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

Abstract

Equilibrium thermochemical measurements using the ion mobility drift cell technique have been utilized to investigate the binding energies and entropy changes for the stepwise association of HCN molecules with the pyridine and pyrimidine radical cations forming the C5H5N(+·)(HCN)n and C4H4N2 (+·)(HCN)n clusters, respectively, with n = 1-4. For comparison, the binding of 1-4 HCN molecules to the protonated pyridine C5H5NH(+)(HCN)n has also been investigated. The binding energies of HCN to the pyridine and pyrimidine radical cations are nearly equal (11.4 and 12.0 kcal/mol, respectively) but weaker than the HCN binding to the protonated pyridine (14.0 kcal/mol). The pyridine and pyrimidine radical cations form unconventional carbon-based ionic hydrogen bonds with HCN (CH(δ+)⋯NCH). Protonated pyridine forms a stronger ionic hydrogen bond with HCN (NH(+)⋯NCH) which can be extended to a linear chain with the clustering of additional HCN molecules (NH(+)⋯NCH··NCH⋯NCH) leading to a rapid decrease in the bond strength as the length of the chain increases. The lowest energy structures of the pyridine and pyrimidine radical cation clusters containing 3-4 HCN molecules show a strong tendency for the internal solvation of the radical cation by the HCN molecules where bifurcated structures involving multiple hydrogen bonding sites with the ring hydrogen atoms are formed. The unconventional H-bonds (CH(δ+)⋯NCH) formed between the pyridine or the pyrimidine radical cations and HCN molecules (11-12 kcal/mol) are stronger than the similar (CH(δ+)⋯NCH) bonds formed between the benzene radical cation and HCN molecules (9 kcal/mol) indicating that the CH(δ+) centers in the pyridine and pyrimidine radical cations have more effective charges than in the benzene radical cation.

Nucleophilic Aromatic Addition in Ionizing Environments: Observation and Analysis of New C–N Valence Bonds in Complexes between Naphthalene Radical Cation and Pyridine

“…The H + (pyridine) 2 is not in equilibrium with the Naph +• or the Naph +• (Pyr) ions, and therefore it has no effect on the equilibrium between the Naph +• reactant and the Naph +• (Pyr) product. 52,53 As the temperature of the drift cell increases, the ion intensity of the Naph +• (Pyr) 2 cluster decreases and eventually disappears ( achieved when a constant ratio is obtained. 52,53 Equilibrium is also verified by observing identical ATDs of the C 10 H 8 +• (C 5 H 5 N) n and C 10 H 8 +• (C 5 H 5 N) n+1 ions since at equilibrium, the ATDs of the reactant and product ions must be identical.…”

“…52,53 As the temperature of the drift cell increases, the ion intensity of the Naph +• (Pyr) 2 cluster decreases and eventually disappears ( achieved when a constant ratio is obtained. 52,53 Equilibrium is also verified by observing identical ATDs of the C 10 H 8 +• (C 5 H 5 N) n and C 10 H 8 +• (C 5 H 5 N) n+1 ions since at equilibrium, the ATDs of the reactant and product ions must be identical. 52,53 The equilibrium constants for reactions (1) and 2 Table 1.…”

Rotational spectroscopy of imidazole: improved rest frequencies for astrophysical searches

“…Botta & Bada 2002;Martins 2018, and references therein), implying that these compounds can be produced under interstellar conditions. For example, gas-phase acetylene polymerisation in the presence of HCN is thought to generate nitrogen that contains rings up to large polycyclic aromatic structures (Frenklach & Feigelson 1989;Ricca et al 2001;Hamid et al 2014). Moreover, grain-mantle reactions of NH 3 in H 2 O ices doped with small quantities of polycyclic aromatic hydrocarbons produce traces of N-and O-containing heterocycles and nucleobases under UV irradiation (Nuevo et al 2014;Materese et al 2015;Bera et al 2017).…”

LOW TEMPERATURE FORMATION OF NITROGEN-SUBSTITUTED POLYCYCLIC AROMATIC HYDROCARBONS (PANHs)—BARRIERLESS ROUTES TO DIHYDRO(iso)QUINOLINES

“…An alternative reaction mechanism has recently been proposed in which nitrogen atoms are incorporated within a bicyclic aromatic system through a series of radical-mediated gas phase reactions (Parker et al 2015). Here, (iso)quinoline (C 9 H 7 N; Scheme 1) was shown to be formed through the hydrogen-abstraction acetylene-addition (HACA) pathway involving pyridyl radicals (C 5 H 4 N) and acetylene (C 2 H 2 ) at elevated temperatures of up to 1500 K. Nitrogen atom incorporation has been shown to occur in mono and bicyclic six-membered carbon rings via ion-molecule reactions (Hamid et al 2014a(Hamid et al , 2014b, however, not via bimolecular reactions under single collision conditions as is characteristic for cold molecular clouds. Current models of circumstellar envelopes of dying carbon stars propose a versatile and robust environment in which small unsaturated molecules and their radicals undergo a series of radical-mediated cyclization reactions leading to (N)PAHs (Ziurys 2006;Cherchneff 2011).…”