A density functional theory based approach for predicting melting points of ionic liquids

Physical Chemistry Chemical Physics
Accurate prediction of melting points of ILs is important both from the fundamental point of view and from the practical perspective for screening ILs with low melting points and broadening their utilization in a wider temperature range. In this work, we present an ab initio approach to calculate melting points of ILs with known crystal structures and illustrate its application for a series of 11 ILs containing imidazolium/pyrrolidinium cations and halide/polyatomic fluoro-containing anions. The melting point is determined as a temperature at which the Gibbs free energy of fusion is zero. The Gibbs free energy of fusion can be expressed through the use of the Born–Fajans–Haber cycle via the lattice free energy of forming a solid IL from gaseous phase ions and the sum of the solvation free energies of ions comprising IL. Dispersion-corrected density functional theory (DFT) involving (semi)local (PBE-D3) and hybrid exchange–correlation (HSE06-D3) functionals is...  Read more

The effect of defect types on the electronic and optical properties of graphene nanoflakes physisorbed by ionic liquids

Physical Chemistry Chemical Physics
Defect engineering and non-covalent interaction strategies allow for dramatically tuning the optoelectronic properties of graphene. Using ab initio density functional theory (M06-2X/cc-pVDZ), we find that the nature of defects on the graphene nanoflakes (GNFs) and the size of defective GNF (DGNF) surfaces affect the binding energy (ΔEb) of ionic liquids (ILs) and the UV-Vis absorption spectra of DGNF⋯IL complexes. Further, our results indicate that increasing the size of DGNFs affects the geometrical structure of the surfaces and increases the binding energy of ILs by about 10%. Analysis based on AIM and EDA shows that the interactions between ILs and DGNFs are non-covalent in nature (dispersion energy being dominant) and associated with charge transfer between the IL and nanoflakes. A comparison between the ΔEb values of ILs on DGNFs, GNFs, and h-BN nanoflakes (h-BNNF) shows that the...  Read more

Hydration of the simplest α-keto acid: a rotational spectroscopic and ab initio study of the pyruvic acid–water complex

Physical Chemistry Chemical Physics
Intermolecular interactions between pyruvic acid, the simplest α-keto acid, and water are important in bio- and atmospheric chemistry. In this context, the pure rotational spectrum of the pyruvic acid–water complex was measured from 7 to 15 GHz using a cavity-based Fourier-transform microwave spectrometer. In the detected isomer, water acts as a hydrogen bond donor and acceptor, bridging the acidic hydrogen and the keto oxygen. Both a- and b-type transitions were observed; however, c-type transitions were not observed, due to vibrational averaging of the effectively barrier-less wagging motion of the free hydrogen of the water subunit, which results in an effective ground state structure with a plane of symmetry. The mass distribution out of the ab-plane, corrected for the out-of-plane hydrogen atoms of the methyl group, confirms that the complex has a plane of symmetry. The observed transitions exhibit splittings due to internal rotations of the...  Read more

Alternative Route Toward Nitrones: Experimental and Theoretical Findings

The Journal of Organic Chemistry
Nitrones are important building blocks for natural and biologically active compounds, used as spin-trap reagents and therapeutic agents. All this makes nitrones intriguing and valuable compounds for fundamental studies and as useful chemicals in various synthetic strategies. Therefore, nitrones are still of great interest and in the limelight of researches. With our initial goal to solve synthetic problems toward 5-phenyl-2,2′-bipyridine (Phbpy), we found that this reaction can proceed through the formation of 6-phenyl-3-(pyridin-2-yl)-1,2,4-triazin-4(3H)-ol (4-OH), which rapidly isomerizes to a 3,4-dihydro-1,2,4-triazine-based nitrone, namely 6-phenyl-3-pyridin-2-yl-2,3-dihydro-1,2,4-triazin-4-oxide (4′), This encouraged us to study condensation of hydrazonophenylacetaldehyde oxime (2), obtained from 2-isonitrosoacetophenone (1), with other aldehydes. The reaction with both salicylaldehyde and p-tolualdehyde leads to the open-chain...  Read more

Computation of the Isotropic Hyperfine Coupling Constant: Efficiency and Insights from a New Approach Based on Wave Function Theory

Journal of Chemical Theory and Computation
The present paper reports an original computational strategy for the computation of the isotropic hyperfine coupling constants (hcc). The algorithm proposed here is based on an approach recently introduced by some of the authors, namely, the first-order breathing orbital self-consistent field (FOBO-SCF). The approach is an almost parameter-free wave function method capable to accurately treat the spin delocalization together with the spin polarization effects while staying in a restricted formalism and avoiding spin contamination. The efficiency of the method is tested on a series of small radicals, among which four nitroxide radicals and the comparison with high-level ab initio methods show very encouraging results. On the basis of these results, the method is then applied to compute the hcc of a challenging system, namely, the DEPMPO-OOH radical in various conformations. The reference values obtained on such a large system allows us to validate a cheap computational method...  Read more

Stabilization of a Ga-adlayer structure with the zincblende stacking sequence in the GaN(0 0 0 −1) surface at the nanoscale

Profile imaging by in situ high-resolution transmission electron microscopy is used to elucidate reconstructions of the GaN(0 0 0 −1) surface during annealing in the TEM. We have successfully captured a detailed process of a change from the stacking sequence of the wurtzite to that of the zincblende structure in the topmost three Ga layers for the surface with nanoscale hill-and-valley structures. For ab initio calculations of the change in the sequence, a model structure is approximated by the addition of a 1 × 1 Ga layer on the GaN(0 0 0 −1) surface (i.e., 1 × 1 Ga-adlayer structure). The ab initio calculations predict that, as the surface size decreases, the 1 × 1 Ga-adlayer structure with the wurtzite stacking sequence in the topmost three Ga layers becomes destabilized against the adlayer with the zincblende stacking sequence in the surface layers, which well elucidates the experimental observation.Read more

The annular tautomerism of lithium 1,2,3-triazolate

New Journal of Chemistry
The annular tautomerism of lithium 1,2,3-triazolate (Li-TR) is experimentally investigated in the solid state by X-ray diffraction and in methanolic solution by NMR spectroscopy. DFT-based ab initio molecular dynamics simulations are additionally carried out to characterize the solvation structure of Li-TR in methanol and to determine the free energy landscape of the two tautomers.Read more

Defect States and Charge Transport in Quantum Dot Solids

Chemistry of Materials
Defects at the surface of semiconductor quantum dots (QDs) give rise to electronic states within the gap, which are detrimental to charge transport properties of QD devices. We investigated charge transport in silicon quantum dots with deep and shallow defect levels, using ab initio calculations and constrained density functional theory. We found that shallow defects may be more detrimental to charge transport than deep ones, with associated transfer rates differing by up to 5 orders of magnitude for the small dots (1–2 nm) considered here. Hence, our results indicate that the common assumption, that the ability of defects to trap charges is determined by their position in the energy gap of the QD, is too simplistic, and our findings call for a reassessment of the role played by shallow defects in QD devices. Overall, our results highlight the key importance of taking into account the atomistic structural properties of QD surfaces when investigating transport properties.Read more

Novel Two-Dimensional Silicon Dioxide with in-Plane Negative Poisson’s Ratio

Nano Letters
Silicon dioxide or silica, normally existing in various bulk crystalline and amorphous forms, was recently found to possess a two-dimensional structure. In this work, we use ab initio calculation and evolutionary algorithm to unveil three new two-dimensional (2D) silica structures whose thermal, dynamical, and mechanical stabilities are compared with many typical bulk silica. In particular, we find that all three of these 2D silica structures have large in-plane negative Poisson’s ratios with the largest one being double of penta graphene and three times of borophenes. The negative Poisson’s ratio originates from the interplay of lattice symmetry and Si—O tetrahedron symmetry. Slab silica is also an insulating 2D material with the highest electronic band gap (>7 eV) among reported 2D structures. These exotic 2D silica with in-plane negative Poisson’s ratios and widest band gaps are expected to have great potential applications in nanomechanics and nanoelectronics.Read more

Probing the Origin of Interfacial Carriers in SrTiO3–LaCrO3 Superlattices

Chemistry of Materials
Emergent phenomena at complex oxide interfaces could provide the basis for a wide variety of next-generation devices, including photovoltaics and spintronics. To date, detailed characterization and computational modeling of interfacial defects, cation intermixing, and film stoichiometry have helped to explain many of the novel behaviors observed at a single heterojunction. Unfortunately, many of the techniques employed to characterize a single heterojunction are less effective for a superlattice made up of a repeating series of interfaces that induce collective interfacial phenomena throughout a film. These repeating interfaces present an untapped opportunity to introduce an additional degree of complexity, such as confined electric fields, that cannot be realized in a single heterojunction. In this work, we explore the properties of SrTiO3–LaCrO3 superlattices to understand the role of defects, including variations in cation stoichiometry of individual layers...  Read more

Manipulation of Optoelectronic Properties and Band Structure Engineering of Ultrathin Te Nanowires by Chemical Adsorption

ACS Applied Materials and Interfaces
Band structure engineering is a powerful technique both for the design of new semiconductor materials and for imparting new functionalities to existing ones. In this article, we present a novel and versatile technique to achieve this by surface adsorption on low dimensional systems. As a specific example, we demonstrate, through detailed experiments and ab initio simulations, the controlled modification of band structure in ultrathin Te nanowires due to NO2 adsorption. Measurements of the temperature dependence of resistivity of single ultrathin Te nanowire field-effect transistor (FET) devices exposed to increasing amounts of NO2 reveal a gradual transition from a semiconducting to a metallic state. Gradual quenching of vibrational Raman modes of Te with increasing concentration of NO2 supports the appearance of a metallic state in NO2 adsorbed Te. Ab initio simulations attribute these observations to the appearance of midgap states in...  Read more

An Estimation of Hybrid Quantum Mechanical Molecular Mechanical Polarization Energies for Small Molecules Using Polarizable Force-Field Approaches

Journal of Chemical Theory and Computation
In this work, we report two polarizable molecular mechanics (polMM) force field models for estimating the polarization energy in hybrid quantum mechanical molecular mechanical (QM/MM) calculations. These two models, named the potential of atomic charges (PAC) and potential of atomic dipoles (PAD), are formulated from the ab initio quantum mechanical (QM) response kernels for the prediction of the QM density response to an external molecular mechanical (MM) environment (as described by external point charges). The PAC model is similar to fluctuating charge (FQ) models because the energy depends on external electrostatic potential values at QM atomic sites; the PAD energy depends on external electrostatic field values at QM atomic sites, resembling induced dipole (ID) models. To demonstrate their uses, we apply the PAC and PAD models to 12 small molecules, which are solvated by TIP3P water. The PAC model reproduces the QM/MM polarization energy with a R2 value...  Read more

Adsorption and Intermolecular Interaction of Cobalt Phthalocyanine on CoO(111) Ultrathin Films: An STM and DFT Study

The Journal of Physical Chemistry C
We investigate the adsorption of cobalt phthalocyanine (CoPc) molecules on a thin layer of cobalt oxide grown on Ir(100). To that end we compare the results of low-temperature scanning tunneling microscopy (STM) with those of ab initio density functional theory (DFT) calculations and reveal the adsorption geometry. We find that the CoPc molecules lie flat on the surface and that their central cobalt atom forms a chemical bond to a substrate oxygen ion. However, this bond contributes only modestly to the adsorption energy, while van der Waals forces dominate the potential landscape and determine to a large extent the geometry as well as the distortion of substrate and molecule. Furthermore, they lead to attractive molecule–molecule interactions at higher molecular coverages. The DFT calculations predict energetic positions of the molecular orbitals, which are compared to scanning tunneling spectroscopy (STS) measurements.Read more

Structural Dependence of the Ising-type Magnetic Anisotropy and of the Relaxation Time in Mononuclear Trigonal Bipyramidal Co(II) Single Molecule Magnets

Inorganic Chemistry
This paper describes the correlation between Ising-type magnetic anisotropy and structure in trigonal bipyramidal Co(II) complexes. Three sulfur-containing trigonal bipyramidal Co(II) complexes were synthesized and characterized. It was shown that we can engineer the magnitude of the Ising anisotropy using ligand field theory arguments in conjunction with structural parameters. To prepare this series of compounds, we used, on the one hand, a tetradentate ligand containing three sulfur atoms and one amine (NS3tBu) and on the other hand three different axial ligands, namely, Cl, Br, and NCS. The organic ligand imposes a trigonal bipyramidal arrangement with the three sulfur atoms lying in the trigonal plane with long Co–S bond distances. The magnetic properties of the compounds were measured, and ab initio calculations were used to analyze the anisotropy parameters and perform magneto-structural correlations. We...  Read more

Conformational structures of jet-cooled acetaminophen–water clusters: a gas phase spectroscopic and computational study

Physical Chemistry Chemical Physics
Jet-cooled acetaminophen (AAP)–water clusters, AAP–(H2O)1, were investigated by mass-selected resonant two-photon ionization (R2PI), ultraviolet–ultraviolet hole-burning (UV–UV HB), infrared-dip (IR-dip), and infrared–ultraviolet hole-burning (IR–UV HB) spectroscopy. Each syn- and anti-AAP rotamer has three distinctive binding sites (–OH, >CO, and >NH) for a water molecule, thus 6 different AAP–(H2O)1 conformers are expected to exist in the molecular beam. The origin bands of the AAP(OH)–(H2O)1 and AAP(CO)–(H2O)1 conformers (including their syn- and anti-conformers) in the R2PI spectrum are shifted to red and blue compared to those of the AAP monomer, respectively. These frequency shifts upon complexation between a...  Read more

Pursuit of Record Breaking Energy Barriers: A Study of Magnetic Axiality in Diamide Ligated DyIII Single-Molecule Magnets

Journal of the American Chemical Society
DyIII single-ion magnets (SIMs) with strong axial donors and weak equatorial ligands are attractive model systems with which to harness the maximum magnetic anisotropy of DyIII ions. Utilizing a rigid ferrocene diamide ligand (NNTBS), a DyIII SIM, (NNTBS)DyI(THF)2, 1-Dy (NNTBS = fc(NHSitBuMe2)2, fc = 1,1′-ferrocenediyl), composed of a near linear arrangement of donor atoms, exhibits a large energy barrier to spin reversal (770.8 K) and magnetic blocking (14 K). The effects of the transverse ligands on the magnetic and electronic structure of 1-Dy were investigated through ab initio methods, eliciting significant magnetic axiality, even in the fourth Kramers doublet, thus demonstrating the potential of rigid diamide ligands in the design of new SIMs with defined magnetic axiality.Read more

An ab initio investigation of photoswitches adsorbed onto metal oxide surfaces: the case of donor–acceptor Stenhouse adduct photochromes on TiO2 anatase

Journal of Materials Chemistry C
The recent discovery of donor–acceptor Stenhouse adducts (DASAs) as a novel class of visible light activated photochromes has received considerable attention due to their high potential for original applications. Up to now, the experimental and theoretical studies have mainly focused on the intrinsic physico-chemical properties and photoswitching process of these molecules, as well as on the incorporation of DASAs in multi-organized structures. In this context, we propose here the first study, by means of first-principle calculations, of DASA photochromes adsorbed onto titanium dioxide anatase surfaces. By analyzing the density of states, we show that DASAs are electronically more sensitive to the surfaces in the closed compact form than in the extended π-conjugated one, which is unusual for photochromes. This finding is also supported by an analysis of the energy band gap. A charge transfer study is performed to quantify the interactions taking place within the total systems. In...  Read more

The initial stages of melting of graphene between 4000 K and 6000 K

Physical Chemistry Chemical Physics
Graphene and its analogues have some of the highest predicted melting points of any materials. Previous work estimated the melting temperature for freestanding graphene to be a remarkable 4510 K. However, this work relied on theoretical methods that do not accurately account for the role of bond breaking or complex bonding configurations in the melting process. Furthermore, experiments to verify these high melting points have been challenging. Practical applications of graphene and carbon nanotubes at high temperatures will require a detailed understanding of the behavior of these materials under these conditions. Therefore, we have used reliable ab initio molecular dynamics calculations to study the initial stages of melting of freestanding graphene monolayers between 4000 and 6000 K. To accommodate large defects, and for improved accuracy, we used a large 10 × 10 periodic unit cell. We find that the system can be heated up to 4500 K for 18 ps without melting, and 3-rings...  Read more

Oxygenate-Induced Tuning of Aldehyde-Amine Reactivity and Its Atmospheric Implications

The Journal of Physical Chemistry A
Atmospheric aerosols often contain a significant fraction of carbon–nitrogen functionality, which makes gas-phase aldehyde-amine chemistries an important source of nitrogen containing compounds in aerosols. Here we use high-level ab initio calculations to examine the key determinants of amine (ammonia, methylamine, and dimethylamine) addition onto three different aldehydes (acetaldehyde, glycolaldehyde, and 2-hydroperoxy acetaldehyde), with each reaction being catalyzed by a single water molecule. The model aldehydes reflect different degrees of oxygenation at a site adjacent to the carbonyl moiety, the α-site, and represent typical oxygenates that can arise from atmospheric oxidation especially under conditions where the concentration of NO is low. Our results show that the reaction barrier is influenced not only by the nature of the amine but also by the nature of the aldehyde. We find that, for a given amine, the reaction barrier decreases with increasing oxygenation of the...  Read more

Full Mechanism of Zeolite Dealumination in Aqueous Strong Acid Medium: Ab Initio Periodic Study on H-Clinoptilolite

The Journal of Physical Chemistry C
Periodic quantum-chemistry methods as implemented in the Crystal14 code were considered to study the mechanism of the dealumination of H-clinoptilolite in aqueous acid medium. A series of models consisting of the primitive cell of a monoaluminated structure of HEU (heulandite) framework type together with water molecules and HCl as catalyzer were considered. Stable and transition state structures together with their relative Gibbs free energy values were computed at the hybrid density functional theory (DFT) level of theory with dispersion correction (PBE0-D2) employing atomic basis sets. Four different steps for the mechanism have been considered. Each of them corresponds to the hydrolysis of the bonds that connect the Al atom with the framework providing as final product Al(OH)3 adsorbed on a silanol nest. Results show that the free reaction energy for the whole acid dealumination is about −49 kJ mol–1. The crucial way the HCl...  Read more