Mass-Selected Ion Mobility Studies of the Isomerization of the Benzene Radical Cation and Binding Energy of the Benzene Dimer Cation. Separation of Isomeric Ions by Dimer Formation

Abstract

A mass-selected, ion mobility drift-cell technique has been used to study the isomerization of the benzene radical cation generated by electron impact (EI) ionization. Evidence is presented for the generation of both the benzene and fulvene cations, with a lower limit for the barrier of isomerization E a(benzene+• → fulvene+•) estimated as E a > 1.6 eV. The reduced mobilities of the benzene and fulvene cations are nearly similar, making it difficult to separate the two ions in pure helium. However, because of the large difference in the bonding of the benzene and fulvene cations with neutral benzene, the two isomeric ions are readily separated in the presence of neutral benzene in the drift cell. This suggests that isomer ions with similar mobilities can be separated on the basis of the degree of their associations with their neutral precursor in the drift cell. This separation concept allows us to discover the “hidden isomerization” process responsible for the generation of the fulvene cations via EI ionization of benzene. This also allows us to measure the equilibrium constant for the formation of the benzene dimer cation without interference from the fulvene isomer. The resulting binding energies (17.6 and 17.4 kcal/mol for (C6H6)2 +• and (C6D6)2 +, respectively) are smaller than that reported by Hiraoka and co-workers (J. Chem. Phys. 1991, 95, 8413) and similar to that measured by Meot-Ner et al. (J. Am. Chem. Soc. 1978, 100, 5466), both using pulsed high-pressure mass spectrometry. The binding energies of the proton-bound dimers in pyridine and triethylamine systems have been measured as 25.2 ± 1 and 20.9 ± 1 kcal/mol, respectively, and the binding energies of the protonated methanol clusters H+(CH3OH) n with n = 3−5 have been measured as 21.0, 14.3, and 11.3 kcal/mol, respectively, in good agreement with literature values. The combination of thermochemical properties with isomer identification provides valuable information on the structure−property relationship of molecular cluster ions. The novel concept of the separation of isomeric ions by dimer formation is expected to be of general application and may help to resolve important outstanding issues in the separation, structure, and chemistry of hydrocarbon radical cations.

Ion Mobility Collision Cross Section Compendium

“…A significant number of the small molecules CCS values consists of aromatic hydrocarbons. 98,119,139,162–164 Starting in the 1990s, interest in inorganic compounds (metal salts, atomic and molecular clusters) began to emerge. Very few inorganic compound CCS values were reported between 2000 and 2010, with a resurgence of interest starting in 2013 which were primarily focused on gaining fundamental insights into the structures of inorganic salt and metal clusters.…”

The influence of drift gas composition on the separation mechanism in traveling wave ion mobility spectrometry: insight from electrodynamic simulations

“…In more dramatic examples, doping the drift gas with a small amount of neutral modifier agent can effectuate the separation of structural (Rusyniak et al 2003) and chiral isomers (Dwivedi et al 2006). This separation enhancement is a result of intrinsic differences in the ion-neutral interaction which in turn alters the mobilities, and by association, the collision cross-section of the ions differently.…”

Communication: Ion mobility of the radical cation dimers: (Naphthalene)2+• and naphthalene+•-benzene: Evidence for stacked sandwich and T-shape structures

“…[7][8][9] In homodimer radical cations such as benzene and naphthalene dimers ((Bz·Bz) +· and (Naph·Naph) +· , respectively), the unpaired electron is considered to be equally distributed between the two moieties providing extra charge resonance stabilization in addition to the ion-induced dipole and dispersion interactions. 3 The binding energy of the homodimer cations such as (Bz·Bz) +· and (Naph·Naph) +· is significantly higher (17 kcal/mol and 17.8 kcal/mol, respectively) 10,11 than the recently measured value of the naphthalene +· ·benzene heterodimer (8 kcal/mol). 12 The smaller binding energy of the naphthalene +· ·benzene heterodimer is attributed to two factors: (1) the delocalization of the charge on the larger naphthalene cation as compared to the benzene cation could lead to weaker charge-induced dipole interactions with the neutral benzene molecule and (2) the lack of charge resonance interaction in the naphthalene +· ·benzene heterodimer as a result of the large a) Author to whom correspondence should be addressed.…”

“…10,11,[15][16][17][18] Therefore, homodimer radical cations are believed to adopt face-to-face sandwich-like structures as was shown for the benzene radical cation dimer. [15][16][17][18][19] The structural minima and nature of bonding in the naphthalene +· ·benzene heterodimer were recently identified using a combination of quenching ab initio molecular dynamics (QAIMD) starting from random initial orientations, followed by gradient-based local optimizations with density functional theory (DFT) using the cc-pVTZ basis set.…”