Infrared spectroscopy of ions and ionic clusters upon ionization of ethane in helium droplets

Abstract

Helium droplets are unique hosts for isolating diverse molecular ions for infrared spectroscopic experiments. Recently, it was found that electron impact ionization of ethylene clusters embedded in helium droplets produces diverse carbocations containing three and four carbon atoms, indicating effective ion-molecule reactions. In this work, similar experiments are reported but with saturated hydrocarbon precursor, such as ethane. In distinction to ethylene precursors, no characteristic bands of larger covalently bound carbocations were found, indicating inefficient ion-molecule reactions. Instead, the ionization in helium droplets leads to formation of weaker bound dimers such as (C2H4)(C2H5) +, (C2H6)(C2H5)+, and (C2H6)(C2H6)+, as well as larger clusters containing several ethane molecules attached to C2H4+, C2H5+, and C2H6+ ionic cores. The spectra of larger clusters resemble those for neutral, neat ethane clusters. This work shows the utility of the helium droplets to study small ionic clusters at ultra-low temperatures.

Electron-induced hydroamination of ethane as compared to ethene: implications for the reaction mechanism

“…A previous radiolysis study 69 also suggested the formation of covalently bound adducts of C 2 H 5 + and C 2 H 6 and their decay by loss of H 2 . In contrast, the He droplet study 68 gave only evidence of clusters of the cation fragments with neutral C 2 H 6 . Our study confirms the formation of covalent dimers.…”

“…66 Later, the formation of butane from C 2 H 6 was investigated by MS of H 2 gas with an admixture of C 2 H 6 at temperatures between 128 K and 625 K 67 and by infrared spectroscopy of He droplets (0.4 K) doped with C 2 H 6 and irradiated with 100 eV electrons. 68 The former work 67 detected C 4 H 11 + and its decay to C 4 H 9 + and monitored the temperature dependence of the reaction. A previous radiolysis study 69 also suggested the formation of covalently bound adducts of C 2 H 5 + and C 2 H 6 and their decay by loss of H 2 .…”

“…66 Later, the formation of butane from C 2 H 6 was investigated by MS of H 2 gas with an admixture of C 2 H 6 at temperatures between 128 K and 625 K 67 and by infrared spectroscopy of He droplets (0.4 K) doped with C 2 H 6 and irradiated with 100 eV electrons. 68 The former work 67 dimers. As a tentative explanation for the discrepancy with the He droplet study, 68 we may propose that the temperature of the He droplets was too low to enable the reaction.…”

“…68 The former work 67 dimers. As a tentative explanation for the discrepancy with the He droplet study, 68 we may propose that the temperature of the He droplets was too low to enable the reaction. At 128 K, the reaction of C 2 H 5 + with C 2 H 6 was observed to proceed rapidly yielding C 4 H 11 + as the major product.…”