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 σ C - 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.

Computational chemistry: A useful (sometimes mandatory) tool in mass spectrometry studies

“…Different ab initio approaches indicate that the global minimum corresponds to the non‐classical bridged structure 38 + , which at the MP4/6‐311G** level is predicted to be 18.4 kJ mol −1 more stable than the classical one, 39 + (Carneiro et al, 1994). This estimate does not change significantly when the CCSD(T) method is used (East et al, 1998). The controversy arises because the existence of only these two isomers is difficult to reconcile with the experimental evidence.…”

“…On the other hand, although this process should take place without an activation barrier, the dissociation of structure 39 + involves an activation barrier of 38 kJ mol −1 (Carneiro et al, 1994). This controversy was partially solved when a third minimum, corresponding to a loose solvated‐ion complex C 2 H 5 + ···H 2 , was found to exist as a stable species (East et al, 1998). This would explain the barrierless dissociation observed in the aforementioned experiments.…”

Properties of C−C Bonds inn-Alkanes: Relevance to Cracking Mechanisms

“…Two of the three general isomers of C 2 H 7 + ion have been detected in the gas phase. 37 Proponium and butonium ions have been recently studied computationally by Mota and co-workers, 22,38,39 demonstrating multiple possible geometries for protonation, as well as providing useful energetics. Calculations on larger carbonium ions have been limited in accuracy and/or scope.…”

Study of solvents effect on distribution of configurational and conformational isomers of apo-tirucallol theoretically and experimentally

“…The dynamic investigations of conformations have achieved great progress by using NMR or HPLC method [3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21]. During the last decades, Lunazzi, Mazzanti, Casarini and coworkers have reported a series of conformational studies with some interesting discoveries [5][6][7][8][9].…”

“…Conformational studies of other compounds, such as the bioactive cyclic heptapeptide from marine origin [13], acyclic hydrazines [14], etc. have been done [15][16][17][18][19][20]. Among the compounds, the condensation products from ketone/ aldehyde with alcohols are well known in organic chemistry.…”