Infrared spectroscopy of carbocations upon electron ionization of ethylene in helium nanodroplets

Abstract

The electron impact ionization of helium droplets doped with ethylene molecules and clusters yields diverse CXHY+ cations embedded in the droplets. The ionization primarily produces C2H2+, C2H3+, C2H4+, and CH2+, whereas larger carbocations are produced upon the reactions of the primary ions with ethylene molecules. The vibrational excitation of the cations leads to the release of bare cations and cations with a few helium atoms attached. The laser excitation spectra of the embedded cations show well resolved vibrational bands with a few wavenumber widths—an order of magnitude less than those previously obtained in solid matrices or molecular beams by tagging techniques. Comparison with the previous studies of free and tagged CH2+, CH3+, C2H2+, C2H3+, and C2H4+ cations shows that the helium matrix typically introduces a shift in the vibrational frequencies of less than about 20 cm−1, enabling direct comparisons with the results of quantum chemical calculations for structure determination. This work demonstrates a facile technique for the production and spectroscopic study of diverse carbocations, which act as important intermediates in gas and condensed phases.

Time-Domain Rotational Raman Spectroscopy of the Ethylene Dimer and Trimer by Coulomb Explosion Imaging of Rotational Wave Packets

“…The strong, focused signals found in the lower middle were attributed to the C 2 H 4 + including one 13 C and N 2 + from the residual N 2 in the chamber. C 2 H 5 + , known species in the mass spectrum of ionized ethylene gas may also contribute. The signals from the ethylene clusters are found in the rest of images.…”

“…First, they represent the smallest assembled π system and are an important model for studying intermolecular interactions between π systems. Numerous theoretical and experimental studies have been conducted on their structures and properties, and high-resolution spectral data provide useful references for testing and constructing intermolecular potentials. − Second, several groups have studied the ethylene dimer and trimer using high-resolution (sub-GHz resolution) IR spectroscopy, − which will facilitate the validation of the present results. Finally, it is worth noting that the yield of ethylene clusters is known to be high in supersonic jet beam sources. , …”

Deciphering the Impact of Helium Tagging on Flexible Molecules: Probing Microsolvation Effects of Protonated Acetylene by Quantum Configurational Entropy

“…In both tagging and HENDI spectroscopy, the impact of helium on the ro-vibrational spectra of the spectroscopic species has been under investigation for a long time. Molecular spectroscopy of many different but mostly quasi-rigid neutral molecules (such as SF 6 , OCS, and CO 2 to name but a few) has shown an impact of helium on their ro-vibrational spectra in the sense that effective rotational constants get renormalized − and vibrational frequencies shift mostly in dependence on the number of solvating helium atoms. − Recently, molecular cations have been studied in the mid-IR frequency regime in the microsolvation and bulk solvation limits using the respective messenger and HENDI techniques to investigate the impact of helium on OH, NH, or CH stretching bands for instance of a protonated dipeptide, the protonated ammonia–water dimer, protonated acetic acid, protonated carbon dioxide, several protonated water clusters including the dimer , as well as various carbocations. , …”

“…13−17 molecular cations have been studied in the mid-IR frequency regime in the microsolvation and bulk solvation limits using the respective messenger and HENDI techniques to investigate the impact of helium on OH, NH, or CH stretching bands for instance of a protonated dipeptide, 18 the protonated ammonia−water dimer, 19 protonated acetic acid, 20 protonated carbon dioxide, 21 several protonated water clusters including the dimer 18,22 as well as various carbocations. 23,24 So far, however, little is known about the possibly perturbing effects of helium (micro)solvation directly on the structure of molecules or molecular complexes by investigating the tagging response of the intramolecular degrees of freedom. A notable exception is early work on the impact of helium atoms on the internal rotation of the −OH 2 and −NH 3 groups within the NH 4 + •H 2 O heterodimer.…”

Microsolvation of H2O+, H3O+, and CH3OH2+ by He in a cryogenic ion trap: structure of solvation shells

“…This technique has been pioneered and extensively applied by Scheier and his group to a plethora of cations, 51,52 including metal, PAH, and fullerene cations and their clusters. [53][54][55][56][57][58] Few groups have also started recently to perform elegant ion spectroscopy in large He droplets, [59][60][61][62][63][64][65][66][67] although this approach does not provide details about the local X + Á Á ÁHe interaction potential (radial strength and angular anisotropy).…”