On the theory of collision-induced vibrational transitions in polyatomic molecules: The mode-matching model

A modification of the SSH theory for collision-induced vibrational transitions in polyatomic molecules is proposed. The breathing-sphere model assumptions are avoided by considering the angular relation between the direction of approach and the normal modes displacements of the atoms involved in the contract (mode-matching). The results, as compared to the breathing-sphere model, indicate a considerable, mode specific reduction of the transition probabilities. Intramolecular transitions of CH₃Cl are studied as an example.     

Ab initio MRD-CI study of ethane: The 14–25 eV PES region and Rydberg states of positive ions

Ab initio calculations of the MRD-CI type are reported for various states of the C₂H₆⁺ ion in two different nuclear geometries and the results are compared with the experimentally observed ethane PES in the 14–25 eV region. The calculated vertical IP values for ionization out of the 1e_u, 2a_2u and 2a_1g MO's respectively agree well with the locations of the three ionization maxima in this spectral range. The analogous findings for excitation out of the relaxed ionic ground state find several relatively low-lying species which occupy a 2pσ* MO in addition to states resulting from simple ionization of the neutral molecule. A number of Rydberg states are also calculated at the relaxed-ion geometry, from which results it is determined that the quantum defects for such species are from 0.40–0.45 units smaller than for their counterparts in neutral systems; these findings are clearly consistent with a decrease in the core penetrability of the Rydberg electron as the effective charge is increased to Z = 2.     

Mehrfachbindungen zwischen hauptgruppenelementen und übergangsmetallen: IV. Chrom-schwefel- und chrom-selen-bindungen: Knüpfung,umwandlungsreaktionen und alkylidenaddition

New dinuclear chromium—sulfur complexes containing one or two sulfur atoms are formed upon reaction of the ionic complex K[(η⁵-C₅Me₅)Cr(CO)₃] (Me = CH₃; 1) with trithiazyl trichloride at temperatures below ambient. The μ(η¹ :η²)-disulfur complex 2a of composition (η⁵-C₅Me₅)₂Cr₂(CO)₅S₂ undergoes concomitant CO-elimination and desulfurization on heating or in the presence of triphenylphosphine; the CrSCr-complex (η⁵-C₅Me₅)₂Cr₂(CO)₄S (2c) is formed in these cases. When diazomethane is treated with 2a, a novel μ,η²-thio- formaldehyde complex (2d) results, again following CO- and S-extrusion. While the triple-bonded CrSCr-skeleton present in 2c does not allow alkylidene additions, such reactions, which proceed cleanly, are observed with the corresponding μ⁵-seleno derivative (η⁵-C₅H₅)₂Cr₂(CO)₄Se (4) synthesized from Na[(η⁵-C₅H₅)Cr(CO)₃] and sodium selenite in the presence of hydrochloric acid.     

Photochemical Regulation of an Artificial Hydrolase by a Backbone Incorporated Tertiary Structure Switch

A stilbene chromophore has been incorporated into the turn region of a 42 amino acid peptide, linking two helical peptide sections. Spatial proximity between these sections, as well as aggregation into dimers, is required to facilitate the catalytic function of this artificial hydrolase. Photomodulation of the hydrolase activity results in an increase of the activity of 42 % upon switching from the trans to the cis isomer of the chromophore. This is rationalized by a change in the aggregation state of the peptidomimetic, which is supported by diffusion coefficients obtained from PFG‐NMR experiments. The results show that incorporation of a small, relatively flexible chromophore into a large peptide is capable of inducing a considerable change in tertiary structure and thus, functionality.     

Extending the coordination chemistry of molecular P(4)S(3): the polymeric Ag(P(4)S(3))(+) and Ag(P(4)S(3))(2)(+) cations

Upon reacting P(4)S(3) with AgAl(hfip)(4) and AgAl(pftb)(4) [hfip = OC(H)(CF(3))(2); pftb = OC(CF(3))(3)], the compounds Ag(P(4)S(3))Al(hfip)(4) 1 and Ag(P(4)S(3))(2)(+)[Al(pftb)(4)](-) 2 formed in CS(2) (1) or CS(2)/CH(2)Cl(2) (2) solution. Compounds 1 and 2 were characterized by single-crystal X-ray structure determinations, Raman and solution NMR spectroscopy, and elemental analyses. One-dimensional chains of [Ag(P(4)S(3))(x)](infinity) (x = 1, 1; x = 2, 2) formed in the solid state with P(4)S(3) ligands that bridge through a 1,3-P,S, a 2,4-P,S, or a 3,4-P,P eta(1) coordination to the silver ions. Compound 2 with the least basic anion contains the first homoleptic metal(P(4)S(3)) complex. Compounds 1 and 2 also include the long sought sulfur coordination of P(4)S(3). Raman spectra of 1 and 2 were assigned on the basis of DFT calculations of related species. The influence of the silver coordination on the geometry of the P(4)S(3) cage is discussed, additionally aided by DFT calculations. Consequences for the frequently observed degradation of the cage are suggested. An experimental silver ion affinity scale based on the solid-state structures of several weak Lewis acid base adducts of type (L)AgAl(hfip)(4) is given. The affinity of the ligand L to the silver ion increases according to P(4) < CH(2)Cl(2) < P(4)S(3) < S(8) < 1,2-C(2)H(4)Cl(2) < toluene.     

Antioxidant oligomeric proanthocyanidins from Cistus salvifolius

The purified proanthocyanidin oligomers of Cistus salvifolius herb extract accounted for 78% of the total proanthocyanidins and 73% of the total antioxidant activity of this extract. To elucidate the structure of the oligomer, it was depolymerized by acid catalysis in the presence of phloroglucinol. The structures of the resulting flavan-3-ols and phloroglucinol adducts were determined on the basis of 1D- and reverse 2D-NMR (HSQC, HMBC) experiments of their peracetylated derivatives, MALDI-TOF-MS and CD spectroscopy. These observations resulting from the degradation with phloroglucinol were confirmed by 13C NMR spectroscopy of the oligomer. The mean molecular weight of the higher oligomeric fraction was estimated to be 5-6 flavan-3-ol-units.     

Substituent effects on the basicity of 3,7-diazabicyclo[3.3.1]nonanes

Basicity constants for a series of 3,7-diazabicyclo[3.3.1]nonane derivatives in acetonitrile with a variation over 13 orders of magnitude have been determined using a spectrophotometric titration technique. An excellent correlation between basicity and calculated proton affinities obtained at PCM-B3LYP/6-31+G(d)//B3LYP/6-31G(d) level was found. The results are discussed in terms of substituent effects and compared to (15)N NMR chemical shifts.