Nucleation in a supercooled or a supersaturated medium is a stochastic event, and hence statistical analyses are required for the understanding and prediction of such events. The development of reliable statistical methods for quantifying nucleation probability is highly desirable for applications where control of nucleation is required. The nucleation of gas hydrates in supercooled conditions is one such application. We describe the design and development of a high pressure automated lag time apparatus (HP-ALTA) for the statistical study of gas hydrate nucleation and growth at elevated gas pressures. The apparatus allows a small volume (≈150 μl) of water to be cooled at a controlled rate in a pressurized gas atmosphere, and the temperature of gas hydrate nucleation, T(f), to be detected. The instrument then raises the sample temperature under controlled conditions to facilitate dissociation of the gas hydrate before repeating the cooling-nucleation cycle again. This process of forming and dissociating gas hydrates can be automatically repeated for a statistically significant (>100) number of nucleation events. The HP-ALTA can be operated in two modes, one for the detection of hydrate in the bulk of the sample, under a stirring action, and the other for the detection of the formation of hydrate films across the water-gas interface of a quiescent sample. The technique can be applied to the study of several parameters, such as gas pressure, cooling rate and gas composition, on the gas hydrate nucleation probability distribution for supercooled water samples.

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The solvolysis of 21 different 3‐ or 17‐androstanyl tosylates or chlorides with halogen, hydroxy or oxo substituents in 17‐ or 3‐position shows rate constant variations in hexafluoro isopropanol of up to 6·3 with regular differences for epimeric substituents. Similar variations ae seen over a distance of ∼10 Å in 13C‐NMR shifts at C‐17‐ones of androstan‐17‐ones with varied halogen substituents in C‐3, whereas the shielding observed at other distant carbon atoms excludes significant through bond effects. It is shown how, on the basis of suitable Coulomb‐type equations and of force field minimized geometries, both the stability variation of carbocationic transition states and the carbonyl group polarization are predictable by linear electric field effects, using the same parametrization.

Solvolysis in hexalluoroisopropyl alcohol of 17a-(tosyloxy)androstane as well as of the corresponding 18-norsteroid proceeds faster than that of the 17P-isomers by a factor of > lo4 (25 "C), and also considerably faster than that of cyclopentyl or cyclohexyl tosylates.The same 1,2-methyl migration products are observed with both epimers. Kinetic comparison with steroids, cyclohexane and cyclopentane derivatives with vicinal methyl groups or hydrogen shows that neither bridging with antiperiplanar C -C bonds, nor hydrogen rearrangements leading to charges at tertiary centers can be responsible for the. reactivity differences. Chlorides instead of tosylates show a substantial decrease of the epimeric rate ratios. The epimers are characterized by k,/k, ratios differing by a factor of Id to lo3.Molecular mechanics calculations indicate that the fast reactions of the 17u-isomers are due to steric acceleration; the 17P-epimers react much slower than predicted by the applied force field model, which is in accord with a strong steric hindrance to solvation. Preparations of, e.g., 18-norsteroids and "C NMR spectra are described. In spite of almost countless investigations the either classical (steric) or nonclassical (charge delocalization) origin of large reactivity differences in the solvo-