RESUMO
A systematic mechanistic survey was performed for the CH3OH + OH reaction on ice. ONIOM(ωB97X-D/Def2-TZVP:AMOEBA09) calculations suggested a range of binding energies for the CH2OH radical (0.29-0.69 eV) and CH3OH (0.15-0.72 eV) molecule on hexagonal water ice (Ih) and amorphous solid water (ASW). Computed average binding energies of CH2OH radical (0.49 eV) and CH3OH (0.41 eV) are relatively stronger compared to the CH3O radical binding energies (0.32 eV, Sameera et al., J. Phy. Chem. A, 2021, 125, 387-393). Thus, the CH3OH molecule, CH2OH and CH3O radicals can adsorb on ice, where the binding energies follow the order CH2OH > CH3OH > CH3O. The multi-component artificial force-induced reaction (MC-AFIR) method systematically determined the reaction mechanisms for the CH3OH + OH reaction on ice, where two reaction paths, giving rise to CH2OH and CH3O radicals, were confirmed. A range of reaction barriers, employing the ωB97X-D/Def2-TZVP level of theory, was found for each reaction (0.03-0.11 eV for CH2OH radical formation, and 0.03-0.44 eV for CH3O radical formation). Based on the lowest energy reaction paths, we suspect that both reactions operate on ice. The computed data in this study evidence that the nature of the binding site or the reaction site has a significant effect on the computed binding energies or reaction barriers. Thus, the outcomes of the present study will be very useful for the computational astrochemistry community to determine reliable binding energies and reaction barriers on ice.
RESUMO
The existence of molecular orientational order in nanometer-thick films of molecules has long been implied by surface potential measurements. However, direct quantitative determination of the molecular orientation is challenging, especially for metastable amorphous thin films at low temperatures. This study quantifies molecular orientation in amorphous N2O at 6 K using infrared multiple-angle incidence resolution spectrometry (IR-MAIRS). The intensity ratio of the weak antisymmetric stretching vibration band of the 14N15NO isotopomer between the in-plane and out-of-plane IR-MAIRS spectra provides an average molecular orientation angle of 65° from the surface normal. No discernible change is observed in the orientation angle when a different substrate material is used (Si and Ar) at 6 K or the Si substrate temperature is changed in the range of 6-14 K. This suggests that the transient mobility of N2O during physisorption is key in governing the molecular orientation in amorphous N2O.
RESUMO
The surface of most aerial plant organs is covered with the cuticle, a membrane consisting of a variety of organic compounds, including waxes, cutin (a polyester) and polysaccharides. The cuticle serves as the multifunctional interface between the plant and the environment, and plays a major role in protecting plants against various environmental stress factors. Characterization of the molecular arrangements in the intact cuticle is critical for the fundamental understanding of its physicochemical properties; however, this analysis remains technically challenging. Here, we describe the nondestructive characterization of the intact cuticle of Brassica oleracea L. leaves using polarization modulation-infrared (IR) reflection-absorption spectroscopy (PM-IRRAS). PM-IRRAS has a probing depth of less than several hundreds of nanometers, and reveals the crystalline structure of the wax covering the cuticle surface (epicuticular wax) and the nonhydrogen-bonding character of cutin. Combined analysis using attenuated total reflection-IR spectra suggested that hemicelluloses xylan and xyloglucan are present in the outer cuticle region close to the epicuticular wax, whereas pectins are dominant in the inner cuticle region (depth of ≤2 µm). PM-IRRAS can also determine the average orientation of the cuticular molecules, as indicated by the positive and negative spectral peaks. This unique advantage reveals the orientational order in the intact cuticle; the hydrocarbon chains of the epicuticular wax and cutin and the backbones of hemicelluloses are oriented perpendicular to the leaf surface. PM-IRRAS is a versatile, informative and easy-to-use technique for studying plant cuticles because it is nondestructive and does not require sample pretreatment and background measurements.
