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Time-resolved photodissociation (TRPD) rate measurements on nickelocene cation were performed at two different W wavelengths in the ion cyclotron resonance (ICR) mass spectrometer. For completely thermalized ions, one-photon absorption at 266 nm gave a dissociation rate of 168 s-', and two-photon absorption at 355 nm gave a dissociation rate of (1.7 & 0.3) x lo6 s-l, which is the fastest dissociation rate yet measured in our lab. The TRPD results were modeled by RRKM calculations. The critical energy and the activation entropy at 1000 K were determined to be EO = 3.24 eV and A$looo~ = 6.34 eu. From the RRKM fit the CpNi+-Cp bond strength was assigned as 313 kJ/mol, which is significantly weaker than for ferrocene ion (357 kJImo1). The dissociation rate was used as a thermometric probe to measure the internal energies and cooling rates of initially hot ions. The radiative cooling rate in the internal energy range 1.0-0.3 eV was 0.48 s-'. The radiative cooling rate constant has the same order of magnitude as previously found for comparable ions. It is about twice that of ferrocene cation (0.28 s-').

The decomposition of p-aminosalicylic acid hydrochloride involves initial dehydrochlorination followed by decarboxylation to give m-aminophenof hydrochloride. The solid-state decarboxylation of p-aminosalicylic acid (I)presumably follows the same mechanism as the solution reaction (proton electrophilic substitution) since no decarboxylation of several meta-substituted salicylic acids was found. In addition, analyses of the crystal packing of (I) revealed several carboxy-protons within 5 A of the nucleophilic ring carbon atom. A solid-state electrophilic substitution involving one of these protons would be consistent with available data concerning the solidstate reaction of (I).The crystal structure of (I) [a = 7.209(2), b = 3.786(1), c = 25.109(9) A, (3 = 103.22". space group PZ,/c] is reported and the published structure corrected. 0.044 for 1 044 observed reflections. 25.109(9) A, p = 103.22(3)", U = 667.

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The application of the continuous ejection technique to observe unimolecular dissociation of metastable organometallic ions in a Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometer is demonstrated. A radio-frequency ejection voltage is continuously applied at a frequency corresponding to the exact mass of a metastable adduct ion, [MC6H6 +]* (M = Cr and Mn), formed in the reactions of Cr+ and Mn+ with C6H6. By monitoring the initial metal ion intensity as a function of ion ejection time, metastable ion lifetimes and unimolecular dissociation rate constants are obtained. The bond dissociation energies D 0(Cr+−C6H6) = 1.75 ± 0.1 eV and D 0(Mn+−C6H6) = 1.50 ± 0.1 eV are derived from RRKM modeling and are in excellent agreement with other literature values.