Showing papers in "International Journal of Molecular Sciences in 2002"

Theoretical Calculation of Absolute Radii of Atoms and Ions. Part 2. The Ionic Radii

Abstract: The theoretical method of determination of absolute atomic size, discussed in Int. J. Mol. Sci. 2002, 3, 87-113, is exploited to calculate absolute radii of the ions whose experimental radii are published by Shanon. The computed radii are found to reproduce the expected periodic variation of size in periods and in groups and nicely reproduce the d-block and f-block contractions in the respective series. It is pointed out that experimental radii of d and f block transition metal ions make erroneous and misleading representation of the size behaviour of the respective series. A detailed comparative study of the crystal radii vis-a-vis the theoretical radii is reported. A rationale of the double hump curve of the experimental radii of 3 d-block transition metal ions is put forward in terms of the crystal field theory and Jahn-Teller distortion. The theoretical radii are exploited to calculate the diamagnetic susceptibility, polarizability and chemical hardness of the ions and compared with available experimental data. The fact of good agreement between the experimental and computed global hardness of ions and correct demonstration of d-block and f-block contraction by the computed radii are used as benchmark to test the validity of the values of the computed theoretical radii of the ions as their representative sizes. It is concluded that the theoretically computed radii of ions are visualizable size representation of ions and can be used as their absolute radii at the respective oxidation states.

Can Unrestricted Density-Functional Theory Describe Open Shell Singlet Biradicals?

Abstract: Unrestricted density functional theory (UDFT) can be used for the description of open-shell singlet (OSS) biradicals provided a number of precautions are considered. Biradicals that require a two-determinantal wave function (e.g. OSS state of carbenes) cannot be described by UDFT for principal reasons. However, if the overlap between the open-shell orbitals is small (the single electrons are located at different atomic centers) errors become small and, then, the principal failure of UDFT in these cases is not apparent and may even be disguised by the fact that UDFT has the advantage of describing spin polarization better than any restricted open shell DFT method. In the case of OSS biradicals with two- or multiconfigurational character (but a onedeterminantal form of the leading configuration), reasonable results can be obtained by broken-symmetry (BS)-UDFT, however in each case this has to be checked. In no case is it reasonable to lower the symmetry of a molecule to get a suitable UDFT description. Hybrid functionals such as B3LYP perform better than pure DFT functionals in BS-UDFT calculations because the former reduce the self-interaction error of DFT exchange functionals, which mimics unspecified static electron correlation effects, so that the inclusion of specific static electron correlation effects via the form of the wavefunction becomes more effective.

The O-H Bond Dissociation Energies of Substituted Phenols and Proton Affinities of Substituted Phenoxide Ions: A DFT Study

Abstract: The accurate O-H bond dissociation enthalpies for a series of meta and para substituted phenols (X-C 6 H 4 -OH, X=H, F, Cl, CH 3 , OCH 3 , OH, NH 2 , CF 3 , CN, and NO 2 ) have been calculated by using the (RO)B3LYP procedure with 6-311G(d,p) and 6-311++G(2df,2p) basis sets. The proton affinities of the corresponding phenoxide ions (X-C 6 H 4 -O - ) have also been computed at the same level of theory. The effect of change of substituent position on the energetics of substituted phenols has been analyzed. The correlations of Hammett’s substituent constants with the bond dissociation enthalpies of the O-H bonds of phenols and proton affinities of phenoxide ions have been explored. Keywords: Substituted Phenol, Bond Dissociation Energy, Acidity, DFT. I. Introduction Phenols are widely used as synthetic organic materials and also as antioxidants in living organisms [1]. Phenoxyl radicals are known as important intermediates in many biological and industrial applications [2]. Phenols are of special interest in organic chemistry, since their acid-base equilibria have often been used as reference values in establishing linear free energy relationships [3]. Consequently, much effort has been put to understand the factors governing the O-H bond dissociation energies, BDE(O-H), and acidities of substituted phenols, both in the solution and gas phase [4-11].

Chemical Reactivity as Described by Quantum Chemical Methods

Abstract: Density Functional Theory is situated within the evolution of Quantum Chemistry as a facilitator of computations and a provider of new, chemical insights. The importance of the latter branch of DFT, conceptual DFT is highlighted following Parr's dictum "to calculate a molecule is not to understand it". An overview is given of the most important reactivity descriptors and the principles they are couched in. Examples are given on the evolution of the structure-property-wave function triangle which can be considered as the central paradigm of molecular quantum chemistry to (for many purposes) a structure-property-density triangle. Both kinetic as well as thermodynamic aspects can be included when further linking reactivity to the property vertex. In the field of organic chemistry, the ab initio calculation of functional group properties and their use in studies on acidity and basicity is discussed together with the use of DFT descriptors to study the kinetics of SN2 reactions and the regioselectivity in Diels Alder reactions. Similarity in reactivity is illustrated via a study on peptide isosteres. In the field of inorganic chemistry non empirical studies of adsorption of small molecules in zeolite cages are discussed providing Henry constants and separation constants, the latter in remarkable good agreement with experiments. Possible refinements in a conceptual DFT context are presented. Finally an example from biochemistry is discussed : the influence of point mutations on the catalytic activity of subtilisin.

Implicit and Explicit Coverage of Multi-reference Effects by Density Functional Theory

Abstract: Multi-reference effects can be covered by density functional theory (DFT) either implicitly via the exchange-correlation functional or explicitly via the form of the Kohn-Sham wave function. With the help of the exchange hole it is shown that the self-interaction error of the exchange functional will mimic long-range electron correlation effects if restricted Kohn-Sham theory is used. Functionals based on Slater or Becke exchange have a relatively large self-interaction error and, therefore, lead to a relatively large implicit coverage of long-range correlation, which, because of the possibility of double- counting of electron correlation, has to be considered when using these functionals in connection with two- or multi-configurational descriptions based on ensemble DFT methods such as REKS (spin- Restricted Ensemble-referenced KS-DFT). Arguments are given that a REKS description of a multi- reference problem avoids a double-counting of long-range correlation effects, in particular as in this situation the self-interaction error of the exchange functional simulates more short- rather than long- range correlation effects. There is, however, no guarantee that the short-range effects are not double- counted, namely once via the exchange and once via the correlation functional. Therefore, one should use hybrid functionals such as B3LYP in connection with multi-reference DFT methods because for hybrid functionals the self-interaction error and by this the implicit coverage of long(short)-range correlation effects is reduced due to the admixture of exact exchange. This rule applies also to broken-symmetry UDFT, which performs better with hybrid rather than GGA functionals. A way of avoiding the implicit coverage of multi-reference effects is given by the combination of wave function theory and DFT methods. The advantages and disadvantages of CAS-DFT are discussed and it is shown that an effective reduction of a double-counting of correlation effects is possible within this

Genotoxic Pyrrolizidine Alkaloids — Mechanisms Leading to DNA Adduct Formation and Tumorigenicity

Abstract: Plants that contain pyrrolizidine alkaloids are widely distributed in the world. Although pyrrolizidine alkaloids have been shown to be genotoxic and tumorigenic in experimental animals, the mechanisms of actions have not been fully understood. The results of our recent mechanistic studies suggest that pyrrolizidine alkaloids induce tumors via a genotoxic mechanism mediated by 6,7-dihydro-7-hydroxy-1-hydroxymethyl-5Hpyrrolizine (DHP)-derived DNA adduct formation. This mechanism may be general to most carcinogenic pyrrolizidine alkaloids, including the retronecine-, heliotridine-, and otonecinetype pyrrolizidine alkaloids. It is hypothesized that these DHP-derived DNA adducts are potential biomarkers of pyrrolizidine alkaloid tumorigenicity. The mechanisms that involve the formation of DNA cross-linking and endogenous DNA adducts are also discussed.

Use of Local Softness for the Interpretation of Reaction Mechanisms

Abstract: The application of reactivity parameters derived from density functional theory in a local sense, in particular the softness and Fukui function, to interpret and predict the mechanisms of various organic reactions has been discussed. Local softness is shown to be successful in determining the site-selectivity and regiochemistry and can be used as an alternative to the traditional frontier orbital theory. Keywords: Fukui Function, Local Softness, Reactivity Indices, Reaction Mechanism, 1,3-Dipolar Cycloadditions. 1. Introduction According to the density functional theory (DFT), the properties of a molecular system are determined by its total electron density [1]. However, electron density alone could not describe all the chemical phenomena. Its sensitivities to structural perturbations and responses to the changes in external conditions are rather more important in reflecting the chemical reactivity of a system than its absolute values. Therefore much effort has recently been devoted to the development and application of reactivity indices derived from electron density. Many of the well known, empirical but important, chemical concepts such as the chemical potential (µ), electronegativity (χ), hardness (η), softness (S)… appear naturally within the framework of DFT

To Multireference or not to Multireference: That is the Question?

Abstract: I present a personal viewpoint on multi-reference coupled-cluster theory, its pros and cons. I also suggest some criteria that should be satisfied by multi-reference CC, not the least of which is to develop a tool that will be (almost!) as easy to apply as today’s powerful array of single reference coupled-cluster methods. Some approaches like the equation of motion CC method offers a multi-reference description of some target states, while being entirely single reference in execution. Perhaps it offers a model for further generalization to a wider array of multi-reference problems.

State Selective Equation of Motion Coupled Cluster Theory: Some Preliminary Results

Abstract: A multireference variant of coupled cluster theory is described that applies to systems that can qualitatively be described by deleting two electrons from a closed shell determinant, for example biradicals, single bond breaking processes, or valence excited states. The theory can be generalized to arbitrary open-shell systems and takes a form that is akin to equation-of-motion coupled cluster theory, but where all wave function parameters are explicitly optimized for the state of interest. The implementation of the present methods was accomplished in an automated fashion using the recently developed Automatic Program Generator (APG). We present benchmark results for the O 2 and F 2 molecules and investigate the behaviour of a number of closely related variants within the same general framework. Keywords: Multireference electronic structure; Coupled cluster; Electronically excited states; Automatic code generation. I. Introduction Single reference coupled cluster theory, [1,2] especially CCSD(T) [3,4], is a highly successful approach to describe the electronic structure of systems that are qualitatively reasonably well described by a single determinant (many closed-shell molecules and high-spin open-shell systems) [5-8]. Likewise coupled cluster response theory (CCLRT [9-11]) and its cousin equation-of-motion coupled cluster theory (EOM-CC [5,12]) are well suited to describe singly excited states and radicals. An important prerequisite for the applicability of the CC response approach is the presence of a nearby state that can serve as the reference in the parent state CCSD calculation. Most commonly the reference state is a closed-shell system that differs by up to one electron from the states of interest. The

The Concept of Density Functional Theory Based Descriptors and its Relation with the Reactivity of Molecular Systems: A Semi-Quantitative Study

Abstract: In this present paper, we have made an attempt to explain the theoretical basis for the empirical hardness/softness concepts to address the reactivity of molecular complexes in a semi-quantitative way within the framework of density functional theory A model based on local hard-soft-acid-base principle has been proposed The results obtained using some prototype charge transfer complexes, Lewis acid-base complexes and hydrogen-bonded complexes as examples, are in good agreement with the standard ab initio values Although the model contains ad hoc parameters, it may form the basis of semi-quantitative description of inter-molecular interactions using hardness/softness parameters The limitation, weakness and other critical issues of the present model are also discussed

Zeolite-Catalyzed Hydrocarbon Formation from Methanol: Density Functional Simulations

Abstract: We report detailed density functional theory (DFT) calculations of important mechanisms in the methanol to gasoline (MTG) process in a zeolite catalyst. Various reaction paths and energy barriers involving C-O bond cleavage and the first C-C bond formation are investigated in detail using all-electron periodic supercell calculations and recently developed geometry optimization and transition state search algorithms. We have further investigated the formation of ethanol and have identified a different mechanism than previously reported [1], a reaction where water does not play any visible role. Contrary to recent cluster calculations, we were not able to find a stable surface ylide structure. However, a stable ylide structure built into the zeolite framework was found to be possible, albeit a very high reaction barrier.

The State-Universal Multi-Reference Coupled-Cluster Theory: An Overview of Some Recent Advances

Abstract: Some recent advances in the area of multi-reference coupled-cluster the-ory of the state-universal type are overviewed. An emphasis is placed on the follow-ing new developments: (i) the idea of combining the state-universal multi-referencecoupled-cluster singles and doubles method (SUMRCCSD) with the multi-referencemany-body perturbation theory (MRMBPT), in which cluster amplitudes of the SUM-RCCSD formalism that carry only core and virtual orbital indices are replaced by theirfirst-order MRMBPT estimates; and (ii) the idea of combining the recently proposedmethod of moments of coupled-cluster equations with the SUMRCC formalism. It isdemonstrated that the new SUMRCCSD(1) method, obtained by approximating theSUMRCCSD cluster amplitudes carrying only core and virtual orbital indices by theirfirst-orderMRMBPTvalues, providestheresultsthatarecomparabletothoseobtainedwith the complete SUMRCCSD approach.Keywords: Coupled-cluster theory; Multi-reference formalism; State-universal multi-reference coupled-cluster method; Many-body perturbation theory; Method of mo-ments of coupled-cluster equations c 2002 by MDPI, Basel, Switzerland. Reproduction for noncommercial purposes permitted.

State-specific multi-reference perturbation theories with relaxed coefficients: molecular applications

Abstract: We present in this paper two new versions of Rayleigh-Schr¨odinger (RS) and the Brillouin-Wigner (BW) state-specific multi-reference perturbative theories (SSMRPT) which stem from our state-specific multi-reference coupled-cluster formalism (SS-MRCC), developed with a complete active space (CAS). They are manifestly sizeextensive and are designed to avoid intruders. The combining coefficients cμ for the model functions φμ are completely relaxed and are obtained by diagonalizing an effective operator in the model space, one root of which is the target eigenvalue of interest. By invoking suitable partitioning of the hamiltonian, very convenient perturbative versions of the formalism in both the RS and the BW forms are developed for the second order energy. The unperturbed hamiltonians for these theories can be chosen to be of both Mφller-Plesset (MP) and Epstein-Nesbet (EN) type. However, we choose the corresponding Fock operator fμ for each model function φμ, whose diagonal elements are used to define the unperturbed hamiltonian in the MP partition. In the EN partition, we additionally include all the diagonal direct and exchange ladders. Our SS-MRPT thus utilizes a multi-partitioning strategy. Illustrative numerical applications are presented for potential energy surfaces (PES) of the ground (1Σ+) and the first delta (1Δ) states of CH+ which possess pronounced multi-reference character. Comparison of the results with the corresponding full CI values indicates the efficacy of our formalisms.

A Protons Exchanged Montmorillonite Clay as an Efficient Catalyst for the Reaction of Isobutylene Polymerization

Abstract: “Maghnite” a montmorillonite sheet silicate clay, exchanged with protons to produce “H-Maghnite” is an efficient catalyst for cationic polymerization of many vinylic and heterocyclic monomers (Belbachir, M. U.S. Patent. 066969.0101 –2001) . The structure compositions of both “Maghnite” and “H-Maghnite” have been developed. Isobutylene monomer, wich is polymerizable only by cationic process (Odian,G. La Polymerisation :principes et Applications ; Ed.Technica: New York, 1994; pp 222-226), was used to elucidate the cationic character of polymerization. The polymerization was performed under suitable conditions at isobutylene vaporization temperature (–7°C). Experiments revealed that polymerization induced by “H-Maghnite” proceed in bulk and in solution. In contrast to findings with methylene chloride CH 2 Cl 2 as a polar solvent, polymerization yields with hexane C 6 H 14 non-polar solvent is very significant. In bulk polymerization, Isobutylene conversion increases with increasing “H-Maghnite” proportion.

Effect of Dissolved Humic Substances on the Photochemical Degradation Rate of 1-Aminopyrene and Atrazine

Abstract: Humic substances (HS) are ubiquitous in the environment, and can act as photosensitizers in the redox reactions of some photochemical processes The influence of HS in these reactions varies with the HS type and concentration The total organic carbon content (TOC) of some commercial HS (such as soil and river humic acid, and fulvic acid) was studied 1-Aminopyrene (1-AP) and 1-hydoxypyrene (1-HP) are carcinogenic and slightly water-soluble polycyclic aromatic hydrocarbons (PAH) The impact of PAH on natural environment is related to their photolysis rates and photoproducts; therefore, it is of interest to study the photolysis of these compounds Our previous study showed that the photolysis rate of 1-HP was inhibited by HS In this study, photolysis of 1-AP was conducted with pure water, natural river water, and pure water containing commercial HS It was found that the photolysis rate of 1-AP can be inhibited or enhanced by HS, depending on the type and concentration The first order photolysis rate constant of 1-AP (10 μM) in phosphate buffer (pH 70, 1 mM) containing a humic acid (20-80 ppm) was enhanced by up to 5 folds With a fulvic acid (20-80 ppm), it was enhanced by about 2 folds With a soil humic acid, it was enhanced by about 2 folds at the concentration of 20 ppm and was inhibited by up to 4 folds at the concentration of 80 ppm Atrazine (2-chloro-4-ethylamine-6-isopropylamino-s-triazine) is a widely used herbicide It is toxic, often bioaccumulative and persistent In this study, the effect of HS on the photochemical fate of atrazine was also studied The results showed that photolysis of atrazine can be enhanced by humic acid, depending on the type and concentration of humic acid The fulvic acid has no effect on its photolysis within 10 days

Mercury Induces Cytotoxicity and Transcriptionally Activates Stress Genes in Human Liver Carcinoma (HepG2) Cells

Abstract: Mercury is a non-essential element that exhibits a high degree of toxicity to humans and animals. Exposure to mercury has been associated with a significant number of adverse health effects including: cardiovascular disease, anemia, developmental abnormalities, neurobehavioral disorders, kidney and liver damage, and cancer in some cases. In several studies, the toxicity of mercury has been attributed to its high affinity to protein-containing sulfhydryl groups. However, little is known regarding the molecular mechanisms by which mercury exerts its toxicity, mutagenesis, and carcinogenesis. This research was therefore designed to assess the cellular and molecular responses of human liver carcinoma cells following exposure to mercury. Cytotoxicity was evaluated using the MTT-assay for cell viability, while the gene profile assay was performed to measure the transcriptional activation of stress genes in thirteen different recombinant cell lines generated from HepG2 cells. Cytotoxicity experiment yielded a LD50 value of 3.5 ± 0.6 μg/mL upon 48 hours of exposure, indicating that mercury is highly toxic. A dose response relationship was recorded with respect to both cytotoxicity and gene induction. Overall, nine out of the thirteen recombinant cell lines tested showed inductions to statistically significant levels (p 0.05) were observed for CYP1A1, XRE, NFkBRE, and RARE.

Differential Cytotoxicity and Gene Expression in Human Liver Carcinoma (HepG2) Cells Exposed to Arsenic Trioxide, and Monosodium Acid Methanearsonate (MSMA)

Abstract: Research in our laboratory has demonstrated that a trivalent form of arsenic such as arsenic trioxide (AT) has the ability to cause significant cytotoxicity, and induction of a significant number of stress genes in human liver carcinoma cells (HepG2). However, the literature also indicates that the toxicity of arsenic depends on its chemical form. To test this hypothesis, we further evaluated the cellular and molecular responses of HepG2 cells following exposure to monosodium acid methanearsonate (MSMA), a pentavalent and organic form of arsenic. Cytotoxicity was evaluated using the MTT-assay for cell viability, while the gene profile assay was performed to measure the degree of gene induction in 13 different recombinant cell lines generated from a parental HepG2 cell line. Cytotoxicity experiments yielded LC50 values of 11.9 + 2.6 μg/mL for AT, and 257.3 + 51.4μg/mL for MSMA; indicating that AT was about 20 times more toxic than MSMA. Exposure of HepG2 cells to MSMA also resulted in a significant reduction (p < 0.05) in the number of stress genes induced, compared to AT. Upon MSMA exposure, only 2 (HMTIIA and HSP70) out of the 13 constructs evaluated yielded inductions to statistically significant levels (p < 0.05), compared to 11 (GSTYa, XRE, HMTIIA, c-fos, NF-kBRE, HSP70, p53RE, GADD153, GADD45, and GRP78) for AT. These results greatly support the hypothesis that the toxicity of arsenic compounds highly depends on their chemical forms; with the inorganic forms being more potent than the organic ones.

Health Risk Assessment of Pesticide Usage in Menia El-Kamh Province of Sharkia Governorate in Egypt

Abstract: Menia El-Kamh province of the Sharkia Governorate constitutes one of the largest agricultural areas in Egypt. About 88% of the nearly 472,000 people living in this province rely on agricultural activities for subsistence. Several pesticides including organochloride, organophosphorus, carbamate, and pyrethroid insecticides, fungicides, and herbicides are commonly used in citrus, vegetable and other crop-growing areas to increase agricultural productivity. However, their use has also been associated with several cases of pesticide poisoning. In this research, we conducted a field survey to assess the knowledge, attitudes, and practices of the farmer’s community regarding the safe use of pesticides. We also evaluated the residual concentrations of selected pesticides in water, soil, milk, fish, and orange samples, and estimated the potential health risks associated with the exposure to these pesticides. Data obtained from the field survey indicate that more than 95% of farm workers do not practice safety precautions during pesticide formulation and application; leading to a considerable prevalence of pesticide-related illnesses in this agricultural community. Pesticide residues in various environmental samples varied greatly; from below detection levels (3-5 ng) to as high as 325 ppb depending on the matrix of interest, and the specific pesticide of concern. The analysis of health risk estimates indicated that chlorpyrifos, DDT, dimethoate, methomyl, and larvin did not pose a direct hazard to human

Applications of the Information Theory to Problems of Molecular Electronic Structure and Chemical Reactivity

Abstract: Recent studies on applications of the information theoretic concepts to molecular systems are reviewed. This survey covers the information theory basis of the Hirshfeld partitioning of molecular electron densities, its generalization to many electron probabilities, the local information distance analysis of molecular charge distributions, the charge transfer descriptors of the donor-acceptor reactive systems, the elements of a “thermodynamic” description of molecular charge displacements, both “vertical” (between molecular fragments for the fixed overall density) and “horizontal” (involving different molecular densities), with the entropic representation description provided by the information theory. The average uncertainty measures of bond multiplicities in molecular “communication” systems are also briefly summarized. After an overview of alternative indicators of the information distance (entropy deficiency, missing information) between probability distributions the properties of the “stockholder” densities, which minimize the entropy deficiency relative to the promolecule reference, are summarized. In particular, the surprisal analysis of molecular densities is advocated as an attractive information-theoretic tool in the electronic structure theory, supplementary to the familiar density difference diagrams. The subsystem information density equalization rules satisfied by the Hirshfeld molecular fragments are emphasized: the local values of alternative information distance densities of subsystems are equal to the corresponding global value, characterizing the molecule as a whole. These local measures of the information content are semi-quantitatively related to the molecular density difference function. In the density functional theory the effective external potentials of molecular fragments are defined, for which the subsystem densities are the ground-state densities. The nature of the energetic and “entropic” equilibrium conditions is reexamined and the entropy representation forces driving the charge transfer in molecular systems are introduced. The latter combine the familiar Fukui functions of subsystems with the information densities, the entropy representation “intensive” conjugates of the subsystem electron densities, and are shown to exactly vanish for the “stockholder” charge distribution. The proportionality relations between charge response characteristics of reactants, e.g., the Fukui functions, are derived. They are shown to follow from the minimum entropy deficiency principles formulated in terms of both the subsystems electron densities and Fukui functions, respectively.

Method of Moments of Coupled-Cluster Equations: Externally Corrected Approaches Employing Configuration Interaction Wave Functions

Abstract: A new approach to the many-electron correlation problem, termed the method of moments of coupled-cluster equations (MMCC), is further developed and tested. The main idea of the MMCC theory is that of the noniterative energy corrections which, when added to the energies obtained in the standard coupled-cluster calculations, recover the exact (full configuration interaction) energy. The MMCC approximations require that a guess is provided for the electronic wave function of interest. The idea of using simple estimates of the wave function, provided by the inexpensive configuration interaction (CI) methods employing small sets of active orbitals to define higher–than–double excitations, is tested in this work. The CI-corrected MMCC methods are used to study the single bond breaking in HF and the simultaneous breaking of both O–H bonds in H2O.

Proton NMR Chemical Shift Behavior of Hydrogen-Bonded Amide Proton of Glycine-Containing Peptides and Polypeptides as Studied by ab initio MO Calculation

Abstract: NMR chemical shifts of the amide proton of a supermolecule, an Nmethylacetamide hydrogen-bonded with a formamide, were calculated as functions of hydrogen-bond length RN…O and hydrogen-bond angles by FPT-GIAO method within the framework of HF/STO 6-31++G(d,p) ab initio MO method. The calculations explained reasonably the experimental data reported previously that the isotropic proton chemical shifts move downfield with a decrease in RN…O. Further, the behavior of proton chemical shift tensor components depending on the hydrogen-bond length and hydrogen-bond angle was discussed.

Quantum Chemical and FTIR Spectroscopic Studies on the Linkage Isomerism of Carbon Monoxide in Alkali-Metal-Exchanged Zeolites: A Review of Current Research

Abstract: When adsorbed (at a low temperature) on alkali-metal-exchanged zeolites, CO forms both M(CO) + and M(OC) + carbonyl species with the extra-framework alkali-metal cation of the zeolite Both quantum chemical and experimental results show that C-bonded adducts are characterized by a C−O stretching IR band at a frequency higher than that of 2143 cm -1 for free CO, while for O-bonded adducts this IR band appears below 2143 cm -1 The cation-CO interaction energy is higher for M(CO) + than for M(OC) + carbonyls, although the corresponding difference decreases substantially when going from Li + to Cs + By means of variable-temperature FTIR spectroscopy, this energy difference was determined for several alkali-metal cations, and the existence of a thermal equilibrium between M(CO) + and M(OC) + species was established The current state of research in this field is reviewed here, with a view to gain more insight into the thermal isomerization process Keywords: Alkali metal carbonyls, CO adsorption, FTIR spectroscopy, Linkage isomerism, Zeolites

Bioorganic Studies in AIDS: Synthetic Antifungals Against Pneumocystis carinii Based on the Multivalency Concept

Abstract: We report the syntheses of antifungals containing the novel pharmacophores: oxaziridines, sulfonyloxaziridines, nitrones and nitronyl nitroxides. We hypothesized that multiple copies of the pharmacophore per molecule might be a prerequisite to enhance efficacy against the opportunistic pathogen, Pneumocystis carinii. Therefore structural optimization of the leads was based on this new “multivalency” approach. All bisoxaziridines were inactive, but a trisoxaziridine caused ca. 50% reduction of the number of P. carinii tropozoites, compared to TMP-SMX, and a hexaoxaziridine at 1 μg/ml showed activity comparable to the currently used drug, TMP-SMX. Insertion of three units of the nitronyl nitroxide pharmacophore per molecule afforded an antifungal triradical with activity comparable to TMP-SMX at 1 μg/ml; at 25 μg/ml and at 10 μg/ml the triradical was better. The results lend further support to the oxidoredox pharmacophore hypothesis, and the enhancement of activities observed demonstrates the high potential and benefits of applying the concept of multivalency to drug development.

Multi-state Multireference Rayleigh–Schrödinger Perturbation Theory for Mixed Electronic States: Second and Third Order

Abstract: The formalism for multi-state multireference configuration-based Rayleigh-Schrodinger perturbation theory and procedures for its implementation for the second-order and third-order energy within a multireference configuration interaction computer program are reviewed. This formalism is designed for calculations on electronic states that involve strong mixing between different zero-order contributions, such as avoided crossings or mixed valence-Rydberg states. Such mixed states typically display very large differences in reference-configuration mixing coefficients between the reference MCSCF wave function and an accurate correlated wave function, differences that cannot be reflected in state-specific (diagonalize-then-perturb) multireference perturbation theory through third order. A procedure described in detail applies quasidegenerate perturbation theory based on a model space of a few state-averaged MCSCF functions for the states expected to participate strongly in the mixing, and can be characterized as a “diagonalize-then-perturb-thendiagonalize” approach. It is similar in various respects to several published methods, including an implementation by Finley, Malmqvist, Roos, and Serrano-Andres [Chem. Phys. Lett. 1998, 288, 299–306].

New Transcorrelated Method Improving the Feasibility of Explicitly Correlated Calculations

Abstract: We recently developed an explicitly correlated method using the transcorrelated Hamiltonian, which is preliminarily parameterized in such a way that the Coulomb repulsion is compensated at short inter-electronic distances. The extra part of the effective Hamiltonian features short-ranged, size-consistent, and state-universal. The localized and frozen nature of the correlation factor makes the enormous three-body interaction less important and enables us to bypass the complex nonlinear optimization. We review the basic strategy of the method mainly focusing on the applications to single-reference many electron theories using modified Moller-Plesset partitioning and biorthogonal orbitals. Benchmark calculations are performed for 10-electron systems with a series of basis sets.

Chemical Reactivity Dynamics and Quantum Chaos in Highly Excited Hydrogen Atoms in an External Field: A Quantum Potential Approach

Abstract: Dynamical behavior of chemical reactivity indices like electronegativity, hardness, polarizability, electrophilicity and nucleophilicity indices is studied within a quantum fluid density functional framework for the interactions of a hydrogen atom in its ground electronic state (n = 1) and an excited electronic state (n = 20) with monochromatic and bichromatic laser pulses. Time dependent analogues of various electronic structure principles like the principles of electronegativity equalization, maximum hardness, minimum polarizability and maximum entropy have been found to be operative. Insights into the variation of intensities of the generated higher order harmonics on the color of the external laser field are obtained. The quantum signature of chaos in hydrogen atom has been studied using a quantum theory of motion and quantum fluid dynamics. A hydrogen atom in the electronic ground state (n = 1) and in an excited electronic state ( n = 20) behaves differently when placed in external oscillating monochromatic and bichromatic electric fields. Temporal evolutions of Shannon entropy, quantum Lyapunov exponent and Kolmogorov – Sinai entropy defined in terms of the distance between two initially close Bohmian trajectories for these two cases show marked differences. It appears that a larger uncertainty product and a smaller hardness value signal a chaotic behavior.

Detection of Riddelliine-Derived DNA Adducts in Blood of Rats Fed Riddelliine

Abstract: We have previously shown that riddelliine, a naturally occurring genotoxic pyrrolizidine alkaloid, induces liver tumors in rats and mice through a genotoxic mechanism mediated by the formation of a set of eight 6,7-dihydro-7-hydroxy-1-hydroxymethyl-5 H -pyrrolizine ( DHP)-derived DNA adducts. In this study we report the formation of these DHP-derived DNA adducts in blood DNA of rats fed riddelliine. In an adduct formation and removal experiment, male and female F344 rats (8 weeks of age) were administered riddelliine by gavage at a single dose of 10.0 mg/kg body weight in 0.1 M phosphate buffer. At 8, 24, 48, and 168 hrs after dosing, the levels of DHP-derived DNA adduct in blood and liver were determined by 32 P-postlabeling/HPLC. Maximum DNA adduct formation occurred at 48 hr after treatment. From 48 to 168 hours, the adduct levels in female rat blood were 4-fold greater than those in male rats. In a dose response experiment, female rats were gavaged 0.1 and 1.0 mg/kg doses of riddelliine for three consecutive days and the DHP-derived DNA adducts in blood DNA were assayed. The levels of the DHP-derived DNA adducts in blood of rats receiving 0.1 and 1.0 mg/kg doses were 12.9 and 51.8 adducts/10

Ab Initio Calculations of 31P NMR Chemical Shielding Anisotropy Tensors in Phosphates: Variations Due to Ring Formation

Abstract: Ring formation in phosphate systems is expected to influence both the magnitude and orientation of the phosphorus ( 31 P) nuclear magnetic resonance (NMR) chemical shielding anisotropy (CSA) tensor. Ab initio calculations of the 31 P CSA tensor in both cyclic and acyclic phosphate clusters were performed as a function of the number of phosphate tetrahedral in the system. The calculation of the 31 P CSA tensors employed the GAUSSIAN 98 implementation of the gauge-including atomic orbital (GIAO) method at the Hartree-Fock (HF) level. It is shown that both the 31 P CSA tensor anisotropy, and the isotropic chemical shielding can be used for the identification of cyclic phosphates. The differences between the 31 P CSA tensor in acyclic and cyclic phosphate systems become less pronounced with increasing number of phosphate groups within the ring. The orientation of the principal components for the 31 P CSA tensor shows some variation due to cyclization, most notably with the smaller, highly strained ring systems.

Self-energy Effects on Nuclear Magnetic Resonance Parameters within Quantum Electrodynamics Perturbation Theory

Abstract: A theory for the calculation of self-energy corrections to the nuclear magnetic parameters is given in this paper. It is based on the S-matrix formulation of bound-state quantum electrodynamics (QED). Explicit expressions for the various terms of the S-matrix are given. The interpretation of the self-energy, one- and two-vertex terms and some perspective for possible future developments are discussed.

15N Chemical Shifts in Energetic Materials: CP/MAS and ab Initio Studies of Aminonitropyridines, Aminonitropyrimidines, and Their N-Oxides

Abstract: Solid state 15N NMR chemical shift measurements have been performed on a series of nitro- and amino-substituted nitrogen-containing heterocycles that are of interest as potential new insensitive explosives. Due to low solubilities, many of these compounds are not amenable to study by solution state methods. Theoretical calculations of 15N chemical shift parameters have been performed on the structures of interest and are reported herein. The calculated and experimental values are in good agreement. The use of a model that includes intermolecular effects and allows the proton positions of the nearest neighbors to be optimized leads to the best agreement between calculated and experimental values. The theoretical models accurately predict the effects of nitro and amino substituents on ringnitrogen chemical shifts, explaining a seeming reversal in trend that is noted in the pyridine and pyridine-1-oxide chemical shifts of the highly substituted compounds.