Chang Kon Kim

Prediction of densities for solid energetic molecules with molecular surface electrostatic potentials

The densities of high energetic molecules in the solid state were calculated with a simplified scheme based on molecular surface electrostatic potentials (MSEP). The MSEP scheme for density estimation, originally developed by Politzer et al., was further modified to calculate electrostatic potential on a simpler van der Waals surface. Forty‐one energetic molecules containing at least one nitro group were selected from among a variety of molecular types and density values, and were used to test the suitability of the MSEP scheme for predicting the densities of solid energetic molecules. For comparison purposes, we utilized the group additivity method (GAM) incorporating the parameter sets developed by Stine (Stine‐81) and by Ammon (Ammon‐98 and ‐00). The absolute average error in densities from our MSEP scheme was 0.039 g/cc. The results based on our MSEP scheme were slightly better than the GAM results. In addition, the errors in densities generated by the MSEP scheme were almost the same for various molecule types, while those predicted by GAM were somewhat dependent upon the molecule types.

Effects of Basis Set Superposition Error on Optimized Geometries and Complexation Energies of Organo-Alkali Metal Cation Complexes

Theoretical studies were performed to study the binding of alkali metal cations, X(+) (X = Li, Na, K), to poly(ethylene oxide) (PEO, I), poly(ethylene amine) (PEA, II), and poly(ethylene N-methylamine) (PEMA, III) by the Hartree-Fock (HF) and B3LYP methods using the 6-31G(d) and 6-311+G(d,p) basis sets. Two types of complex were considered in this study: a singly coordinated system (SCS) and a doubly coordinated system (DCS). Complexation energies were calculated both without and with basis set superposition error (BSSE). Because of the strong charge-dipole interactions, the complexation energies were largely negative and decreased in the order Li(+) > Na(+) > K(+). Three possible counterpoise (CP) approaches were examined in detail. In the case of the function CP (fCP) correction, the complexation energies exhibited an unusual trend because of the deformation of the subunits. This problem was solved by including geometry relaxation in the CP-corrected (GCP) interaction energies. The effects on the structures and vibrational frequencies were small when the complexes were reoptimized on the CP-corrected potential energy surface (PES).

Comparative studies on the reactions of acetyl and thioacetyl halides with NH3 in the gas phase and in aqueous solution: a theoretical study.

The reactions of acetyl halides, CH3C(═ O)X and corresponding sulfur analogues, thioacetyl halides, CH3C(=S)X, where X = F and Cl, with NH3 nucleophile were studied theoretically, at the QCISD level of theory, in the gas phase and in aqueous solution. All reactions occurred via the tetrahedral species, and reactions through neutral intermediates both in the gas phase and in aqueous solution could be ruled out, except for the case of the gas-phase reaction of acetyl fluoride. The tetrahedral structure was a transition state (TS) in the reactions of acetyl chloride, while it was a stable intermediate in reactions of thioacetyl halides. These differences could be caused by the π-bond strength of C ═ O and C ═ S. In the case of acetyl fluoride, the T(±)-type species was neither a saddle point nor an energy minimum in the gas phase, but existed as a stable intermediate in aqueous solution due to solvation. Moreover, in reactions of thioacetyl chloride, the rate-limiting step changed from the first step in the gas phase to the second step in aqueous solution, since the zwitterionic intermediates become more stabilized in aqueous solution. However, lower activation energies (ΔG(‡)) in aqueous solution were not caused by the solvent effects, but smaller deformation effects, in going from reactants through the TS.

Comparative Studies on the Reactions of Carbamyl and Thiocarbamyl Halides with NH 3 in the Gas Phase and in Aqueous Solution: A Theoretical Study

The reactions of acetyl halides, CH3C(═O)X and corresponding sulfur analogues, thioacetyl halides, CH3C(=S)X, where X = F and Cl, with NH3 nucleophile were studied theoretically, at the QCISD level of theory, in the gas phase and in aqueous solution. All reactions occurred via the tetrahedral species, and reactions through neutral intermediates both in the gas phase and in aqueous solution could be ruled out, except for the case of the gas-phase reaction of acetyl fluoride. The tetrahedral structure was a transition state (TS) in the reactions of acetyl chloride, while it was a stable intermediate in reactions of thioacetyl halides. These differences could be caused by the π-bond strength of C═O and C═S. In the case of acetyl fluoride, the T±-type species was neither a saddle point nor an energy minimum in the gas phase, but existed as a stable intermediate in aqueous solution due to solvation. Moreover, in reactions of thioacetyl chloride, the rate-limiting step changed from the first step in the gas pha...

Reexamination of the π-bond strengths within H2C=XHn systems: A theoretical study

The accurate determination of π‐bond energies, Dπ, in doubly‐bonded species has been an important issue in theoretical chemistry. The procedure using the divalent state stabilization energy defined by Walsh has been suggested, and the procedure seems to be conceptually reasonable and applicable to all kinds of doubly‐bonded species. Therefore, the aim of this study was to examine whether the procedure could be a reliable methodology for estimating the Dπ values for a variety of H2C=XHn species. To achieve a higher accuracy, the Dπ values were estimated at QCISD(T)/6‐311++G(3df,2p) level of theory combined with isogyric correction. The Dπ values estimated in this work were in excellent agreement with the extant literature values. On the other hand, in determining accurate Dπ values for doubly bonded species, especially in species with lone‐pair electrons such as H2C=O, it has been found that consideration of highly sophisticated electron correlation effects could be important. However, sufficiently accurate Dπ values have been obtainable at QCISD(T) or CCSD(T) levels with a 6‐311++G(3df,2p) basis set on geometries at relatively inferior correlated levels such as MP2 and B3LYP levels with a 6‐31+G(d) basis set.

Anion Complexation by Calix(4)pyrrole in Solid Polymer Electrolytes

ConclusionsIn this paper, we describe a prospective approach to increasing the metal ionic conductivity of solid polymer electrolytes by using the anion complexation between the anion of the metal salt and the solvent matrix. We have prepared a new type of solid polymer electrolyte (CP/LiCl/ PVC) and demonstrated its excellent ionic conductivity. The high ionic conductivity of this material is attributed to anion complexation between the anions and their receptors (CPs) dissolved in the PVC matrix. The anion complexation of CP, which should help to increase both the concentration of free Li+ and its mobility, is more favorable with Cl− than ClO4−. Consequently, such systems have high cation conductivity and characteristics of high performance electrolytes that are required to achieve high power density and good rechargeability for lithium batteries.

Effects of entropy on the gas-phase pyrolysis of ethyl N,N-dimethylcarbamate.

In this study, we examined the gas‐phase pyrolysis of ethyl N,N‐dimethylcarbamate theoretically at various theoretical levels. The reaction consists of a two‐step mechanism, with N,N‐dimethylcarbamic acid and ethylene as reaction intermediates. In the first step, the reaction proceeds via a six‐membered cyclic transition state (TS), which is more favorable than that via a four‐membered cyclic TS. Here, the contribution of entropy to the overall potential energy surface was found to play an important role in determining the rate‐limiting step, which was found to be the second step when viewed in terms of the enthalpy of activation (ΔH≠), but the first step when entropy changes (−TΔS≠) were considered. These results are consistent with experimental findings. Moreover, the experimental activation entropy can be reproduced by using the hindered rotor approximation, which converts some low vibration frequencies that correspond to internal rotational modes into hindered rotors.

Theoretical studies on the base-catalysed rearrangement of 4,4′-disubstituted benzils in the gas phase and aqueous solution

The base-catalysed rearrangements of disubstituted aryl benzils in the gas phase and in solution (water) have been investigated theoretically by the AM1 method. The solvent effects were accounted for using the Cramer–Truhlar SM2.1 solvation model. The calculated Gibbs free energy changes (ΔG‡) for the overall reactions (kobs) are dissected into component parts, i.e. for the equilibrium step(K) of reactants ⇌ intermediate (ΔGIo) and rearrangement step (k2) of intermediate → transition state (ΔGI‡); ΔG‡=ΔGIo+ΔGI‡. The Hammett ρ values for the migrating (ρX) and non-migrating rings (ρY) are then calculated for each step. Thus, the overall ρ(kobs) value can be dissected into two components, ρ(K)+ρ(k2). It has been shown that (i) the substituent effects of the 4,4′-disubstituted aryl benzils are not in general additive, ρ(kobs)≠ρX+ρY, due to the cross-interaction term, ρXY, and (ii) in the gas phase, the equilibrium step is considerably more important than the rearrangement step, ρ(K)ρ(k2), whereas in solution (water) the two steps contribute comparably, ρ(K)≃ρ(k2). It is also notable that in the rearrangement step (k2) the carbonyl carbon of the non-migrating ring actually becomes more electron-deficient [ρY(k2) 0]. Fair agreement of the ρ(kobs) values are obtained between those for the theoretical solution phase (6.7 in water) and for the experimentally observed [5.7 in 7%(v/v) aqueous Me2SO] for the symmetrically disubstituted aryl benzils (σX=σY).