Browsing by Author "Liebman, Joel F."
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Item Indenone and cyclopentadienone energetics via mass spectrometry and computations: Are these species antiaromatic or “merely” nonaromatic?(ScienceDirect, 2014-06-14) Fattahi, Alireza; Liebman, Joel F.; Miranda, Margarida S.; Morais, Victor M.F.; Matos, M. Agostinha R.; Lis, Lev; Kass, Steven R.The heat of hydrogenation of indenone was measured via two partially independent thermodynamic cycles by carrying out energetic measurements (i.e., electron affinities, proton affinities and ionization potentials) on both negative and positive ions (ΔH°(H2) = 17.8 ± 5.5 and 17.5 ± 5.7 kcal mol⁻¹, respectively). High level G3 computations were also carried out to provide the heats of formation of indenone (16.8 kcal mol⁻¹) and cyclopentadienone (14.0 kcal mol⁻¹). These 4n π electron systems are found to be nonaromatic in contrast to previous views. A recent report on cyclopropenyl anion (J. Org. Chem. 2013, 78, 7370–7372) indicates that this ion is also nonaromatic, and suggests that NMR ring currents and nucleus independent chemical shift (NICS) calculations do not correlate with the energetic criterion for antiaromatic compounds.Item Paradigms and Paradoxes: The Ionization Potential of Atomic Astatine (Z =85), Polonium (Z = 84) and some other Elements: What Does This Value Tell Us About the Energetics of Atomic and Diatomic Halogens(Research Square, 2021-02-25) Ponikvar-Svet, Maja; Edwards, Kathleen F.; Liebman, Joel F.The ionization potential of atomic astatine is discussed and compared with that of polonium and the other halogens. Some surprises remain.Item Paradoxes and paradigms: elements and compounds: similar names, very different energetics. Part 5, selected examples of exotic species and isotopes (H, He, C, F, U)(Springer, 2023-08-18) Ponikvar‑Svet, Maja; Edwards, Kathleen Frances; Liebman, Joel F.In this paper we continue to review the phonetic similarity of trivial names of chemical substances, in the particular, the names of isotopes of selected elements (H, He, C, F, U) of the periodic table. Thermochemical properties are explicitly considered.Item Photochemical Valence Isomerization to High Energy Products—Bicyclobutanes and Oxabicyclobutanes(Wiley Online Library, 2021-06-18) Rágyanszki, Anita; Fiser, Béla; Lee-Ruff, Edward; Liebman, Joel F.DFT calculations were used to determine the structures and energies of bicyclobutane and oxabicyclobutane as valence isomers derived from electronic excitation of their corresponding precursors, 1,3-butadiene and acrolein, respectively. Proton affinities of these strained compounds were determined and compared with their simple ring components, cyclopropane and ethylene oxide. The basicity as determined from proton affinities showed that bicyclobutane is the most basic saturated hydrocarbon, even more basic than oxabicyclobutane. Strain energies of these valence tautomers were computed which showed oxabicyclobutane to be significantly more strained than bicyclobutane. Qualitative reasons are provided to account for the difference in strain energies.Item Protonated heterocyclic derivatives of cyclopropane and cyclopropanone: classical species, alternate sites, and ring fragmentation(Canadian Science, 2015-02-19) Miranda, Margarida S.; Duarte, Darío J.R.; Silva, Joaquim C.G. Esteves da; Liebman, Joel F.A computational study has been performed for protonated oxygen- or nitrogen-containing heterocyclic derivatives of cyclopropane and cyclopropanone. We have searched for the most stable conformations of the protonated species using density functional theory with the B3LYP functional and the 6-31G(2df,p) basis set. More accurate enthalpy values were obtained from G4 calculations. Proton affinities and gas-phase basicities were accordingly derived.Item Strained Small Nitrogen Heterocycles-Azabicyclobutanes and Azirines(Wiley, 2023-07-10) Rágyanszki, Anita; Fiser, Béla; Lee-Ruff, Edward; Liebman, Joel F.Small ring nitrogen heterocycles, azabicyclobutanes and azirines, were investigated by computational methods in order to address the discrepancy between their regioisomers 1- and 2-azabicyclobutane and 1H- and 2H-azirines. Both 1-azabicyclobutane and 2H-azirine are well known synthetic starting points to larger nitrogen heterocycles whereas 2-azabicyclobutane and 1H-azirine and their derivatives have yet to be reported as isolable compounds. Calculated parameters such as structure, base strength (proton affinities), NICS values and enthalpies of formation from which strain energies are derived are reported. The destabilization of the less stable regioisomers is attributed to homoantiaromaticity in 2-azabicyclobutane and antiaromaticity in 1H-azirine. Two stereoisomers exist for 2-azabicyclobutane with the endo- stereoisomer being more stable. This phenomenon is indicative of the hydrogen bond acceptor properties of the neighboring cyclpropane and the π-bond character of the central bond in 2-azabicyclobutane.Item Three-membered ring amides — a calculational and conceptual study of the structure and energetics of 1,2-oxaziridine-3-one and aziridine-2,3-dione(Canadian Science, 2015-04) Miranda, Margarida S.; Ferreira, Paulo J.O.; Silva, Joaquim C.G. Esteves da; Liebman, Joel F.Species with three-membered rings and the amide linkage are well studied. A quick perusal of the literature with SciFinder finds some 50 000 references to cyclopropanes and almost 300 000 references to amides. In the current paper, we discuss the structure and energetics of two understudied three-membered ring amides, 1,2-oxaziridine-3-one (5) (simultaneously describable as the simplest cyclic carbamate and simplest hydroxamate) and aziridine-2,3-dione (7) (simultaneously describable as the simplest imide and simplest -ketoamide), with but 5 and nearly 10 references, respectively, for these two classes of compounds. Neither 1,2-oxaziridine-3-one (5) nor aziridine-2,3-dione (7), nor any derivative thereof, has been isolated. Calculational theory ameliorates the paucity of experimental information. The current study reports our computational findings for these and related species (e.g., enols and imidols) where we have used the G3(MP2)//B3LYP method.