Iridium isotope

There are two iridium isotopes, ir and Ir, suitable for Mossbauer spectroscopy. Each of them possesses two nuclear transitions with which nuclear resonance absorption has been observed. Figure 7.58 (from [266]) shows the (simplified) nuclear decay schemes for both iridium Mossbauer isotopes the Mossbauer transitions are marked therein with bold arrows. The relevant nuclear data known to date for the four Mossbauer transitions are collected in Table 7.1 at the end of the book. 

In another type of treatment for prostate cancer, long needles containing iridium-192 are placed in the tumor. However, the needles are removed after 5 to 10 min, depending on the activity of the iridium isotope. Compared to permanent brachytherapy, temporary brachytherapy can deliver a higher dose of radiation over a shorter time. The procedure may be repeated in a few days. 

Gammagraphic weld inspection in the lower range of steel thicknesses has been done with Iridium and Ytterbium isotope sources throughout the past. The large majority of applications has been using Iridium due to the unfavourable economical parameters of Ytterbium, obviously with non-optimal results at thin wall inspections. 

The GammaMat M isotope pipeline crawlers previously have been used with exposure cameras for iridium the models M6 and Ml 8 used exposure units designed for a maximum loading of 2.2 TBq (60Ci) and 3.7 TBq (lOOCi), respectively. 

For organometailic compounds, the situation becomes even more complicated because the presence of elements such as platinum, iron, and copper introduces more complex isotopic patterns. In a very general sense, for inorganic chemistry, as atomic number increases, the number of isotopes occurring naturally for any one element can increase considerably. An element of small atomic number, lithium, has only two natural isotopes, but tin has ten, xenon has nine, and mercury has seven isotopes. This general phenomenon should be approached with caution because, for example, yttrium of atomic mass 89 is monoisotopic, and iridium has just two natural isotopes at masses 191 and 193. Nevertheless, the occurrence and variation in patterns of multi-isotopic elements often make their mass spectrometric identification easy, as depicted for the cases of dimethylmercury and dimethylplatinum in Figure 47.4. 

The lr Mossbauer experiments are usually carried out in transmission geometry with both source and absorber kept at liquid helium temperature and a Ge(Li) diode or a 3 mm Nal(Tl) crystal used to detect the 73 keV y-rays. The absorbers typically contain 50-500 mg cm of natural iridium, which contains 62.7% of the Mossbauer isotope lr. The isomer shifts are generally given with respect to iridium metal (the isomer shift between Os/Os and Ir metal is (0.540 0.004) mm s at 4.2 K ([268]). 

The two Mossbauer levels of Pt, 99 keV and 130 keV, are populated by either EC of Au(fi/2 = 183 days) or isomeric transition of Pt(fi/2 = 4.1 days). Only a few authors, e.g., [323, 324] reported on the use of Pt, which is produced by thermal neutron activation of " Pt via " Pt(n, y) Pt. The source used in the early measurements by Harris et al. [322, 325] was carrier-free Au diffused into platinum metal. Walcher [326] irradiated natural platinum metal with deuterons to obtain the parent nuclide Au by (d, xn) reactions. After the decay of short-lived isotopes, especially Au(fi/2 = 6.18 days), Au was extracted with ethyl acetate, and the Au/Pt source prepared by induction melting. Buym and Grodzins [323] made use of (a, xn) reactions when bombarding natural iridium with... 

The results obtained with nickel raised the question whether the relation found between rate of exchange and particle size holds also for other metals of group VIII. We therefore carried out the benzene-D2 reaction on some iridium catalysts widely differing in particle size. We chose iridium because we knew from earlier experiments that iridium black gives a very characteristic cyclohexane isotopic distribution pattern with a maximum for C6H4Ds, whereas the patterns of Ni, Ru, Pd, and Pt show a maximum for the d6 compound. 

Two of the three catalysts, viz, Ir-8 and Ir-45, have also been used in the infrared experiments. Ir-600 is an iridium-black catalyst. Figure 15 shows the cyclohexane isotopic distributions for the iridium catalysts. 

A histogram of the benzene isotopic distribution pattern obtained on the three iridium catalysts is shown in Fig. 16. 

The weighted-average atomic mass of the element iridium is just slightly more than 192 u. The mass of the first isotope is a bit less than 191 u. Hence, the mass of the second isotope must more than 192 u that isotope must be 193 Ir. 

The peaks in the m/z 50-57 range of the 1-butene El spectrum could be misinterpreted as a complex isotopic pattern if no formula were available on the plot (Fig. 3.8). However, there is no element having a comparable isotopic pattern and in addition, all elements exhibiting broad isotopic distributions have much higher mass. Instead, the 1-butene molecular ion undergoes H, H2 and multiple H2 losses. The m/z 57 peak, of course, results from In a similar fashion the peaks at m/z 39 and 41 appear to represent the isotopic distribution of iridium, but this is impossible due to the mass of iridium (cf. Appendix). However, these peaks originate from the formation of an allyl cation, CsHs, m/z 41, which fragments further by loss of H2 to form the CsHs" ion, m/z 39 (Chap. 6.2.4). 

Fig. 9.17. Partial LT-FAB mass spectmm of the reaction mixture containing the iridium complexes 1 and 2 in toluene. In addition to the changes in mass, the isotopic pattern changes upon exchange of Cl by Br. By courtesy of P. Hofmann, University of Heidelberg.

Example Selective activation of C-H bonds is rarely observed in saturated alkyl groups, but the iridium complex 1 does react by C-H insertion of the metal into a ligand bond upon treatment with LiBr in solution. The reaction can be tracked by LT-FAB-MS (Fig. 9.17). A decreasing intensity of the molecular ion of 1, m/z 812.4, and increasing of 2, m/z 856.4, indicate the progress of this reaction. Furthermore, the halogen exchange is indicated by the isotopic pattern. 

Scheme 12 Isotopic labeling and crossover experiments in iridium-catalyzed couplings of 1,1-dimethylallene under transfer hydrogenation conditions...

Extensive studies of kinetics and isotope effects by Hartwig and coworkers support the mechanism shown in Scheme 5 for the lr(I)/dtbpy catalyzed borylation [81]. In particular, these studies indicate that the iridium(III) trisboryl bipyridine complex (10) is the species that activates the arene C-H bond this is in agreement with DFT calculations by Sakaki et al. predicting the key intermediacy of the trisboryl complex and the seven-coordinated Ir(V) species resulting from C-H addition [82]. C-H addition to Ir(III) was also proposed in the (Ind)Ir(COD)/ phosphine-catalyzed borylation by Smith et al. [76]. 

ISOTOPES There are 55 Isotopes of Iridium, two of which are stable and account for the element s total existence on Earth. Those two are lr-191, which makes up 37.3% of the amount In the Earth s crust, and lr-193, which constitutes 62.7% of Iridium s existence on Earth. All the other 53 Isotopes of Iridium are radioactive with half-lives ranging from a few microseconds to a few hours or days and up to a few hundred years. These unstable Isotopes are all artificially produced. 

Fig. 5.5. Decomposition of Solar System abundances into r and s processes. Once an isotopic abundance table has been established for the Solar System, the nuclei are then very carefully separated into two groups those produced by the r process and those produced by the s process. Isotope by isotope, the nuclei are sorted into their respective categories. In order to determine the relative contributions of the two processes to solar abundances, the s component is first extracted, being the more easily identified. Indeed, the product of the neutron capture cross-section with the abundance is approximately constant for aU the elements in this class. The figure shows that europium, iridium and thorium come essentially from the r process, unlike strontium, zirconium, lanthanum and cerium, which originate mainly from the s process. Other elements have more mixed origins. (From Sneden 2001.)... 

The most widely used radioactive isotopes in medical and industrial applications are cobalt 60 ( C), cesium 137 ( Cs), and iridium 192 ( lr). The half-life of is 5.3 years, that of Cs is 30 years, and the half-life of Ir is 74 days. When used for irradiation the isotope is generally in the form of a pellet size, 1.5 x 1.5 mm, loaded into a stainless steel capsule and sealed. Unlike electron beam or x-rays, gamma rays cannot be turned off. 

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