1.002 + R-R0) where R is the ratio in the sample in question and Ra is the ratio in unlabeled material. Good agreement was found between measurements of the two pairs of peaks. A fragment peak at m/e 127 is also observed, attributed to the loss of benzoyl group. The ratio 129/127 in all samples was within experimental error identical with that in unlabeled material. This establishes that the 180 label is in the carbonyl oxygen. First-order rate constants for exchange were evaluated as the slopes of plots of In (excess 180) vs. time.Acknowledgment. Continued financial support of the Natural Sciences and Engineering Research Council is gratefully acknowledged.Registry No. 2 (major isomer), 86239-10-3; 2 (minor isomer),

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