Measuring diffusivity in supercooled liquid nanoscale films using inert gas permeation. II. Diffusion of Ar, Kr, Xe, and CH4 through methanol

We present an experimental technique to measure the diffusivity of supercooled liquids at temperatures near their T(g). The approach uses the permeation of inert gases through supercooled liquid overlayers as a measure of the diffusivity of the supercooled liquid itself. The desorption spectra of the probe gas are used to extract the low temperature supercooled liquid diffusivities. In the preceding companion paper, we derived equations using ideal model simulations from which the diffusivity could be extracted using the desorption peak times for isothermal or peak temperatures for temperature programmed desorption experiments. Here, we discuss the experimental conditions for which these equations are valid and demonstrate their utility using amorphous methanol with Ar, Kr, Xe, and CH(4) as probe gases. The approach offers a new method by which the diffusivities of supercooled liquids can be measured in the experimentally challenging temperature regime near the glass transition temperature.     

Vibrational dynamics of deoxyguanosine 5'-monophosphate following UV excitation

The relaxation dynamics of the DNA nucleotide deoxyguanosine 5'-monophosphate (dGMP) following 266 nm photoexcitation has been studied by transient IR spectroscopy with femtosecond time resolution. The induced dynamics of the amide I (carbonyl) stretch, the asymmetric guanine ring stretch and the phosphate asymmetric stretch are monitored in the region 1000-1800 cm(-1). Excitation and subsequent rapid internal conversion to a "hot" ground state is reflected by depletion of the vibrational ground states of the amide I stretch and guanine ring stretch. However, the vibrational ground state of the phosphate is left unperturbed, indicating the absence of vibrational coupling between the guanine ring system and the phosphate group. The vibrational ground state of the amide I is repopulated in 2.5 ps (±0.2 ps) while it takes 3.7 ps (±0.5 ps) to repopulate the guanine ring vibration. This article discusses two possible relaxation pathways of dGMP, as well as the implications of the weak phosphate dynamics.     

Insight into the Binding Mode of Agonists of the Nicotinic Acetylcholine Receptor from Calculated Electron Densities

Insect nicotinic acetylcholine receptors (nAChRs) are among the most prominent and most economically important insecticide targets. Thus, an understanding of the modes of binding of respective agonists is important for the design of specific compounds with favorable vertebrate profiles. In the case of nAChRs, the lack of available high‐resolution X‐ray structures leaves theoretical considerations as the only viable option. Starting from classical homology and docking approaches, binding mode hypotheses are created for five agonists of the nAChR, covering insecticides in the main group 4 of the Insecticide Resistance Action Committee (IRAC) mode of action (MoA) classification, namely, neonicotinoids, nicotine, sulfoxaflor, and butenolides. To better understand these binding modes, the topologies of calculated electron densities of small‐model systems are analyzed in the framework of the quantum theory of atoms in molecules. The theoretically obtained modes of binding are very much in line with the biology‐driven IRAC MoA classification of the investigated ligands.     

Crystallization kinetics and excess free energy of H2O and D2O nanoscale films of amorphous solid water

Temperature-programmed desorption (TPD) and reflection absorption infrared spectroscopy (RAIRS) are used to investigate the crystallization kinetics and measure the excess free energy of metastable amorphous solid water films (ASW) of H(2)O and D(2)O grown using molecular beams. The desorption rates from the amorphous and crystalline phases of ASW are distinct, and as such, crystallization manifests can be observed in the TPD spectrum. The crystallization kinetics were studied by varying the TPD heating rate from 0.001 to 3 K/s. A coupled desorption-crystallization kinetic model accurately simulates the desorption spectra and accurately predicts the observed temperature shifts in the crystallization. Isothermal crystallization studies using RAIRS are in agreement with the TPD results. Furthermore, highly sensitive measurements of the desorption rates were used to determine the excess free energy of ASW near 150 K. The excess entropy obtained from these data is consistent with there being a thermodynamic continuity between ASW and supercooled liquid water.     

Subnatural linewidth single photons from a quantum dot

The observation of quantum-dot resonance fluorescence enabled a new solid-state approach to generating single photons with a bandwidth approaching the natural linewidth of a quantum-dot transition. Here, we operate in the small Rabi frequency limit of resonance fluorescence--the Heitler regime--to generate subnatural linewidth and high-coherence quantum light from a single quantum dot. The measured single-photon coherence is 30 times longer than the lifetime of the quantum-dot transition, and the single photons exhibit a linewidth which is inherited from the excitation laser. In contrast, intensity-correlation measurements reveal that this photon source maintains a high degree of antibunching behavior on the order of the transition lifetime with vanishing two-photon scattering probability. Generating decoherence-free phase-locked single photons from multiple quantum systems will be feasible with our approach.     

Determination of A0 from the ν4 Raman band of CD3Br

The principles and methods of determining A₀ from the Raman spectra of degenerate fundamentals of C_3v molecules are outlined, and applied to the ν₄ band of CD₃Br to yield a value of A₀ = 2.6004 ± 0.0010 cm^?1.     

Leucine Oxidation in vivo: Inter-and Intraindividual Variation in Healthy Subjects as Assessed by Oral L-[1-13C]Leucine Loads

Interindividual variability of leucine catabolism was studied in eight overnight fasted healthy volunteers by means of a controlled oral L-[1-¹³C]leucine loading test (5 mg/kg body weight). With the exception of total CO² expiration (CV = 5%),a considerable variation (CV generally < or ?10%)was observed with all other metabolic parameters: increase of leucine/2-oxoisocaproate in serum, ¹³C-enrichment in serum metabolites and ¹³CO² in expired air, and estimated oxidation rates. The degree of variation was only partially referable to individual differences. A comparable high degree of variability was found when two probands underwent multiple loading tests (n = 4). Possible reasons and implications are discussed.     

Computer-aided quality assurance and certification of reference materials in the DANREF network

A network between 6 Danish laboratories with experience in quality assurance and quality control has been established with the aim of supporting the use and production of high-quality certified reference materials in Denmark. One of the tasks of the network was to develop a PC-program for computer-aided quality assurance and certification of reference materials produced by the network laboratories. This paper describes the design and features of the DANREF PC-program and shows examples of computer-aided evaluation of selected studies on reference materials. The program provides the network laboratories with the possibility to carry out harmonized statistical evaluation and documentation of homogeneity, stability and intercomparison studies in accordance with international guidelines and standards, e.g. ISO guide 35.     

Knoevenagel condensation catalyzed by K2NiP2O7. Synthesis of (E)-methyl-α-cyanocinnamates in high yields

The Knoevenagel reaction of benzaldehyde and several chloroderivatives with methyl cyanoacetate catalyzed by K₂NiP₂O₇ leads to methyl (E)--cyanocinnamate derivatives in 40 min with yields of 71.65–83.45%. Pure products are easily obtained in crystalline form, uncontaminated by side products or by stereoisomers. Methyl 2-chlorocyanocinnamate, methyl 2,4-dichlorocyanocinnamate, and a new polymorph of methyl cyanocinnamate have been characterized by single crystal X-ray diffraction.