Combined Experimental and Theoretical Study of the Protonation of Polyfluorobenzenes [C6H6-nFn] (n=0-6)

Autor: J. Hrušák, T. B. McMahon, J. E. Szulejko
Rok vydání: 1997
Předmět:
Zdroj: Journal of Mass Spectrometry. 32:494-506
ISSN: 1096-9888
1076-5174
DOI: 10.1002/(sici)1096-9888(199705)32:5<494::aid-jms508>3.0.co;2-y
Popis: In a recent high-pressure mass spectrometric revision to the gas-phase basicity scale (J. E. Szulejko and T. B. McMahon, J. Am. Chem. Soc. 115, 7839 (1993)), it was observed that the proton affinity for hexafluorobenzene was 24 kcal mol -1 (1 kcal = 4.184 kJ) lower than the accepted National Institute of Science and Technology (NIST) database value of 177.7 kcal mol -1 (S. G. Lias et al., J. Phys. Chem. Ref Data 17, Suppl. 1 (1988)). Furthermore, the proton affinities for most other polyfluorobenzenes were also found to differ substantially from the NIST values. For many of the polyfluorobenzenes large protonation entropy changes were noted, which were substantially greater than then those expected from rotational symmetry number changes alone. In view of these observations, MI2/6-31G ** //HF/6-31G ** ab initio calculations were undertaken to investigate further the proton affinity and entropy changes with respect to the degree of fluorine substitution. The present proton affinity variations found for the polyfluorobenzenes (hexaflorobenzene excepted) can be interpreted with the aid of the ab initio results in terms of a simple additivity scheme. Each fluorine substituent para, meta, ortho or ipso to the ring protonation site will induce an incremental proton affinity change with respect to benzene of 1.5, -7.0, -1.5 and -19.0 kcal mol -1 , respectively. This additivity scheme can also be used to rationalize the re-evaluated proton affinities for the polymethylbenzenes and m- and o-fluorotoluenes. The corresponding methyl increments are 7.5, 5.5, 2.5 and 1.0 kcal mol -1 for para, meta, ortho and ipso protonation respectively. From the present ab initio statistical thermodynamic analysis of the various protonation entropy components, it was concluded that the lowfrequency vibrations are almost exclusively responsible for the large excess entropy changes observed experimentally. Ab initio calculated barriers for 1,2 proton shifts in arenium species available in the literature are concluded to be too large to allow the existence of a so-called dynamic proton Large excess protonation entropies are noted from the literature for polymethylbenzenes, naphthalene and 1-methylnaphthalene.
Databáze: OpenAIRE
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