Intermediates lifetime

DR. ENDICOTT In fact, the cobalt-eyelam-oxygen adduct can have a very short lifetime depending on the competition reactions. If the cobalt dioxygen adduct is generated in the presence of greater concentrations of CoAA([l4]aneN ), the intermediate lifetime is clearly shortened. The classical studies made by Ralph Wilkins, for instance, were carried out under conditions in which intermediates only achieved small stationary state concentrations. 

At binding ratios r > 0.27, both linear and supercoiled DNAs show evidence of a marked structural change. A component with intermediate lifetime (t 5 ns) appears in the ethidium fluorescence decay, which may represent a partially intercalated species. The apparent torsion constants become highly nonuniform and exhibit considerably altered values. The long-range torsion constant increases appreciably for the linear DNA, but decreases for the supercoiled DNAs, which are substantially positively supercoiled at that point.(53)... 

If the distribution of transient intermediate lifetimes is similar to the time required for rf ejection, some of these species will be ejected from the FTICR cell before undergoing unimolecular dissociation and the overall intensity corresponding to the abundance of ejected ions will be lost from the second spectrum. The difference spectrum then corresponds to the mass spectrum of unimolecular dissociation products of those transient species that did live long enough to be ejected from the cell. If the intermediate is weakly bound, and/or contains relatively few atoms, a... 

This notion that reaction mechanisms are strictly enforced by the intermediate lifetime implies the existence of a narrow borderline region and a sharp change in reaction mechanism with changing lifetime of the carbocation intermediate. However, a narrow borderline region is not observed in all cases. The problem is... 

The understanding of chemical reactions taking place in the normal laboratory time scale, seconds-to-days, requires insight into much faster processes, with ultimate consequences in the normal time scale. For example, free radicals frequently have lifetimes in the microsecond or millisecond time scale. There are normally two approaches to study short-lived intermediates. In one, the experimental conditions are adjusted, so as to lengthen the intermediate lifetime to the point... 

The very short-lived ions (< 10 s) will decompose in the source of the mass spectrometer and will not be detected as such, while long-lived ions (> 10 s) are accelerated through the mass spectrometer to reach the detector intact. The ions of intermediate lifetime (10 5 to 10 6) are accelerated from the source with mass m but before reaching the mass analyser decompose to give a smaller ion of mass m. In the case of a magnetic sector instrument a broad metastable ion peak (m ) is observed at a non-integral m/z value given by the relationship... 

From the quantum yields of the direct cis-trans photoisomerization of 2,4-hex-adienes it is seen that a one-step conversion of the trans-trans isomer into the cis-cis isomer or vice versa does not occur. A common intermediate can therefore be excluded, and at least two different intermediates have to be assumed as indicated in Scheme 1 (Saltiel et al., 1970). In contrast to this singlet reaction with very fast return to S, the triplet reaction is characterized by intermediate lifetimes long enough to allow for interconversion. The result of the benzo-phenone-sensitized photoisomerization of 2,4-hexadienes is therefore in agreement with a common triplet intermediate for the isomerization of both double bonds (Saltiel et al., 1969). 

Intermediate lifetime. The lifetime of the enzyme-oxocarbenium ion complex in Dn An mechanisms cannot be determined by KIE studies. It must exist for at least... 

If the NH exchange rate is low, the NH peak is still broad because the electrical quadrupole moment of the nitrogen nucleus induces a moderately efficient spin relaxation and, thus, an intermediate lifetime for the spin states of the nitrogen nucleus. The proton thus sees three spin states of the nitrogen nucleus (spin number = 1), which are changing at a moderate rate, and the proton responds by giving a broad peak. In this case, coupling of the NH proton to the adjacent protons is observed. Such is the case for pyrroles, indoles, secondary and primary amides, and carbamates (Fig. 4.34). 

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