Solvation Stages of HCl and HBr in Crystalline Phases with Methanol and Small Ethers: Acid−Ether Cluster Complexes in Amorphous and Crystal Phases

Evidence for Oxonium Ions in Ethereal "Hydrogen Chloride"

Although solutions of hydrogen chloride in ethereal solvents like diethyl ether or dioxane are commonly employed in the laboratory, the solution structure of these reagents has yet to be firmly established. Here, we analyze solutions of ethereal hydrogen chloride or deuterium chloride in toluene, in dichloromethane, or in the absence of a co-solvent by in situ infrared spectroscopy. The resulting spectra are inconsistent with free HCl or often-proposed 1:1 HCl-ether complexes but closely match the predicted spectra of oxonium ions generated via protonation of diethyl ether. Molecular dynamics simulation of the oxonium chloride complexes provides evidence for an outer-sphere contact ion pair. These results suggest new approaches for tuning the acidity of strong Brønsted acids in organic solvents and demonstrate the importance of conducting spectroscopic measurements under reaction-relevant conditions.

Integrated experimental and theoretical approach to probe the synergistic effect of ammonia in methanesulfonic acid reactions with small alkylamines

While new particle formation events have been observed worldwide, our fundamental understanding of the precursors remains uncertain. It has been previously shown that small alkylamines and ammonia (NH3) are key actors in sub-3 nm particle formation through reactions with acids such as sulfuric acid (H2SO4) and methanesulfonic acid (CH3S(O)(O)OH, MSA), and that water also plays a role. Because NH3 and amines co-exist in air, we carried out combined experimental and theoretical studies examining the influence of the addition of NH3 on particle formation from the reactions of MSA with methylamine (MA) and trimethylamine (TMA). Experiments were performed in a 1 m flow reactor at 1 atm and 296 K. Measurements using an ultrafine condensation particle counter (CPC) and a scanning mobility particle sizer (SMPS) show that new particle formation was systematically enhanced upon simultaneous addition of NH3 to the MSA + amine binary system, with the magnitude depending on the amine investigated. For the MSA + TMA reaction system, the addition of NH3 at ppb concentrations produced a much greater effect (i.e. order of magnitude more particles) than the addition of ∼12 000 ppm water (corresponding to ∼45-50% relative humidity). The effect of NH3 on the MSA + MA system, which is already very efficient in forming particles on its own, was present but modest. Calculations of energies, partial charges and structures of small cluster models of the multi-component particles likewise suggest synergistic effects due to NH3 in the presence of MSA and amine. The local minimum structures and the interactions involved suggest mechanisms for this effect.

Molecular perspectives on solid-state phase transformation and chemical reactivity of drugs: metoclopramide as an example

Here, I provide an overview of the solid-state characteristics, phase transformations and chemical reactions of metoclopramide hydrochloride monohydrate (MCP HCl H2O). Three unique techniques, including thermoanalytical methods, one-step simultaneous differential scanning calorimetry (DSC) and Fourier transform infrared (FTIR) microspectroscopy, and hot-stage microscopic (HSM) imaging, have been applied to study the solid-state phase transitions of MCP HCl H2O in continuous dehydration, amorphization and recrystallization processes. I also review the effects of grinding or heating on ion-exchange reactions, milling, compression or colyophilization on Maillard reactions, and γ-ray irradiation or electron beams on radiolysis in the solid state. I also report the exposure of MCP HCl H2O in solution to light, irradiation, oxidants or π-acceptors. This review will serve as a useful keynote for the evolving realm of solid-state chemistry research.

CO2 and C2H2 in cold nanodroplets of oxygenated organic molecules and water

Recent demonstrations of subsecond and microsecond timescales for formation of clathrate hydrate nanocrystals hint at future methods of control of environmental and industrial gases such as CO2 and methane. Combined results from cold-chamber and supersonic-nozzle [A. S. Bhabhe, "Experimental study of condensation and freezing in a supersonic nozzle," Ph.D. thesis (Ohio State University, 2012), Chap. 7] experiments indicate extremely rapid encagement of components of all-vapor pre-mixtures. The extreme rates are derived from (a) the all-vapor premixing of the gas-hydrate components and (b) catalytic activity of certain oxygenated organic large-cage guests. Premixing presents no obvious barrier to large-scale conditions of formation. Further, from sequential efforts of the groups of Trout and Buch, a credible defect-based model of the catalysis mechanism exists for guidance. Since the catalyst-generated defects are both mobile and abundant, it is often unnecessary for a high percentage of the cages to be occupied by a molecular catalyst. Droplets represent the liquid phase that bridges the premixed vapor and clathrate hydrate phases but few data exist for the droplets themselves. Here we describe a focused computational and FTIR spectroscopic effort to characterize the aerosol droplets of the all-vapor cold-chamber methodology. Computational data for CO2 and C2H2, hetero-dimerized with each of the organic catalysts and water, closely match spectroscopic redshift patterns in both magnitude and direction. Though vibrational frequency shifts are an order of magnitude greater for the acetylene stretch mode, both CO2 and C2H2 experience redshift values that increase from that for an 80% water-methanol solvent through the solvent series to approximately doubled values for tetrahydrofuran and trimethylene oxide (TMO) droplets. The TMO solvent properties extend to a 50 mol.% solution of CO2, more than an order of magnitude greater than for the water-methanol solvent mixture. The impressive agreement between heterodimer and experimental shift values throughout the two series encourages speculation concerning local droplet structures while the stable shift patterns appear to be useful indicators of the gas solubilities.

HCl dissociation in methanol clusters from ab initio molecular dynamics simulations and inner-shell photoelectron spectroscopy

HCl dissociation in methanol clusters is studied by ab initio molecular dynamics simulations and experimentally by X-ray photoelectron spectroscopy. From theoretical simulations of HCl in oligomers and medium-sized clusters of methanol, two states of solvation are identified for HCl: an intermediate proton-sharing (ion pair) state and a fully dissociated state. Lowering the temperature from 150 to 100 K is found to promote full dissociation over the proton-sharing state. The dissociation of HCl is well reflected in the experimental chlorine 2p photoelectron spectrum recorded for a beam of clusters formed by adiabatic expansion of the vapor over a solution of HCl in methanol. In order to reproduce the observed Cl 2p spectrum by means of theoretical line-shape modeling, one needs to take into account both the intermediate proton-sharing state and the fully dissociated state.

Interaction in the ternary complexes of HCl-methanol-X, X = H2O or NH3: Ab initio calculations and on-the-fly molecular dynamics

Dynamics, structures, energetics, and vibrational spectra of the ternary complexes of hydrogen chloride with either methanol and water or methanol and ammonia were investigated by on-the-fly molecular dynamics and ab initio and density functional theory (DFT) with aug-cc-pvDZ basis sets. Addition of CH3OH to the HCl-NH3 system catalyzes the proton transfer from HCl to NH3. However, the dynamics of the system show that the proton is not localized on NH3; rather, it is shared between N and Cl.

HCl hydrates as model systems for protonated water

Ab initio molecular dynamics simulations are presented of vibrational dynamics and spectra of crystal HCl hydrates. Depending on the composition, the hydrates include distinct protonated water forms, which in their equilibrium structures approximate either the Eigen ion H3O+(H2O)3 (in the hexahydrate) or the Zundel H2O...H+...OH2 ion (in the di- and trihydrate). Thus, the hydrates offer the opportunity to study spectra and dynamics of distinct species of protonated water trapped in a semirigid solvating environment. The experimentally measured spectra are reproduced quite well by BLYP/DZVP-level calculations employing Fourier transform of the system dipole. The large overall width (800-1000 cm-1) of structured proton bands reflects a broad range of solvating environments generated by crystal vibrations. The aqueous HCl solution was also examined in search of an objective criterion for separating the contributions of "Zundel-like" and "Eigen-like" protonated forms. It is suggested that no such criterion exists since distributions of proton-related structural properties appear continuous and unimodal. Dipole derivatives with respect to OH and O...H+ stretches in water and protonated water were also investigated to advance the understanding of the corresponding IR intensities. The effects of H bonding and solvation on the intensities were analyzed with the help of the Wannier centers' representation of electron density.

Elusive structure of HCl monohydrate

The study addresses the structure of crystalline HCl monohydrate which is composed of H3O+ and Cl-. The published x-ray diffraction patterns indicate an element of disorder, the nature of which is debated in the literature and is addressed in the present study. The computational investigations include searches for alternative crystal structures employing an empirical potential, and on-the-fly simulations as implemented in the density functional code QUICKSTEP employing Gaussian basis sets. The experimental work focuses on Fourier-transform infrared (FTIR) spectra of crystal nanoparticles. Simulations of FTIR spectra and of the x-ray diffraction patterns are consistent with crystal monohydrate structure composed of ferroelectric domains, joined by "boundary tissue" of antiferroelectric structure.

HCl Solvation at the Surface and within Methanol Clusters/Nanoparticles II: Evidence for Molecular Wires

Condensed-phase solvation of HCl on and within methanol nanoparticles was investigated by Fourier transform infrared (FTIR) spectroscopy, on-the-fly molecular dynamics as implemented in the density functional code Quickstep (which is part of the CP2K package), and ab initio calculations. Adsorption and solvation stages are identified and assigned with the help of calculated infrared spectra obtained from the simulations. The results have been further checked with MP2-level ab initio calculations. The range of acid solvation states extends from the single-coordinated slightly stretched HCl to proton-sharing with Zundel-like methanol O...H+...X- states, and finally to MeOH2+...Cl- units with full proton transfer. Furthermore, once the proton moves to methanol, it is mobilized along methanol molecular chains. Since the proton dynamics reflects the evolving local structures, the "proton" spectra display broad bands usually with underlying continua.