Singlet state density functional calculations

The relative stabilities of singlet and triplet electronic states of three different oxyallyl systems and the closed form of cyclopropanone, bicyclo[1.1.0]butanone, and bicyclo[2.1.0]pentan-5-one have been smdied by density-functional calculations (B3LYP/6-31G ). The results of these calculations are in good agreement with predictions of calculations based on multi-determinant methods. 

Recently, stereospecific addition of bis(triisopropylsilyl)srlylene (i-Pr3Si)2Si to cis-2-butene has been observed. Since a triplet ground state has been predicted on the basis of density functional calculations, stereospecific addition could be due to a violation of the Skell rule. Alternatively, the Skell rule could be obeyed if the triplet ground state is siphoned off via more rapid reactions of the low-lying excited singlet (which, according to the calculations, lies only 1.7 kcalmoH above the triplet ) that may be in equilibrium with the triplet. 

Ah initio methods are applicable to the widest variety of property calculations. Many typical organic molecules can now be modeled with ah initio methods, such as Flartree-Fock, density functional theory, and Moller Plesset perturbation theory. Organic molecule calculations are made easier by the fact that most organic molecules have singlet spin ground states. Organics are the systems for which sophisticated properties, such as NMR chemical shifts and nonlinear optical properties, can be calculated most accurately. 

The possibility of N2 coordination to up to four (six) iron atoms has been proposed by Dance on the basis of restricted frozen-core Kohn-Sham calculations on a FeMoco model (32,33). It was found that a binding mode intermediate between p Vn and p4,r 2 coordination to be most stable. However, these propositions are not necessarily the final answer since open-shell states are most likely to become important and exact exchange was not present in the density functional chosen to cure the singlet preference of the pure density functional (cf. discussion in the Appendix). 

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