Ab initio MRD–CI calculations on cubane (neutral, carbocation, carboanion) and dissociation of nitrocubanes based on localized orbitals

Abstract

AbstractAb initio MRD–CI calculations based on localized orbitals were carried out for cubane (neutral, carbocation, carboanion) both in our customary MODPOT basis set and in an all‐electron 4–31G basis set. The calculated MRD–CI charge distributions on C1 (the skeletal atom from which the H− or H+ was removed) (ab initio MODPOT neutral 4.221, carbocation 3.796, carboanion 4.282; all‐electron 4–31G neutral 6.171, carbocation 5.717, carboanion 6.078) indicate that the + or ‐ charge does not remain localized on C1 but redistributes itself. This has significant implications for preparative reactions of energetically substituted cubanes. The MRD–CI population analyses differ somewhat from the SCF population analyses, especially in the calculated total overlap populations. To investigate this effect on electrostatic molecular potential contour (EMPC) maps generated from SCF or MRD–CI wave functions, we wrote additional routines to calculate EMPC maps from MRD–CI wave functions. The EMPC maps generated from SCF or MRD–CI wave functions are different if the molecule needs an MRD–CI multideterminant wave function to describe it adequately. The EMPC map is a one‐electron property. One‐electron properties are derived from the 1‐matrix. The 1‐matrix is different for SCF or MRD–CI wave functions. Thus, all the one‐electron properties (EMPC maps, population analyses, bond deviation indices, etc.) are different when calculated from SCF or MRD–CI wave functions if MRD–CI wave functions are necessary to describe a system properly. We calculate these one‐electron properties from the 1‐matrix from the final natural orbitals. Our preliminary calculations for the dissociation pathway indicate it takes more energy to dissociate a bond in 1‐nitrocubane than in octanitrocubane. Even in their ground electronic states at equilibrium geometry, both 1‐nitrocubane and octanitrocubane require MRD–CI wave functions to describe them properly. The c2 of the single determinant SCF wave function is only 0.8401 for 1‐nitrocubane and 0.8300 for octanitrocubane. There are contributions from skeletal excitations as there are for cubane itself as well as excitations involving the nitrogroup. As the bond in nitrocubane is dissociated to 8.00 bohrs, the c2 of the SCF contribution drops to only 0.4606 (1‐nitrocubane) and 0.4445 (octanitrocubane). At this C1N1 intermolecular distance, the largest excitations are in the C1N1 bond: (C1N1)2 → (C1N1*)2, (C1N1) → (C1N1*). We also calculated the first electronically excited state for the dissociation pathway for selected points for both 1‐nitrocubane and octanitrocubane.