reference mass spectrum of unlabeled propane and that of the labeled mixture. It is assumed that the 'isotope effect" 86 does not influence the fragmentation process. A part of the reference spectrum ml to m7 have relative intensities II to I7 as shown in Figure 9. From the measured peaks p 1 to p4 of the labeled mixture, one can calculate the fractionsf, to&. Each of the measured intensities p is the sum of the fraction multiplied by the intensity of the unlabeled component. This follows from Figure 9 and is shown in eq 1:(1) This can be written in matrix form for all the four peaks measured:The fractions fi to f4 are calculated from the inverse matrix (M-I) containing the intensities II to I7 from eq 2:Finally the fractionsf, (i = 1-4) are normalized according to:In principle, from any n peaks from the mass spectrum of C,Hw2 this calculation can be performed; however, experimental errors are relatively low when differences between intensities in the reference spectrum are large. Therefore, we used the masses m / z 44-41 for propane, m / r 30-32 for ethane, and m / z 31-29 for the C2 fragment of propane. By comparing the analyses of the C2 and the C3 fragments of propane, it could be concluded that in the singly labeled product mainly the primary atom is labeled.Acknowledgment. We gratefully thank Henri M. J. Snijders for his assistance at the high resolution mass spectrometer and Jef L. Willigers for his experimental help at the microflow reactor. The Dutch Organisation of Fundamental Chemical Research, SON, is acknowledged for its financial support. Registry No. CH,, 74-82-8; C2H4, 74-84-0; C2H2, 74-86-2; C3H8, 11 5-07-1; CHICHICH3, 74-98-6; CH3(CH2)2CHJ, 106-97-8; CHJ(C-H2)3CH,, 109-66-0; "CH4, 6532-48-5; CH3I3CH3, 6145-17-1; "CH,"CH3, 52026-74-1; CH,CH,"CH,, 17251-65-9; CHI"CH2CH3,
AbstractThe aerobic oxidative dehydrogenation of a-terpinene to p-cymene catalyzed by the mixed-addenda heteropolyanion PV2Molo04~-has been used to investigate the oxidation mechanism catalyzed by this class of compounds. The kinetics of the reaction show it to be zero order in a-terpinene, first order in dioxygen, and second order in the heteropolyanion catalyst. The kinetic results along with the use of UV-vis, ESR, 3'P NMR, and IR spectroscopy have enabled the formulation of a reaction scheme involving the formation of a stable substrate-catalyst complex in the catalyst reduction (substrate oxidation) stage and a p-peroxo catalyst intermediate in the catalyst reoxidation stage where dioxygen is reduced to water in a four-electron redox reaction.
IntroductionSince the discovery in 1826 by Berzelius of the first heteropoly compound,' the vast field of heteropolyanion chemistry has been centered mostly around the preparation, structure, properties, and analytical chemistry applications of these compounds.2 Heteropoly compounds are soluble polyoxoion salts of anions having the general formula [X,M,,,O,]r ( x I m), where X is a heteroatom and M is an addenda atom. A common and important class of these salts and those used in the maj...
