Half-life, rotational isomers

The properties of 99Y suggest that this nucleus is a classical symmetric rotor. Thus, configurations can be assigned to all the bands in accordance with the predictions of the Nilsson model for A 100 and a deformation of e - 0.3. Also the mixing ratios 6 for the AI = 1 members of the bands can be accounted for in the classical picture of rotational nuclei. The half-life of the isomer at 2142 keV is obviously due to K forbiddenness. 

In a few cases, single-bond rotation is so slowed that cis and trans isomers can be isolated even where no double bond exists (see also p. 230). One example is N-methyl-A -benzylthiomesitylide (69 and 70), the isomers of which are stable in the crystalline state but interconvert with a half-life of 25 h in CDCI3 at 50°C. This type of isomerism is rare it is found chiefly in certain amides and thioamides, because resonance gives the single bond some double-bond character and slows rota-tion. (For other examples of restricted rotation about single bonds, see pp. 230-233). 

The ratio of count rates in the two ionization chambers at a known rotational speed of the wheel (800-1,400 rpm) allowed to calculate the half-life of the spontaneously fissioning nuclides. The value obtained was 0.02 s - about lo times shorter than expected for a normal spontaneously fissioning nuclide of this fissility parameter (see O Fig. 4.4). (The shortest-lived normal spontaneously fissioning nuclide that could be considered is pm formed in the reaction ( 0,2n) Fm. Its fissility parameter would be Z /A = 39.7. The half-life extrapolated from the systematics (O Fig. 4.4) would be of the order of 10 -10 s. The species observed here was later identified to be a shape isomer of Am (fissility parameter of 37.3). The ground state of Am decays by P decay and electron capture with a half-life of 16 h.)... 

Although we knew from the preparative separation that the rotational isomers were relatively stable to equilibration, the actual barrier was measured (P). The barrier was found to be 28.5 kcal/mol, similar to the expected value for a trisubstituted biaryl bond. This correlates to a half-life of 2.8 years at room... 

---