Optimizing the performance of the multiconfiguration molecular mechanics method

Multiconfiguration molecular mechanics (MCMM) is a general algorithm for constructing potential energy surfaces for reactive systems (Kim, Y.; Corchado, J. C.; Villà, J.; Xing, J.; Truhlar, D. G. J. Chem. Phys. 2000, 112, 2718). This paper illustrates how the performance of the MCMM method can be improved by adopting improved molecular mechanics parameters. We carry out calculations of reaction rate constants using variational transition state theory with optimized multidimensional tunneling on the MCMM PESs for three hydrogen transfer reactions, and we compare the results to direct dynamics. We find that the MCMM method with as little as one electronic structure Hessian can describe the dynamically important regions of the ground-electronic state PES, including the corner-cutting-tunneling region of the reaction swath, with practical accuracy.     

Polynomials whose coefficients coincide with their zeros

In this paper we consider monic polynomials such that their coefficients coincide with their zeros. These polynomials were first introduced by S. Ulam. We combine methods of algebraic geometry and dynamical systems to prove several results. We obtain estimates on the number of Ulam polynomials of degree N. We provide additional methods to obtain algebraic identities satisfied by the zeros of Ulam polynomials, beyond the straightforward comparison of their zeros and coefficients. To address the question about the existence of orthogonal Ulam polynomial sequences, we show that the only Ulam polynomial eigenfunctions of hypergeometric type differential operators are the trivial Ulam polynomials \(\{x^N\}_{N=0}^\infty \). We propose a family of solvable N-body problems such that their stable equilibria are the zeros of certain Ulam polynomials.     

Unexpected behaviour of tosylated and acetylated imidazolinium salts

Tosylated and acetylated imidazolinium salts revealed an unexpected reactivity when treated with methyl iodide or benzyl bromide. Moreover an unprecedented acid-catalysed rearrangement for an acetylated imidazolinium salt was observed during an anion metathesis.     

Isobornylchalcones as Scaffold for the Synthesis of Diarylpyrazolines with Antioxidant Activity

The pyrazoline ring is defined as a “privileged structure” in medicinal chemistry. A variety of pharmacological properties of pyrazolines is associated with the nature and position of various substituents, which is especially evident in diarylpyrazolines. Compounds with a chalcone fragment show a wide range of biological properties as well as high reactivity which is primarily due to the presence of an α, β-unsaturated carbonyl system. At the same time, bicyclic monoterpenoids deserve special attention as a source of a key structural block or as one of the pharmacophore components of biologically active molecules. A series of new diarylpyrazoline derivatives based on isobornylchalcones with different substitutes (MeO, Hal, NO₂, N(Me)₂) was synthesized. Antioxidant properties of the obtained compounds were comparatively evaluated using in vitro model Fe²⁺/ascorbate-initiated lipid peroxidation in the substrate containing brain lipids of laboratory mice. It was demonstrated that the combination of the electron-donating group in the para-position of ring B and OH-group in the ring A in the structure of chalcone fragment provides significant antioxidant activity of synthesized diarylpyrazoline derivatives.     

Failure of immunocompetitive capillary electrophoresis assay to detect disease-specific prion protein in buffy coat from humans and chimpanzees with Creutzfeldt-Jakob disease

The emergence of a new environmentally caused variant of Creutzfeldt-Jakob disease (vCJD), the result of food-born infection by the causative agent of bovine spongiform encephalopathy (BSE), has stimulated research on a practical diagnostic screening test. The immunocompetitive capillary electrophoresis (ICCE) assay has been reported to detect disease-specific, proteinase-resistant prion protein (PrPres) in the blood of scrapie-infected sheep. We have applied this method to blood from CJD-infected chimpanzees and humans. The threshold of detection achieved with our ICCE was 0.6 nM of synthetic peptide corresponding to the prion protein (PrP) C-terminus, and 2 nM of recombinant human PrP at the optimized conditions. However, the test was unable to distinguish between extracts of leucocytes from healthy and CJD-infected chimpanzees, and from healthy human donors and patients affected with various forms of CJD. Thus, the ICCE assay as presently performed is not suitable for use as a screening test in human transmissible spongiform encephalopathies (TSEs).     

A very large Rh-Rh bond shortening on excitation of the [Rh2(1,8-diisocyano-p-menthane)4]2+ ion by time-resolved synchrotron X-ray diffraction

A very large Rh-Rh contraction of approximately 0.85 A occurs on excitation of the [Rh(2)(1,8-diisocyano-p-menthane)(4)](2+) ion to its triplet state.     

Electronic factors in (4 + 2)-cycloaddition reactions of phosphaalkenes

Reactions involving the (4 + 2)-cycloaddition of phosphaalkenes to 1,3-butadiene have been characterized in the framework of the frontier-orbital approach. MNDO and nonempirical calculations in the 4-31G basis have shown that most of these reactions take place under normal electronic conditions. The -electronic stabilization energies of the transition states of the reactions have been evaluated. The influence of substituents on the relative rate and regioselectivity of the reactions of phosphaalkenes with derivatives of 1,3-butadiene has been considered.Institute of Organic Chemistry, Academy of Sciences of the Ukrainian SSR, Kiev. Scientific-Research Institute of Technical Physics, Far-Eastern University. Vladivostok. Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 27, No. 4, pp. 462–466, July–August, 1991. Original article submitted October 12, 1990.     

How well can density functional methods describe hydrogen bonds to pi acceptors?

We employed four newly developed density functional theory (DFT) methods for the calculation of five pi hydrogen bonding systems, namely, H2O-C6H6, NH3-C6H6, HCl-C6H6, H2O-indole, and H2O-methylindole. We report new coupled cluster calculations for HCl-C6H6 that support the experimental results of Gotch and Zwier. Using the best available theoretical and experimental results for all five systems, our calculations show that the recently proposed MPW1B95, MPWB1K, PW6B95, and PWB6K methods give accurate energetic and geometrical predictions for pi hydrogen bonding interactions, for which B3LYP fails and PW91 is less accurate. We recommend the most recent DFT method, PWB6K, for investigating larger pi hydrogen bonded systems, such as those that occur in molecular recognition, protein folding, and crystal packing.     

Mechanism of the oxidation reaction of Cu with N2O via nonadiabatic electron transfer

We treat the present work as an attempt to elucidate the mechanism of the oxidation reaction of the Cu atom by nitrous oxide based on our recent work (Kryachko, E. S.; Vinckier, C.; Nguyen, M. T. J Chem Phys 2001, 114, 7911) on the electron attachment to this molecule. We suggest that the title reaction in its Arrhenius regime occurs via the nonadiabatic electron transfer from Cu to the oxygen atom at the crossing of the potential energy surfaces Cu(4s ²S_1/2) + N₂O(X ¹Σ⁺) and Cu⁺ + N₂O^?, where the latter is linked to the complex N₂O^? originated from the higher‐energy T‐shape N₂O molecule and discovered in the aforementioned work. The calculations performed in the present work using a variety of quantum chemical methods support the proposed model. We also show the existence of other reaction pathways of the title reaction that, we believe, contribute to its non‐Arrhenius behavior observed experimentally at T > 1190 K. © 2002 Wiley Periodicals, Inc. Int J Quantum Chem, 2002     

Novel Ligands Incorporating Phosphinoxide Groups as Particularly Efficient Sensitizers for Lanthanide Emission

Helical molecular assemblies of bile acids and their derivatives were systematically investigated. These molecules have a common asymmetric structure with three different directions, and form characteristic inclusion crystals with the following hierarchical structure. First, such molecules arrange along their helical axes to yield the helical tapes with a definite direction. Second, the helical tapes are combined in a parallel fashion by using their side-chains through hydrogen bonds to produce chiral sheets. Third, the sheets stack together through van der Waals forces in a parallel or antiparallel fashion. Fourth, there exist chiral spaces for including guests among their sheets. Such hierarchical structure enables us to explain a role of the side-chains with different hydrogen bonding groups and length.