Generic implementation of semi-analytical CI gradients for NDDO-type methods

Generic semi-analytical energy gradients are derived and implemented for NDDO-type methods, by using numerical integral and Fock matrix derivatives in the context of an otherwise analytical approach for configuration interaction (CI) and other non-variational treatments. The correctness, numerical precision, and performance of this hybrid approach are established through comparisons with fully numerical and fully analytical calculations. The semi-analytical evaluation of the CI gradient is generally much faster than the fully numerical computation, but somewhat slower than a fully analytical calculation, which however shows the same scaling behavior. It is the method of choice whenever a fully analytical CI gradient is not available due to the lack of analytical integral derivatives. The implementation is generic in the sense that it can easily be extended to any new NDDO-type Hamiltonian. The present development of a semi-analytical CI gradient will facilitate studies of electronically excited states with recently proposed NDDO methods that include orthogonalization corrections. Dedicated to Professor Karl Jug on the occasion of his 65th birthday     

The Equilibrium Structure of Methyl Fluoride

The equilibrium structure of CH₃F has been determined using new sets of accurate rotational constants that have been determined by taking into account all the interactions between the excited vibrational states. This experimental structure is in excellent agreement with the equilibrium geometry calculated at the CCSD(T) level of theory with the cc-pV(5, Q)Z basis set (including corrections for the core correlation and for the effect of diffuse functions on fluorine). Finally, the experimental and ab initio structures have been combined by a least-squares analysis. The results are , and L _e(HCH) = 110.2 (1)°, where the uncertainties shown in parentheses correspond to three standard deviations.     

The palladium-catalyzed cross-coupling reaction of carboxylic anhydrides with arylboronic acids: a DFT study

The mechanism of the cross-coupling of phenylboronic acid with acetic anhydride, a viable model of the widely used Suzuki reaction, has been studied by DFT calculations at the BP86/6-31G level of theory. Two alternative catalytic cycles have been investigated, one starting from a neutral Pd(0)L(2) complex, the other from an anionic "Jutand-type" [Pd(0)L(2)X](-) species. The reaction profiles are in good agreement with the experimental findings, as both pathways require only moderate activation energies. Both pathways are dominated by cis-configured square-planar palladium(II)diphosphine intermediates. Despite careful investigations, we did not find in this model reaction any evidence for five-coordinate palladium(II) intermediates, which are commonly believed to cause the profound effects of counterions in palladium-catalyzed transformations. Instead, our calculations suggest that the higher catalytic activity of anionic complexes, such as [Pd(PMe(3))(2)OAc](-), may arise from their stronger ability to coordinate to carbon electrophiles. The transmetalation sequence is the same for both catalytic cycles, involving the dissociation of one phosphine ligand from the palladium. In the decisive transition state, in which the phenyl group is transferred from boron to palladium, the acetate base is found to be in a bridging coordination between these two atoms.     

Self-assembly of a fluorescent chiral zinc(II) complex that leads to supramolecular helices

The chiral Zn(II) complex [ZnLCl(2)], 1 {L = 4-methyl-2,6-di[(S)-(+)-1-phenylethyliminomethyl] phenol}, self-assembles via C-H...Cl hydrogen bonding into supramolecular helices. Complex 1 exhibits emission in solution at room temperature in the visible range. Crystal data for 1: orthorhombic space group P2(1)2(1)2(1), a = 9.614(2) A, b = 13.825(3) A, c = 18.667(3) A, V = 2481.1(8) A(3), Z = 4.     

Preparation of [5,6]- and [6,6]-oxahomofullerene derivatives and their interconversion by Lewis acid assisted reactions of fullerene mixed peroxides

[60]Fullerene mixed peroxides C60(O)(OOtBu)4 exhibit chemo- and regioselective reactions under mild conditions. The epoxy moiety is opened by ferric chloride to form vicinal hydroxy chloride C60Cl(OH)(OOtBu)4. BF3 is also effective in opening the epoxy moiety. The O-O bond of the fullerene mixed peroxide is cleaved by aluminum chloride to form both [5,6]- and [6,6]-fullerene hemiketals (oxohomo[60]fullerenes). A Hock-type rearrangement is proposed for the formation of the hemiketals, in which a fullerene C-C bond is cleaved. Lewis acids and/or visible light can initiate isomerization of the hemiketal isomers. Single-crystal X-ray analysis and theoretical calculations confirmed the results.     

Rationalizing perhydrolase activity of aryl-esterase and subtilisin Carlsberg mutants by molecular dynamics simulations of the second tetrahedral intermediate state

The perhydrolysis reaction in hydrolases is an important example of catalytic promiscuity and has many potential industrial applications. The mechanisms of perhydrolase activity of a subtilisin Carlsberg mutant and of an aryl-esterase mutant have been investigated using classical molecular dynamics simulations of the second tetrahedral intermediate (TI) state. The simulations demonstrated that hydrogen bonding between the second TI of the perhydrolysis reaction is possible in the mutants but not wild type. The stabilization by hydrogen bonds was specific for the perhydrolysis intermediate and either no hydrogen bonding or only weakened hydrogen bonding to the second TI state of the hydrolysis reaction was observed. Furthermore, a significant hindrance to the formation of the catalytically important hydrogen bond between His64 and Ser221 in the catalytic triad by competing hydrogen bonds was found for the subtilisin mutant but not wild type enzyme in case of the hydrolysis intermediate. The opposite was observed in case of the perhydrolysis intermediate. The result offers a qualitative explanation for the overall reduced hydrolysis activity of the subtilisin mutant. In addition, the simulations also explain qualitatively the perhydrolysis activity of the enzyme variants and may be helpful for designing enzyme mutants with further improved perhydrolysis activity.     

Computing discrete logarithms in real quadratic congruence function fields of large genus

The discrete logarithm problem in various finite abelian groups is the basis for some well known public key cryptosystems. Recently, real quadratic congruence function fields were used to construct a public key distribution system. The security of this public key system is based on the difficulty of a discrete logarithm problem in these fields. In this paper, we present a probabilistic algorithm with subexponential running time that computes such discrete logarithms in real quadratic congruence function fields of sufficiently large genus. This algorithm is a generalization of similar algorithms for real quadratic number fields.