Rate Of Back Reaction Research Articles

Measurement of Both the Equilibrium Constant and Rate Constant for Electronic Energy Transfer by Control of the Limiting Kinetic Regimes

Electronic energy transfer can fall into two limiting cases. When the rate of the energy transfer back reaction is much faster than relaxation of the acceptor excited state, equilibrium between the donor and acceptor excited states is achieved and only the equilibrium constant for the energy transfer can be measured. When the rate of the back reaction is much slower than relaxation of the acceptor, the energy transfer is irreversible and only the forward rate constant can be measured. Herein, we demonstrate that with trans-[Cr(d(4)-cyclam)(CN)(2)](+) as the donor and either trans-[Cr([15]ane-ane-N(4))(CN)(2)](+) or trans-[Cr(cyclam)(CN)(2)](+) as the acceptor, both limits can be obtained by control of the donor concentration. The equilibrium constant and rate constant for the case in which trans-[Cr([15]ane-ane-N(4))(CN)(2)](+) is the acceptor are 0.66 and 1.7 x 10(7) M(-1) s(-1), respectively. The equilibrium constant is in good agreement with the value of 0.60 determined using the excited state energy gap between the donor and acceptor species. For the thermoneutral case in which trans-[Cr(cyclam)(CN)(2)](+) is the acceptor, an experimental equilibrium constant of 0.99 was reported previously, and the rate constant has now been measured as 4.0 x 10(7) M(-1) s(-1).

Influence of Sodium Cations of N3 Dye on the Photovoltaic Performance and Stability of Dye‐Sensitized Solar Cells

We report on the effect of substituting the two tetrabutyl ammonium counter ions of the standard N719 dye by sodium ions on the performance and stability of dye-sensitized solar cells (DSCs). The disodium analogue of N719 in conjunction with a non-volatile electrolyte gives a conversion efficiency of 7.6% under standard global AM 1.5 sunlight. Devices maintain 99% of their initial performance after 1000 h under full sunlight aging at 50 degrees C. Electrochemical impedance spectroscopy and photovoltage transient decay studies reveal the evolution of the solar cell parameters during aging. Remarkably, upon aging a decrease in the rate of electron back reaction with the triiodide ions across the TiO(2)/electrolyte interface appears as well as enhanced electronic conduction in the TiO(2) film.

Controlled Synthesis of Photochromic Polymer Brushes by Atom Transfer Radical Polymerization

This work reports on the grafting of methyl methacrylate polymer brushes containing spirobenzopyran pendant groups from flat silica surfaces and colloidal particles utilizing atom transfer radical polymerization (ATRP). The reaction conditions were optimized with respect to the kind of surface bound initiator, the type of halide and ligand used in the catalytic complex, the presence/absence of untethered initiator, and solvent type. This enabled synthesis of coatings up to 80 ± 3 nm thick with controlled spirobenzopyran content. While polymerization kinetics indicate the presence of chain termination reactions, the “living” character of the process is confirmed by controlled formation of block copolymer brushes. UV/vis spectroscopy was used to characterize the UV-induced isomerization of spirobenzopyran to zwitterionic merocyanine and the thermal back-reaction. Spectral and kinetic analyses of this latter bleaching process points to the existence of free and associated merocyanines in the polymeric brush in both tetrahydrofuran and toluene. However, stabilization of merocyanine species by the polymer matrix is considerably greater in toluene with thermal back-reaction rates approaching those determined for solid dry films.

Evidence for a non-planar conformer and conformational isomerization of o-fluoroanisole in a low-temperature Ar matrix

FT-IR spectra of o-fluoroanisole were measured in a low-temperature Ar matrix with and without UV irradiation. The observed bands, without irradiation, were assigned to the trans conformer with the aid of the quantum chemical calculations at the B3LYP/cc-pVTZ level. By comparing the IR spectrum, after irradiation, with the calculated spectra, the formation of a non-planar conformer was established. The non-planar conformer gradually decayed by a unimolecular process under dark conditions at 16 K, while the trans conformer increased. The back-reaction rate was estimated, and the reaction dynamics between the trans and non-planar conformers was discussed.

How Important is the Back Reaction of Electrons via the Substrate in Dye-Sensitized Nanocrystalline Solar Cells?

The role of the conducting glass substrate (fluorine-doped tin oxide, FTO) in the back reaction of electrons with tri-iodide ions in dye-sensitized nanocrystalline solar cells (DSCs) has been investigated using thin-layer electrochemical cells that are analogues of the DSCs. The rate of back reaction is dependent on the type of FTO and the thermal treatment. The results show that this back-reaction route cannot be neglected in DSCs, particularly at lower light intensities, where it is the dominant route for the back transfer of electrons to tri-iodide. This conclusion is confirmed by measurements of the intensity dependence of the photovoltages of DSCs with and without blocking layers. It follows that blocking layers should be used to prevent the back reaction in mechanistic studies in which the light intensity is varied over a wide range. Conclusions based on studies of the intensity dependence of the parameters of DSCs such as photovoltage and electron lifetime in cells without blocking layers, must be critically re-examined.

Anchoring Group and Auxiliary Ligand Effects on the Binding of Ruthenium Complexes to Nanocrystalline TiO2 Photoelectrodes

The thermodynamics and kinetics of binding to nanocrystalline TiO_2 were investigated for five ruthenium complexes that differed structurally in the number of possible anchoring carboxy groups (one, two, four, or six) attached to coordinated bipyridyl ligands and in the number of auxiliary ligands (bipyridine, CN-, or SCN-). Diffuse reflectance infrared spectroscopic data indicated that the dyes predominantly bound to TiO_2 in a bridging mode in which the oxygen atoms of an attached carboxy group were bound to separate titanium atoms on the TiO_2 surface. Furthermore, in the dry state, complexes with only one monocarboxy or dicarboxy ligand used essentially all of their available carboxy groups to bind to the surface. However, complexes having two or three dicarboxy ligands used on average two carboxylato groups in binding to TiO_2. The structural differences between the complexes were manifested chemically in that the five dyes yielded similar maximum coverages (>100 nmol cm^(-2)) on nanocrystalline TiO_2 electrodes, but exhibited different binding constants (10^3−10^5 M^(-1)) and different adsorption and desorption kinetics (3−11) × 10^3 M^(-1) h^(-1) and 1−100 h, respectively). The binding constant for the monocarboxy dye was significantly lower than the binding constants for dyes with dicarboxy ligands, correlating primarily with an increase in the desorption rate of the monocarboxy complex. The adsorption rate constants were similar for all of the dyes, suggesting that formation of the first bond to TiO_2 was rate limiting. Binding of the dyes from an ethanolic solution that contained pyridine and pyridinium as an acidic proton activity buffer yielded lower coverages than binding from a nonbuffered ethanol solution, even though the binding constants were up to 100 times greater under buffered conditions. The lower equilibrium dye coverage in buffered ethanol did not correlate with changes in the protonation state of the dyes but rather indicated competition for, and/or deactivation of, TiO_2 active sites in buffered ethanol. The more weakly bound monocarboxy dye displayed the lowest short-circuit current density and open-circuit voltage under simulated solar illumination in a photoelectrochemical cell containing 0.50 M LiI, 0.040 M I_2, 0.020 M pyridine, and 0.020 M pyridinium triflate in acetonitrile. Additionally, even at constant coverage, the integrated quantum yield for photocurrent flow was lowest for TiO_2 sensitized with the monocarboxy dye. The potential required to drive 0.1 mA cm^(-2) of cathodic current density in the dark on dye-sensitized TiO_2 photoelectrodes was least negative for the monocarboxy dye, indicating more facile electron transfer between reduced TiO_2 and the solution redox couple. Hence, in this series of ruthenium carboxy-bipyridyl dyes, the most weakly bound species (i.e., the monocarboxy dye) yielded inferior photoelectrode properties, whereas differences between the dyes that contained at least one dicarboxy ligand resulted primarily from differences in the light absorption and energetic properties of the metal complexes. These observations suggest an important role for the linkage to the TiO_2 surface in achieving temporal stability as well as in tuning both the steady-state quantum yield and the magnitude of the predominant back-reaction rate in dye-sensitized TiO_2-based photoelectrochemical solar cells.

Solvent-dependent dynamics of the MQ*-->ReII excited-state electron transfer in [Re(MQ+)(CO)3(dmb)]2+.

The Re-->MQ(+) MLCT excited state of [Re(MQ(+))(CO)(3)(dmb)](2+) (MQ(+) = N-methyl-4,4'-bipyridinium, dmb = 4,4'-dimethyl-2,2'-bipyridine), which is populated upon 400-nm irradiation, was characterized by picosecond time-resolved IR and resonance Raman spectroscopy, which indicate large structural differences relative to the ground state. The Re-->MQ(+) MLCT excited state can be formulated as [Re(II)(MQ*)(CO)(3)(dmb)](2+). It decays to the ground state by a MQ*-->Re(II) back-electron transfer, whose time constant is moderately dependent on the molecular nature of the solvent, instead of its bulk parameters: formamides approximately DMSO approximately MeOH (1.2-2.2 ns) < THF, aliphatic nitriles (3.2-3.9 ns) << ethylene-glycol approximately 2-ethoxyethanol (4.2-4.8 ns) < pyridine (5.7 ns) < MeOCH(2)CH(2)OMe (6.9 ns) < PhCN (7.5 ns) < MeNO(2) (8.6 ns) <<< CH(2)Cl(2), ClCH(2)CH(2)Cl (25.9-28.9 ns). An approximate correlation was found between the back-reaction rate constant and the Gutmann donor number. Temperature dependence of the decay rate measured in CH(2)Cl(2), MeOH, and BuCN indicates that the inverted MQ*-->Re(II) back-electron transfer populates a manifold of higher vibrational levels of the ground state. The solvent dependence of the electron transfer rate is explained by solvent effects on inner reorganization energy and on frequencies of electron-accepting vibrations, by interactions between the positively charged MQ(+) pyridinium ring and solvent molecules in the electron-transfer product, that is the [Re(MQ(+))(CO)(3)(dmb)](2+) ground state.

Dissolution and growth of [formula omitted] calcite in flowing water: estimation of back reaction rates via kinetic Monte Carlo simulations

Although calcite is an important mineral for many processes, there have been relatively few simulations of its growth and dissolution behavior. Such simulations are complicated by the multitude of defect types and by the asymmetry of the crystal. The present work combined a kinetic Monte Carlo technique with the Kossel crystal (1 0 0) simple cubic concept and the Blasius boundary layer model to simulate the simultaneous growth and dissolution of the (1 0 1 ̄ 4) calcite cleavage surface in flowing water. The activation energies of the back reaction (growth) were determined from those of the forward reaction (dissolution) by obtaining agreement with cleavage-step morphologies and step dissolution velocities over a range of flow rates previously measured using an atomic force microscope. The kinetics were dominated by diffusion events on the solid/fluid interface and in the fluid, as expected. The relative magnitudes of the desorption and adsorption activation energies were consistent with experimental data, entropic arguments, and crystal roughening theories. Quantitative agreement with measured step velocities was best when the boundary layer parameters were given physically reasonable values, indicating that the model is self-consistent.

Photochemical nitric oxide precursors: synthesis, photochemistry, and ligand substitution kinetics of ruthenium salen nitrosyl and ruthenium salophen nitrosyl complexes.

Described are syntheses, characterizations, and photochemical reactions of the nitrosyl complexes Ru(salen)(ONO)(NO) (I, salen = N,N'-ethylenebis(salicylideneiminato) dianion), Ru(salen)(Cl)(NO) (II), Ru((t)Bu(4)salen)(Cl)(NO) (III,(t)Bu(4)salen = N,N'-ethylenebis(3,5-di-tert-butylsalicylideneiminato) dianion), Ru((t)Bu(4)salen)(ONO)(NO) (IV), Ru((t)Bu(2)salophen)(Cl)(NO) (V, (t)Bu(2)salophen = N,N'-1,2-phenylenediaminebis(3-tert-butylsalicylideneiminato) dianion), and Ru((t)Bu(4)salophen)(Cl)(NO) (VI, (t)Bu(4)salophen = N,N'-1,2-phenylenebis(3,5-di-tert-butylsalicylideneiminato) dianion). Upon photolysis, these Ru(L)(X)(NO) compounds undergo NO dissociation to give the ruthenium(III) solvento products Ru(L)(X)(Sol). Quantum yields for 365 nm irradiation in acetonitrile solution fall in a fairly narrow range (0.055-0.13) but decreased at longer lambda(irr). The quantum yield (lambda(irr) = 365 nm) for NO release from the water soluble complex [Ru(salen)(H(2)O)(NO)]Cl (VII) was 0.005 in water. Kinetics of thermal back-reactions to re-form the nitrosyl complexes demonstrated strong solvent dependence with second-order rate constants k(NO) varying from 5 x 10(-4) M(-1) s(-1) for the re-formation of II in acetonitrile to 5 x 10(8) M(-1) s(-1) for re-formation of III in cyclohexane. Pressure and temperature effects on the back-reaction rates were also examined. These results are relevant to possible applications of photochemistry for nitric oxide delivery to biological targets, to the mechanisms by which NO reacts with metal centers to form metal-nitrosyl bonds, and to the role of photochemistry in activating similar compounds as catalysts for several organic transformations. Also described are the X-ray crystal structures of I and V.

Calculation of High-Temperature Interaction of Carbon Particles with Carbon Dioxide

The gasification of a porous carbon particle, which proceeds in accordance with the reaction equation C + CO2 = 2CO, is investigated in view of the finite rate of back reaction. The rate of gasification of a spherical particle with a varying degree of porosity is calculated in the limiting cases of small and large radius, as compared with the effective thickness of the chemical reaction zone. An expression is derived for the rate of gasification of a large particle in the mode of mixed kinetics.

Transient EPR studies of Photosystem I containing non-native quinones

The quinone acceptors in both type I and type II reaction centres play a pivotal role in light induced photosynthetic electron transfer. Although, the quinones found in the various reaction centres are structurally similar, the rates of electron transfer and nature of the subsequent acceptors vary greatly. The relationship between the properties of the quinone binding-site, the structure of the quinone and the rates of electron transfer are still not well understood. Transient EPR data for the forward and back reaction kinetics will be presented for Photosystem I, in which the native phylloquinone has been replaced by a series of foreign quinones. Our results suggest that the ability of quinones to function in the electron transport chain is governed primarily by their midpoint potentials. We find that the less negative the midpoint potential of a given quinone is the slower the forward electron transfer rate. The back reaction from the iron sulphur centres is also influenced by the presence of a foreign quinone in the A1 site clearly showing that the back reaction from FA/FB proceeds via A1. In contrast to the forward rate, the back reaction rate increases as the midpoint potential of the quinone becomes less negative. The EPR data also suggest that naphthoquinone derivatives bind more strongly to the A1 site than benzoquinone derivatives do. This observation is can be understood as a result of differences in the proposed p-stacking arrangement between PsaA-W697 (PsaB-W677) and phylloquinone.

Electron transport and back reaction in dye sensitised nanocrystalline photovoltaic cells

The transport and back reaction of electrons in dye sensitised nanocrystalline solar cells (DSNC) has been studied by frequency resolved optical perturbation techniques. Intensity modulated photocurrent spectroscopy (IMPS) has been used to obtain values of the electron diffusion coefficient, D n, as a function of illumination intensity. It was found that D n increased with intensity ( D n∝ I −0.5). Intensity-modulated photovoltage spectroscopy (IMVS) has been used to measure the electron lifetime, τ n, which is determined by the rate of back reaction with I 3 − ions in the electrolyte. It was found that τ n decreased with light intensity ( τ n∝ I −0.5). The electron diffusion length, L n=( D n τ n) 1/2, is therefore only weakly dependent on light intensity. The values of L n were used to calculate the theoretical IPCE of the cell. Experimental measurements confirmed the prediction that the IPCE should remain almost constant over five orders of magnitude of light intensity. Possible reasons for the opposite trends in D n and τ n are discussed and related to the fundamental processes taking place in the DSNC.

A Novel Material for Spectral Hole-Burning Applications: Photochemical Properties of meso-Diphenyl-Tribenzonaphtoporphine

We have studied the photochemical properties of meso-diphenyl-tribenzo- naphtoporphine (P2TriBNP) as a material exhibiting spectral hole-burning (SHE) that are important for its use in optical data storage and imaging spectros- copy. We measured the Debye-Waller factor, the hole-burning efficiency, the in- homogeneous and homogeneous line width, the back reaction rates of the photoproduct, and the dipole moment differences between ground and excited state of this compound at liquid helium temperatures. Poly(vinyl butyral) was used as the matrix. The hole-burning efficiency is higher by orders of magnitude compared with commonly used systems like chlorin (3.5 ± 1.5% vs. 0.03%) which brings about the possibility to perform storage experiments with much lower light levels and on shorter time scales than before.

Chlororespiration revisited: Mitochondrial-plastid interactions in Chlamydomonas

The changes of the redox state of plastoquinone and of the rate of back-reaction between the primary photoproducts of PS II centres have been studied following addition of myxothiazol to whole cells of C. reinhardtii. Using appropriate mutant strains it is shown that these changes, namely a slow reduction of plastoquinone and a comparatively fast increase in the rate of back-reaction, are a consequence of the inhibition of mitorespiration at the level of cytochrome bc 1 complex. These new data are discussed in relation with our previous model of chlororespiration (Bennoun, P. (1982) Proc. Natl. Acad. Sci. USA 79, 4352–4356). In contrast to these effects, a decrease in oxygen concentration affecting only slightly the rate of back-reaction induces a fast reduction of plastoquinone, suggesting that the chloroplast oxidase has a low affinity for oxygen as compared to mitochondrial oxidases. The large variations in the rate of back-reaction that we could observe raise many questions relevant to their origin and to their possible implications, that are discussed in detail. If, as seems likely, these variations result from changes in the electrochemical gradient built up across thylakoid membranes, then the existence of a new gradient generator should be postulated.

Effect of the arginine-82 to alanine mutation in bacteriorhodopsin on dark adaptation, proton release, and the photochemical cycle.

The pH dependence of the rate constant of dark adaptation (thermal isomerization from all-trans- to 13-cis-bR) drastically changes when Arg82 of bacteriorhodopsin is replaced by an alanine. In the wild type (WT) the rate decreases sharply between pH 2.5 and pH 5. In R82A the sharp decrease is shifted to pH > 7. This correlates with the shift in the pK of the purple-to-blue transition from pH 2.6 in the wild type to pH 7.2 in the mutant (in 150 mM KCl). We propose that the same group that controls the purple-to-blue transition, namely, Asp85, catalyzes dark adaptation. The rate of dark adaptation in the R82A mutant is proportional to the fraction of protonated Asp85, indicating that dark adaptation occurs when Asp85 is transiently protonated. Thermal isomerization is at least 2 x 10(3) times more likely when Asp85 is protonated (blue membrane) than when it is deprotonated (purple membrane). The pH dependence of dark adaptation in the WT can be explained by a model in which the rate of dark adaptation in the WT is also proportional to the fraction of protonated Asp85 and that the pK of Asp85 depends on some other group, X, which deprotonates (or moves away from Asp85) with pK9 and causes the shift in the pK of Asp85 from 2.6 to 7.2. The quantum yield of light adaptation is at least an order of magnitude less in R82A as compared to the WT. The rise time of M formation is very fast in R82A and, unlike the WT, pH independent (1 microsecond versus 85 and 6 microseconds in the WT at pH 7 and 10, respectively). The activation energy of the L to M transition is 6.9 kcal/mol versus 13.5 kcal/mol in the WT. Thus the loss of a positive charge in the active site greatly increases the rate of light-induced deprotonation of the Schiff base. In the R82A mutant, the M decay at pH > 8.8 is much faster than the recovery of initial bR, which suggests a decrease in the rate of back-reaction from N to M. In a suspension of R82A membranes the rate of proton release as measured by the pH-sensitive dye pyranine is delayed by at least 20-fold (in 2 M KCl), while the uptake of protons did not change much (12 ms in the WT versus 8 ms in R82A).(ABSTRACT TRUNCATED AT 400 WORDS)

Nucleation of RNA Chain Formation by Escherichia coli DNA-dependent RNA Polymerase

We have studied the early steps in RNA synthesis. The kinetic behaviour of the nascent RNA, having chain lengths between 3 and 11 bases, and the transcription fidelity were analysed using the bacteriophage T7 Al promoter. By moving the stop-inducing base at position + 12 in the wild-type template in single base steps upstream, a set of closely related templates was constructed which allowed stalling of the complexes in the registers 11, 10, 9 and 8. Using this set of templates σ-factor release was determined. It occurs when RNA synthesis has proceeded to base position + 9. Analysis of the RNA synthesis both with and without heparin yielded the following results: there are three kinds of complexes, (a) the well-known abortively transcribing complex, which is present until the RNA has reached a length of 5 bases, (b) an intermediate complex having RNA chain lengths between 6 and 8 bases, which is stably bound but has high forward as well as back reaction rates, (c) complexes with RNA chains consisting of more than 8 bases, which are stably bound and do not contain the σ-factor. In general, the likelihood of chain elongation and the stability of the complexes increases with increasing RNA chain length in the early stages of RNA synthesis. Also the transcription fidelity increases correspondingly. Lack of fidelity leads to additional RNA products during the abortive state of transcription. "Read through" of RNA polymerase at stall positions of + 8 to + 11 also result from misincorporation.

A photophysical model for diphenylhexatriene fluorescence decay in solvents and in phospholipid vesicles

The fluorescence decay of 1,6-diphenyl-1,3,5-hexatriene (DPH) in pure solvents and in phospholipid vesicles has been measured using frequency domain fluorometry. Data analysis uses a model with two energetically close excited states. The model explains the high quantum yield and the double exponential decay of DPH observed in some pure solvents and in phospholipid vesicles. This model assumes that after excitation to a first excited state, there is a rapid interconversion to a lower excited state and that most of the emission occurs from this state. The interconversion rates between the two excited states determine the average lifetime. For DPH in solvents, we find that the interconversion rates are solvent and temperature dependent. For DPH in phospholipid vesicles, we find that the back reaction rate from excited state 2 to excited state 1 (R12) is what determines the fluorescence properties. The phospholipid phase transition affects only this back reaction rate. The model was analyzed globally for a range of solvents, temperatures and vesicle composition. Of the six parameters of the model, only two, the interconversion rates between the two excited states, varied in all different samples examined. For DPH in phospholipid vesicles, there is an additional feature of the model, which is related to the apparent distribution of the rate R12. Significantly better fits were obtained using a continuous lorentzian distribution of interconversion rates. The resulting lifetime distribution was asymmetric and showed a definite narrowing above the phase transition.

Effect of vesicular interfacial potential on electron transfer between N,N,N′,N′-tetramethylbenzidine and quinones

Photoinduced electron transfer reactions between N,N,N′,N′-tetramethylbenzidine (TMB) and various quinones (duroquinone (DQ), benzoquinone (BQ)) and vitamin K 1(VK 1) embedded in the lipidic phase of dihexadecylphosphate vesicles (DHP), and the corresponding back electron transfer reactions, are studied by laser flash spectroscopy. The results show that TMB + cation and Q − anion formation are principally a result of the direct electron transfer between the TMB singlet excited state and Q. The back reaction between TMB + cation and Q − anion takes place by two decays. The fast decay, corresponding to a reaction site located in the bilayer, is independent of the interfacial electrical potential ΔΨ, and depends on the redox potential of the quinone. The best electron acceptor (VK 1) yields the slowest rate. A slow back-reaction rate is observed when the two reactants are separated by the vesicular interface after migration of Q − anions towards the aqueous phase, and the transfer rate constant turns out to be an exponential function of ΔΨ.

Chlorophyll a fluorescence transient as an indicator of active and inactive photosystem II in thylakoid membranes

Upon illumination, a dark-adapted photosynthetic sample shows time-dependent changes in chlorophyll (Chl) a fluorescence yield, known as the Kautsky phenomenon or the OIDPS transient. Based on the differential effects of electron acceptors such as 2,5-dimethyl- p-benzoquinone (DMQ) and 2,6-dichloro- p-benzoquinone (DCBQ) on Chl a fluorescence transients of spinach thylakoids, we suggest that the OID phase reflects the reduction of the electron acceptor Q A to Q − A in the inactive PS II (see Graan, T. and Ort, D. (1986) Diochim. Biophys. Acta 852, 320–330 [14]). In spinach thylakoids, heat-induced increase of the Chl a fluorescence yield is also differentially sensitive to the addition of DMQ and DCBQ suggesting that this increase is mainly on the ‘I’ level, and thus heating is suggested to convert active PS II to inactive PS II centers. The kinetics of decay of Q − A, calculated from variable Chl a fluorescence, was analyzed into three exponential components (365–395 μs; 6–7 ms; and 1.4–1.7 s). In heated samples, the decay rate of variable Chl a fluorescence is slower than the normal back-reaction rate; there is a preponderance of the slow component that may be due, partly, to the inactive centers undergoing slow back reaction between Q − A and the S 2 state of the oxygen-evolving complex.

Sequential clustering reactions of SiD+3 with SiD4 and SiH+3 with SiH4: Another case of arrested growth of hydrogenated silicon particles

Sequential clustering reactions of SiD+3 with SiD4 and SiH+3 with SiH4 are observed in the ion cell of a Fourier transform mass spectrometer. Clustering occurs either by addition of SiD2 or SiH2 accompanied by loss of D2 or H2, or by the formation and stabilization of the bimolecular adducts. All of the clustering reactions are highly inefficient and lead to bottleneck structures at small silicon cluster sizes containing two to four silicon atoms. Rates are measured for both the addition and association products for each step of the reaction. Back reaction rates are monitored via silicon-29 isotope exchange. Ab initio electronic structure calculations of the reaction pathways including intermediates, transition states and products have been performed by Raghavachari and are presented in his companion paper. The overall reaction mechanisms are similar for each reaction step. First an intermediate complex is formed between the ion and neutral which is strongly bound by a bridging deuterium or hydrogen atom. Collisional stabilization of this complex leads to formation of the observed bimolecular adduct products. These bimolecular adducts do not react further with SiD4 (SiH4) on the time scale of our experiments. Elimination of D2 or H2 leading to the SiD2 (SiH2) addition products occurs via a thermoneutral transition state. Sequential growth by addition of SiD2 (SiH2) arrests at Si3D+7 (Si3H+7). Ab initio calculations find that this occurs because Si3D+7 (Si3H+7) assumes a highly stable cyclic structure. Phase space theoretical modeling of the experimentally measured reaction rates is performed to quantitatively test energies of the reaction intermediate complexes and transition states calculated by Raghavachari. Excellent agreement within 0.13 eV is obtained between the phase space and ab initio energies. Phase space derived kinetic isotope effects on the reaction rates of protiated and deuterated species also correspond well with experiment. Reaction rates at typical temperature and pressure conditions in silane plasmas are also calculated. These results strongly suggest that sequential clustering of SiH+3 with SiH4 does not lead to formation of the deleterious hydrogenated silicon dust observed in silane plasmas.