Soo Gyeong Cho

A computational study of imidazole, 4-nitroimidazole, 5-nitroimidazole and 4,5-dinitroimidazole

Abstract We have examined the molecular structure of imidazole (I), 4-nitroimidazole (II), 5-nitroimidazole (III) and 4,5-dinitroimidazole (IV) with semi-empirical, ab initio and density functional theory (DFT) calculations. Compared with experimental data, both B3LYP and MP2 calculations with the 6-31G∗ basis set furnish excellent geometric features with average errors of less than 0.01 A in bond lengths and of less than 1 ° in bond angles. Although the SCF results are slightly worse than those with electron correlation effects, HF/3-21G and HF/6-31G∗ calculations also provide reasonable geometries and are probably useful in a practical sense. Geometries obtained from semi-empirical methods have large errors in some of bond lengths and angles. Rotational barriers of the nitro group in II and III have been estimated to be 6.4 and 10.8 kcal/mol, respectively, at the QCISD(T)/6-31G∗//HF/6-31G∗ level. These barriers are largely overestimated at the SCF and B3LYP levels, although utilizing an extremely large basis set such as 6-311++G(3pd,3df) at the B3LYP level reduces the barriers to a comparable range with those of QCISD(T)/6-31G∗//HF/6-31G∗. On the other hand, both AM1 and PM3 underestimate these barriers considerably. According to the analysis of natural bond orbitals, a substantial difference in rotational barriers of between II and III is attributed to the relative stabilization energy due to π orbital conjugations between the C4C5 bond and NO bonds. In the global minimum of IV predicted by both ab initio and DFT calculations, both nitro groups are skewed, with more deformation of the nitro group attached to the C5 atom, although the degree of twisted angles in nitro groups is quite variant depending on the calculational levels. Both semi-empirical methods predict that the nitro group attached to the C4 atom is eclipsed and the one attached to the C5 atom is planar. The orientation of nitro groups in IV can be understood by a compromise between 1. (1) an increase of electrostatic repulsions as two electronegative nitro groups approach and 2. (2) a destabilization due to less π orbital overlaps as nitro groups are skewed.

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.

Analysis of explosives using corona discharge ionization combined with ion mobility spectrometry-mass spectrometry.

Corona discharge ionization combined with ion mobility spectrometry-mass spectrometry (IMS-MS) was utilized to investigate five common explosives: cyclonite (RDX), trinitrotoluene (TNT), pentaerythritol tetranitrate (PETN), cyclotetramethylenetetranitramine (HMX), and 2,4-dinitrotoluene (DNT). The MS scan and the selected ion IMS analyses confirmed the identities of the existing ion species and their drift times. The ions observed were RDX·NO3(-), TNT(-), PETN·NO3(-), HMX·NO3(-), and DNT(-), with average drift times of 6.93 ms, 10.20 ms, 9.15 ms, 12.24 ms, 11.30 ms, and 8.89 ms, respectively. The reduced ion mobility values, determined from a standard curve calculated by linear regression of (normalized drift times)(-1) versus literature K0 values, were 2.09, 1.38, 1.55, 1.15, 1.25, and 1.60 cm(2) V(-1) s(-1), respectively. The detection limits were found to be 0.1 ng for RDX, 10 ng for TNT, 0.5 ng for PETN, 5.0 ng for HMX, and 10 ng for DNT. Simplified chromatograms were observed when nitrogen, as opposed to air, was used as the drift gas, but the detection limits were approximately 10 times worse (i.e., less sensitivity of detection).

Energetic salts based on nitroiminotetrazole-containing acetic acid

2-(5-Nitroiminotetrazol-1-yl)acetic acid (4) was synthesized from 100% nitric acid and ethyl 2-(5-aminotetrazol-1-yl)acetate (2), which was easily obtained by reaction of ethyl aminoacetate hydrochloride, sodium hydroxide, and cyanogen azide. Compound 4 was also formed with 100% nitric acid and 2-(5-aminotetrazol-1-yl)acetic acid which was prepared from sodium 5-aminotetrazolate and 2-chloroacetic acid. New energetic materials comprised of nitroiminotetrazolate salts with nitroiminotetrazolate and carboxylate anions have been characterized spectroscopically as well as with single crystal X-ray diffraction and elemental analyses. In addition, the heats of formation (ΔHf), and detonation pressures (P) and velocities (D) were calculated. All compounds were insensitive (>40 J) for impact with BAM Fallhammer.

Energetic salts based on 1-methoxy-5-nitroiminotetrazole

Eight new energetic salts based on 1-methoxy-5-nitroiminotetrazole, which was obtained by nitration of 1-methoxy-5-aminotetrazole, were synthesized. Guanidinium (5), aminoguanidinium (6), diaminoguanidinium (7), triaminoguanidinium (8), carbohydrazidinium (9), 3-amino-1,2,4-triazolium (10), 4-amino-1,2,4-triazolium (11), and 3,5-diamino-1,2,4-triazolium (12) salts were characterized by vibrational spectroscopy (IR, Raman), multinuclear spectroscopy (1H, 13C, 15N), elemental analysis, and single crystal X-ray diffraction analysis. Salts 5·1/3H2O and 7–9 crystallize in the triclinic space group P, whereas compounds 6 and 10 crystallize in the monoclinic space groups C2/c and P2(1)/n, respectively. Compound 11 is in orthorhombic group P2(1)2(1)2(1). In addition, the heats of formation (ΔHf), and detonation properties (pressure and velocity) were calculated using Gaussian 03 and EXPLO5 programs, respectively. Thermal stabilities were obtained by DSC measurements and the sensitivities toward impact and friction were determined by BAM methods.

Rotational barriers of disilane, hexafluorodisilane, and hexamethyldisilane: Ab initio, density functional, and molecular mechanics (MM3) studies

We investigated structures, vibrational frequencies, and rotational barriers of disilane (Si2H6), hexafluorodisilane (Si2F6), and hexamethyldisilane (Si2Me6) by using ab initio molecular orbital and density functional theories. We employed four different levels of theories (i.e., HF/6–31G*, MP2/6–31G*, BLYP/6–31G*, and B3LYP/6–31G*) to optimize the structures and to calculate the vibrational frequencies (except for Si2Me6 at MP2/6–31G*). MP2/6–31G* calculations reproduce experimental bond lengths well, while BLYP/6–31G* calculations largely overestimate some bond lengths. Vibrational frequencies from density functional theories (BLYP/6–31G* and B3LYP/6–31G*) were in reasonably good agreement with experimental values without employing additional correction factors. We calculated the ΔG‡(298 K) values of the internal rotation by correcting zero‐point vibration energies, thermal vibration energies, and entropies. We performed CISD/6–31G*//MP2/6–31G* calculations and found the ΔG‡(298 K) values for the internal rotation of Si2H6, Si2F6, and Si2Me6 to be 1.36, 2.06, and 2.69 kcal/mol, respectively. The performance of this level was verified by using G2 and G2(MP2) methods in Si2H6. According to our theoretical results, the ΔG‡(298 K) values were marginally greater than the ΔE‡(0 K) values in Si2F6 and Si2Me6 due to the contribution of the entropy. In Si2H6 the ΔE‡(0 K) and ΔG‡(298 K) values were coincidently similar due to a cancellation of two opposing contributions between zero‐point and thermal vibrational energies, and entropies. Our calculated ΔG‡(298 K) values were in good agreement with experimental values published recently. In addition, we also performed MM3 calculations on Si2H6 and Si2Me6. MM3 calculated rotational barriers and thermodynamic properties were compared with high level ab initio results. Based on this comparison, MM3 calculations reproduced high level ab initio results in rotational barriers and thermodynamic properties of Si2H6 derivatives including vibrational energies and entropies, although large errors exist in some vibrational frequencies. © 1997 John Wiley & Sons, Inc. J Comput Chem 18: 1523–1533, 1997

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.

Ti(N5)4 as a Potential Nitrogen-Rich Stable High-Energy Density Material

We have studied molecular structures and kinetic stabilities of M(N5)3 (M = Sc, Y) and M(N5)4 (M = Ti, Zr, Hf) complexes theoretically. All of these compounds are found to be stable with more than a 13 kcal/mol of kinetic barrier. In particular, Ti(N5)4 showed the largest dissociation energy of 173.0 kcal/mol and thermodynamic stability. This complex had a high nitrogen content (85% by weight), and a significantly high nitrogen to metal ratio (20:1) among the neutral M(N5)n species studied here and in the literature. Ti(N5)4 is thus forecasted to be a good candidate for a nitrogen-rich high-energy density material (HEDM). We reveal in further detail using ab initio molecular dynamics simulations that the dissociation pathways of M(N5)n involve the rearrangements of the bonding configurations before dissociation.

An ab initio and X-ray study of morpholinone derivatives: the intermolecular 1,3-dipolar cycloaddition of a derived ylide with maleimide

Abstract Conformational analysis of N-acetyl 5-phenyl-morpholin-2-one, shows two main conformers. That which is also observed in the crystal structure determination contains the C(5)-phenyl substituent in a pseudo-axial position. The favored conformations of the paraformaldehyde-derived ylide, 1, were similarly established and two conformers were also found distinguished by the position of the C(5)-phenyl group either in a pseudo-axial or equatorial position. Four reaction pathways for the cycloaddition of maleimide with each ylide conformer, have been examined by ab initio methods (HF/6-31+G∗ and B3LYP/6-31+G∗) and our calculated transition state energies are in good agreement with the ratio of products observed experimentally.

Fabrication and characterization of porous silicon nanowires

We report the synthesis of porous silicon nanowires through the metalassisted chemical etching of porous silicon in a solution of hydrofluoric acid and hydrogen peroxide. The morphology of porous silicon nanowires was characterized by scanning electron microscopy and transmission electron microscopy. The etch rate of the porous silicon nanowires was faster than that of silicon nanowires, but slower than that of porous silicon. The porous silicon nanowires distributed uniformly on the entire porous silicon layer and the tips of the porous silicon nanowires congregated together. The single crystalline and sponge-like porous structure with the pore diameters of less than 5 nm was confirmed for the porous silicon nanowires.