Elusive structure of HCl monohydrate

Pyrolysis behavior, kinetics, and thermodynamics of waste pharmaceutical blisters under CO2 atmosphere

The disastrous impact of COVID-19 pandemic has caused a significantly increased production and use of pharmaceutical drugs, which is accompanied by the rapid generation of waste pharmaceutical blisters (WPBs). Nonetheless, its treatment has not gained appropriate attentions and a perceptible process development was not achieved. In this study, the WPBs pyrolysis in CO₂ atmosphere was conducted as well as the thermodynamics and kinetics were investigated. The thermogravimetric analysis revealed that the WPBs decomposition could be divided into two stages of 25 - 365 °C and 365 - 900 °C with mass loss of 56.5 - 60.5 wt% and 22.5 - 25.9 wt%, respectively. Fourier-transform infrared spectroscopy analysis indicated the dechlorination process initiating at ∼300 °C. The simultaneous asymmetric stretching of HCl and stretching vibration of C-Cl bond was detected in the range of 2600 - 3250 cm^-1 and 660 - 750 cm^-1, respectively. The dechlorination reactions were almost complete at ∼520 °C and minor peaks (2900 -3100 cm^-1) due to C-H vibrations were observed. Gas chromatography-mass spectrometry analysis indicated that the evolved products included alkanes, benzene, olefin, as well as HCl. The cycloalkenes content significantly increased during the second conversion stage, implying the addition reactions between alkanes and olefins. The apparent activation energy was calculated using three model-free methods and the values from Flynn-Wall-Ozawa model increased from 142.0 to 255.8 kJ·mol^-1 with an average value of 147.4 kJ·mol^-1. The methods of Coats-Redfern as well as Malek were applied to determine the reaction mechanism. The one-dimensional diffusion model was more reliable to describe the WPBs pyrolysis. This study will represent a significant reference case for the thermochemical conversion of multilayer packing waste and facing the increasing demand for the medical waste recycling.

Fundamental frequency from classical molecular dynamics

We give a theoretical validation for calculating fundamental frequencies of a molecule from classical molecular dynamics (MD) when its anharmonicity is small enough to be treated by perturbation theory. We specifically give concrete answers to the following questions: (1) What is the appropriate initial condition of classical MD to calculate the fundamental frequency? (2) From that condition, how accurately can we extract fundamental frequencies of a molecule? (3) What is the benefit of using ab initio MD for frequency calculations? Our analytical approaches to those questions are classical and quantum normal form theories. As numerical examples we perform two types of MD to calculate fundamental frequencies of H2O with MP2/aug-cc-pVTZ: one is based on the quartic force field and the other one is direct ab initio MD, where the potential energies and the gradients are calculated on the fly. From those calculations, we show comparisons of the frequencies from MD with the post vibrational self-consistent field calculations, second- and fourth-order perturbation theories, and experiments. We also apply direct ab initio MD to frequency calculations of C-H vibrational modes of tetracene and naphthalene. We conclude that MD can give the same accuracy in fundamental frequency calculation as second-order perturbation theory but the computational cost is lower for large molecules.

A sodium atom in a large water cluster: electron delocalization and infrared spectra

Ab initio molecular dynamics simulations modeling low-energy collisions of a sodium atom with a cluster with more than 30 water molecules are presented. We follow the dynamics of the atom-cluster interaction and the delocalization of the valence electron of sodium together with the changes in the electron binding energy. This electron tends to be shared by the nascent sodium cation and the water cluster. IR spectra of the sodium-water cluster are both computationally and experimentally obtained, with a good agreement between the two approaches.

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.

When is a molecule properly solvated by a continuum model or in a cluster ansatz? A first-principles simulation of alanine hydration

In order to test the validity of the cluster ansatz approach as well as of the continuum model approach and to learn about the solvation shell, we carried out first-principles molecular dynamics simulations of the alanine hydration. Our calculations contained one alanine molecule dissolved in 60 water molecules. Dipole moments of individual molecules were derived by means of maximally localized Wannier functions. We observed an average dipole moment of about 16.0 D for alanine and of about 3.3 D for water. In particular, the average water dipole moment in proximity of alanine's COO(-) group decayed continously with increasing distance, while, surprisingly, close to the CH3 and NH3+ group, the dipole moment first rose before its value dropped. In a cluster ansatz approach, we considered snapshots of alanine surrounded by different water molecule shells. The dipole moments from the cluster approaches utilizing both maximally localized Wannier functions as well as natural population analysis served to approximate the dipole moments of the total trajectory. Sufficient convergence of the cluster ansatz approach is found for either of the two solvent shells around the polar groups and one solvent shell around the apolar groups or two solvent shells around the polar groups surrounded by a dieletric continuum.