Bond-Stretch Isomerism in Tetrasilabicyclo[1.1.0]butane Derivatives

Bond-stretch isomerism is predicted not to occur in the parent compound Si4H6 and the substituted 1,3-di-tert-butyl derivative, although the existence of bond-stretch isomerism in the 1,3-dimethyl derivative is a possibility. Bulky bridgehead moieties induce a preference for a short Si−Si bridge bond, while a longer bridge bond length (2.840 Å at the MP2/6-31G(d,p) level) is predicted for the unsubstituted molecule

Cation−Cation π−π Stacking in Small Ionic Clusters of 1,2,4-Triazolium

The existence of cation−cation π−π stacking in the 1,2,4-triazolium−dinitramide tetramer and 1,2,4-triazolium−chloride tetramer (two cations and two anions) is predicted based on the structures optimized using second-order perturbation theory (MP2). In the most stable tetramer structure of 1,2,4-triazolium−dinitramide, π−π stacking is formed with an interplane distance of ∼3.2 Å and a parallel displacement of ∼1.4 Å. In the most stable tetramer structure of 1,2,4-triazolium−chloride, π−π stacking is formed with an interplane distance of ∼2.9 Å and a parallel displacement of ∼1.0 Å

Cubic Fuels?

The cubic molecules C4O4, N8, and Si8R8 are considered as possible high-energy propellants. The potential energy surface minimum for C4O4 is found to be incapable of supporting any bound vibrational levels. N8\u27s decomposition to 4 N2 is mapped in detail, but the initial barrier of its rather complicated dissociation is too modest to allow hope for its handling in bulk quantity. Si8H8 is found to be the most promising additive among the four R\u27s considered, but simulation of its combustion in an LH2/LOX rocket gives only a 6-s specific impulse enhancement

Electronic Structure Studies of Tetrazolium-Based Ionic Liquids

New energetic ionic liquids are investigated as potential high energy density materials. Ionic liquids are composed of large, charge-diffuse cations, coupled with various (usually oxygen containing) anions. In this work, calculations have been performed on the tetrazolium cation with a variety of substituents. Density functional theory (DFT) with the B3LYP functional, using the 6-311G(d,p) basis set was used to optimize geometries. Improved treatment of dynamic electron correlation was obtained using second-order perturbation theory (MP2). Heats of formation of the cation with different substituent groups were calculated using isodesmic reactions and Gaussian-2 calculations on the reactants. The cation was paired with oxygen rich anions ClO4-, NO3-, or N(NO2)2- and those structures were optimized using both DFT and MP2. The reaction pathway for proton transfer from the cation to the anion was investigated

Triazolium-Based Energetic Ionic Liquids

The energetic ionic liquids formed by the 1,2,4-triazolium cation family and dinitramide anion are investigated by ab initio quantum chemistry calculations, to address the following questions:  How does substitution at the triazolium ring\u27s nitrogen atoms affect its heat of formation, and its charge delocalization? What kind of ion dimer structures might exist? And, do deprotonation reactions occur, as a possible first step in the decomposition of these materials

Pentazole-Based Energetic Ionic Liquids: A Computational Study

The structures of protonated pentazole cations (RN5H+), oxygen-containing anions such as N(NO2)2-, NO3-, and ClO4- and the corresponding ion pairs are investigated by ab initio quantum chemistry calculations. The stability of the pentazole cation is explored by examining the decomposition pathways of several monosubstituted cations (RN5H+) to yield N2 and the corresponding azidinium cation. The heats of formation of these cations, which are based on isodesmic (bond-type conserving) reactions, are calculated. The proton-transfer reaction from the cation to the anion is investigated

Accurate Methods for Large Molecular Systems

Three exciting new methods that address the accurate prediction of processes and properties of large molecular systems are discussed. The systematic fragmentation method (SFM) and the fragment molecular orbital (FMO) method both decompose a large molecular system (e.g., protein, liquid, zeolite) into small subunits (fragments) in very different ways that are designed to both retain the high accuracy of the chosen quantum mechanical level of theory while greatly reducing the demands on computational time and resources. Each of these methods is inherently scalable and is therefore eminently capable of taking advantage of massively parallel computer hardware while retaining the accuracy of the corresponding electronic structure method from which it is derived. The effective fragment potential (EFP) method is a sophisticated approach for the prediction of nonbonded and intermolecular interactions. Therefore, the EFP method provides a way to further reduce the computational effort while retaining accuracy by treating the far-field interactions in place of the full electronic structure method. The performance of the methods is demonstrated using applications to several systems, including benzene dimer, small organic species, pieces of the α helix, water, and ionic liquids

Structure and Thermodynamics of Carbon and Carbon/Silicon Precursors to Nanostructures

The structures at the Hartree−Fock level, as well as the energetics, are reported for the unsaturated system C36H16, its Si-doped analogue C32Si4H16, and several smaller, unsaturated fragments. Structural effects on the electronic distribution are discussed in terms of a localized orbital energy decomposition. The standard heats of formation are calculated based on homodesmic and isodesmic reactions and the G2(MP2,SVP) method with a valence double-ζ plus polarization basis. The origin of the observed explosion of the all-carbon system (C36H16) to form carbon nanotubes was investigated by exploring a possible initial reactive channel (dimerization), which could lead to the formation of the observed onion-type nanostructures.Reprinted (adapted) with permission from Journal of the American Chemical Society 124 (2002): 6144, doi:10.1021/ja012301u. Copyright 2002 American Chemical Society.</p