title: 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.
authors.affiliation: Department of Chemistry, Virginia Commonwealth University, Richmond, Virginia 23284, USA.
authors.forename: Ahmed M
authors.initials: AM
authors.lastname: Hamid
authors.affiliation: Department of Chemistry, Virginia Commonwealth University, Richmond, Virginia 23284, USA.
authors.forename: M Samy
authors.initials: MS
authors.lastname: El-Shall
authors.affiliation: Department of Chemistry, Faculty of Science, King Abdulaziz University, Jeddah 21589, Saudi Arabia.
authors.forename: Rifaat
authors.initials: R
authors.lastname: Hilal
authors.affiliation: Department of Chemistry, Faculty of Science, King Abdulaziz University, Jeddah 21589, Saudi Arabia.
authors.forename: Shaaban
authors.initials: S
authors.lastname: Elroby
authors.affiliation: Department of Chemistry, Faculty of Science, King Abdulaziz University, Jeddah 21589, Saudi Arabia.
authors.forename: Saadullah G
authors.initials: SG
authors.lastname: Aziz
mesh_terms: D001665:Binding Sites; D002412:Cations; D003198:Computer Simulation; D005609:Free Radicals / Q000737:chemistry; D005740:Gases / Q000737:chemistry*; D006860:Hydrogen Bonding; D006856:Hydrogen Cyanide / Q000737:chemistry*; D008956:Models, Chemical; D008958:Models, Molecular; D009930:Organic Chemicals / Q000737:chemistry*; D044367:Phase Transition; D011522:Protons; D011725:Pyridines / Q000737:chemistry*; D011743:Pyrimidines / Q000737:chemistry*; D021241:Spectrometry, Mass, Electrospray Ionization / Q000379:methods*
chemical_list: D002412:Cations; D005609:Free Radicals; D005740:Gases; D009930:Organic Chemicals; D011522:Protons; D011725:Pyridines; D011743:Pyrimidines; D006856:Hydrogen Cyanide; C030986:pyrimidine; C023666:pyridine