Isotope shift

Isotope effects acting over a network of more than three bonds have been illustrated in the case of methyl groups contributing in a steric fashion to the shielding of another carbon nucleus (at least five bonds away, but close in 

3 Isotope Shifts. - Isotope shifts are intimately related to rovibrational corrections. Thus, with the same shielding surfaces used for rovibrational averaging, one can extract expected changes in chemical shift due to isotope substitution. This is once again illustrated in Dransfeld s work, which is cited near the beginning of the preceding section. Here too, some cautionary words are in 

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Although this limit is not always reaehed. The same is true for the eoherenee of the radiation. Eaeh of these properties ean be exploited for partieular ehemieal applieations. The monoeliromatieity ean be used to initiate a ehemieal reaetion of partieular moleeules in a mixture. The laser isotope separation of and in nafriral abimdanee exploits the isotope shift of moleeular vibrational frequeneies. At 10-50 em, the eorresponding shift of IR absorption wavenumbers is large eompared to the speetral width of the CO2 laser... 

The existence of isotope shifts and of tunable lasers with narrow Hnewidth leads to the possibHity of separating isotopes with laser radiation (113,114). This can be of importance, because isotopicaHy selected materials are used for many purposes in research, medicine, and industry. In order to separate isotopes, one needs a molecule that contains the desired element and has an isotope shift in its absorption spectmm, plus a laser that can be tuned to the absorption of one of the isotopic constituents. Several means for separating isotopes are avaHable. The selected species may be ionized by absorption of several photons and removed by appHcation of an electric field, or photodissociated and removed by chemical means. 

The overall biosynthetic pathway to the tetracychnes has been reviewed (74). Studies (75—78) utilising labeled acetate and malonate and nmr analysis of the isolated oxytetracycline (2), have demonstrated the exclusive malonate origin of the tetracycline carbon skeleton, the carboxamide substituent, and the folding mode of the polyketide chain. Feeding experiments using [1- 02] acetate and analysis of the nmr isotope shift effects, led to the location of... 

Vibrational spectra including Raman data of 3,3-dimethyldiaziridine and its hexadeutero compound were recorded in the gas phase and in the crystalline state. Assuming C2 symmetry and employing isotopic shifts and comparison with azetidine, a classification of bands which regarded 33 normal modes could be given (75SA(A)1509). 

Isotope shifts for most elements are small in comparison with the bandwidth of the pulsed lasers used in resonance ionization experiments, and thus all the isotopes of the analyte will be essentially resonant with the laser. In this case, isotopic analysis is achieved with a mass spectrometer. Time-of flight mass spectrometers are especially well-suited for isotopic analysis of ions produced by pulsed resonance ionization lasers, because all the ions are detected on each pulse. 

For easier reference to earlier work the shift for (239-240) was adopted as the reference for tabulation and where necessary derived from the (240-244) isotope shift by dividing the latter by 2.531, a constant factor determined by Blaise et al. (15). Over 9500 isotope shifts have now been measured, 50% of them in the infrared. 

The number of energy levels found to date, with the aid of the Zeeman effect and the isotope shift data, is 605 even and 586 odd levels for Pu I and 252 even and 746 odd for Pu II. The quantum number J has been determined for all these levels, the Lande g-factor for most of them, and the isotope shift for almost all of the Pu I levels and for half of those of Pu II. Over 31000 lines have been observed of which 52% have been classified as transitions between pairs of the above levels. These represent 23 distinct electron configurations. 

Isotope shift for various configurations in Pu I plotted as a function of the central electron density, 4m ig(0). 

Table I. The Lowest Level of Each Configuration of Pu I and Pu II with their Corresponding Lande g-Values and Experimental Isotope Shifts (E in cm-l and IS in 10-2 cm-l).

Isotope shift for Pu(I) configurations as a function of central electron... 

Z. Physik 126. 344 (1944) (change in value of isotope shift in atomic spectra) G. Scharff-Goldhaber, Phys. Rev. 90, 587 (1953) (excitation energy to first excited states of even-even nuclei). 

Figure 5.3. Model describing expected carbon isotopic shifts in direction of matrix values for browsers and grazers. Large arrows represent large shifts, small arrows the converse.

Taken from Ref. (191). Isotope shifts are the average of data collected with 457.9-, 488.0-, and 514.5-nm excitation. 

The S isotope shifts was reported as zero (17), but the data are of poor quality and the band is not well resolved. The isotope shift needs to be reinvestigated in light of isotope shifts observed for the equivalent band in the high-quality data obtained for aconitase and Pf Fd. 

Based both on the determined isotopic shifts and the comparison of the radical IR spectrum with the spectra of various substituted benzenes, the bands have been assigned to the normal modes and the force field of the benzyl radical calculated (Table 8). 

Carbon isotope fractionation was examined during the aerobic degradation of TCE by Burkholderia cepacia strain G4 that possesses toluene monooxygenase activity (Barth et al. 2002). There were substantial differences in values of isotope shifts during degradation, from 57 to 17 ppm, and when the data were corrected to correspond to the same amount of substrate reduction the Releigh enrichment factor was 18.2. 

It can be seen from Figures 3.7 and 3.8 that the calculations reproduce very well not only the experimental spectra but also the experimentally observed isotopic shifts indicating a high reliability of the computational method. According to this comparison, definite attribution can be made for even the difficult Raman bands that cannot be assigned based solely on the experimental results. It is, however, necessary to mention at this point that the calculated Raman spectrum provided directly by the ab initio computations correspond to the normal Raman spectrum with the band intensity determined by the polarizability of the correlating vibration. Since the intensity pattern exhibited by the experimentally recorded resonance Raman spectrum is due to the resonance enhancement effect of a particular chromophore, with no consideration of this effect, the calculated intensity pattern may, in many... 

Similar experiments were conducted with deuterium. Comparing the results of these experiments with those obtained with protium, a conclusion was made that the ratio of calculated coefficients of surface diffusion of these particles is close to Vz. In other words, this ratio is a manifestation of a usual isotopic shift for diffusivity given by as one might expect assuming that surface diffusion... 

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