Configurations 1s Research Articles

Generalized Levinson theorem: Applications to electron-atom scattering.

A recent formulation provides an absolute definition of the zero-energy phase shift \ensuremath{\delta} for multiparticle single-channel scattering of a particle by a neutral compound target in a given partial wave l. This formulation, along with the minimum principle for the scattering length, leads to a determination of \ensuremath{\delta} that represents a generalization of Levinson's theorem. In its original form that theorem is applicable only to potential scattering of a particle and relates \ensuremath{\delta}/\ensuremath{\pi} to the number of bound states of that l. The generalized Levinson theorem relates \ensuremath{\delta}/\ensuremath{\pi} for scattering in a state of given angular momentum to the number of composite bound states of that angular momentum plus a calculable number that, for a system described in the Hartree-Fock approximation, is the number of states of that angular momentum excluded by the Pauli principle. Thus, for example, for electron scattering by Na, with its (1s${)}^{2}$(2s${)}^{2}$(2p${)}^{6}$3s configuration and with one L=0 singlet composite bound state, \ensuremath{\delta} would be \ensuremath{\pi}+2\ensuremath{\pi} for s-wave singlet scattering, 0+3\ensuremath{\pi} for s-wave triplet scattering, and 0+\ensuremath{\pi} for both triplet and singlet p-wave scattering; the Pauli contribution has been listed first. The method is applicable to a number of ${\mathit{e}}^{\ifmmode\pm\else\textpm\fi{}}$-atom and nucleon-nucleus scattering processes, but only applications of the former type are described here. We obtain the absolute zero-energy phase shifts for ${\mathit{e}}^{\mathrm{\ensuremath{-}}}$-H and ${\mathit{e}}^{\mathrm{\ensuremath{-}}}$-He scattering and, in the Hartree-Fock approximation for the target, for atoms that include the noble gases, the alkali-metal atoms, and, as examples, B, C, N, O, and F, which have one, two, three, four, and five p electrons, respectively, outside of closed shells. In all cases, the applications provide results in agreement with expectations. \textcopyright{} 1996 The American Physical Society.

Uncatalysed (thermal) and Lewis acid-promoted asymmetric hetero-Diels–Alder reaction of 1-thiabuta-1,3-dienes (thiochalcones) with di-(–)-menthyl fumarate. Configuration determination by X-ray crystallographic analysis of (2S,3R,4R)-(+)-2,3-bis[(–)-menthoxycarbonyl]-4,6-diphenyl-3,4-dihydro-2H-thiopyran and conversion of cycloadducts into optically pure diols

1-Thiabuta-1,3-dienes 1 (thiochalcones) underwent asymmetric hetero-Diels–Alder reaction with di-(–)-menthyl fumarate 2 to afford a mixture of 3,4-cis-(3 and 4) and 3,4-trans-dihydrothiopyrans (5 and 6) in good-to-excellent chemical yield with fair diastereoselectivities, the stereoisomer 3 predominating. An endo(3,4-cis): exo(3,4-trans) selectivity was observed in the ratio 98–85:2–15. The uncatalysed reaction at 20–40°C showed 10–48% de in diastero-π-facial selectivity, while the selectivity was improved to 62–71% de by using a suitable Lewis acid under appropriate reaction conditions. The 3,4-cis-stereoisomer formed favourably in both thermal and Lewis acid-promoted reactions, had the same configuration 2S,3R,4R(3). The configuration was unequivocally established by X-ray crystallographic structural analysis using the isomer 3a (R1= R2= Ph). The isomer 3, which could be obtained stereochemically homogeneous by chromatography and recrystallization, was consecutively transformed to optically pure diols 7 and 8 or 9 by LiAlH4 reduction, followed by reductive desulfurization with Raney Ni.

Total Synthesis of Zaragozic Acid A (Squalestatin S1). Degradation to a Relay Compound and Reassembly of the Natural Product

Zaragozic acid A (squalestatin S1) (1) was converted into the simpler derivative 2, which was reconverted into the natural product, thus establishing 2 as a viable relay compound for total synthesis of 1. The degradation (Scheme 1) consisted of formation of the tri-tert-butyl ester (3), from which the two side chains were sequentially removed to obtain 8. Aldehyde 8 was converted into dimethyl acetal 2 in standard fashion. The C6 acyl side chain 14 was prepared from (S)-2-methylbutanol (“active amyl alcohol”), and the desired 4S configuration was obtained by use of Evans asymmetric enolate methylation (Scheme 2). The C1 alkyl side chain was prepared as stannane 23a from (R)-2-methyl-3-phenylpropanol (21) as shown in Scheme 5. For conversion of 2 back into zaragozic acid A, the dimethyl acetal was first converted into the cyclic acetal 17, thus protecting the C7 hydroxyl group. The remaining hydroxyl group was then acylated with acid 14 to obtain 18, which was transformed into aldehyde 20 (Scheme 4). The C1 alkyl chain was elaborated by the addition of a chiral α-alkoxyorganocerium reagent, obtained from 23a, to aldehyde 20. The resulting mixture of diastereomeric secondary alcohols was converted into zaragozic acid A (1) in six steps (Scheme 6).

First X-ray Determination of an Oxaziridinium Salt: (1S,2R,3R,4S)-2,3-Dimethyl-4-phenyl-1,2,3,4-tetrahydro-1,2-epoxyisoquinolinium Tetrafluoroborate and (1S,2R,3R,4S)-3-Methyl-4-phenyl-1,2,3,4-tetrahydro-2,3-epoxyisoquinoline

The chiral oxaziridinium salt (1), C 17 H 18 NO + .BF 4- , transfers the O atom enantioselectively to nucleophilic substrates. The configuration (1S,2R) for the O atom has been established. This is the first X-ray determination for an oxaziridinium salt. Its geometry is quite superimposable with that of the parent oxaziridine (2), C 16 H 15 NO. The N-O bond length of 1.468(8)A in the oxaziridinium salt is shortened compared with the mean bond length of 1.508 (16) A observed in 40 X-ray structures of oxaziridine. The oxaziridine ring is perpendicular to the isoquinoline ring, the former making a dihedral angle of 97° with the mean plane of the latter in (1), and dihedral angles of 94 and 93° in the two independent molecules of (2).

Simultaneous assay of four stereoisomers of diltiazem hydrochloride. Application to in vitro chiral inversion studies

The simultaneous stereospecific assay of four stereoisomers of diltiazem hydrochloride in bulk drug and aqueous solution was developed using HPLC on a Chiralcel OF column. The four isomers were quantitated with good precision by the internal standard method. The chiral inversion of (+)-cis-diltiazem hydrochloride in vitro, stability of its (2S, 3S) configuration in the solid and aqueous states was examined by HPLC. Chiral inversion of (+)-cis-diltiazem hydrochloride was not observed in the solid state, and its (2S, 3S) configuration was stable to heat, humidity and light. Chiral inversion of (+)-cis-diltiazem hydrochloride (2S, 3S) was observed in aqueous solution under UV, but not in aqueous solution stored at 80°C for 5 h nor under visible light for 10 h. The (+)-cis-diltiazem hydrochloride (2S, 3S) epimerized to (+)-trans-diltiazem hydrochloride (2R, 3S) with a half-life of 5 h in aqueous solution under UV but the reverse chiral inversion of (+)-trans-diltiazem hydrochloride (2R, 3S) to (+)-cis-diltiazem hydrochloride (2S, 3S) was not observed.

Synthetic microbial chemistry, XXVIII. Synthesis and absolute configuration of (−)‐koninginin A

AbstractThe absolute configuration of the microbial metabolite (−)‐koninginin A (2‐hexyloctahydro‐2H‐3,9a‐epoxy‐1‐benzoxe‐pine‐6,9‐diol, 1) is shown to have the configuration 2S,3S,6R,9S by its synthesis employing the Sharpless asymmetric dihydroxylation.

7-Azetidinylquinolones as antibacterial agents. 3. Synthesis, properties and structure-activity relationships of the stereoisomers containing a 7-(3-amino-2-methyl-1-azetidinyl) moiety.

A series of stereochemically pure 7-(3-amino-2-methyl-1-azetidinyl)-1,4- dihydro-6-fluoro-4-oxoquinoline- and -1,8-naphthyridine-3-carboxylic acids, with varied substituents at the 1-, 5-, and 8-positions, was prepared to determine the effects of chirality on potency and in vivo efficacy relative to the racemic mixtures (for part 2, see: J. Med. Chem. 1994, 37, 4195-4210). A series of chiral 9-fluoro-2,3-dihydro-3-methyl-7-oxo-10-(substituted-1- azetidinyl)-7H-pyrido[1,2,3- de]-1,4-benzoxazine-6-carboxylic acids was synthesized to study the effect of the azetidine moiety on tricyclic quinolone antibacterial agents. A series of amino acid prodrugs of chiral naphthyridines 24a and 24b and quinolone 33a (cetefloxacin) was prepared and evaluated for antibacterial activity, solubility, and pharmacokinetic behavior. The absolute configuration of the new azetidinylquinolones was established by X-ray analysis of one of the diastereomeric salts of the resolved azetidinols (15) and of compound 25a (E-4767), which showed the best in vitro and in vivo overall profile. Structure-activity relationship studies indicated that the absolute stereochemistry at the asymmetric centers of both the azetidine and the oxazine rings was critical to increase in vitro activity and oral efficacy. The 3S configuration in the pyridobenzoxazine series and the (2S,3R) configuration of the 3-amino-2-methylazetidine moiety for all new compounds conferred the best antibacterial activity.

Compton Profile of Ag2O

AbstractThe Compton profile (CP) of polycrystalline silver oxide is measured with the use of 59.54 keV γ‐radiation from a 241Am source. The experimental data are compared with the renormalized free atom (RFA) model calculation using Watson's +1 well solution for the oxygen 2p state. Oxygen 2p6 and hybridization of different 4d–5s configurations of Ag are considered in the theoretical calculations. The theoretical values from such calculations agree well with experimental results.

Stereochemistry of 3-deoxyoctulosonate 8-phosphate synthase.

(Z)- and (E)-[3-2H]phosphoenolpyruvate were prepared chemically by the reductive deuteration of (Z)- and (E)-3-bromophosphoenolpyruvate, respectively, and were converted into 3-deoxyoctulosonate 8-phosphates deuterated at the C-3 position by incubation with unlabeled D-arabinose 5-phosphate in the presence of the enzyme, 3-deoxyoctulosonate 8-phosphate synthase (EC4.1.2.16) purified from Escherichia coli K-12 containing the plasmid pMW101. Analysis of the stereochemistry of the two 3-deoxyoctulosonate 8-phosphates deuterated at the C-3 position by 1H NMR showed that the (Z)-[3-2H]phosphoenolpyruvate had produced [3-2H]-3-deoxyoctulosonate 8-phosphate of predominantly the 3S configuration and that the E isomer had given predominantly (3R)-[3-2H]-3-deoxyoctulosonate 8-phosphate. The 3-deoxyoctulosonate 8-phosphate synthase reaction is therefore stereospecific with respect to the C-3 of phosphoenolpyruvate. The results indicate a si face attack from the C-3 of phosphoenolpyruvate, a result identical to that reported for 3-deoxyheptulosonate 7-phosphate synthase (EC 4.1.2.15), an enzyme catalyzing an identical aldol-type condensation, except that it takes place between phosphoenolpyruvate and D-erythrose 4-phosphate. The stereochemistry with respect to the face of the carbonyl of the attacked aldehyde, in both 3-deoxyoctulosonate 8-phosphate synthase and 3-deoxyheptulosonate 7-phosphate synthase, is re. On the basis of the results of the studies reported herein, the presence of a transient methyl group at the C-3 of phosphoenolpyruvate as part of the reaction mechanism seems unlikely.

Infrared coronal emission lines and the possibility of their laser emission in Seyfert nuclei

Infrared coronal emission lines are providing a new window for observation and analysis of highly ionized gas in such Galactic and extragalactic sources as Seyfert nuclei and classical novae shells. These lines are expected to be primary coolants in colliding galaxies, galaxy cluster cooling flows, cometary-compact H II regions, and supernova remnants. In this paper, we compile a complete list of infrared (λ > 1 micron) lines due to transitions within the ground configurations 2S^2^2p^k^ and 3S^2^3p^k^ (k = 1-5) or the first excited configurations 2s2p and 3s3p of highly ionized (χ >= 100 eV) astrophysically abundant [n(X)/n(H) >= 10^-6^] elements. Included are approximately 74 lines in ions of O, Ne, Na, Mg, Al, Si, S, Ar, Ca, Fe, and Ni spanning a wavelength range of approximately 1-280 microns. We present results from detailed balance calculations, critical densities for collisional deexcitation, intrinsic photon rates, branching ratios, and excitation temperatures for the majority of the compiled transitions. The temperature and density parameter space for dominant cooling via infrared coronal lines is presented, and the relationship of infrared and optical coronal lines is discussed. We find that under physical conditions found in Seyfert nuclei, 14 of 70 transitions examined have significant population inversions in levels that give rise to infrared coronal lines. Laser gain lengths and corresponding column densities are calculated for dense [10^6^ <= n_e_ (cm^-3^) <= 10^9^] collisionally ionized plasmas. Application of these results to cooler plasmas photoionized by power-law continuum radiation fields is also discussed. We find that several infrared coronal line transitions have laser gain lengths that correspond to column densities of 10^24-25^ cm^-2^ which are modeled to exist in Seyfert nuclei. Observations that can reveal inverted level populations and laser gain in infrared coronal lines are also suggested.

Quantum chemical and molecular mechanics studies on the binding of stereoisomers of the oligopeptide antibiotics amidinomycin and noformycin to the minor groove of B-DNA.

Ab initio calculations (Hartree-Fock) using the 6-31G basis set have been performed on two chiral oligopeptide antitumor antibiotics amidinomycin 5 and noformycin 6. The latter are DNA minor groove binding agents related to the A.T recognizing netropsin 4 and distamycin 3 but, unlike the latter, bear stereocenters (two for 5 and one for 6) that may be expected to affect binding to the B-DNA receptor. Geometry optimized conformations, energies and distribution of electrostatic charges within the molecules were derived. The rotational barrier for bond C3-C6 in 6 was calculated to be ca. 6 kcal.mole-1 and the dipole moment for 6 was 7.69D and for 5 was 5.58D. The ab initio derived parameters of the geometry optimized conformations of the different possible stereoisomeric forms of 5 and 6 were used to interpret their different interactions with the minor groove of DNA at both A.T and G.C sequences and the results were compared with molecular mechanics calculations. The order of binding of the four stereoisomers of 5 at the preferred (A.T)n sequences by both ab initio and molecular mechanics calculations is 1S,3R > RR > RS > SS. The predicted energy differences for complexation with DNA of the other stereoisomers from that of 1S,3R are: RR (4.2%); RS (6.7%) and SS (21.5%). In the case of noformycin the 4R structure binds more effectively than the enantiomer. Considerations of phasing in the computed distances between hydrogen bond donating sites in the DNA-bound antibiotics provide further insight into the binding processes. In the complexes of noformycin 6 the N-N1-N4 and N1-N5 distances (9.05 and 9.15 A respectively for 4R-6 and 9.23 and 9.26 A respectively for 4S-6) are close to the optimum value of 9.1 A for effective binding. In the case of amidinomycin 5 the best agreement with the optimum value occurs with the strongest binding diastereomer 1S,3R (N1-N3 = 8.91, N1-N4 = 9.41 A). The unexpected result, consistent in both ab initio and molecular mechanics treatments, is that, in contrast to the cases of kikumycin 1 and anthelvencin 2, the natural 3S configuration of 5 and 4S of 6 do not confer maximal binding efficiency. This suggests that biogenetic factors in the generation of the oligopeptide antibiotics lead to maximum DNA binding in the cases of kikumycin and anthelvencin but not in the cases of amidinomycin and noformycin.

Scalar and tensor polarizabilities for the 1s2s2p 4P0 states of He- and lithium

The differential Stark shift of the 1s2s2p 4P0 states of He- and Li is studied. The scalar and tensor polarizabilities are calculated using a multiconfiguration interaction wavefunction. Polarization to the 4S, 4P and 4D configurations is considered. For He- the scalar polarizability is calculated to be 8169.13 a30 and the tensor polarizability is -4463.5 a30. They are 45.64 and 6.86 a30, respectively, for lithium. The dynamic polarizability is calculated for the frequency range from zero to near the ionization threshold for He- and to the first excitation threshold for lithium.

Hyperfine-structure constants for 47Ti I 3d24s2 3P.

We report magnetic dipole hyperfine-structure constants, obtained by doing nonrelativistic configuration-interaction calculations, for $^{47}\mathrm{Ti}$ i 3${\mathit{d}}^{2}$4${\mathit{s}}^{2}$ $^{3}$P, which are in excellent agreement with experiment [theory (experiment) ${\mathit{A}}_{2}$=-23.150 (-25.215) MHz, ${\mathit{A}}_{1}$=2.202 (2.665) MHz] in contrast with Hartree-Fock and some semiempirical values. Second-order electrostatic effects, arising mainly from the 3${\mathit{d}}^{3}$4s configuration, are crucial to understanding this system. The nuclear quadrupole coupling constant is -138.4 MHz and 69.2 MHz\ifmmode\times\else\texttimes\fi{}Q (in b) (J=1,2). By combining with measurement, we predict Q=0.346 b. Hyperfine-structure constants are also given for the 3${\mathit{d}}^{3}$4s and 3${\mathit{d}}^{4}$ $^{3}$P terms.

Krypton 4p, 4s, and 3d partial photoionization cross sections below a photon energy of 260 eV.

The photoionization of krypton 4p, 4s, and 3d subshells has been studied theoretically from the 4p threshold and experimentally from the 3d threshold up to a photon energy of 260 eV. In converting the measured count rates of the 4p, 4s, and 3d main photoelectron lines into partial cross sections, we have used recent photoion yield measurements to estimate the contribution of multiple-excitation processes to the total photoabsorption cross section. Theoretical partial cross sections and angular asymmetry parameters have been calculated using the multichannel Dirac-Fock method. Eighteen leading single-excitation channels related to the 4p, 4s, 3d, and 3p hole configurations of the final ionic state were included. Relaxation was taken into account by using separately optimized initial- and final-state orbitals and by including all the resulting overlap integrals in the transition amplitudes. The relaxation effect was found to depend strongly on the photon energy in the near-threshold regions. For the 3d subshell the relaxation reduces the 3d cross section by 35--40 % at the maximum, whereas it is less significant for the 4p and 4s cross sections.

SC-MEH-CI calculations on the (NH4) 4CuCl6 (D2h) cluster in (NH4)2CuCl4

Calculations of electronic structure and bonding via the SCMEH-MO method with limited Cu 3d, 4s, and 4p configuration interaction, have been carried out on the tetragonally coordinated (D2h symmetry) CuCl6(NH4)4 structural cluster unit in crystalline (NH4)2CuCl4. The results are compared with those of other calculations. The calculated electronic spectrum, electric field gradient (EFG), and magnetic data are compared with experiment. © 1992 John Wiley & Sons, Inc.

A new sesquiterpene from Maytenus boaria (Celastraceae); crystal structure and absolute configuration

Hitherto all the agarofuran sesquiterpenes substituted at C-4 isolated from the Celastraceae had a 4S configuration but now a sesquiterpene diol with a 4R configuration has been isolated and studied by x ray diffraction analysis and application of the Horeau method.

Synthesis of 6R(β)-tritylaminopenicillanic-3R(β)-alcohol, a versatile stereoisomer of natural β-lactams

In an attempt to rationalize a synthesis of penicillin analogs modified at C(3), we have isolated the 3R(β)-carbinolamide derivative 4a. The trityl substituent on N(6′) seems to be responsible for the inversion of configuration which occurs at C(3) during the acid hydrolysis of the isocyanate intermediate. An hydrogen bond is formed on the β-face of the bell-shaped bicyclic skeleton between the N(6′)-nitrogen lone pair and the C(3) hydroxyl group. On standing, the carbinolamide analog slowly isomerizes to its expanded bicyclic isomer 4b, but the starting material may be easily recovered by treatment with acid. The postulated intermediate during isomerization, i.e., the open aldehyde form, does not accumulate. Substitutions of the hydroxyl group at C(3) lead to a variety of compounds with the biologically active 3S(α) configuration. These may be used to study the importance of the carboxyl group of penicillins in their interaction with β-lactamases at the molecular level.

Parametric description of 3dN 4s configurations using orthogonal operators

Parametric fits to the 3dN 4s configurations in the III to VI spectra of the iron sequence are reported, using a set of orthogonal operators consisting of electrostatic operators up to four-body character and magnetic operators up to two-body character. The results are presented for all operators. The parameter values behave regularly among neighbouring ions and, for the 3dN core, compare well to those for the corresponding 3dN configurations. For comparison, ab initio calculated values of some operators are given as well.

Excitation of Ar I 3p54p, 3p53d and 3p55s configurations by electrons in the distorted-wave approximation

Excitation cross sections for all fine-structure levels of Ar I 3p54p, 3d and 5s configurations are calculated for the range of electron energies 16-100 eV in the distorted-wave Born approximation with exchange. Non-empirical intermediate-coupling wavefunctions are used and their quality is checked by calculating the dipole transition probabilities. The results are compared with the experimental data obtained by electrical and optical methods.

Enantiospecific Syntheses of Sphingosine and Ceramide Stereoisomers with 3S Configuration from D-Glucose.

Utilizing the carbon framework (C3-C6) and chirality (at C4 and C5) of D-glucose, (2S, 3S, 4E and 2R, 3S, 4E)-2-amino-4-octadeceiie-1, 3-diol (sphingosine analogues) and four Stereoisomers (2S, 3S, 4E-, 2S, 3S, 4Z-, 2R, 3S, 4E-, and 2R, 3S, 4Z-) of the ceramide derivatives were synthesized in a stereocontrolled way.