Molecular electrostatic potentials and electron densities in nitrocubanes C8H8-α(NO2)α (α=1-8) :: ab initio and density functional study

Charge distributions in nitro derivatives of cubane, C8H8-alpha(NO2)alpha (alpha = 1-8) have been derived employing the ab initio Hartree-Fock and hybrid density functional calculations using the topography of the molecular electrostatic potential (MESP) and the electron density as a tool. The electron-rich regions around the oxygen atoms of the nitro group emerged from the electrostatic potential get smaller with increasing number of nitro groups in the series. Within different isomers of nitrocubane, the lowest energy isomer engender large electron-rich regions relative to the other isomers, which has been supported by the shallow MESP minima near oxygen atoms of nitro group. The electrostatic potential at the center of the cube (cage critical point) on the other hand increases on encompassing from mono- to octa-nitrocubane. Thus it has been inferred that the successive replacement of cubyl hydrogens by NO2 groups pushes the added electrons along a cube with an increase of curvature of the C-C bonds. It has been found that the higher value of the dipole moment for the most destabilized isomer stems from the large electron-rich regions in the electrostatic potential. Further the lowest frequency vibration and energy gap of the highest occupied and the lowest unoccupied molecular orbital in the series of octa-nitrocubane suggest it to be most labile in the series. The electron density at the bond critical point and the curvature of the C-C bonds in a cubanoid increases from mono- to octa-nitrocubane. Heats of formation derived from the isodesmic reaction approach increase with increased destabilization in the nitrocubane isomers. (C) 2004 Elsevier B.V. All rights reserved.