As a prototype of the SiOOOSi bonding region for silica modeling, especially the highly flexible SiOOOSi angle deformation, we studied the structure of disiloxane (H 3 SiOOOSiH 3 ), with ab initio calculations on the SCF, MBPT (2), CCSD, and CCSD(T) levels of theory. The convergence of the results is studied for a series of basis sets of increasing quality. Large basis sets including f functions are necessary to obtain reliable results for the structure and the barrier to linearization of the molecule. The following structure and energy parameters are the results of CCSD(T)-fc/cc-pVTZ calculations: SiOO distance is 1.645Å, the SiOOOSi angle 145.3°, and the barrier to linearization 0.48 kcal/mol.

A previously unreported channel in the spin-allowed reaction path for the CH+N2 reaction that involves the HNNC radical is presented. The structures and energetics of the HNNC radical and its isomers HCNN and HNCN and the relevant intermediates and transition states that are involved in the proposed mechanism are obtained at the coupled cluster singles and doubles level of theory with noniterative triples correction (CCSD(T)) using a converging series of basis sets aug-cc-pVDZ, aug-cc-pVTZ, and aug-cc-pVQZ. The aug-cc-pVQZ basis is used for all the final single point energy calculations using the CCSD(T)/aug-cc-pVTZ optimized geometries. We find the HNNC radical to have a heat of formation of DeltafH0 (HNNC)=116.5 kcal mol(-1). An assessment of the quality of computed data of the radical species HNCN and HCNN is presented by comparison with the available experimental data. We find that HNNC can convert to HNCN, the highest barrier in this path being 14.5 kcal mol(-1) above the energy of the CH+N2 reactants. Thus, HNNC can play a role in the high-temperature spin-allowed mechanism for the reaction of CH+N2 proposed by Moskaleva, Xia, and Lin (Chem. Phys. Lett. 2000, 331, 269).

In the potential solution observation of the long-sought-after pentazole anion (N(5)(-)), the principal experimental tool used for detection is NMR. However, in two experiments, very different conclusions were reached. To assist in the interpretation, we report predictive level coupled-cluster results for the spin-spin coupling constants and chemical shifts for all of the key species, which include NO(3)(-), N(5)(-), HN(5), N(3)(-), and MeOC(6)H(5)N(3). In the case of the shifts, an empirical estimate based on the molecule polarity enables comparison of gas-phase and observed values with expected error bars of approximately +/-10 ppm. For the scalar couplings, the evidence is that the solution effects are modest, enabling the gas-phase values (with error bars are approximately +/-5 Hz) to be accurate. The latter supports the observation of centrally (15)N labeled N(3)(-) in the cerium(IV) ammonium nitrate (CAN) solution which could only occur if the pentazole anion had been created in the experiment, yet with too short a lifetime to be observed in NMR.