Prediction of ternary vapor–liquid equilibria for 33 systems by molecular simulation

A set of molecular models for 78 pure substances from prior work is taken as a basis for systematically studying vapor–liquid equilibria (VLE) of ternary systems. All 33 ternary mixtures of these 78 components for which experimental VLE data are available are studied by molecular simulation. The mixture models are based on the modified Lorentz–Berthelot combining rule that contains one binary interaction parameter which was adjusted to a single experimental binary vapor pressure of each binary subsystem in prior work. No adjustment to ternary data is carried out. The predictions from the molecular models of the 33 ternary mixtures are compared to the available experimental data. In almost all cases, the molecular models give excellent predictions of the ternary mixture properties.     

A computational study of the intramolecular deprotonation of a carbon acid in aqueous solution

Proton transfer reactions are the rate-limiting steps in many biological and synthetic chemical processes, often requiring complex cofactors or catalysts to overcome the generally unfavourable thermodynamic process of carbanion intermediate formation. It has been suggested that quantum tunnelling processes enhance the kinetics of some of these reactions, which when coupled to protein motions may be an important consideration for enzyme catalysis. To obtain a better fundamental and quantitative understanding of these proton transfer mechanisms, a computational analysis of the intramolecular proton transfer from a carbon acid in the small molecule, 4-nitropentanoic acid, in aqueous solution is presented. Potential-energy surfaces from gas-phase, implicit and QM/MM (quantum mechanical/molecular mechanical) explicit solvation quantum chemistry models are compared, and the potential of mean force, for the full reaction coordinate, using umbrella-sampling molecular dynamics is analysed. Semi-classical multidimensional tunnelling corrections are also used to estimate the quantum tunnelling contributions and to understand the origin of the primary deuterium kinetic isotope effects (KIEs). The computational results are found to be in excellent agreement with the KIEs and the energetics obtained experimentally.     

Switching individual molecules by light and electrons: From isomerisation to chirality flip

Molecular electronics offers a promising way for constructing nano-electronic devices in future with faster performance and smaller dimensions. For this aim, electronic switches are essential as basic components for storage and logical operations. The main requirements for a molecular switch are reversibility and bistability. This necessitates the existence of at least two different thermally stable forms of a molecule that may be changed repeatedly from one state to the other one through an external stimulus. The transition should then be connected to a measurable change in molecular properties. The development of such molecular switches on the single molecule level is a major challenge on the path towards incorporating molecules as building units into nanoelectronic circuits. Since isomers may differ significantly in physical and chemical properties, isomerisation opens a way for a molecular switch.In this article, an overview is provided over those isomerisation reactions of single molecules adsorbed on surfaces that are investigated with a scanning tunnelling microscope and that have a potential as a molecular switch in future molecular electronics. These are mainly, but not exclusively, constitutional, configurational, and geometric isomerisation reactions. The external stimulus is either light or the possible interaction with the tip of a scanning tunnelling microscope, i.e. electrons, electric field, or mechanical force. Some reactions are similar to those observed for the molecule in the liquid phase, but some are observed or even possible only on a surface. The detailed investigation of the isomerisation yield dependence on several parameters gives insight into the underlying processes of the reaction.     

Reflection spectroscopy on monoclinic crystals

The properties of the dielectric tensor of a monoclinic crystal are reviewed and the nature of transverse excitations is pointed out. Reflection equations for normal incidence on the (010) plane and (001) plane are presented, by use of which full information on all tensor components may be obtained from reflection measurements and Kramers- Kronig analysis. A reflection equation for oblique incidence on faces (h₁, O, h₃) with plane of incidence (010) and transverse magnetic polarization allows the comparison of reflectance spectra calculated from the dielectric tensor components with respective experimental spectra. Simple model calculations related to exciton structures in molecular crystals and quasi one-dimensional materials are displayed.     

A generalized quantum chemical approach for elastic and inelastic electron transports in molecular electronics devices

A generalized quantum chemical approach for electron transport in molecular devices is developed. It allows one to treat devices where the metal electrodes and the molecule are either chemically or physically bonded on equal footing. An extension to include the vibration motions of the molecule has also been implemented which has produced the inelastic electron-tunneling spectroscopy of molecular electronics devices with unprecedented accuracy. Important information about the structure of the molecule and of metal-molecule contacts that are not accessible in the experiment are revealed. The calculated current-voltage (I-V) characteristics of different molecular devices, including benzene-1,4-dithiolate, octanemonothiolate [H(CH2)8S], and octanedithiolate [S(CH2)8S] bonded to gold electrodes, are in very good agreement with experimental measurements.     

Neuromorphic Liquids,Colloids, and Gels: A Review

Advances in flexible electronic devices and robotic software require that sensors and controllers be virtually devoid of traditional electronic components, be deformable and stretch-resistant. Liquid electronic devices that mimic biological synapses would make an ideal core component for flexible liquid circuits. This is due to their unbeatable features such as flexibility, reconfiguration, fault tolerance. To mimic synaptic functions in fluids we need to imitate dynamics and complexity similar to those that occurring in living systems. Mimicking ionic movements are considered as the simplest platform for implementation of neuromorphic in material computing systems. We overview a series of experimental laboratory prototypes where neuromorphic systems are implemented in liquids, colloids and gels.     

An analytical determination of the proton second moment from tunnelling methyl groups,together with a numerical estimate of its modification due to the experimental signal-to-noise ratio

The proton second moments M₂, from both a unidirectional and an isotropic distribution of tunnelling methyl groups, are evaluated analytically in terms of the individual transitions of the proton spectrum. They are shown, in principle, to be independent of the tunnelling splitting, Δ, of the methyl group, and in the latter case, equal to the polycrystalline van Vleck rigid lattice second moment. In practice, the effects of a finite signal-to-noise ratio modify the observed M₂ and in order to account for this, second moments are evaluated numerically as a function of signal to noise ratio, for methyl groups with tunnelling splittings in the range Δ < E_D to Δ >E_D, where E_D is the intra methyl-group nuclear dipolar energy. These effects, which are most significant when Δ ≈ E_D, are linked to the experimental situation by calculating, in a representative case, the increase in M₂ with decreasing temperature, due to the inherent temperature dependence of both the spectrum and the signal-to-noise ratio. As a result the unequivocal importance of accounting for the signal-to-noise ratio when extracting tunneling splitting information from second moment data is clearly seen for that very range of methyl group barriers which can best be studied by spectral experiments.     

A Molecular Circuit Regenerator to Implement Iterative Strand Displacement Operations

The predictable chemistry of Watson–Crick base-pairing imparts a unique structural programmability to DNA, enabling the facile design of molecular reactions that perform computations. However, many of the current architectures limit devices to a single operational cycle. Herein, we introduce the design of the “regenerator”, a device based on coupled enthalpic and entropic reactions that permits the regeneration of molecular circuit components.     

Self assembled monolayers on silicon for molecular electronics

We present an overview of various aspects of the self-assembly of organic monolayers on silicon substrates for molecular electronics applications. Different chemical strategies employed for grafting the self-assembled monolayers (SAMs) of alkanes having different chain lengths on native oxide of Si or on bare Si have been reviewed. The utility of different characterization techniques in determination of the thickness, molecular ordering and orientation, surface coverage, growth kinetics and chemical composition of the SAMs has been discussed by choosing appropriate examples. The metal counterelectrodes are an integral part of SAMs for measuring their electrical properties as well as using them for molecular electronic devices. A brief discussion on the variety of options available for the deposition of metal counterelectrodes, that is, soft metal contacts, vapor deposition and soft lithography, has been presented. Various theoretical models, namely, tunneling (direct and Fowler-Nordheim), thermionic emission, Poole-Frenkel emission and hopping conduction, used for explaining the electronic transport in dielectric SAMs have been outlined and, some experimental data on alkane SAMs have been analyzed using these models. It has been found that short alkyl chains show excellent agreement with tunneling models; while more experimental data on long alkyl chains are required to understand their transport mechanism(s). Finally, the concepts and realization of various molecular electronic components, that is, diodes, resonant tunnel diodes, memories and transistors, based on appropriate architecture of SAMs comprising of alkyl chains (sigma- molecule) and conjugated molecules (pi-molecule) have been presented.     

Molecular models for 267 binary mixtures validated by vapor–liquid equilibria: A systematic approach

By assessing a large number of binary systems, it is shown that molecular modeling is a reliable and robust route to vapor–liquid equilibria (VLE) of mixtures. A set of simple molecular models for 78 pure substances from prior work is taken to systematically describe all 267 binary mixtures of these components for which relevant experimental VLE data is available. The mixture models are based on the modified Lorentz–Berthelot combining rule. Per binary system, one state independent binary interaction parameter in the energy term is adjusted to a single experimental vapor pressure. The unlike energy parameter is altered usually by less than 5% from the Berthelot rule. The mixture models are validated regarding the vapor pressure at other state points and also regarding the dew point composition, which is a fully predictive property in this work. In almost all cases, the molecular models give excellent predictions of the mixture properties.     

Two-dimensional functional molecular nanoarchitectures – Complementary investigations with scanning tunneling microscopy and X-ray spectroscopy

Functional molecular nanoarchitectures (FMNs) are highly relevant for the development of future nanotechnology devices. Profound knowledge about the atomically controlled construction of such nanoscale assemblies is an indispensable requirement to render the implementation of such components into a real product successful. For exploiting their full potential the architectures’ functionalities have to be characterized in detail including the ways to tailor them. In recent years a plethora of sophisticated constructs were fabricated touching a wide range of research topics.     

Viscosity of Ionic Liquids: Application of the Eyring’s Theory and a Committee Machine Intelligent System

Accurate determination of the physicochemical characteristics of ionic liquids (ILs), especially viscosity, at widespread operating conditions is of a vital role for various fields. In this study, the viscosity of pure ILs is modeled using three approaches: (I) a simple group contribution method based on temperature, pressure, boiling temperature, acentric factor, molecular weight, critical temperature, critical pressure, and critical volume; (II) a model based on thermodynamic properties, pressure, and temperature; and (III) a model based on chemical structure, pressure, and temperature. Furthermore, Eyring’s absolute rate theory is used to predict viscosity based on boiling temperature and temperature. To develop Model (I), a simple correlation was applied, while for Models (II) and (III), smart approaches such as multilayer perceptron networks optimized by a Levenberg–Marquardt algorithm (MLP-LMA) and Bayesian Regularization (MLP-BR), decision tree (DT), and least square support vector machine optimized by bat algorithm (BAT-LSSVM) were utilized to establish robust and accurate predictive paradigms. These approaches were implemented using a large database consisting of 2813 experimental viscosity points from 45 different ILs under an extensive range of pressure and temperature. Afterward, the four most accurate models were selected to construct a committee machine intelligent system (CMIS). Eyring’s theory’s results to predict the viscosity demonstrated that although the theory is not precise, its simplicity is still beneficial. The proposed CMIS model provides the most precise responses with an absolute average relative deviation (AARD) of less than 4% for predicting the viscosity of ILs based on Model (II) and (III). Lastly, the applicability domain of the CMIS model and the quality of experimental data were assessed through the Leverage statistical method. It is concluded that intelligent-based predictive models are powerful alternatives for time-consuming and expensive experimental processes of the ILs viscosity measurement.     

Viscosities of Binary Liquid Mixtures of 2-Methylpropane Nitrile and Some Aromatic Hydrocarbons at 303.15 K and 308.15 K

Viscosities, ??, of five binary mixtures of 2-methylpropane nitrile with aromatic hydrocarbons, viz. benzene, methylbenzene, 1,4-dimethylbenzene, chlorobenzene and 1,2-dichlorobenzene, have been measured at temperatures 303.15 and 308.15?K. Deviations in viscosities, ????, from the linear mixing rule have been determined for all of the binary mixtures studied at both experimental temperatures. The ???? values are negative for all the five systems at both experimental temperatures. Parameters of the Redlich?CKister and Myers and Scott equations have been fitted to the ???? values and standard deviations, ??(????), have also been evaluated. The correlating performances of several viscosity models have been evaluated. The Hind?CMcLaughlin?CUbbelohde viscosity model gives fairly constant interaction parameters with composition. Results are discussed in terms of unlike molecular interactions between the components of the binary mixtures.     

Chemical tuning of spin clock transitions in molecular monomers based on nuclear spin-free Ni(ii)

We report the existence of a sizeable quantum tunnelling splitting between the two lowest electronic spin levels of mononuclear Ni complexes. The level anti-crossing, or magnetic “clock transition”, associated with this gap has been directly monitored by heat capacity experiments. The comparison of these results with those obtained for a Co derivative, for which tunnelling is forbidden by symmetry, shows that the clock transition leads to an effective suppression of intermolecular spin–spin interactions. In addition, we show that the quantum tunnelling splitting admits a chemical tuning via the modification of the ligand shell that determines the crystal field and the magnetic anisotropy. These properties are crucial to realize model spin qubits that combine the necessary resilience against decoherence, a proper interfacing with other qubits and with the control circuitry and the ability to initialize them by cooling.

We have directly monitored spin level anti-crossings, or “clock transitions”, in Ni(ii) molecular monomers and shown that the quantum tunnelling gap admits a chemical tuning.     

Charge Transport in Mesomorphic Phthalocyanines Studied by Pulse-Radiolysis Time-Resolved Microwave Conductivity

The pulse-radiolysis time-resolved microwave conductivity (PR-TRMC) technique has been used to obtain information on the transport of charge within columnarly stacked, peripherally octaalkoxy-substituted phthalocyanines. Data are presented on the one-dimensional, intracolumnar charge mobility and on the timescale of quasi-two-dimensional intercolumnar electron tunnelling. Particular attention is paid to materials that are liquid-crystalline at room temperature, because of their potential technological importance in optoelectric charge transport layers and molecular semiconductor devices. The relevance of the data to channeled charge transport in aligned thin layers is discussed.     

Electrochemistry of complex molecular and biomolecular scale entities

Structural mapping of intermediate size and large molecules and biomolecules at ultra-high resolution using single-crystal electrodes and in situ scanning tunnelling microscopy continues to disclose surprising findings. In situ scanning tunnelling spectroscopy has also recently disclosed new electrochemical conductivity features at the level of the single molecule. We overview briefly elements of this development over the last few years, with focus on three recent discoveries: (1) a new packing mode of a core monolayer target thiol, the amino acid cysteine (Cys) on Au(100)-electrodes, quite different from Cys packing on Au(111)- and Au(110)-electrodes; (2) transition of a core ferrocene in situ scanning tunnelling spectroscopy probe from stochastic single-molecule to macroscopic behaviour, a concept at the heart of nanoscience; and (3) unexpected behaviour of the large molybdenum enzyme sulfite oxidase, when going from macroscopic to single-molecule electrochemistry. We compare these studies with other recent discoveries of single-molecule protein conductivity and molecular scale inorganic nanostructures.     

Propagator theory of quantum-wire transmission

In the tight-binding formalism, the Green function (GF) is cast in the site representation, and can be interpreted as a propagator of information from one site to another. A general propagator theory is developed that encompasses the present GF treatments of electron transport in molecular wires and the scanning tunnelling and ballistic electron emission microscopes. Finally, the question of a multi-contact quantum circuit is addressed.