Diatomic monocations and dications containing germanium and a rare gas atom.

Diatomic dications and monocations GeY2+ and GeY+, where Y = Ne, Ar, Kr or Xe, have been detected on the microsecond time scale. They are characterized via appearance energies, charge stripping of GeY+ ions, and mass-analysed ion kinetic energy spectroscopy. The formation of GeAr+ is found to involve an excited state of Ar in a Hornbeck-Molnar process.     

Three-component coupling based on the "cation pool" method

Sequential one-pot three-component coupling reactions have been developed based on the "cation pool" method. An N-acyliminium ion generated by the "cation pool" method adds to an electron-rich carbon-carbon double bond, such as enamine derivatives and vinyl sulfides, to form the second "cation pool". The addition of nucleophiles such as allylsilanes, enol silyl ethers, Grignard reagents, and organoaluminum compounds led to the formation of the corresponding three-component coupling products.     

Electron capture dissociation and infrared multiphoton dissociation of oligodeoxynucleotide dications

We report electron capture dissociation (ECD) and infrared multiphoton dissociation (IRMPD) of doubly protonated and protonated/alkali metal ionized oligodeoxynucleotides. Mass spectra following ECD of the homodeoxynucleotides polydC, polydG, and polydA contain w or d "sequence" ions. For polydC and polydA, the observed fragments are even-electron ions, whereas radical w/d ions are observed for polydG. Base loss is seen for polydG and polydA but is a minor fragmentation pathway in ECD of polydC. We also observe fragment ions corresponding to w/d plus water in the spectra of polydC and d(GCATGC). Although the structure of these ions is not clear, they are suggested to proceed through a pentavalent phosphorane intermediate. The major fragment in ECD of d(GCATGC) is a d ion. Radical a- or z-type fragment ions are observed in most cases. IRMPD primarily results in base loss, but backbone fragmentation is also observed. IRMPD provides more sequence information than ECD, but the spectra are more complex due to extensive base and water losses. It is proposed that the smaller degree of sequence coverage in ECD, with fragmentation mostly occurring close to the ends of the molecules, is a consequence of a mechanism in which the electron is captured at a P=O bond, resulting in a negatively charged phosphate group. Consequently, at least two protons (or alkali metal cations) must be present to observe a w or d fragment ion, a requirement that is less likely for small fragments.     

Effects of chain length on the rates of C-C bond dissociation in linear alkanes and polyethylene

Molecular dynamics modeling of C-C bond dissociation is performed for a series of linear alkanes and polyethylene macromolecules with the chain lengths ranging from one to a thousand constituent ethylene monomers (PE-1-PE-1000). The rate constants obtained in molecular dynamics calculations are compared with those determined using variational transition state theory with the same potential energy surface. The results of simulations demonstrate a significant accelerating effect of chain length on the rates of C-C bond scission. Per-bond rate constant values increase with the increasing chain length, up to an order of magnitude, in the sequence of linear alkanes from PE-1 (ethane) to PE-5 (decane); this dependence becomes saturated for longer chain lengths. Stiffening the potentials of bending and especially the torsional degrees of freedom diminishes the accelerating effect of chain length, while constraining the bond distances for all C-C bonds except the one undergoing dissociation has no effect. The results of the calculations are compared with existing experimental data on the dependences of the rates of thermal decomposition of linear alkanes on the alkane chain length.     

Dynamic equilibrium between dissociation and regeneration of the C-C bond in trispiro-conjoined cyclopropane compound

-The oxidation of 2,2-di(3,5-di-t-butyl-4-hydroxyphenyl)indan-1,3-dione by potassium hexacyanoferrate(III) afforded a trispiro-conjoined cyclopropane compound due to the bond formation between the ipso-carbons. Its cyclopropane ring was found to exist in solution in dynamic equilibrium with biradical species by the dissociation of the C-C bond, which is as long as 1.595 Å as revealed by X-ray crystal analysis.     

Relationship of charge to antiaromaticity in bis-fluorenyl dications and fluorenyl monocations: experimental support for theoretical calculations

The antiaromaticity of fluorenyl cations is dependent on the magnitude of the charge of the system. Theoretical assessments of antiaromaticity and charge were supported by experimental NMR chemical shifts. Delocalization was related to antiaromaticity, and evaluated through the standard deviation of the charges on carbons of the fluorenyl systems.     

Ab initio study on the structures, energies, and vibrational frequencies of acetone complexes with metal monocations and dications

The structures, interaction energies, and vibrational frequencies of acetone and its complexes with proton and various metal monocations/dications such as H⁺, Li⁺, Na⁺, K⁺, Rb⁺, Cs⁺, Be²⁺, Mg²⁺, Ca²⁺, and Zn²⁺ have been investigated from the ab initio calculations. The linear or bent structure for the cation-acetone complexes has been found to be deeply influenced by the amount of charge transfer. The amount of red-shift of CO stretching frequencies of acetone is almost equivalent regardless of the kind of alkali metal monocations. This behavior can be attributed to the electrostatic nature of the interactions rather than orbital interactions. This phenomenon has been supported by the highest occupied molecular orbitals of the acetone complexes investigated, charge transfer based on natural bond orbital (NBO) atomic charges, and the inversely proportional behavior of the interaction energies to the interatomic distances r(M⁺O).     

Symmetric and unsymmetric "dumbbells" of Ru2-alkynyl units via C-C bond formation reactions

Oxidative homocoupling (Glaser) reaction of Ru2 compounds bearing peripheral ethyne resulted in symmetric dimers. Cross-coupling (Sonogashira) reaction between Ru2 compounds bearing peripheral iodo and ethyne groups yielded an unsymmetric dimer. Voltammetric data indicated that Ru2 units in the symmetric dimers are noninteracting, and the unsymmetric dimer is best described as a weakly coupled push-pull compound.     

Calculations of bond dissociation energies. New select applications of an old method

Application of Sanderson's definition of electronegativity as stability ratios (SRs), which BE = [E(i) + E(DA)] (IC) + E(cov) had been applied in the past to a wide variety of organic and nontransitional metal inorganic compounds with very good success, has been revived, modified so as to be applied to any types of molecules, including those containing transition metals, lanthanides, and actinides. This paper is limited to a demonstration of the method which is applied to a few metal cyclopentadienyl compounds, plus specific emphasis on the U(III) metallocene (CpSiMe(3))(3)U-AlCp* recently prepared by Arnold and co-workers having no experimental bond energies available. It is shown that computed bond energies of pertinent metallocyclopentadieneyls are in excellent agreement with the available experimental data. Calculated bond energies for all essential bonds in the uranium metallocene cited above are provided together with a further analysis of the bonding and magnetic properties of this unique compound.     

Indirect cation pool method. Rapid generation of alkoxycarbenium ion pools from thioacetals

A sequential one-pot indirect cation pool method has been developed. The method involves the electrochemical generation and accumulation of ArS(ArSSAr)+ at low temperature (step 1) and the follow-up reaction with a thioacetal to generate an alkoxycarbenium ion pool (step 2), which reacts with various carbon nucleophiles (step 3). Steps 2 and 3 are extremely fast. The electrogenerated ArS(ArSSAr)+ was well-characterized by 1H NMR and CSI-MS. The alkoxycarbenium ion pool generated by the present indirect method exhibited 1H and 13C NMR spectra and thermal stability similar to those of the alkoxycarbenium ion pool generated by the direct electrochemical method.     

Breathing orbital valence bond method in diffusion Monte Carlo: C-H bond dissociation of acetylene

This study explores the use of breathing orbital valence bond (BOVB) trial wave functions for diffusion Monte Carlo (DMC). The approach is applied to the computation of the carbon-hydrogen (C-H) bond dissociation energy (BDE) of acetylene. DMC with BOVB trial wave functions yields a C-H BDE of 132.4 +/- 0.9 kcal/mol, which is in excellent accord with the recommended experimental value of 132.8 +/- 0.7 kcal/mol. These values are to be compared with DMC results obtained with single determinant trial wave functions, using Hartree-Fock orbitals (137.5 +/- 0.5 kcal/mol) and local spin density (LDA) Kohn-Sham orbitals (135.6 +/- 0.5 kcal/mol).     

Reduction of a "cation pool": a new approach to radical mediated C--C bond formation.

Carbocations, carbon radicals, and carbanions are important reactive carbon intermediates in organic chemistry, and their interconversions can be carried out by redox processes. Although, such relationships have been well recognized, experimental work has been limited to analytical studies on highly stabilized intermediates. In this study such interconversions were examined using electrochemical reduction of "cation pools". Acyliminium cations, which were generated by low-temperature electrolysis of carbamates, were reduced electrochemically in the absence of radical acceptors. The homo coupling products formed effectively, suggesting that the one-electron reduction of the acyliminium cation produced the corresponding carbon-centered radical. Next, the electrochemical reduction of the acyliminium cations in the presence of electron-deficient olefins was examined. The cross coupling products were obtained in good-to-moderate yields. A mechanism involving radical addition to the double bond followed by the reduction of the resulting radical to the carbanion was suggested. The overall transformation serves as redox-mediated formal addition of C-H to C=C. The present strategy opens new opportunities to manipulate reactive carbon species using redox processes in organic synthesis.     

A new method for estimation of homolytic C-H bond dissociation enthalpies

In this work we have quantitatively analyzed substituent effects on the homolytic bond dissociation enthalpy of 79 different compounds using a method based on discrete distance dependent atomic contributions to a molecular property. The resulting empirical relationship can be used to predict C-H bond dissociation enthalpies (within +/-10 kJ mol(-1)) for molecules where resonance contributions and captodative stabilization are insignificant. For species where captodative stabilization of the corresponding C-centered radical is possible, the method clearly overestimates the C-H bond dissociation enthalpy.     

Reductive generation of enolates from enones using elemental hydrogen: catalytic C-C bond formation under hydrogenative conditions

Exposure of enones to elemental hydrogen in the presence of a Rh(I) catalyst enables reductive enolate generation, as evidenced by electrophilic trapping of the enolate by appendant and exogenous aldehyde partners. The significance of these findings resides in the ability to regioselectivity generate and transform transition metal enolates under catalytic conditions that circumvent formation of stoichiometric byproducts.     

Pd-catalyzed sequential C-C bond formation and cleavage: evidence for an unexpected generation of arylpalladium(II) species

A Pd(II)-catalyzed reaction engaging alkenyl β-keto esters is reported that leads to the formation of 1-naphthols and an unexpected generation of arylpalladium(II) species. Interception of the in situ generated arylpalladium(II) species in a Mizoroki-Heck reaction, together with additional mechanistic studies, provided strong evidence in support of the first aromatization-driven β-carbon elimination process. A single Pd catalyst served to promote a series of both C-C bond forming and cleavage events in an unprecedented manner.     

The effect of bond functions on dissociation energies

The procedure employing bond functions recently suggested by Wright and Buenker has been applied to the N₂ X ¹Σ_g⁺ potential curve within the CAS SCF+MRSD Ci treatment of electron correlation. The basis set used herein is identical to that employed by these authors in their SCF+CI calculations. The D_e and the shape of the resulting potential curve, as judged by the computed vibrational levels, is not so accurate as would be expected from the results reported by Wright and Buenker. Our results indicate that using the CI superposition errors associated with bond functions to cancel basis set incompleteness depends on the treatment of the electron correlation.     

Thermochemistry and bond dissociation energies of ketones

Ketones are a major class of organic chemicals and solvents, which contribute to hydrocarbon sources in the atmosphere, and are important intermediates in the oxidation and combustion of hydrocarbons and biofuels. Their stability, thermochemical properties, and chemical kinetics are important to understanding their reaction paths and their role as intermediates in combustion processes and in atmospheric chemistry. In this study, enthalpies (ΔH°(f 298)), entropies (S°(T)), heat capacities (C(p)°(T)), and internal rotor potentials are reported for 2-butanone, 3-pentanone, 2-pentanone, 3-methyl-2-butanone, and 2-methyl-3-pentanone, and their radicals corresponding to loss of hydrogen atoms. A detailed evaluation of the carbon-hydrogen bond dissociation energies (C-H BDEs) is also performed for the parent ketones for the first time. Standard enthalpies of formation and bond energies are calculated at the B3LYP/6-31G(d,p), B3LYP/6-311G(2d,2p), CBS-QB3, and G3MP2B3 levels of theory using isodesmic reactions to minimize calculation errors. Structures, moments of inertia, vibrational frequencies, and internal rotor potentials are calculated at the B3LYP/6-31G(d,p) density functional level and are used to determine the entropies and heat capacities. The recommended ideal gas-phase ΔH°(f 298), from the average of the CBS-QB3 and G3MP2B3 levels of theory, as well as the calculated values for entropy and heat capacity are shown to compare well with the available experimental data for the parent ketones. Bond energies for primary, secondary, and tertiary radicals are determined; here, we find the C-H BDEs on carbons in the α position to the ketone group decrease significantly with increasing substitution on these α carbons. Group additivity and hydrogen-bond increment values for these ketone radicals are also determined.