title: 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.
authors.affiliation: Department of Chemistry, Virginia Commonwealth University, Richmond, Virginia 23284-2006, USA. mselshal@vcu.edu
authors.forename: M Samy
authors.initials: MS
authors.lastname: El-Shall
authors.forename: Yehia M
authors.initials: YM
authors.lastname: Ibrahim
authors.forename: Edreese H
authors.initials: EH
authors.lastname: Alsharaeh
authors.forename: Michael
authors.initials: M
authors.lastname: Meot-Ner Mautner
authors.forename: Simon P
authors.initials: SP
authors.lastname: Watson
mesh_terms: D001554:Benzene / Q000737:chemistry*; D002412:Cations / Q000737:chemistry; D003198:Computer Simulation; D019281:Dimerization; D006571:Heterocyclic Compounds / Q000737:chemistry*; D006841:Hydrocarbons, Aromatic / Q000737:chemistry*; D008956:Models, Chemical; D008958:Models, Molecular; D011522:Protons; D011725:Pyridines / Q000737:chemistry*
chemical_list: D002412:Cations; D006571:Heterocyclic Compounds; D006841:Hydrocarbons, Aromatic; D011522:Protons; D011725:Pyridines; D001554:Benzene; C023666:pyridine