The Three Isomers of Protonated Ethane, C₂H₇⁺

Abstract

The fluxional behavior of the protonated ethane ion was examined using both static and dynamic modeling. Static ab initio calculations, including perturbation theory (MP2), coupled cluster (CCSD(T)), and density functional theory, were used to locate various minima, saddle points, and G2-quality relative energies on the potential energy surface for atomic motions. In tandem, Car−Parrinello molecular dynamics simulations were performed to aid the stationary-point search and to examine the stabilities of various isomers at different temperatures. Predicted infrared spectra were also obtained from both techniques. Unlike most previous experimental and theoretical investigations which have focused upon the relative energies and stabilities of σC-C-protonated (bridged) structures and σC-H-protonated (open or “classical”) structures, this work establishes the existence of a third isomer, the ion−molecule or solvated-ion complex C2H5+···H2, which is the more likely candidate for the second isomer of experiments by Hiraoka and Kebarle and by Yeh, Price, and Lee. The open isomer may still be experimentally unknown. Peculiar discrepancies remain, however, and further experimental work is needed to resolve them.