Reactions between Aromatic Hydrocarbons and Heterocycles: Covalent and Proton-Bound Dimer Cations of Benzene/Pyridine

Abstract

Despite the fact that benzene (Bz) and pyridine (Py) are probably the most common and extensively studied organic molecules, the observation of a covalent adduct in the ionized benzene/pyridine system has never been reported. This Article reports the first experimental and theoretical evidence of a covalent (Bz x Py)(*+) adduct that results from the reaction of Bz(*+) with pyridine or Py(*+) with benzene. These reactions are studied using mass-selected ion mobility, chemical reactivity, collisional dissociation, and ab initio calculations. The (Bz x Py)(*+) adduct does not exchange ligands with Bz to form Bz(2)(*+) or with Py to form (Py)(2)H(+) despite the strong bonds in these homodimers. The thermochemistry then suggests that the (Bz x Py)(*+) heterodimer is bonded covalently with a bonding energy of>33 kcal/mol. Correspondingly, ab initio calculations identify covalently bonded propeller-shaped isomers of (Bz x Py)(*+) with bonding energies of 31-38 kcal/mol, containing a C-N bond. The mobility of the (Bz x Py)(*+) adduct in helium is consistent with these covalent dimers. As to noncovalent adducts, the computations identify novel distonic hydrogen-bonded complexes (C(5)H(5)NH(+) x C(6)H(5)(*)) where the charge resides on one component (PyH(+)), while the radical site resides on the other component (C(6)H(5)(*)). Collisional dissociation suggests that the covalent and distonic dimers may interconvert at high energies. The most stable distonic (C(5)H(5)NH(+) x C(6)H(5)(*)) complex contains a hydrogen bond to the phenyl radical carbon site with a calculated dissociation energy of 16.6 kcal/mol. This bond is somewhat stronger than the NH(+) x pi hydrogen bonds of PyH(+) to the pi system of the phenyl radical and of the benzene molecule. For this NH(+) x pi bond in the PyH(+) x Bz dimer, the measured binding energy is 13.4 kcal/mol, and ab initio calculations identify two T-shaped isomers with the NH(+) pointing to the center of the benzene ring or to the negatively charged C atoms of the ring. In contrast, the more stable proton-bound PyH(+) x Py dimer contains a linear NH(+)...N hydrogen bond. The formation of the (benzene/pyridine)(*+) adduct may represent a general class of addition reactions that can form complex heterocyclic species in ionizing environments.

Nucleophilic Aromatic Addition in Ionizing Environments: Observation and Analysis of New C–N Valence Bonds in Complexes between Naphthalene Radical Cation and Pyridine

“…However, we instead found that the most stable isomer of Bz +• (Pyr) includes a covalent C-N bond, and has a dissociation enthalpy that we determined experimentally as > 33 kcal/mol, and computationally as 34.9 kcal/mol. 13 Very recently, we found that Bz +• (Pyrim) corresponds to two families of covalent and noncovalent isomers characterized by two ion mobility peaks corresponding to small and large collision cross sections in helium (67.7±2.2 Å 2 and 76.0±1.8 Å 2 , respectively) consistent with two types of compact covalent and extended non-covalent structures, respectively. 21 DFT calculations at the M06-2X/6-311++G** level showed the most stable Bz +• (Pyrim) isomer contains a covalent C-N bond with a binding energy of 49.7 kcal/mol and a calculated collision cross section of 69.2 Å 2 , in excellent agreement with the value obtained from the higher mobility peak observed experimentally.…”

“…25 It is also significantly smaller than the binding of the Bz +• (Pyr) heterodimer (>33 kcal/mol). 13 The addition of the second pyridine molecule to the Naph +• (Pyr) heterodimer results in nearly 50% drop in the sequential binding enthalpy which appears to stay nearly constant at the 10- To gain insight into the structure and the nature of the interaction in the Naph +• (Pyr) heterodimer and to understand the interesting changes in bond strengths compared to these related systems, it is essential to characterize the potential energy surface. For this purpose, we have carried out DFT calculations as discussed below.…”

“…Again, no formation of new covalent bonds is observed, and the third pyridine molecule arranges on the same side of the naphthalene ring where the other two pyridine fragments are, in a pattern that maximizes the number of hydrogen bond interactions between the N terminal of the pyridine and different hydrogen atoms in the system. We identified more than ten minima with diverse structures and hydrogen bond patterns, but with energies that differ by less than 0.1 kcal/mol between each other, resulting in binding energies of roughly [11][12][13] kcal/mol for each structure. The potential energy surface for the addition of the fourth pyridine is obviously even flatter, and the yet larger size of the system makes it computationally challenging to explore.…”

“…9, 10 The formation of PAHs and PANs can also involve ion chemistry, as observed in ionic association and polymerization reactions. [11][12][13][14][15][16][17][18][19][20][21] In ionizing environments, PAH +• and PANH +• radical cations can interact with neutral PAH and PANH molecules. In fact, their dimers may contribute to interstellar IR bands.…”

Growth kinetics and formation mechanisms of complex organics by sequential reactions of acetylene with ionized aromatics

“…These experiments can identify the chains of reactions leading to the formation of PAHs and PANHs found in soot, combustion, organic aerosols, interstellar clouds, and meteorites [16][17][18][19][20][21][22][23][24][25][26][27]. They can also provide detailed information on the reactivity, kinetics, growth mechanisms, energy barriers, and structures of large organic ions as well as on their complexes with associated solvent molecules in different environments [28][29][30].…”