EXAMINATION OF THE HUND RULE IN CLOSED-SHELL SYSTEMS - INVESTIGATION OF SPIN CORRELATION-EFFECTS

The validity of the Hund rule in atomic orbitals (AOs) of the carbon a toms inside closed-shell molecules, such as acetylene, ethylene, and e thane, is examined. Electron-pair populations and contributions of the two-electron covalent structures with parallel (up-up) and antiparall el (up-or-down) spins are calculated by multielectron population analy sis of MO + CI wave functions. Such an analysis, which allows the visu alization of various cooperative electronic effects in some target Aos , is extended on the basis of (strictly orthogonal) hybrid orbitals. A lthough the HF level shows, incorrectly, that the Hund rule is not sat isfied, the CI clearly shows a preference for (up-up) spins to those o f (up-or-down): This holds for both the electron-pair populations [tho se of (up-or-down) spins diminish with the CI more drastically than th ose of (up-up) spins], as well as for contributions of the two-electro n covalent structures [those of (up-up) spins increase with the CI mor e drastically than do those of (up-or-down) spins]. The calculation of the correlation functions (or dependent functions) in AO space allows the comparison of repulsive or ''attractive'' behavior of (up-up) and (up-or-down) spins in various AO couples. Mutual dependence of the tw o electrons inside the sigma system increases in the series ethane < e thylene < acetylene. Also found is that parallel spins in (pure) AOS o f sigma systems are preferred to the antiparallel spins when going fro m ethane to acetylene. The preference parallel - antiparallel spins in AOs belonging to two different atoms, including hydrogens, is also ex amined.