Contacts of the type C-H‚‚‚X where X ) F, Cl, Br, I, O, S, or N were analyzed statistically using structures available in the Cambridge Structural Database. For this analysis, the cone correction method was extended to an isotropic density correction. This method corrects for both angular and distance effects by normalizing isotropic contact densities from the experimental distributions. It was possible to show the widespread occurrence of C-H‚‚‚X interactions in the crystalline state using this method. Distances of highest incidence, at which the largest number of contacts were sampled after normalization of isotropic distribution, were determined for each interaction type. These distances were compared to the classically accepted van der Waals cutoff radii, often used to distinguish between hydrogen-bonded interactions and van der Waals contacts. Generally, distances of highest incidence did not occur at the sum of van der Waals cutoff radii but occurred either below or above that distance depending on the specific interaction analyzed. This methodology, while of course not yielding quantitative data on hydrogen bond strength, allowed for a qualitative ordering of the interactions studied, in terms of expected interaction strengths.
The successful [Ph 2 PN( i Pr)PPh 2 ]Cr-catalyzed trimerization and tetramerization of ethylene to 1-hexene and 1-octene requires the presence of a cocatalyst, of which methylaluminoxane (MAO) is particularly relevant. Density functional theory (DFT) calculations are reported on the interaction of various MAO models with chromacycloheptane intermediates. Chromacycloheptanes are well established to be important intermediates during the selective chromium-catalyzed trimerization and tetramerization of ethylene, effectively resembling appropriate models for a study of MAO interaction with chromium complexes during active catalysis. A systematic study is presented evaluating different (AlOMe) n cage structure models for MAO, as both "classic" MAO cages and cages activated by interaction with trimethylaluminium (TMA), comparing methylation aptitudes of TMA versus MAO models and evaluating the interaction of MAO models with chromacycloheptane intermediates. From the results the importance of the use of realistic ligand and large MAO models is shown to be a prerequisite for obtaining accurate catalyst activation data. In particular, use of a "stripped-down" ligand [Me 2 PN(Me)PMe 2 ] on chromacycloheptane in combination with a relatively small MAO cage [(AlOMe) 6 ‚(AlMe 3 )] results in the optimization of formally coordinated chromacycloheptane-MAO complexes, even with increased steric congestion on chromium upon coordination of an additional ethylene moiety. In contrast, use of the "full" ligand [Ph 2 PN( i Pr)PPh 2 ] and larger MAO cages [(AlOMe) 9 ‚(AlMe 3 )] shows that while the formation of formally coordinated chromacycloheptane-MAO complexes are successfully optimized in the absence of additional ethylene, only dissociated ion-pair complexes are present when an additional ethylene molecule is introduced. From these results important insight is gained on the role of MAO during catalysis, as well as the model requirements for both MAO and chromium complexes to conduct fundamental theoretical studies in selective chromium-catalyzed ethylene oligomerization.
Fischer–Tropsch synthesis of hydrocarbons from CO and H2 is an established industrial process, during which the C–O bond must break. The preferred mechanism and sites at which CO is activated and hydrocarbon products are formed remains under debate. Density functional theory calculations are used to investigate direct and H-assisted dissociation of CO at kink and step sites on FCC Co(321) and Co(221), at which direct dissociation is demonstrated to be intrinsically preferred at low coverage. The CO dissociation rate is predicted to be higher at defect sites than on extended facets on FCC Co, with dissociation at the kink site having the highest relative rate of all FCC Co site types predicted to be exposed on FCC Co nanocrystals. Nevertheless, the increase in activity is not sufficient to exceed that of highly active HCP Co sites. However, the intrinsically preferred mechanism of CO activation at low coverage, direct rather than H assisted, is concluded to be the same on FCC and HCP Co.
Density functional theory calculations have been used to investigate CO adsorption on three surface terminations of -Fe 5 C 2 in the presence of carbon vacancy sites. CO did not show a strong energetic preference for a particular adsorption site on each surface, since similar adsorption energies were obtained for structurally distinct adsorption configurations. In addition, it was found that the adsorption of CO in a vacancy site is not necessarily more favorable energetically, compared with adsorption in alternative Fe sites. The presence of a subsurface carbon atom directly below a 4-fold site was found to inhibit or significantly destabilize adsorption of CO in that site. The role of step sites in activating CO has been investigated by comparing the calculated adsorption energies, structural properties, and vibrational stretching frequencies of CO adsorbed in equivalent sites in the presence or absence of steps. Coordination of CO to the surface through both ends of the molecule was associated with a lengthening of the C-O bond and a red-shift of the C-O stretching frequency, and such geometries were readily obtained for adsorption at the bottom of a step. Activation energies were calculated for the dissociation of CO initially adsorbed in a vacancy site in the presence and absence of steps. The step sites were found to lower the activation energy by at least 0.3 to 0.6 eV, without destabilizing the initial state.
A multi-site microkinetic model for the Fischer-Tropsch synthesis (FTS) reaction up to C products on a FCC cobalt catalyst surface is presented. This model utilizes a multi-faceted cobalt nanoparticle model for the catalyst, consisting of the two dominant cobalt surface facets Co(111) and Co(100), and a step site represented by the Co(211) surface. The kinetic parameters for the intermediates and transition states on these sites were obtained using plane-wave, periodic boundary condition density functional theory. Using direct DFT data as is, the microkinetic results disagree with the expected experimental results. Employing an exploratory approach, a small number of microkinetic model modifications were tested, which significantly improved correspondence to the expected experimental results. Using network flux and sensitivity analysis, an in-depth discussion is given on the relative reactivity of the various sites, CO activation mechanisms, the nature of the reactive chain growth monomer, the probable C formation mechanism, the active site ensemble interplay and the very important role of CO* surface coverage. The findings from the model scenarios are discussed with the aim of guiding future work in understanding the FTS mechanism and subsequent controlling kinetic parameters.
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