Three series of SiC/AlN samples were prepared by pyrolysis of “poly(aluminosilazane)”, the product of the reaction between hexamethylcyclotrisilazane and triethylaluminum. The Si/Al ratio was varied for one series (with constant pyrolysis temperature) and the pyrolysis temperature was varied for the other two series, with Si/Al = 1 and 3. 27Al and 29Si magic angle spinning NMR were used to analyze these samples in terms of coordination number, nearest neighbor atoms, and crystallinity. NMR results indicate that higher crystallinity and phase separation into discrete SiC and AlN crystallites accompany an increase in pyrolysis temperature. Disorder and increased heterogeneity in connectivity are observed with increasing Si/Al ratio of the initial reactants.

Cl,J-18 indicates the singlet state) outline appropriate mechanistic possibili ties.26In Scheme 11, X could result from either proton or electron transfer, as mentioned above. If electron transfer is involved, then a subsequent transfer of a hydrogen atom would be required for hydrogen exchange to occur. The appropriate expressions for dex, the quantum yield of hydrogen exchange, are given as eq 3 for Scheme I and eq 4 for Scheme 11. In these equations, [MI and [D] refer to monomeric and dimeric TFA concentrations, respectively.We argue in favor of the second possibility as follows.Quation 3 can 'be transformed into eq 5, where CY = It4/ (It4 + k5). Since dr is found experimentally to increase with the stoichiometric concentration of TFA5~27 the value of (26) For simplicity, the reverse of the quenching steps whose rate constants are kz and ka (Scheme I) and k2 (Scheme 11) are omitted. (27) G. Lodder, Thesis, University of Leiden, 1971, pp 110-3. The thesis provides the raw data on which the graphs of ref 4 are based. ~J ( c Y dr) increases with [TFA]. Likewise, the value of (1 + KsvK[M]2)/[M] must increase with increasing [TFA].Consequently, this model can never compute a constant value for kz7, and in practice the calculated value of kz7 rises monotonically with [TFA].By contrast, a good fit to Lodder's dataz7 (r = 0.997) is found when l/& is plotted against 1/[D] as required by eq 4. Scheme I1 thus accounts for the experimental data.Spillane7 has raised the question that it may be the excimer of naphthalene that undergoes hydrogen exchange rather than the monomeric singlet. This possibility is clearly excluded by Lodder's data:' which show the relationship 1/dex 0: [CloHa], which is to be expected if the monomer is reactive and the excimer unreactive. The reverse situation leads to a linear relationship betweenWe conclude that both fluorescence quenching and hydrogen exchange in the system naphthaleneTFA in alkane solvents involve the dimer of TFA and not the monomer.Further insight into the mechanism of hydrogen exchange in this system must await more information on the nature of the intermediate formed in the initial interaction between the TFA dimer and the naphthalene singlet.Acknowledgment. We thank the NSERC of Canada for financial support, Professor E. Havinga for kindly making available a copy of the thesis of G.The pyrolysis of nitrobenzene was examined over the temperature range of 275-630 O C . Liquid products were determined by using a GC-MS combination. Evaluation of the gaseous products was done by using an on-line IR gas cell. At the lower temperatures, nitrobenzene decomposes via direct rupture of the arene-nitro bond to give phenyl radical and NO2 as primary products. As the temperature is raised, the yield of NOz decreases.There still exists uncertainty on the question of what are the primary products resulting from the pyrolysis of nitrobenzene. Two reports highlight this question. In the earlier report,' Fields and Meyerson, utilizing a flowthrough Vycor reactor, postulated that at 600 "C ...

Producers with wheat stored on-farm for a few months are concerned about unexpected decreases in protein content measurements obtained from commercial laboratories. These differences can adversely affect the price when the wheat is sold. This study evaluated the contribution of measurement errors in giving a false indication of protein change during storage. Eleven bins of wheat were sampled at three in-bin positions during one storage season and five of these bins were refilled and sampled during the second season to evaluate differences in protein measurements. Samples were analyzed for protein content using four measurement instruments. Additional wheat was stored in the laboratory and evaluated over two years with two instruments. Data showed that the variation between protein measuring instruments was significant with an expected variation of +0.74% protein content (95% confidence interval) during the field tests. The variation over time for measurements with the FGIS instrument was +0.3% protein for an eight-month period, when measuring successive samples taken from the same positions. Measurements from the other three instruments varied by +0.8% protein or more during the same time. Variation with in-bin position was not significantly different (a = 0.05) than the variation between instruments. The greater consistency for the FGIS instrument was likely due to the rigorous standardization and maintenance procedures employed by FGIS for their near-IR protein instruments. These results suggest that a similar rigorous system is needed to obtain the same consistency for other instruments used in the wheat marketing system.

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