Energy Distribution Of Pairs Research Articles

Intermolecular interactions in binary mixtures of formamide and acetone

Trajectories from molecular dynamics simulations of binary mixtures of formamide (FA) and acetone (AT) have been analyzed across the entire concentration range by means of various pair distributions and topological network analysis techniques to probe the structure of the intermolecular interaction networks existing in these mixtures. Radial pair distributions clearly indicated H-bonding interactions between the two molecule types through acetone oxygen and the cis- and/or trans-hydrogens of formamide. Decomposition of pair energy distributions into possible intermolecular interaction types revealed that the most stable formamide-acetone dimers existing in the binary mixtures interact via an H-bonding. Orientational pair distributions have shown that both the AT-AT and the FA-FA pairs tend to adopt an antiparallel dipole orientation, as their respective pair interaction energies get smaller. And, the FA-AT pairs interacting stronger than −6.3 kcal/mol are found to adopt an orientation such that the angle between the dipole of AT and the C-N bond of FA is larger than 130°. Network neighborhood analysis evidently revealed that the formamides do not show a strong preference for either molecule types. As acetone mole fraction of the mixture is increased, the added ATs are found to be capable of substituting the formamides, to a great extent, in the region spanning their second-order neighborhood. Conversely, as formamide content of the mixture is increased, the added FAs fail to adequately substitute the ATs in the region spanning their second-order neighborhood. Guided by our molecular dynamics based observations regarding the different types of intermolecular interactions existing between formamide and acetone, five FA-AT dimeric structures are highlighted. Their interaction energies are computed with density functional theory at the mPW2PLYP/def2-TZVP level. The most stable dimer is found to have an interaction energy of −7.5 kcal/mol, to involve an O⋯HC hydrogen-bonding, and to have nearly coplanar formamide and acetone. In addition, the DFT structures and interaction energies of trimers of the form AT⋯FA⋯AT and FA⋯AT⋯FA are also reported.

Structure of the Hydrogen-Bond Network in Binary Mixtures of Formamide and Methanol

Molecular dynamics simulations in conjunction with a number of network analysis techniques have been carried out to explore the structure of the hydrogen-bond (H-bond) network in mixtures of formamide (FA) and methanol (MeOH) across the entire composition range. Radial distribution functions and pair energy distributions have been utilized for a general insight into the localized interactions. A comprehensive understanding of the structural properties of the H-bond networks including FA…FA, MeOH…MeOH, and FA…MeOH H-bonding interactions has been obtained collectively from the distribution of H-bonds, extended neighborhood analysis, and the geodesic distance distribution. These analyses indicate that formamide molecules prefer formamide molecules not only as immediate hydrogen-bonded neighbors but also as neighbors in their extended neighborhood that spans the neighbors of the immediate neighbors. Methanol molecules are found not to show a strong preference for either of the molecule types. The network analyses utilized in this study reveal that the population and the topological length of chain-like H-bond pathways formed by FA molecules increase with the addition of small amounts of methanol until the mole fractions of each species are equal.

Cosolvent or Antisolvent? A Molecular View of the Interface between Ionic Liquids and Cellulose upon Addition of Another Molecular Solvent

Ionic liquids (ILs) are promising nonderivatizing solvents for the dissolution of cellulose and lignin in biomass pretreatment processes, which are, however, retarded by sluggish dynamics. Recent investigations showed that cosolvents such as dimethyl sulfoxide (DMSO) can accelerate the dissolution dramatically. On the other hand, water is used as a common antisolvent to regenerate cellulose from solutions. To understand the co-/antisolvent effects in dissolving cellulose by ILs, we performed molecular dynamics simulations of the interfaces between an Iβ cellulose crystal and different solvent systems, including ILs, DMSO, water, and mixed solvent systems. The density profiles and pair energy distributions (PEDs) show that the anions interact much more strongly with the cellulose surface than the cations, which is responsible for the dissolution of cellulose. It was found that the number of chloride ions in contact with cellulose does not cause the co-/antisolvent effect. In contrast, the cellulose-chloride PEDs are sensitive to the addition of molecular solvents, such as DMSO and water. Detailed analyses show that multiple hydrogen-bond (HB) patterns are formed between chloride and the hydroxyl groups of cellulose that are noticeably changed in the presence of DMSO or water. A combined analyses of both the PEDs and HB patterns can provide valuable information about the enhancement of cellulose dissolution. The simulation results in this work present useful knowledge for the design of solvent systems for dissolving cellulose or other types of biomass.

THE PAIR BEAM PRODUCTION SPECTRUM FROM PHOTON-PHOTON ANNIHILATION IN COSMIC VOIDS

Highly beamed relativistic e ±-pair energy distributions result in double photon collisions of the beamed gamma rays from TeV blazars at cosmological distances with the isotropically distributed extragalactic background light (EBL) in the intergalactic medium. The typical energies k 0 10–7 in units of mec 2 of the EBL are more than 10 orders of magnitude smaller than the observed gamma-ray energies k 1 ≥ 107. Using the limit k 0 k 1, we demonstrate that the angular distribution of the generated pairs in the lab frame is highly beamed in the direction of the initial gamma-ray photons. For the astrophysically important case of power-law distributions of the emitted gamma-ray beam up to the maximum energy M interacting with Wien-type N(k 0)∝kq 0exp (– k 0/Θ) soft photon distributions with total number density N 0, we calculate analytical approximations for the electron production spectrum. For distant objects with luminosity distances dL r 0 = (σ T N 0)–1 = 0.49N –1 0 Mpc (with Thomson cross section σ T ), the implied large values of the optical depth τ0 = dL /r 0 indicate that the electron production spectra differ at energies inside and outside the interval [(Θln τ0)–1, τ0/Θ], given the maximum gamma-ray energy M Θ–1. In the case M Θ–1, the production spectrum is strongly peaked near E Θ–1, being exponentially reduced at small energies and decreasing with the steep power law ∝E –1 – p up to the maximum energy E = M – (1/2).

Theoretical Studies of the In-Solution Isomeric Protonation of Non-Aromatic Six-Member Rings with Two Nitrogens

For exploring the preferred site for hydrogen bond formation, theoretical calculations have been performed for a number of six-member, nonaromatic rings allowing for alternative protonation on the ring nitrogens. Gas-phase protonation studies for test molecules indicate that the B3LYP/aug-cc-pvtz and QCISD(T)(CBS) calculations approach the experimental values within about 1 kcal/mol with considerable improvement for relative enthalpies and free energies. Relative free energies calculated at the IEF-PCM/B3LYP/aug-cc-pvtz level predict favorable protonation on the tertiary rather than on the secondary nitrogen both in aqueous solution and in a dichloromethane solvent for saturated rings. Protonation on a nitrogen atom next to a C═C bond is disfavored due to a large increase in internal energy. Monte Carlo simulations considering a counterion and Ewald summation for the long-range electrostatic effects for a 0.1 molar model system predict ΔG(solv)/MC values generally less negative than from the IEF-PCM calculations. These results make the protonation on the tertiary nitrogen even more favored. The solute-solvent pair-energy distribution depends sensitively on the applied model. In conclusion, the freely moving anion has been considered as the most relevant model with overall neutrality for the system and applying the least restrictions.

MONTE CARLO SIMULATION OF I<sup>-</sup>, Br<sup>-</sup>, AND Cl<sup>-</sup> IN WATER USING AB INITIO PAIR POTENSIAL FUNCTIONS

Monte Carlo simulations were performed for I-, Br- and Cl-, in water using ab initio pair potential. The systems consisting of one anion in 215 solvent molecules have been simulated at 298 K. Anion-water pair potentials have been newly developed based on ab initio calculations of split valence basis set plus polarization quality. The structure of the solvated ion is discussed in terms of radial distribution functions, coordination number and pair potential distribution. Structural properties were investigated by means of radial distribution functions and their running integration numbers, leading for the first solvation shell to an average 12.60 H2O around I- with I--O distance of 3.74 Å and I--H distance of 2.86 Å, 11.84 H2O around Br- with Br--O distance of 3.40 Å and Br--H distance of 2.42 Å, and 10.68 H2O around Cl- with Cl--O distance of 3.20 Å and Cl--H distance of 2.24 Å, respectively. The structure of the water-anion complexes are agreed with dipole orientation. Pair energy distribution of hydrated anions showed that the pair interaction energy are increase from I-, Cl-, to Br-, namely, -6.28, -9.98 and -13.67 kcal/mol, respectively. The coordination number distribution analysis for the first solvation shell of the all hydrated anions indicated a high exchange rate for the first solvation shell ligands. Keywords: Monte Carlo simulation, halogen anion, ab initio

Competition between magnetic and superconducting pairing exchange interactions in confined systems

We analyze the competition between magnetic and pairing interactions in confined systems relevant to either small superconducting grains or trapped ultracold atomic gases. The response to the imbalance of the chemical potential for the two spin states leads to various inhomogeneous profiles of the pair energy distribution. We show that the position in the energy spectrum for the unpaired particles can be tuned by varying the filling or the pairing strength. When small grains are considered, the antiferromagnetic exchange stabilizes the pair correlations, whereas for Fermi gases, a transition from a mixed configuration to a phase-separated one beyond a critical polarization threshold appears, as does an unconventional phase with a paired shell around a normal core.

Computational study of structural and dynamical properties of formamide-water mixtures

A molecular dynamics simulation study of structural and dynamical properties in liquid mixtures of formamide and water is presented. Site-site radial pair distribution functions, local mole fractions, pair energy distributions, and tetrahedral orientational order are the quantities analyzed to investigate the local structure in the simulated mixtures, along with a review of the intermolecular structure in terms of the distribution of hydrogen bonds. Our results indicate that there is a substitution of formamide molecules by water in the hydrogen bonds and a formation of a common hydrogen bond network. By analyzing the extent of tetrahedral order in the liquid as a function of composition, it is observed that whereas the tetrahedral network of liquid water is progressively lost by increasing the formamide concentration, the water structure within the first coordination shell is preserved and somewhat enhanced. The hydrogen-bond mean lifetimes were estimated by performing a time integration of the autocorrelation functions of bond occupation numbers. The lifetimes associated with hydrogen bonds between water, formamide, and interspecies pairs are found to increase with increasing formamide concentration. The lifetimes of the water hydrogen bonds show the largest variations, supporting the picture of an enhancement of the water structure among the nearest neighbors within the first coordination shell. We have used two different force field models for water, SPC/E [J. C. Berendsen et al., J. Phys. Chem. 91, 6269 (1987)] and TIP4P/2005 [J. L. F. Abascal and C. Vega, J. Chem. Phys. 123, 234505 (2005)]. Our results for structural and dynamical properties yield very small differences between those models, the TIP4P/2005 predicting a slightly more structured liquid and, consequently, exhibiting a slightly slower translational and librational dynamics.

Molecular Dynamics Study of Polarizability Anisotropy Relaxation in Aromatic Liquids and Its Connection with Local Structure

The collective polarizability anisotropy dynamics in a set of three aromatic liquids, benzene (Bz), hexafluorobenzene (HFB), and 1,3,5-trifluorobenzene (TFB), has been studied by molecular dynamics simulation. These liquids have very similar shapes, but different electrostatic interactions due to opposite polarities of C-H and C-F bonds, giving rise to different local intermolecular structures in the liquid phase. We have investigated how these structural arrangements affect polarizability anisotropy dynamics observed in optical Kerr-effect (OKE) spectroscopy. We have modeled the interaction-induced polarizability with the first-order dipole-induced dipole approximation, with the molecular polarizability distributed over the carbon sites. Local contributions to the librational OKE spectrum were computed separately for molecules participating in parallel or perpendicular relative orientations within the first coordination shell. We found that the relative locations of parallel and perpendicular librational bands of the OKE spectra are closely related to the corresponding pair energy distributions of the closest four neighbors of a given molecule, corresponding to a model of a harmonic oscillator in a cage of nearest neighbors. This model predicts higher librational frequencies for more attractive intermolecular interactions, which in all three liquids correspond to parallel local arrangements. On the diffusive orientational time scale, all three liquids exhibit slower relaxation of molecules in parallel arrangements, although the difference in relaxation rates is substantial only in TFB, which has the strongest tendency toward parallel stacking. The analysis of the collective polarizability relaxation was performed using two different approaches, the projection scheme (J. Chem. Phys. 1980, 72, 2801) and the theory developed by Steele (Mol. Phys. 1987, 61, 1031) for the second time derivatives applied to collective time correlations. Both approaches allow the decomposition of the OKE response into contributions from orientational relaxation and other dynamical processes. We find that they lead to different predictions on how the response depends on collective reorientation and processes arising from fluctuations in the interaction-induced polarizability. We discuss the reasons for these differences and the advantages and disadvantages of the two analysis schemes.

Double photoionisation spectra of methane, ammonia and water

Complete double photoionisation spectra of methane, ammonia and water measured by the TOF-PEPECO method are reported. The electron pair energy distributions and low quantum yields suggest that for methane and ammonia the double photoionisation is mainly direct. In addition to the direct process, water undergoes two indirect dissociative double ionisation processes, one involving autoionisation of an oxygen atom in course of, or after, dissociation. The existence of these processes explains discrepancies between previous determinations of the ionisation threshold. Results on these and related species lead to a general conclusion that molecular double photoionisation in small molecules is mainly direct, whereas the same process in complex atoms is mainly indirect.

Studies of double photoionisation using a fast pulsed VUV source

A lamp giving 5 ns pulses of 10–50 eV photons at 10 kHz repetition rate has been developed and used with a magnetic bottle electron time-of-flight (TOF) spectrometer to study double photoionisation of atoms and molecules. In addition to the spectra of the dications and complete electron pair energy distributions, quantum yields of double ionisation, as defined in this paper, can be determined. Electron pair energy distributions in double photoionisation of Hg, N 2 and CS 2 are presented as representative of results on many other atoms and molecules. Double photoionisation is found to be mainly indirect, dominated by autoionisation of intermediate monocations, for complex atoms, but mainly direct for small molecules in the valence energy range. The difference arises because superexcited states of molecular monocations decay by dissociation to singly charged products.

CHARMM fluctuating charge force field for proteins: I parameterization and application to bulk organic liquid simulations.

A first-generation fluctuating charge (FQ) force field to be ultimately applied for protein simulations is presented. The electrostatic model parameters, the atomic hardnesses, and electronegativities, are parameterized by fitting to DFT-based charge responses of small molecules perturbed by a dipolar probe mimicking a water dipole. The nonbonded parameters for atoms based on the CHARMM atom-typing scheme are determined via simultaneously optimizing vacuum water-solute geometries and energies (for a set of small organic molecules) and condensed phase properties (densities and vaporization enthalpies) for pure bulk liquids. Vacuum solute-water geometries, specifically hydrogen bond distances, are fit to 0.19 A r.m.s. error, while dimerization energies are fit to 0.98 kcal/mol r.m.s. error. Properties of the liquids studied include bulk liquid structure and polarization. The FQ model does indeed show a condensed phase effect in the shifting of molecular dipole moments to higher values relative to the gas phase. The FQ liquids also appear to be more strongly associated, in the case of hydrogen bonding liquids, due to the enhanced dipolar interactions as evidenced by shifts toward lower energies in pair energy distributions. We present results from a short simulation of NMA in bulk TIP4P-FQ water as a step towards simulating solvated peptide/protein systems. As expected, there is a nontrivial dipole moment enhancement of the NMA (although the quantitative accuracy is difficult to assess). Furthermore, the distribution of dipole moments of water molecules in the vicinity of the solutes is shifted towards larger values by 0.1-0.2 Debye in keeping with previously reported work.

Time-dependent modeling of high energy emission of TeV blazars

We present a self-consistent and time-dependent model in order to explain the variability behaviors of extreme or TeV-blazars. This model is based on the ‘Two-Flow model’ involving a relativistic pair plasma beam surrounded by a classical MHD structure. Particular symbiosis between the ultra-relativistic pairs and the turbulence generated by the MHD jet may lead to a preys–predators comportment. We also consider that the pair energy distribution is not a power-law but a ‘pile-up’ one.

Two electron photoemission processes in the valence band of solids

We present experimental evidence on photo double ionization processes in the valence band of solids. Applying a 'Time-of-Flight' electron spectroscopy technique, we measured the energy distribution of pairs of correlated electrons emitted from clean Cu(001)- and Ni(001)- crystals upon absorption of linearly polarized photons of 45eV energy. Data were taken for normal and grazing orientation of the light wave vector with respect to the crystal surfaces. The energy sharing distributions of electron pairs emitted from the vicinity ofthe Fermi-level show significant differences for Cu and Ni. For interpretation of our data, we consider a direct double ionization mechanism in competition with a two step process involving single photoexcitation and a subsequent collision of the photoelectron with an additional band electron. A theoretical approach by J. Berakdar including both mechanisms is in good agreement with our data. In this theory the differences between Cu and Ni are traced back to distinct relative contributions of both mechanisms in the two metals due to the different average electron density.

Role of the C−H···O Hydrogen Bonds in Liquids: A Monte Carlo Simulation Study of Liquid Formic Acid Using a Newly Developed Pair-Potential

We performed a Monte Carlo simulation study on liquid formic acid employing a recently developed five-site pair-potential. We found that similarly to the crystalline phase, besides the strong O−H···O interactions, C−H···O hydrogen bonds are also present in the liquid and play an important role in the hydrogen-bonding structure. The presence of C−H···O hydrogen bonds is manifest in the results of the structural analysis based on the partial pair correlation functions, the pair-energy distribution function, and the statistical examination of the hydrogen-bonding network. Although, the C−H···O bonds are weaker than the O−H···O interactions and their geometry is somewhat more distorted, the basic geometric preferences are the same for all hydrogen bonds: the X−H···O (X = C,O) bonds tend to be linear in the plane of the acceptor molecule. On the other hand, the hydrogen bond donor molecule appears to rotate freely around the hydrogen bond. Interestingly enough, the acceptor atom both in the O−H···O and in the...

Excitation spectra for the photoassociation of Kr–F and Xe–I collision pairs in the ultraviolet (208–258 nm)

Photoassociation, the bound←free absorption of a photon by a colliding pair of atoms, has been studied experimentally in the Kr–F and Xe–I systems and is demonstrated to be an effective tool for deducing molecular interaction potentials and for determining the dependence of the relative electronic transition moment on internuclear separation (R) in small molecules having dissociative ground states. Specifically, the excitation (‘‘action’’) spectrum for the photoassociation of thermalized Kr(4p6 1S0)–F(2p5 2P) collision pairs, measured at 300 K in the 208–250 nm wavelength region, reveals deeply modulated Franck–Condon structure associated with transitions of Kr–F pairs from the ground state’s thermal and vibrational continua to the lowest 27 bound vibrational states (v′=0–26) of the KrF(B 2Σ) excited state. Under these conditions, photoassociation occurs in a narrow (ΔR=0.7–0.8 Å) Franck–Condon region in which the difference potential is a single valued function of R. Similar spectra have been observed for Xe(1S0)–I(2P) pairs in the 208–258 nm spectral region that arise from bound←free transitions to the lowest ∼40 vibrational levels of the XeI(B) state. Since the Kr–F and Xe–I collision pairs are photoexcited from the vibrational continuum of the ground state, the atomic pair energy distribution is thermalized and the envelope of the excitation spectrum can be interpreted directly in terms of the relative B–X transition moment, μ(R). Consequently, the behavior of μ(R) has been determined over a limited range in R and, for XeI, μ is found to fall by a factor of 2 in the ∼0.4 Å region extending from R−ReB≊0.25–0.62 Å. The XeI ground state (X 2Σ1/2+) potential in the 3.0≤R≤5.0 Å interval, derived from photoassociation spectra, is similar to that deduced from scattering experiments. Excited and ground state structural constants derived from numerical quantum simulations of the experimental spectra are presented.

High-Energy Interference Effect of Bremsstrahlung and Pair Production in Crystals

With the use of monocrystalline targets for bremsstrahlung and pair production experiments, interference phenomena are expected to occur which will markedly change the $\ensuremath{\gamma}$-ray spectrum as well as the pair energy distribution, for certain angles giving enhancement of radiation and pairs. The effect increases with energy; it sets in at $\ensuremath{\delta}\ensuremath{\lesssim}{n}_{0}(\frac{2\ensuremath{\pi}}{a})$, where $\ensuremath{\delta}$ is the minimum momentum transfer to the target atoms, $a$ the lattice constant, and ${n}_{0}$ a number of the order 2 or 3. This corresponds to \ensuremath{\sim}200-Mev primary energy for bremsstrahlung, \ensuremath{\sim}1 Bev for pair production. The effect is confined to angles between the primary beam and a line of atoms, of order $\ensuremath{\theta}\ensuremath{\lesssim}(\frac{{a}_{\mathrm{screen}}}{{\ensuremath{\lambda}}_{C}})(\frac{m{c}^{2}}{E})$. The temperature motion of the lattice reduces the interference effect to some extent. The Born approximation was used for the quantitative analysis of the problem.

Determination of the Energy Distribution of Bremsstrahlung from 19.5-Mev Electrons

The conventional target of a 22-Mev betatron has been replaced by a 0.005-in. platinum target and the intensity spectrum of x-rays produced in the platinum by betatron-accelerated electrons has been measured. The kinetic energy of the monoenergetic electrons at the time of expansion was 19.5 Mev. The x-rays were collimated and observed in the forward direction by studying the electron-positron pairs produced in a cloud chamber filled with air. About 1300 pairs were measured. The intensity spectrum of x-rays is calculated from the pair energy distribution and is compared with the bremsstrahlung theory of Bethe and Heitler. The experimental intensity in the region of 10 Mev is found to be higher than the theoretical value by an amount which is larger than the statistical uncertainty of the experiment. Some details of the pair production process are given, and methods of correcting for instrumental discrimination are discussed and applied.