Computational studies of the thermal fragmentation of P-arylphosphiranes: Have arylphosphinidenes been generated by this method?

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 Caspar. 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 reaction at the temperatures at which this phosphirane has been pyrolyzed. © 2005 American Chemical Society.link_to_subscribed_fulltex

Photoelectron Spectroscopy and Theoretical Studies of PCSe- , AsCS- , AsCSe- , and NCSe- : Insights into the Electronic Structures of the Whole Family of ECX- Anions (E=N, P, As; X=O, S, Se)

The newly synthesized phosphorus- and arsenic-containing analogues of the thio- and seleno-cyanate anions, PCSe- , AsCS- , and AsCSe- , as well as the known ion NCSe- were investigated in the gas phase by negative-ion photoelectron spectroscopy (NIPES), velocity-map imaging (VMI) spectroscopy, and quantum-chemical computations. The electron affinities (EA), spin-orbit (SO) splittings, and "symmetric"/"asymmetric" stretching frequencies of the neutral radicals ECX. (E=N, P, As; X=S, Se), generated by electron detachment from the corresponding anions, were obtained from the spectra. The calculated EAs, SO splittings, and vibrational frequencies are in excellent agreement with the experimental measurements. These newly obtained values, when combined with those previously determined for the lighter analogues, show interesting trends on descending the pnictogen and chalcogen series. These trends are rationalized based on electronegativity arguments, the electron distributions in the HOMOs, and NBO/NRT analyses

Stoichiometric oxidations of σ-bonds: Radical and possible non-radical pathways

Many transition metal complexes accomplish or catalyze the oxidation of C{single bond}H, O{single bond}H, and other σ-bonds. Under aerobic conditions, metal complexes typically modulate an autoxidation radical chain. In anaerobic reactions, a metal complex can be the reactive species that attacks the σ-bond, in many cases by abstracting a hydrogen atom from the substrate. Examples described here include the oxidation of alkylaromatic compounds by ruthenium oxo complexes and reactions of deprotonated iron(III) complexes. In general, these reactions occur with addition of H+ to a ligand and e- to the metal center. Rate constants for such hydrogen-atom transfer reactions can, in many cases, be predicted by the Marcus cross relation. Autoxidation and metal-mediated radical mechanisms are so prevalent that proposals of non-radical oxidations of C{single bond}H bonds carry a higher burden of proof. It is argued here that the oxidation of H2 by OsO4 occurs by a non-radical, [3 + 2] mechanism. OsO4 oxidizes alkanes under similar aqueous conditions. For example, isobutane is oxidized to tert-butanol, and cyclohexane to adipate and succinate. The alkane oxidations do not have the hallmarks of a radical mechanism but sufficient questions remain that a radical pathway cannot be excluded at this time. © 2006 Elsevier B.V. All rights reserved.link_to_subscribed_fulltex

Switching radical stability by pH-induced orbital conversion

In most radicals the singly occupied molecular orbital (SOMO) is the highest-energy occupied molecular orbital (HOMO); however, in a small number of reported compounds this is not the case. In the present work we expand significantly the scope of this phenomenon, known as SOMO-HOMO energy-level conversion, by showing that it occurs in virtually any distonic radical anion that contains a sufficiently stabilized radical (aminoxyl, peroxyl, aminyl) non-pi-conjugated with a negative charge (carboxylate, phosphate, sulfate). Moreover, regular orbital order is restored on protonation of the anionic fragment, and hence the orbital configuration can be switched by pH. Most importantly, our theoretical and experimental results reveal a dramatically higher radical stability and proton acidity of such distonic radical anions. Changing radical stability by 3-4 orders of magnitude using pH-induced orbital conversion opens a variety of attractive industrial applications, including pH-switchable nitroxide-mediated polymerization, and it might be exploited in nature