Activation energies and potentials of mean force for water cluster evaporation

Activation energies for water cluster evaporation are of interest in many areas of chemical physics. We present the first computation of activation energies for monomer evaporation of small water clusters using the formalism of dynamical nucleation theory (DNT). To this end, individual evaporation rate constants are computed for water clusters (H(2)O)(i), where i=2-10 for temperatures ranging from 243 to 333 K. These calculations employ a parallel sampling technique utilizing a Global Arrays toolkit. The resulting evaporation rate constants for each cluster are then fitted to Arrhenius equations to obtain activation energies. We discuss DNT evaporation rate constants and their relation to potentials of mean force, activation energies, and how to account for nonseparability of the reaction coordinate in the reactant state partition function.     

Biotransformation of substituted pyridines with dioxygenase-containing microorganisms

A series of 2-, 3- and 4-substituted pyridines was metabolised using the mutant soil bacterium Pseudomonas putida UV4 which contains a toluene dioxygenase (TDO) enzyme. The regioselectivity of the biotransformation in each case was determined by the position of the substituent. 4-Alkylpyridines were hydroxylated exclusively on the ring to give the corresponding 4-substituted 3-hydroxypyridines, while 3-alkylpyridines were hydroxylated stereoselectively on C-1 of the alkyl group with no evidence of ring hydroxylation. 2-Alkylpyridines gave both ring and side-chain hydroxylation products. Choro- and bromo-substituted pyridines, and pyridine itself, while being poor substrates for P. putida UV4, were converted to some extent to the corresponding 3-hydroxypyridines. These unoptimised biotransformations are rare examples of the direct enzyme-catalysed oxidation of pyridine rings and provide a novel synthetic method for the preparation of substituted pyridinols. Evidence for the involvement of the same TDO enzyme in both ring and side-chain hydroxylation pathways was obtained using a recombinant strain of Escherichia coli (pKST11) containing a cloned gene for TDO. The observed stereoselectivity of the side-chain hydroxylation process in P. putida UV4 was complicated by the action of an alcohol dehydrogenase enzyme in the organism which slowly leads to epimerisation of the initial (R)-alcohol bioproducts by dehydrogenation to the corresponding ketones followed by stereoselective reduction to the (S)-alcohols.     

Ground state properties of CsCoCl3

The electronic spectra of CsCoCl₃ are fit to a Hamiltonian that includes terms for interelectron repulsion, octahedral and trigonal crystal fields, and spin-orbit coupling. The fit adequately accounts for both the optical spectrum and the electronic Raman spectrum. The fitted parameters give empirical estimates of the radial expectation values 〈r^?1〉 and 〈r^?3〉 as well as the charge on the cobalt. The ground state wave functions generated from the fit are used to calculate the following properties: parallel and perpendicular g factors, Co hyperfine field, ⁵⁹Co quadrupole splitting, anisotropy of magnetic exchange, the magnetic moment of Co²⁺, and the spin flop field. The agreement between calculated values and observed values for this variety of independently obtained properties is reasonable in all cases.     

A new look at an old technique for the measurement of fatigue crack-growth rates

An automated technique is described by which a sequence of ‘beachmarks’ (clam-shell markings) can be produced in a specimen during laboratory tests on the propagation of fatigue cracks. The subsequent use of the beachmarks for the measurement of the lengths of the cracks is described, and the intrinsic accuracy of the method is estimated. It is found that, although fewer data points are obtained by this method, they are more accurate and better characterized than those obtained by the more commonly used ‘indirect’ methods. In addition, valuable supplementary information is obtained from which it is possible to assess both the confidence intervals for the results, and the degree of crack front curvature.