“…Sun and Li [140] used DFT to investigate intramolecular proton-transfer reactions in propylene derivatives (X-CH = Y, X = CH 2 , NH, O; Y = CH 2 , NH, O, S, Se) and pentadiene derivatives (X-CH = CH-CH = Y, X = CH 2 , NH, O; Y = CH 2 , NH, O, S, Se). The calculations revealed that all proton-transfer reactions were exothermic except for those involving propylene derivatives with X = O and Y = S and Se, and for those involving pentadiene derivatives with X = O and Y = S. For a given X and Y, how constant is the difference of the enthalpies of formation of X-CH = CH-CH = Y and X-CH = Y?…”