Study of phonon dispersion in silicon and germanium at long wavelengths using picosecond ultrasonics

We have studied the dispersion of long wavelength longitudinal phonons in silicon and germanium using ultrasonic techniques. For long wavelengths, the acoustic phonon dispersion relation is of the form omega(k) approximately ck-gammak(3), where c is the speed of sound and gamma measures the lowest-order phonon dispersion. By sending an ultrasonic pulse of length a few hundred angstroms into a crystal and measuring the change of the pulse shape with propagation distance, we are able to determine the parameter gamma. The results are compared with lattice dynamics models.     

Engineering hydrogen-bonded molecular crystals built from derivatives of hexaphenylbenzene and related compounds

Hexakis[4-(2,4-diamino-1,3,5-triazin-6-yl)phenyl]benzene (4) incorporates a disc-shaped hexaphenylbenzene core and six peripheral diaminotriazine groups that can engage in hydrogen bonding according to established motifs. Under all conditions examined, compound 4 crystallizes as planned to give closely related noninterpenetrated three-dimensional networks built from sheets in which each molecule has six hydrogen-bonded neighbors. In the structure of compound 4, the number of hydrogen bonds per molecule and the percentage of volume accessible to guests approach the highest values so far observed in molecular networks. Analogue 5 (which has the same hexaphenylbenzene core but only four diaminotriazine groups at the 1,2,4,5-positions) and analogue 7 (in which the two unsubstituted phenyl groups of compound 5 are replaced by methyl groups) crystallize according to a closely similar pattern. Analogues with flatter pentaphenylbenzene or tetraphenylbenzene cores crystallize differently, underscoring the importance of maintaining a consistent molecular shape in attempts to engineer crystals with predetermined properties.     

Methylsalicylate: a rotational spectroscopy study

We report the free-jet rotational spectra of methylsalicylate, a molecule with a possible tautomeric and conformational equilibrium. In the ground electronic state, the molecule adopts a form stabilized by an intramolecular hydrogen bond between the phenolic hydrogen and the carbonylic oxygen, and this structure is characterized as the lowest-energy form by quantum chemical calculations. All rotational transitions are split because of the internal rotation of the methyl group, and the value of the barrier for this motion was determined to be V(3) = 5.38 kJ mol(-1).     

Mild Organosolv Delignification of Residual Aspen Bark after Extractives Isolation as a Step in Biorefinery Processing Schemes

European aspen (Populus tremula (L.) (Salicaceae)) bark is a promising raw material in multi-step biorefinery schemes due to its wide availability and higher content of secondary metabolites in comparison to stem wood biomass. The main objective of this study was to investigate the major cell wall component-enriched fractions that were obtained from aspen bark residue after extractives isolation, primarily focusing on integration of separated lignin fractions and cellulose-enriched bark residue into complex valorization pathways. The “lignin first” biorefinery approach was applied using mild organosolv delignification. The varying solvent systems and process conditions for optimal delignification of residual aspen bark biomass were studied using a response surface methodology approach. The conditions for maximum process desirability at which the highest amount of lignin-enriched fraction was separated were as follows: 20-h treatment time at 117 °C, butanol/water 4:1 (v/v) solvent system with solid to liquid ratio of 1 to 10. At optimal separation conditions, lignin-enriched fraction exhibited a higher content of β–O–4 linkages vs. C–C linkages content in its structure as well as a high amount of hydroxyl groups, being attractive for its further valorization. At the same time, the content of glucose in products of cellulose-enriched residue hydrolysis was 52.1%, increased from 10.3% in untreated aspen bark. This indicates that this fraction is a promising raw material for obtaining cellulose and fermentable glucose. These results show that mild organosolv delignification of extracted tree bark can be proposed as a novel biorefinery approach for isolation of renewable value-added products with various application potentials.     

Alkaloids of the Genus Datura: Review of a Rich Resource for Natural Product Discovery

The genus Datura (Solanaceae) contains nine species of medicinal plants that have held both curative utility and cultural significance throughout history. This genus’ particular bioactivity results from the enormous diversity of alkaloids it contains, making it a valuable study organism for many disciplines. Although Datura contains mostly tropane alkaloids (such as hyoscyamine and scopolamine), indole, beta-carboline, and pyrrolidine alkaloids have also been identified. The tools available to explore specialized metabolism in plants have undergone remarkable advances over the past couple of decades and provide renewed opportunities for discoveries of new compounds and the genetic basis for their biosynthesis. This review provides a comprehensive overview of studies on the alkaloids of Datura that focuses on three questions: How do we find and identify alkaloids? Where do alkaloids come from? What factors affect their presence and abundance? We also address pitfalls and relevant questions applicable to natural products and metabolomics researchers. With both careful perspectives and new advances in instrumentation, the pace of alkaloid discovery—from not just Datura—has the potential to accelerate dramatically in the near future.     

Shapes and Internal Dynamics of the 1:1 Adducts of Ammonia with trans and gauche Ethanol: A Rotational Study

Two 1:1 adducts of ammonia with ethanol have been characterized by using pulsed‐jet FT microwave spectroscopy. They are formed with two different (trans and gauche), stable conformers of ethanol. Several internal‐dynamics effects are reflected in the features of the rotational spectra. The trans complex shows the tunneling effects owing to internal rotation of both ammonia and the methyl group. The rotational transitions of the gauche species exhibit a small splitting that is related to tunneling through the potential‐energy barrier between the two equivalent minima.     

Phase-equivalent transformation which does not affect bound state properties and its manifestation in many-body systems

We propose a phase-equivalent transformation of NN interaction of a new type, the DET-PET transformation, which does not affect the wave function of the bound system (deuteron). The DET-PET properties and its manifestation in many-body systems are studied. In particular, we investigate the correlation of the ³H and ⁴He binding energies (the Tjon line) in calculations with NN potentials obtained by means of DET-PET from the JISP16 NN interaction.