No abstract

The 2-hydroxyhomotropylium cation 10 and 8-endo-methyl-, 8-exo-methyl-, 8,8-dimethyl-, 1,8,8-trimethyl-, and 3,8,8-trimethyl-2-hydroxyhomotropylium cations, 12,11,13,14, and 15, respectively, were prepared by protonation of the corresponding 2,3-homotropones in FS03H. On the basis of a comparison of the 'H NMR spectra of the 2-hydroxyhomotropylium ions with nonaromatic systems it is concluded that they can properly be regarded as homoaromatic cations. Ions 10, 11, 12, and 13 isomerized when irradiated in FSO3H to give the corresponding I-hydrgxyhomotropylium cations 19, 21, 22, and 23, respectively. The thermal isomerization of these ions has been investigated. Cation 22 was shown to isomerize to 21 at -39.5 "C ( k = 4.3 X 10-4 s-I). An equilibrium was set up between these two ions consisting of 6% 22 and 94% 21 at 0 "C. At higher temperatures 21 rearranged to protonated I-phenylpropanal ( k = 2.5 X s-I at 37 "C). The 8,8-dimethyl cation 23 isomerized back to 13 ( k = 3.1 X I 0-4 S-I at -23 "C), which underwent a further series of rearrangements to give eventually protonated 8,8-dimethylbicyclo[ 3.1 .O]octa-3,6-dien-2-one (29). The symmetrical 8,8-dimethyl-4-hydroxyhomotropylium cation 32 was observed as an intermediate in the isomerization of 13 to 29. On the basis of the thermal isomerizations of 14 and 15 it was concluded that the 8,8-dimethyl-3-hydroxyhomotropylium cation must also be a n intermediate in the conversions of 13 to 29 and 13 to 32.

1305of the solvents. When the solvent forms hydrogen bonds, the splitting constant9 deviate above the linear lines. The slope and intersection of the line can be qualitatively accounted for by the Huckel MO calculation on the radical. solvent into account. When the neutral radical forms no hydrogen bonds with the solvent, the expression gives a good linear relationship between the hyperfine splitting constants and the Block and Walker parameters (eq 17) sgCo chemical shifts, line widths, viscosities, and electronic spectral data are reported for solutions of potassium hexacyanocobaltate(II1) in mixed solvents involving dimethyl sulfoxide, water, formic acid, trifluoroacetic acid, and propionic acid. The chemical shifts cover a range of some 350 ppm, and it in concluded that hydrogen-bonding effects are responsible for the differences. The larger high-field shifts and by inference the stronger hydrogen bonds are associated with the stronger acids. There in evidence for preferential solvation of the hexacyanmbaltate anion in mixtures of dimethyl sulfoxide and acids, but not in waterlacid mixtures. In the dimethyl sulfoxide mixtures the hydrogen bonds become progressively weaker as more acid molecules are incorporated into the second coordination sphere. Interactions with potassium or hydrogen cations do not contribute significantly to the shifts or line widths. The energy of the fmt d-d electronic transition shows parallel changes. The NMR line widths show only modest changes in mixed aqueous solutions, but larger changes (factors of up to several hundred) in dimethyl sulfoxide solutions. Since TI is reduced by a similar factor, "chemical exchange" is not responsible for the broadening. The larger changes in relaxation rate are associated with the formation of the stronger hydrogen bonds. Relaxation rates are not proportional to viscosity, showing that changes in relaxation mechanism rather than simple variations of the correlation time are involved. These results are interpreted in terms of changes in the lifetimes of the hydrogen bonds. The stronger hydrogen bonds (with trifluoroacetic acid) have lifetimes greater than the rotational correlation time of the complex leading to an efficient quadrupolar relaxation mechanism for complexes with unsymmetrically substituted second coordination spheres. Estimates of the quadrupole coupling constants and the rotational correlation times show that this mechanism suffices to account for the observed line widths. The weaker hydrogen bonds (with water) have lifetimes shorter than the rotational correlation times, and other relaxation mechanisms dominate. Formic acid shows intermediate behavior.

SynopsisThin-film composite membranes containing photoreactive groups have been developed. The presence of these groups facilitates the subsequent photochemical conversion of the membrane to include a range of different chemical moieties. For example, a cationic or an anionic membrane can be made from the same initial composite membrane. In this paper the selection of an appropriate photolabile group is investigated.

.J. Chcm. 61, 856 (1983).The photoisomcrizations of protonated 5-mcthylhcx-3-en-2-one and 4-methylpent-2-cnoic acid have been exalnined. In each case a relatively rapid c~i,s/trrrtl.s ison~crization about the carbon-carbon partial doublc bond and a slower conversion to protonated dihydrofuran 2H or prvtonatcd lactonc 5H, respcctivcly, were observed. The transformation of the acyclic cations to the five-membered ring cations was shown to occur by a photo-initiated. intramolecular hydride shift from the y to P carbon atom. This shift is fully in accord with an excited state twisting/chasge localization model for the excited states of these systems. Control experiments and the thermal chemistry of these ions is described.