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

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+center dot(HCN)(n) and C4H4N2+center dot(HCN)(n) clusters, respectively, with n = 1-4. For comparison, the binding of 1-4 HCN molecules to the protonated pyridine C5H5NH+center dot(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 delta+center dot center dot center dot NCH). Protonated pyridine forms a stronger ionic hydrogen bond with HCN (NH+center dot center dot center dot NCH) which can be extended to a linear chain with the clustering of additional HCN molecules (NH+center dot center dot center dot NCH center dot center dot center dot NCH center dot center dot center dot 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 delta+center dot center dot center dot NCH) formed between the pyridine or the pyrimidine radical cations and HCN molecules (11-12 kcal/mol) are stronger than the similar (CH delta+center dot center dot center dot NCH) bonds formed between the benzene radical cation and HCN molecules (9 kcal/mol) indicating that the CH delta+ centers in the pyridine and pyrimidine radical cations have more effective charges than in the benzene radical cation. (C) 2014 AIP Publishing LLC.