An Electronic Structure Perspective of the Promoter Modes in Proton Transfer Reactions

The changes in the electronic structure are analyzed along the IRC of proton transfer reactions in two model systems, formic acid dimer and cyclic HF trimer. The IRC consists of three regions. In the first region, the two units between which a proton is exchanged approach each other. The vibrational mode associated with this movement is called the promoter mode. Computational evidence is provided to suggest that during this period the electrons in the lone pair of the acceptor atom delocalize significantly into the antibonding orbital of the covalent bond between the donor and hydrogen atoms. This reduces the barrier to the proton transfer. The force constants associated with the migrating protons also decrease as a result of the delocalization of lone pair electrons into the sigma* orbital, and the associated stretching vibrational frequencies decrease along the IRC up to the transition state. In the second part of the IRC, the proton migration occurs. Finally, the two monomers retreat to the new equilibrium positions. On the basis of this, it is suggested that the role of the promoter mode is to enhance the flow of the lone pair electrons into the sigma* orbital such that the barrier to the actual proton migration comes down. This can be used to identify them.