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The uncoupled anharmonic OH-stretching vibrational frequency for the layered mineral Mg[(OH).sub.2] (brucite) has been calculated in the pressure range 0-22 GPa. Quantum-mechanical electronic structure (DFT) calculations were performed, followed by quantum-mechanical vibrational energy calculations. The following findings emerged: (1) The calculated dv(OH)/dP slope is -4 [cm.sup.-1]/GPa, in agreement with the experimental literature value [taken as the average between the Raman and IR-measured slopes for Mg[(OH).sub.2]]. (2) The calculated v(OH) vs. R(O***O) correlation is linear and the slope is much smaller than that of traditional H-bond correlation curves in the literature. (3) The main origin of the small dv/dP and dv/dR(O***O) slopes is the small electric field variation as the mineral layers are pressed toward each other. (4) At high pressure, the OH- ions show s[O.sup.m]e tendency to be tilted with respect to the c axis, and a larger tilt angle leads to a larger v(OH) downshift. (5) The pressure variation of the D quadrupole coupling constant is approximately -1 kHz/GPa. Keywords: Brucite, anharmonic OH frequencies, high pressure, electric field, hydrogen bonding, correlation curve, hydrous mineral DOI: 10.2138/am.2009.3188

Growth of large-area, few-layer graphene has been reported recently through the catalytic decomposition of methane (CH4) over a Cu surface at high temperature. In this study, we used ab initio calculations to investigate the minimum energy pathways of successive dehydrogenation reactions of CH4 over the Cu (111) surface. The geometries and energies of all the reaction intermediates and transition states were identified using the climbing image nudged elastic band method. The activation barriers for CH4 decomposition over this Cu surface are much lower than those in the gas phase; furthermore, analysis of electron density differences revealed significant degrees of charge transfer between the adsorbates and the Cu atoms along the reaction path; these features reveal the role of Cu as the catalytic material for graphene growth. All the dehydrogenation reactions are endothermic, except for carbon dimer (C2) formation, which is, therefore, the most critical step for subsequent graphene growth, in particular, on Cu (111) surface. [ABSTRACT FROM AUTHOR]

A two-dimensional quantum-mechanical vibrational model has been used to calculate the anharmonic OH vibrational frequencies in the layered Mg(OH)2 (brucite) crystal. The underlying potential energy surface was generated by density functional theory (DFT) calculations. The resulting OH frequencies are upshifted (blueshifted) by about +75 cm-1 with respect to the gas-phase OH frequency (+120 cm-1 in experiments; the discrepancy is mainly due to inadequacies in the DFT and pseudopotential models). The Raman-IR split is about 50 cm-1, both in the calculations and in experiments. We find that the blueshift phenomenon in brucite can qualitatively be explained by a parabolalike “OH frequency versus electric field” correlation curve pertaining to an OH- ion exposed to an electric field. We also find that it is primarily the neighbors within the Mg(OH)2 layer that induce the blueshift while the interlayer interaction gives a smaller (and redshifting) contribution. [ABSTRACT FROM AUTHOR]

Anharmonic vibrational frequencies for the Raman-active (A1g) and the IR-active (A2u) modes have been calculated for the LiOH crystal within a plane-wave density functional theory (DFT) framework. We find that a two-dimensional quantum-mechanical vibrational approach, allowing for anharmonic coupling between symmetric and antisymmetric OH stretching modes, produces OH frequencies—both absolute frequencies and gas-to-solid frequency shifts—in good agreement with experiment. Remaining errors in the absolute frequencies are largely a consequence of the DFT model chosen. A one-dimensional normal-mode following vibrational treatment, on the other hand, fails to reproduce both absolute anharmonic frequencies and gas-to-solid frequency shifts. [ABSTRACT FROM AUTHOR]

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