INTERACTING MOLECULAR-ORBITALS AND THE MASS-SPECTRUM OF N-BUTANE

A simple bond index is developed which permits predicting weak bonds i n a radical cation. The model is based on the perturbed first-order wa ve function resulting from a bond elongation in a radical cation. By r eferring to the first-order wave function, the model contains implicit ly the relaxation term of the force constant but also important terms of the anharmonicity constant. The band strength is related to an inte raction between occupied and unoccupied molecular orbitals. The bond s tretch force operator is applied to show that an interaction is bond w eakening provided the occupied and the unoccupied orbital both have a nodal surface in the bond region. When only the occupied orbital is bo nding but the unoccupied orbital is antibonding a weak bond is not ind icated. The theoretical scheme is applied to estimate carbon bond stre ngths in the n-butane radical cation. The bond index indicates that a rupture of the terminal carbon bond is involved in the main fragmentat ion pathway as observed in the mass spectrum. This qualitative result is supported by ab initio Hartree-Fock calculations. They show in addi tion that the preferred fragmentation pathway comprises also a hydroge n shift leading to low-energy fragments.