Ab Initio Modeling of Proton Transfer in Phosphoric Acid Clusters

Development of superior electrolytes for fuel cells that enable operation at temperatures above 120 degrees C without external humidification will benefit from molecular-level understanding of proton conduction mechanisms in neat acid systems possessing little or no water. The energetics and collective molecular effects associated with proton transfer in clusters consisting of two to six phosphoric acid (H3PO4) molecules are examined with electronic structure calculations. Global minimum-energy structures are determined at the B3LYP/6-311G** level for each cluster from many chemically rational initial configurations. Binding energies are computed and found to correlate with the number and type of hydrogen bonds present in the cluster and show an increase in the strength of the interactions up to and including (H3PO4)(6). This suggests that more than six molecules may be required to fully encompass the binding in bulk phosphoric acid. Potential energy profiles and associated energetic penalties for proton transfer are determined at the B3LYP/6-31G** level under four different constraints on the positions of surrounding atoms. The endothermicities decrease with increasing cluster size, suggesting that several molecules facilitate proton transfer. Calculation of partial atomic charges with the CHELPG scheme both prior to and following proton transfer indicates a higher degree of charge delocalization in the larger clusters and thereby a smaller energetic penalty.