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Notes: In the propagation step of the cationic polymerization, oxetane reacts with the protonated oxetane formed in the initiation step, with concomitant ring opening of the protonated oxetane. To describe properly bond making or bond breaking, it is necessary to use MC-SCF or CI calculations. We have carried out ab initio MODPOT/VRDDO MRD-CI calculations (by the multireference double excitation-configuration interaction technique of Buenker and Peyerimhoff into which we have also meshed a number of desirable computational options for ab initio calculations on large molecules). The CI calculations were carried out on strictly orthogonalized localized occupied and virtual orbitals in the reaction region, with the remainder of the occupied molecular orbitals being folded into an effective CI Hamiltonian. The calculated potential energy surfaces indicate that a preferred pathway for this reaction resembles an SN2 reaction with the oxygen of the oxetane attacking linearly along the C4—O direction of the protonated oxetane and inversion of the hydrogens around the C4 atom.

Notes: Recently we extended our strategy for MRD-CI (multireference double excitation-configuration interaction) calculations, based on localized/local orbitals and an “effective” CI Hamiltonian, for molecular decompositions of large molecules to breaking a chemical bond in a molecule in a crystalline or other solid environment.Our technique begins with an explicit quantum chemical SCF calculation for a reference molecule surrounded by a number of other molecules in the multipole environment of more distant neighbors. The resulting canonical molecular orbitals are then localized, and the localized occupied and virtual orbitals in the region of interest are included explicitly in the MRD-CI with the remainder of the occupied localized orbitals being folded into an “effective” CI Hamiltonian. The MRD-CI calculations are then carried out for breaking a bond in the reference molecule. This method is completely general in that the space treated explicitly, as well as the surrounding space, may contain voids, defects, deformations, dislocations, impurities, dopants, edges and surfaces, boundaries, etc.Dimethylnitramine is the smallest prototype of the energetic R2N - NO2 nitramines, such as the 6-member ring RDX or the 8-member ring HMX. Decomposition of energetic compounds is initiated in the solid by a breaking of the target bond. Thus, it is crucial to know the difference in energy between breaking a bond in an isolated energetic molecule versus in the molecule in a solid. In the present study, we have carried out MRD-CI calculations for the Me2N - NO2 dissociation of dimethylnitramine in a dimethylnitramine crystal. The cases we investigated were one dimethylnitramine molecule (surrounded by 53 and 685 neighboring dimethylnitramine molecules represented by multipoles), three dimethylnitramine molecules, and three dimethylnitramine molecules (surrounded by 683 neighbors). All multipoles were cumulative atomic multipoles up through quadrupoles. The MRD-CI calculations on dimethylnitramine required large numbers of reference configurations from which were allowed all single and double excitations.

Notes: Recently we derived, implemented, tested, and used successfully a new computational strategy for ab-initio MRD-CI (multireference double excitation - configuration interaction) calculations for molecular decompositions of large molecules and intermolecular reactions of large systems. We carry out the ab-initio SCF for the entire system, then transform the canonical delocalized molecular orbitals to localized orbitals and include explicitly in the MRD-CI only the localized occupied and virtual orbitals in the region of interest, folding the remainder of the occupied localized orbitals into an “effective” CI Hamiltonian. The advantage is that the transformations from integrals over atomic orbitals to integrals over molecular orbitals (the computer time-, computer core-, and external storage- consuming part of the CI calculations) only have to be carried out for the localized orbitals included explicitly in the MRD-CI calculations. The challenge arose to extend our MRD-CI technique based on localized/local orbitals and “effective” CI Hamiltonian to the breaking of a chemical bond in a molecule in a crystal (or other solid environment). This past year we have derived, implemented, and used successfully a procedure for doing this. Our technique involves solving a quantum chemical ab-initio SCF explicitly for a system of a reference molecule surrounded by a number of other molecules in the multipole environment of yet more further out surrounding molecules. The resulting canonical molecular orbitals are then localized and the localized occupied and virtual orbitals in the region of interest are included explicitly in the MRD-CI with the remainder of the occupied localized orbitals being folded into an “effective” CI Hamiltonian. The MRD-CI calculations are then carried out for breaking a bond in the reference molecule. This method is completely general. The space treated explicitly quantum chemically and the surrounding space can have defects, deformations, dislocations, impurities, dopants, edges, and surfaces, boundaries, etc. We have applied this procedure successfully to the H3C - NO2 bond dissociation of nitromethane with extensive testing of the number of molecules that have to be included explicitly in the SCF and how many further out molecules have to be represented by multipoles. To check the goodness of the model cluster approximation for crystalline nitromethane, we carried out ab-initio crystal orbital (XTLORB) calculations using our POLY-CRYST program. The difference in the XTLORB total energies between the 4 nitromethane molecules/unit cell and the 3 nitromethane molecules/unit cell (Table VIII), ER = E4 - E3 = -48.0609079 a.u., corresponds very closely to the reduced energy per nitromethane molecule, ER = (-;48.0605)9 a.u., calculated from explicit SCF calculations on the model nitromethane cluster in the multipole field of farther out nitromethane molecules for the model cluster. Thus, the multipole approximation for describing the effect of further out molecules on the SCF cluster energies is quite good.

Notes: Recently we extended our strategy for MRD-CI (multireference double excitation-configuration interaction) calculations based on localized/local orbitals and an “effective” CI Hamiltonian for molecular decompositions of large molecules to breaking a chemical bond in a molecule in a crystal or other solid environment. Our technique involves solving a quantum chemical ab-initio SCF explicitly for a system of a reference molecule surrounded by a number of other molecules in the multipole environment of more distant neighbors. The resulting canonical molecular orbitals are then localized and the localized occupied and virtual orbitals in the region of interest are included explicitly in the MRD-CI with the remainder of the occupied localized orbitals being folded into an “effective” CI Hamiltonian. The MRD-CI calculations are carried out for breaking a bond in the reference molecule. This method is completely general. The space treated explicitly quantum chemically and the surrounding space can have voids, defects, deformations, dislocations, impurities, dopants, edges and surfaces, boundaries, etc. We previously applied this procedure successfully to the H3C—NO2 bond dissociation of nitromethane in a nitromethane crystal with extensive testing of the number of molecules that have to be included explicitly in the SCF and how many molecules have to be represented by more distant multipoles. The results indicated that it took more energy to dissociate the H3C—NO2 bond when the nitromethane molecule was in the crystal than it did to dissociate that bond in the free nitromethane molecule. In this present study we have investigated the effect of voids (both in the nitromethane molecules treated explicitly in the SCF and those in the environment represented by multipoles) on the calculated H3C—NO2 bond dissociation energies.

Notes: Compact representation of molecular charge distribution in molecular crystals has been derived from ab-initio crystal orbital wavefunction within the framework of cumulative atomic multipole moment (CAMM) expansion. Results for HF, CO2, and cubane C8H8 crystals have been compared with values calculated for corresponding clusters within conventional LCAO MO SCF approach. CAMM technique has also been used to represent molecular charge distribution in electronic excited states using CI wavefunction obtained from multireference configuration interaction (MRD-CI) calculations. This approach supplements previously introduced uncorrelated and correlated CAMM's and allows accurate nonempirical modeling of electrostatic effects involving molecules in excited states or in crystalline environment. Improved point charge model derived from CAMM has been also described.