The three isomers of protonated ethane, C2H7+

The fluxional behavior of the protonated ethane ion was examined using both static and dynamic modeling. Static ab initio calculations, including pert urbation theory (MP2), coupled cluster (CCSD(T)), and density functional th eory, were used to locate various minima, saddle points, and G2-quality rel ative energies on the potential energy surface for atomic motions. In tande m, Car-Parrinello molecular dynamics simulations were performed to aid the stationary-point search and to examine the stabilities of various isomers a t different temperatures. Predicted infrared spectra were also obtained fro m both techniques. Unlike most previous experimental and theoretical invest igations which have focused upon the relative energies and stabilities of s igma(C-C)-protonated (bridged) structures and sigma(C-H)-protonated (open o r "classical") structures, this work establishes the existence of a third i somer, the ion-molecule or solvated-ion complex C2H5+... H-2, which is the more likely candidate for the second isomer of experiments by Hiraoka and K ebarle and by Yeh, Price, and Lee. The open isomer may still be experimenta lly unknown. Peculiar discrepancies remain, however, and further experiment al work is needed to resolve them.