For Whom a Puddle Is the Sea? Adsorption of Organic Guests on Hydrated MCM-41 Silica

Amino Acid Polymerization on Silica Surfaces

The polymerization of unactivated amino acids (AAs) is an important topic because of its applications in various fields including industrial medicinal chemistry and prebiotic chemistry. Silica as a promoter for this reaction, is of great interest owing to its large abundance and low cost. The amide/peptide bond synthesis on silica has been largely demonstrated but suffers from a lack of knowledge regarding its reaction mechanism, the key parameters, and surface features that influence AA adsorption and reactivity, the selectivity of the reaction product, the role of water in the reaction, etc. The present review addresses these problems by summarizing experimental and modeling results from the literature and attempts to rationalize some apparent divergences in published results. After briefly presenting the main types of silica surface sites and other relevant macroscopic features, we discuss the different deposition procedures of AAs, whose importance is often neglected. We address the possible AA adsorption mechanisms including covalent grafting and H-bonding and show that they are highly dependent on silanol types and density. We then consider how the adsorption mechanisms determine the occurrence and outcome of AA condensation (formation of cyclic dimers or of long linear chains), and outline some recent results that suggest significant polymerization selectivity in systems containing several AAs, as well as the formation of specific elements of secondary structure in the growing polypeptide chains.

Structures and Dynamics of Complex Guest Molecules in Confinement, Revealed by Solid-State NMR, Molecular Dynamics, and Calorimetry

This review gives an overview of current trends in the investigation of confined molecules such as water, small and higher alcohols, carbonic acids, ethylene glycol, and non-ionic surfactants, such as polyethylene glycol or Triton-X, as guest molecules in neat and functionalized mesoporous silica materials employing solid-state NMR spectroscopy, supported by calorimetry and molecular dynamics simulations. The combination of steric interactions, hydrogen bonds, and hydrophobic and hydrophilic interactions results in a fascinating phase behavior in the confinement. Combining solid-state NMR and relaxometry, DNP hyperpolarization, molecular dynamics simulations, and general physicochemical techniques, it is possible to monitor these confined molecules and gain deep insights into this phase behavior and the underlying molecular arrangements. In many cases, the competition between hydrogen bonding and electrostatic interactions between polar and non-polar moieties of the guests and the host leads to the formation of ordered structures, despite the cramped surroundings inside the pores.

Probing the Isolobal Relation between Cp'''NiP3 and White Phosphorus by Experimental Charge Density Analysis

An in-depth analysis of the description of bonding within Cp'''Ni-cyclo-P3 (Cp'''=1,2,4-tri-tert-butylcyclopentadienyl, [Ni]P3) employing X-ray diffraction based multipolar modeling, density functional theory (DFT) as well as an "experimental wavefunction" obtained from X-ray restrained wavefunction (XRW) fitting is presented. The results are compared to DFT calculations on white phosphorus - an isolobal analogue to [Ni]P3. A complementary bonding analysis shows insights into the reactivity of [Ni]P3. The isolobal principle is reflected in every aspect of our analysis and the employed methods seamlessly predict the differences in reactivity of [Ni]P3 and P4. Crystallographic modeling, solid-state NMR, and DFT calculations describe the dynamic behavior of the cyclo-P3 unit in the title molecule.

Very Large Pore Mesoporous Bioactive Silicate Glasses: Comparison of Behavior toward Classical Mesoporous Bioactive Glasses in Terms of Drug Loading/Release and Bioactivity

Considering the increase in patients who suffer from osteoporosis and the bone defects that occur in these patients, bone tissue regeneration is a promising option to solve this problem. To achieve a synergistic effect between the synthesis of a proper structure and bioactive/pharmaceutical activity, ions with a physiological effect can be added to silica structures, such as Ca2+, thanks to its bioactive behavior, and Ga3+ for its antibacterial and anticancer action. In this work, the synthesis of large pore mesoporous silica (LPMS), potential bioactive glasses containing Ca2+ and Ga3+, has been studied. Corresponding structures, in terms of composition, have been synthesized following the Sol-Gel EISA (Evaporation Induced Self-Assembly) process (obtaining Classical Mesoporous Silica, MS). Pore structure characterization of LPMSs and MSs has been performed using N2 adsorption/desorption and Hg-porosimetry, showing the presence of pores for LPMSs in the range of 20-60 and 200-600 nm. Nisin, a polycyclic antibacterial peptide, has been used for load tests. The load and release tests performed highlight a higher loading and releasing, doubled for LPMSs if compared to MSs. To confirm the maintenance of the structure of LPMSs and their mechanical strength and resistance, scanning electron microscopy images were acquired before and after release tests. Ca and Ga release in SBF has been studied through inductively coupled plasma-optical emission spectroscopy (ICP-OES), showing a particularly high release of these ions performed with LPMSs. The bioactive behavior of Ca-containing structures has been confirmed using FT-IR (Fourier-transform infrared spectroscopy), SEM-EDS (Scanning Electron Microscope-Energy Dispersive Spectroscopy), and X-ray powder diffraction (XRDP). In conclusion, LPMSs showed better loading and releasing properties compared with classical MS and better release in terms of active ions. In addition, it has also been demonstrated that LPMSs have bioactive behavior (a well-known characteristic of MSs).

Very Large Pores Mesoporous Silica as New Candidate for Delivery of Big Therapeutics Molecules, Such as Pharmaceutical Peptides

The synthesis of a scaffold that can accommodate big molecules with a pharmaceutical role is important to shield them and maintain their biological activity. In this field, silica particles with large pores (LPMS) are innovative supports. Large pores allow for the loading of bioactive molecules inside the structure and contemporarily their stabilization and protection. These purposes cannot be achieved using classical mesoporous silica (MS, pore size 2-5 nm), because their pores are not big enough and pore blocking occurs. LPMSs with different porous structures are synthesized starting from an acidic water solution of tetraethyl orthosilicate reacting with pore agents (Pluronic^® F127 and mesitylene), performing hydrothermal and microwave-assisted reactions. Time and surfactant optimization were performed. Loading tests were conducted using Nisin as a reference molecule (polycyclic antibacterial peptide, with dimensions of 4-6 nm); UV-Vis analyses on loading solutions were performed. For LPMSs, a significantly higher loading efficiency (LE%) was registered. Other analyses (Elemental Analysis, Thermogravimetric Analysis and UV-Vis) confirmed the presence of Nisin in all the structures and its stability when loaded on them. LPMSs showed a lower decrease in specific surface area if compared to MS; in terms of the difference in LE% between samples, it is explained considering the filling of pores for LPMSs, a phenomenon that is not allowed for MSs. Release studies in simulated body fluid highlight, only for LPMSs, a controlled release, considering the longer time scale of release. Scanning Electron Microscopy images acquired before and after release tests shows the LPMSs' maintenance of the structure, demonstrating strength and mechanical resistance of structures. In conclusion, LPMSs were synthesized, performing time and surfactant optimization. LPMSs showed better loading and releasing properties with respect to classical MS. All collected data confirm a pore blocking for MS and an in-pore loading for LPMS.

Creating Solid Solutions of Metallocenes: Migration of Nickelocene into the Ferrocene Crystal Lattice in the Absence of a Solvent

Ferrocene and nickelocene do not react with each other in solution; however, the large impact of the paramagnetic component on the ferrocene 1H NMR signal linewidth and relaxation times has been quantified. Co-crystallization of ferrocene and nickelocene at any ratio from a solvent can be explained with both pure substances crystallizing in the same space group P21/n. As a new phenomenon, when a ferrocene single crystal is exposed to polycrystalline nickelocene in the absence of a solvent, the nickelocene migrates into the ferrocene crystal lattice and a mixed crystal is formed that retains its macroscopic shape. This process has been proven visually by cutting the single crystal. Mixing polycrystalline ferrocene with polycrystalline nickelocene at different molar ratios with a mortar and pestle leads to crystalline solid solutions with the corresponding molar ratios of both components. This migration of one organometallic component into an existing crystal lattice of another at ambient temperature in the absence of a solvent has not been described previously. Paramagnetic 1H solid-state NMR spectroscopy of static and rotating samples of dry ferrocene/nickelocene mixtures at varying ratios is used to prove and quantify the mixing of both metallocenes at the molecular level. A single-crystal X-ray structure of a 50/50 mixed crystal corroborates the NMR results that nickelocene and ferrocene are randomly distributed in the lattice and that the space group P21/n is retained. All ferrocene molecules in the mixed crystal lattice show a broadening of their 1H wideline signals and residual magic-angle spinning (MAS) lines at ambient temperature. The broadening of the ferrocene signals correlates with the nickelocene content. 1H T1 relaxation time measurements for the signals of ferrocene in samples with different amounts of nickelocene corroborate the assumption that the signal broadening is due to paramagnetic dipole-dipole relaxation of ferrocene molecules in the vicinity of nickelocene. Spatially separated ferrocene and nickelocene powders in one rotor show the solid-state NMR characteristics of the individual polycrystalline metallocenes. The described formation of solid solutions of metallocenes in the absence of a solvent will open new pathways to homogeneously mixed nanoparticles with desired metal ratios and dual-atom catalysts.

Leucine on Silica: A Combined Experimental and Modeling Study of a System Relevant for Origins of Life, and the Role of Water Coadsorption

Leucine on silica constitutes an interesting system from the point of view of origins of life studies since leucine coadsorbed on SiO₂ together with glutamic acid can give rise to rather long linear polypeptides upon activation. It is also an ideal system to test methods of molecular characterization of biomolecules deposited on mineral surfaces since it combines a small-scale model of peptides and proteins, which are among the most important components of biodevices, with one of the most widely used inorganic materials. We have deposited l-leucine on a high surface fumed silica in the submonolayer range and characterized it by a multipronged approach including macroscopic information (thermogravimetry, X-ray diffraction), in situ spectroscopic methods (IR, multinuclear solid-state NMR including single-pulse and CP-MAS, 2-D HETCOR), and molecular modeling by density functional theory (DFT), including calculation of NMR parameters. Specific information can be obtained on the adsorption and interaction mechanism. Leucine is rather strongly adsorbed without any covalent bonds, through the formation of a specific lattice of H-bonds that often involve coadsorbed water molecules. Its state is indeed strongly dependent on the drying procedure: insufficient drying results in liquid-like surroundings for the leucine functional groups, while vacuum drying only retains a limited number of waters (of the order of 5 per leucine molecule). The most stable models have zwitterionic leucine interacting directly with surface silanols through their ammonium group, while the carboxylate interacts through bridging waters. Experimental NMR chemical shifts are satisfactorily predicted for these models, and leucine can be viewed as a probe for specific groups of surface sites known as silanol nests.

Molecular Dynamics and Surface Interactions of Nickelocene Adsorbed on Silica: A Paramagnetic Solid-State NMR Study

When grinding nickelocene with silica in the absence of a solvent at room temperature, it adsorbs on the surface within the pores. This has also been demonstrated visually by adsorbing green nickelocene in the pores of a large colorless silica gel specimen. While this dry adsorption and translational mobility of nickelocene within the pores is proven visually, the site-to-site mobility of the nickelocene molecules and their orientation toward the surface are not yet understood. In this contribution, mesoporous silica is used as the support material for a systematic solid-state NMR study of these issues. Paramagnetic ¹H VT solid-state NMR and T₁ relaxation times have been powerful tools for studying the dynamics of nickelocene on the silica surface. Herewith, the mobility of the surface-adsorbed nickelocene molecules in the pores could be quantified on the molecular scale. According to the obtained data, the nickelocene molecules move like a liquid on the surface. Isotropically moving molecules exchange places rapidly with surface-attached molecular states of nickelocene in a sample with submonolayer surface coverage. This finding is corroborated by a macroscopic visualization experiment. The states of the surface-attached horizontally oriented nickelocene molecules that are prevalent at temperatures below 200 K have been quantified. The temperature dependencies of the rate k in coordinates of ln(k) versus 1/T and ln(k/T) versus 1/T form ideal straight lines that allow the determination of the kinetic parameters E_act = 5.5 kcal/mol, A = 1.1 × 10¹⁰, ΔH^‡ = 5.0 kcal/mol, and ΔS^‡ = -15 eu. Investigating a sample with equal amounts of nickelocene and ferrocene in a submonolayer amount of 80% overall surface coverage shows that the different metallocenes mix on the molecular level on the silica surface.

NMR Properties of the Cyanide Anion, a Quasisymmetric Two-Faced Hydrogen Bonding Acceptor

The isotopically enriched cyanide anion, (13C≡15N)−, has a great potential as the NMR probe of non-covalent interactions. However, hydrogen cyanide is highly toxic and can decompose explosively. It is therefore desirable to be able to theoretically estimate any valuable results of certain experiments in advance in order to carry out experimental studies only for the most suitable molecular systems. We report the effect of hydrogen bonding on NMR properties of 15N≡13CH···X and 13C≡15NH···X hydrogen bonding complexes in solution, where X = 19F, 15N, and O=31P, calculated at the ωB97XD/def2tzvp and the polarizable continuum model (PCM) approximations. In many cases, the isotropic 13C and 15N chemical shieldings of the cyanide anion are not the most informative NMR properties of such complexes. Instead, the anisotropy of these chemical shieldings and the values of scalar coupling constants, including those across hydrogen bonds, can be used to characterize the geometry of such complexes in solids and solutions. 1J(15N13C) strongly correlates with the length of the N≡C bond.

Actual Symmetry of Symmetric Molecular Adducts in the Gas Phase, Solution and in the Solid State

This review discusses molecular adducts, whose composition allows a symmetric structure. Such adducts are popular model systems, as they are useful for analyzing the effect of structure on the property selected for study since they allow one to reduce the number of parameters. The main objectives of this discussion are to evaluate the influence of the surroundings on the symmetry of these adducts, steric hindrances within the adducts, competition between different noncovalent interactions responsible for stabilizing the adducts, and experimental methods that can be used to study the symmetry at different time scales. This review considers the following central binding units: hydrogen (proton), halogen (anion), metal (cation), water (hydrogen peroxide).

1,3,5-Triaza-7-Phosphaadamantane (PTA) as a 31P NMR Probe for Organometallic Transition Metal Complexes in Solution

Due to the rigid structure of 1,3,5-triaza-7-phosphaadamantane (PTA), its ³¹P chemical shift solely depends on non-covalent interactions in which the molecule is involved. The maximum range of change caused by the most common of these, hydrogen bonding, is only 6 ppm, because the active site is one of the PTA nitrogen atoms. In contrast, when the PTA phosphorus atom is coordinated to a metal, the range of change exceeds 100 ppm. This feature can be used to support or reject specific structural models of organometallic transition metal complexes in solution by comparing the experimental and Density Functional Theory (DFT) calculated values of this ³¹P chemical shift. This approach has been tested on a variety of the metals of groups 8-12 and molecular structures. General recommendations for appropriate basis sets are reported.

Electric field effect on 31P NMR magnetic shielding

Magnetic shielding depends on molecular structure and noncovalent interactions. This study shows that it is also measurably dependent on the electric field generated by surrounding molecules. This effect has been observed explicitly for ³¹P nucleus using the adduct under field approach. The results obtained indicate that the field strength experienced by molecules in crystals consisting of molecules with large dipole moments is similar to that in polar solvents. Therefore, magnetic shielding should explicitly depend on solvent polarity. It is important to note that this effect cannot be reproduced correctly within the polarizable continuum model approach.

NMR Study of Intercalates and Grafted Organic Derivatives of H2La2Ti3O10

The protonated perovskite-like titanate H2La2Ti3O10 has been used to produce organic-inorganic hybrids with simple organic molecules: methylamine, methanol, monoethanolamine, and n-butylamine. The optimal pathways for the preparation of such hybrids are summarized. Solid-state NMR, combined with thermal analysis, Raman, and IR spectroscopy, has been applied to determine the bonding type in the obtained organic-inorganic hybrids. It has been found that, in the methanolic hybrid, the organic residues are covalently bound to the inorganic matrix. In contrast, in the methylamine and n-butylamine hybrids, the organic molecules are intercalated into the inorganic matrix in cationic forms. The structure of the monoethanolamine hybrid is composite and includes both the covalently bound and intercalated organic species.

Inter- vs. Intramolecular Hydrogen Bond Patterns and Proton Dynamics in Nitrophthalic Acid Associates

Noncovalent interactions are among the main tools of molecular engineering. Rational molecular design requires knowledge about a result of interplay between given structural moieties within a given phase state. We herein report a study of intra- and intermolecular interactions of 3-nitrophthalic and 4-nitrophthalic acids in the gas, liquid, and solid phases. A combination of the Infrared, Raman, Nuclear Magnetic Resonance, and Incoherent Inelastic Neutron Scattering spectroscopies and the Car-Parrinello Molecular Dynamics and Density Functional Theory calculations was used. This integrated approach made it possible to assess the balance of repulsive and attractive intramolecular interactions between adjacent carboxyl groups as well as to study the dependence of this balance on steric confinement and the effect of this balance on intermolecular interactions of the carboxyl groups.