Structures, Energetics, Relative Stabilities, and Out-of-Plane Distortivities of Skeletally Disubstituted Benzenes, (CH)4X2 (X = N, P, C-, Si-, O+, and S+): An ab Initio and DFT Study

Potential of nonporous adaptive crystals for hydrocarbon separation

Hydrocarbon separation is an important process in the field of petrochemical industry, which provides a variety of raw materials for industrial production and a strong support for the development of national economy. However, traditional separation processes involve huge energy consumption. Adsorptive separation based on nonporous adaptive crystal (NAC) materials is considered as an attractive green alternative to traditional energy-intensive separation technologies due to its advantages of low energy consumption, high chemical and thermal stability, excellent selective adsorption and separation performance, and outstanding recyclability. Considering the exceptional potential of NAC materials for hydrocarbon separation, this review comprehensively summarizes recent advances in various supramolecular host-based NACs. Moreover, the current challenges and future directions are illustrated in detail. It is expected that this review will provide useful and timely references for researchers in this area. Based on a large number of state-of-the-art studies, the review will definitely advance the development of NAC materials for hydrocarbon separation and stimulate more interesting studies in related fields.

The Conversion among Various B4C Clusters: A Density Functional Theoretical Study

Geometry optimizations and vibration frequencies of B4C clusters were performed with Becke-3LYP method using 6-31G(d) basis set. We have found 14 stable isomers, and the most stable structure among them is the five-member ring containing two three-member boron rings. We also analyzed these stable isomers in detail, and the results show that the structures containing three-member boron rings are predominant in energy for B4C clusters. In terms of MO and NBO analysis, the three-centered bond and the pi-electron delocalization play an important role in stabilizing the planar five-member rings of these B4C clusters. Our calculations suggest that isomer4 can be converted into isomer7 with only an energy barrier of 0.31 kJ mol(-1) at the B3LYP/6-311G+(3df) level. Although the planar structures of the five-member rings (isomers12-14) can be converted with each other, the conversions of isomer14 to isomer13 and isomer13 to isomer12 have high-energy barriers of 70.99 and 68.51 kJ mol(-1) at the B3LYP/6-31G(d) level, respectively.

A study by ultraviolet spectroscopy on the self-association of diazines in aqueous solution

The self-association of pyridazine and pyrazine was studied in aqueous solution at acidic, neutral and basic pH values, by ultraviolet spectroscopy. The spectra of pyridazine in the mid-ultraviolet region did not show any variation in molar absorptivity upon concentration of this compound, indicating that self-association is not important. By contrast, deviations from Beer-Lambert law with increasing concentration were found in pyrazine at the pH values studied. A single hypochromic effect was detected at pH<0, 0.6 and 11.0, while a double hypochromic effect was observed at pH 6.9. These results were interpreted in terms of self-association of pyrazine leading to the formation of dimers at acidic and basic pH, and of dimers and polymers at neutral pH. From the fitting of the experimental curves of hypochromic effects, self-association constants were calculated. The self-association of 3-methylpyridazine, 2-methylpyrazine, and 2,3-dimethylpyrazine was also analyzed, concluding that the introduction of a methyl group has no influence upon the self-association of the pyridazine ring, but the self-association of pyrazine is enhanced by the introduction of two methyl groups. A comparative discussion of the self-association behavior of pyridazine, pyrazine and pyrimidine in aqueous solution has been carried out in terms of the differences in the molecular structure of the three diazines.

Silaaromaticity in polycyclic systems: a computational study

Density functional theory (B3LYP) calculations were performed to examine the effect of Si substitution on the aromaticity of some polycyclic hydrocarbons using geometric criterion (HOMA), isodesmic isomerization reactions, homodesmotic equations, NICS values, chemical hardness, and out-of-plane distortive tendencies. The HOMA values are lower and the NICS values are higher in the Si-substituted rings compared to those in the hydrocarbon counterpart, whereas the homodesmotic equations predict little loss of aromaticity upon Si replacement in polycylic systems. The chemical hardness values decrease and the out-of-plane distortive tendency increases upon silicon substitution. The relative energies of the positional isomers and the causative factors are analyzed. The high reactivity of some silaaromatics toward dimerization is explained based on local softness indices.

A theoretical study of the structures, energetics, stabilities, reactivities, and out-of-plane distortive tendencies of skeletally substituted benzenes (CH)(5)XH and (CH)(4)(XH)(2) (X = B(-), N(+), Al(-), Si, P(+), Ga(-), Ge, and As(+))

Ab initio molecular orbital theory at Hartree-Fock (HF), post-Hartree-Fock (MP2 and CCSD(T)), and the hybrid density functional theory (B3LYP) calculations were done on the mono-(CH)(5)XH and diskeletally substituted (CH)(4)(XH)(2) benzenes (X = B(-), N(+), Al(-), Si, P(+), Ga(-), Ge, and As(+)). The computed relative energies of the disubstituted isomers show interesting trends. While the ortho-isomer is the most stable for X = Ga(-), Ge, and As(+), meta was found to be the most stable for X = B(-), N(+), Al(-) and Si, and para was found to be the most stable for X = P(+). Various intricate factors that govern the relative stabilities, such as the sum of bond strengths in the twin Kekule forms, rule of topological charge stabilization (TCS), and electrostatic repulsion were critically examined. The sum of bond strengths in the twin Kekule forms was proved to be quite a successful measure in predicting the relative stability orders between ortho- and meta-/para-isomers. The rule of TCS breaks down especially in the presence of overwhelming factors such as the differences in the cumulative bond strengths of the two positional isomers; however, the stability ordering between the para- and meta-isomers is successfully predicted in most cases. The tendency for ring puckering increases a great deal especially when the substituents are from 3rd or 4th row. Extension of the popular inverse relationship between the thermodynamic stability and reactivity was found to be inapplicable for this class of compounds. The computed singlet-triplet energy differences and the chemical hardness (eta) values indicate that the skeletal substitution weakens the pi-strength of the benzenoid system and increases their reactivity.