Theoretical studies of conjugate and substituent effects on the intramolecular proton transfer: an HF/CIS study

To find the effect of conjugate and substituents on the intramolecular proton transfer, fourteen systems are studied using HF and CIS methods. The results indicate that the intramolecular proton transfer barrier decreases firstly, and changes a little for larger conjugate systems when the combined phenyl cycles are added from three to five. Hereby, we conjecture that the larger conjugate systems (more than three combined phenyl cycles) have little effect on the proton transfer barrier. Theoretical studies on substituents effect show that electronic donor groups enhance the H-bond and electronic acceptor groups reduce the H-bond. The position of substituents is another factor in the proton transfer reaction. The effect of all the substituents on the proton transfer barrier is small, so the character of mother molecule is crucial to the proton transfer reaction. Moreover, the linear analysis results show that the charges on the acceptor oxygen and the differences between the labile hydrogen and the acceptor oxygen have good linear relationship with the proton transfer barrier.