The carvoxime system: IV. heat capacities and enthalpies of melting of dl-carvoxime, l-carvoxime and standard n-heptane

Heat capacities between 160 and 385 K and enthalpies of melting of dl-carvoxime and l-carvoxime were determined by adiabatic calorimetry. The performance of the apparatus was checked on standard n-heptane. The enthalpy of melting values are dl-carvoxime, 22.70 ± 0.06; l-carvoxime, 17.02 ± 0.02; n-heptane, 14.059 ± 0.010 kJ mol⁻¹.     

Nonexistence of conformally flat slices in Kerr and other stationary spacetimes

It is proved that stationary solutions to the vacuum Einstein field equations with a nonvanishing angular momentum have no Cauchy slice that is maximal, conformally flat, and nonboosted. The proof is based on results coming from a certain type of asymptotic expansion near null and spatial infinity--which also show that the development of Bowen-York-type data cannot have a development admitting a smooth null infinity--and from the fact that stationary solutions do admit a smooth null infinity.     

Polarization dependence of vibrational coupling signals in femtosecond stimulated Raman spectroscopy

The polarization dependence of vibrational coupling signals seen in femtosecond stimulated Raman spectroscopy (FSRS) is investigated. Changing the polarization of a pulse used to impulsively excite coherent low frequency chlorine bending motion in CDCl(3) has a dramatic effect on the line shape of vibrational sidebands which arise from the anharmonic coupling of the pumped modes at 262 and 365 cm(-1) with the higher frequency symmetric stretching mode at 652 cm(-1). The asymmetric bend sideband (652+262 cm(-1)) changes sign and magnitude as the impulsive pulse polarization is rotated relative to the Raman pulses, while the symmetric bend sideband (652+365 cm(-1)) is relatively polarization independent. These experiments demonstrate the ability of FSRS to obtain time-resolved information on not only the vibrational coupling strength but also the symmetry of anharmonically coupled modes.     

Increasing the open circuit voltage of bulk-heterojunction solar cells by raising the LUMO level of the acceptor

We report the synthesis, characterization, and electrochemical properties of ten new fullerene derivatives for usage in organic solar cells. The phenyl ring of PCBM was substituted with electron-donating and electron-withdrawing substituents to study their influence on the LUMO level of the parent fullerene. We varied the LUMO level over a range of 86 mV and show a small but significant change of the open circuit voltage upon application in MDMO-PPV:methanofullerene bulk-heterojunction photovoltaic cells. [structure: see text].     

Development and Validation of a LC-MS/MS Technique for the Analysis of Short Chain Fatty Acids in Tissues and Biological Fluids without Derivatisation Using Isotope Labelled Internal Standards

The gut microbiota is critical to the maintenance of physiological homeostasis and as such is implicated in a range of diseases such as colon cancer, ulcerative colitis, diabetes, cardiovascular diseases, and neurodegenerative diseases. Short chain fatty acids (SCFAs) are key metabolites produced by the gut microbiota from the fermentation of dietary fibre. Here we present a novel, sensitive, and direct LC-MS/MS technique using isotopically labelled internal standards without derivatisation for the analysis of SCFAs in different biological matrices. The technique has significant advantages over the current widely used techniques based on sample derivatization and GC-MS analysis, including fast and simple sample preparation and short LC runtime (10 min). The technique is specific and sensitive for the quantification of acetate, butyrate, isobutyrate, isovalerate, lactate, propionate and valerate. The limits of detection were all 0.001 mM except for acetate which was 0.003 mM. The calibration curves for all the analytes were linear with correlation coefficients r² > 0.998. The intra- and inter-day precisions in three levels of known concentrations were <12% and <20%, respectively. The quantification accuracy ranged from 92% to 120%. The technique reported here offers a valuable analytical tool for use in studies of SCFA production in the gut and their distribution to host tissues.     

Femtosecond stimulated Raman spectroscopy – guided library mining leads to efficient singlet fission in rubrene derivatives

Chromophores undergoing singlet fission are promising candidates for harnessing solar energy as they can generate a pair of charge carriers by the absorption of one photon. However, photovoltaic devices employing singlet fission are still lacking practical applications due to the limitations within the existing molecules undergoing singlet fission. Chemical modifications to acenes can lead to efficient singlet fission devices, but the influence of changes to molecular structure on the rate of singlet fission is challenging to model and predict. Using femtosecond stimulated Raman spectroscopy we have previously demonstrated that the triplet separation process during singlet fission in crystalline rubrene is associated with the loss of electron density from its tetracene core. Based on this knowledge, we mined a library of new rubrene derivatives with electron withdrawing substituents that prime the molecules for efficient singlet fission, without impacting their crystal packing. Our rationally chosen crystalline chromophores exhibit significantly improved singlet fission rates. This study demonstrates the utility and strength of a structurally sensitive spectroscopic technique in providing insights to spectroscopy-guided materials selection and design guidelines that go beyond energy arguments to design new singlet fission-capable chromophores.

In the race to find efficient singlet fission materials, picking a winner is not easy. Femtosecond stimulated Raman spectroscopy can help us choose the best candidates, as demonstrated here in choosing from a library of rubrene derivatives.