Transition state localization by a density functional method. Applications to isomerization and symmetry-forbidden reactions

Summary A new transition state algorithm incorporated into the density functional code deMon, has been used to study thetrans cis-hydroxymethylene andtrans-hydroxymethylene formaldehyde isomerization processes and to localize the transition state for the decomposition of 1,3-diazacyclobutane to methyleneimine. Calculations have been performed using both local (LSD) and nonlocal spin density (NLSD) gradient-corrected approximations. Two different basis sets of small and large size have been used for the study in order to analyze the effect of the basis set dimension other than that of the nonlocal corrections on the activation energy barrier value. The results seem to confirm that density functional methods can be considered practical and reliable tools for the localization of transition states.     

A spectral solution method for a boundary-value problem for a mixed-type equation with two singularity lines

We consider a boundary-value problem for a mixed-type equation with two perpendicular singularity lines given in a domain whose elliptic part is a rectangle, while the hyperbolic one is a vertical half-strip. This problem differs from the Dirichlet one by the fact that at the left boundary of the rectangle and the half-strip we specify the vanishing order of the desired function rather than its value. We find a solution to the problem by a spectral method with the use of the Fourier–Bessel series and prove the uniqueness of the solution. We substantiate the uniform convergence of the corresponding series under certain requirements to the problem statement.     

Density functional treatment of water–carbon dioxide van der waals complex

LCGTO–LSD and LCGTO–NLSD methods have been tested for the study of water–carbon dioxide weakly bound binary complex. Different local and nonlocal exchange-correlation energy functionals and many grid radial points have been used. Results show that both nonlocal corrections and a large number of radial points in the grid are mandatory for well reproducing the experimental data. © 1994 John Wiley & Sons, Inc.     

(salen)MnIII compounds as nonpeptidyl mimics of catalase. Mechanism-based tuning of catalase activity: a theoretical study

We present the results of the first theoretical investigation of salen-manganese complexes as synthetic catalytic scavengers of hydrogen peroxide molecules that mimic catalase enzymes. Catalase mimics can be used as therapeutic agents against oxidative stress in treatment of many diseases, including Alzheimer's disease, stroke, heart disease, aging, and cancer. A ping-pong mechanism approach has been considered to describe the H2O2 dismutation reaction. The real compounds reacting with a peroxide molecule were utilized in our BP density functional calculations to avoid uncertainties connected with using incomplete models. Part I of the dismutation reaction-converting a peroxide molecule into a water molecule with simultaneous oxidation of the metal atom of the catalyst-can be done quite effectively at the Mn catalytic center. To act as catalytic scavengers of hydrogen peroxide, the oxomanganese salen complexes have to be deoxidized during part II of the dismutation reaction. It has been shown that there are two possible reaction routes for the second part of the dismutation reaction: the top and the side substrate approach routes. Our results suggest that the catalyst could be at least temporarily deactivated (poisoned) in the side approach reaction route due to the formation of a kinetically stable intermediate. Overall, the side approach reaction route for the catalyst recovery is the bottleneck for the whole dismutation process. On the basis of the detailed knowledge of the mode of action of the (salen)MnIII catalase mimics, we suggest and rationalize structural changes of the catalyst that should lead to better therapeutic properties. The available experimental data support our conclusions. Our findings on the reaction dismutation mechanism could be the starting point for further improvement of salen-manganese complexes as synthetic catalytic scavengers of reactive oxygen species.     

Intrasphere Transformations of Platinum Nitrocomplexes in Phosphoric Acid Solutions as Method of Synthesis of Different Oligomeric Platinum Phosphates

Abstract

Platinum phosphatocomplexes of various types were for the first time obtained in multi-stage redox-interaction of isomers Pt(NH₃)₂ (NO₂)₂ with conc.H₃PO₄: phosphatonitroso-mines cis-HPt (NH₂NONH₃)NO₂ (H₂PO₄) ₂H₃PO₄ (I), trans- Pt(NH₃)₂ · NO₂NO(H₂PO₄)₂H₃PO₄(II); phosphatonitrodi'amines cis-(H_n) · [Pt(NH₃)₄(μ-HPO₄) (NO₂)₂] (H₃PO₄)₄(H₂0)₂(III), trans-(H_n) · [Pt(NH₃)₂(-NO₂)] (H₂PO₄)_1.25(H₂0) _1.5(IV); phosphatonitromo-nolamines (H)Pt₂(NH₃)₂(μ-NO₂) (μ-HPO₄) ₂ · 1.5H₂O (V)t tetra-phosphates (NH₄)₂[Pt₂(HPO₄)₄(H₂0)₂1 (VI) etc, (H)_2+n (NH₄)₂Pt₂(HPO₄4)(H₂20)(VII); phosphatodiamines cis-(H)_n[Pt₂ (NH₃ · L)₂(μ-HPO₄)₂] (VIII, IX), (L=NH₃, H₂0), n O. Molecular structures of the compounds IV-IX were derived from analysis of AB, IR, XPS, ESR and RDF spectra: binuclear clusters (VI, VII) and oligomeric chains consisting of platinum atoms bonded both by direct metal-metal interaction and by bridging groups (IV, V, VIII, IX) (1). 111, IV, VIII, IX are classifed as platinum blues of a new type with inorganic anions as bridging ligands: NO^? ₂(IV), HPO^2- ₄(III, VIII, IX). IV is the first trans-platinum blue. cis-Diamines form adducts in which the medium molecules are bonded with cis-ammonias. trans-Diamines do not form such adducts. Paper (1) presents a general mechanism of compound formations consisting in generation of intermediate Pt(III) forms followed by interaction with environmental species.