Hai Whang Lee

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.

Prediction of physicochemical properties of organic molecules using van der Waals surface electrostatic potentials

The generalized interaction properties function (GIPF) methodology developed by Politzer and coworkers, which calculated molecular surface electrostatic potential (MSESP) on a density envelope surface, was modified by calculating the MSESP on a much simpler van der Waals (vdW) surface of a molecule. In this work, vdW molecular surfaces were obtained from the fully optimized structures confirmed by frequency calculations at B3LYP/6‐31G(d) level of theory. Multiple linear regressions for normal boiling point, heats of vaporization, heats of sublimation, heats of fusion, liquid density, and solid density were performed using GIPF variables from vdW model surface. Results from our model are compared with those from Politzer and coworkers. The surface‐dependent β (and γ) values are dependent on the surface models but the surface‐independent α and regression coefficients (r) are constant when vdW surface and density surface with 0.001 a.u. contour value are compared. This interesting phenomenon is explained by linear dependencies of GIPF variables.

Kinetics and mechanism of the aminolysis of thiophenyl methylacetates in acetonitrile

and cross-interaction constant ρXZ is relatively large and positive (0.90). These trends are consistent with the rate-limiting breakdown of a tetrahedral intermediate, T ± . The proposed mechanism is also supported by adherence of the rate data to the reactivity-selectivity principle (RSP). The kinetic isotope effects, kH/kD, are greater than unity (1.3-1.4) suggesting a possibility of hydrogen-bonded four-centered transition state. The activation parameters, ΔH ≠ and ΔS ≠ , are consistent with this transition-state structure.

Nucleophilic substitution reactions of anilino thioethers with anilines in methanol

Kinetic studies have been carried out on the solvolysis and aminolysis with anilines of anilino thioethers, N-methyl-N-[(Z-phenylthio)methyl]-Y-anilines, YC6H4N(CH3)CH2SC6H4Z, I, in methanol at 45.0°C. In contrast to the iminium cations with significant lifetimes produced in water, the solvolysis proceeds by a direct displacement (SN2) mechanism in methanol. In the aminolysis, both bond formation and cleavage are well under way in a late SN2 transition state with high ρX (βX) and ρZ (βZ) values. However, cross interactions between the nucleophile and leaving group are extensive with large negative constants (ρXZ=−1.7, βXZ=−0.27), which are suggestive of an SN2 reaction with frontside attack. The inverse secondary kinetic isotope effects involving deuterated anilines (kH/kD=0.84–0.88) along with low ΔH≠ (4.2–5.2 kcal mol−1) and ΔS≠ (−46–−59 e.u.) values are consistent with the proposed mechanism.

Theoretical studies on the formation mechanism and explosive performance of nitro‐substituted 1,3,5‐triazines

To develop new highly energetic materials, we have considered the design of molecules with high nitrogen content. Possible candidates include 1,3,5‐triazine derivatives. In this work, we studied potential synthetic routes for melamine using the MP2/6‐31+G(d,p)//B3LYP/6‐31G(d) level of theory. The mechanisms studied here are stepwise mechanism beginning with the dimerization of cyanamide and one‐step termolecular mechanism. The same type of mechanism is also applied to nitro‐substituted 1,3,5‐triazines. Values for the heat of formation in the solid phase were predicted from density functional theory calculations. Densities were estimated from a regression equation obtained by molecular surface electrostatic potentials. The Cheetah program was used to study the explosive performance of these compounds. In this study, we found that the explosive properties of 2‐amino‐4, 6‐dinitro‐1, 3,5‐triazine (ADNTA), and 2,4,6‐trinitro‐1,3,5‐triazine (TNTA) are similar to those of RDX and HMX, respectively.

Nucleophilic substitution reactions of α-chloroacetanilides with benzylamines in dimethyl sulfoxide

Kinetic studies of the reactions of alpha-chloroacetanilides (YC6H4NRC(=O)CH2Cl; R = H (5) and CH3 (6)) with benzylamines (NH2CH2C6H4X) were carried out in dimethyl sulfoxide at 55.0 degrees C. The Brønsted betaX values were in the range from 0.6 to 0.9 and cross-interaction constants phoXY were positive: phoXY = +0.21 and +0.18 for 5 and 6, respectively. The rates were faster with 6 than with 5 and inverse secondary kinetic isotope effects involving deuterated benzylamine (ND2CH2C6H4X) nucleophiles, kH/kD < 1.0, were obtained. Based on these and other results, a stepwise mechanism with rate-limiting expulsion of the chloride leaving group from a zwitterionic tetrahedral intermediate, T+/-, is proposed. In this mechanism, a prior carbonyl addition to T+/- is followed by a bridged type transition state to expel the chloride. An enolate-like transition state in which the developing negative charge on C(alpha) delocalizes toward the carbonyl group (nC-->pi*(C=O) interaction) is not feasible for the present series of reactions due to a stronger charge transfer involving the lone pair on the anilino nitrogen (nAN-->pi*(C=O) interaction).

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.

Reactivity–selectivity relationship and kinetic solvent isotope effects in nucleophilic substitution reactions

Selectivity plots, log(Kx1/Kx2)vs. σy, have been obtained for various nucleophilic substitution reactions: (i) with anilines (N) in methanol (S), kN/kS, using literature data for YC6H4CH2Cl, YC6H4COCl, YC6H4SO2Cl, YC6H4COCH2Br, YC6H4CH(CH3)Cl and YC6H4CH[C(CH3)3]OSO2C6H4NO2; and (ii) with kinetic solvent isotope effects (KSIE) in water and methanol, kSOH/kSOD, determined in this work for the same compounds (except for the latter two). The two selectivity plots are shown to be equivalent in applying mechanistic criteria based on four reactivity–selectivity regions identified by the, slopes of the selectivity plots. Δρy. Straightforward applications were possible in most cases except for the aminolysis of phenacyl bromides and 1-phenylethyl chlorides. For these two, detailed analyses of kinetic results were required, in addition to the reactivity-selectivity considerations, before deciding the reaction mechanism.

Kinetics and mechanism of the anilinolysis of aryl phenyl isothiocyanophosphates in acetonitrile.

Summary Kinetic studies on the reactions of Y-aryl phenyl isothiocyanophosphates with substituted X-anilines and deuterated X-anilines were carried out in acetonitrile at 55.0 °C. The free-energy relationships with X in the nucleophiles were biphasic concave upwards with a break region between X = H and 4-Cl, giving unusual positive ρX and negative βX values with less basic anilines (X = 4-Cl and 3-Cl). A stepwise mechanism with rate-limiting bond breaking for more basic anilines and with rate-limiting bond formation for less basic anilines is proposed based on the positive and negative ρXY values, respectively. The deuterium kinetic isotope effects involving deuterated anilines (XC6H4ND2) showed primary normal and secondary inverse DKIEs for more basic and less basic anilines, rationalized by frontside attack involving hydrogen-bonded four-center-type TSf and backside attack TSb, respectively. The positive ρX values with less basic anilines are substantiated by the tight TS, in which the extent of the bond formation is great and the degree of the bond breaking is considerably small.

Thermodynamic studies on the structure of iso-dielectric binary mixtures of methanol with ethylene glycol, acetonitrile, nitrobenzene and nitromethane

The relative partial molal enthalpies of component solvents have been measured at 25 °C in CH3OH–(CH2OH)2, CH3OH–CH3CN, CH3OH–C6H5NO2 and CH3OH–CH3NO2 binary mixtures using an LKB-2277 microcalorimeter, and the molar excess enthalpies have been determined. The structure of binary mixtures is discussed by calculating local mole fractions around an added component solvent based on derived non-random two-liquid type expressions.