to be interpreted as hydrogen bonds of approximately equal strength. The enthalpies of these liquid-state H-bonds, estimated with the use of empirical correlations, amount to 1.2-1.5 kcal mol-1, while the analogous estimates obtained for the gas-phase charged species [Emim][BF4]2- and [Emim]2[BF4]+ increase to 3.6-3.9 kcal mol-1.

Formulation of Microwave-Assisted Natural-Synthetic Polymer Composite Film and Its Physicochemical Characterization

This study is aimed at microwave-assisted synthesis of sodium carboxymethylcellulose and Eudragit L100 composite film and its physicochemical characterization. The film was developed with varying quantities of each polymer and treated with microwave at a fixed frequency of 2450 MHz with a power of 350 Watts for 60 and 120 s. All formulations were characterized for thickness/weight uniformity, moisture adsorption, erosion and water uptake, tensile strength, and vibrational, thermal, and surface morphological analysis in comparison with untreated film samples. Results indicated that microwave treatment for 60 s significantly improved the tensile strength, reduced the water adsorption, delayed erosion, and reduced the water uptake in comparison with the untreated and 120 s treated film formulations. The vibrational analysis revealed rigidification of hydrophilic domains at OH/NH moiety and fluidization of hydrophobic domains at asymmetric and symmetric CH moieties, which is envisaged to be due to the formation of new linkages between the two polymers. These were later confirmed by thermal analysis where a significant rise in transition temperature, as well as enthalpy of the system, was recorded. The microwave treatment for 60 s is thus advocated to be the best treatment condition for developing sodium carboxymethylcellulose and Eudragit L100 composite polymeric films.

Perfect and Defective 13C-Furan-Derived Nanothreads from Modest-Pressure Synthesis Analyzed by 13C NMR.

The molecular structure of nanothreads produced by the slow compression of 13C4-furan was studied by advanced solid-state NMR. Spectral editing showed that >95% of carbon atoms were bonded to one hydrogen (C-H) and that there were 2-4% CH2, 0.6% C═O, and <0.3% CH3 groups. Alkenes accounted for 18% of the CH moieties, while trapped, unreacted furan made up 7%. Two-dimensional (2D) 13C-13C and 1H-13C NMR identified 12% of all carbon in asymmetric O-CH═CH-CH-CH- and 24% in symmetric O-CH-CH═CH-CH- rings. While the former represented defects or chain ends, some of the latter appeared to form repeating thread segments. Around 10% of carbon atoms were found in highly ordered, fully saturated nanothread segments. Unusually slow 13C spin-exchange with sites outside the perfect thread segments documented a length of at least 14 bonds; the small width of the perfect-thread signals also implied a fairly long, regular structure. Carbons in the perfect threads underwent relatively slow spin-lattice relaxation, indicating slow spin exchange with other threads and smaller amplitude motions. Through partial inversion recovery, the signals of the perfect threads were observed and analyzed selectively. Previously considered syn-threads with four different C-H bond orientations were ruled out by centerband-only detection of exchange NMR, which was, on the contrary, consistent with anti-threads. The observed 13C chemical shifts were matched well by quantum-chemical calculations for anti-threads but not for more complex structures like syn/anti-threads. These observations represent the first direct determination of the atomic-level structure of fully saturated nanothreads.

Synergy of Solid-State NMR, Single-Crystal X-ray Diffraction, and Crystal Structure Prediction Methods: A Case Study of Teriflunomide (TFM).

In this work, for the first time, we present the X-ray diffraction crystal structure and spectral properties of a new, room-temperature polymorph of teriflunomide (TFM), CSD code 1969989. As revealed by DSC, the low-temperature TFM polymorph recently reported by Gunnam et al. undergoes a reversible thermal transition at −40 °C. This reversible process is related to a change in Z’ value, from 2 to 1, as observed by variable-temperature 1H–13C cross-polarization (CP) magic-angle spinning (MAS) solid-state NMR, while the crystallographic system is preserved (triclinic). Two-dimensional 13C–1H and 1H–1H double-quantum MAS NMR spectra are consistent with the new room-temperature structure, including comparison with GIPAW (gauge-including projector augmented waves) calculated NMR chemical shifts. A crystal structure prediction procedure found both experimental teriflunomide polymorphs in the energetic global minimum region. Differences between the polymorphs are seen for the torsional angle describing the orientation of the phenyl ring relative to the planarity of the TFM molecule. In the low-temperature structure, there are two torsion angles of 4.5 and 31.9° for the two Z’ = 2 molecules, while in the room-temperature structure, there is disorder that is modeled with ∼50% occupancy between torsion angles of −7.8 and 28.6°. These observations are consistent with a broad energy minimum as revealed by DFT calculations. PISEMA solid-state NMR experiments show a reduction in the C–H dipolar coupling in comparison to the static limit for the aromatic CH moieties of 75% and 51% at 20 and 40 °C, respectively, that is indicative of ring flips at the higher temperature. Our study shows the power of combining experiments, namely DSC, X-ray diffraction, and MAS NMR, with DFT calculations and CSP to probe and understand the solid-state landscape, and in particular the role of dynamics, for pharmaceutical molecules.

Charge-by-Charge Ion Pairing Preserves Fluorescence Emission

In this issue of Chem, Flood and co-workers demonstrate spatially and electronically isolated cationic dyes in crystalline charge-by-charge ion-pairing systems based on sandwiched [2 + 1]-type anion complexes of a π-electronic cyanostar macrocycle; these dyes exhibit fluorescent colors and intensities similar to those in solution.

Effect of formal CH/P exchange on tandem Cope rearrangement and [2 + 2] cycloaddition of cis-1,2-diethylnylcyclopropane and its mono-hetero analogues

Effect of formal one and two CH/P exchange(s) on tandem Cope rearrangement and [2 + 2] cycloaddition of cis-1,2-diethylnylcyclopropane and its mono-hetero analogues has been investigated theoretically in the gas phase at the DFT (B3LYP/6-31+G(d)) level. The activation free energy for the first step, i.e., Cope rearrangement decreases successively on replacing one and two CH moieties of cis-1,2-diethylnylcyclopropane and its mono-hetero analogues with P atom(s), which could be rationalized on the basis of weaker C = P π bond than C = C π bond. However, this trend is not maintained in the second step, i.e. [2 + 2] cycloaddition of the initially formed intermediate; instead, the activation free energy for monophospha- and diphospha- analogues is slightly higher, which may be attributed to the lesser stability of monophospha- and diphosphacyclobutene rings than cyclobutene. The CASSCF calculations indicate that except the intermediates generated from the Cope rearrangement of cis-diethynylthiirane and cis-bis(phosphaethynyl)cyclopropane which are expected to follow concerted mechanism for the [2 + 2] cycloaddition, in all other cases, apparently, there is no preference for the concerted or stepwise mechanism. Inclusion of the dispersion correction in the DFT functional does not cause significant changes in energetics.

Microwave assisted chitosan-polyethylene glycol hydrogel membrane synthesis of curcumin for open incision wound healing.

Chitosan and polyethylene glycol hydrogel membranes containing curcumin were synthesized using microwave technology at fixed frequency, power and time of 2450 MHz, 500 Watt and 120 s. Polymers were solubilized separately, combined with drug and mixed in two different ratios i.e. F1=80:20 and F2=85:15. The untreated and microwave treated hydrogel membranes were analyzed for degree of swelling, degree of degradation, tensile strength, surface morphology, vibrational and thermal analysis and in vitro drug release. Results indicated that F2(micro) showed a significantly high degree of swelling (96.49±1.21 %), low degradation (9.88±1.68 %), sustained drug release through slow erosion (55.1±3.11 %) via non-Fickian diffusion. The vibrational and thermal analysis revealed rigidification of hydrophilic domains of the polymers by formation of hydrogen bonds between chitosan and PEG moieties (OH/NH) and elasticity of hydrophobic domains (asymmetric and symmetric CH moieties and/or C=O moieties) which not only significantly increased the transition temperature and enthalpy (297.2±3.2 °C and 4.24±1.4 J/g) of the chitosan moiety but also resulted in enhanced tensile strength (18.2±1.3 Mpa). In vivo wound healing study revealed significantly faster wound healing in the F2(micro) treated animal group in comparison to a control animal group where at day 14, a significant re-epithelization (87.26 %) with smaller wound size was observed. Hence microwave assisted chitosan-PEG hydrogel membrane of curcumin is advocated to be a suitable plate form for wound healing applications.

Phenolic Hydrogen Transfer by Molecular Oxygen and Hydroperoxyl Radicals. Insights into the Mechanism of the Anthraquinone Process.

Hydrogen atom transfer (HAT) by 3O2 and HO2• from arenols (ArOH), aryloxyls (ArO•), their tautomers (ArH), and auxiliary compounds has been investigated by means of CBS-QB3 computations. With 3O2, excellent linear correlations have been found between the activation enthalpy and the overall reaction enthalpy. Different pathways have been discerned for HATs involving OH or CH moieties. The results for ArOH + HO2• → ArO• + H2O2 neither afford a linear correlation nor agree with the experiment. The precise mechanism for the liquid-phase autoxidation of anthrahydroquinone (AnH2Q) appears to be not fully understood. A kinetic analysis shows that the HAT by chain-carrying HO2• occurs with a high rate constant of ≥6 × 108 M–1 s–1 (toluene). The second propagation step pertains to a diffusion-controlled HAT by 3O2 from the 10-OH-9-anthroxyl radical. Oxanthrone (AnOH) is a more stable tautomer of AnH2Q with a ratio of 13 (298 K) in non-hydrogen-bonding (HB) solvents, but the reactivity toward 3O2/HO2• is much lower. Combination of the computed free energies and Abrahams’ HB donating (α2H) and accepting (β2H) parameters has afforded an α2H(HO2•) of 0.86 and an α2H(H2O2) of 0.50.

Dual turn-on fluorescence signal-based controlled release system for real-time monitoring of drug release dynamics in living cells and tumor tissues.

Controlled release systems with capabilities for direct and real-time monitoring of the release and dynamics of drugs in living systems are of great value for cancer chemotherapy. Herein, we describe a novel dual turn-on fluorescence signal-based controlled release system (CDox), in which the chemotherapy drug doxorubicin (Dox) and the fluorescent dye (CH) are conjugated by a hydrazone moiety, a pH-responsive cleavable linker. CDox itself shows nearly no fluorescence as the fluorescence of CH and Dox is essentially quenched by the C=N isomerization and N-N free rotation. However, when activated under acidic conditions, CDox could be hydrolyzed to afford Dox and CH, resulting in dual turn-on signals with emission peaks at 595 nm and 488 nm, respectively. Notably, CDox exhibits a desirable controlled release feature as the hydrolysis rate is limited by the steric hindrance effect from both the Dox and CH moieties. Cytotoxicity assays indicate that CDox shows much lower cytotoxicity relative to Dox, and displays higher cell inhibition rate to cancer than normal cells. With the aid of the dual turn-on fluorescence at different wavelengths, the drug release dynamics of CDox in living HepG2 and 4T-1 cells was monitored in double channels in a real-time fashion. Importantly, two-photon fluorescence imaging of CDox in living tumor tissues was also successfully performed by high-definition 3D imaging. We expect that the unique controlled release system illustrated herein could provide a powerful means to investigate modes of action of drugs, which is critical for development of much more robust and effective chemotherapy drugs.

Functionalized NHC-incorporated Te−Fe−Cu clusters: Facile synthesis, electrochemistry, and catalytic reaction

Two novel TeFe3(CO)9-containing functionalized NHC dicopper complexes, [TeFe3(CO)9{Cu(Me2-bimy)}2] (1) and [TeFe3(CO)9{Cu(iPr2-bimy)}2] (2), were prepared from the reaction of the ternary Te−Fe−Cu complex [TeFe3(CO)9{Cu(MeCN)}2] with bis-N-methyl- or bis-N-isopropyl-substituted benzimidazol-2-ylidene (Me2-bimy or iPr2-bimy) under appropriate conditions, respectively. X-ray analysis showed that Me2-bimy-introduced complex 1 displayed a TeFe3(CO)9Cu(Me2-bimy) trigonal-bipyramidal core geometry having the Fe2Cu face capped by a Cu(Me2-bimy) fragment with the two bonded Cu atoms, whereas the bulkier iPr2-bimy-incorporated complex 2 exhibited a similar TeFe3(CO)9Cu(iPr2-bimy) core having the Fe3 triangle coordinated by a Cu(iPr2-bimy) fragment with the two unbonded Cu atoms. On the other hand, when [TeFe3(CO)9{Cu(MeCN)}2] reacted with the 4,5-dichloro-substituted 1,3-dimethylimidazolium salt (Me2-Cl2-imy·HI) in the presence of KOtBu, a different type of [TeFe3(CO)9{Cu(Me2-Cl2-imy)}2] (3) was produced in good yields. Cluster 3 was shown to possess a tetrahedral TeFe3 core geometry in which the two Fe−Fe edges were each bridged by a Cu(Me2-Cl2-imy) group. According to solid-state packings, complex 1 revealed a cluster-based 3D-supramolecular framework and complexes 2 and 3 each formed a 1D-supramolecular chain, which was stabilized by intermolecular C−H…O interactions between COs and CH moieties of the corresponding NHC ligands. Further, these di-Cu(I) based complexes 1−3 exhibited catalytic activities toward the homocoupling of arylboronic acid with high yields. Importantly, the catalytic efficiencies of this series of functionalized NHC-incorporated TeFe3(CO)9Cu2-based complexes perfectly paralleled the ease of their first oxidation and the greater electron density of the carbene atom in the NHC ligands, which was elucidated by electrochemistry, 13C NMR, and DFT calculations.

A solid-state 1H-NMR study of the dynamic structure of ZIF-8 and its role in the adsorption of bulky molecules

In order to clarify the adsorption mechanism of the zeolitic imidazolate framework, ZIF-8, for bulky molecules larger than its aperture diameter, the local structure of ZIF-8 and its host–guest interactions were studied by adsorption kinetics, potential energy analysis, and solid-state 1H nuclear magnetic resonance (1H-NMR) spectroscopy. Adsorption kinetics measurements lead to apparent diffusion coefficients for CS2, CDCl3, and CCl4 in ZIF-8 that provide residence times for each molecule in the ZIF-8 micropores. Potential energy analyses using van der Waals pairwise interactions reveal two kinds of feasible sites for bulky molecules in the ZIF-8 micropores, where there are minimal intermolecular interactions between an adsorbate and the pore wall. Each site lies on one of the four diagonal axes of the cubic lattice of the ZIF-8 unit cell. One set of sites corresponds to the side of the micropore where methyl groups form a bottleneck, and the other to the side where CH moieties form the bottleneck. Second-moment analysis of the 1H-NMR resonance line reveals that the 2-methylimidazolate rings undergo flip motions, with flip angles of ~55°, in addition to the rapid axial rotations of their methyl groups. Temperature-dependent spin–lattice relaxation time analyses indicate that adsorptions of these molecules reduce the motions of the 2-methylimidazolate rings. In particular, steric hindrance from bulky molecules such as CDCl3 and CCl4 significantly lowers the flipping frequency of the 2-methylimidazolate ring. The flipping of the 2-methylimidazolate ring results in instantaneous aperture expansion that enables bulky molecules to pass through the ZIF-8 aperture.

Vibrational frequencies of hydrogenated silicon carbonitride: A DFT study

Due to their good chemical and thermal inertness, SiCxNy:H films are suitable for a variety of applications in electronic, tribology, optic, photovoltaic and more recently gas separation membranes. For these applications, film evolution can be attractively probed by FTIR spectroscopy. In this work, a systematic quantum mechanical study of the vibrational modes position in the pattern of a-SiCxNy(O):H is presented. Vibrational frequencies of SiC, SiN, CN, SiH, CH and NH moieties have been calculated at DFT/B3LYP level of theory using the 6-311++G(3df,3pd) basis set. Characteristic absorption domains have been compared with FTIR data from the literature. In particular, DFT calculations provide guidelines to discriminate SiH from CN stretching bands, which are calculated to lie below and above ~2230cm−1, respectively. As an example, the oxidation of a microwave PECVD SiCxNy:H films during ageing was evidenced in this work through the progressive increase of SiO stretching band (~1040cm−1). The simultaneous decay of the band centered at 2170cm−1 was attributed to the vanishing of SiH bond upon oxidation. This example illustrates that unambiguous band assignment is required to provide a molecular description of the ageing process, which is in turn required to optimize the material composition and stability. Results of these calculations will be helpful to identify both the chemical moieties and their environment in future investigations on a-SiCxNy:H materials but also on materials containing additional elements such as B- or O-doped SiCN-based systems.

Ternary Se[sbnd]Ru[sbnd]Cu clusters: Facile synthesis, electrochemistry, and UV/Vis absorption

New ternary SeRuCu octahedral-based complexes, namely, CuX-capped SeRu5 clusters [SeRu5(CO)14(μ3-CuX)]2− (2a–2c; X = Cl, Br, I), bis-CuX-capped SeRu5 clusters [SeRu5(CO)14(μ3-CuX)2]2− (3a, 3b; X = Cl, Br), Cu4X2-bridged di-SeRu5 clusters [{SeRu5(CO)14}2(μ6-Cu4X2)]2− (4a, 4b; X = Cl, Br), and bis-CuX-incorporated Se2Ru4 clusters [Se2Ru4(CO)10(μ-CuX)2]2− (5a–5c; X = Cl, Br, I), were prepared from the reaction of the binary cluster [SeRu5(CO)14]2− (1) with CuX under appropriate conditions. Their solid-state packing formed cluster-based 1D-supramolecular chains, which were stabilized by CH⋯X/Y (X = halides; Y = O) H-bonding between CuX/COs of cluster anions and the CH moieties of the [PPh4]+ or [PPN]+ cations. In addition, electrochemical studies showed that the successive anodic shifts were found for 1–4b, tuned by the incoming CuBr fragments introduced into the parent SeRu5 cluster, supported by their decreasing HOMO levels and increasing ionization energies. Solid-state diffuse reflectance spectra showed these SeRuCu complexes exhibited semiconducting behaviors with the low and tunable energy gaps of 0.82–1.08 eV. Significantly, the solid-state absorptions of 2b and 4b were red-shifted compared with the corresponding Te analogues, which was explained by the larger numbers of the density of states (DOS) close to the HOMO for the selenide cases.

Oxidation and reduction of nanodiamond particles in colloidal solutions by laser irradiation or radio-frequency plasma treatment

Functionalized nanodiamond particles (NDs) represent carbon nanomaterial with unique properties for various applications. Here we report on a new approach to surface modification of NDs by their exposure to radio frequency (RF) plasma or laser irradiation (LI) plasma directly in aqueous solution. By using grazing angle reflectance Fourier transform infrared spectroscopy and supporting analysis by X-ray photoelectron spectroscopy, zeta-potential, and Kelvin force microscopy we show that surface chemistry of NDs produced by detonation process (DNDs) or high-pressure high-temperature process (HPHT NDs) works in different way. Moieties on as-received NDs are dominated by COOH and COC groups due to wet chemical cleaning procedures. On DNDs, both RF and LA treatment lead to removal of sp2 shell and additional oxidation of the surface to C OC groups. On HPHT NDs the RF treatment leads to reduction of COC groups that are transformed into COH and CH moieties. Thus at least partial hydrogenation of colloidal HPHT NDs seems feasible.