Low temperature dynamics of H + HeH+→ H2+ + He reaction: On the importance of long-range interaction.

While the growing realization of the importance of long-range interactions is being demonstrated in cold and ultracold bimolecular collision experiments, their influence on one of the most critical ion-neutral reactions has been overlooked. Here, we address the non-Langevin abrupt decrease observed earlier in the low-energy integral cross-sections and rate coefficients of the astrochemically important H + HeH+→ H2+ + He reaction. We attribute this to the presence of artificial barriers on existing potential energy surfaces (PESs).

Temperature driven transformations of glycine molecules embedded in interstellar ice.

The formation of glycine amino acid on ice grains in space raises fundamental questions about glycine chemistry in interstellar media. In this work, we studied glycine conformational space and the related tautomerization mechanisms in water media by means of QM/MM molecular dynamics simulations of four glycine conformational isomers (, , , and ). Interstellar low density amorphous (LDA) ice and = 20 K were considered as representative for a cold interstellar ice environment, while temperatures of 250 and 450 K were included to model rapid local heating in the ice.

Atomistic simulations of the aggregation of small aromatic molecules in homogenous and heterogenous mixtures.

The relatively weak London dispersion forces are the only interactions that could cause aggregation between simple aromatic molecules. The use of molecular dynamics and high-level ab initio computer simulations has been used to describe the aggregation and interactions between molecular systems containing benzene, naphthalene and anthracene. Mixtures containing one type of molecule (homogenous) and more than one type of molecule (heterogenous) were considered.

Pressure dependent low temperature kinetics for CN + CH3CN: competition between chemical reaction and van der Waals complex formation.

The gas phase reaction between the CN radical and acetonitrile CH3CN was investigated experimentally, at low temperatures, with the CRESU apparatus and a slow flow reactor to explore the temperature dependence of its rate coefficient from 354 K down to 23 K. Whereas a standard Arrhenius behavior was found at T > 200 K, indicating the presence of an activation barrier, a dramatic increase in the rate coefficient by a factor of 130 was observed when the temperature was decreased from 168 to 123 K. The reaction was found to be pressure independent at 297 K unlike the experiments carried out at 52 and 132 K.

Rate constants and branching ratios for the reaction of CH radicals with NH3: a combined experimental and theoretical study.

The reaction between CH radicals and NH(3) molecules is known to be rapid down to at least 23 K {at which temperature k = (2.21 ± 0.17) × 10(-10) cm(3) molecule(-1) s(-1): Bocherel ; et al.

Direct infrared absorption spectroscopy of benzene dimer.

The direct infrared (IR) absorption spectrum of benzene dimer formed in a free-jet expansion was recorded in the 3.3 μm region for the first time. This has led to the observation of the C-H stretching fundamental mode ν(13) (B(1u)), which is both IR and Raman forbidden in the monomer.

A theoretical analysis of the reaction between CN radicals and NH3.

The reaction between CN radicals and NH3 molecules has been studied experimentally over an unusually wide range of temperature (25-716 K). Below 295 K, the rate constant exhibits a strong negative dependence on temperature; that is, it increases sharply as the temperature is lowered. The present work analyses the kinetics of this reaction theoretically, both to explain this unusual temperature-dependence and to identify the major products of the reaction--which have not been well established by experiment.

Correction for dispersion and Coulombic interactions in molecular clusters with density functional derived methods: application to polycyclic aromatic hydrocarbon clusters.

The density functional based tight binding (DFTB) is a semiempirical method derived from the density functional theory (DFT). It inherits therefore its problems in treating van der Waals clusters. A major error comes from dispersion forces, which are poorly described by commonly used DFT functionals, but which can be accounted for by an a posteriori treatment DFT-D.

Interaction between n-Alkane Chains: Applicability of the Empirically Corrected Density Functional Theory for Van der Waals Complexes.

The geometries, interaction energies, and vibrational frequencies of a series of n-alkane dimers up to dodecane have been calculated using density functional theory (DFT) augmented with an empirical dispersion energy term (DFT-D). The results obtained from this method for ethane to hexane dimers are compared with those provided by the MP2 level of theory and the combined Gaussian-3 approach with CCSD(T) being the highest correlation method [G3(CCSD(T))]. Two types of dimer isomers have been studied.