Proton transfer methodology development

As noted in Sections 3.1 and 4.1, the atmospheric relevance of the two acids often extends beyond the issue of their surface dissociation in the conditions that we have considered here. Thus, for example, the widely occurring sulfate aerosols are concentrated sulfuric acid solutions and nitric acid trihydrate (NAT) is also clearly fairly concentrated. Elucidation of the acid ionization state of such aerosol surfaces—and thus the theoretical characterization of the mechanisms and rates of chemical reaction occurring on them—requires other tools to deal with the multiple proton transfer possibilities present in these systems. One such tool, a generalization of available reaction Monte Carlo methodologies [72] to treat such multiple proton transfers, is under development [73]. 

Since the SECM response depends on rates of both heterogeneous reactions (26) and (27), their kinetics can be measured using methodologies developed previously for ET processes. The mass-transfer rate for IT measurements by SECM is similar to that for heterogeneous ET measurements, and the standard rate constants of the order of 1 cm/s should be measurable. With relatively large pipet probes used in Ref. 49 (i.e., n > 10 /Ain), the mass-transfer coefficient was <0.01 cm/s, i.e., too low to measure the standard rate constant of potassium transfer [1.3 cm/s (51)]. The SECM response was diffusion controlled. In contrast, a kinetically controlled regeneration of the mediator was observed for a slower reaction of proton transfer assisted by 1,10-phenanthroline. 

This approach is able to model a variety of condensed phase chemical reactions with essentially experimental accuracy [16]. We did And, however, one specific experimental system for which this methodology was not able to reproduce experimental results, and that is proton transfer in benzoic acid crystals. In developing a physical understanding of this system, we first identified the concept of the promoting vibration. 

In this chapter, we have described the fundamental parameters that should be obtained when characterising an electronic, singlet or triplet, excited state and how to determine them experimentally including methodologies and required equipment. These characteristics include electronic energy, quantum yields, lifetimes and number and type of species in the excited state. Within this last context, i.e., when excited state reactions give rise to additional species in the excited state we have explored several excited state kinetic schemes, found to be present when excimers, exciplexes are formed and (intra and intermolecular) proton transfer occurs. This includes a complete formalism (with equations) for the steady-state and dynamic approaches for two and three-state systems, from where all the rate constants can be obtained. Additionally, we have explored additional recent developments in photophysics the competition between vibrational relaxation and photochemistry, and the non-discrete analysis (stretched-exponential) of fluorescence decays. 

This review summarises and discusses the advances of computational photochemistry in 2012 and 2013 in both methodology and applications fields. The methodological developments of models and tools used to study and simulate non-adiabatic processes are highlighted. These developments can be summarised as assessment studies, new methods to locate conical intersections, tools for representation, interpretation and visualisation, new computational approaches and studies introducing simpler models to rationalise the quantum dynamics near and in the conical intersection. The applied works on the topics of photodissociation, photostability, photoisomerisations, proton/charge transfer, chemiluminescence and bioluminescence are summarised, and some illustrative examples of studies are analysed in more detail, particularly with reference to photostability and chemi/ bioluminescence. In addition, theoretical studies analysing solvent effects are also considered. We finish this review with conclusions and an outlook on the future. 

Much remains to be done before a clear, quantitative mechanistic picture will be available to describe enzymatic proton transfers. From the methodological point of view, the development of models for intramolecular proton transfers appear to be necessary for the quantification of the influence of H-bond strength and bond orienta-... 

Synthetic methodology for the preparation of switchable rotaxanes has been primarily developed by Stoddart and coworkers [4]. In collaboration with Stoddart, the author s group reported in 1994 a molecular shuttle that exhibited a novel feature dual control. In this rotaxane the average position of the bead could be controlled by both proton- and electron-transfer reactions... 

The sequential trans-addition of a carbon nucleophile and a carbon electrophile across an arene double bond in (arene)Cr(CO)3 was first reported in 1983 [35]. Since then this methodology has undergone extensive development, with recent efforts mainly directed towards enantioenriched products [36]. Anionic (cy-clohexadienyl)Cr(CO)3 complexes are very soft nucleophiles and this places restrictions on the electrophiles that can be used in this sequence. Specifically these reactions are successful when carbanion dissociation from the intermediate anionic cyclohexadienyl complex is slow compared to the reaction with the carbon electrophile. The sequential addition is usually carried out as a one-pot reaction and the proposed reaction sequence is that shown in Scheme 11. In contrast to the nucleophile addition/protonation sequence, products form with excellent 1,2-regioselectivity. It is likely that this is due to an irreversible transfer of the acyl, allyl, or propargyl group to one of the two termini of the cyclohexadienyl ligand. 

Parallel to the developments achieved in methodology and hardware, the conventional methods and some of the new approaches have been employed to study several types of photoinduced processes which are relevant mainly in biology and nanotechnology. In particular, important contributions have been made related to the topics of photodissociations, photostability, photodimerizations, photoisomerizations, proton/hydrogen transfer, photodecarboxylations, charge transport, bioexcimers, chemiluminescence and bioluminescence. In contrast to earlier studies in the field of computational photochemistry, recent works include in many cases analyses in solution or in the natural environment (protein or DNA) of the mechanisms found in the isolated chromophores. In addition, semi-classical non-adiabatic molecular dynamics simulations have been performed in some studies to obtain dynamical attributes of the photoreactions. These latter calculations are however still not able to provide quantitative accuracy, since either the level of theory is too low or too few trajectories are generated. Within this context, theory and hardware developments aimed to decrease the time for accurate calculations of the PESs will certainly guide future achievements in the field of photodynamics. 

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