Gas Phase Hydration and Deprotonation of the Cyclic C₃H₃⁺ Cation. Solvation by Acetonitrile, and Comparison with the Benzene Radical Cation

Abstract

The binding energies of the first 5 H2O molecules to c-C3H3+ were determined by equilibrium measurements. The measured binding energies of the hydrated clusters of 9-12 kcal/mol are typical of carbon-based CH+...X hydrogen bonds. The ion solvation with the more polar CH3CN molecules results in stronger bonds consistent with the increased ion-dipole interaction. Ab initio calculations show that the lowest energy isomer of the c-C3H3+(H2O)4 cluster consists of a cyclic water tetramer interacting with the c-C3H3+ ion, which suggests the presence of orientational restraint of the water molecules consistent with the observed large entropy loss. The c-C3H3+ ion is deprotonated by 3 or more H2O molecules, driven energetically by the association of the solvent molecules to form strongly hydrogen bonded (H2O)nH+ clusters. The kinetics of the associative proton transfer (APT) reaction C3H3+ + nH2O --> (H2O)nH+ + C3H2* exhibits an unusually steep negative temperature coefficient of k = cT(-63+/-4) (or activation energy of -37 +/- 1 kcal mol(-1)). The behavior of the C3H3+/water system is exactly analogous to the benzene+*/water system, suggesting that the mechanism, kinetics and large negative temperature coefficients may be general to multibody APT reactions. These reactions can become fast at low temperatures, allowing ionized polycyclic aromatics to initiate ice formation in cold astrochemical environments.

Fragmentation, auto-modification and post ionisation proton bound dimer ion formation: the differential mobility spectrometry of low molecular weight alcohols

“…Proton bound dimers were also not reported within 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 15 The m/z 39 fragment (C3H3 + ) was observed with PTRMS studies at electric fields of 138 Td [27], and is thought to result from the sequential loss of H2 (8) and at this stage the tentative assignment for C3H3 + is a cyclic entity [34,35].…”

Hydration of the pyrimidine radical cation and stepwise solvation of protonated pyrimidine with water, methanol, and acetonitrile

“…The thermochemical and ion mobility experiments were performed using the VCU mass-selected ion mobility spectrometer. The details of the instrument can be found in several publications, [16][17][18][19][20] and only a brief description of the experimental procedure is given here (see Fig. S1, supplementary material).…”

“…[9][10][11][16][17][18][19][20] The hydration energies of these organic ions vary over an unexpected wide range from 8 to 18 kcal/mol depending on the nature of the hydrogen bonding interaction. [9][10][11][16][17][18][19][20] For example, benzene radical cation interacts with water molecules by relatively weak CH δ+ • • O hydrogen bonds resulting in stepwise hydration energies, H • n−1,n , where n is the number of water molecules, that are nearly constant at 8.5 ± 1 kcal/mol for n = 1-6. 9,10 Dissociative proton transfer is observed in the C 6 H 6 +.…”

“…. 17 The hydration energy can also identify the nature of the charge on the organic ion and distinguish between a conventional radical cation and a distonic ion. For example, the measured hydration energies of the pyridine and 2-F pyridine radical cations (15-16 kcal/mol) are consistent with distonic structures of these ions ( • C 5 H 4 NH + and • C 5 H 3 FNH + , respectively) where the protonated nitrogen sites form stronger NH + • • OH 2 hydrogen bonds similar to protonated pyridines.…”

Unconventional hydrogen bonding to organic ions in the gas phase: Stepwise association of hydrogen cyanide with the pyridine and pyrimidine radical cations and protonated pyridine

“…4 For example, carbon-based CH δ+ • • •O IHBs appear in the hydration of ionized aromatics such as benzene (C 6 H 6 + • ), cyclic C 3 H 3 + , and phenyl acetylene (C 8 H 6 + • ). [7][8][9][10] In addition to water, other polar molecules containing lone pair of electrons such as hydrogen cyanide can participate in hydrogen bonding interactions with the ring hydrogen atoms (CH δ+ ) of ionized aromatics. Hydrogen cyanide is a useful probe of non-covalent interactions because it is a highly polar molecule (μ = 2.98 D), and it can serve both as a hydrogen donor and as a lone-pair hydrogen acceptor in hydrogen bonds.…”