Vibrational dynamics of carboxylic acid dimers in gas and dilute solution

Ultrafast mid-IR transient absorption spectroscopy has been used to study the vibrational dynamics of hydrogen-bonded cyclic dimers of trifluoroacetic acid and formic acid in both the gas and solution phases (0.05 M in CCl(4)). Ultrafast excitation of the broad O-H cyclic dimer band leads, in the gas phase, to large-scale structural changes of the dimer creating a species with a distinct free O-H stretching band on 20 ps and 200 ps timescales. These timescales are assigned to ring-opening and dissociation of the dimer, respectively. In the solution phase, no such structural rearrangement occurs and our results are consistent with previous studies. The gas phase dynamics are insensitive to both the specific excitation energy (over a span of 550 cm(-1)) and the chemical identity of the dimer.     

Next generation techniques in the high resolution spectroscopy of biologically relevant molecules

Recent advances in the technology of test and measurement equipment driven by the computer and telecommunications industries have made possible the development of a new broadband, Fourier-transform microwave spectrometer that operates on principles similar to FTNMR. This technique uses a high sample-rate arbitrary waveform generator to construct a phase-locked chirped microwave pulse that gives a linear frequency sweep over a wide frequency range in 1 μs. The chirped pulse efficiently polarizes the molecular sample at all frequencies lying within this band. The subsequent free induction decay of this polarization is measured with a high-speed digitizer and then fast Fourier-transformed to yield a broadband, frequency-resolved rotational spectrum, spanning up to 11.5 GHz and containing lines that are as narrow as 100 kHz. This new technique is called chirped-pulse Fourier transform microwave (CP-FTMW) spectroscopy. The technique offers the potential to determine the structural and dynamical properties of very large molecules solely from fully resolved pure rotational spectra. FTMW double resonance techniques employing a low-resolution UV laser facilitate an easy assignment of overlapping spectra produced by different conformers in the sample. Of particular interest are the energy landscapes of conformationally flexible molecules of biological importance, including studies of their interaction with solvent and/or other weakly bound molecules. An example is provided from the authors' work on p-methoxyphenethylamine, a neurotransmitter, and its complexes with water.     

Rotationally resolved C2 symmetric conformers of bis-(4-hydroxyphenyl)methane: prototypical examples of excitonic coupling in the S1 and S2 electronic states

Rotationally resolved microwave and ultraviolet spectra of jet-cooled bis-(4-hydroxyphenyl)methane (b4HPM) have been obtained using Fourier-transform microwave and UV laser/molecular beam spectrometers. A recent vibronic level study of b4HPM [Rodrigo, C. P.; Mu?ller, C. W.; Pillsbury, N. R.; James, W. H., III; Plusquellic, D. F.; Zwier, T. S. J. Chem. Phys. 2011, 134, 164312] has assigned two conformers distinguished by the orientation of the in-plane OH groups and has identified two excitonic origins in each conformer. In the present study, the rotationally resolved bands of all four states have been well-fit to asymmetric rotor Hamiltonians. For the lower exciton (S(1)) levels, the transition dipole moment (TDM) orientations are perpendicular to the C(2) symmetry axes and consist of 41(2):59(2) and 34(2):66(2)% a:c hybrid-type character. The S(1) levels are therefore delocalized states of B symmetry and represent the antisymmetric combinations of the zero-order locally excited states of the p-cresol-like chromophores. The TDM polarizations of bands located at ≈132 cm(-1) above the S(1) origins are exclusively b-type and identify them as the upper exciton S(2) origin levels of A symmetry. The TDM orientations and the relative band strengths from the vibronic study have been analyzed within a dipole-dipole coupling model in terms of the localized TDM orientations, μ(loc), on the two chromophores. The out-of-the-ring plane angles of μ(loc) are both near 20° and are similar to results for diphenylmethane [Stearns, J. A.; Pillsbury, N. R.; Douglass, K. O.; Mu?ller, C. W.; Zwier, T. S.; Plusquellic, D. F. J. Chem. Phys. 2008, 129, 224305]. The in-plane angles are, however, rotated by 14 and 18° relative to DPM and, in part, explain the smaller than expected exciton splittings of these two conformers.     

Superconductivity in non-cubic crystal structures

Superconductivity has been observed in alloy samples of the zirconium-germanium hafnium-germanium, yttrium-germanium and lanthanum-germanium systems. The samples were prepared on heated substrates, under varying conditions by r.f. sputtering technique. The composition span was from 20 to 90 at.% Ge. Superconductivity was found in all four of these systems and was identified with the digermanide phase by X-ray analysis. Four terminal resistive measurements show the following superconducting transitions: 8 K (ZrGe₂), ≈ 2.2 K (HfGe₂), 3.8 K (YGe₂) and 2.65 K (LaGe₂). Annealing studies indicate that the superconducting ZrGe₂ and HfGe₂ phases are metastable, and depend strongly on the conditions of synthesis.     

Tautomerism in quinaldyl ketones

The quinaldyl ketone, 4-phenyl-3-(quinolin-2-yl)-butan-2-one was prepared by two methods: (a) benzylation of 1-(1H-quinolin-2-ylidene)propan-2-one in the presence of sodium hydride in dimethylformamide and (b) by the benzylative demethoxycarbonylation of methyl 2-(1H-quinolin-2-ylidene)-3-oxobutanoate in the presence of lithium bromide in hexamethylphosphoramide at 135°. In the absence of acid, the compound exists exclusively in the tautomeric form, 4-phenyl-3-(1H-quinolin-2-ylidene)butan-2-one.     

A flexible enantioselective synthesis of the isofurans

Recently, the isolation of a new class of human arachidonic acid tetrahydrofuran oxidation products, the isofurans (IsoF's), was reported. These new compounds are available from natural sources only in microgram quantities as mixtures. The enantioselective preparation of a versatile epoxide intermediate and its conversion to the enantiomerically pure isofurans SC-Delta(13)-9-IsoF and 15-epi-SC-Delta(13)-9-IsoF are described. This synthesis will make these metabolites available for physiological evaluation.     

Synthesis of (+)-sulcatine G

The total synthesis of (+)-sulcatine G is described. A key structural feature of sulcatine G is the highly functionalized, enantiomerically pure cyclobutane ring. We have prepared (+)-sulcatine G using a new strategy for bicyclic ring construction, Rh-mediated intramolecular C-H insertion followed by intramolecular alkylation.     

Retro Diels-Alder reactions of 5,6-disubstituted-7-oxabicyclo

Several 5,6-disubstituted-7-oxabicyclo[2.2.1]hept-2-enes (1-4) were synthesized on > or = 0.1 mol scale. The heat-induced retro Diels-Alder (rDA) decomposition of these derivatives was studied by thermal analysis, and the kinetics of the rDA were measured for 4. First-order rate constants (k = 1.91-14.2 x 10(-5) s(-1)), measured at four temperatures between 124 and 150 degrees C, were used to calculate Arrhenius activation parameters Ea (34.5 +/- 0.5 kcal/mol) and ln A (1.77 +/- 0.03 x 10(4)). The observed activation energy was significantly larger (by 9.5 kcal/mol) than that previously measured for the maleic anhydride adduct 1, and this was attributed to the difference in LUMO energies for the two dienophiles. Modeling of the activation parameters found for 4 with density functional theory (DFT) calculations for similar compounds 5 and 6 gave close quantitative correlations for deltaH double dagger, deltaG double dagger, deltaS double dagger. The rDA reactions studied were found to be entropy-driven.     

Kinetics of wetting of a compacted powder by aqueous solutions of surface active agents

Summary The capillary rise of aqueous solutions of anionic wetting agents into a compacted powder of an organic chromium complex is discussed on the basis of the Washburn-Rideal equation expanded to account for the porous structure parameters.At the porosity 1 —/6, corresponding to the loosest packing of monodisperse spheres the penetration rate is found to be mostly governed by the effective tensions operative at the moving three phase line of contact in case no deflocculation intervenes.

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Zusammenfassung Der Kapillaraufstieg wässeriger Lösungen anionischer Netzmittel in gepreßtem Pulver eines organischen Chrom-Komplexes wird durch die erweiterte Gleichung vonWashburn-Rideal wiedergegeben, in der die Parameter der porösen Struktur mit einbezogen werden.Bei der Porosität 1 —/6, die der lockersten Anordnung gleichförmiger Kugeln entspricht, wird die Eindringgeschwindigkeit meistens durch die an der sich bewegenden Dreiphasengrenze wirkenden Grenzflächenspannungen bestimmt, wenn keine Peptisation eintritt.

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Paper presented at the XII Intern. Congr. Fat Research, Milan, September 1974.