Selenium calculation

Figure V-2 Comparison between the saturated pressure of selenium calculated from the selected thermodynamic quantities and the total pressure measured in various investigations. A logarithmic unit of 0.1 corresponds approximately to a relative error of 25%.

The selenium analogue [PhCNSeSeN] and cyano-functionalized diselenadiazolyl radicals adopt cofacial dimeric structures, e.g., 11.4 (E = Se), with unequal Se Se interactions of ca. 3.15 and 3.35 A. In the latter case the radical dimers are linked together by electrostatic CN Se contacts.Tellurium analogues of dithiadiazolyl radicals (or the corresponding cations) are unknown, but calculations predict that the radical dimers, e.g., 11.4 (E = Te), will be more strongly associated than the sulfur or selenium analogues. ... 

In the various editions of his textbook, and in the publications dealing specifically with his predictions, Mendeleev repeatedly gives the example of calculating the atomic weight of the element selenium, a value that was known at the time and which could thus be used to test the reliability of his method. 

The selenious acid does not need to be freshly prepared. A larger amount does not increase the yield. The submitters specified selenium dioxide as the oxidizing agent, but the checkers prepared their material by evaporating an aqueous solution to dryness on the water bath. They, therefore, have considered the oxidizing agent to be selenious acid and have calculated the... 

The structure of 11 2IBr is (TBP, TBP), which represents the first example of a T-shaped adduct of a selenium compound with IBr.39a The anomalous bond distances in ll-2IBr (Se-Br (A)/Se-I (A) 2.897(2)/2.606(2) and 2.766(2)/ 2.629(2) for the two Br-Se-I units, respectively) have been explained on the basis of different polarities of the two bonds and on the basis of the involvement of Br in intramolecular hydrogen bondings. Reactions and/or structures are similarly investigated for A-methylthiazolidine-2(3H)-selone (12), N-methylbenzothiazole-2(3H)-selone (13), tV,7V -dimethylbenzoimidazole-2(3H)-thione (14), and -selone (15) with bromine,39b,c and more recently, for 14 and 15 with IBr.39d In this last case, the prediction of a 10-1-2 adduct for 14 IBr and a 10-Se-3 adduct for 15 IBr from the DFT calculations has been experimentally confirmed.39d... 

No new calculations were specifically devoted to this heterocylic system since CHEC-II(1996). Redox properties of chalcogeno-ureas possessing this heterocyclic skeleton and resulting from the reaction of Arduengo carbenes such as 108 with sulfur or selenium was investigated through semi-empirical calculations <2000EJI1935>. 

Prescribed Limit The measured absorbance at 420 nm is not greater than that of a solution prepared by treating 5 ml of selenium standard solution (1 ppm Se) in the same manner (5 ppm, calculated as Se). 

The mechanism of the asymmetric methoxyselenenylation of alkenes has been investigated using competition experiments and computational methods (Scheme 8). The experiments have demonstrated that the formation of the intermediate seleniranium ion (48) is reversible. Ions of type (49), generated in the addition of chiral selenium electrophiles to alkenes, are the key intermediates in the asymmetric methoxyselenenylation their stability is strongly dependent on the strength of the selenium-heteroatom interaction. Calculations have been carried out to determine the relative stabilities of the diastereoisomeric seleniranium ions (49). The results obtained from the calculations support the experimental flndings. ... 

In the above-described displacement reactions (Sh2) of sulfur- and selenium-containing compounds, the mechanism could either be a synchronous or a stepwise process (Scheme 4.2). Thus, in the Sh2 stepwise mechanism an intermediate sulfuranyl or selenyl radical is formed, followed by a-cleavage [56]. Based on competitive studies, a stepwise process was suggested to occur in the reaction of (TMS)3Si radical with -decylphenylselenide [43]. On the other hand, ab initio calculations supported the synchronous process and the same observations were explained in terms of an overall reversible reaction [57]. 

Raman scattering from crystalline S3, SCg, and cyclic selenium sulfides is very intense. The Raman spectrum of 1,2,3-86385 is shown in Fig. 3 as a typical example of the spectra of mixed Se Sg species. The first Raman spectra of liquid and solid sulfur-selenjum phases were reported in 1968 by Ward For a mixture of composition SCqojSo js he observed nine weak Raman lines not present in the spectrum of Sg and assigned them to different Se Sg (n = 0-4) molecules by comparison with the spectra of 8g and 8eg. According to the more recent calculations the assignment given in Table 4 is the most likely one. 

Table 4. The nine weak Raman lines of liquid and solid sulfur-selenium phase with composition S oo5 o95 assignment of the lines is given according to the calculations by the present...

The Raman spectrum of the reaction product of titanocene pentasulfide and dichlorodiselane shows the presence of l,2,3-Se3Sj as the main species (sea Fig. 3). The normal coordinate treatment using a modified Urey-Bradley force field with 12 independent force constants resulted in a complete assignment of the spectrum and in a very good agreement between the observed and calculated wave numbers. The composition of the phase as determined from the HPLC data, molecular weight measurement, and selenium analysis agrees with the vibrational analysis. 

Figure 8.1 Theoretically calculated f and f" values as a function of wavelength in the vicinity of the K absorption edge of selenium (a) and the L edge of holmium (b).

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