The electronic and vibrational structures of C,, and C,,, have been calculated at the PM3 semiempirical level. C, has a partially delocalized structure, while C,, has both a localized set and a delocalized set of MOs. As with AM1 and MNDO, PM3 predicts the heat of formation of C,, to be greater than that of C,,,, and that C,,, is the thermodynamically more stable species. Calculation of the normal modes was accelerated over 40 times by limited use of symmetry theory.

No abstract

The deuterium isotope effect was applied to condensed-phase thermochemical reactions of HMX and HMX-

MIND0/3, MNDO, AM1, and PM3 calculations of molecular vibrational frequencies are reported for 61 molecules. All techniques were applied to both well-behaved and badly behaved systems. Overall, MIND0/3 and MNDO were found to contain rather large errors whereas AM1 and PM3 were relatively accurate. Since no technique does well for all molecules, the technique used should be chosen based on the molecular vibration of interest. in general, AM1 and PM3 together provide fairly accurate resuits.

The structures and thermodynamic stabilities of X(YH4)3, X and/or Y being boron or aluminum, were calculated using ab initio molecular orbital methods. The structures were determined at the Hartree-Fock level using the 6-31G* basis set and confirmed to be minima by vibrational analysis. Correlation effects were estimated using MP4(SDTQ) calculations at the Hartree-Fock minima. The correlated energies were used to estimate AH0 and AG°for the decomposition of these dodecahydrides into various products. No Hartree-Fock minimum could be found for B(A1H4)3. The symmetries of the other structures were found to be rotated prisms. Only A1(BH4)3, which is known experimentally, was stable with respect to all the decomposition modes studied. Comparison of the calculated vibrational spectra of these dodecahydrides with those of analogous covalently bonded molecules and ionically bonded molecules suggests that B(BH4)3, A1(BH4)3, and A1(A1H4)3 are all primarily covalently bonded.