Master of Science (MS)
Semester of Degree Completion
Carol A. Deakyne
G2 and G2(MP2) theories have been used to study the bond dissociation energies D0m of a series of AB2, AB2+, AB2-, AB, AB+ and AB- systems. A total of fifty-four molecules and ions have been examined, for which B = O, S or Se and A = C, S, Ge, N+, P+, As+, B-, Al- or Ga-. Each triatomic system has sixteen valence electrons and each diatomic system has ten valence electrons. Both triplet and singlet spin states have been considered for each molecule and ion. For the triatomic systems, the preferred connectivity, i.e. B-A-B or B-B-A, has been determined.
It has been observed experimentally that the ratio D0m(AB2)/2D0m(AB) is a nearly constant value of 0.8 for these neutral systems. One question of interest is whether the analogous ratios D0m(AB2+)/2D0m(AB+) and D0m(AB2-)/2D0m(AB-) for the isoelectronic cations and anions are also nearly constant. We have used ab initio molecular orbital calculations to probe this question and to find an explanation for the observed ratios.
The results show that the ground state of each of these diatomic and triatomic molecules and ions is a singlet. The B-A-B connectivity is strongly favored for the ground state species but both B-A-B and B-B-A connectivities are found for the excited state species. Although stable cyclic, bent and linear structures have been located, the structures of the ground state singlet B-A-B molecules and ions are uniformly linear. These observations have been rationalized based on the compositions of the highest occupied and lowest unoccupied molecular orbitals.
The computations reproduce the observed bond dissociation energy ratios for the neutral systems within experimental error. The agreement between experiment and theory for the actual bond dissociation energies is less satisfactory. In addition, the computations show that the ratios for the anionic and cationic systems differ in magnitude from those for the neutral systems (by -0.1 and +0.1, respectively) but are also nearly constant. The bond dissociation energy ratios are nearly constant for these systems because the ratio of the dissociation energy of the A=B bond to that of the A≡B bond is nearly constant.
Xu, Danyu, "Constancy of Bond Dissociation Energy Ratios for AB₂/AB Systems" (1997). Masters Theses. 1781.