RESUMO
Metal-organic frameworks (MOFs) have emerged as promising tailor-designed materials for developing next-generation solid-state devices with applications in linear and nonlinear coherent optics. However, the implementation of functional devices is challenged by the notoriously difficult process of growing large MOF single crystals of high optical quality. By controlling the solvothermal synthesis conditions, we succeeded in producing large individual single crystals of the noncentrosymmetric MOF Zn(3-ptz)2 (MIRO-101) with a deformed octahedron habit and surface areas of up to 37 mm2. We measured the UV-vis absorption spectrum of individual Zn(3-ptz)2 single crystals across different lateral incidence planes. Millimeter-sized single crystals have a band gap of E g = 3.32 eV and exhibit anisotropic absorption in the band-edge region near 350 nm, whereas polycrystalline samples are fully transparent in the same frequency range. Using solid-state density functional theory (DFT), the observed size dependence in the optical anisotropy is correlated with the preferred orientation adopted by pyridyl groups under conditions of slow crystal self-assembly. Our work thus paves the way for the development of optical polarization switches based on metal-organic frameworks.
RESUMO
Reactions involving C and N play an essential role in the chemistry around the surface of a hypersonic spacecraft during its atmospheric re-entry. The collision of CN with other molecules and atoms has particular interest in aerothermodynamic modeling. This work focuses on the study of the CN + N â N2 + C reaction in the triplet manifold 3Aâ³ of CN2. A high-level full-dimensional potential energy surface for this system is developed from ab initio calculations at the MRCI-F12 + Q level of theory. This surface is employed in quasiclassical trajectory calculations, and thermal rate coefficients from 100 to 20,000 K are computed. The rates for the formation of N2 are compared with the available experimental data, and good agreement is found. At low and intermediate temperatures, the N2 formation is more efficient than the N-exchange process, while at high temperatures, the rates for both processes are comparable. Finally, analytically modified Arrhenius expressions for the reaction rates of N2 formation and N-exchange are reported.
RESUMO
The HOC+ molecule has for long been detected in several regions of the interstellar medium (ISM). The collisional ro-vibrational rate coefficients of this molecule with the most common colliders in the ISM are then required for applying nonlocal thermal equilibrium models. However, this molecule has a low bending frequency (249 cm-1), and the use of the rigid rotor approximation is therefore limited to low collision energies. Also, the complete determination of the ro-vibrational rate coefficients of HOC+ in collision with He requires including the bending motion in the analytical model of the potential energy surface (PES) of the system. The first goal of this work is then to develop the first rigid bender four-dimensional PES for the interaction between HOC+ and He. To this aim, a large grid of ab initio energies are computed at the CCSD(T)-F12b/aug-cc-pVQZ level of theory and an analytical representation of the PES is obtained using a combination of least square and reproducing kernel Hilbert space procedures. The global minimum of this PES is found to be reached for a linear configuration of the complex. In the second part of this study, rigid rotor close-coupling calculations are performed at low collision energy, and the calculated rate coefficients are compared to those previously determined for the collisions of He with its HCO+ isomer.