Thermal Decomposition of Gas-Phase Bis(1,2,4-oxadiazole)bis(methylene) Dinitrate (BODN): A CCSD(T)-F12/DFT-Based Study of Reaction Pathways.

Electronic structure methods based on density functional theory and coupled-cluster theory were employed to characterize elementary steps for the gas-phase thermal decomposition of bis(1,2,4-oxadiazole)bis(methylene) dinitrate (BODN). As typically found for nitrate ester-functionalized compounds, NO2 and HONO eliminations were the most energetically favorable unimolecular paths for the parent molecule's decomposition. From there, sequences of unimolecular reactions for daughters of the initiation steps were postulated and characterized. For intermediates found to have barriers to unimolecular decomposition that would make their rate at the temperatures and time scales of interest negligible, their decomposition via H-atom abstraction and radical-addition reactions was characterized. Creating a comprehensive network that can be employed to develop a detailed finite-rate chemical kinetics mechanism for simulating BODN's decomposition, the results provide a basis for modeling BODN's combustion, as well as its response to thermal loads germane to its aging, storage, and handling.

Correlated Molecular Orbital Theory Study of the Al + CO2 Reaction.

Density functional theory (DFT) and correlated molecular orbital electronic structure calculations were used to study the Al + CO2 → AlO + CO reaction on the electronic ground-state potential-energy surface (PES). Geometries were optimized using DFT (M11/jun-cc-pV(Q+d)Z) and more accurate energies were obtained using the composite Weizmann-1 theory with Brueckner doubles (W1BD). The results comprise the most complete, most systematic characterization of the Al + CO2 reaction surface to date and are based on consistent application of high-level methods for all stationary points identified. The pathways from Al + CO2 to AlO + CO on the electronic ground-state PES all involve formation of one or more stable AlCO2 complexes denoted η-AlCO2, trans-AlCO2, and C2v-AlCO2, among which η-AlCO2 and C2v-AlCO2 are the least and most stable, respectively. We report a new minimum-energy pathway for the overall reaction, namely formation of η-AlCO2 from reactants and dissociation of that same complex to products via a bond-insertion reaction that passes through a fourth (weakly metastable) AlCO2 complex denoted cis-OAlCO. Natural Bond Orbital analysis was applied to study trends in charge distribution and the degree of charge transfer in key structures along the minimum-energy pathway. The process of aluminum insertion into CO2 is discussed in the context of analogous processes for boron and first-row transition metals.

MRMP2, CCSD(T), and DFT Calculations of the Isomerization Barriers for the Disrotatory and Conrotatory Isomerizations of 3-Aza-3-ium-dihydrobenzvalene, 3,4-Diaza-3-ium-dihydrobenzvalene, and 3,4-Diaza-diium-dihydrobenzvalene.

The isomerizations of 3-aza-3-ium-dihydrobenzvalene, 3,4-diaza-3-ium-dihydrobenzvalene, and 3,4-diaza-diium-dihydrobenzvalene to their respective cyclic-diene products were studied using electronic structure methods with a multiconfigurational wave function and several single reference methods. Transition states for both the allowed (conrotatory) and forbidden (disrotatory) pathways were located. The conrotatory pathways of each structure all proceed through a cyclic intermediate with a trans double bond in the ring: this trans double bond destroys the aromatic stabilization of the π electrons due to poor orbital overlap between the cis and trans π bonds. The 3,4-diaza-3-ium-dihydrobenzvalene structure has C1 symmetry, and there are four separate allowed and forbidden pathways for this structure. The 3-aza-3-ium-dihydrobenzvalene structure is Cs symmetric, and there are two separate allowed and forbidden pathways for this structure. For 3,4-diaza-3,4-diium-dihydrobenzvalene, there was a single allowed and single forbidden pathway due to the C2v symmetry. The separation of the barrier heights for all three molecules was studied, and we found the difference in activation barriers for the lowest allowed and lowest forbidden pathways in 3,4-diaza-3-ium-dihydrobenzvalene, 3-aza-3-ium-dihydrobenzvalene, and 3,4-diaza-diium-dihydrobenzvalene to be 9.1, 7.4, and 3.7 kcal/mol, respectively. The allowed and forbidden barriers of 3,4-diaza-diium-dihydrobenzvalene were separated by 3.7 kcal/mol, which is considerably less than the 12-15 kcal/mol expected based on the orbital symmetry rules. The addition of the secondary ammonium group tends to shift the conrotatory and disrotatory barriers up in energy (∼12-14 kcal/mol (conrotatory) and 5-10 kcal/mol (disrotatory) per secondary NH2 group) relative to the barriers of dihydrobenzvalene, but there is negligible effect on E,Z to Z,Z isomerization barriers, which remain in the expected range of greater than 4 kcal/mol.

Isomerization barriers for the disrotatory and conrotatory isomerizations of 3-aza-benzvalene and 3,4-diaza-benzvalene to pyridine and pyridazine.

The isomerizations of 3-aza-benzvalene to pyridine and 3,4-diaza-benzvalene to pyridazine have been studied using ab initio methods with a multiconfigurational wavefunction. Transition states for both the allowed disrotatory and forbidden conrotatory pathways were located. The forbidden pathways proceed through an intermediate consisting of pyridine or pyridazine with a trans double bond in the ring: this trans double bond destroys the aromatic stabilization of the π electrons due to poor orbital overlap between the cis and trans π bonds. Due to the Cs molecular point group, there are two separate allowed and forbidden pathways for 3-aza-benzvalene. The separation of the barrier heights was of particular interest: the difference in activation barriers for the lowest allowed and lowest forbidden pathways in 3-aza-benzvalene was only 1.3 kcal mol(-1), and the lowest forbidden pathway actually had a 1.5 kcal mol(-1) lower barrier than the highest allowed one. The 3-aza-benzvalene structure allows energy crossing of the allowed and forbidden barriers. For 3,4-diaza-benzvalene, there was only a single allowed and single forbidden pathway, due to the C2v point group, and they were separated by 8.4 kcal mol(-1), more in line with the orbital symmetry rules.

Isomerization barriers and strain energies of selected dihydropyridines and pyrans with trans double bonds.

The stability of cis,trans-dihydropyridines and cis,trans-pyrans has been studied using ab initio methods. The strain introduced by the trans double bond has been determined relative to the cis,cis-isomers and introduces 58-69 kcal x mol(-1) of strain energy, at the G3 level of theory, depending on the particular isomer. Double bond rotation barriers have been calculated at the MRMP2/MCSCF level and range from 2.2 kcal x mol(-1) to 11.0 kcal x mol(-1), significantly lower than butadiene (50.3 kcal x mol(-1)). Evidence of resonance through delocalization of the pi electrons is present for the conjugated double bond isomers which lowers the activation barriers. The transition states for trans double bond rotation have significant biradical character but markedly less than that for butadiene. The early transition states with H-C=C-H dihedral angles of 130-150 degrees, as opposed to 90 degrees for butadiene, are consistent with the reduction in the natural orbital occupation numbers. We could not locate a minimum for a structure having both double bonds in the trans configuration and so report that one trans bond is the most the rings can accommodate.