Interconversion rates

LR and LRX dissociate with half-times comparable to those seen in the absence of interconversion (rate 3, t, 10 s rate 5,... 

Individual Spin-State Interconversion Rate Studies. 119... 

Several observations indicate spin-state interconversion rates faster than could be measured with the applied technique. Thus an upper limit for the relaxation time of 30 ns has been set for [Fe((CH3)2NC(S)S)3] [34], whereas the estimate r < 1 ns has been obtained for the complex [Fe(benzac2trien)]PFg... 

The methano-dimer of a-tocopherol (28)50 was formed by the reaction of o-QM 3 as an alkylating agent toward excess y-tocopherol. It is also the reduction product of the furano-spiro dimer 29, which by analogy to spiro dimer 9 occurred as two interconvertible diastereomers,28 see Fig. 6.23. However, the interconversion rate was found to be slower than in the case of spiro dimer 9. While the reduction of furano-spiro dimer 29 to methano-dimer 28 proceeded largely quantitatively and independently of the reductant, the products of the reverse reaction, oxidation of 28 to 29, depended on oxidant and reaction conditions, so that those two compounds do not constitute a reversible redox pair in contrast to 9 and 12. 

Fig. 1 Jablonski diagram of energy level for describing processes absorption, fluorescence and phosphorescence in complex molecules where kf and /c arc the radiative and nonradiative rates of fluorescence, respectively, kj and kTnr are the radiative and nonradiative rates of phosphorescence, respectively, k-lsc is the interconversion rate, and kmt is the rate of intermolecular processes Av denotes the Stokes shift of fluorescence...

The second and third relaxation processes were coupled, where the observed rate constants differed by a factor of 3 to 7 and the rate constant for each relaxation process varied linearly with the DNA concentration.112 This dependence is consistent with the mechanism shown in Scheme 2, where 1 binds to 2 different sites in DNA and an interconversion between the sites is mediated in a bimolecular reaction with a second DNA molecule. For such coupled kinetics, the sum and the product of the two relaxation rate constants are related to the individual rate constants shown in Scheme 2. Such an analysis led to the values for the dissociation rate constants from each binding site, one of the interconversion rate constants and the association rate constant for the site with slowest binding dynamics (Table 2).112 The dissociation rate constant from one of the sites was similar to the values that were determined assuming a 1 1 binding stoichiometry (Table 1). 

Rotamer distribution and interconversion rates give biochemically-relevant information on hydrogen-bonding and steric interligand interactions in [Pt(Me3dien)(guanine)]2+ (230). 

The lability inherent in the planar, tetrahedral equilibria which nearly all involve Ni(II) requires that nmr line broadeningor photochemical perturbation methods be used for their kinetic resolution. First-order interconversion rate constants for... 

For a complete quantitative description of the solvent effects on the properties of the distinct diastereoisomers of dendrimers 5 (G = 1) and 6 (G = 1), a multiparameter treatment was used. The reason for using such a treatment is the observation that solute/solvent interactions, responsible for the solvent influence on a given process—such as equilibria, interconversion rates, spectroscopic absorptions, etc.—are caused by a multitude of nonspecific (ion/dipole, dipole/dipole, dipole/induced dipole, instantaneous dipole/induced dipole) and specific (hydrogen bonding, electron pair donor/acceptor, and chaige transfer interactions) intermolecular forces between the solute and solvent molecules. It is then possible to develop individual empirical parameters for each of these distinct and independent interaction mechanisms and combine them into a multiparameter equation such as Eq. 2, "... 

The energy barrier for internal rotation in dipeptides is generally high (89.6 kJ mor in L-hystidyl-L-proline) (82MI6) but decreases when the dipeptide unit is introduced in internal positions of a longer sequence. This is seen (82MI6), for example, in the octapeptide angiotensin II, in which the L-hystidyl-L-prolyl unit is in position 6 or 7, and the interconversion rate becomes 70 times faster than in the isolated dipeptide. 

The far-infrared spectrum of Fe(EtPhDtc)3 as a function of pressure shows that the intensity of the band assigned to the 2 V2 state increases relative to that assigned to the 6A t state upon increasing pressure (93b) (38). For the Fe(n-Pr2 -Dtc)3 complex, the Fe—S vibration at 367 cm"1 was assigned to the low-spin-state (6i4j) isomer. On the basis of these results, the spin-state equilibrium was adopted as the true model with a spin-state interconversion rate lower than the vibrational time scale ( 10" 3 sec). 

A number of observations indicate interconversion rates for some iron(III) complexes too fast to measure with existing techniques. Relaxation times less than the 30-nsec limit set by the heating rise time of the laser temperature-jump technique were observed for [Fe(benzac2trien)]+, [Fe(Salmeen)2]+, and [Fe(Me2dtc)3] (45, 128). From ultrasonic observations a limit of less than 1 nsec was placed on the relaxation time for the first of these compounds,... 

The EPR spectrum of a spin-equilibrium complex can be used to establish a lower limit to the spin state lifetimes of the order of 10 10 second. In an important paper in 1976, Hall and Hendrickson reported observation of EPR signals for both the high-spin and the low-spin isomers of iron(III) dithiocarbamate complexes at 4 12 K as powders, glasses, and doped solids (71). This resolved the question whether these complexes possess distinct high-spin and low-spin states. It also sets a lower limit on their interconversion lifetimes. Similarly, the observation of signals for both the high-spin and low-spin states of [Co(terpy)22+] (97) leads to the same conclusions about this complex. In both cases the interconversion rates in solution have proved too fast to measure, with lifetimes of less than 10-9 second indicated. The solution measurements were undertaken, of course, at room temperature and the EPR measurements at close to 4 K. Significant differences in the rates of solid and solutions at room temperature are still possible. 

Interconversion rates have also been measured recently using a photoperturbation technique on solid samples. Irradiation of a sample... 

At this point NOE and NOESY experiments are needed. In this way we learn which proton is close to which other proton, and the picture becomes sharper. Sometimes, when the correlation times governing cross relaxation are unfavorable, ROE or ROESY experiments can be a valid alternative. Both NOE and ROE types of experiment also provide information on the presence of chemical equilibria when the interconversion rate is of the order of T. It is also clear that NOE and ROE types of experiment may not always provide a unique picture, because they ignore chemical bonds. COSY and TOCSY experiments provide the information on which proton is chemically bound to which other proton. 

Brouwer and van Doom51 used this approach in the early 1970s. In the NMR study of the interconversion rates of alkyl tetrahydrofuryl ions 7 and 8 (Scheme 1.2), proceeding via dicarbenium ion intermediates, they measured the overall rate of rearrangement in various superacid combinations of HF, HS03F, and SbF5. 

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