Manipulation and control of hydrogen bond dynamics in absorbed ice nanoclusters

Inelastic electron tunneling is used to explore the dynamics of ice nanoclusters adsorbed on Ag(111). The diffusion of entire nanoclusters or internal hydrogen bond rearrangement can be selectively controlled by injecting electrons either directly into the clusters themselves or indirectly ("indirect inelastic electron tunneling") into the substrate at distances of up to 20 nm from them; a reaction probability that oscillates with the tip-cluster lateral distance presents evidence that surface state electrons mediate the excitation. Density functional theory calculations reveal a strong sensitivity of the computed activation energies of the individual processes to the applied electrical field.     

Practical aspects of fast HPLC separations for pharmaceutical process development using monolithic columns

A large number of samples can be generated during pharmaceutical process development. Fast separation for these samples is usually challenging due to the complexity of sample matrix, which requires high efficiency as well as high speed. Monolithic columns (E. Merck, Germany) were investigated as a possible tool for reducing separation time in reversed-phase HPLC without significantly sacrificing efficiency or resolution. Both van Deemter plots and separations of alkyl benzenes and in-process samples showed that monolithic columns were suitable for fast separations without significantly compromising resolution. Practical parameters including the pressure drop, retention factor, selectivity, and tailing factor of monolithic columns (Chromolith type) were compared to those of conventional YMC 150 mm × 4.6 mm (3-μm particles) and 250 mm × 4.6 mm (5-μm particles) packed columns. The batch-to-batch reproducibility of the 100 mm × 4.6 mm Chromolith columns from five randomly ordered batches was also compared to the 250 mm × 4.6 mm YMC particle-packed columns. Fast and efficient separations of complicated process samples including crude drug substances, reaction mixtures, and crystallized mother liquors were demonstrated for both monolithic columns and conventional packed columns. The analysis times were decreased by three to seven times on the coupled monolithic columns, while maintaining the comparable resolution to typical 5-μm particle-packed 250 mm × 4.6 mm columns.     

Hyperfine structures of the 2 (3)Sigma(g) (+), 3 (3)Sigma(g) (+), and 4 (3)Sigma(g) (+) states of Na(2)

The hyperfine structures of the 2 (3)Sigma(g) (+), 3 (3)Sigma(g) (+), and 4 (3)Sigma(g) (+) states of Na(2) have been resolved with sub-Doppler continuous wave perturbation facilitated optical-optical double resonance spectroscopy via A (1)Sigma(u) (+) approximately b (3)Pi(u) mixed intermediate levels. The hyperfine patterns of these three states are similar. The hyperfine splittings of the low rotational levels are all very close to the case b(betaS) limit. As the rotational quantum number increases, the hyperfine splittings become more complicated and the coupling cases become intermediate between cases b(betaS) and b(beta J) due to spin-rotation interaction. We present a detailed analysis of the hyperfine structures of these three (3)Sigma(g) (+) states, employing both case b(betaS) and b(beta J) coupling basis sets. The results show that the hyperfine splittings of the (3)Sigma(g) (+) states are mainly due to the Fermi-contact interaction. The Fermi contact constants for the two d sigma Rydberg states, the 2 (3)Sigma(g) (+) and 4 (3)Sigma(g) (+), are 245+/-5 MHz and 225+/-5 MHz, respectively, while the Fermi contact constant of the s sigma 3 (3)Sigma(g) (+) Rydberg state is 210+/-5 MHz. The diagonal spin-spin and spin-rotation constants, and nuclear spin-electronic spin dipolar interaction parameters of the 3 (3)Sigma(g) (+) and 4 (3)Sigma(g) (+) states are also obtained.     

On barrier strategy dividends with Parisian implementation delay for classical surplus processes

In this paper, we apply a single barrier strategy to optimise dividend payments in the situation where there is a time lag d>0 between decision and implementation. Using a classical surplus process with exponentially distributed jumps, we obtain the optimal barrier b^* which maximises the expected present value of dividends.     

Globally flexible functional forms: The neural distance function

The output distance function is a key concept in economics. However, its empirical estimation often violates properties dictated by neoclassical production theory. In this paper, we introduce the neural distance function (NDF) which constitutes a global approximation to any arbitrary production technology with multiple outputs given by a neural network (NN) specification. The NDF imposes all theoretical properties such as monotonicity, curvature and homogeneity, for all economically admissible values of outputs and inputs. Fitted to a large data set for all US commercial banks (1989–2000), the NDF explains a very high proportion of the variance of output while keeping the number of parameters to a minimum and satisfying the relevant theoretical properties. All measures such as total factor productivity (TFP) and technical efficiency (TE) are computed routinely. Next, the NDF is compared with the Translog popular specification and is found to provide very satisfactory results as it possesses the properties thought as desirable in neoclassical production theory in a way not matched by its competing specification.     

Direct assessment of quantum nuclear effects on hydrogen bond strength by constrained-centroid ab initio path integral molecular dynamics

The impact of quantum nuclear effects on hydrogen (H-) bond strength has been inferred in earlier work from bond lengths obtained from path integral molecular dynamics (PIMD) simulations. To obtain a direct quantitative assessment of such effects, we use constrained-centroid PIMD simulations to calculate the free energy changes upon breaking the H-bonds in dimers of HF and water. Comparing ab initio simulations performed using PIMD and classical nucleus molecular dynamics (MD), we find smaller dissociation free energies with the PIMD method. Specifically, at 50 K, the H-bond in (HF)(2) is about 30% weaker when quantum nuclear effects are included, while that in (H(2)O)(2) is about 15% weaker. In a complementary set of simulations, we compare unconstrained PIMD and classical nucleus MD simulations to assess the influence of quantum nuclei on the structures of these systems. We find increased heavy atom distances, indicating weakening of the H-bond consistent with that observed by direct calculation of the free energies of dissociation.     

The effect of native vowel processing ability and frequency discrimination acuity on the phonetic training of English vowels for native speakers of Greek

The perception and production of nonnative phones in second language (L2) learners can be improved via auditory training, but L2 learning is often characterized by large differences in performance across individuals. This study examined whether success in learning L2 vowels, via five sessions of high-variability phonetic training, related to the learners' native (L1) vowel processing ability or their frequency discrimination acuity. A group of native speakers of Greek received training, while another completed the pre-/post-tests but without training. Pre-/post-tests assessed different aspects of their L2 and L1 vowel processing and frequency acuity. L2 and L1 vowel processing were assessed via: (a) Natural English (L2) vowel identification in quiet and in multi-talker babble, and natural Greek (L1) vowel identification in babble; (b) the categorization of synthetic English and Greek vowel continua; and (c) discrimination of the same continua. Frequency discrimination acuity was assessed for a nonspeech continuum. Frequency discrimination acuity was related to measures of both L1 and L2 vowel processing, a finding that favors an auditory processing over a speech-specific explanation for individual variability in L2 vowel learning. The most efficient frequency discriminators at pre-test were also the most accurate both in English vowel perception and production after training.