Adiabatic pathways

Deuteration has been previously shown to cause an increase in the lifetime of triplet free-base porphyrins ( 7). This has been attributed to the strong coupling of N-H tautomerism with nonradiative decay. In the case of mesoporphyrin IX the increase upon deuteration is approximately two-fold ( ) As indicated in Table III deuteration of the picket fence porphyrin results in little change in the photostationary state composition but an almost twofold increase in the quantum yield of 4,0 -> 3>1. As stated above there is no measurable deuterium isotope effect on the thermal reaction the proportionate increase in quantum yield and triplet lifetime upon deuteration of the picket fence porphyrin is thus completely consistent with the adiabatic mechanism described above. Although the evidence amassed does not completely rule out other possibilities, it seems that the photoatropisomerization is to date best described by the adiabatic pathway in which the porphyrin ground and excited state potential surfaces are modified much as illustrated in Figure 3. 

There are large families of chemical compounds between those of DMABN and ethylene that are either polar or nonpolar in S0, but after photoexcitation the twisted charge-separated biradical excited states develop via adiabatic pathways (Section III). This has been displayed by experimental2 and theoretical14 studies. 

Perez-Prieto, J., Stiriba, S.E., Gonzalez-Bejar, M., Domingo, L.R. and Miranda, M.A. (2004) Mechanism of triplet photosensitized Diels-Alder reaction between indoles and cyclohexadienes theoretical support for an adiabatic pathway. Organic Letters, 6, 3905-3908. 

Saltiel, J., Waller, A.S., and Sears, D. F., Jr. (1992) Dynamics of cis-stilbene photoisomerization the adiabatic pathway to excited trans-stilbene, J. Photochem. Photobiol., A 65, 29-40. 

Experimental estimates of <5r/c , are relatively difficult to obtain. While they can, in principle, be extracted from temperature-dependence studies, this approach is complicated by uncertainties in the entropic term (Sect. 4.3). An alternative method has recently been described for some Cr(III) reductions which involves comparing the work-corrected rate constants, kco , with unimolecular rate constants, ket, for structurally related reactants that reduce via ligand-bridged pathways [30]. Provided that the corresponding outer- and inner-sphere pathways involve the same activation barrier (Sect. 4.6) and the latter also follow adiabatic pathways, we can write [30]... 

Variation in the metal surface composition is, then, generally expected to yield large variations in the observed rate constant for inner-sphere pathways since the reaction energetics will be sensitive to the chemical nature of the metal surface. For outer-sphere reactions, on the other hand, the rate constants are anticipated to be independent of the electrode material after correction for electrostatic work terms provided that adiabatic (or equally non-adiabatic) pathways are followed. Although a number of studies of the dependence of the rate constants for supposed outer-sphere reactions on the nature of the electrode material have been reported, relatively few refer to sufficiently well-defined conditions where double-layer corrections are small or can be applied with confidence [111-115]. Several of these studies indeed... 

Although the adiabatic pathway appears unlikely, we note that the shape resonance or single particle resonance at low energy ca. 0-7 eV) have several interacting pathways, including vibrational. 

As described before, substitution of an aromatic group with a low triplet excitation energy on an unsaturated bond leads to one-way cis - trans isomerization of the unsaturated bond in an adiabatic pathway. Here we deal with intramolecular hydrogen bonding between two heteroaromatic groups substituted on the cis side of an unsaturated bond which can result in one-way trans - cis isomerization. 

A DFT-based computational study has established the thermal and photochemical isomerisation mechanisms from the terminal P-bound phosphinidene oxide complex [Ru(tpy)(bpy)(POPh)] to the corresponding O-bound, [Ru(tpy)(bpy)(OPPh)] . Thermal isomerisation was found to be both kinetically and thermodynamically unfavourable, while photoisomerisation can readily take place by either adiabatic or non-adiabatic pathways. The different absorption spectra of the two isomers and the bi-stability of the system make this complex a good candidate for photochromism. ... 

Pre-association and limiting electron-transfer behaviour is also observed in the oxidation of stellacyanan, St(i), by [Co(edta)] at pH 7.0. The large negative entropy of activation for electron transfer is interpreted as indicative of a non-adiabatic mechanism. When binding of [Co(edta)] to the protein is prevented, either at pH 10.0 or by the addition of edta to block the reactive site, electron transfer takes place through an alternative adiabatic pathway. Under these conditions a Marcus self-exchange rate constant of 3 x 10 s ... 

Figure 4.10 includes constant entropy or adiabatic pathways in the pV plane. The lower panel transformations are more complicated as they call for extra attachments not represented in Figure 4.3. For a system to be directed along an adiabatic path, the heat reservoirs must be cast aside and the surrounding walls covered with insulation. As with isotherms, if the piston compresses the gas, > 0. Since the "new energy cannot leak out, the temperature rises sooner or later. If the piston moves upward, then dW <0. In this case, thermal enei y is depleted and the temperature...

Equation (4.28) would be appropriate to isothermal or adiabatic pathways, to name two. 

Isothermal and adiabatic pathways were discussed in Section 4.2 of Chapter 4. One learned that the following statements hold for the isotherms of Figure 5.4 ... 

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