Cationic adducts oxygen bases

As noted previously, the formation of approximately tetrahedral bonds to nitrogen occurs principally in ammonium cations (R,N+), amine oxides (R3N+—O"), and Lewis add-base adducts (e.g., R3N+—B X3). In the amine oxides and these adducts, the bonds must have considerable polarity in the amine oxides, for instance, N — O donation cannot be effectively counterbalanced by any back-donation to N. In accord with this, the stability of amine oxides decreases as the R3N basidty decreases, since the ability of N to donate to O is the major factor. Similarly, R3N —> BX3 complexes have stabilities that are roughly parallel to R3N basicity for given BX3. When R is fluorine, basicity is minimal and F3N —> BX3 compounds are unknown. It is, therefore, curious that F3NO is an isolable compound. Evidently the extreme electronegativity of fluorine coupled with the availability otpir electrons on oxygen allows the structures in (9-V) to contribute to stability. 

The electron-impact mass spectra of bromides, iodides, and fluorobo-rates of the 2,4,6-triphenyl-substituted cations 8 and 9 have the base peak at the mass number of the cation (74OMS80). No molecular ion peak of an adduct between the cation and the anion has been found the fluoroborates show also weak peaks with the elemental composition of an adduct between the cation and F". On the contrary, the spectra of perchlorates do not show the peaks at the mass number of the cation but peaks indicating the addition of an oxygen atom and the removal of a hydrogen atom. From ionization potential measurements it has been shown that the bromides, iodides, and fluoroborates of 8 and 9 are thermally reduced in the mass spectrometer to volatile free radicals 50 and 51 prior to evaporation, presumably with concomitant oxidation of the anion. In the presence of a nonoxidizable anion, e.g., perchlorate, reduction of the cations to free radicals does not take place. Interestingly, the order of ionization potentials of the radicals, 50 < 51, indicates that the LUMO energy level of pyrylium is higher than that of thiopyrylium, consistent with electrochemical studies (Section II,D). 

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