The negative p-value means that the reaction rate is increased by the substitution of groups which increase the electron density on the diazocarbon. It is interesting to note2 that the rate is also increased by the substitution of electron-withdrawing groups on the benzoic acid.In the five cases where the effect of a substituent on one ring was compared to the effect of two of the same substituents on different rings, it was found that the -values are satisfactorily additive.s" The good correlation for 3-nitro-4 '-methyldiphenyldiazomethane, where the nitro group with a strong deactivating effect and the methyl group with a moderate activating effect are both influencing the same reaction center, offers striking proof of the additive principle of substituent effects.The results show that an increase in the electron density on the diazocarbon increases the rate of the reaction. This fact is consistent with either of the mechanisms shown in Fig. 1. However, considera-

predicted curves is that they depend almost entirely on the rate of helix propagation for the longer oligomers, when the temperature jumps are limited to the melting zone, and are relatively insensitive to the rate of helix nucleation. We expect that it will be possible to use this approach to measure the rate constant for propagation and eventually to work out the kinetics of a defined loop migration reaction (see Figure 2).The studies of oligomers which are described here have been a collaborative effort between Drs.

for this spectrum. (4) Due to miscalibration, the nmr signals previously reported for this compound are in error; we now find a complex group of signals at S (CCI4) 1.5-2.3 ppm with three sharp signals at 0.93, 0.98, and 1.20 ppm.

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