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Stepwise Association of Hydrogen Cyanide with the Pyridine and Pyrimidine Radical Cations and Protonated Pyridine Ahmed M

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Stepwise Association of Hydrogen Cyanide with the Pyridine and Pyrimidine Radical Cations and Protonated Pyridine Ahmed M Virginia Commonwealth University VCU Scholars Compass Chemistry Publications Dept. of Chemistry 2014 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 Ahmed M. Hamid Virginia Commonwealth University M. Samy El-Shall Virginia Commonwealth University, [email protected] Rifaat Hilal King Abdulaziz University Shaaban Elroby King Abdulaziz University FSaoallodulwl ahthi Gs a. ndAz iazdditional works at: http://scholarscompass.vcu.edu/chem_pubs King Abdulaziz University Part of the Chemistry Commons Hamid, A. M., El-Shall, M. S., & Hilal, R., et al. 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. The Journal of Chemical Physics, 141, 054305 (2014). Copyright © 2014 AIP Publishing LLC. Downloaded from http://scholarscompass.vcu.edu/chem_pubs/66 This Article is brought to you for free and open access by the Dept. of Chemistry at VCU Scholars Compass. It has been accepted for inclusion in Chemistry Publications by an authorized administrator of VCU Scholars Compass. For more information, please contact [email protected]. THE JOURNAL OF CHEMICAL PHYSICS 141, 054305 (2014) 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 Ahmed M. Hamid,1 M. Samy El-Shall,1,a) Rifaat Hilal,2 Shaaban Elroby,2 and Saadullah G. Aziz2 1Department of Chemistry, Virginia Commonwealth University, Richmond, Virginia 23284, USA 2Department of Chemistry, Faculty of Science, King Abdulaziz University, Jeddah 21589, Saudi Arabia (Received 11 June 2014; accepted 4 July 2014; published online 5 August 2014) 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 bind- ing 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 (CHδ+···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 (CHδ+···NCH) formed between the pyridine or the pyrimidine radical cations and HCN molecules (11–12 kcal/mol) are stronger than the similar (CHδ+···NCH) bonds formed between the benzene radical cation and HCN molecules (9 kcal/mol) indicating that the CHδ+ centers in the pyridine and pyrimidine radical cations have more effective charges than in the benzene radical cation. © 2014 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4890372] I. INTRODUCTION π systems.4 For example, carbon-based CHδ+···O IHBs ap- pear in the hydration of ionized aromatics such as benzene Hydrogen bonding is one of the most important inter- (C H + · ), cyclic C H +, and phenyl acetylene (C H + · ).7–10 molecular interactions in chemistry and biology both in gas 6 6 3 3 8 6 In addition to water, other polar molecules containing lone phase and condensed phase systems.1–3 A special class of pair of electrons such as hydrogen cyanide can participate in this interaction, usually referred to as ionic hydrogen bonds hydrogen bonding interactions with the ring hydrogen atoms (IHBs), involves hydrogen bonding between radical ions or (CHδ+) of ionized aromatics. Hydrogen cyanide is a useful protonated molecules and neutral molecules.4 IHBs have probe of non-covalent interactions because it is a highly polar bond strengths higher than the typical conventional hydro- molecule (μ = 2.98 D), and it can serve both as a hydro- gen bond in neutral systems and they could reach up to 35 gen donor and as a lone-pair hydrogen acceptor in hydrogen kcal/mol, nearly a third of the strength of covalent bonds. bonds. Furthermore, HCN is an important atmospheric com- These strong interactions are critical in many fields such as pound known to be produced by biomass burning, and it can ion induced nucleation, ionic clusters, ion solvation, radiation be produced in interstellar/nebula environments by the reac- chemistry, electrochemistry, acid-base chemistry, and self- tions of ammonia and methane.11 In fact, HCN polymers have assembly in supramolecular chemistry.1–6 IHBsarealsoim- been shown to exist in meteorites, comets, planets, moons, portant in biological systems including protein folding, proton and in circumstellar envelops.12–14 Ion-molecule interactions transport, membranes, enzyme active centers, and molecular involving HCN, particularly those that lead to the formation recognition.1–6 of larger species either through chemical addition or asso- Unconventional carbon-based IHBs are formed when the ciation reactions, are of particular interest for the formation hydrogen donors are ionized hydrocarbons containing CH of complex organics, clustering, and polymerization in astro- groups and the hydrogen acceptors are electron lone pairs on chemical environments.15, 16 hetero atoms such as O or N, olefin double bonds, or aromatic We recently studied the stepwise association of HCN a) with benzene, substituted benzene, and phenylacetylene Author to whom correspondence should be addressed. Electronic mail: 17–19 ·+ [email protected]. radical cations. In benzene (HCN)n clusters the lig- 0021-9606/2014/141(5)/054305/11/$30.00 141, 054305-1 © 2014 AIP Publishing LLC This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 128.172.48.59 On: Mon, 12 Oct 2015 16:14:30 054305-2 Hamid et al. J. Chem. Phys. 141, 054305 (2014) and molecules are bonded to the benzene hydrogens by ature of the drift cell can be controlled to better than ±1K –CHδ+···N hydrogen bonds, but linear hydrogen bonded using four temperature controllers. Liquid nitrogen flowing HCN···HCN···HCN chains are also formed with further through solenoid valves is used to cool down the drift cell. 17, 18 ·+ HCN molecules. In the phenylacetylene (HCN)n The reaction products can be identified by scanning a second clusters, the dominant interaction was hydrogen bonding quadrupole mass filter located coaxially after the drift cell. between the C–H acetylenic hydrogen and the nitrogen The arrival time distributions (ATDs) are collected by mon- atom of the HCN ligand.19 Here also subsequent ligand itoring the intensity of each ion as a function of time. The molecules are added to form linear hydrogen bonded reaction time can be varied by varying the drift voltage. The phenylacetylene ·+(NCH···NCH···) chains.19 Such chains injection energies used in the experiments (10–14 eV, labora- + were suggested previously in protonated (HCN)nH clus- tory frame) are slightly above the minimum energies required ters (HCN···(HCN···H+···NCH)···NCH) where binding to introduce the ions into the cell against the HCN/He outflow enthalpies indicated completion of solvent shells by two from the entrance orifice. Most of the ion thermalization oc- (first shell) or four (second shell) HCN molecules about the curs outside the cell entrance by collisions with the HCN/He proton.4, 15, 16 gases escaping from the cell entrance orifice. At a cell pres- Ionized aromatics containing N heteroatoms such as the sure of 0.2 Torr, the number of collisions that the ion encoun- pyridine and the pyrimidine radical cations can also partic- ters with the neutral molecules within the 1.5 ms residence ipate in hydrogen bonding interactions.20, 21 The hydration time inside the cell is about 104 collisions, which is sufficient of these ions has been investigated both experimentally and to ensure efficient thermalization of the molecular ions. theoretically.20, 21 However, no data exist on the interactions HCN is prepared by adding 8 g of sodium cyanide of these ions with HCN in spite of their importance in biol- (NaCN) (Sigma-Aldrich, 97%) into a 500 ml stainless steel ogy and origin of life. Here, we present the first study of the bubbler which is then placed in liquid nitrogen and evacuated + · stepwise association of HCN with the pyridine (C5NH5 ) followed by the addition of 4 ml of pure sulfuric acid (H2SO4) + · and pyrimidine (C4N2H4 ) radical cations and protonated (Aldrich, 99.999%) through a stainless steel tube extension of + pyridine (C5NH5)H . We apply equilibrium thermochemical the inlet valve of the bubbler. Following the reaction of sulfu- measurements to study the association of 1-4 HCN molecules ric acid with the sodium cyanide salt inside the bubbler, HCN with the pyridine and pyrimidine radical cations and compare gas evolves and the bubbler is allowed to warm up to room their thermochemistry with the HCN association with the pro- temperature. The pressure in the HCN line is monitored by a tonated pyridine. We also provide Density Functional Theory Baratron pressure gauge (MKS-626A13TBD). + · (DFT) calculations of the structures of the C5NH5 (HCN)n, The equilibrium reactions (taken pyridine radical cation + · + + · C4N2H4 (HCN)n, and C5NH5H (HCN)n clusters with C5H5N as an example) are represented by Eq.
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