Ab initio molecular-orbital calculations using the 3-21 G basis set are performed to study the protonation reaction of propylene and isobutene by zeolite bridged hydroxyls ZOH, which are simulated by a cluster model which consists of two Si tetrahedra and one A1 tetrahedron. We have extended this study to include clusters with different compositions by introducing B and Ga in TI1' positions. The calculations show that in all reactions the adsorption of the olefin molecule on the acidic OH group takes place, leading to a stable r-complex with a structure similar to those of the isolated olefin and clusters constituents. The r-complex is transformed into a zeolite-alkoxide of covalent characteristics. The surface alkoxides are the most stable structures with reaction energies in the range of -1 5 to -1 7 kcal/mol. These reactions take place through transition states whose organic fragment has a geometry and charge distribution resembling those of the 2-propyl and tert-butyl classical carbenium ions for zeolite-propylene and -isobutene, respectively. The obtained activation energies (30-40 kcal/mol) are of the same order of magnitude in all the reactions considered. The bifunctional mechanism of the reactions is rather complicated and implies a concerted process involving the proton transfer from the zeolite toward a carbon of the olefin double bond, and the simultaneous C-O bond formation at the adjacent oxygen on the zeolite structure. The reaction mechanism and the properties of the transition states are practically the same, irrespective of the framework TI1' atom and the increase in propyleneisobutene branching. It is then proposed that the geometric conformation of transition state can be more determinant than the chemical composition of the zeolite. In this respect, the flexibility of the zeolite structure plays an important role. IntroductionThe use of zeolites as acidic catalysts has raised interest in the structure and properties of their active sites.t-s The sources of Brbnsted acidity in zeolites are bridged hydroxyls which arise from the presence of A1 or more generally TI1' atoms replacing Si in their structure (see Scheme I).Its is generally accepted that interaction of olefins with strong acid hydroxyls in zeolites results in the formation of adsorbed carbenium ions and their evolution can explain the mechanism of the reactions catalyzed by solid acids such as cracking, isomerization, and alkylation.Ig6 This mechanism is apparently indistinguishable from that proposed on homogeneous acid catalysts, since in the formal mechanisms, the solid acid catalysts are not considered explicitly, and therefore their influence on the formation and reactivity of the carbocations is not considered.Kazansky et al.'-I2 have shown that this traditional point of view is too simplified. Their ab initio quantum chemical calculation of ethylene interaction with zeolite OH groups, simulated by the simplest cluster HO(H)AI(OH)2OH, and the semiempirical study of the electronic structure and geometry of different surface alko...

We present a theoretical investigation of the electronic structure of oligorylenes (from perylene to heptarylene, including also the naphthalene molecule) and their corresponding polymer poly(peri-naphthalene) (PPN) using the nonempirical valence effective (VEH) method. The geometry of the unit cell used to generate the polymer is extrapolated from the PM3-optimized molecular geometries of the longest oligorylenes. That geometry shows some bond alternation along the perimeter carbon chains and a bond length of ≊1.46 Å is calculated for the peri bonds connecting the naphthalene units. The VEH one-electron energy level distributions calculated for oligorylenes are used to interpret the experimental trends reported for the first ionization potentials, redox potentials, and lowest energy optical transitions. An excellent agreement is found between theoretical estimates and experimental values. The VEH band structure calculated for an isolated chain of PPN is interpreted in terms of the molecular orbitals of naphthalene. The ionization potential, electron affinity, and bandwidths obtained for PPN suggest a large capacity to form conducting p- or n-type materials. The small band gap of 0.56 eV predicted for PPN from VEH band structure calculations is in good agreement with theoretical and experimental estimates calculated by extrapolating the data reported for the oligomers.

4853' b 4: IEO-AI b-IEo+hl -+ U -Eo Figure 11. The dynamics underlying the solvent time scale function +(z) (eq 111-19 and . We start with a fluctuation of the solvation coordinate at the transition state (curve crossing) U = -Eo. If the system is in the la) state (R, and R4), this fluctuation will relax to S,(U) with a characteristic time scale s [ ( E o + A)/A]. If the system is in the Ib) state (R, and Rz), it will relax to S,(U) and the characteristic time scale7 [ ( E 0 -X)/A] is thus the average time it takes for a solvent fluctuation at the transition state (U = -Eo) to relax to thermal equilibrium in the state Ib), whereas 7[(E0 + X)/A] is the average time it takes for the same fluctuation to relax to thermal equilibrium in the state la). This is represented schematically in Figure 11. If these times are fast, the fourth-order contribution to the rate vanishes and the rate is adiabatic. The transition from the nonadiabatic to the adiabatic limit is therefore a result of the finite relaxation time of the solvent which results in a change of the distribution of the solvation coordinate Uduring the course of the ARTICLES rate process. It should be stressed that eq VII-8 were obtained by a careful evaluation of the nonlinear response functions. We did not have to assume a priori that the reaction takes place at the transition-state configuration U = -Eo.It should further be noted that, in fluorescence measurements, the Stokes shift depends on solvent relaxation in the excited state (R, and R2). In hole burning, we probe a difference between the ground and the excited states and therefore all pathways R,, R2, R3, and R4 contribute. Hole-burning spectroscopy is thus a probe for ground-state as well as excited-state r e l a~a t i o n .~~The present formulation is based on a generalized master equation, and we have derived a frequency-dependent rate K(s).The s scale over which K(s) varies is determined by the solvation time scales. The values of s, relevant in the generalized master equation, are approximately equal to the inverse reaction time scale (the rate). Reactions with large activation barriers are slow, and a separation of time scales is expected to hold, resulting in ordinary rate equations (eq 11-22), For barrierless reactions this separation of time scales may not hold. Several optically induced electron-transfer and isomerization reactions show a time evolution which does not follow a simple rate Our generalized rate equation provides an adequate method for treating these reactions by keeping the s dependence of K(s) and allowing for an initial nonequilibrium distribution of the solvation coordinate.The interaction of copper and silver monoions with acetylene has been studied including the effect of electron correlation. Geometries of the minima and binding energies have been determined by using properly localized molecular orbitals in the configuration interaction. Although the main interaction is due to the presence of a positive charge, inclusion of electron correlation is needed if accurate res...

No abstract

No abstract