© 2014 American Chemical Society. A hybrid quantum mechanics/molecular mechanics (QM/MM) model and the quasiclassical trajectory (QCT) method have been combined to study the reaction of alkene methylation by methanol catalyzed by the zeolite H-MFI. The rate-limiting step of this reaction is the methylation of the alkene, and the apparent activation energy calculated at the ωB97X-D/6-31G(d,p)//ωB97X-D/6-311++G(3df,3pd) level of theory for this step agrees well with experiment and previous full QM studies. Following the ethene methylation transition state toward the products along the intrinsic reaction coordinate reveals the existence of a protonated cyclopropane (PCP+) carbocation intermediate. A similar protonated methylcyclopropane (mPCP+) carbocation intermediate is found for propene methylation. The intermediates produced during the alkene methylation reaction are metastable with a lifetime of O(1 ps) obtained from QCTs. Because of the short lifetime of these intermediates, the available energy in the carbocation is not in thermal equilibrium distribution with the zeolite lattice before subsequent reaction occurs. The qualitative difference between product distributions obtained by static and dynamic reaction pathways suggests the pathways of zeolite-catalyzed reactions proceed through high-temperature pathways that differ from the 0 K potential energy surface. The transformation of the m-PCP+ intermediate to the longer-lived secondary 2-butyl carbocation observed during QCTs suggests that more stable carbocations can properly thermalize and exist as reaction intermediates for longer than 1 ps.

Abstract Temp.‐programmed desorption spectroscopic studies show that while the chemisorption of NO is suppressed with increasing catalyst reduction temp. in a manner similar to that for H 2 and C0 chemisorption, the amount of NO adsorbed is always greater than that of H 2 or C0.

The cost of calculating nuclear hessians, either analytically or by finite difference methods, during the course of quantum chemical analyses can be prohibitive for systems containing hundreds of atoms. In many applications, though, only a few eigenvalues and eigenvectors, and not the full hessian, are required. For instance, the lowest one or two eigenvalues of the full hessian are sufficient to characterize a stationary point as a minimum or a transition state (TS), respectively. We describe here a method that can eliminate the need for hessian calculations for both the characterization of stationary points as well as searches for saddle points. A finite differences implementation of the Davidson method that uses only first derivatives of the energy to calculate the lowest eigenvalues and eigenvectors of the hessian is discussed. This method can be implemented in conjunction with geometry optimization methods such as partitioned-rational function optimization (P-RFO) to characterize stationary points on the potential energy surface. With equal ease, it can be combined with interpolation methods that determine TS guess structures, such as the freezing string method, to generate approximate hessian matrices in lieu of full hessians as input to P-RFO for TS optimization. This approach is shown to achieve significant cost savings relative to exact hessian calculation when applied to both stationary point characterization as well as TS optimization. The basic reason is that the present approach scales one power of system size lower since the rate of convergence is approximately independent of the size of the system. Therefore, the finite-difference Davidson method is a viable alternative to full hessian calculation for stationary point characterization and TS search particularly when analytical hessians are not available or require substantial computational effort.

Abstract The catalytic activity of secondary amines supported on mesoporous silica for the self‐condensation of n ‐butanal to 2‐ethylhexenal can be altered significantly by controlling the Brønsted acidity of MOH species present on the surface of the support. In this study, MOH (M=Sn, Zr, Ti, and Al) groups were doped onto the surface of SBA‐15, a mesoporous silica, prior to grafting secondary propyl amine groups on to the support surface. The catalytic activity was found to depend critically on the synthesis procedure, the nature and amount of metal species introduced and the spatial separation between the acidic sites and amine groups. DFT analysis of the reaction pathway indicates that, for weak Brønsted acid groups, such as SiOH, the rate‐limiting step is CC bond formation, whereas for stronger Brønsted acid groups, such as Ti and Al, hydrolysis of iminium species produced upon CC bond formation is the rate‐limiting step. Theoretical analysis shows further that the apparent activation energy decreases with increasing Brønsted acidity of the MOH groups, consistent with experimental observation.

Detailed mechanisms for the synthesis of p-xylene as well as the primary byproducts observed experimentally, 2,5-hexadione and 2,5-dimethyl-3-[(4-methyl-1,3-cyclohexadien-1-yl)methyl]furan, from ethylene and 2,5-dimethylfuran (DMF) mediated by H-BEA are obtained using an extended QM/MM model containing 208 tetrahedral atoms. The formation of p-xylene proceeds via Diels-Alder cycloaddition of ethylene and DMF, which is rate-limiting, followed by Brønsted acid-catalyzed dehydration. Secondary addition of DMF to the substrate following the Diels-Alder reaction leads to 2,5-dimethyl-3-[(4-methyl-1,3-cyclohexadien-1-yl)methyl]furan. The analysis of the free energies associated with the mechanisms suggests that the secondary addition can be eliminated by introducing n-heptane as an inert solvent to decrease the loading of DMF in the zeolite or by using a weak Brønsted acid site to facilitate the dehydration of the Diels-Alder product, for which the rate is determined by the deprotonation via the conjugate base of the active site. Water formed in the dehydration process can react directly with DMF to form 2,5-hexadione, thereby decreasing the yield of p-xylene. However, the free-energy barriers for the formation of 2,5-heaxdione compared to the Diels-Alder reaction indicate that DMF and 2,5-hexadione will be equilibrated. Therefore, the 2,5-hexadione yield can be minimized by operating at a high conversion of DMF.

Anheften eines geeigneten “Türsteher”-Moleküls an den Rand einer Graphenpore verhindert das Passieren eines nicht erwünschten Enantiomers, während dessen Spiegelbild durchgelassen wird. In der Zuschrift auf S. 10117 ff. berichten A. W. Hauser, P. Schwerdtfeger et al., dass ein kleiner Unterschied in der Geometrie des temporären Dimerkomplexes, der vom “Türsteher” und dem durchtretenden Molekül gebildet wird, in einem deutlichen Unterschied der Durchtrittsbarriere resultiert.

Abstract The strengths of H 2 and CO chemisorption (by TPD) and the catalytic activity of Pd/SiO2 for hydrogenation of CO to CH 4 have been investigated as a function of dispersion.

Abstract Upon several tested sulfonic acid, para‐toluene sulfonic acid shows the highest reactivity in the reaction of furfural (I) and 2‐methylfuran (II).

Abstract Whether annual evapotranspiration of native ecosystems is increasing or decreasing with time as CO 2 concentrations are rising, the climate is warming and rainfall experiences booms and busts, remains an unanswered question in the field of global change biology. To answer this question, we measured evapotranspiration and carbon dioxide exchange over and under an oak savanna and over an annual grassland in the Mediterranean climate of California, USA, from 2001 through 2019 with the eddy covariance method; during this 19‐year period, CO 2 rose 40 ppm, air temperature increased by 1°C and annual rainfall ranged between 133 and 890 mm/year. No temporal trend in evapotranspiration or water use efficiency was observed over this time duration. Many competing positive and negative feedbacks among stomatal sensitivity to carbon dioxide concentrations, soil moisture, and vapor pressure deficit, the impact of temperature on saturation vapor pressure and access to groundwater muted the response of evapotranspiration to its changing world when integrated to the ecosystem scale and annual time steps. At the intra‐annual time scale, we found that plants transmit information on soil moisture status through their influence on the vapor pressure deficit of the atmospheric boundary layer. The inter‐annual variations in evaporative water use by the savanna and annual grassland were relatively decoupled from the booms and busts in rainfall. Instead, variations in length of growing season and access to groundwater explained much of this year‐to‐year variation in annual evapotranspiration. The access of groundwater by the oak savanna may make these ecosystems more robust in a warmer world, than was previously thought. This is a scale emergent property that needs better consideration in coupled climate‐ecosystem models.

By means of /sup 29/Si NMR spectroscopy, it is established that the distribution of silicate anions in alkaline silicate solutions is a moderate function of base composition. At a fixed SiO/sub 2/ concentration and silicate ratio, the proportion of Si present in oligomeric and cage-like structures increases in progressing from Li to Cs hydroxide. Interactions between alkali metal cations and silicate anions are investigated using NMR spectroscopy of the cations; in this way the concentration of ion pairs is measured as a function of cation size. As a result the silicate redistribution is ascribed to cation-silicate anion pairing and to a higher selectivity for ion pairing by large silicate anions as cation size increases. 19 refs., 4 figs.

This is the FLUXNET-CH4 version of the carbon flux data for the site US-Sne Sherman Island Restored Wetland.