Study of structure and spectroscopy of water–hydroxide ion clusters: A combined simulated annealing and DFT-based approach

Energetics and spectroscopic studies of CNO (-) (H 2 O) n $$ {\mathbf{CNO}}^{\left(\hbox{-} \right)}{\left({\mathbf{H}}_{\mathbf{2}}\mathbf{O}\right)}_{\mathbf{n}} $$ clusters and the temperature dependencies of the isomers: An approach based on a combined recipe of parallel tempering and quantum chemical methods

A system associated with several number of weak interactions supports numerous number of stable structures within a narrow range of energy. Often, a deterministic search method fails to locate the global minimum geometry as well as important local minimum isomers for such systems. Therefore, in this work, the stochastic search technique, namely parallel tempering, has been executed on the quantum chemical surface of theCNO (-) (H 2 O) n $$ {\mathrm{CNO}}^{\left(\hbox{-} \right)}{\left({\mathrm{H}}_2\mathrm{O}\right)}_n $$ system forn = 1 $$ n=1 $$ -8 to generate global minimum as well as several number of local minimum isomers. IR spectrum can act as the fingerprint property for such system to be identified. Thus, IR spectroscopic features have also been included in this work. Vertical detachment energy has also been calculated to obtain clear information about number of water molecules in several spheres around the central anion. In addition, in a real experimental scenario, not only the global but also the local minimum isomers play an important role in determining the average value of a particular physically observable property. Therefore, the relative conformational populations have been determined for all the evaluated structures for the temperature range between 20K and 400K. Further to understand the phase change behavior, the configurational heat capacities have also been calculated for different sizes.

ABEEM/MM OH- Models for OH-(H2O)n Clusters and Aqueous OH-: Structures, Charge Distributions, and Binding Energies

Based on the atom-bond electronegativity equalization method fused into molecular mechanics (ABEEM/MM), two fluctuating charge models of OH^--water system were proposed. The difference between these two models is whether there is charge transfer between OH^- and its first-shell water molecules. The structures, charge distributions, charge transfer, and binding energies of the OH^-(H₂O)_n (n = 1-8, 10, 15, 23) clusters were studied by these two ABEEM/MM models, the OPLS/AA force field, the OPLS-SMOOTH/AA force field, and the QM methods. The results demonstrate that two ABEEM/MM models can search out all stable structures just as the QM methods, and the structures and charge distributions agree well with those from the QM calculations. The structures, the charge transfer, and the strength of hydrogen bonds in the first hydration shell are closely related to the coordination number of OH^-. Molecular dynamics simulations on the aqueous OH^- solution are performed at 298 and 278 K using ABEEM/MM-I model. The MD results show that the populations of three-, four-, and five-coordinated OH^- are 29.6%, 67.1%, and 3.4% at 298 K, respectively, and those of two-, three-, four-, and five-coordinated OH^- are 10.8%, 44.9%, 39.2%, and 4.9% at 278 K, respectively; the average hydrogen bond lengths and the hydrogen bond angle in the first shell increase with the temperature decreasing.

Activation of CO2 in Photoirradiated CO2-H2O Clusters: Direct Ab Initio Molecular Dynamics (MD) Study

Carbon dioxide (CO₂) is one of the stable and inactive molecules that contribute to greenhouse gases. The development of new reactions of CO₂ activation, chemical fixation, and conversion is a very important issue. In this report, the reactions of CO₂-H₂O binary clusters were investigated using a direct ab initio molecular dynamics (AIMD) method to find a new reaction of CO₂ activation. Clusters composed of carbon dioxide and water molecules, CO₂(H₂O) _n ( n = 2-5), were utilized as a model of the binary cluster. The reaction dynamics of [CO₂(H₂O) _n]⁺ following the ionization of parent neutral clusters were also investigated. Two electronic states of [CO₂(H₂O) _n]⁺ were examined for direct AIMD surfaces: CO₂[(H₂O) _n]⁺ (ground state) and (CO₂)⁺(H₂O) _n (excited charge transfer (CT) state). After the ionization of the clusters, a proton-transfer (PT) reaction occurred within the (H₂O) _n⁺ moiety at the ground state, whereas the reactive HCO₃ radical was formed at the CT state for OH addition to CO₂⁺: CO₂⁺(H₂O) _n → HCO₃ + H⁺(H₂O) _n-1. The mechanisms of the PT process and the HCO₃ radical formation were discussed based on the theoretical results.

Recycling of isopropanol for cost-effective, environmentally friendly production of carboxymethylated cellulose nanofibrils

An approach to recycling isopropanol used in the carboxymethylation of pulp fiber was investigated as a cost-effective and environmentally friendly method of producing cellulose nanofibrils (CNF). Carboxymethylation of pulp fiber was carried out using isopropanol (IPA) as the sole solvent. IPA was recovered after carboxymethylation reaction and recycled in the next carboxymethylation reaction. Simple recycling of IPA decreased the reaction efficiency of carboxymethylation due to the increase of water content in the IPA. To dehydrate the recovered IPA, a 4 Å molecular sieve was used as a drying material. It was shown that dehydration restored carboxymethylation efficiency to the same level as when fresh IPA was used. The characteristics of the carboxymethylated CNFs produced using the recycled IPA were evaluated, including fibrillation tendency, average width, and width distribution, and it was shown that the use of recycled IPA after dehydration treatment did not cause any changes in carboxymethylated CNF properties. Recycling IPA after simple dehydration using a molecular sieve is thus a cost-effective and environmentally friendly method of producing carboxymethylated CNF.

An adaptive mutation simulated annealing based investigation of Coulombic explosion and identification of dissociation patterns in (CO2)n2+ clusters

In this communication, we would like to discuss the advantages of adaptive mutation simulated annealing (AMSA) over standard simulated annealing (SA) in studying the Coulombic explosion of (CO₂)_n²⁺ clusters for n = 20–68, where ‘n’ is the size of the cluster.

An investigation on the structure, spectroscopy and thermodynamic aspects of Br2((-))(H2O)n clusters using a conjunction of stochastic and quantum chemical methods

In this work we obtained global as well as local structures of Br2((-))(H2O)n clusters for n = 2 to 6 followed by the study of IR-spectral features and thermochemistry for the structures. The way adopted by us to obtain structures is not the conventional one used in most cases. Here we at first generated excellent quality pre-optimized structures by exploring the suitable empirical potential energy surface using stochastic optimizer simulated annealing. These structures are then further refined using quantum chemical calculations to obtain the final structures, and spectral and thermodynamic features. We clearly showed that our approach results in very quick and better convergence which reduces the computational cost and obviously using the strategy we are able to get one [i.e. global] or more than one [i.e. global and local(s)] energetically lower structures than those which are already reported for a given cluster size. Moreover, IR-spectral results and the evolutionary trends in interaction energy, solvation energy and vertical detachment energy for global structures of each size have also been presented to establish the utility of the procedure employed.

Structural and spectroscopic studies of iodine dimer radical anion hydrated clusters: an approach using a combination of stochastic and quantum chemical methods

Structures of I₂⁽⁻⁾(H₂O)₅ clusters after evaluation by simulated annealing and subsequent DFT calculation respectively.