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J. Phys. Chem. 1981, 85, 3746-3748
tetrazine have relative fluorescence excitation intensities of 100, 2, 0.3, 0.8, 33, and 41%, respectively, at room temperature.16 In a supersonic expansion at 4 atm through a 100-pm nozzle the corresponding relative intensities are 100, 0.6, 0.04, 0.08, 2, and 2%. Rice and co-workers16have noticed similar behavior in the excited electronic state vibrational relaxation in benzene. In other molecules, the vibrational relaxation (15) Room temperature fluorescence excitation intensities of tetrazine were obtained from our own measurements, the absorption measurements of K. K. Innes, L. H.Franks, A. J. Merer, G. K. Vemulapalli, T. Cassen, and J. Lowry, J. Mol. Spectrosc., 66, 465 (1977), and the lifetime measurements of J. Langelaar, D. Bebelaar, M. W. Leeuw, J. J. F. Ramaekers, and R. P. H. Rettachnick in “Proceedings of the 2nd International Conference on Picosecond Phenomena”, Springer-Verlag,Berlin, 1980, p 171. The room temperature fluorescence excitation intensities of benzene were obtained from the absorption intensities of ref 4 and the quantum yields of K. G. Spears and S. A. Rice, J. Chem. Phys., 56, 5561 (1971). (16) C. Jouvet, M. Sulkes,and S. A. Rice, Chem. Phys. Lett., in press. (17) A. Savitzky and M. J. F. Golay, Anal. Chem., 36, 1627 (1964).
cross section increases with decreasing temperature and becomes very large at the low translational temperature in the downstream part of the supersonic expansion. This produced a much faster vibrational-translational energy transfer than is usually observed in room temperature static gases. In the case of benzene, no increase in the vibrational relaxation cross section was observed as the temperature was lowered. This is consistent with our lack of vibrational cooling of ground electronic state benzene. At this time we have no explanation for this anomalous behavior of benzene. Acknowledgment. This material is based upon work supported by the National Science Foundation under Grant CHE-7825555, by the US.Public Health Service under Grant 5-R01-GM25907, and by the donors of the Petroleum Research Fund, administered by the American Chemical Society. C.A.H. was supported by the Fannie and John Hertz Foundation.
Additivity of Fluorine Substituent Effects in the Gas-Phase Basicities of Fluorinated Acetones D. F. Drummond and T. B. McMahon” Department of Chemistw, UniversiV of New Brunswick, Fredericton, New Brunswick, Canada E3B 6E2 (Received: July 27, 1981)
Gas-phase basicities of six fluorinated acetones have been determined from proton transfer equilibrium and bracketing experiments with ion cyclotron resonance spectroscopy. A very regular decrease in proton affinity of 6.1 f 0.4 kcal mol-1 for each successive fluorine substituent is found. This result is interpreted in terms of a substituent effect which is almost purely inductive and correlates extremely well with electronegativity of the methyl substituents. In addition, a small stabilizing interaction of 2-3 kcal mol-l is revealed in each of the protonated fluoroacetones due to formation of an intramolecular hyrogen bond. Intr
