Anisotropic surface tension and the liquid drop model

The liquid drop model assumes that the surface tension of all nuclei is isotropic. The consequences of relaxing this assumption are studied. First a macroscopic theory of anisotropic surface tension is presented. Then with a particular choice for the functional form of the surface tension tensor, this theory is applied to a charged liquid drop. A small anisotropy in the surface tension can significantly distort a highly charged drop and lower its fission barrier.     

Lanthanide induced paramagnetic shifts—derivation of equilibrium constants and absolute shifts in Eu(DPM)3 systems

Relationships between the magnitude of induced ¹H NMR shifts and the equilibrium conditions in solutions containing lanthanide shift reagents are discussed and applied to the shifts induced in several organic molecules by Eu(DPM)₃. Evidence that monoadduct formation is the predominant equilibrium in these solutions, and values of absolute shifts and equilibrium constants are reported. The results indicate that a contact mechanism makes the major contribution to the observed shifts in 4-methylpyridine. In the case of 2,4,6-trimethylpyridine the observed shifts are largely pseudocontact in origin.     

The role of partial saturation of the λ3 ladder in SF6 absorption under high power infrared laser excitation

A rate process model is developed to interpret the experimental results of Smith, Schmid, Tablas and Kompa on the time and intensity dependence of the infrared absorption of SF₆. In this model incoherent excitation of the λ₃ molecular mode by the processes of absorption and stimulated emission is followed by radiationless energy transfer from a maximum level to an inactive mode heatbath. This achieves control of the level population by an empirical transfer rate parameter. Good theoretical fits of the experiment absorption decays are obtained with values of approximately 0.1 ns.     

Computational study of the oxidation and decomposition of dibenzofuran under atmospheric conditions

The atmospheric degradation of dibenzofuran (DF) initiated by OH addition has been studied by using density functional theory (B3LYP method). Site C1 in DF is predicted to be the favored site for OH addition, with a branching ratio of 0.61 to produce a DF-OH(1) adduct. The calculated reaction rate constant for OH addition to DF has been used to predict the atmospheric lifetime of DF to be 0.45 day. Three different modes of attack of O2 ((3)Sigma(g)) on DF-OH(1) have been examined. Abstraction of hydrogen gem to OH in DF-OH(1) by O2 ((3)Sigma(g)) (producing 1-dibenzofuranol I) and dioxygen addition in the three radical sites in cis and trans orientation (relative to the ispo-added OH) of the pi-delocalized electron system of DF-OH(1) are feasible under atmospheric conditions. The free energy of activation (at 298.15 K) for the formation of 1-dibenzofuranol is 15.1 kcal/mol with a free energy change of -36.3 kcal/mol, while the formation of DF-OH(1)-O2 adducts are endergonic by 9.2-21.8 kcal/mol with a 16.3-23.6 kcal/mol free energy of activation. On the basis of the calculated reaction rate constants, the formation of 1-dibenzofuranol is more important than the formation of DF-OH-O2 adducts. The results presented here are a first attempt to gain a better understanding of the atmospheric oxidation of dioxin-like compounds on a precise molecular basis.     

Quantum chemical and kinetic study of formation of 2-chlorophenoxy radical from 2-chlorophenol: unimolecular decomposition and bimolecular reactions with H, OH, Cl, and O2

This study investigates the kinetic parameters of the formation of the chlorophenoxy radical from the 2-chlorophenol molecule, a key precursor to polychlorinated dibenzo-p-dioxins and dibenzofurans (PCCD/F), in unimolecular and bimolecular reactions in the gas phase. The study develops the reaction potential energy surface for the unimolecular decomposition of 2-chlorophenol. The migration of the phenolic hydrogen to the ortho-C bearing the hydrogen atom produces 2-chlorocyclohexa-2,4-dienone through an activation barrier of 73.6 kcal/mol (0 K). This route holds more importance than the direct fission of Cl or the phenolic H. Reaction rate constants for the bimolecular reactions, 2-chlorophenol + X --> X-H + 2-chlorophenoxy (X = H, OH, Cl, O2) are calculated and compared with the available experimental kinetics for the analogous reactions of X with phenol. OH reaction with 2-chlorophenol produces 2-chlorophenoxy by direct abstraction rather than through addition and subsequent water elimination. The results of the present study will find applications in the construction of detailed kinetic models describing the formation of PCDD/F in the gas phase.     

Quantum chemical investigation of formation of polychlorodibenzo-p-dioxins and dibenzofurans from oxidation and pyrolysis of 2-chlorophenol

Density functional theory (DFT) calculations have been used to obtain thermochemical parameters for formation of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/PCDF) from the oxidation of 2-chlorophenol. Formation mechanisms of PCDD through radical-radical coupling have been investigated in detail. The sequence of 2-chlorophenoxy radical coupling has been studied. The formation of chlorinated bis keto dimers which results from cross coupling of 2-chlorophenoxy at the ortho carbon bearing hydrogen (a known direct route for PCDF formation) passes through a tight transition structure whose barrier is 9.4 kcal/mol (0 K). Three routes for the formation of the most abundant PCDD/PCDF species (viz., 4,6-dichlorodibenzofuran, 4,6-DCDF, and 1-monochlorodibenzo-p-dioxin, 1-MCDD) in oxidation and pyrolysis of 2-chlorophenol are discussed. In the case of 4,6-DCDF, formation through H or HO + keto-keto <==> H2 or H2O + keto-keto* <==> H2 or H2O + enol-keto* <==> H2 or H2O + 4,6-DCDF + HO is shown to be the preferred route. The other two routes proceed via closed shell processes (keto-keto <==> enol-keto <==> enol-enol <==> H2O + 4,6-DCDF) and (keto-keto <==> enol-keto <==> (H-,OH-) 4,6-DCDF <==> H2O + 4,6-DCDF). Results indicate that 1-MCDD should be the favored product in 2-chlorophenol pyrolysis in agreement with experimental findings. According to our results, tautomerization (inter-ring hydrogen transfer) and intra-annular displacement of HCl would not be competitive with paths deriving from H abstraction from the phenolic oxygen and the benzene ring followed by displacement of Cl in the formation of dibenzo-p-dioxin (DD) and 1-MCDD. The results presented here will assist in construction of detailed kinetic models to account for the formation of PCDD/PCDF from chlorophenols.