Hydrogen Bonding Interactions of Pyridine^•+ with Water:  Stepwise Solvation of Distonic Cations

Abstract

The solvation energies of the pyridine*+ radical cation by 1-4 H2O molecules were determined by equilibrium measurements in a drift cell. The binding energies of the pyridine*+(H2O)n clusters are similar to the binding energies of protonated pyridine-water clusters, (C5H5NH+)(H2O)n, which involve NH+..OH2 bonds and different from those of the solvated benzene radical cation-water clusters, C6H6*+(H2O)n, which involve CHdelta+..OH2 bonds. These relations indicate that the observed pyridine*+ ions have the distonic *C5H4NH+ structures that can form NH+..OH2 bonds. The observed thermochemistry and ab initio calculations show that these bonds are not affected significantly by an unpaired electron at another site of the ion. Similar observations also identify the 2-fluoropyridine*+ distonic ion. The distonic structure is also consistent with the reactivity of pyridine*+ in H atom transfer, intra-cluster proton transfer and deprotonation reactions. The results present the first measured stepwise solvation energies of distonic ions, and demonstrate that cluster thermochemistry can identify distonic structures.

Stabilities of nanohydrated thymine radical cations: insights from multiphoton ionization experiments and ab initio calculations

“…For n = 1, we obtain a value of 0.77 eV, slightly higher than that measured by Ibrahim et al [20] for the pyridine radical cation (0.59 eV) and consistent with water being bound to an NH group in both cases and to the adjacent C=O group in thymine (see Fig. 3).…”

“…We chose the uMP2/6-31+g(d, p) level of theory as this has been demonstrated to provide enthalpy changes for water evaporation from nanohydrated pyridine in very good agreement with experimental values [20]. All calculations have been performed with the Gaussian 09 package [21].…”

Hydration of the pyrimidine radical cation and stepwise solvation of protonated pyrimidine with water, methanol, and acetonitrile

“…1 For example, organic ions can interact with water molecules by either relatively weak carbon-based CH δ+ -O hydrogen bonds, such as in benzene radical cation, or via stronger hydrogen bonds, such as in protonated pyridine where the NH + -O hydrogen bonds are formed. [9][10][11] Insight into the basic molecular interactions can be obtained from the energies and structures of the key species a) Author to whom correspondence should be addressed. Electronic mail: mselshal@vcu.edu involved in the stepwise association of polar molecules with organic ions.…”

Stepwise Association of Hydrogen Cyanide and Acetonitrile with the Benzene Radical Cation: Structures and Binding Energies of (C6H6•+)(HCN)n, n = 1–6, and (C6H6•+)(CH3CN)n, n = 1–4, Clusters

“…For example, the measured hydration energies of the pyridine and 2-F pyridine radical cations (15−16 kcal/mol) are consistent with distonic structures of these ions ( • C 5 H 4 NH + and • C 5 H 3 FNH + , respectively) where the protonated nitrogen sites form stronger NH + ···OH 2 hydrogen bonds similar to protonated pyridines. 16 The stepwise hydration of different organic ions can thus provide prototypical models for understanding structural and energy changes associated with hydration, which could lead to a molecular level understanding of complicated phenomena in the condensed phase such as hydrophobic hydration and clathrate formation. 2,4 In addition to water, other polar molecules containing a lone pair of electrons such as hydrogen cyanide (HCN), acetonitrile (CH 3 CN), and ammonia (NH 3 ) can participate in hydrogen bonding interactions with the ring hydrogen atoms (CH δ+ ) of ionized aromatics.…”