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The dimerization of molecules in helium nanodroplets is known to preferentially yield structures of higher energy than the global energy minimum structure for a number of quite different monomers. Here, we explore dimerization in this environment using an atomistic model within statistically converged molecular dynamics (MD) trajectories, treating the solvent implicitly through the use of a thermostat, or more explicitly by embedding one monomer in a He100 cluster. The focus is on the two simplest carboxylic acids, formic and acetic, both of which have been studied experimentally. While the global minimum structure, which comprises two CO⋯HO hydrogen bonds, is predicted to be the most abundant dimer in the absence of the helium solvent, this is no longer the case once helium atoms are included. The simulations confirm the importance of kinetic trapping effects and also shed light on the occurrence of specific dynamical effects, leading to the occasional formation of high-energy structures away from minima, such as saddle configurations. Theoretically predicted infrared spectra, based on the MD statistics, are in good agreement with the experimental spectra. [ABSTRACT FROM AUTHOR]

We report quantum calculations involving the dynamics of rotational energy-transfer processes, by collision with He atoms in interstellar environments, of the title molecular species which share the presence of the CN backbone and are considered of importance in those environments. The latter structural feature is taken to be especially relevant for prebiotic chemistry and for its possible role in the processing of the heterocyclic rings of RNA and DNA nucleobases in the interstellar space. We carry out ab initio calculations of their interaction potentials with He atoms and further obtain the state-to-state rotationally inelastic cross sections and rate coefficients over the relevant range of temperatures. The similarities and differences between such species and other similar partners which have been already detected are analyzed and discussed for their significance on internal state populations in interstellar space for the two title molecular radicals. [ABSTRACT FROM AUTHOR]

The vibrational quenching cross sections and corresponding low-temperature rate constants for the ν = 1 and ν = 2 states of CN−(1Σ+) colliding with He and Ar atoms have been computed ab initio using new three-dimensional potential energy surfaces. Little work has been carried out so far on low-energy vibrationally inelastic collisions for anions with neutral atoms. The cross sections and rates calculated at energies and temperatures relevant for both ion traps and astrochemical modeling are found by the present calculations to be even smaller than those of the similar C 2 − /He and C 2 − /Ar systems, which are in turn of the order of those existing for the collisions involving neutral diatom–atom systems. The implications of our finding in the present case mainly focus on the possible role of small computed rate constants in the dynamics of molecular cooling and the evolution of astrochemical modeling networks. [ABSTRACT FROM AUTHOR]

We present an extensive range of accurate ab initio calculations, which map in detail the spatial electronic potential energy surface that describes the interaction between the molecular anion N H 2 − (1A1) in its ground electronic state and the He atom. The time-independent close-coupling method is employed to generate the corresponding rotationally inelastic cross sections, and then the state-changing rates over a range of temperatures from 10 to 30 K, which is expected to realistically represent the experimental trapping conditions for this ion in a radio frequency ion trap filled with helium buffer gas. The overall evolutionary kinetics of the rotational level population involving the molecular anion in the cold trap is also modelled during a photodetachment experiment and analyzed using the computed rates. The present results clearly indicate the possibility of selectively detecting differences in behavior between the ortho- and para-anions undergoing photodetachment in the trap. [ABSTRACT FROM AUTHOR]

The potential energy surface of N-acetyl-L-tryptophan-N-methylamide (NATMA) is analyzed both in the gas phase and in solution, while the minima are identified using the density functional theory (DFT) with the 6-31 g(d) basis set. The results have concluded that, with two exceptions, the single residue DFT calculations have yielded minima that correspond to the populated regions of the native state values.

A robust method of identifying the sampling of the phase space in classical dynamical simulations is presented. The method provides an efficient way to detect structural changes, isomerizations, during the simulations even for those cases where the bond lengths may not show significant activity.

The relation between thermochromism in polythiophenes and the internal rotation was studied by density functional theory (DFT). It was found that the internal rotation alone was not sufficient to explain the mechanism of thermochromism.

Ab initio calculations can be used to investigate thermodynamical stability of all possible isomers of small oligomers of thiophene. Thermodynamical stability of small oligomers can distinguish various growth possibilities.

A study was conducted to analyze cyclobutadiene and benzocyclobutenes fused to o and p-quinone by computational techniques. The GIAO technique was utilized to determine absolute nuclear magnetic resonance (NMR) shielding values. The fused-ring system between cyclobutadiene and quinone were also examined. Experimental results indicated the preference of cyclobutadiene to fuse to benzene or to fuse to quinone. In addition, the 13C NMR chemical shift correlated with experimental data.

We employ potential energy surfaces (PES) from ab initio quantum chemistry methods to describe the interaction of the CN−(1Σ) molecule, one of the small anions often studied at low temperatures, with other possible gases which can be employed as buffer in cold ion traps: the He and Ar atoms and the p-H2 molecule. These PESs are used to calculate from quantum multichannel dynamics the corresponding state-changing rate constants between the populated rotational states of the anion, the latter being in its electronic and vibrational ground states. The different cross sections for the collision-driven quenching and excitation processes at low temperatures are compared and further used to model CN− cooling (de-excitation) efficiency under different trap conditions. The interplay of potential coupling strength and mass-scaling effects is discussed to explain the differences of behaviour among the buffer gases. The advantages of being able to perform collisional cooling at higher trap temperatures when using Ar and p-H2 as buffer gases are also discussed. [ABSTRACT FROM AUTHOR]

The collisional cooling of the internal rotational states of the nonlinear anion NH2− (1A1), occurring at the low temperature of a cold ion trap under helium buffer gas cooling, is examined via quantum dynamics calculations and ion decay rate measurements. The calculations employ a novel ab initio potential energy surface that describes the interaction anisotropy and range of action between the molecular anions and the neutral He atoms. The state changing integral cross sections are employed to obtain the state-to-state rate coefficients, separately for the ortho- and the para-NH2− ions. These rates are in turn used to compute the state population evolution in the trap for both species, once photodetachment by a laser is initiated in the trap. The present work shows results for the combined losses of both species after the photodetachment laser is switched on and analyzes the differences of loss kinetics between the two hyperfine isomers. [ABSTRACT FROM AUTHOR]

The unimolecular dissociation of a model triatomic molecule is analyzed. The model, which is loosely based on the radical HO2, shows mostly regular dynamical behavior although it has enough energy to dissociate. By analyzing the progress towards dissociation, we find that dissociation takes place due to concerted and correlated motions as in the Slater theory of unimolecular reactions rather than the steady accumulation of energy in the dissociative mode. [ABSTRACT FROM AUTHOR]

Abstract: Polyethyleneimine (PEI) is rarely recognized as a luminescent polymer but is frequently used for the production of cationic nanoparticles and tagged with an organic fluorophore to be tracked optically. Herein, a strongly luminescent, branched PEI‐superparamagnetic iron oxide nanoparticle (bPEI‐SPION) without a traditional fluorophore is reported. A tremendous enhancement (1200 times) in the weak blue luminescence of bPEI is achieved only if it is adsorbed on a SPION during the synthesis of nanoparticles, which is improved further upon protonation, irreversibly. This is quite unexpected since SPIONs are strong absorbers in the visible region. All reaction parameters, different synthetic methods, as well as protonation are studied as independent factors to understand the origin of such enhancement. Detailed spectroscopic analysis and density functional theory calculations indicate that partial amine oxidation and Fe3+ reduction takes place during the synthesis, which significantly contributes to the luminescence enhancement. In addition, PEI‐SPION exhibits excitation wavelength dependent emission and maintains its magnetic properties. [ABSTRACT FROM AUTHOR]

The solvation of Buckminsterfullerene $$\hbox {C}_{60}$$ by para-hydrogen and ortho-deuterium clusters has been modeled using a dedicated potential and path-integral molecular dynamics simulations at low temperature (2 K). The solvation shell obtained from the distribution of radial distances is found to be complete near 50 molecules, in agreement with recent mass spectrometry measurements. Deuteration increases the shell size by one, indicating a denser shell owing to less prominent vibrational delocalization for this heavier isotope. [ABSTRACT FROM AUTHOR]

We present a detailed SCS-MP2 study on the potential energy curves (PEC) for interactions between diatomic halogen molecules and pyrene. BSSE corrected CCSD[T] energies at equilibrium distances are computed and compared to CCSD(T) energies. The most stable conformation of these weakly bound van der Waals complexes is almost linear in the perpendicular direction to the pyrene plane. The complexes of highly polarizable bromine and iodine molecules with pyrene are very stable and they carry rather large number of vibrational states. Despite its small size, F2 also forms strong halogen bonding similar to Br2 and I2. The interaction between Cl2 and pyrene is the weakest and it is attributed to the highest polarizability /molar mass ratio of chlorine among the others. I2-pyrene is found to be the most stable complex due to the strongest mutual polarization effects and is carrying more than 60 vibrational states. Due to the rather large number of electrons in some complexes, the relativistic corrections are also considered. © 2016 Wiley Periodicals, Inc. [ABSTRACT FROM AUTHOR]

Aspirin-aspirin and aspirin-leucine interactions are studied by the density functional theory (DFT) and high level ab initio calculations with second order Moller-Plesset perturbation theory (MP2). The rotational isomers of aspirin are identified by their relative stability both in gaseous phase and in water using the polarizable continuum method (PCM). Local minima of aspirin monomers in water are found to be all highly populated compared to the gas phase behavior. Homodimers of aspirin form hydrogen bonds with bond energies of 10 kcal/mol. Weak hydrogen bonds utilizing phenyl and methyl groups are also found. The interaction between aspirin and leucine is stronger with relatively short bond lengths compared to homodimeric aspirin interactions. The potential energy surface has several minima with comparable stability. This study shows the significance of diverse bonding schemes, which are important for understanding complete interaction mechanisms of aspirin. [ABSTRACT FROM AUTHOR]

Quantum dynamics of superelastic collisions involving vibrational levels of MgH(XΣ) ions in cold traps, interacting with He(S) as a buffer gas at relative temperatures down to millikelvins, is discussed using an ab initio computed potential energy surface. The relative efficiency of collisional cooling with respect to collisional quenching of the internal vibrations is examined from the results of the relative sizes of the relevant cross sections in relation to predicting actual behaviour in cold traps. The present study indicates the feasibility of cooling vibrationally 'hot', trapped ions with the buffer gas. [Figure not available: see fulltext.] [ABSTRACT FROM AUTHOR]

Flow behavior of nanosize alumina suspensions in the presence of polyalcohols was investigated. Viscosity of high solids content (35 vol%) alumina nanopowders was measured at room temperature. It was shown that after the first advancing shear rate branch, viscosity measurements show excellent reproducibility. All the suspensions showed the formation of large clusters, which are broken down as shear rate increases indicating reversible cluster breaking and reforming. Viscosity of suspensions decreases dramatically with up to 10 wt% polyalcohol addition. Only ethylene glycol showed an increase in viscosity indicating no interaction with the particle surface. The addition of polyalcohols is believed to prevent the formation of large and/or strong clusters by modifying the alumina particle surface. The exact nature and quantitative description of viscosity reduction by polyalcohol addition have yet to be established. [ABSTRACT FROM AUTHOR]

Vibrational spectra of oligothiophenes with structural defects are calculated within the density-functional-theory methodology. The effects of the defective αβ linkages on the infrared (IR) and Raman spectra are characterized from calculations of all isomers up to the hexamer. The signatures of αβ linked monomers can be found in IR spectra from broken symmetry arguments which result in absorptions localized in the defective region. The positions of the absorption peaks in the Raman spectra seem to be unaffected by the presence of such defects; however, strong reductions in the intensities are observed because of the shortening of the conjugation length. [ABSTRACT FROM AUTHOR]