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Examination of the Hund rule in closed‐shell systems: Investigation of spin correlation effects
Author(s) -
Parrondo Ramón Martín,
Karafiloglou Padeleimon,
Marcos Enrique Śnchez
Publication year - 1993
Publication title -
international journal of quantum chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.484
H-Index - 105
eISSN - 1097-461X
pISSN - 0020-7608
DOI - 10.1002/qua.560470304
Subject(s) - spins , atomic orbital , acetylene , chemistry , molecular orbital , electronic correlation , antiparallel (mathematics) , electron , open shell , atomic physics , physics , condensed matter physics , quantum mechanics , molecule , magnetic field , organic chemistry
The validity of the Hund rule in atomic orbitals ( AO s) of the carbon atoms inside closed‐shell molecules, such as acetylene, ethylene, and ethane, is examined. Electron‐pair populations and contributions of the two‐electron covalent structures with parallel (⇈) and antiparallel (↑↓) spins are calculated by multielectron population analysis of MO + CI wave functions. Such an analysis, which allows the visualization of various cooperative electronic effects in some target AO s, is extended on the basis of (strictly orthogonal) hybrid orbitals. Although the HF level shows, incorrectly, that the Hund rule is not satisfied, the CI clearly shows a preference for (⇈) spins to those of (↑↓): This holds for both the electron‐pair populations [those of (↑↓) spins diminish with the CI more drastically than those of (⇈) spins], as well as for contributions of the two‐electron covalent structures [those of (⇈) spins increase with the CI more drastically than do those of (⇈) spins]. The calculation of the correlation functions (or dependent functions) in AO space allows the comparison of repulsive or “attractive” behaviour of (⇈) and (↑↓) spins in various AO couples. Mutual dependence of the two electrons inside the sigma system increases in the series ethane < ethylene < acetylene. Also found is that parallel spins in (pure) AO s of sigma systems are preferred to the antiparallel spins when going from ethane to acetylene. The preference parallel–antiparallel spins in AO s belonging to two different atoms, including hydrogens, is also examined. © 1993 John Wiley & Sons, Inc.