Initial steps for several proposed decomposition mechanisms of NTO have been studied theoretically in order to determine the energies of various reaction intermediates. The methods applied were restricted Hartree-Fock self-consistent field (SCF), single-and double-excitation configuration interaction (CISD), CISD corrected for unlinked quadruple excitations (CISD+Q), and coupled cluster including all single and double substitutions (CCSD) with a double-ζ plus polarization (DZP) basis set. Harmonic vibrational frequencies were computed to characterize the species at the stationary points. The previously reported mechanisms examined include decomposition initiated by C-NO 2 homolysis, migration of ring substituents, and ring-opening processes. On the basis of the energetics of the various schemes, our computations suggest that C-NO 2 bond homolysis is the most probable initial step for unimolecular decomposition of NTO.

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Ab initio quantum mechanical methods have been employed to study the mechanisms for the hydrolysis of the pyrophosphate and p-monothiopyrophosphate (MTP) anions at the self-consistent-field (SCF) and secondorder perturbation ( M E ) levels of theory using double zeta plus polarization (DZP) and DZP plus diffuse functions (DZP+diff) basis sets. Metaphosphate is found to be a kinetically important species in the hydrolysis of pyrophosphate for three mechanisms examined in this study ( S N~ dissociation, acid catalysis, and Mg2+ catalysis). The transition state for the unimolecular reaction of hanion to yield a PO3-anion is dissociative, with the reaction coordinate distance as long as 2.91 8, at the DZP SCF level. This dissociation reaction has a gas phase classical barrier of 25 kcal mol-' (DZP MP2) and 26 kcal mol-' (DZP+diff SCF).For the proton-catalyzed hydrolysis of pyrophosphates, the potential energy surface of the pyrophosphate is dissociative if the bridging oxygen atom is protonated. This dissociation thus yields a metaphosphate and an orthophosphoric acid (H3P04). A hypothesis is formulated to explain the catalytic effect of the Mg2+ cation on the hydrolysis of the pyrophosphate. We predict that one of the P-0 bridging bonds is activated in the Mg2+*H2P2G2-complex. Upon hydrolysis, the Mg2+*H2P2G2-complex may isomerize to the HzFQ-*M~Z+*PO~-complex initially; then the H2P04-*Mg2+*P03-complex may be captured by a water molecule to form the H2P04-*Mg2+*H2P04-complex. The classical activation barrier for the isomerization of Mg2+*H2P2072-to H2P04-*Mg2+*P03-is 7.8 kcal mol-' at the DZP MP2 level. The H2P04-*Mg2+*P03-complex lies 8.9 kcal mol-' (DZP SCF) lower in energy than the Mg2+*H2P2072-complex. Therefore, the isomerization of Mg2+*H2P2072-to H2P04-*Mg2+*P03-is both thermochemically and kinetically feasible.

6) Baumeister, E.; Oberhammer, H.; Schmidt, H.; Steudel, R. (20) Schwarz, W. H. E.; Mewching, L.; Valtazanos, P.; Von Niessen, W. (21) Frisch, M., Ed. GAUSSIAN86, User's Guide; Carnegie-Mellon (22) Donohue, J.; Schomaker, V.Ab initio molecular electronic structure theory has been applied in an investigation of the oxywater-hydrogen peroxide isomerization. Oxywater, hydrogen peroxide, and the transition state connecting them have been located using the selfconsistent-field (SCF), configuration interaction including all single and double excitations (CISD), and coupled cluster with single and double excitations (CCSD) methods with several basis sets, the largest being of triple-zeta plus double-polarization (including f functions on the oxygen atoms) quality (TZ2P+O. Harmonic vibrational frequencies have been evaluated at the SCF, CISD, and CCSD levels of theory and the stationary points characterized as minima or transition states. The CCSD method with connected triple excitations [CCSD(T)] also has been used to obtain oxywater's equilibrium geometry and frequencies as well as to compute singlepoint energies of all CCSD-optirmzed structures. A classical barrier to isomerization of 5.7 kcal mol-' has been predicted at the highest level of theory. After correction for zero-point vibrational energies, the comparable ground-state activation energy is 3.2 kcal mol-'. Although these ab initio predictions could be decreased by 1 or 2 kcal mol-' at yet higher levels of theory, there can be little doubt that oxywater is a genuine minimum on the H202 potential energy hypersurface.

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