Performance of binary-encounter-Bethe (BEB) theory for electron-impact ionization cross sections of molecules containing heavy elements (\u3cem\u3eZ\u3c/em\u3e \u3e 10)

The binary-encounter-Bethe (BEB) theory developed by Kim and coworkers has been successful for computing electron-impact ionization cross sections of many molecules. However, some recent publications have stated that BEB theory performs poorly for molecules that contain heavier elements such as chlorine and sulfur. We have found that the BEB calculations in those publications were performed incorrectly. When performed correctly, BEB predictions are as good for heavy-element molecules as for light-element molecules. We recommended recently that an alternative, less-confusing procedure be used for molecules that contain heavier elements. The alternative procedure, based upon effective core potentials (ECPs), does not require explicit kinetic energy corrections. For peak cross sections of a group of 18 molecules, the root-mean-square difference between BEB predictions and experimental values is 13%. Results are presented for CCl3CN, C2Cl6, C2HCl5, C2Cl4, both isomers of C2H2Cl4, CCl4, TiCl4, CBr4, CHBr3, CH2Br2, GaCl, CS2, H2S, CH3I, Al(CH3)3, Ga(CH3)3, and hexamethyldisiloxane. Incorrect BEB calculations have been reported in the literature for several of these molecules

Unimolecular Decomposition of 5-Aminotetrazole and its Tautomer 5-Iminotetrazole: New Insight from Isopotential Searching

Aminotetrazole compounds have become attractive ingredients in gas generating compositions, solid rocket propellants, and green pyrotechnics. Therefore, a fundamental understanding of their thermal decomposition mechanisms and thermodynamics is of great interest. In this study, the specular reflection isopotential searching method was used to investigate the unimolecular decomposition mechanisms of 5-iminotetrazole (5-ITZ), 1H-5-aminotetrazole (1H-5-ATZ), and 2H-5-aminotetrazole (2H-5-ATZ). Subsequent thermochemical analysis of the unimolecular decomposition pathways was performed at the CCSD(T)/aug-cc-pVTZ//B3LYP/6- 311++G(3df,3pd) level of theory. Based upon the relative reaction barriers predicted in this study, the initial gaseous products of 5-ITZ unimolecular decomposition are HN3 and NH2CN (calculated activation barrier equal to 199.5 kJ/mol). On the other hand, the initial gaseous products of 1H-5-ATZ and 2H-5-ATZ unimolecular decomposition are predicted to be N2 and metastable CH3N3 (calculated activation barriers equal to 169.2 and 153.7 kJ/mol, respectively). These predicted unimolecular decomposition products and activation barriers are in excellent agreement with thermal decomposition experiments performed by Lesnikovich et al. [Lesnikovich, A. I.; Ivashkevich, O. A.; Levchik, S. V.; Balabanovich, A. I.; Gaponik, P. N.; Kulak, A. A. Thermochim. Acta 2002, 388, 233], in which the apparent activation barriers were measured to be approximately 200 and 150 kJ/mol, respectively, for 5-ITZ and 1H-5-ATZ/2H-5-ATZ

Aminoxyl (Nitroxyl) Radicals in the Early Decomposition of the Nitramine RDX

The explosive nitramine RDX (1,3,5-trinitrohexahydro-<i>s</i>-triazine) is thought to decompose largely by homolytic N–N bond cleavage, among other possible initiation reactions. Density-functional theory (DFT) calculations indicate that the resulting secondary aminyl (R₂N·) radical can abstract an oxygen atom from NO₂ or from a neighboring nitramine molecule, producing an aminoxyl (R₂NO·) radical. Persistent aminoxyl radicals have been detected in electron-spin resonance (ESR) experiments and are consistent with autocatalytic “red oils” reported in the experimental literature. When the O-atom donor is a nitramine, a nitrosamine is formed along with the aminoxyl radical. Reactions of aminoxyl radicals can lead readily to the “oxy-<i>s</i>-triazine” product (as the <i>s</i>-triazine <i>N</i>-oxide) observed mass-spectrometrically by Behrens and co-workers. In addition to forming aminoxyl radicals, the initial aminyl radical can catalyze loss of HONO from RDX

Gas-Phase Energetics of Thorium Fluorides and Their Ions

Gas-phase thermochemistry for neutral ThF_<i>n</i> and cations ThF_<i>n</i>⁺ (<i>n</i> = 1–4) is obtained from large-basis CCSD­(T) calculations, with a small-core pseudopotential on thorium. Electronic partition functions are computed with the help of relativistic MRCI calculations. Geometries, vibrational spectra, electronic fine structure, and ion appearance energies are tabulated. These results support the experimental results by Lau, Brittain, and Hildenbrand for the neutral species, except for ThF. The ion thermochemistry is presented here for the first time

Thermochemistry of HO₂ + HO₂ → H₂O₄: Does HO₂ Dimerization Affect Laboratory Studies?

Self-reaction is an important sink for the hydroperoxy radical (HO₂) in the atmosphere. It has been suggested (Denis, P. A.; Ornellas, F. R. <i>J. Phys. Chem. A</i>, <b>2009</b>, <i>113</i> (2), 499–506) that the minor product hydrogen tetroxide (HO₄H) may act as a reservoir of HO₂. Here, we compute the thermochemistry of HO₂ self-reactions to determine if either HO₄H or the cyclic hydrogen-bound dimer (HO₂)₂ can act as reservoirs. We computed electronic energies using coupled-cluster calculations in the complete basis set limit, CCSD­(T)/CBS[45]//CCSD­(T)/cc-pVTZ. Our model chemistry includes corrections for vibrational anharmonicity in the zero-point energy and vibrational partition functions, core–valence correlation, scalar relativistic effects, diagonal Born–Oppenheimer, spin–orbit splitting, and higher-order corrections. We compute the Gibbs energy of dimerization to be (−20.1 ± 1.6) kJ/mol at 298.15 K (2σ uncertainty), and (−32.3 ± 1.5) kJ/mol at 220 K. For atmospherically relevant [HO₂] = 10⁸ molecules per cm³, our thermochemistry indicates that dimerization will be negligible, and thus H₂O₄ species are atmospherically unimportant. Under conditions used in laboratory experiments ([HO₂] > 10¹² molecules per cm³, 220 K), H₂O₄ formation may be significant. We compute two absorption spectra that could be used for laboratory detection of HO₄H: the OH stretch overtone (near-IR) and electronic (UV) spectra