Showing papers by "Ernest R. Davidson published in 2005"
TL;DR: In this article, the extent of spin polarization in unrestricted Hartree-Fock and density functional B3LYP calculations has been determined for a few monoradicals as well as the corresponding diradicals containing these monorads as functional groups.
Abstract: The extent of spin polarization in unrestricted Hartree–Fock and density functional B3LYP calculations has been determined for a few monoradicals as well as the corresponding diradicals containing these monoradicals as functional groups. Simple relations are found that explain the very high value for the average spin after projecting out the triplet component from a broken-spin calculation on the singlet state. The implications of these results for estimating the energy of the low-spin diradical by spin projection or by the Heisenberg Hamiltonian are discussed. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2005
45 citations
TL;DR: Osmium tetroxide is reduced by molecular hydrogen in the presence of ligands in both polar and nonpolar solvents and the observation that ligands also accelerate H2 addition to OsO4(L)n highlights the analogy between these two reactions.
Abstract: Osmium tetroxide is reduced by molecular hydrogen in the presence of ligands in both polar and nonpolar solvents. In CHCl3 containing pyridine (py) or 1,10-phenanthroline (phen), OsO4 is reduced by H2 to the known Os(VI) dimers L2Os(O)2(μ-O)2Os(O)2L2 (L2 = py2, phen). However, in the absence of ligands in CHCl3 and other nonpolar solvents, OsO4 is unreactive toward H2 over a week at ambient temperatures. In basic aqueous media, H2 reduces OsO4(OH)nn- (n = 0, 1, 2) to the isolable Os(VI) complex, OsO2(OH)42-, at rates close to that found in py/CHCl3. Depending on the pH, the aqueous reactions are exergonic by ΔG = −20 to −27 kcal mol-1, based on electrochemical data. The second-order rate constants for the aqueous reactions are larger as the number of coordinated hydroxide ligands increases, kOsO4 = 1.6(2) × 10-2 M-1 s-1 < kOsO4(OH)- = 3.8(4) × 10-2 M-1 s-1 < kOsO4(OH)22- = 3.8(4) × 10-1 M-1 s-1. The observation of primary deuterium kinetic isotope effects, kH2/kD2 = 3.1(3) for OsO4 and 3.6(4) for OsO4(OH)...
33 citations
TL;DR: Electronic structure calculations on fragmentation of anti-cis-2,3-dimethyl-P-mesitylphosphirane ( cis-1Me) to triplet mesitylph phosphinidene (MesP) indicate that this reaction should be more stereospecific, in agreement with the experimental results of Li and Gaspar.
Abstract: CASSCF, CASPT2, CCSD(T), and (U)B3LYP electronic structure calculations have been performed in order to investigate the thermal fragmentation of P-phenylphosphirane (1) to phenylphosphinidene (PhP) and ethylene. The calculations show that generation of triplet PhP via a stepwise pathway is 21 kcal mol-1 less endothermic and has a 12 kcal mol-1 lower barrier height than concerted fragmentation of 1 to give singlet PhP. The formation of singlet PhP via a concerted pathway is predicted to be stereospecific, whereas formation of triplet PhP is predicted to occur with complete loss of stereochemistry. However, calculations on fragmentation of anti-cis-2,3-dimethyl-P-mesitylphosphirane (cis-1Me) to triplet mesitylphosphinidene (MesP) indicate that this reaction should be more stereospecific, in agreement with the experimental results of Li and Gaspar. Nevertheless, with a predicted free energy of activation of 42 kcal mol-1, the formation of MesP from cis-1Me is not likely to have occurred in an uncatalyzed rea...
18 citations