J. Phys. Chem. 1990, 94, 5391-5398 by a particularly rapid formation but by extraordinarily slow rates
of consecutive condensation reactions of the respective ions. PAH' ions are not formed via a one-lane road. Otherwise the formation of the first relatively stable ion would be a bottleneck for the formation of larger PAH+, which is not the case. Other PAH+ which are not so inert to further condensation reactions are consumed both by growth and by thermal decomposition (or partial oxidation). With increasing temperature thermal decomposition soon becomes the dominating consumption for ions up to a mass of about 325 u. The reason for the increasing maximum concentrations of ions in the mass range 100-200 u is probably the still increasing total positive ion concentration in the respective flame zone and the apparent tendency of smaller aromatic ions to decompose faster. Between 7 and 9 mm (Figure 6) the PAH+ in the range 200-325 u (including those ions which are relatively inert to growth) decompose almost simultaneously. If they were consumed only by growth through addition of C2H2,for example, then, with increasing mass, the decreasing flanks of their profiles should be graduated to greater heights in the flame and their maximum concentrations should decrease monotonously. This is the result of some general computer simulations. The picture is different for PAH+ with masses larger than about 325 u. Their profiles do just show the behavior expected for consumption by growth only. Apparently these large species are not, or are much less, decomposed thermally. Their growth could be followed to several thousand mass units into the range where coagulation to soot particles began.I4 However, this does not solve the problem why PAH+ formation and growth was so much faster in the oxidation zone than in the burned gas (factor of lo2)and why it stopped in the postflame
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gas at an average mass of about 250 u although the PAH+ in both flame zones were qualitatively identical (as far as mass spectrometry can prove). An explanation for the large difference in rate would be the participation of free radicals in the condensation reactions of the ions. Their contribution to the growth of ions would become relatively greater in the same degree as the reactivity of the latter to nonradicalic alkynes decreased with increasing ion mass. Unfortunately nothing is known about the reactions of large ions with small hydrocarbon radicals. However, the following order-of-magnitude considerations show that this is a realistic assumption if the rate constants are of the order of 1 X lo-" to I X cm3/s or larger: In a flame with C / O = I . 12, u, = 42 cm/s the maximum concentration of C29H15+was 3 X IO6 and the average rate of formation of a PAH+ with 32 C atoms was 1 X lo9 cm-3 s-I. Within the above range of rate constants a C3-radical concentration of 3 X 1012-3 X loi3cmV3 corresponding to mole fractions of 2 X 10-5-2 X lo4 would be necessary. Delfau et al. have reported a C3H3mole fraction of (3-6) X IO-" in the oxidation zone of a low-pressure flame, depending on This could explain the continued growth of PAH+ in the radical-rich oxidation zone whereas the growth in the burned gas ceases since the radical concentration is too low.
Acknowledgment. This work received substantial support from the Deutsche Forschungsgemeinschaft and also from the Fonds der Chemischen Industrie which is gratefully acknowledged. We also thank S. Loffler for helpful discussions. Registry No. C2H4,74-85-1. (41) Delfau, J. L.; Vovelle, Ch. Combusr. Sci. Techno/. 1984, 41, I .
Structure and Dynamics of the Urea-Trloxane Inclusion Compound Phases, Studied by *H NMR Spectroscopy E. Gelerinter,+ Z. Luz,* R. Poupko, The Weizmann Institute of Science, 76 100 Rehovot. Israel
and H. Zimmermann Max- Planck- Institut fur Medizinische Forschung, AG Molekiilkristalle, Jahnstrasse 29, 0-6900 Heidelberg, Federal Republic of Germany (Received: December 1 , 1989) Deuterium NMR measurements are reported on the solid phases of the urea-trioxane inclusion compound (UTIC) prepared from isotopically normal urea and perdeuterated trioxane. The results are interpreted in terms of the dynamic state and orientational disorder of the trioxane sublattice in the various solid phases of the UTIC. At the low-temperature region of phase IV (
