Structural requirements in chiral diphosphine-rhodium complexes. X asymmetric homogeneous hydrogenation of z-N-acetyldehydroamino acids and esters with (1R,2R)-trans-1,2-bis(diphenylphosphinomethyl)cyclobutane/rhodium(I) complexes.

Z--acetamidocinnamate esters were hydrogenated with neutral rhodium(I) complexes containing (1R,2R)-trans-1,2-bis(diphenylphosphinomethyl)cyclobutane. Increasing the steric bulk of the alcohol moiety in the unsaturated esters had little influence upon the optical purity of the N-acetylphanylalanine ester products. In the series Me, Et, i-Pr, and t-Bu the optical purity decreased from 44 % ee-(R) [Me] to 40 % ee-(R) [t-Bu]. The chiral cyclobutane diphosphine appears to be only slightly more effective than the heterocyclic DIOP when ring-substituted Z--acetamidocinnamic acids are hydrogenated with neutral rhodium(I) complexes without the addition of triethylamine. Addition of triethylamine to the solvent blend seems to be more beneficial to the cyclobutane analogue than to DIOP.     

Spectroscopic and electrochemical study of the adsorption of [Co(en)2Cl2]Cl on gamma-alumina: influence of the alumina ligand on Co(III)/(II) redox potential

UV-visible and Raman spectroscopies as well as electrochemical techniques have been used to characterize cis- and trans-[Co(III)(en)2Cl2]Cl (en=ethylenediamine) complexes and the gamma-alumina-supported cis-Co((III)) complex. It is shown that the electrochemical reduction of these complexes occurs according to a multistage mechanism involving two electrochemical steps, with the formation of a dimer that was characterized by UV-visible spectroscopy (intervalence band at 670 nm). The apparent standard redox potential for each step has been determined, and experimental results reveal that cis and trans complexes present similar electrochemical characteristics. It is also shown that the deposition of trans-[Co(III)(en)2Cl2]+ on gamma-alumina leads to an inner-sphere complex (ISC) in a cis configuration in which Cl- ligands are substituted by OH or O- surface groups of alumina. These changes in the coordination sphere of the complex induce a substantial decrease of its apparent redox potential since it is -0.63 V/SCE (saturated calomel electrode) for the gamma-alumina-supported cis-Co(III) complex, whereas values of -0.17 and -0.35 V/SCE were determined in dimethyl sulfoxide (DMSO) for the trans and cis precursor complexes, respectively.     

Infrared spectra of protonated uracil, thymine and cytosine.

The gas-phase structures of protonated uracil, thymine, and cytosine are probed by using mid-infrared multiple-photon dissociation (IRMPD) spectroscopy performed at the Free Electron Laser facility of the Centre Laser Infrarouge d'Orsay (CLIO), France. Experimental infrared (IR) spectra are recorded for ions that were generated by electrospray ionization, isolated, and then irradiated in a quadrupole ion trap; the results are compared to the calculated infrared absorption spectra of the different low-lying isomers (computed at the B3LYP/6-31++G(d,p) level). For each protonated base, the global energy minimum corresponds to an enolic tautomer, whose infrared absorption spectrum matched very well with the experimental IRMPD spectrum, with the exception of a very weak IRMPD signal observed at about 1800 cm(-1) in the case of the three protonated bases. This signal is likely to be the signature of the second-energy-lying oxo tautomer. We thus conclude that within our experimental conditions, two tautomeric ions are formed which coexist in the quadrupole ion trap.     

The gas-phase basicity and proton affinity of 1,3,5-cycloheptatriene--energetics,structure and interconversion of dihydrotropylium ions

The hitherto unknown gas-phase basicity and proton affinity of 1,3,5-cycloheptatriene (CHT) have been determined by Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry. Several independent techniques were used in order to exclude ambiguities due to proton-induced isomerisation of the conjugate cyclic C(7)H(9)(+) ions, [CHT + H](+). The gas-phase basicity obtained by the thermokinetic method, GB(CHT) = 799 +/- 4 kJ mol(-1), was found to be identical, within the limits of experimental error, with the values measured by the equilibrium method starting with protonated reference bases, and with the values resulting from the measurements of the individual forward and reverse rate constants, when corrections were made for the isomerised fraction of the C(7)H(9)(+) population. The experimentally determined gas-phase basicity leads to the proton affinity of cycloheptatriene, PA(CHT) = 833 +/- 4 kJ mol(-1), and the heat of formation of the cyclo-C(7)H(9)(+) ion, deltaH(f)(0)([CHT + H](+)) = 884 +/- 4 kJ mol(-1). Ab initio calculations are in agreement with these experimental values if the 1,2-dihydrotropylium tautomer, [CHT + H((1))](+), generated by protonation of CHT at C-1, is assumed to be the conjugate acid, resulting in PA(CHT) = 825 +/- 2 kJ mol(-1) and deltaH(f)(0)(300)([CHT + H((1))](+)) = 892 +/- 2 kJ mol(-1). However, the calculations indicate that protonation of cycloheptatriene at C-2 gives rise to transannular C-C bond formation, generating protonated norcaradiene [NCD + H](+), a valence tautomer being 19 kJ mol(-1) more stable than [CHT + H((1))](+). The 1,4-dihydrotropylium ion, [CHT + H((3))](+), generated by protonation of CHT at C-3, is 17 kJ mol(-1) less stable than [CHT + H((1))](+). The bicyclic isomer [NCD + H](+) is separated by relatively high barriers, 70 and 66 kJ mol(-1) from the monocyclic isomers, [CHT + H((1))](+) and [CHT + H((3))](+), respectively. Therefore, the initially formed 1,2-dihydrotropylium ion [CHT + H((1))](+) does not rearrange to the bicyclic isomer [NCD + H](+) under mild protonation conditions.     

Simplified models for the temperature dependence of linewidths at elevated temperatures and applications to CO broadened by Ar and N2

The broadening coefficients for i.r. lines of CO perturbed by Ar are calculated in the temperature range 300–3500 K using the formalism previously developed by two of us (D.R. and J.B.). The results are compared with high-resolution spectroscopic measurements of shock-heated CO-Ar gas mixtures. A simplified model is proposed to describe the temperature dependence of the linewidths. The resulting model is applied to CO broadened by N₂ and the results are critically discussed.     

Cold storage and cryopreservation of hops (Humulus l.) shoot cultures through application of standard protocols

The USDA-ARS National Clonal Germplasm Repository (NCGR) stores the global diversity of Humulus for the US Plant Germplasm System as trellised plants in a field genebank. In vitro storage and cryopreservation are now considered excellent ways to provide medium and long-term storage for plant collections. Developing a new cryopreservation or cold storage protocol for every accession or genus of large multi-crop collections can be a very time consuming and long-term activity. We propose that standard cold storage and cryopreservation techniques used for other temperate crop genera would be successful for additional crops with few modifications. This study was initiated to determine if a large collection of hops germplasm could be successfully stored with techniques developed for unrelated genera. In this study we characterized the response of diverse Humulus genotypes to in vitro storage under low light at 4 degree C following techniques used for strawberry and mint plants, and cryopreservation in liquid nitrogen by slow cooling with a pear protocol. The average storage time without transfer for the 70 genotypes evaluated was 14 +/- 3.5 months with a range of 6 to 26 months. Mean recovery of cryopreserved shoot tips of accessions with 1-wk cold acclimation was 41 +/- 18 percent and increased to 54 +/- 13 percent with 2-wk cold acclimation. This demonstrates that application of a well-tested standard technique can provide a quick start for storing additional germplasm collections.     

Reactivity and selectivity of homolytic substitution reactions on the peroxyacid group

Peracids in solution may undergo homolytic decomposition leading mostly to mixtures of acid and alcohols, these Latters being formed from either the peracid or the solvent

The peracid behaves as source of radicals and also as substrate thus presenting two radicophile centers

- with a nucleophilic radical (R' from peracid or S' from solvent) the site of attack is the 0-0 bond of the peracid yielding an alcohol (NuOH). The rate of this reaction increases with increasing nucleophilicity of the radical ; this reactivity is interpreted in terms of frontier orbital perturbation theory, taking into account both the orbital energy level difference and the orbital overlap.- when the radical is electrophilic, it abstracts rather the hydrogen of the peroxidic group Leading to decomposition of the peracid into acid.Hydrogen abstraction from the solvent may be also observed in appropriate cases : this process can be used for free radical hydroxylation of hydrocarbons.The relative importance of these three chain reactions in competition : 0-0 bound attack, hydrogen abstraction from the peracid and solvent transfer reaction were studied as a function of structure of the radicals, solvent properties and temperature.     

Pressure broadening mechanisms of the ammonia lines: Rotation,inversion and rotation-inversion spectra of NH3 perturbed by monoatomic gases

The half-widths of rotation, inversion and rotation-inversion spectral lines of NH₃ perturbed by rare gases have been calculated. These calculations take into account the R^-7 dispersion potential energy, which was previously studied by the present authors. A detailed analysis of the contribution of various induced collisional processes has shown that the ΔK = ±3 transitions induced by the dispersion potential are of importance for both the pure rotation spectrum and the inversion and rotation-inversion spectrum.     

Unimolecular reactivity of strong metal-cation complexes in the gas phase: ethylenediamine-Cu(+)

The gas-phase reactions between ethylenediamine (en) and Cu(+) have been investigated by means of mass spectrometry techniques. The MIKE spectrum reveals that the adduct ions [Cu(+)(H(2)NCH(2)CH(2)NH(2))] spontaneously decompose by loosing H(2), NH(3) and HCu, the loss of hydrogen being clearly dominant. The spectra of the fully C-deuterated species show the loss of HD, NH(3) and CuD but no losses of H(2), D(2), NH(2)D, NHD(2), ND(3) or CuH are observed. This clearly excludes hydrogen exchange between the methylene and the amino groups as possible mechanisms for the loss of ammonia. Conversely, methylene hydrogen atoms are clearly involved in the loss of molecular hydrogen. The structures and bonding characteristics of the Cu(+)(en) complexes as well as the different stationary points of the corresponding potential energy surface (PES) have been theoretically studied by DFT calculations carried out at B3LYP/6-311+G(2df,2p)//B3LYP/6-311G(d,p) level. Based on the topology of this PES the most plausible mechanisms for the aforementioned unimolecular fragmentations are proposed. Our theoretical estimates indicate that Cu(+) strongly binds to en, by forming a chelated structure in which Cu(+) is bridging between both amino groups. The binding energy is quite high (84 kcal mol(-1)), but also the products of the unimolecular decomposition of Cu(+)(en) complexes are strongly bound Cu(+)-complexes.