Methane hydrate: Modeling and assessing the experimental data of incipient stability conditions

Accurate knowledge of equilibrium conditions for methane hydrates dissociation is crucial to implement an appropriate hydrate-based technology/process. Hence, much importance is attached to utilize techniques for the modeling of hydrate stability zones and the existence of reliable data as well. This research employs the published experimental data, from 1940 to 2016, for modeling the incipient stability conditions of methane hydrate in pure water using Classification and Regression Tree (CART) as a novel methodology in gas hydrates research. Least squares support vector machine (LSSVM) and artificial neural network (ANN) methods were selected as a basis for comparison. Furthermore, the Leverage mathematical approach was used for evaluating the quality of the data as well as the correctness of the CART model. Results confirmed that the developed tree-based model provides excellent outcomes and no model can rival it for accuracy. In addition, applying the Leverage algorithm specified that: (a) the CART model is statistically valid and correct; (b) the published experimental data in the literature are different in quality; (c) there is no doubtful data in the studied database; and (d) the published data in the temperature range from 148.8 to 238.8?K are not sufficient compared with the whole of the collected database.     

Theoretical Study of Solvent Effects on the Cis-to-Trans Isomerization of [Pd(C6Cl2F3)I(PH3)2]

In this work, the effect of different solvents on the mechanism of the cis-to-trans isomerization of [Pd(C₆Cl₂F₃)(I)(PH₃)₂] has been investigated theoretically in detail. Using a quantum mechanical approach, different pathways were investigated. A three-pathway mechanism has already been proposed which consists of two PH₃-sensitive steps (k ₃, k ₄) and one PH₃-insensitive (k _diss) step. Since in the k ₃ pathway the solvent has two types of explicit and implicit effects, this path was investigated for both the gas phase and solvent systems (using the PCM model). In this path, solvents with larger donor numbers (necessary condition) and smaller dielectric constant and dipole moment values (sufficient condition) are more appropriate. In the k ₄ pathway, the solvent has an implicit effect only, and the smaller are the dielectric constant and dipole moment of a solvent, the more appropriate it is. To find the best solvent, a parameter, called the average activation energy, was defined, which considers the contribution of each path in the mechanism.     

Theoretical investigation on geometries and aromaticity of heterocyclic platinabenzenes

The electronic structure and properties of the heterocyclic platinabenzenes isomers have been investigated using the hybrid density functional B3LYP theory. Basic measures of aromatic character were derived from the structure and nucleus-independent chemical shift (NICS). The energetic criterion suggests that the ortho-isomer enjoy conspicuous stabilization where heteroatom is P or As. But the meta-isomer is most stable isomer, where heteroatom is N. The NICS values calculated at several points above the ring center to gave the result consistent with that cased on the relative energy. The atoms in molecules analysis indicates a correlation between NICS(1.0) and the electron density of ring critical point in all species. The natural bond orbital analysis has been used to study the bond characterizations in all species.     

A theoretical study of the interaction between [HB≡CH]<Superscript>−</Superscript>, [H<Subscript>2</Subscript>B=CH<Subscript>2</Subscript>]<Superscript>−</Superscript>, and boratabenzene anions with alkali and alkaline earth metals: properties and structures

The nature of [HB≡CH]⁻, [H₂B=CH₂]⁻, and boratabenzene interactions with alkaline and alkaline earth metals are studied by ab initio calculations. The interaction energies are calculated at the B3LYP/6-311++G(d,p) level. The calculations suggest that the cation size and charge are two influential factors that affect the nature of the interaction. AIM and NBO analyses of the complexes indicate that the variation of densities and the extent of charge transfers upon complexation correlate well with the obtained interaction energies.     

Assessment of substituent effects on the parameters of 35Cl nuclear quadrupole resonance in para‐substituted benzene‐sulphenyl chloride via quantum chemical calculations

In this research, substituent effects on the parameters of ³⁵Cl nuclear quadrupole resonance (NQR) in para‐substituted benzene‐sulphenyl chloride were studied at M062X/6‐311G(d,p) theory level. The ³⁵Cl NQR parameters of the quadrupole coupling constant (QCC) and electric‐field gradient (EFG) tensor, as well as an asymmetric parameter, were shown to be correlated with Hammett constant following their calculations. The frontier orbital energy levels, HOMO‐LUMO gaps, hardness, electrophilicity, and chemical potential values of these molecules were calculated as well. natural bond orbital (NBO) analysis was applied for calculating natural populations at chlorine atoms.     

Solvent effect on the stability and properties of platinum-substituted borirene and boryl isomers: The polarizable continuum model

The structure and properties of platinum borirene complex trans-[Cl(PMe₃)₂Pt{μ-BN(SiMe₃)₂C=C}Ph] and its isomer the platinum boryl complex trans–[Cl(PMe₃)₂PtBN(SiMe₃)₂C≡CPh] were investigated theoretically. The solvent effect on the stability, structural parameters, frontier orbital energies, HOMO–LUMO gaps, and hardness of isomers was investigated using the polarizable continuum model (PCM). It was found that borirene isomer is the most stable isomer in the gas phase and solvent. The calculated results show that the presence of solvent reduces the frontier orbital energy of the studied molecules. Geometries obtained from calculations were used to perform NBO analysis.     

Theoretical Study of Substituent Effect in Aryl Group Migration in (<Emphasis Type="Italic">para</Emphasis>-C<Subscript>6</Subscript>H<Subscript>4</Subscript>X)Mn(CO)5 Complexes

In this study, we report substituent effect on aryl group migration in (para-C₆H₄X)Mn(CO)₅ complexes using mpw1pw91 quantum chemical calculations. These calculations reveal good linear relationships between barrier energy (ΔE), activation energy (ΔH^?), activation free energy (ΔG^?) values and rate constants with Hammett constants of X-substituents. The occupancy values of Mn–CO_cis and Mn–C(O)-(para-C₆H₄X) bonds in reactant, transition state and product were calculated by Natural bond orbital (NBO) method.     

Theoretical Study of Tautomerization in 1,5-Dimethyl-6-Thioxo-1,3,5-Triazinane-2,4-Dione

In this work, the tautomeric transformations of a 1,5-dimethyl-6-thioxo-1,3,5-triazinane-2,4-dione molecule are explored at the M062X/6-311G(d,p) level of theory in gas and solution phases. These calculations show that the 1,5-dimethyl-6-thioxo-1,3,5-triazinane-2,4-dione isomer is more stable than its tautomer (4-hydroxy-1,5-dimethyl-6-thioxo-5,6-dihydro-1,3,5-triazin-2(1H)-one) in gas and solution phases. The frontier molecular orbitals and band gap energy calculations are performed at the M062X/6-311G(d,p) level in gas and various solvents. Solvent effects are analyzed using the self-consistent reaction field method based on the polarizable continuum model in chloroform, chlorobenzene, tetrahydrofurane, dichloromethane, and quinoline. The solvent effect on the N–H and C=O vibrations is explored. Also, natural bond orbital (NBO) analysis was used to understand the structure and bonding of the molecule.     

Analysis of the Interaction Between the C<Subscript>20</Subscript> Cage and <Emphasis Type="Italic">cis</Emphasis>-Ptcl<Subscript>2</Subscript>(NH<Subscript>3</Subscript>)<Subscript>2</Subscript>: A DFT Investigation of the Solvent Effect,Structures, Properties,and Topologies

This study investigates the interaction between C²⁰ and the cis-PtCl₂(NH₃)₂ complex using MPW1PW91 quantum chemical calculations in gas and solution phases. Two interaction modes between C²⁰ and the cis-PtCl₂(NH₃)₂ complex are considered: I-isomer (η₂-C²⁰) and II-isomer (η₁-C²⁰). It also determines the effects of the solvent polarity on the dipole moment, electronic spatial extent (ESE), structural parameters, and frontier orbital energies of two possible isomers of the C²⁰…cis-PtCl₂(NH₃)₂ complex. The bonding interaction between C²⁰ and the cis-PtCl₂(NH₃)₂ complex was examined through energy decomposition analysis (EDA). The metal–ligand bonds are evaluated using the percentage composition of the specific groups of frontier orbitals. The quantum theory of atoms in molecules (QTAIM) analysis is applied to assess the Pt–C bonds within the complex. Finally, the Pt–C spin-spin coupling constants are calculated using the gauge independent atomic orbital (GIAO) method.     

QM study on regioselectivity in the synthesis of pyrimido[4,5-b][1,4]benzothiazines: kinetic and thermodynamic point of view

In the present study, the experimentally observed regioselectivity in pyrimido[4,5-b][1,4]benzothiazine synthesis has been modeled using density functional theory method at M06/6-311++G** level. Also the tautomerism of all four tautomers of 6-ethyl-2-thio(1H) pyrimidin-(3H)-4-one as an important intermediate through the reaction path in pyrimido[4,5-b][1,4]benzothiazine synthesis has been investigated extensively in the gas and solution phase. The stability order of tautomers was found as A > D > B > C, in gas and chloroform solvent; however, changing to more polar solvents this order was changed as A > B > D > C using polarized continuum model. In the next step, we focused on another significant feature of this synthesis which has major role in the observed regioselectivity. Two proposed reaction paths with two different transition states that it seems the mode of its intermolecular cyclization has a special role in regioselectivity of the final product have been considered. Comparison of our calculated NMR and IR spectrum with those already reported for some pyrimido[4,5-b][1,4]benzothiazines demonstrates a reliable agreement. Moreover, all obtained results in the gas and solution phase confirm that the synthesis of above-mentioned compound is thermodynamically more favorable than the possible regioisomeric product.