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

Authors

Ahmed M. Hamid, Virginia Commonwealth University
M. Samy El-Shall, Virginia Commonwealth UniversityFollow
Rifaat Hilal, King Abdulaziz University
Shaaban Elroby, King Abdulaziz University
Saadullah G. Aziz, King Abdulaziz University

Document Type

Article

Original Publication Date

2014

Journal/Book/Conference Title

The Journal of Chemical Physics

Volume

141

Issue

5

DOI of Original Publication

10.1063/1.4890372

Comments

Originally published at http://dx.doi.org/10.1063/1.4890372

Date of Submission

October 2015

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)nand 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 ionichydrogen 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 bifurcatedstructures 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.

Rights

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.

Is Part Of

VCU Chemistry Publications