Anatomy of relativistic energy corrections in light molecular systems

Relativistic energy corrections which arise from the use of the Dirac-Coulomb Hamiltonian, and the Gaunt and Breit interaction operators, plus Lamb-shift effects have been determined for the global minima of the ground electronic states of C₂H₆, NH₃, H₂O, [H,C,N], HNCO, HCOOH, SiC₂, SiH^? ₃, and H₂S, and for barrier characteristics for these molecular systems (inversion barrier of NH₃ and SiH^? ₃, barrier to linearity of H₂O, H₂S, and HNCO, rotational barrier of C₂H₆, difference between conformations of HCOOH (Z/E) and SiC₂ (linear/T-shaped), and isomerization barrier of HCN/HNC). The relativistic calculations performed at the Hartree-Fock and the highly correlated CCSD(T) levels employed a wide variety of basis sets. Comparison of the perturbational and the four-component fully variational results indicate that the Coulomb-Pauli Hamiltonian and the lowest order Hamiltonian of direct perturbation theory (DPT(2)) are highly successful for treating the relativistic energy effects in light molecular systems both at a single point on the potential energy hypersurface and along the surface. Electron correlation contributions to the relativistic corrections are relatively small for the systems studied, and are comparable with the 2-electron Darwin correction. Corrections beyond the Dirac-Coulomb treatment are usually rather small, but may become important for high accuracy ab initio calculations.     

Reactions of closo-1,5-C2B3H5 with Cl2 and with BMe3

Reaction of closo-1,5-C₂B₃H₅ with Cl₂ under reduced temperatures in an inert solvent gives 2-Cl-1,5-C₂B₃H₄. Using a hot/cold reactor a mixture of BMe₃ and 1,5-C₂B₃H₅ is converted to a combination of B-mono-, di-, and tri-methyl derivatives of this smallest closo carborane. In addition, B-mono-, di-, tri-, and tetramethyl derivatives of 2,2$\text{-}\text{?}$C₂B₃H₄C₂B₃H₄, as well as the parent dimer, are produced.     

Degradation of passive layers of iron studied by conversion electron Mössbauer spectroscopy

Integral electron Mössbauer spectroscopy (ICEMS) and additionally some electrochemical methods were used to characterize the passivation process of iron (low carbon steel) in sulfate, sulfate+sulfite (a possible model solution of acid rain) solutions and in phospate buffer. The phase compositions and thicknesses of the passive layers formed due to the electrochemical polarizations were analyzed in dependence on the duration of the anodic passivations and on the pH of the used electrolytes. The passive layer, as determined from the Mössbauer spectra, consists mainly of -FeOOH, however in sulfite containing sulfate aqueous solution at pH 3.5 Fe₃C and despite ex-situ circumstances FeSO₄·H₂O was detected after the shortest polarization time. The film thickness, which was found to grow nearly linearly with polarization time in pure sulfate solution and in phospate buffer, reached a maximum of 60–160 nm (depending on pH) in sulfate+sulfite solution after a passivation time of about 4 hours. It has been proved, that HSO₃ ^–-ion, which is contained by acid rain, initiate pit formation under acid conditions and so enforces the corrosion of iron. The experimental results furthermore suggest, that not the whole oxidic layer is responsible for the passivity but only a very thin intermediate layer formed between an inner oxide layer of a cubic structure and the rhombic oxide (-FeOOH) cover.     

Deuteration of closo-1,2- and 1,7-C2B10H12 using C6D6/AlCl3: Mechanistic considerations

Regioselective deuteration of 1-X-C₂B₁₀H₁₂ (X = 2, 7) cage systems with C₆D₆/AlCl₃ is correlated to ab initio calculational results on a [C₂B₁₀H₁₃]⁺ intermediate. Full geometry optimizations of pertinent [C₂B₁₀H₁₃]⁺ isomers, derived from each of the two 1-X-C₂B₁₀H₁₂ carborane isomers, result in cage geometries not unlike the (nearly) icosahedral starting carborane. Each isomer contains a BH₂ group having an acute H-B-H angle, long B–H bonds, and a very short H · · · H distance, hinting that the pertinent boron shares the electrons of a hydrogen molecule σ pair. It is suggested that the structural differences between the BH₂ group of [C₂B₁₀H₁₃]⁺ and the CH₂ group of the benzenium ion, [C₆H₇]⁺ (the intermediate strongly intimated upon protonation of benzene), can be explained, in part, by (a) the availability of the π-ring electrons for bonding to the (extra) proton in the latter and (b) the unavailability of π electrons from the carborane. Thus, the C₂B₁₀H₁₂ cage is most probably very reluctant to give up a cage electron pair in order to assist in bonding to an (externally bound) proton. Instead, it is more probable that “hydridic” B–H sigma electrons could very well play the important role in providing bonding to the attacking proton. © 1998 John Wiley & Sons, Inc. Heteroatom Chem 9:95–102, 1998     

VIBRATION ANALYSIS OF A DAMPED ARCH USING AN ITERATIVE LAMINATE MODEL

A new model is presented for the dynamic analysis of a laminated circular ring segment. The differential equations which govern the free vibrations of a circular ring segment and the associated boundary conditions are derived by Hamilton's principle having consideration for the bending and shear deformation of all layers. The author uses a new iterative process to successively refine the stress/strain field in the sandwich arch. The model includes the effects of transverse shear and rotatory inertia. The iterative model is used to predict the modal frequencies and damping of simply-supported sandwich circular arch. The solutions for a three-layer circular arch are compared with a three-layer approximate model.     

Acknowledgement to referees

Acknowledgements

Acknowledgement to referees     

Asymmetric barrier model for heavy ion fusion and its relation to channel coupling

A new asymmetric parabolic effective fusion barrier model for heavy ion fusion is developed.