Adsorption substituent effects

First-principle quantum chemical methods have advanced to the stage where they can now offer qualitative, as well as, quantitative predictions of structure and energetics for adsorbates on surfaces. Cluster and periodic density functional quantum chemical methods are used to analyze chemisorption and catalytic surface reactivity for a series of relevant commercial chemistries. DFT-predicted adsorption and overall reaction energies were found to be within 5 kcal/mol of the experimentally known values for all systems studied. Activation barriers were over-predicted but still within 10 kcal/mol. More specifically we examined the mechanisms and reaction pathways for hydrocarbon C-H bond activation, vinyl acetate synthesis, and ammonia oxidation. Extrinsic phenomena such as substituent effects, bimetallic promotion, and transient surface precursors, are found to alter adsorbate-surface bonding and surface reactivity. 

For small solutes, much study has centered on the generation of data for classes of compounds [596]. The retention behavior of 38 alkyl-substituted benzenes was studied on silica and alumina columns (A = 200-300 nm) using hexane as the eluent. A strong orrto-substituent effect was noted. This effect is common in adsorption chromatography and will be discussed again below. Retention, given as logk values, was tabulated for all compounds used in the study. 

Less ambiguous conclusions can be reached if the polarographic characteristics obtained under conditions which exclude adsorption and the effect of protolytic equilibria are compared. We therefore carried out such an analysis for the half-wave potential of the first wave formed by these compounds in the polarography of dimethyl-formamide solutions. In fact, under these conditions, we were able to observe the transmission of the substituent effect in the first series, which in accordance with the views expressed earlier indicated decrease of conjugation (p = +0.1). The polarographic data can thus be explained if a nonplanar conformation of the hydrazone molecules is adopted. 

Selective reduction of dienes may be influenced by the substituents, which can change the substrate orientation during adsorption on the catalyst surface (equation 65)158. It has to be mentioned that this effect worked only if low amounts of catalyst were used at higher amounts the selectivity decreased. 

The results of the alkylbenzene series may also be readily explained in terms of ir complex adsorption. In this series, the molecular orbital symmetry of individual members remains constant while the ionization potential, electron affinity, and steric factors vary. Increased methyl substitution lowers the ionization potential and consequently favors IT complex adsorption. However, this is opposed by the accompanying increase in steric hindrance as a result of multiple methyl substitution, and decrease in electron affinity (36). From previous data (Tables II and III) it appears that steric hindrance and the decreased electron affinity supersede the advantageous effects of a decreased ionization potential. The results of Rader and Smith, when interpreted in terms of tt complex adsorption, show clearly the effects of steric hindrance, in that relative adsorption strength decreases with increasing size, number, and symmetry of substituents. 

The old and lasting problem of heterogeneous catalysis, the mechanism of alkene hydrogenation, has also been approached from the viewpoint of structure effects on rate. In 1925, Lebedev and co-workers (80) had already noted that the velocity of the hydrogenation of the C=C bond decreases with the number of substituents on both carbon atoms. The same conclusion can be drawn from the narrower series of alkenes studied by Schuster (8J) (series 52 in Table IV). Recently authors have tried to analyze this influence of substituents in a more detailed way, in order to find out whether the change in rate is caused by polar or steric effects and whether the substituents affect mostly the adsorptivity of the unsaturated compounds or the reaetivity of the adsorbed species. Linear relationships have been used for quantitative treatment. 

A similar system, (CH3)2C=CH X, was studied by Endrysova and Kraus (55) in the gas phase in order to eliminate the possible leveling influence of a solvent. The rate data were separated in the contribution of the rate constant and of the adsorption coefficient, but both parameters showed no influence of the X substituents (series 61). A definitive answer to the problem has been published by Kieboom and van Bekum (59), who measured the hydrogenation rate of substituted 2-phenyl-3-methyl-2-butenes and substituted 3,4-dihydro-1,2-dimethylnaphtalenes on palladium in basic, neutral, and acidic media (series 62 and 63). These compounds enabled them to correlate the rate data by means of the Hammett equation and thus eliminate the troublesome steric effects. Using a series of substituents with large differences in polarity, they found relatively small electronic effects on both the rate constant and adsorption coefficient. 

The conclusions on the mechanism of the double bond hydrogenation on metallic catalysts can be summarized as follows (1) with respect to structure effects on rate, all transition metals behave similarly (2) the reactivity of the unsaturated compounds is governed mostly by the number and size of the substituents on the carbon atoms of the double bond through their influence on adsorptivity (3) the electronic nature of the substituents plays a minor if any role. 

Vasudevan, D., and A. T. Stone, Adsorption of catechols, 2-aminophenols, and 1,2-phenylenediamines at the metal (hydr)oxide-water interface Effects of ring substituents on the adsorption onto Ti02 , Environ. Sci. Technol., 30, 1604-1613 (1996). 

Two extensive sets of data on the reactivity of various amines in deamination and disproprotionation on alumina give an insight into the influence of structure on rate [149,152]. However, the picture is complicated by different effects of the number and nature of the alkyl substituents on the reaction rate coefficient and on the adsorption coefficient. 

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