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The reactions of N-unsubstituted triazoles with sulfonyl chlorides afforded mixtures of regioisomeric 1- and 2-sulfonyl-1,2,3-triazoles. In some cases, pure regioisomers were obtained by crystallization of mixtures of isomers. 1,2,3-Triazoles bearing thiadiazole, isoxazole and benzene rings react with mesyl chloride and tosyl chloride to form mainly 1-substituted 1,2,3-triazoles. Conversely, reactions of triazoles 1a,b and oxadiazolyl triazole 1h with more bulky 1-(2,4-dimethylphenyl)sulfonyl chloride affords mainly 2-substitued products. Oxadiazole 1b furnishes an equal mixture of the regioisomeric products 3 and 4 as a result of the reaction with mesyl and tosyl chlorides. Higher temperatures increase the ratio of isomers in favor of the 2-substituted triazole. The ratio of isomers depends on both the nature of the azolyl ring and on the size of the substituent in the sulfonyl chloride. Based on the results of experimental and theoretical data, 1-substituted and 2-substituted 1,2,3-triazoles can be considered as the kinetic and thermodynamic products.

The oxidation of the renewable diols isosorbide and isomannide was successfully achieved using a TEMPO/laccase system. Furthermore, various TEMPO-derivatives were tested leading to conversions of up to >99% for the oxidation of isosorbide, isomannide, indanol and a halohydrin to the corresponding ketone.

Prochiral bicyclic diketones were transformed to a single diastereomer of 3-substituted cyclohexylamine derivatives via three consecutive biocatalytic steps. The two chiral centres were set up by a CC hydrolase (6-oxocamphor hydrolase) in the first step and by an ω-transaminase in the last step. The esterification of the intermediate keto acid was catalysed by a lipase in the second step if possible. For two substrates the CC hydrolytic step as well as the esterification could be run simultaneously in a one-pot cascade in an organic solvent. In one example, the reaction mixture of the first two steps could be directly subjected to bio-amination in an organic solvent without the need to change the reaction medium. Depending on the choice of the ω-transaminase employed and the substrate the cis- as well as the trans-diastereomers could be obtained in optically pure forms.

In this work we studied the capability of several diene/dienophile pairs to undergo Diels-Alder (DA) reactions leading to benzo[b]thiophenes. A variety of synthetically and commercially available nitrothiophenes were chosen as dienophiles. Methyl 5-nitro-3-thiophenecarboxylate was selected as a potential strong electrophilic candidate based on some DFT-based properties and the substitution pattern of the expected product. The mechanistic details concerning the participation of this dienophile in polar DA reactions were investigated through a theoretical point of view. The results were compared with the experimental outcomes. This methodology should allow synthetic chemists to analyze DA reactions in detail in a stage prior to the synthetic job. Fil: Brasca, Romina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Universidad Nacional del Litoral; Argentina Fil: Kneeteman, Maria Nelida. Universidad Nacional del Litoral; Argentina Fil: Mancini, Pedro Maximo Emilio. Universidad Nacional del Litoral; Argentina Fil: Fabian, Walter M.F.. Karl-franzens-universitat Graz; Austria

Density functional theory (M06-2X) calculations on divanadium complexes [(VO)2(bpbp)(A)]2+ of the oxo anions formate, phosphate, and arsenate (A = HCOO−, H 2 PO 4 - , H 2 AsO 4 - ) as well as the diaquo complex [(VO)2(bpbp)(OH2)2]3+ indicate increased binding in the series H2O H 2 PO 4 - H 2 AsO 4 - . Aqueous solvation enhances the experimentally observed selective recognition of arsenate over phosphate indicating differences in hydration enthalpies of the respective complexes as the main reason for the surprising differentiation between dihydrogenphosphato and dihydrogenarsenato by the divanadyl complex [(VO)2(bpbp)(OH2)2](ClO4)3·H2O. Deprotonation of the dihydrogenphosphato/arsenato complexes to [(VO)2(bpbp)(HXO4)]+ further increases preferential solvation of the arsenato over the phosphato complex.

The Quantum Cluster Equilibrium (QCE) model was applied to the liquid phase for the first few members of the homologous series of unbranched aliphatic primary alcohols, methanol, ethanol, propan-1-ol, and butan-1-ol. Cluster structures and energies were calculated by density functional theory [B3LYP/6-311++G(2d,2p)]. For butan-1-ol the dispersion interaction was also considered with the B3LYP-D method. In agreement with previous findings, cyclic cluster structures are the most probable ones. In addition, weak C-H...O interactions as well as dispersion interactions between the longer alkyl chains were found to be important in the cluster formation. The reliability of the model was assessed by the calculated constant pressure heat capacity (C(p)) values. Larger deviations between theory and experiment were found for higher homologes (propan-1-ol, butan-1-ol) with the B3LYP method. When the B3LYP-D method was applied for butan-1-ol, adequate agreement was found between experimental and calculated C(p) values.

Diels-Alder reactions between the substituted furans 1a-1c and Danishefsky's diene (2), yielding 5-hydroxybenzofurans, were studied by density functional theory [B3LYP/6-31G(d) level] in benzene as solvent (SP/CPCM). The mechanistic details of these reactions, especially with respect to regio-, site-, and stereochemistry were analyzed in detail. Moreover, the extrusion of nitrous acid from the Diels-Alder cycloadducts and the formation of the aromatic products were also explained in order to give a total description of the complete domino processes. Some Diels-Alder reactions leading to 5-hydroxybenzofurans were studied at the B3LYP/6-31G(d) level of theory in benzene as solvent (SP/CPCM). DFT calculations on the transition structures were carried out for all the possible reaction channels (site, regio-, and stereoisomeric channels were included in this study). Additionally, a description of the complete domino processes was performed. Fil: Brasca, Romina. Universidad Nacional del Litoral. Facultad de Ingeniería Química. Departamento de Química; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina Fil: Kneeteman, Maria Nelida. Universidad Nacional del Litoral. Facultad de Ingeniería Química. Departamento de Química; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina Fil: Mancini, Pedro Maximo Emilio. Universidad Nacional del Litoral. Facultad de Ingeniería Química. Departamento de Química; Argentina Fil: Fabian, Walter M. F.. Karl Franzens Universität Graz,; Austria

Aqueous acid dissociation constants of substituted areneseleninic, areneselenenic, arenesulfinic, and benzoic acids are calculated by ab initio (MP2) and DFT (B3LYP) methods in combination with bulk solvation models (IEFPCM, CRSrad) from appropriate thermodynamic cycles. Mean absolute deviations (MAD) between experimental and calculated pK(a) values are quite large for basis sets without diffuse functions; however, trends are reasonably well described. Best agreement with experiment as described by MAD as well as correlation coefficient and slope of the correlation equation pK(a) = a*ΔG(calc)/RT ln(10) + b is obtained with the CPCM solvation model using the defaults optimized within COSMO-RS (CRSrad; MAD = 1.54, R(2) = 0.94, a = 0.83). Sulfenic (selenenic) acid tautomers are significantly more stable than the corresponding sulfoxide (selenoxide) forms.

The quantum cluster equilibrium (QCE) theory was applied for liquid methanol using MP2(fc)/6-31+G(d,p) cluster geometries and MP2/6-311++G(d,p) energies. Three approaches for the determination of the empirical parameters inherent in the QCE model, amf and bxv, were considered, while the molar volumes of both the liquid and the vapor were obtained from experiment. Model 1, in which amf and bxv were optimized for the vapor and liquid phase, respectively, gave the best results for thermochemical parameters (constant pressure heat capacity C and entropy S) of methanol, especially for the liquid state. In line with Pauling’s suggestions concerning the structure of liquid methanol, cyclic hexamers cyclo-(MeOH)6 are the dominant species, even near the boiling point (∼50%) increasing to ∼90% at the freezing temperature.

Three approaches of computational chemistry [quantum mechanics (QM) calculations, docking, and molecular dynamics (MD) simulations] were used to investigate the redox cycle of bovine erythrocyte glutathione peroxidase from class 1 (GPx1, EC 1.11.1.9). The pKa calculations for two redox states of the active-site selenocysteine of GPx1 (selenol, Sec45-SeH, and selenenic acid, Sec45-SeOH) were estimated using a bulk solvent model (B3LYP-IEFPCM and B3LYP-CPCM-COSMO-RS). The calculated pKa values of Sec45-SeH and Sec45-SeOH were corrected via a simple linear fit to a training set of organoselenium compounds, which consisted of aliphatic selenols and aromatic selenenic acids with available experimental pKa values. Based on docking calculations, binding sites for both molecules of the cofactor glutathione (GSH) are described. MD simulations on the dimer of GPx1 have been performed for all chemical states of the redox cycle: without GSH and with one or two molecules of GSH bound at the active site. Conformational analyses of MD trajectories indicate high mobility of the Arg177 and His79 residues. These residues can approach the vicinity of Sec45 and take part in the catalytic mechanism. On the basis of the calculated data, new atomistic details for a generally accepted mechanism of GPx1 are proposed.

A biocatalytic redox-neutral reaction cascade was designed for the deracemisation of racemic mandelic acid to yield optically pure L -phenylglycine employing three enzymes. The cascade consisted of three steps: a racemisation, an enantioselective oxidation and a stereoselective reductive amination. The enantioselective oxidation of D -mandelic acid to the corresponding oxo acid was coupled with the stereoselective reductive amination of the latter; thus the oxidation as well as the reduction reactions were performed simultaneously. The formal hydrogen abstracted in the first step -the oxidation - was consumed in the reductive amination allowing a redox-neutral cascade due to a cascade-internal cofactor recycling. The enantiomers of the starting material were interconverted by a racemase (mandelate racemase) ensuring that in theory 100% of the starting material can be transformed. Using this set-up racemic mandelic acid was transformed to optically pure L -phenylglycine (ee > 97% ) at 94% conversion without the requirement of any additional redox reagents in stoichiometric amounts.

The adsorption of aniline on SWCNTs has been investigated using three different DFT methods, PW91LYP, the hybrid m-GGA MPWB1K, and the dispersion-corrected BP86-D functionals. The BSSE-corrected adsorption energies for top orientation of aniline to the nanotube surface are Eads = 9.7 kcal mol−1 and Eads = 1.4 kcal mol−1 with BP86-D/SVP and MPWB1K/6-311+G(d), respectively. Results validated that the adsorption energy of aniline depends on the diameter of the nanotube with a pronounced manner, especially for small diameters. The TDDFT calculations of UV/vis spectra predict two electronic transitions resulting from nanotube excitations in the visible spectra (>500 nm) and one imperceptible aniline → SWCNT excitation at ca. 1200 nm. Inclusion of solvent effects (MeOH) by explicit solvent molecules leads to a pronounced red shift of the weak longest wavelength aniline → nanotube transition, whereas the strong intratube excitations are only a little influenced. Fluorescence excitation spectra (λem = 586 nm) of ...

Various types of biocatalysts like oxidases, alcohol dehydrogenases, and microbial cells were tested for the oxidation of benzyl alcohol. Oxidases in combination with molecular oxygen led to low conversion. Alcohol dehydrogenases and microbial cells were tested in a hydrogen transfer reaction employing acetaldehyde, chloroacetone, and acetone as hydrogen acceptor. Excellent conversion (95%) could be achieved employing lyophilised cells of Janibacter terrae DSM 13953 at a substrate concentration of 97 mM.

Quasi-irreversible oxidation of sec-alcohols was achieved via biocatalytic hydrogen transfer reactions using alcohol dehydrogenases employing selected ketones as hydrogen acceptors, which can only be reduced but not oxidized. Thus, only 1 equiv of oxidant was required instead of a large excess. For the oxidation of both isomers of methylcarbinols a single nonstereoselective short-chain dehydrogenase/reductase from Sphingobium yanoikuyae was identified and overexpressed in E. coli.

Density functional theory [B3LYP/6-31G(d) with inclusion of solvent effects by the Onsager SCRF approximation] is used to investigate the mechanism of the hydrogen halide induced cyclisation of α-diazonitriles to 5-halo-1,2,3-triazoles. The reaction occurs stepwise. The first step, addition of the hydrogen halide onto the nitrile group to form diazo imidoyl halides, is rate determining. Barriers are calculated to be in the range 20–30 kcal mol −1 . The largest ones were found for 2-diazomalononitrile and 2-trifluoroacetyl-2-diazoacetonitrile. The second step, cyclisation of these intermediates to the triazoles, has a significantly lower barrier, −1 .

The mechanism of reaction of isothiocyanates with diazopyrazoles to 4-imino-4H-pyrazolo[5,1-d] [1,2,3,5]thiatriazines was investigated. It was suggested that the reaction proceeds via a pseudopericyclic cycloaddition with activation barrier of ∼31 kcal mol−1 (B3LYP/6-31G*). The mechanism was confirmed by NBO and ACID schemes. The question of regioselectivity was also explored. It was shown that an acyl substituent on the isothiocyanate led to a stabilization of the final product due to S… O interaction. © 2007 Wiley Periodicals, Inc. Int J Quantum Chem, 2007

The tautomeric enol imine--enaminone (phenol--quinone) equilibrium of the 1-hydroxy-2-naphthaldehyde Schiff base (2-phenyliminomethyl-naphthalen-1-ol) was investigated by density functional theory (B3LYP) and ab initio (MP2) methods in the IEF-PCM polarizable continuum dielectric solvent approximation and by a combined ab initio + FEP/MC study by considering an explicit solvent model. Special emphasis was put on the effect of solvation on this equilibrium by using an apolar (CCl4), polar aprotic (CH3CN), and polar protic (CH3OH) solvent. Compared with experimental tautomerization Gibbs free energies, the IEF-PCM/B3LYP calculations apparently overestimate the stability of the quinone form both when the 6-31G(d,p) and the 6-311++G(d,p) basis sets are applied. IEF-PCM/MP2 studies with the above basis sets predict the preference of the aromatic phenol tautomer, in contrast to the experiment in methanol and acetonitrile solvent. Calculation of the total relative free energy as DeltaG(tot) = DeltaE(int)(IEF-PCM/QCISD(T)/6-31G(d)) + DeltaG(solv, FEP/MC) + DeltaG(thermal) provided agreement with the experimental values up to 0.6 kcal/mol in the three solvents, and the predominant tautomer was always correctly predicted. In-solution relevant atomic charges, derived by a fit to the molecular electrostatic potential generated by the IEF-PCM/B3LYP/6-31G(d,p) wave function, show strong dependence on the fitting procedure (CHELPG or RESP) and are fairly insensitive to the chemical nature of the actual solvent. Use of the CHELPG charges in FEP/MC simulations revealed to be superior in comparison with the use of the RESP charge set.

The product of the reaction between o-phenylenediamine and 4-dimethylaminobenzaldehyde, mp=179°, is unambiguously shown to be 1-(4-dimethylaminobenzyl)-2-(4-dimethylaminophenyl)-benzimidazole by X-ray crystallography. It crystallises in the monoclinic space group P21/c, with a=18.327(5) A, b=6.318(15) A, c=18.204(4) A, β=110.73(2)°, R1=0.0647. Quantum chemical calculations [density functional theory, B3LYP/6-31G(d)] are used to propose a reaction mechanism for the rearrangement of the initially formed bis-Schiff base via a two-step process involving cyclization (TS1, ΔG≠=32 kcal mol–1) to a tetrahedral intermediate (3, ΔGreact=30 kcal mol–1). A formal (1,3)-shift (TS2, ΔG≠=38 kcal mol–1 with respect to 3) then yields in a strongly exothermic reaction (ΔGreact=−17 kcal mol–1) the rearranged product, 1-(4-dimethylaminobenzyl)-2-(4-dimethylaminophenyl)-benzimidazole (2).

For a set of 7 selected stable intermediates and derived radicals for the formation of 1,3,6,8- and 1,3,7,9-tetrachlorodibenzo- p -dioxins from 2,4,6-trichlorophenol, energy minimum structures, total atomic spin densities ρ , harmonic vibrational wavenumbers ν ˜ , principal moments of inertia I A , I B , and I C , heat capacities C p o ( T ) , entropies S o ( T ) , enthalpy increments H o ( T ) − H o ( 0 ) , and standard enthalpies of formation Δ f H o ( T ) were calculated at the G3MP2B3 level of theory and presented for T = 298.15 K . A set of 7 reaction paths is characterized by means of enthalpies of reaction Δ r H o ( 298.15 ) .

Semi-empirical (ZINDO-SOS), time-dependent density functional theory and ab initio quadratic response function (DDRPA) calculations on a series of donor–acceptor substituted salen-type Schiff bases are used to aid in the design of dyes with useful optical nonlinearities (molecular quadratic hyperpolarizabilities β vec ). 4-Phenylazo-2-phenyliminomethylphenols 1 are calculated to be less suitable as nonlinear optical materials than the isomeric 5-phenylazo-2-phenyliminomethylphenols 2 . The largest hyperpolarizabilities are predicted for compounds with an acceptor-containing phenylazo and a donor-substituted phenyliminomethyl moiety. The calculations also clearly indicate the intramolecular charge transfer nature of the first ππ ∗ -transition in the investigated Schiff bases.

1,7-Dipolar cyclizations of 1-diazo-2,4-pentadiene 1a and its heteroanalogues 1b,c and 4c were studied using density functional theory (DFT). Although the heteroanalogue 1c has an appropriate electronic structure to allow for pseudopericyclic cyclization, natural bond order (NBO) analysis has provided evidence for the electrocyclic ring closure. Magnetic criteria (anisotropy of the induced current density [ACID], nucleus-independent chemical shifts [NICS]) confirmed the pericyclic character of the located transition states 2a,c and 5c. The activation barriers for the cyclization of 1-diazo-2,4-pentadiene 1a and its aza analogues 1c, 4c are 3.3, 8.2, and 12.3 kcal/mol at the B3LYP/6-31G(d) level, respectively. The higher barrier of the 1c3c and 4c3c reactions compared with 1a3a is in line with the Hammond postulate. The out-of-plane distorted geometry of the cyclic product is an additional factor arguing against a pseudopericyclic mechanism. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2006

Nucleophilic vinylic substitutions of 4 H -pyran-4-one and 2,6-dimethyl-4 H -pyran-4-one with hydroxide ion in aqueous solution were calculated by the density functional theory (B3LYP) and ab initio (MP2) methods using the 6-31+G(d) and 6-31G(d) basis sets. The aqueous solution was modelled by a supermolecular approach where 11 water molecules were involved in the reaction system. The calculations confirmed a different addition-elimination mechanism of the reaction compared with that in the gas phase or non-polar solution. Addition of hydroxide ion at the C2 vinylic carbon of the pyranone ring with activation barrier of 10–11 kcal mol −1 (B3LYP) was shown to be rate determing in good quantitative and qualitative agreement with experimental kinetics. Solvent effects increase the activation barrier of the addition step, and in opposite, decrease the barrier of the elimination step.

Two alternative directions for thermal transformation of 6-amino-4-oxopyrano[3,4-d] [1,2,3]thiadiazoles 1, leading either to 6-hydroxy-4-oxo-[1,2,3]thiadiazolo[4,5-c]pyridines 2 or 2-cyano-2-(1,2,3-thiadiazol-5-yl)acetamide 4b, were observed. The first one represents a new, Dimroth-type rearrangement and proceeds by thermal opening of the pyrane ring, followed by the simultaneous rotational isomerization of the ketene intermediate 7 (s-cis) to 7 (s-trans) and its recyclization onto the amido group to form the pyridin-2-one cycle.The first step of the rearrangement has a calculated [B3LYP/6-31G(d)] activation barrier of 24–34 kcal/mol, the involvement of amines reduces it by ca. 5 kcal/mol [PCM-B3LYP/6-31G(d), DMSO]. In contrast, the recyclization step onto the amido group is calculated to occur essentially barrierless (ΔE < 2 kcal/mol). The influence of substituents, in particular of those capable of intramolecular hydrogen bonding, on the preferred reaction path was also studied. The alternative ring opening of the pyrane cycle was calculated to be at least 5 kcal/mol [B3LYP/6-31G(d)] less favorable than the Dimroth rearrangement. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)

The enol imine ⇄ enaminone tautomerization constants, KT, and thermodynamic parameters, ΔHT and ΔST, of 1-hydroxy-2-naphthaldehyde Schiff bases are determined by UV/vis spectroscopy. Polar solvents shift the equilibrium toward the quinone form (for the unsubstituted derivative 1c, KT = 0.20 (cyclohexane) and KT = 1.49 (ethanol)). Both donor (MeO, NMe2) and acceptor (CN, NO2) substituents lead to a decreased KT independent of solvent polarity. In apolar solvents, for all derivatives 1a−1e, the enol imine ⇄ enaminone equilibria are endergonic but exothermic. Linear solvation energy relationships allow extrapolation of ΔGT to the gas phase. Density functional theory calculations (B3LYP/6-311+G**) yield good agreement with these extrapolated ΔGT values. Solvent effects on 1c are also successfully reproduced by the calculations. Geometric (O···N distance) and energetic criteria (conformer energy differences, homodesmotic reactions) establish the importance of intramolecular hydrogen bonding for the tautomerism...

The synthesis and photophysical data of new carbostyrils (quinoline-2(1H)-ones) with the longest hitherto observed absorption- and emission wavelengths are described. Introduction of 6-amino, 7-MeO, and 4-(CF3) substituents enabled us to rise the absorption and fluorescence maxima up to 414 and 557 nm, respectively. Supported by semi-empirical and ab initio calculations, the 6,7-(1,4-diazine)-fused carbostyril 23b displayed absorption maxima at up to 440 nm, with quantum yields of up to 0.9 and large Stokes shifts (>100 nm), comparable to the best coumarin chromophores known. The new fluorophore is neither pH-sensitive between pH 6 and 10 nor susceptible to O2 quenching. At pH 3, the emitted light appears greenish-white, which arises from three different stages of protonation.

Semi-empirical ZINDO SOS and ab initio quadratic response function (DDRPA) calculations on a series of donor–acceptor substituted π-conjugated chromophores based on styryl benzothiazoles are used to aid in the design of dyes with high non-linear optical properties. Structural modifications studied include nature and position of the donor and/or acceptor group as well as the length of the connecting π-bridge. The most worthwhile target for synthesis are 2-tricyanovinyl-6-p-donor-styryl benzothiazoles and heteroaryl derivatives thereof.

For a set of 17 selected chlorinated and polychlorinated phenols and cyclo-C5 compounds, energy minimum structures, harmonic vibrational wavenumbers ωe, principal moments of inertia IA, IB, and IC, heat capacities Cpo(T), entropies So(T), thermal energy contents Ho(T)−Ho(0), and standard enthalpies of formation ΔfHo(T) were calculated at the ab initio G3MP2B3 level of theory and presented for T=298.15 K. The eight radicals of this set are characterized, in addition, by total atomic spin densities ρ as well as by isotropic hyperfine coupling constants Aiso. The relationships between the radicals and their parent molecules are investigated, and the corresponding bond dissociation energies are presented.

Nucleophilic vinylic substitutions of 4H-pyran-4-one and 2-methyl-4H-pyran-4-one with ammonia were calculated by the B3LYP method using the 6-31G(d,p) basis set. Bulk solvent effects of aqueous solution were estimated by the polarized continuum and Poisson–Boltzmann self-consistent reaction field models using the 6-311+G(d,p) basis set. In the gas phase different mechanisms were found for the two reaction systems calculated. The reaction of 4H-pyran-4-one proceeds through enol, whereas a feasible path for the less reactive 2-methyl-4H-pyran-4-one is the mechanism through a keto intermediate. Addition of ammonia in concert with proton transfer is the rate-determining step ofthe reaction. The mechanism proceeding either by a bimolecular nucleophilic substitution (SN2) or by one involving a tetrahedral zwitterionic intermediate is shown to be unlikely in the gas phase or nonpolar solution. The effects of bulk solvent not only consist in a reduction of the various activation barriers by about 25–40 kJ mol−1 but also in a change in the reaction mechanism.

Concerted as well as stepwise reaction pathways for cyclization of 3-azido-propenal to isoxazole have been investigated by density functional (B3LYP) and ab initio methods up to CCSD(T)/cc-pVQZ methods. These calculations clearly establish the pathway with concerted albeit asynchronous nitrogen extrusion and ring closure as the most feasible mechanism. Barriers for cyclization increase in the order Hartree–Fock

Three novel chiral packing materials for high-performance liquid chromatography were prepared by covalently binding of (2S)-N-(3,5-dimethylphenyl)-2-[(4-chloro-3,5-dinitrophenyl)carbonylamino]propan-amide (7), (2S)-N-(3,5-dimethylphenyl)-2-[(4-chloro-3,5-dinitrophenyl)carbonylamino]-4-methylpentanamide (8), and (2S)-N-(3,5-dimethylphenyl)-2-[(4-chloro-3,5-dinitrophenyl)carbonyl-amino]-2-phenylacetamide (9) to aminopropyl silica. The resulting chiral stationary phases (CSPs 1-3) proved effective for the resolution of racemic 4-aryl-3,4-dihydro-2(1H)-pyrimidone derivatives (TR 1-14). The mechanism of their enantioselection, supported by the elution order of (S)-TR 13 and (R)-TR 13 and molecular modeling of the complex of the slower running (S)-TR 13 with CSP 1 is discussed.

A 3D QSAR analysis (quantitative structure activity relationships) of a set of 2,2-disubstituted epoxides, substrates for epoxide hydrolases originating from four different organisms, was conducted by CoMFA (comparative molecular field analysis) and CoMSIA (comparative molecular similarity indices analysis), with respect to the enantioselective ring opening to the corresponding vicinal diol. Structural variations of the substrates include alkyl chains of different lengths, unsaturated moieties ((E)- and (Z)-alkenyl, alkinyl, aryl) and electronegative groups (ether oxygens, halogen atoms) at different locations within the 2-substituent group. Generally, all four organisms, namely Rhodococcus ruber NCIMB 11216, Rhodococcus ruber DSM 43338, Rhodococcus ruber DSM 44540 and Rhodococcus ruber DSM 44539, preferentially react with the (S)-enantiomer of the epoxide. Enantioselectivities (enantiomeric ratio, ln E values) show a rather large variation, ranging from almost no (ln E 5.3). In addition, the response of the epoxide hydrolases stemming from the four organisms towards structural modifications of the substrate is different. Models for the enantioselectivity (enantiomeric ratio, ln E values) obtained by CoMFA and CoMSIA are of different but reasonable predictive power, e.g., q2 CV=0.701 and r2=0.937 for the CoMFA model of Rhodococcus ruber DSM 43338. Enantiomeric ratios for the test molecules can be well predicted. Plots of steric and electrostatic CoMFA (CoMSIA) fields allow conclusions to be drawn for the choice of the most suitable organism for a specific type of substrate.

Density functional theory (B3LYP-D3, M06-2X) has been used to calculate the structures, interaction energies and vibrational frequencies of a set of 93 methanol-water clusters of different type (cubic, ring, spiro, lasso, bicyclic), size and composition. These interaction energies have been used within the framework of the Quantum Cluster Equilibrium Theory (QCE) to calculate cluster populations as well as thermodynamic properties of binary methanol-water mixtures spanning the whole range from pure water to pure methanol. The necessary parameters amf and bxv of the QCE model were obtained by fitting to experimental isobars of MeOH-H2O mixtures with different MeOH content. The cubic and spiro motifs dominate the distribution of methanol-water clusters in the mixtures with a maximum of mixed clusters at x(MeOH) = 0.365. Reasonable agreement with experimental data as well as earlier molecular dynamics simulations was found for excess enthalpies H(E), entropies S(E) as well as Gibbs free energies of mixing G(E). In contrast, heat capacities Cp and C showed only poor agreement with experimental data.

Substituent and solvent effects on tautomeric equilibria of isomeric phenylazo naphthols and the analogous Schiff bases are calculated by various quantum chemical methods and solvation models. Special emphasis is put on the reliability of the computational procedures used. The results obtained should aid in resolving discrepancies of experimental data resulting from UV/Vis and NMR spectroscopy.

Correlations of AM1 and PM3 theoretical data, 13C-NMR substituent chemical shifts (13C-SCS) and IR carbonyl group wave numbers [ν(C3â•ÂO)] were studied using dual substituent parameter (DSP) models for 5-substituted indole-2,3-diones. For the C7 atom a reverse substituent effect attributed to extended À-polarization was observed. On the other hand, the DSP approaches for the C3 atom showed normal substituent effects with some contribution of reverse effect supported strongly by 13C-SCS correlations. In the ν(C3â•ÂO) and p(C3â•ÂO) DSP correlations the field effect contribution predominates over the resonance effect, which justifies the using of earlier suggested vibrational coupling (V-C) model for 5- and 6-substituted indole-2,3-diones.

This paper deals with the influence of various electron acceptors, which contain at least two cyano groups (obtained from reactions with malononitrile, tetracyanoethene, tetracyanoquinodimethane, dimeric malononitrile), on the generation of second harmonic light of donor–acceptor-substituted anilines. For EFISH measurements, which were performed on solutions in 1,4-dioxane and acetone a Yag–Nd laser was used. The influence of the solvent on the SHG efficiency is discussed shortly and compared to solvatochromic effects. The measurement data were corrected for absorptive effects so that absorbing compounds can be compared to non-absorbing ones. Increasing the acceptor strength not only bathochromically shifted the longest wavelength UV–vis absorption band but also leaded to an enhancement of μβ -values obtained by the EFISH measurements. Trends of hyperpolarizabilities calculated by semi-empirical molecular orbital methods by and large conformed those found experimentally.

The infrared absorption bands of the S=O and N-H stretching vibrations as well as the natural abundance 13 C and the 17 O NMR chemical shifts were measured for fifteen substituted benzenesulfonamides in CDCl 3 and acetone-d 6 . For the unsubstituted compound the natural abundance 33 S NMR chemical shift was also measured in acetone-d 6 PM3 charge densities and bond orders were calculated for the completely optimised geometry. Mutual correlations are reported between the wave numbers of the asymmetric SO 2 stretching vibration, the arithmetic means of symmetric and asymmetric NH 2 stretching vibrations, Hammett † or † + substituent constants, dissociation constants, 1 H, 13 C, 15 N and 17 O NMR chemical shifts as well as the PM3 S=O and N-H bond orders and hydrogen, nitrogen and oxygen atom charge densities. The results of the correlation analysis showed that the factors determining the electronic structure and controlling the dissociation equilibria, the IR and NMR spectral properties of substituted benze...

Ab initio [Hartree–Fock (HF)] and density functional theory (B3LYP) ECP calculations on intermediates and transition states of the reduction of phenylseleninic acid with hydrogen sulfide and of seleninic acid with benzenethiol as models for the species involved in the redox cycle of selenoenzymes are presented. Selenenyl sulfide is found to be the final product of the reduction reaction with sulfides. Further reduction to the selenol requires surmounting a substantial barrier (∼50 kcal mol−1). The final step of the redox cycle—oxidation of the selenol by hydrogen peroxide—is strongly exothermic. In case of the unsubstituted derivative H2Se three mechanisms for this oxidation are found [concerted and stepwise 1,2-hydrogen and oxygen shift (TS6A, TS6B, resp.) and a four-membered transition state TS6C]. For the oxidation of phenylselenol only TS6A and TS6B are obtained. For nearly all calculated structures the HF wave functions show a triplet instability. In contrast, at the B3LYP level only TS6C has a significant triplet instability. On the basis of QCISD(T)//QCISD results a mechanism involving TS6C seems highly unlikely. © 2002 Wiley Periodicals, Inc. Int J Quantum Chem, 2002

The effect of structural variations of either the donor (number and position of methoxy groups, replacement of the aryl by Fischer's base) or the acceptor moiety (bisacylation of the amino group, extension of the conjugated system by condensation with dimethylformamide–dimethylacetale) of a series of dyes containing malononitrile dimer as acceptor is investigated both experimentally and computationally. Semiempirical (AM1) and ab initio (HF/6-31G ∗ and HF/3-21G) calculations are used for a conformational analysis of the acceptor groups and model systems thereof. Electronic excitation energies are obtained by ZINDO, ab initio CIS/6-31+G ∗ and time-dependent density functional theory (TDDFT(B3LYP)/6-31+G ∗ ) computations. Inclusion of solvent effects is important for a reliable description of the trends observed in the experimental absorption spectra by ZINDO. Methoxyaryl, even polysubstituted, derivatives absorb at significantly shorter wavelengths than the corresponding p-dimethylamino compounds. Replacement of the p-dimethylaminoaryl donor by Fischer's base leads to a bathochromic shift, as does the bisacylation of malononitrile dimer's amino group. Functionalization by condensation with dimethylformamide–dimethylacetale generally has only a small and irregular effect on absorption spectra.

Treatment of dipivaloylketene 1 and its dimers 2 and 3 with 4-(dimethylamino)pyridine (DMAP) in acetonitrile affords the alpha -oxoketene-pyridine zwitterion 4 as a pale yellow solid. This is the first example of a stable zwitterion obtained from a true alpha -oxoketene. In anhydrous solution at room temp., compound 4 largely cleaves into the reactants 1 and DMAP. At -60 degreesC the equilibrium is shifted entirely towards the zwitterion 4, as shown by C-13 NMR measurements. Calculations using density functional theory (B3LYP/6-311+G**) are in excellent agreement with the formation and relative stability of 4. N-15 labelling experiments demonstrate that the ring nitrogen atom of DMAP is involved in generating the new zwitterionic C-N bond. Reactions of 4 with NH and OH nucleophiles in solution afforded the corresponding dipivaloyl acetic acid derivatives 6 and 7, whereas acetone or benzylidene aniline undergo cycloaddition reactions of the hetero-Diels-Alder type.

The complexation of a series of new Schiff bases containing theN-phenylaza-15-crown-5 (Ph-A15C5) moiety with alkali and alkaline-earth metal ions (incl. Be2+ and Mg2+, is studied by means of UV-Vis spectroscopy. The corresponding stability constant values are determined and discussed from two aspects: the position of the Ph-A15C5 moiety in the ligand molecule and the nature of the metal ion.

The results of ab initio (HF, MP2) and density functional (B3LYP) calculations on a series of γ-oxocarboxylic acids [(Z)-3-acetyl or benzoylacrylic and 2-acetyl or benzoylbenzoic acids] A and their cyclic tautomers B are presented. For the open-chain tautomers, different rotamers were considered. Bulk solvent effects were considered by using the polarised continuum model (PCM) and were found to be essential for a proper description of the ring-chain tautomeric equilibrium. Acetyl derivatives generally prefer the cyclic form, whereas for benzoyl derivatives the preference depends on the nature of the link. Introduction of substituents (methyl, phenyl) at C2 and/or C3 for acrylic acid derivatives substantially shifts the equilibrium towards the cyclic form B.

The mechanism of the first steps of the degradation of 4H-1-benzopyran-4-one in alkaline media, i.e., nucleophilic addition of hydroxide ion at the C2 carbon of 4H-1-benzopyran-4-one followed by ring-opening, is investigated by ab initio [HF/6-31+G(d) and MP2/6-31+G(d)] and density functional theory [B3LYP/6-31+G(d)] methods. In addition, bulk solvent effects are estimated by the polarised continuum (overlapping spheres) model (PCM) and the polarizable conductor PCM model (CosmoPCM). Depending on the level of theory used three reaction coordinates are found: path A (1→TS1→2→TS2A→3A), path B (1→TS1→2→TS2B→3B) and path C (1→TS1→2→TS2A→3C). The ring-opening step passing through TS2B has a lower activation energy than the step proceeding through TS2A. These second barriers are higher compared with the barrier of the first step. Therefore fission of the γ-pyrone ring (2→TS2B→3B) seems to be the rate-determining step in the gas phase. Contrary to these gas phase results, addition of hydroxide ion to C2 (reactants→TS1→2) is the rate-determining step for the reaction in aqueous solution, in agreement with experimental results.

Ab initio (HF and MP2) and density functional (B3LYP) calculations with different basis sets (6-31G∗, 6-311+G∗∗, 6-311+G(2df,p)) on the various conformers of (Z)-3-formyl-acrylic acid and the corresponding ring-closed tautomer 5-hydroxy-furan-2(5H)-one are performed. At all levels of theory the ring tautomer is predicted to be the most stable one. Inclusion of bulk solvent effects (H2O) by the self-consistent isodensity surface polarized continuum model (SCIPCM) further shifts the ring-chain tautomeric equilibria towards the furanone form. Among the open chain tautomers, the s-cis–s-trans conformation is the most stable.

A UV–Vis spectroscopic study based on the recently developed chemometric approach for quantitative analysis of undefined mixtures is performed on a series of donor and acceptor substituted Schiff bases of 2-hydroxynaphthaldehydes. In CCl4 solution all compounds preferentially exist as the phenol tautomer independent of the nature of the respective substituent. With increasing polarity the tautomeric equilibrium is shifted towards the quinone form. In CHCl3 and, especially, ethanol a clear distinction between the effect of donors (stabilization of the quinone form) and acceptors (stabilization of the phenol tautomer) is evident. Ab initio calculations including solvent effects via the polarized continuum model of solvation as well as the supermolecule approach are used to rationalize the experimental findings.

The effect of conformation (E/Z isomerism), nature (donor/acceptor) of substituents, and endgroups (indandione, pyrazolone, pyrazoledione) on the molecular hyperpolarizabilityvec of dicyanomethylene (hetero)aromatic dyes is investigated by means of semiempirical (AM1, ZINDO) molecular orbital calculations. Unless Z isomers are stabilized by intramolecular hydrogen bonding, generally E conformers have largervec's. Replacement of one nitrile group of the dicyanomethylene moiety by p-aminoaryl rather than p-R-arylamino (RD NMe2 ,M eO, H, NO 2 )i s found to be advantageous. Increasing the acceptor strength of 29 by successively replacing the carbonyl with dicyanovinyl groups leads to a maximum ofvec for the derivative with one rather than two C(CN)2 groups. With respect to endgroups, the indandione moiety generally is the least active group. Solvent effects are treated within the framework of the self-consistent reaction field approximation. In most cases gas-phase tendencies are either parallel or even reinforced if solvent effects are taken into account. The calculated results are compared with electric field induced second harmonic generation (EFISH) measurements. c 2000 John Wiley & Sons, Inc. Int J Quantum Chem 79: 253-266, 2000

Semiempirical computational methods (AM1 for geometries, ZINDO for electronic transition energies) were used to calculate the absorption and fluorescence spectra of a series of oligophenylenes, their heteroaromatic substituted derivatives, and aryl substituted open-chain and bridged polyenes. The calculations were calibrated on a series of 61 (for absorption) and 42 (for fluorescence) compounds and are shown to reproduce experimental data with an accuracy of −1 . Based on these results, calculations on possible synthetic targets for compounds to be used as scintillating fluors in high-energy particle detection were performed, whereby absorption and emission wavelengths were predicted for 15 and 34 compounds, respectively.

Synthesis, absorption and fluorescence spectra, as well as quantum yields of a series of donor-acceptorsubstituted carbostyrils (a quinolin-2(1H )-ones), are reported. Unprecedented strong absorption maxima (ea 10000 ‐ 20000) close to the visible spectrum, large Stokes shifts up to 130 nm, and quantum yields up to 0.7 are obtained with derivatives containing donor substituents at C(6) and C(7), and either one Ph substituent at C(3) or one CF 3 residue at C(4). For analytical applications in biochemistry and medicine, N(1)-functionalization, or amidoacylation at C(3) in the case of the CF 3 derivatives, is possible without a concomitant hypsochromic shift of their absorption and emission maxima. Semiempirical molecular-orbital calculations ( AM1 for structures, ZINDO for electronic transition energies) prove to be a suitable tool for the prediction of absorption properties of these compounds. The crystal-structure analysis of 6,7-dimethoxy-1-methyl-3-nitro-4-(trifluoromethyl)quinolin-2-(1H )-one (7 )( C 13H11F 3N2O5, monoclinic, P21/c, aa 12.372(2), ba 12.154(2), ca 10.119(2)a, ba 112.95(2)8) shows that the NO2 group, squeezed between the CF 3 and the CaO group, is oriented almost perpendicularly (87.8(4)8) to the ring plane. The intramolecular F ··· N distance between the CF 3 and the NO2 group is only 2.513(4)a.

Absorption and fluorescence spectra as well as quantum yields of a series of differently substituted carbostyrils (quinolin-2(1H)-ones) are reported. Especially for compounds containing donor substituents in position 6, substantial bathochromic shifts (comparable to analogous coumarins) of both absorption as well as fluorescence transitions are obtained. High absorption intensities and quantum yields are found for 7-donor substituted isomers. Semiempirical molecular orbital calculations (AM1 for structures, ZINDO for electronic transition energies) prove to be a suitable tool for the prediction of absorption and fluorescence properties of these compounds. Ab initio and density functional calculations establish the lactam form as the dominant tautomer of the parent quinolin-2(1H)-one.

Reaction of 4-methylamino-5,6-dihydropyridinone with acetyl chloride yielded exclusively the 3-acetyl derivative. When the methylamino group of the latter was removed by alkaline hydrolysis, a mixture of two hydroxy derivatives was formed. Those cyclized upon treatment with hydroxylamine exclusively to the isoxazolo[4,5-c]pyridinone, whereas the 4-methylamino analogue yielded, depending on pH, mainly the isoxazolo[4,3-c]pyridinone or the isoxazolo[4,5-c]pyridinone. The configurations of the latter compounds were established by NMR experiments..