Internal isotope effect determination

The differences in rate for the two positions of naphthalene show clearly that an additional-elimination mechanism may be ruled out. On the other hand, the magnitude of the above isotope effect is smaller than would be expected for a reaction involving rate-determining abstraction of hydrogen, so a mechanism involving significant internal return had been proposed, equilibria (239) and (240), p. 266. In this base-catalysed (B-SE2) reaction both k and k 2 must be fast in view of the reaction path symmetry. If diffusion away of the labelled solvent molecule BH is not rapid compared with the return reaction kLt a considerable fraction of ArLi reacts with BH rather than BH, the former possibility leading to no nett isotope effect. Since the diffusion process is unlikely to have an isotope effect then the overall observed effect will be less than that for the step k. ... 

Figure 21. Internal conversion rates of the Si keto state of OHBA (open circles) and ODBA (filled circles), determined by single exponential fits to the 2 band decay. Both show a monotonic increase in rate as a function of the excitation energy, but without a significant isotope effect.

The experimental KIEs were determined for the aliphatic Claisen rearrangement in p-cymene at 120°C and for the aromatic Claisen rearrangement either neat at 170°C or in diphenyl ether at 220°C. Changes in 2H, 13C or 170 composition were determined for unreacted substrates. For carbon analysis of allyl vinyl ether the C5 carbon was used as an internal standard. The C4 atom and rneta aryl protons were used as references in analysis of allyl phenyl ether. The 170 analysis was based on a new methodology. The results are summarized in Table 1, along with predicted isotope effects calculated for experimental temperatures by means of different computational methods. The absolute values of predicted isotope effects for C4 and C5 atoms varied with theoretical level and all isotope effects were rescaled to get reference effects equal to 1.000. 

Intermolecular isotope effects upon ion abundances are, in general, not reliable guides as to intermolecular kinetic isotope effects. Intermolecular kinetic isotope effects kl(E)/ku(E) could be determined by PIPECO, although few measurements have been reported [210]. It would be necessary to conduct measurements at two different photon energies since, in general, the molecules Mj and Mn would not have the same ionization energies (IEs). In this way, the molecular ions Mj" and M could be formed with the same internal energy (E = hv — IE). 

Intermolecular isotope effects upon bromine atom loss have been determined from a comparison of the El mass spectra of C6H5Br and C6D5Br over a range of electron energies near threshold [94, 636]. The inverse isotope effect, which was observed, is attributable to differences between the internal energies, P(E), of the two molecular ions. 

The second method is internal competition. This method is used for tritium and isotope effects where the label is a trace one and only determines isotope effects on V/K for the labeled substrate. It is also used for and isotope effects... 

At high pressures gaseous system most closely resemble the situation in condensed media, and it is instructive to determine the radiative rate constant for this system, and Its variation with temperature. Assuming that internal conversion from Sj is a nonquenchable process which occurs prior to vibrational relaxation from a nonequilibrated state and using results of and 0.j. determined by Cundall and Dunnicliff (105) and values of Tp due to Lockwood (114), the calculated kp values are presented in Table 7 for both CgHg and C Dg. The similarity of rate constants for the protonated and deuterated molecules indicates that the large differences in yield and lifetime for the two isomers are the result of an Isotope effect on a nonradlative transition. 

The accuracy of analysis increases substantially if the compound used as the internal standard is chemically similar to the compound being determined. In a limiting case it is expedient to use as the internal standard the same compound as that being determined, but with a different isotopic composition. The combined use of GC and MS makes it possible to utilize this optimal variant. In recent years, especially in the combined GC—MS analysis of biologically active compounds, wide use has been made of compounds labelled with stable isotopes as internal standards [58], one of the optimal methods being the use of compounds having three or four C atoms. A positive feature of this method is the absence of isotopic effects in the course of the GC separation or in detection during chemical ionization used in MS [59]. Isotope effects also do not manifest themselves in the processes that result in the loss of the sample substance by... 

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