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Photofragment spectroscopy of CH212 at a wavelength of 266 nm shows evidence for a two-photon, twostep dissociation process. The first photon absorption causes transition to a repulsive excited state of BI symmetry that breaks up rapidly to yield highly internally excited CHJ radicals and an I atom. On average, between 80% and 90% of the effective available energy for this process appears in CHJ rovibrational excitation. Once formed, a sizable fraction of the CH 2 1 fragments may absorb a second photon, dissociating to CH 2 radicals and another I atom. Energy distribution analysis indicates that the CH 2 fragments are both translationally and internally excited, although quantitative estimation of their rovibrational excitation is complicated by the possibility of electronic excitation of one or more fragments in the two-step process. An approximate analysis of the CH 2 fragment recoil angular distribution allows a crude resolution of the CH 2 1 fragment internal excitation into its rotational and vibrational components. In the absence of electronic excitation, 40% of the CH 2 1 internal energy is found to be rotational and 60% vibrational, qualitatively consistent with the picture of impulsive recoil of a semirigid radical. Kinetic analysis of the dependence of CH 2 1 and CH 2 fragment production on photolyzing light intensity permits extraction of the cross section for second photon absorption. From this, it is estimated that the extinction coefficient for CH 2 1 radical absorption at 266 nm is 30 I mole-I em-I. uPresumably, at longer wavelengths ground state molecular iOdine could be formed.

5N-and I70-enriched samples of imidodiphosphate (PNP), its tetraethyl ester, and 5'-adenylyl imidodiphosphate (AMP-PNP) have been prepared. The I5N NMR spectra of both PNP and AMP-PNP reveal the presence of approximately 70-Hz 'H-I5N coupling constants for the fully ionized samples, demonstrating an imido tautomeric structure in both cases. For AMP-PNP this coupling persists in the presence of a stoichiometric amount of Mg2+. The 170 NMR chemical shifts of the resonances associated with the phosphoryl oxygens have been assigned. The effect of pH on the resonances for PNP and AMP-PNP is similar to that observed for ATP (Gerlt, J. A.; Demou, P. C.; Mehdi, S. J . Am. Chem. SOC. 1982, 104, 2848) and its thiophosphate and phosphonate structural analogues (Gerlt, J. A.; Reynolds, M. A.; Demou, P. C.; Kenyon, G. L. J . Am. Chem. Soc., preceding paper in this issue), indicating that protonation of the tetraanion of PNP occurs exclusively on the oxygens and that protonation of the tetraanion of AMP-PNP occurs exclusively on the y-phosphoryl oxygens. Although I7O N M R demonstrates that protonation of the monoanion of the tetraethyl ester of PNP occurs predominantly on nitrogen, the corresponding 15N N M R chemical shift change was only 2.50 ppm. Thus, I5N NMR chemical shift changes cannot be used reliably to ascertain the sites of protonation in imidodiphosphates.

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