Negative hyperconjugation in methyl and silyl anions and its effect on the acidities of CH(4-n)Fn CH(4-n)C1n SiH(4-n)Fn and SiH(4-n)Cln

Group separation reactions calculated using an ab intio molecular orbital calculation at the MP4/6-31 + + G(d,p) level of theory, show the negative hyperconjugation between fluorine atoms to be larger in methanes than in silanes. Stabilisation due to negative hyperconjugation is larger in anions than in identically substituted neutral molecules, e.g. 43.1 kcal mol⁻¹ in CF₃⁻ compared with 26.7 kcal mol⁻¹ in CHF₃. By contrast, in chloro-substituted methanes, silanes, methyl anions and silyl anions, group separation energies are approximately zero, indicating no appreciable negative hyperconjugation. An -chloro substituent is more effective than an -fluoro one at delocalising the negative charge of an anion and, as a consequence, the chloromethanes and chlorosilanes are all more acidic than the identically substitued fluoromethanes and fluorosilanes. For chloro-substituted molecules the acidity is linearly dependent on the number of chlorine atoms; for fluoro-substituted molecules stabilisation by negative hyperconjugation results in each additional fluorine atom increasing the acidity by larger increments.     

Nonlinear Effects and a Cascade of Radical Events Leading to Laser-Specific Generation of Active Oxygen Species

Laser photolysis of benzoyl radical precursors (such as PhCOC[OH]R₂) in oxygen-containing solutions leads to a laser-specific cascade of oxidative events in which benzoyl peroxyl radicals undergo self-reaction, ultimately leading to the sequential formation of benzoyloxyl, phenyl and phenyl peroxyl radicals. Thus, photoreactions producing benzoyl (PheO) radicals lead to PhĊO under conditions where benzoyl radicals do not decarbonylate. These reactions require the high-intensity conditions that can only be achieved under pulsed laser excitation; yet, they do not involve any multiphoton processes. Type I mechanisms of this type may be of importance as new laser-based therapeutic technologies are developed.     

Symmetry breaking in the Hartree–Fock approximation for binuclear transition metal compounds—a theoretical investigation based on a variable model operator

The validity of the Hartree–Fock (HF ) approximation in bis(π-pentadienyl)dinickel ( 1 ) and in cyclopentadienyl-allyl-cyclobutadiene-dinickel ( 2 ) has been investigated by means of the Thouless instability conditions in the computational framework of a variable model Hamiltonian. Singlet, nonsinglet (triplet), and nonreal instabilities in 1 and 2 have been studied as a function of the one-electron resonance integral β and as a function of the one- and two-center elements of the electron–electron interaction. The one-center integrals of Coulomb (γ) and exchange-type (K) have been modified by a multiplicative factor; the two-center integrals (γ) have been calculated by means of an exponential interpolation formula with a variable decay amplitude. Additionally the Thouless conditions have been studied for nuclear deformations. The stability domain of the HF solution in the model space spanned by the variable INDO Hamiltonian has been analyzed. The nature of the many-body interactions in the unstable region depends strongly on the parametrization of the model operator. HF instabilities in the high-density region (long-range forces) of 1 have their origin in individual particle–hole fluctuations while negative roots for short-range forces (low-density region) are similar to collective excitations in many-body systems (strong off-diagonal coupling). The opposite behavior is encountered in the Ni complex 2 . The physical origin of these different types of correlation processes are analyzed in a simple two-electron two-orbital model. The nature of the HF fluctuations in 1 and 2 , the importance of spatial and spin correlation, and the coupling of symmetry breaking of the electronic wave function with nuclear deformations are compared with the nature of phase transitions in solid-state systems.     

Further studies of the Daphniphyllum alkaloid polycyclization cascade

The scope of the 2-azadiene intramolecular Diels-Alder cyclization, previously employed for synthesis of the Daphniphyllum alkaloids, has been further investigated. Through a series of 1,5-diol cyclization precursors the substitution pattern of both the dienophile and the 2-azadiene were examined. From these studies it was shown that the cascade reaction is tolerant toward a variety of alkyl-substituted dienophiles. However, it was also demonstrated that this reaction is very sensitive to the substitution pattern of the 2-azadiene. Alterations made to the structure of the 2-azadiene cause either competing side reactions or complete failure of the reaction cascade.     

Abstractive chemisorption of O2 on Al(111)

We present experimental evidence that abstraction is a common mechanism (approximately 50%) in the dissociative chemisorption of oxygen on Al(111) at a translational energy of 0.5 eV. As a result of this mechanism, individual isolated O-atoms are observed in scanning tunneling microscopy (STM). At this translational energy ordinary dissociative chemisorption processes also occur, resulting in pairs of adatoms. The ejected O-atoms originating from the abstraction reaction are detected in the gas phase using laser spectrometry. Together, these observations provide compelling evidence for the abstraction mechanism.     

Determination of mercury in human urine by neutron activation analysis,with lead diethyldithiocarbamate as a preconcentration agent

Lead diethyldithiocarbamate is an effective reagent for preconcentration of mercury in urine for neutron activation analysis. Sodium and bromine are removed from the sample by this procedure. As lead diethyldithiocarbamate is insensitive to neutron activation, radiochemical separation is not needed after neutron irradiation. Results from the analysis of urine collected from workers in caustic soda manufacturing plants are discussed.     

Synthesis of racemic and optically active Δ1- and Δ6-3,4-cis-tetrahydrocannabinols1

Three syntheses of the hitherto unknown (±)-Δ⁶-3,4-cis-THC 4a and the first total synthesis of optically active Δ¹- and Δ⁶-cis-THC's are described. These syntheses utilize a stereospecific intramolecular epoxide opening by phenolate anion; an acid catalyzed equilibration of Δ¹- and Δ⁶-cis-THC acetates; and a kinetically controlled dehydration. The use of HPLC, GLC and NMR for the separation and identification of closely related THC's is discussed.     

Synthesis of new fundamental heterocycles. Part VII. Synthesis of 2-azaxanthene

2-Azaxanthone ( 5 ) has been conveniently prepared by condensation of the morpholine enamine of 1-benzyl-4-piperidone witli salicylaldehyde followed by chromium trioxide oxidation and subsequent aromatization. Lithium aluminum hydride reduction of 5 afforded the new fundamental heterocycle 2-azaxanlhene.