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Unimolecular electronic measurements

Molecular electronics" (ME) (sensii stricto), or molecular-scale electronics" or unimolecular electronics" (UE) is the study of electrical and electronic processes measured or controlled om a molecular scale or on the nanometer scaleJ A wider definition of molecular electronics sensu lato), or "molecule-based electronics" encompasses electronic p ocesses by molecular assemblies of any scale, including macroscopic crystals and conducting polymers/ This article deals with UE and focuses on electrical conduction (asymmetric or not), through single molecules or through a monolayer of molecules measured in parallel. [Pg.1525]

Cytochrome c can be strongly adsorbed in its native state on tin oxide electrodes at coverages of several tenths of a monolayer. Measurements of unimolecular electron transfer rate constants for adsorbed cytochrome c can be readily made in the absence of solution cytochrome c using cyclic voltammetry. The kinetic results are consistent with an interfacial model involving electrostatic interaction between the tin oxide and the exposed heme edge of the cytochrome as well as an electrostatically driven adsorbate reorientation capability to account for the anomalous dependence of rate constant on scan rate. Other possible explanations for the... [Pg.73]

Detailed analyses of the above experiments suggest that the apparent steps in k E) may not arise from quantized transition state energy levels [110.111]. Transition state models used to interpret the ketene and acetaldehyde dissociation experiments are not consistent with the results of high-level ab initio calculations [110.111]. The steps observed for NO2 dissociation may originate from the opening of electronically excited dissociation chaimels [107.108]. It is also of interest that RRKM-like steps in k E) are not found from detailed quantum dynamical calculations of unimolecular dissociation [91.101.102.112]. More studies are needed of unimolecular reactions near tln-eshold to detennine whether tiiere are actual quantized transition states and steps in k E) and, if not, what is the origin of the apparent steps in the above measurements of k E). [Pg.1035]

In general, intramolecular isomerization in coordinatively unsaturated species would be expected to occur much faster than bimolecular processes. Some isomerizations, like those occurring with W(CO)4CS (47) are anticipated to be very fast, because they are associated with electronic relaxation. Assuming reasonable values for activation energies and A-factors, one predicts that, in solution, many isomerizations will have half-lives at room temperature in the range 10 7 to 10 6 seconds. The principal means of identifying transients in uv-visible flash photolysis is decay kinetics and their variation with reaction conditions. Such identification will be difficult if not impossible with unimolecular isomerization, particularly since uv-visible absorptions are not very sensitive to structural changes (see Section I,B). These restrictions do not apply to time-resolved IR measurements, which should have wide applications in this area. [Pg.285]

A number of publications in recent years have demonstrated an active interest in the theoretical aspects of electron transfer (ET) processes in biological systems (1.-9). This interest was stimulated by the extensive experimental information regarding the temperature dependence of ET rates measured over a broad range of temperatures (10-16). The unimolecular rate of cyto-chrome-c oxidation in Chromatium (10-12), for example, exhibits the Arrhenius type dependence and changes by three orders of... [Pg.216]

In order to measure the magnitude of the chemical interactions between various ions and buffer gases, approaches that are based on the measurements of either equilibrium or rate constants for ionic processes can be envisioned. An example of a kinetic method is described in the following. The unimolecular kinetic process known as thermal electron detachment (TED) for negative ions (NT -> M + e), should be particularly sensitive to a chemical effect of the buffer gas. This is because the rate of TED will be given by = constant x where the electron... [Pg.228]

Queen153 has recently measured the activation parameters A//, A5 and ACp for the hydrolyses of Me, Et, /z-Pr, /-Pr, and phenyl chloroformates and dimethyl carbamyl chloride in pure water (Table 22), and concluded that a change of mechanism (SN2 to SN1) takes place with increasing electron donation to the chlorocarbamyl group the data, which include solvent isotope effects, are consistent with a unimolecular hydrolysis of isopropyl chloro-formate and dimethyl carbamyl chloride and a bimolecular hydrolysis of the other four compounds. [Pg.252]

Troe and his co-workers [27] have recently measured directly the lifetimes of excited molecules undergoing unimolecular decomposition, under essentially collision-free conditions. In these experiments, cyclo-heptatriene, 7-methylcycloheptatriene, 7,7-dimethylcycloheptatriene and 7-ethylcycloheptatriene were each excited electronically with a short pulse of laser radiation. This is followed by a rapid internal conversion to generate highly vibrationally excited, electronic ground state molecules which absorb in the ultraviolet, at longer wavelengths than the unexcited parent. Their decay (isomerisation to alkylbenzenes) was monitored directly with a continuous background source. [Pg.355]

Benzoic acid and most mono-substituted benzoic acids are stable with respect to decarboxylation in aqueous solution, even at a temperature of 100 °C. However, decarboxylation may occur with a measurable rate if either strong electron-withdrawing or strong electron-releasing substituents are present in the aromatic acid. The decarboxylation rate of 2,4,6-trinitrobenzoic acid is increased by addition of base to the aqueous solution, and it attains a maximum value when the substrate is completely transformed to the anion [236]. A carbon-13 isotope effect of ft, 2/ft, 3 = 1.036 (50 °C) has been observed [237]. There is no D20 solvent isotope effect [238]. These findings indicate that the mechanism of decarboxylation of 2,4,6-trinitrobenzoic acid is a unimolecular electrophilic substitution (SE1), viz. [Pg.73]

The Yukawa-Tsuno equation continues to find considerable application. 1-Arylethyl bromides react with pyridine in acetonitrile by unimolecular and bimolecular processes.These processes are distinct there is no intermediate mechanism. The SnI rate constants, k, for meta or j ara-substituted 1-arylethyl bromides conform well to the Yukawa-Tsuno equation, with p = — 5.0 and r = 1.15, but the correlation analysis of the 5 n2 rate constants k2 is more complicated. This is attributed to a change in the balance between bond formation and cleavage in the 5 n2 transition state as the substituent is varied. The rate constants of solvolysis in 1 1 (v/v) aqueous ethanol of a-t-butyl-a-neopentylbenzyl and a-t-butyl-a-isopropylbenzyl p-nitrobenzoates at 75 °C follow the Yukawa-Tsuno equation well, with p = —3.37, r = 0.78 and p = —3.09, r — 0.68, respectively. The considerable reduction in r from the value of 1.00 in the defining system for the scale is ascribed to steric inhibition of coplanarity in the transition state. Rates of solvolysis (80% aqueous ethanol, 25 °C) have been measured for 1-(substituted phenyl)-l-phenyl-2,2,2-trifluoroethyl and l,l-bis(substi-tuted phenyl)-2,2,2-trifluoroethyl tosylates. The former substrate shows a bilinear Yukawa-Tsuno plot the latter shows excellent conformity to the Yukawa-Tsuno equation over the whole range of substituents, with p =—8.3/2 and r— 1.19. Substituent effects on solvolysis of 2-aryl-2-(trifluoromethyl)ethyl m-nitrobenzene-sulfonates in acetic acid or in 80% aqueous TFE have been analyzed by the Yukawa-Tsuno equation to give p =—3.12, r = 0.77 (130 °C) and p = —4.22, r — 0.63 (100 °C), respectively. The r values are considered to indicate an enhanced resonance effect, compared with the standard aryl-assisted solvolysis, and this is attributed to the destabilization of the transition state by the electron-withdrawing CF3 group. [Pg.320]

If the lifetime of the excited resonance state is too long for direct measurement of the rate via the widths of the spectral features, one can use a third laser (the probe laser in Fig. 11) to resonantly promote the molecules from this level to a rovibrational level in the excited electronic state. The decrease of the total LIF signal as function of the delay time between pump and probe laser reflects the state-specific dissociation rate. The limitation of the SEP technique is that an excited state has to be found, which lives long enough and which is accessible by all three lasers. Molecules, which have been studied by SEP spectroscopy in the context of unimolecular dissociation, are HCO, DCO, HFCO and CH3O. [Pg.131]

The thermal unimolecular cyclization of 3-diazoalkenes to pyrazoles appears to be an intramolecular 1,3-dipolar cycloaddition and the first-order rate coefficients of four substituted // art5-3-diazo-l-phenylpropenes fit the Hammett equation (p = — 0.40). The small value of p, indicating a lack of sensitivity of the cyclization rate to the electronic nature of the substituents supports the belief that the reaction involves a synchronous, cyclic electron shift. Table 11 lists the measured rate coefficients. [Pg.610]

The specific rate constants of interest to the ECD and NIMS are dissociative and nondissociative electron attachment, electron detachment, unimolecular anion dissociation, and electron and ion recombination. The reactions that have been studied most frequently are electron attachment and electron and ion recombination. To measure recombination coefficients, the electron concentration is measured as a function of time. The values are dependent on the nature of the positive and negative ions and most important on the total pressure in the system. Thus far few experiments have been carried out under the conditions of the NIMS and ECD. However, the values obtained under other conditions suggest that there is a limit to the bimolecular rate constant, just as there is a limit to the value of the rate constant for electron attachment. The bimolecular rate constants for recombination are generally large, on the order of 10 7 to 10-6 cc/molecule-s or 1014 to 1015 1/mole-s at about 1 atm pressure. Since the pseudo-first-order rate constants are approximately 100 to 1,000 s 1, the positive-ion concentrations in the ECD and NIMS are about 109 ions/cc. [Pg.132]

The observed rate constant, obs, never equals the rate constant 2 for forward electron transfer in these simple models. The presence of multiple steps in the electron transfer mechanism [keeping in mind that Eq. (20) represents a minimal scheme for an electron transfer reaction] emphasizes the difficulties in extracting 2 values from measurements of obs under steady-state conditions. Rapid kinetic studies provide a more powerful approach for separating the actual kinetics of electron transfer from the association and dissociation steps, but the analysis may still be complex. Owing to difficulties associated with bimolecular kinetics, many recent studies of electron transfer have emphasized unimolecular processes. Physiologically, however, the bimolecular processes can be of considerable importance for the overall electron transfer kinetics. [Pg.57]


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