Ion Pair Dynamics

In photochemical systems the steady-state concentrations of reactive intermediates are very low (typically less than 10 9 M with conventional lamps or less than 10 5 M with intense laser sources). Such low concentrations essentially exclude any kinetically second-order processes between the reactive intermediates (geminate processes dominate) and minimize the probability of re- 

The rates of ion separation from CIP to SSIP are apparently much less structure dependent. Again the solvent polarity plays the dominant role. The typical values of ksep vary from ca. 5 x 108 s 1 in acetonitrile to about 105 s 1 in dichloromethane [50b, 122,123]. The empirical Weller equation [123] (Eq. 8, where rj is the solvent viscosity in cPs-1, r is the ion separation distance within the pair and d = oo) accounts well for the ion dynamics. 

The rates associated with ion-pair dynamics are thus competitive with the rates of BET as well as with the rates of fragmentation. The quantum yield for fragmentation, therefore, usually represents a composite of all these rate constants ( Pm = (km + ks)/(km + ks + kbet), where ks = ksep or k, ). 

The spin status of the ion pair is another crucial variable affecting the overall efficiency of the process. The forward electron transfer from (or to) a diamagnetic molecule is not affected by the spin status of the excited component. The back electron transfer, however, is forbidden within the triplet ion pairs (it would violate Pauli s exclusion principle). In situations like that the intersystem crossing will very often determine the efficiency of BET. In practice, the triplet state acceptors or donors lead to overall efficiencies that are higher than those observed with singlet state acceptors or donors [38,78,102,103,116]. An additional bonus is the fact that triplet states have longer lifetimes [2] and are efficiently ET-quenched with lower concentrations of the ground state component. Quinones and ketones are the most common triplet acceptors, while aromatic amines often serve as triplet donors. 

The ion pair spin multiplicity may be a valuable tool to affect the BET rates and to probe the ion pair dynamics via magnetic field effects [36], Even weak magnetic fields are known to influence relative probabilities of singlet and triplet reactions [34], Chemically induced dynamic nuclear polarization (CIDNP) is a particularly informative technique [12]. Many bond scission reactions and rearrangements in cyclic radical ions have been successfully explored using this approach. Both structural data (spin densities) and approximate kinetic informations are indirectly available from such experiments [12]. 

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Various enol silyl ethers and quinones lead to the vividly colored [D, A] complexes described above and the electron-transfer activation within such a donor/acceptor pair can be achieved either via photoexcitation of charge-transfer absorption band (as described in the nitration of ESE with TNM) or via selective photoirradiation of either the separate donor or acceptor.41 (The difference arising in the ion-pair dynamics from varied modes of photoactivation of donor/acceptor pairs will be discussed in detail in a later section.) Thus, actinic irradiation with /.exc > 380 nm of a solution of chloranil and the prototypical cyclohexanone ESE leads to a mixture of cyclohexenone and/or an adduct depending on the reaction conditions summarized in Scheme 5. 

We emphasize that the critical ion pair stilbene+, CA in the two photoactivation methodologies (i.e., charge-transfer activation as well as chloranil activation) is the same, and the different multiplicities of the ion pairs control only the timescale of reaction sequences.14 Moreover, based on the detailed kinetic analysis of the time-resolved absorption spectra and the effect of solvent polarity (and added salt) on photochemical efficiencies for the oxetane formation, it is readily concluded that the initially formed ion pair undergoes a slow coupling (kc - 108 s-1). Thus competition to form solvent-separated ion pairs as well as back electron transfer limits the quantum yields of oxetane production. Such ion-pair dynamics are readily modulated by choosing a solvent of low polarity for the efficient production of oxetane. Also note that a similar electron-transfer mechanism was demonstrated for the cycloaddition of a variety of diarylacetylenes with a quinone via the [D, A] complex56 (Scheme 12). 

The modulation of the ion-pair dynamics by salt and solvent effects as well as the observation of significant kinetic isotope effects unambiguously establishes that benzylic C—H activation proceeds via a two-step sequence involving reversible electron transfer followed by proton transfer within the contact ion pair, 41c,2°5 (Scheme 18). 

In this connection, attention should be paid to an unusual NMR technique called nuclear magnetic relaxation dispersion (NMRD). In contrast with NMR spectroscopy, the NMRD signal arises from the nuclei of the abundant solvent molecules and not from the dissolved substances. The relaxation properties of the solvent molecules are profoundly modified if the solvent contains paramagnetic particles (see a review by Desreux 2005). A solvent molecule sails in the vicinity of an ion-radical and finds itself in the local magnetic field of this paramagnetic particle. Then, induced magnetism of the solvent molecule dissipates in the solvent bulk. This kind of relaxation seems to be registered by NMR. NMRD is applicable to studies on ion-radical solvation/desolvation, ion-pair dynamics, kinetics of ion-radical accumulation/consumption, and so on. 

Table 2 Selection of Commonly Used Ion-Pairing/Dynamic Liquid-Liquid Ion Exchange/ Mobile-Phase Additive Species, Which at pH <7.0 Modify the Retention Characteristics of Unprotected Peptides on Chemically Bonded Hydrocarbonaceous Stationary Phases3-11...

Femtosecond spectroscopic investigations in the spectral range 400-880 ran have permitted to discriminate specific OH effects on the dynamics of short lived UV excited CTTS states and transient near-IR (HO e )H20 pairs. The complex nature of ultrafast prehydration elementary redox reactions with nascent OH radical (strong acid) must be contemplated in the framework of ion-pairs dynamics, ion-solvent correlation function, short-range ordering water molecules, solvent screening or anisotropic electric field effects and short-time vibronic couplings. 

Although it is not yet possible to assign a spectroscopic fingerprint to a particular GIP structure, above results demonstrate the ability of our experiment to afford new informations about the ion pair dynamics. 

In following papers, this group reported on various aspects of ion pair dynamics of ketone amine systems, such as hydrogen atom transfer [14c, 26] or the activation parameters for ion pair separation of the trans-stilbene-fumaronitrile CIP [27]. 

The reaction mechanism proposed above seems to be the most probable from the kinetic point of view, but must be treated only as a semiquantitative approach until the role of the ion pair dynamics has been exactly recognized. Systematic studies (similar to those performed in acetonitrile solutions) of solvent polarity and viscosity may provide the decisive answer, probably also making possible a more quantitative description with estimates of all the kinetic parameters. 

NOE is widely used in three-dimensional structure determination of molecules, especially for proteins in solution. NOE-based NMR experiments provide an effective means to investigate nanostructural organizations in ionic Hquids. Homonuclear NOE experiments (NOESY) and heter-onuclear NOE experiments (HOESY) have been extensively appfred to probe intra- and intermolecular interactions within ILs and interaction of ILs with solvents and inorganic salts. Due to the abihty to probe dipolar coupled through space interactions, NOE is a perfect choice to probe interionic interactions to understand ion-pair dynamics. Since quantum chemical calculations are a convenient method to study ion-pair interactions and spatial information about them [76-78], combinations of this with NOE experiments would be a powerful approach to understand site-specific ion-pair interactions. 

Bockman, T.M. and Kochi, J.K., Photoinduced electron transfer from enol silyl ethers to quinone. Part 2. Direct observation of ion-pair dynamics by time-resolved spectroscopy, /. Chem. Soc., Perkin Trans. 2, 1633,1996. 

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