DFT study, Electronic Structure, computational chemistry

Using multiwavelets, we have obtained total energies and corresponding atomization energies for the GGA-PBE and hybrid-PBE0 density functionals for a test set of 211 molecules with an unprecedented and guaranteed μHartree accuracy. These quasi-exact references allow us to quantify the accuracy of standard all-electron basis sets that are believed to be highly accurate for molecules, such as Gaussian-type orbitals (GTOs), all-electron numeric atom-centered orbitals (NAOs), and full-potential augmented plane wave (APW) methods. We show that NAOs are able to achieve the so-called chemical accuracy (1 kcal/mol) for the typical basis set sizes used in applications, for both total and atomization energies. For GTOs, a triple-ζ quality basis has mean errors of ∼10 kcal/mol in total energies, while chemical accuracy is almost reached for a quintuple-ζ basis. Due to systematic error cancellations, atomization energy errors are reduced by almost an order of magnitude, placing chemical accuracy within reach also for medium to large GTO bases, albeit with significant outliers. In order to check the accuracy of the computed densities, we have also investigated the dipole moments, where in general only the largest NAO and GTO bases are able to yield errors below 0.01 D. The observed errors are similar across the different functionals considered here.

Recent technological advances in the isolation and transfer of different 2-dimensional (2D) materials have led to renewed interest in stacked Van der Waals (vdW) heterostructures. Interlayer interactions and lattice mismatch between two different monolayers cause elastic strains, which significantly affects their electronic properties. Using a multiscale computational method, we demonstrate that significant in-plane strains and the out-of-plane displacements are introduced in three different bilayer structures, namely graphene-hBN, MoS 2-WS 2 and MoSe 2-WSe 2 , due to interlayer interactions which can cause bandgap change of up to ~300 meV. Furthermore, the magnitude of the elastic deformations can be controlled by changing the relative rotation angle between two layers. Magnitude of the out-of-plane displacements in graphene agrees well with those observed in experiments and can explain the experimentally observed bandgap opening in graphene. Upon increasing the relative rotation angle between the two lattices from 0° to 10°, the magnitude of the out-of-plane displacements decrease while in-plane strains peaks when the angle is ~6°. For large misorientation angles (> 10°), the out-of-plane displacements become negligible. We further predict the deformation fields for MoS 2-WS 2 and MoSe 2-WSe 2 heterostructures that have been recently synthesized experimentally and estimate the effect of these deformation fields on near-gap states. 2D materials such as graphene, hexagonal boron nitride (h-BN), phosphorene, MoS 2 and other transition metal dichalcogenides hold promise for a range of electronic and optoelectronic devices such as field-effect transistors, touch-screen panels, energy harvesting devices and ultrafast lasers. Recent technological advancements in the isolation and transfer of different 2D materials without loss of material quality have led to the renewed interest in stacked/layered materials and heterostructures 1–6. For example, Yu et al. 7 have shown that vertically stacked graphene-MoS 2-graphene and graphene-MoS 2-metal junctions can be tuned to obtain photocurrent with quantum efficiency as high as 55%. Similarly, graphene stacked on hBN has been shown to possess electron mobilities that are at least two orders of magnitudes higher compared to graphene on the conventional substrates such as silica or silicon 8. Other substrate induced phenomenon e.g. band gap opening, Hofstadter butterfly effects and emergence of new Dirac points have also been reported recently 9–11. However, lattice mismatch and interactions between different

First-principles density functional theory (DFT) calculations were carried out to investigate the structural
and electronic properties of beryllium (Be) doped and Be and boron (B) co-doped graphene systems. We
observed that not only the concentration of impurity atoms is important to tune the band-gap to some
desired level, but also the specific substitution sites play a key role. In our system, which consists of
32 atoms, a maximum of 4Be and, in the co-doped state, 2Be and 3B atom substitutions are
investigated. Both dopants are electron deficient relative to C atoms and cause the Fermi level to shift
downward (p-type doping). A maximum band gap of 1.44 eV can be achieved on incorporation of 4Be
atoms. The introduction of Be is more sensitive in terms of geometry and stability than B. However, in
opening the energy gap, Be is more effective than B and N (nitrogen). Our results offer the possibility to
modify the band-gap of graphene sufficiently for utilization in diverse electronic device applications.

The α-effect is a term used to explain the dramatically enhanced reactivity of α-nucleophiles (R−Y−X:−) compared to their parent normal nucleophile (R−X:−) by deviating from the classical Brønsted-type reactivity-basicity relationship. The exact origin of this effect is, however, still heavily under debate. In this work, we have quantum chemically analyzed the α-effect of a set of anionic nucleophiles, including O-, N- and S-based normal and α-nucleophiles, participating in an SN2 reaction with ethyl chloride using relativistic density functional theory at ZORA-OLYP/QZ4P. Our activation strain and Kohn–Sham molecular orbital analyses identified two criteria an α-nucleophile needs to fulfill in order to show α-effect: (i) a small HOMO lobe on the nucleophilic center, pointing towards the substrate, to reduce the repulsive occupied–occupied orbital overlap and hence (steric) Pauli repulsion with the substrate; and (ii) a sufficiently high energy HOMO to overcome the loss of favorable HOMO–LUMO orbital overlap with the substrate, as a consequence of the first criterion, by reducing the HOMO–LUMO orbital energy gap. If one of these two criteria is not fulfilled, one can expect no α-effect or inverse α-effect.

First principles total energy calculations have been applied to describe the ReCN bulk structure and the formation of ReCN monolayers and bilayers. Results demonstrate a strong structural rearrangement in the monolayer due to a reduced dimension effect: an increase in the lattice parameter, accompanied with the contraction of the distance between the C and N planes. On the other hand, a ReCN bilayer has structural parameters similar to those of the bulk. Surface formation energies show that the monolayer is more stable than bilayer geometries. Although bulk ReCN shows a semiconductor behavior, the monolayer ReCN presents a metallic behavior. This metallic character of the ReCN monolayer is mainly due to the d-orbitals of Re atoms. Graphene is a well-known 2D material with outstanding properties, making it one of the most studied materials. It can be obtained by exfoliation 1 , or by epitaxial growth on different substrates 2, 3. It presents physical properties such as substrate-induced band gap opening 3 , a half-metallic behavior 4 , a tunable gap 5 , remarkable electronic mobility 6 , and a topological insulator behavior 7. These properties make graphene suitable for many technological applications 8. For example, it may be used as photodetector 9 , optical modulator 10 , as key constituent in solar cells, in light emitting devices, and in ultrafast lasers 11 , as well as in flexible optoelectronic devices 12 , among other specific applications 13, 14. Since graphene gained such attention, many research groups started to look for new 2D materials with equally interesting properties. As consequence, several 2D materials have been found: silicene 15 , germanene 16 , and tri-layer transition metal dichalcogenides 17–19 are some examples. These 2D materials have excellent properties , which may be exploited to construct new generation devices, or to form heterostructures with graphene to expand its well established applications as well as to tune its unprecedented properties 20–22. Recently, a new two dimensional semiconductor has been obtained: black phosphorene. This novel material has been proposed as a strong competitor to graphene since it has a semiconductor behavior, with a band gap that can be modulated by increasing the number of layers 23. It also can be highly strained without losing its semiconductor character, making it suitable for applications in flexible electronic devices 24. It can change from direct to indirect gap by tensile strain 25. Furthermore, its high carrier mobility makes it a potential material for the channel in the construction of electronic and optoelectronic devices 26–28. Keeping in mind the interest that 2D materials have generated in the past years, we have turned our attention to the ReCN compound. This is a super hard material with an orthogonal bulk structure with two ReCN tri-layers in the unit cell 29. Since two consecutive tri-layers are not bonded by weak van der Waals forces, it is not possible to obtain 2D ReCN by exfoliation. However, it surely may be grown by techniques such as molecular beam epitaxy, chemical vapor deposition, spray pyrolysis or some other chemical or physical growth techniques. To explore this material deeper, we have carried out first principles calculations to characterize the structural and electronic properties of ReCN in bulk and as a 2D material. Method Calculations have been carried out using the density functional theory as implemented in the PWscf code of the Quantum ESPRESSO package 30. The Kohn-Sham states have been expanded in plane waves with a kinetic energy cutoff of 30 Ry, while the charge density cutoff was set to 240 Ry. The generalized gradient approximation with the Perdew-Burke-Ernzerhof parametrization 31 has been used to treat the non-classical exchange and correlation

Halogen bonding (XB) is being extensively explored for its potential use in advanced materials
and drug design. Despite a significant progress in describing this interaction by theoretical and
experimental methods, the chemical nature remains somewhat elusive and, it seems to vary with
selected system. In this work we present a detailed DFT analysis of three-center asymmetric halogen
bond (XB) formed between dihalogen molecules and variously 4-substituted 1,2-dimethoxybenzene.
The energy decomposition, orbital, and electron density analyses suggest that the contribution of
electrostatic stabilization is comparable with that of non-electrostatic factors. Both terms increase
parallel with increasing the negative charge of the electron donor molecule in our model systems.
Depending on the orientation of the dihalogen molecules, this bifurcated interaction may be classified
as ‘σ-hole – lone pair’ or ‘σ-hole – π’ halogen bonds. Arrangement of the XB investigated here
deviates significantly from a recent IUPAC definition of XB and, in analogy to the hydrogen bonding,
term bifurcated halogen bond (BXB) seems to be appropriate for this type of interaction.

Earlier, various bis-NHC ligands have been reported, which are usually linked by alkenic chains be wont to form chelating complexes with metals where NHC with xylene-bridge are surprisingly rare. The allylic substituent at ‘N’ of imidazole, change the metal-organic coordination mode during reaction complex which encouraging to design Pd-complex of xylene bridge allylic substituted bis-NHC ligand. 3,3'-(p-phenylenedimethylene)-bis{1-(2-methylallyl)}imidazolium bromide (L); ligand and its Palladium(II) N-hetreocyclic carbene (NHC) complex 3,3'-(p-phenylenedimethylene)-bis{1-(2-methylallyl)imidazoline}palladium(II) chloride (1) of the same ligand (L) [1] has been synthesized and characterized by Ag(I)-NHC transfer method [2]. The compounds are characterized by spectroscopic techniques and micro-analytical analysis. Single crystal X-ray structure of complex 1 has been determined, shows a square planar geometry.  The solid state structure of Pd(II)-NHC complex 1 is stabilized through different types of H-bond interface along with C-H-Π and Π-Π interactions, and build a supramolecular framework. Complex 1 possesses ring head to tail Π– Π stacking interactions (3.767Å) through imidazole rings. Theoretical  calculation (DFT) and HOMO-LUMO orbital energy has been studied to know the stability of Pd(II) complex in syn and anti-configuration and it shows that anti-configuration of complex 1 is energetically more stable than the syn-configuration.  Complex 1 shows good catalytic activity in Suzuki–Miayura [3] coupling reactions.

The concepts of nucleophilicity and protophilicity are fundamental and ubiquitous in chemistry. A case in point is bimolecular nucleophilic substitution (SN2) and base-induced elimination (E2). A Lewis base acting as a strong nucleophile is needed for SN2 reactions, whereas a Lewis base acting as a strong protophile (i.e., base) is required for E2 reactions. A complicating factor is, however, the fact that a good nucleophile is often a strong protophile. Nevertheless, a sound, physical model that explains, in a transparent manner, when an electron-rich Lewis base acts as a protophile or a nucleophile, which is not just phenomenological, is currently lacking in the literature. To address this fundamental question, the potential energy surfaces of the SN2 and E2 reactions of X−+C2H5Y model systems with X, Y = F, Cl, Br, I, and At, are explored by using relativistic density functional theory at ZORA-OLYP/TZ2P. These explorations have yielded a consistent overview of reactivity trends over a wide range in reactivity and pathways. Activation strain analyses of these reactions reveal the factors that determine the shape of the potential energy surfaces and hence govern the propensity of the Lewis base to act as a nucleophile or protophile. The concepts of “characteristic distortivity” and “transition state acidity” of a reaction are introduced, which have the potential to enable chemists to better understand and design reactions for synthesis.

We have quantum chemically studied the Lewis acid-catalyzed epoxide ring-opening reaction of cyclohexene epoxide by MeZH (Z = O, S, and NH) using relativistic dispersion-corrected density functional theory. We found that the reaction barrier of the Lewis acid-catalyzed epoxide ring-opening reactions decreases upon ascending in group 1 along the series Cs+ > Rb+ > K+ > Na+ > Li+ > H+. Our activation strain and Kohn–Sham molecular orbital analyses reveal that the enhanced reactivity of the Lewis acid-catalyzed ring-opening reaction is caused by the reduced steric (Pauli) repulsion between the filled orbitals of the epoxide and the nucleophile, as the Lewis acid polarizes the filled orbitals of the epoxide more efficiently away from the incoming nucleophile. Furthermore, we established that the regioselectivity of these ring-opening reactions is, aside from the “classical” strain control, also dictated by a hitherto unknown mechanism, namely, the steric (Pauli) repulsion between the nucleophile and the substrate, which could be traced back to the asymmetric orbital density on the epoxide. In all, this work again demonstrates that the concept of Pauli-lowering catalysis is a general phenomenon.

This work constitutes a comprehensive and improved account of electronic-structure and mechanical properties of silicon-nitride (Si3N4) polymorphs via van Leeuwen and Baerends (LB) exchange-corrected local density approximation (LDA) that enforces the exact exchange potential asymptotic behavior. The calculated lattice constant, bulk modulus, and electronic band structure of Si3N4 polymorphs are in good agreement with experimental results. We also show that, for a single electron in a hydrogen atom, spherical well, or harmonic oscillator, the LB-corrected LDA reduces the (self-interaction) error to exact total energy to ∼10%, a factor of 3 to 4 lower than standard LDA, due to a dramatically improved representation of the exchange-potential.

The knowledge of stoichiometries of alkaline-earth metal nitrides, where nitrogen can exist in polynitrogen forms, is of significant interest for understanding nitrogen bonding and its applications in energy storage. For calcium nitrides, there were three known crystalline forms, CaN 2 , Ca 2 N, and Ca 3 N 2 , at ambient conditions. In the present study, we demonstrated that there are more stable forms of calcium nitrides than what is already known to exist at ambient and high pressures. Using a global structure searching method, we theoretically explored the phase diagram of CaN x and discovered a series of new compounds in this family. In particular, we found a new CaN phase that is thermodynamically stable at ambient conditions, which may be synthesized using CaN 2 and Ca 2 N. Four other stoichiometries, namely, Ca 2 N 3 , CaN 3 , CaN 4 , and CaN 5 , were shown to be stable under high pressure. The predicted CaN x compounds contain a rich variety of polynitrogen forms ranging from small molecules (N 2 , N 4 , N 5 , and N 6) to extended chains (N ∞). Because of the large energy difference between the single and triple nitrogen bonds, dissociation of the CaN x crystals with polynitrogens is expected to be highly exothermic, making them as potential high-energy-density materials.