Description: This article discusses qualitative theory, quantitative calculations, and direct experiments. This Perspective describes research, carried out in the authors' labs over the past forty years, aimed at understanding, predicting, and measuring the singlet-triplet energy differences (∆Est) in diradicals. A theory for qualitatively predicting the ground states of diradicals and the use of Negative Ion Photoelectron Spectroscopy (NIPES) for measuring ∆Est are described. The application of this theory, ab initio calculations, and NIPES to the prediction and measurement of ∆Est in a wide variety of organic diradicals is detailed. Among the diradicals that are discussed in this Perspective are HN, CH3N, PhN, CH2, trimethylenemethane (TMM), oxyallyl (OXA), meta-benzoquinodimethane (MBQDM), meta-benzoquinone (MBQ), tetramethyleneethane (TME), 1,2,4,5-tetramethylenebenzene (TMB), and D8h COT were, in fact, collaborations between the research groups of the authors. These two projects both took advantage of the ability to NIPES to provide information about transition states. Transition-state spectroscopy was used to measure the cabonyl stretching frequency in the singlet state of OXA and to establish that D8h COT violates the strictest version of Hund's rule.

Description: This article discusses the selectivity and mechanism of hydrogen atom transfer by an isolable imidoiron (III) complex. In the literature, iron-oxo complexes have been isolated and their hydrogen atom transfer (HAT) reactions have been studied in detail. Iron-imido complexes have been isolated more recently, and the community needs experimental evaluations of the mechanism of HAT from late-metal imido species. The authors report a mechanistic study of HAT by an isolable iron (III) imido complex, LᴹᵉFeNAd (Lᴹᵉ = bulky β-diketiminate ligand, 2,4-bis(2,6-diisopropylphenylimido)pentyl; Ad = 1-adamantyl). HAT is preceded by binding of tert-butylpyridine (ᵗBupy) to form a reactive four-coordinate intermediate LᴹᵉFe(NAd)(ᵗBupy), as shown by equilibrium and kinetic studies. In the HAT step, very large substrate H/D kinetic isotope effects around 100 are consistent with C-H bond cleavage. The elementary HAT rate constant is increased by electron-donating groups on the pyridine additive, and by a more polar medium. When combined with the faster rate of HAT from indene versus cyclohexadiene, this trend is consistent with H⁺ transfer character in the HAT transition state. The increase in HAT rate in the presence of ᵗBupy may be explained by a combination of electronic (weaker Fe=N π-bonding) and thermodynamic (more exothermic HAT) effects. Most importantly, HAT ...

Description: In this article, the authors report the isolation, characterization, and reactions of the unsaturated complex LᵗᴮᵘCo (Lᵗᴮᵘ = bulky β-diketiminate ligand). The unusual slipped kN,ɳ⁶-arene binding mode in LᵗᴮᵘCo interconverts rapidly and reversibly with the traditional k²N,N' ligation mode upon binding of Lewis bases, making it a "masked" two-coordinate complex. The mechanism of this isomerization is demonstrated using kinetic studies. LᵗᴮᵘCo is a stable yet reactive synthon for low-coordinate cobalt(I) complexes and is capable of cleaving the C-F bond in fluorobenzene.

Description: This article discusses the enthalpy of solvation correlations for organic solutes and gases dissolved in 2-propanol, 2-butanol, 2-methyl-1-propanol and ethanol. Abstract: Data have been assembled from the published literature on the enthalpies of solvation for 91 organic vapors and gaseous solutes in 2-propanol, for 73 gaseous compounds in 2-butanol, for 85 gaseous compounds in 2-methyl-1-propanol and for 128 gaseous compounds in ethanol. It is shown that an Abraham solvation equation with five descriptors can be used to correlate the experimental solvation enthalpies to within standard deviations of 2.24 kJ/mole, 1.99 kJ/mole, 1.73 kJ/mole and 2.54 kJ/mole for 2-propanol, 2-butanol, 2-methyl-1-propanol and ethanol, respectively. The derived correlations provide very accurate mathematical descriptions of the measured enthalpy of solvation data at 298 K, which in the case of ethanol span a range of 136 kJ/mole. Division of the experimental values into a training set and a test set shows that there is no bias in predictions, and that the predictive capability of the correlations is better than 3.5 kJ/mole.

Description: Article describing the career of Marie Curie and the discovery of polonium and radium. Tourist information regarding the areas where the Curies were most active are included.