Intramolecular proton exchange

A very similar situation exists for proton addition complexes as in the case of alcohols and other systems involving hydrogen bonds. In the present case the situation is in part even more complicated, since both an intermolecular and an intramolecular proton exchange has been... 

The product is a colorless liquid at room temperature that fumes in air and is quickly colored brown if impure or on exposure to oxygen and moisture. It is a monomer in solution, showing rapid inter- and intramolecular proton exchange. H and C NMR indicate a high negative charge at the ylidic carbon. According to infrared/Raman studies the P=C bond order is about 1.6. The first ionization potential is extremely low (6.8 eV). The gas-phase structure shows a short P=C bond (1.60 A). ... 

In this scheme an intramolecular proton exchange in the immonium ion is apparently required to equilibrate the carbon atoms. However, an alternative mechanism involving an intramolecular proton exchange... 

The hydrogenation of the peripheral double bonds strongly influences the tautomerism. The rates of the intramolecular proton exchange of 4 are smaller than of 3 because only the first trans-tautomer is aromatic, whereas the second trans-tautomer has lost its aromaticity and exhibits a higher energy. It is only observable as an intermediate of the exchange of the two inner protons [24]. 

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