The effect of nonorthogonality in the broken symmetry approach to magnetic coupling has been explicitly considered for the first time in Hartree−Fock and a variety of DFT methods. On the basis of the results for three different systems, representative of a variety of physical situations it is shown that the most often quoted trend concerning the much larger degree of delocalization of magnetic orbitals obtained from DFT, as opposed to Hartree−Fock, is not fully justified. A new and simple way to relate the overlap integral entering into the calculation and the spin density is proposed and tested in a variety of model systems.

No abstract

Ab initio calculations including correlation energy are reported on a series of halogen-substituted carbenes and silylenes: CH 2, CHF, CF 2, CHC1, CC12, CHBr, CBr2, Sill2, SiHF, SiF2, using the difference-dedicated configuration interaction method. All these compounds with the exception of CH 2 have singlet ground states. The singlet-triplet energy differences are in very good agreement with experiment when available, as well as with other high level calculations. The singlet-triplet separation is also determined in trifluoromethyl carbenes, CHCF3, CFCF3, CCICF 3 and CBrCF 3. The CF 3 substituent has little influence on the energy gap since CHCF 3 like CH 2 has a triplet ground state, with a gap of 11"1 kcal mo1-1, and CFCF~, CCICF 3 and CBrCF 3 have singlet ground states, like CHF, CHCI and CHBr, with gaps of -17.6, -5"4, and -3"5 kcal mo1-1.

No abstract

The exchange coupling of Ullman's nitroxide biradicals, bis[2,2'-( 1-oxy-3-oxido-4,4,5,5-tetramethyldihydro-lH-imidazolyl)] (1) and bis[2,2'-( l-oxy-4,4,5,5-tetramethyldihydro-1H-imidazolyl)] (2), has been determined by performing ab initio calculations followed by a specific configuration interaction on model compounds where the methyl groups have been substituted by hydrogen atoms, C1 and C2, respectively. The analysis of the dependence of the coupling on the conformation angle between each nitroxide moiety shows that even in the perpendicular structure the singlet state is found below the triplet. The calculated singlet-triplet gaps for the experimental value of the twist angle, -364 cm-l for C1 and -175 cm-' for C2, are in very good agreement with recent experimental results for 1 and 2.