Negative Thermal Expansion Induced in Tri-graphene and T‑graphene by the Rigid-Unit Modes

Materials which contract on heating (negative thermal expansion, NTE) are of significant interest for advanced applications. Graphene has shown NTE up to 1000 K, which motivates further improvements in two-dimensional carbon to attain superior performance. In this Communication, very large negative thermal expansion coefficients (αT) are reported for tri-graphene (TrG) and T-graphene (TG). Quasi-harmonic approximation calculations show that αT remains negative until 4200 K and 2900 K for TrG and TG, respectively. The high NTE for these systems is understood on the basis of the soft phonon modes, which induce rotation of the 3-membered and the 4-membered rings in TrG and TG, respectively, and ab initio molecular dynamics simulations. The local distortions for the 3–12 rings (in TrG) and 4–8 rings (in TG) have structural resemblance with the rigid-unit modes that are usually envisioned for bulk systems

Metal Free Azide–Alkyne Click Reaction: Role of Substituents and Heavy Atom Tunneling

Metal free click reactions provide an excellent noninvasive tool to modify and understand the processes in biological systems. Release of ring strain in cyclooctynes on reaction with azides on the formation of triazoles results in small activation energies for various intermolecular Huisgen reactions (<b>1</b>–<b>9</b>). Substitution of difluoro groups at the α, α′ position of the cyclooctyne ring enhances the rates of cycloadditions by 10 and 20 times for methyl azide and benzyl azide respectively at room temperature. The computed rate enhancement on difluoro substitution using direct dynamical calculations using the canonical variational transition state theory (CVT/CAG) with small curvature tunneling (SCT) corrections are in excellent agreement with the experimental results. For the intramolecular click reaction (<b>10</b>) notwithstanding its much higher activation energy, quantum mechanical tunneling (QMT) enhances the rate of cycloaddition significantly and increases the N¹⁴/N¹⁵ primary kinetic isotope effect at 298 K. QMT is shown to be rather efficient in <b>10</b> due to a thin barrier of ∼2.4 Å. The present study shows that tunneling effects can be significant for intramolecular click reactions

Negative Thermal Expansion Induced in Tri-graphene and T‑graphene by the Rigid-Unit Modes

Materials which contract on heating (negative thermal expansion, NTE) are of significant interest for advanced applications. Graphene has shown NTE up to 1000 K, which motivates further improvements in two-dimensional carbon to attain superior performance. In this Communication, very large negative thermal expansion coefficients (αT) are reported for tri-graphene (TrG) and T-graphene (TG). Quasi-harmonic approximation calculations show that αT remains negative until 4200 K and 2900 K for TrG and TG, respectively. The high NTE for these systems is understood on the basis of the soft phonon modes, which induce rotation of the 3-membered and the 4-membered rings in TrG and TG, respectively, and ab initio molecular dynamics simulations. The local distortions for the 3–12 rings (in TrG) and 4–8 rings (in TG) have structural resemblance with the rigid-unit modes that are usually envisioned for bulk systems

Negative Thermal Expansion Induced in Tri-graphene and T‑graphene by the Rigid-Unit Modes

Materials which contract on heating (negative thermal expansion, NTE) are of significant interest for advanced applications. Graphene has shown NTE up to 1000 K, which motivates further improvements in two-dimensional carbon to attain superior performance. In this Communication, very large negative thermal expansion coefficients (αT) are reported for tri-graphene (TrG) and T-graphene (TG). Quasi-harmonic approximation calculations show that αT remains negative until 4200 K and 2900 K for TrG and TG, respectively. The high NTE for these systems is understood on the basis of the soft phonon modes, which induce rotation of the 3-membered and the 4-membered rings in TrG and TG, respectively, and ab initio molecular dynamics simulations. The local distortions for the 3–12 rings (in TrG) and 4–8 rings (in TG) have structural resemblance with the rigid-unit modes that are usually envisioned for bulk systems

Supported Sub-Nanometer Gold Cluster Catalyzed Transfer Hydrogenation of Aldehydes to Alcohols

The ability of subnanometer sized Au-clusters to activate small molecule is well-known. Nevertheless, typical experimental situations involve loading of the bare Au_<i>n</i> (<i>n</i> < 10) on oxide surfaces. Recent plethora of literature indicate that supported gold clusters are extremely potent in catalyzing molecular transformation. In this work, we examine the role of the supporting substrate namely TiO₂(110) in enhancing the activity of a small Au₈ cluster for an industrially important reaction, namely, conversion of benzaldehyde into benzyl alcohol. The barrier for rate limiting C–H bond activation of the solvent/H-donor gets reduced by ∼4 kcal/mol over a TiO₂(110) surface with respect to its unsupported analogue. The activation energy (<i>E</i>_a) for the catalytic transfer hydrogenation (CTH) involving transfer of two hydrogens, one each from the solvent and the hydrogenated Au-cluster simultaneously to the aldehyde is also reduced significantly. Strong Au₈–TiO₂(110) interactions result in charge transfer thereby making Au-cluster electron deficient which assists by activating the rate limiting C–H bond cleavage. The present article provides a microscopic picture for catalysis of synthetically important complex reactions by supported Au-clusters

Topological Insulator in Two-Dimensional SiGe Induced by Biaxial Tensile Strain

Strain-engineered two-dimensional (2D) SiGe is predicted to be a topological insulator (TI) based on first-principle calculations. The dynamical and thermal stabilities were ascertained through phonon spectra and ab initio molecular dynamics simulations. 2D SiGe remains dynamically stable under tensile strains of 4 and 6%. A band inversion was observed at the Γ-point with a band gap of 25 meV for 6% strain due to spin–orbit coupling interactions. Nontrivial of the TI phase was determined by its topological invariant (υ = 1). For SiGe nanoribbon with edge states, the valence band and conduction band cross at the Γ-point to create a topologically protected Dirac cone inside the bulk gap. We found that hexagonal boron nitride (h-BN) with high dielectric constant and band gap can be a very stable support to experimentally fabricate 2D SiGe as the h-BN layer does not alter its nontrivial topological character. Unlike other heavy-metal-based 2D systems, because SiGe has a sufficiently large gap, it can be utilized for spintronics and quantum spin Hall-based applications under ambient condition

Negative Thermal Expansion Induced in Tri-graphene and T‑graphene by the Rigid-Unit Modes

Materials which contract on heating (negative thermal expansion, NTE) are of significant interest for advanced applications. Graphene has shown NTE up to 1000 K, which motivates further improvements in two-dimensional carbon to attain superior performance. In this Communication, very large negative thermal expansion coefficients (αT) are reported for tri-graphene (TrG) and T-graphene (TG). Quasi-harmonic approximation calculations show that αT remains negative until 4200 K and 2900 K for TrG and TG, respectively. The high NTE for these systems is understood on the basis of the soft phonon modes, which induce rotation of the 3-membered and the 4-membered rings in TrG and TG, respectively, and ab initio molecular dynamics simulations. The local distortions for the 3–12 rings (in TrG) and 4–8 rings (in TG) have structural resemblance with the rigid-unit modes that are usually envisioned for bulk systems

Negative Thermal Expansion Induced in Tri-graphene and T‑graphene by the Rigid-Unit Modes

Materials which contract on heating (negative thermal expansion, NTE) are of significant interest for advanced applications. Graphene has shown NTE up to 1000 K, which motivates further improvements in two-dimensional carbon to attain superior performance. In this Communication, very large negative thermal expansion coefficients (αT) are reported for tri-graphene (TrG) and T-graphene (TG). Quasi-harmonic approximation calculations show that αT remains negative until 4200 K and 2900 K for TrG and TG, respectively. The high NTE for these systems is understood on the basis of the soft phonon modes, which induce rotation of the 3-membered and the 4-membered rings in TrG and TG, respectively, and ab initio molecular dynamics simulations. The local distortions for the 3–12 rings (in TrG) and 4–8 rings (in TG) have structural resemblance with the rigid-unit modes that are usually envisioned for bulk systems

Understanding of the Buckling Distortions in Silicene

Silicene, the all Si analogue of graphene is structurally different due to the presence of buckling distortions in the individual six membered rings. The sufficiently strong coupling between the unoccupied molecular orbitals (UMOs) with occupied molecular orbitals (OMOs) leads to pseudo-Jahn–Teller distortion (PJT) and the characteristic buckling in silicenes. σ–π separation analyses reveal that the σ-backbone gets stabilized, whereas the π-backbone is destabilized due to buckling. However, the stabilization of puckering σ-backbone overwhelms the π-backbone destabilization. This is exactly opposite to that of graphene. The cations like Li⁺ can suppress the PJT distortions resulting in a planar structure. This leads to opening of band gap (∼1.62 eV). Si substituted benzenes binds more strongly with Li⁺ than benzene. The mutual competition/synergy between the orbital interactions of the ring with the cation and the π-charge density across the surface of molecule governs the stability of these complexes

Negative Thermal Expansion Induced in Tri-graphene and T‑graphene by the Rigid-Unit Modes

Materials which contract on heating (negative thermal expansion, NTE) are of significant interest for advanced applications. Graphene has shown NTE up to 1000 K, which motivates further improvements in two-dimensional carbon to attain superior performance. In this Communication, very large negative thermal expansion coefficients (αT) are reported for tri-graphene (TrG) and T-graphene (TG). Quasi-harmonic approximation calculations show that αT remains negative until 4200 K and 2900 K for TrG and TG, respectively. The high NTE for these systems is understood on the basis of the soft phonon modes, which induce rotation of the 3-membered and the 4-membered rings in TrG and TG, respectively, and ab initio molecular dynamics simulations. The local distortions for the 3–12 rings (in TrG) and 4–8 rings (in TG) have structural resemblance with the rigid-unit modes that are usually envisioned for bulk systems