Trimethylsilylation has proved to be extremely useful in the gas chromatography and the mass spectrometry of hydroxy acid methyl esters. The characteristic mass spectral fragmentation patterns have shown these derivatives to be superior in some respects to others for structural elucidation; in particular, hydroxylation followed by trimethylsilylation provides a useful method of double bond location in unsaturated fatty acid esters.* 593

No abstract

The low-pressure association reactions for the proton bound dimers of acetone and a number of deuterium substituted variants have been studied using Fourier transform ion cyclotron resonance spectrometry. The association rate constants for the reactions (CH3)2COH+ + (CH&CO, (CH&COD+ + (CH&CO, (CD3)2-COH+ + (CD&CO, and (CD&COD+ + (CD&CO have been studied as a function of pressure, and from these data the unimolecular dissociation rate constants, kb, of the intermediate chemically activated adducts as well as their radiative cooling rates, kra, were derived. The kb were measured to be in the range of (1.49-4.9) X lo4 s-I at 47 OC, corresponding to adduct lifetimes of 20-67 ~.ls. These lifetimes were strongly dependent on deuterium labeling of the acetone. The radiative rate constants, k,,, vary between 96 and 147 s-l with little influence of the deuterium labeling in the methyl groups, but a significant dependence when the dimer is bridged by H+ us D+. A decrease in the temperature of the ion cyclotron resonance cell by about 20 OC results in a decrease in kb by about a factor of 2 with only a slight decrease in kra. These results are discussed in terms of RRKM theory, statistical thermodynamics, and the mechanism of spontaneous emission of radiation from chemically activated ions.

Fucoxanthin is an allenic carotenoid. Chemical and spectroscopic studies establish its structure as 3 '-acetoxy-5.6 -epoxy-3,5'-dihydroxy-6',7'-didehydro-5,6,7,8,5',6'-hexahydro-p-caroten-8-one (1 5).FUCOXANTHIN occurs in brown algae (Phaeophyceae) and in diatom^,^^^ and is probably the most abundant carotenoid in nature. Like chlorophyll b it is believed to act as an accessory pigment in photosynthesis.4 A role in the reproduction of brown algae has also been ~uggested.~ Fucoxanthin was first isolated in a crystalline state by Willstatter and Page in 1914 and, until recently, has been assigned a molecular formula of the type C,H,,-,,O, in keeping with the C40 formulation of most other carotenoids.2y6 The melting point of fucoxanthin has been reported as 159-160" and (after purification by chromatography on alumina) as 166-168'.Material with the lower m.p. was shown to be optically active,s but the higher melting product was originally reported to be optically i n a ~t i v e . ~ Since fucoxanthin is stereochemically labile, tends to solvate or occlude solvent, and is unstable to acids, alkalis, and oxygen, it was conceivable that the reported differences in physical properties were due to a structural or stereochemical difference in the two products. However, both melting