Conformations of perfluoroether sulfonic acid side chains for the modelingof Nafion (R)

Optimized geometries for the acid CF3OCF2CF(CF3)O(CF2)(2)SO3H and the acid chain with backbone attachment CF(CF2CF3)(2)- OCF2CF(CF3)OCF2CF2SO3H were d etermined with electronic structure techniques exclusive of hydration effec ts. The minimum energy structures are folded with the two other oxygens in a gauche conformation. Rotation about the outermost ether group provides na tural transition path to the partial unfolding of the side chain. The poten tial energy surface for this this rotation was determined and the results s howed that the staggered fluorine conformation of the head group is unchang ed with rotation. This rotation produced conformations approximately 40% lo nger (8.5 Angstrom)and about 3.5 kcal/mol higher in energy after surmountin g a barrier of about 4.6 kcal/mol. Additional searching for stretched confo rmations of the side chain led to the discovery of an even longer (9.8 Angs trom) minimum energy conformation that was entirely unfolded with the ether oxygens located trans relative to one other. This unfolded conformation is essentially isoenergetic (+0.1 kcal/mol) with the (folded) minimum energy conformation. The existence of a small number of well-defined "short" and " long" side chain conformations, with substantially similar head-group chemi stry, suggests the speculative view that hydration-dependent equilibria bet ween these conformations should be a significant feature in physical models of the function of Nafion(R). Equilibrium geometries were also obtained fo r both the folded and the unfolded acid side chains, each with an associate d single water molecule.