Ion-pair yield

A low ion pair yield of products resulting from hydride transfer reactions is also noted when the additive molecules are unsaturated. Table I indicates, however, that hydride transfer reactions between alkyl ions and olefins do occur to some extent. The reduced yield can be accounted for by the occurrence of two additional reactions between alkyl ions and unsaturated hydrocarbon molecules—namely, proton transfer and condensation reactions, both of which will be discussed later. The total reaction rate of an ion with an olefin is much higher than reaction with a saturated molecule of comparable size. For example, the propyl ion reacts with cyclopentene and cyclohexene at rates which are, respectively, 3.05 and 3.07 times greater than the rate of hydride transfer with cyclobutane. This observation can probably be accounted for by a higher collision cross-section and /or a transmission coefficient for reaction which is close to unity. 

The exceptionally low propagation constants of t-butyl and of phenyl methacrylate are notable. The polymerization of the former monomer was thoroughly examinedS5). At temperatures even as high as 25 °C this reaction, when performed in THF in the presence of salts depressing dissociation of ion-pairs, yields polymers of highly uniform size. The reaction is strictly first order in growing polymers and in monomer, and no... 

About 1910, M. Curie suggested that ions were responsible for the chemical effects of radioactive radiations. Soon thereafter, mainly due to the pioneering work of Lind on gases, the notation M/N was introduced for a quantitative measure of the radiation effect, where N is the number of ion pairs formed and M is the number of molecules transformed—either created or destroyed. This notation, referred to as the ion pair yield, was most conveniently employed in gases where N is a measurable quantity. However, for some time the same usage was extended to condensed systems assuming that ionization did not depend on the phase. This, however, is not necessarily correct. The notation G was introduced by Burton (1947) and others to denote the number of species produced or destroyed per 100 eV absorption of ionizing radiation. In this sense, it is defined... 

Specific ionization times the energy per ion pair yields the stopping power (LET), as shown in Equation 6-3. 

The electron may be transferred back in the radical ion pair yielding either the starting material or an adduct A—D (cage process). Alternatively, the ions diffuse apart and perform their characteristical reactions (escape process). 

Besides cyclobutane formation, alternative ring closures are sometimes observed. One example is the 9,10-dicyanoanthracene sensitized dimerization of 1,1-diphenylethylene (169). The six-membered ring is formed via the 1,4-radical cation, which results from the addition of the free radical cation to diphenylethylene as indicated in Scheme 56, while the 1,4-biradical generated by back electron transfer from the radical ion pair yields tetraphen-ylcyclobutane (170) (Mattes and Farid, 1983). 

Yields were originally measured in terms of the number of molecules M decomposed per ion pair formed by absorption of radiation. This ion pair yield is expressed as MjN where N, the number of ion pairs formed, is usually indeterminate in a condensed system. In this case, the yield is referred to the total energy... 

The sensitivity of fragmentation to the nature of resonances was shown for tetramethylsilane by using the photoion photoion coincidence technique (Morin et al. 1986). In this technique, fragmentation channels are identified by the coincidences of pairs of ions in the time-of-flight mass spectrometer. For tetramethylsilane, two resonances were observed in the ion pair yield curves. The lower energy resonance, below the ionization edge, had been shown from electron spectra to decay into a one-hole state, but the production... 

The conductance study of (Cs+,T )BPh4 in THF led to its dissociation constant, of 1.1 10-4 M at 20 °C. A perfect linear relation was obtained for kp plotted vs. /,fraction of dissociated ion-pairs, yielding k 5.6 M-1s-1 and 1 10-6 M at 20°C. Hence, the kryptated cesium salt is about four times as reactive as the kryptated potassium salt, and the dissociation constant of the former salt is three times larger than of the latter. 

Santamaria et al [42] for synthesizing a-amino nitriles in the alkaloid field and also for preparing 6-cyano-l,2,3,6-tetrahydropyridine from corresponding pyridine nucleus. A similar approach has also been used by Sundberg et al. [43a,43b] for the cyanation of Catharanthine alkaloids. In situ trapping of the iminium cation (37) by allyltrimethylsilanes or silyl enol ethers is also shown [44] recently as a direct —C-C— bond formation methodology at the a-posi-tion of tertiary amines (Scheme 8). The success of this reaction is based on the comparative correlation of ion-pair yield with the AGg, values from amines and enol ethers. 

For the exo isomer 264 the ratio of rate constants determined polari- and titrimetric-ally (k k,) was shown by him to equal about 4. This means that the formation of racemic brosylate from a symmetrical ion pair as a result of internal return is essential for the solvolysis of the exo isomer for the endo isomer 266 the ratio k, k, = 1.0. In using the polarimetrical constants the exo endo rate ratio is equal to 62000. Coi quently, the exo isomer 264 is ionized with anchimeric participation (k ) the resulting ion pair yields racemic (99.9 0.1 %) 2-exo acetate, significant internal return simultaneously taking place. The ionization of the endo brosylate 266 proceeds with the solvent assistance (k ) and results in a classical ion pair. Winstein s formula 265 is a resonance structure corresponding to the same canonical structures as in the ion 265, but with addition of the phenyl cation 267. 

The initial yields of the ions have been determined from gamma radiolysis in cyclohexane at a pressure of 55 torr by isotopic labelling techniques. In Table 6 we show the initial yields of the various ions together with those obtained from mass-spectrometric determinations at very low pressure. The yields are given as ion-pair yields, i.e. the yield of species per ion pair produced. The ion-pair yield M/N can be converted into species produced per unit energy absorbed, by multiplication by the yield of ionization G = 4.4 (lOOeV)" We see that the decomposition of the parent ion is appreciably suppressed in the gas at 55 torr, due to collisional deactivation. 

The decrease in decomposition of of 0.3 in the ion-pair yield is matched by an increase in the fragmentation yield of 0.35. The fragmentation reactions are ... 

The basic advantage of Geiger counter is derived from toe amplification (each primary Ion pair yields close to 100 million secondary ion pairs). The amplification is so great that an extremely simple external electronic ampMer is sufilclent. This simplicity imparts stability to Geiger counters. 

Visual inspection of the distributions shows that the density data distribution is normal for all three cases. The standard deviation becomes smaller with increasing system size. The mean density varies only moderately with the size of the simulation box the smallest box (10 ion pairs) yielded a statistically different mean density (1.0906 g/cm ) than the two larger boxes (1.0951 and 1.0952 g/cm ), which were found to be statistically the same. The box plot visualization of the data set illustrates this finding. 

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