Ab initio calculations of the photoionization of diatomic molecules

A review is presented of the calculation of photoionization spectra, particularly in the spectral range where electron autoionization of diatomic molecules takes place. In addition to some interesting results obtained over years that compare favourably with experiment, the emphasis here is put on the relation between the methods developed for the calculation of observables associated with the continuum energy spectrum of the electrons and the Alchemy system of programs. This system of programs serves as a basis for initial and intermediate calculations. The examples presented show that diatomic molecules not only in gas phase but also oriented in space or physisorbed at surfaces may be studied readily.     

Stereocontrolled preparation of a nonpeptidal (-)-spirobicyclic NK-1 receptor antagonist

The synthesis of a spirobicyclic NK-1 receptor (Substance-P) antagonist 1 antipode is described. Retrosynthetic analysis reveals an allylic halide A bearing the cyclopropoxy-substituted aryl group and a 2-phenyl-3-piperidone B. The stereochemistry in the spirobicyclic system bearing three chiral centers is initially set via a highly diastereoselective zinc-mediated coupling of the allylic bromide 23 to the optically active ketopiperidine 3. The remaining benzylic asymmetric center is set by a diastereoselective hydroboration followed by cyclization to the spirobicyclic system.     

Solvent-dependent dynamic kinetic asymmetric transformation/kinetic resolution in molybdenum-catalyzed asymmetric allylic alkylations

Catalytic asymmetric alkylation reactions of branched racemic carbonates 1a and 1b with sodium dimethyl malonate, promoted by molybdenum and ligand 5, proceed by a kinetic resolution in toluene, THF, tetrahydropyran, i-PrOAc, 1,2-dichloroethane, and MeCN with k(rel) of 7-16. In THF, MeCN, tetrahydropyran, and i-PrOAc using the (S,S)-5 ligand, the fast reacting (S)-carbonate enantiomer provides the branched product with high ee (97-99.5%) and branched/linear selectivity, but the ee erodes as the reaction of the slow-reacting (R)-enantiomer takes place. This implies that the rate of equilibration of the oxidative addition complexes in these solvents is competitive with the subsequent malonate displacement step. In toluene and dichloroethane, the ee and branched/linear ratios diminish during the reaction of the slow-reacting (R)-isomer, but not nearly as much as in the other solvents. This is most likely due to either an increase in the rate of equilibration of the oxidative addition complexes relative to the malonate displacement step, or vice versa. Because of the minimal stereochemical memory effect in toluene and 1,2-dichloroethane, the reactions in these solvents can be carried to completion (dynamic kinetic asymmetric transformation) and still provide product with excellent ee (>95%). The anion of dimethyl methylmalonate also reacts via a kinetic resolution, although the ee's, rates, and k(rel) values differ from those of the reactions with dimethyl malonate.     

Conclusive evidence for a retention-retention pathway for the molybdenum-catalyzed asymmetric alkylation

Enantiomerically enriched, deuterated branched carbonates (Z)-(S)-PhCH(O2COMe)-CH=CHD (1a), (Z)-(R)-PhCH(O2COMe)CH=CHD (2a), and linear carbonate (E)-(S)-PhCH=CHCHD(O2COMe) (12) were employed as probes in the Mo-catalyzed asymmetric allylic alkylation with sodium dimethyl malonate, catalyzed by ligand complex 10 derived from the mixed benzamide/picolinamide of (S,S)-trans-diaminocyclohexane and (norbornadiene)Mo(CO)4. X-ray crystallography, along with solution NMR structural analysis of the pi-allyl intermediate and competition experiments, conclusively established the reaction proceeded via a retention-retention pathway. This mechanism contrasts with that defined for Pd-catalyzed allylic alkylations, which proceed via an inversion-inversion pathway.     

Stereoisomeric separation of pharmaceutical compounds using CE with a chiral crown ether

Optical pure (+)-(18-crown-6)-2,3,11,12-tetracarboxylic acid, a chiral crown ether, was successfully used as a chiral selector for the stereoisomeric separation of numerous real pharmaceutical compounds. Both practical and mechanistic aspects were described. Effects of chiral selector concentration under different pH values of BGE were discussed. Chiral recognition for the enantiomeric compounds with (+)-(18-crown-6)-2,3,11,12-tetracarboxylic acid was investigated through model compounds using CE and infrared spectroscopic techniques. Relations between the enantioselectivity of the chiral crown ether and the structural features of the studied compounds were also investigated. Unusual resolutions of compound-p and its enantiomer as well as compound-o and its 2b epimer were described. These compounds contained only tertiary amine, believed to be nonbinding with crown ethers in general. The possible mechanisms for the interaction between compound-o and the chiral crown ether were investigated using CE, electrospray MS (ESI-MS), and proton ((1)H) NMR spectroscopy. All experiments provided clear evidence that binding between compound-o and the chiral crown ether had occurred. ESI-MS spectra indicated that the complexes had a 1:1 stoichiometric ratio. The advantages and disadvantages of using chiral crown ether for stereoisomeric separations were compared with those using sulfated CDs.     

Interaction between explosive and analyte layers in explosive matrix-assisted plasma desorption mass spectrometry

An HMX/insulin two-layer system was chosen as a model for further investigation of the matrix properties of explosive materials for protein analytes in plasma desorption mass spectrometry. The dependencies of the molecular ion yield and average charge state as a function of the analyte thickness were studied. An increase in the charge state of multiply protonated molecular species was confirmed as the major matrix effect, with the average charge state z at the smallest thickness studied being higher than in matrix-assisted laser desorption/ionization and closer to the value obtained in electrospray ionization under standard acidic conditions. Observed charge state distributions are significantly narrower than the corresponding Poisson distributions, which suggests that the protonation of insulin is limited in plasma desorption by the number of basic sites in the molecule, similar to electrospray ionization. Both the curve displaying total molecular ion yield and the one showing the total charge (proton) yield as a function of the insulin thickness have maxima at a thickness different from an insulin monolayer. These observations diminish the significance of a matrix/analyte interface mechanism for the explosive matrix assistance. Instead, a mechanism related to the chemical energy release during conversion of the explosive after the ion impact is proposed. As additional mechanisms, enhanced protonation of the analyte through collisions with products of the explosive decay is considered, as well as electron scavenging by other products, which leads to a higher survival probability of positively charged protein molecular ions. Copyright 1999 John Wiley & Sons, Ltd.     

Wave scattering effects in elastic percolation models

The study of the propagation of waves in randomly diluted models is presented. Porosity (crack-like) models are simulated by constructing typical elastic percolation networks with random microscopic heterogeneities in order to resemble rock media. Central and bond-bending forces (Born Hamiltonian) models are considered. For each experimental case, the elastic energy of the system is relaxed in equilibrium and then the model is excited by a pulse source in order to produce wave propagation. First, a review is presented of the well established velocity-porosity relationship from rock physics, which shows a linear trend from small porosities up to the critical porosity (percolation threshold) where the rocks fall apart. From the wave propagation analysis a general trend is observed for the attenuation of waves, from the small to the large porosity models, suggesting multiple scattering effects similar to those reported from effective-medium approximations of wave scattering due to random heterogeneities. Finally, the results are compared with those obtained from laboratory experiments on dry rocks with different porosities and different applied stress regimes.     

Practical synthesis of functionalized 1,5-disubstituted 1,2,4-triazole derivatives

A general approach for the synthesis of 1,5-disubstituted-1,2,4-triazole compounds is described. A series of new oxamide-derived amidine reagents can be accessed in excellent yield with minimal purification necessary. Typically, these amidine reagents are stable crystalline solids and in certain cases were found to exist in a cyclic form as determined by NMR spectroscopy. Under optimized conditions, the direct reaction of these prepared reagents with various hydrazine hydrochloride salts efficiently generates the target triazoles. Both aromatic and aliphatic hydrazines react readily with the amidine reagents under very mild reaction conditions, delivering desired 1,5-disubstituted-1,2,4-triazole derivatives in good yields.     

Experimental, Theoretical and Biological Activity Study on the Acyl-Substituted Benzo-18-crown-6, Dibenzo-18-crown-6 and Dibenzo-24-crown-8

Hexadecanoyl, dihexadecanoyl, dioctadecaneoyl, di-10-undecenoyl, and dicis-9-octadecenoyl derivatives of benzo[18]crown-6, dibenzo[18]crown-6 and dibenzo[24]-8 were synthesized by the condensation of carboxylic acids (palmitic, stearic, oleic and undecenoic acid) with benzo and dibenzo crown ethers in the presence of zinc chloride. The extraction equilibrium constants of such macrocyclic ethers with long side chains were estimated using chloroform/water and dichloromethane/water membranes transfer of Na-PAR (4-(2-pyridylazo)-resorcinol mono sodium monohydrate) with UV–Vis spectroscopy. It was found that they were in the range of 10.88–11.71 in dichloromethane and 8.04–11.77 in chloroform. These results actually show that the Na+ binding effect of macrocyclic ethers depends on the type and the length of side chains. The geometrical properties of the molecules were studied employing semi-empirical calculations by simulated annealing technique. The frontier molecular orbital energies and dipole moments were also examined. The biological activity results showed that the synthesized crown ethers have no activity against the studied microorganisms.