Sean P. Platt

What Is the Structure of the Naphthalene–Benzene Heterodimer Radical Cation? Binding Energy, Charge Delocalization, and Unexpected Charge-Transfer Interaction in Stacked Dimer and Trimer Radical Cations

The binding energy of the naphthalene(+•)(benzene) heterodimer cation has been determined to be 7.9 ± 1 kcal/mol for C10H8(+•)(C6H6) and 8.1 ± 1 kcal/mol for C10H8(+•)(C6D6) by equilibrium thermochemical measurements using the mass-selected drift cell technique. A second benzene molecule binds to the C10H8(+•)(C6D6) dimer with essentially the same energy (8.4 ± 1 kcal/mol), suggesting that the two benzene molecules are stacked on opposite sides of the naphthalene cation in the (C6D6)C10H8(+•)(C6D6) heterotrimer. The lowest-energy isomers of the C10H8(+•)(C6D6) and (C6D6)C10H8(+•)(C6D6) dimer and trimer calculated using the M11/cc-pVTZ method have parallel stacked structures with enthalpies of binding (-ΔH°) of 8.4 and 9.0 kcal/mol, respectively, in excellent agreement with the experimental values. The stacked face-to-face class of isomers is calculated to have substantial charge-transfer stabilization of about 45% of the total interaction energy despite the large difference between the ionization energies of benzene and naphthalene. Similarly, significant delocalization of the positive charge is found among all three fragments of the (C6D6)C10H8(+•)(C6D6) heterotrimer, thus leaving only 46% of the total charge on the central naphthalene moiety. This unexpectedly high charge-transfer component results in activating two benzene molecules in the naphthalene(+•)(benzene)2 heterotrimer cation to associate with a third benzene molecule at 219 K to form a benzene trimer cation and a neutral naphthalene molecule. The global minimum of the C10H8(+•)(C6H6)2 heterotrimer is found to be the one where the naphthalene cation is sandwiched between two benzene molecules. It is remarkable, and rather unusual, that the binding energy of the second benzene molecule is essentially the same as that of the first. This is attributed to the enhanced charge-transfer interaction in the stacked trimer radical cation.

Communication: Ion mobility of the radical cation dimers: (Naphthalene)2+• and naphthalene+•-benzene: Evidence for stacked sandwich and T-shape structures

Dimer radical cations of aromatic and polycyclic aromatic molecules are good model systems for a fundamental understanding of photoconductivity and ferromagnetism in organic materials which depend on the degree of charge delocalization. The structures of the dimer radical cations are difficult to determine theoretically since the potential energy surface is often very flat with multiple shallow minima representing two major classes of isomers adopting the stacked parallel or the T-shape structure. We present experimental results, based on mass-selected ion mobility measurements, on the gas phase structures of the naphthalene(+⋅) ⋅ naphthalene homodimer and the naphthalene(+⋅) ⋅ benzene heterodimer radical cations at different temperatures. Ion mobility studies reveal a persistence of the stacked parallel structure of the naphthalene(+⋅) ⋅ naphthalene homodimer in the temperature range 230-300 K. On the other hand, the results reveal that the naphthalene(+⋅) ⋅ benzene heterodimer is able to exhibit both the stacked parallel and T-shape structural isomers depending on the experimental conditions. Exploitation of the unique structural motifs among charged homo- and heteroaromatic-aromatic interactions may lead to new opportunities for molecular design and recognition involving charged aromatic systems.

Proton-bound dimers of nitrogen heterocyclic molecules: Substituent effects on the structures and binding energies of homodimers of diazine, triazine, and fluoropyridine

The bonding energies of proton-bound homodimers BH(+)B were measured by ion mobility equilibrium studies and calculated at the DFT B3LYP/6-311++G** level, for a series of nitrogen heterocyclic molecules (B) with electron-withdrawing in-ring N and on-ring F substituents. The binding energies (ΔH°(dissoc)) of the proton-bound dimers (BH(+)B) vary significantly, from 29.7 to 18.1 kcal/mol, decreasing linearly with decreasing the proton affinity of the monomer (B). This trend differs significantly from the constant binding energies of most homodimers of other organic nitrogen and oxygen bases. The experimentally measured ΔH°(dissoc) for (1,3-diazine)2H(+), i.e., (pyrimidine)2H(+) and (3-F-pyridine)2H(+) are 22.7 and 23.0 kcal/mol, respectively. The measured ΔH°(dissoc) for the pyrimidine(·+)(3-F-pyridine) radical cation dimer (19.2 kcal/mol) is signifcantly lower than that of the proton-bound homodimers of pyrimidine and 3-F-pyridine, reflecting the stronger interaction in the ionic H-bond of the protonated dimers. The calculated binding energies for (1,2-diazine)2H(+), (pyridine)2H(+), (2-F-pyridine)2H(+), (3-F-pyridine)2H(+), (2,6-di-F-pyridine)2H(+), (4-F-pyridine)2H(+), (1,3-diazine)2H(+), (1,4-diazine)2H(+), (1,3,5-triazine)2H(+), and (pentafluoropyridine)2H(+) are 29.7, 24.9, 24.8, 23.3, 23.2, 23.0, 22.4, 21.9, 19.3, and 18.1 kcal/mol, respectively. The electron-withdrawing substituents form internal dipoles whose electrostatic interactions contribute to both the decreased proton affinities of (B) and the decreased binding energies of the protonated dimers BH(+)B. The bonding energies also vary with rotation about the hydrogen bond, and they decrease in rotamers where the internal dipoles of the components are aligned efficiently for inter-ring repulsion. For compounds substituted at the 3 or 4 (meta or para) positions, the lowest energy rotamers are T-shaped with the planes of the two rings rotated by 90° about the hydrogen bond, while the planar rotamers are weakened by repulsion between the ortho hydrogen atoms of the two rings. Conversely, in ortho-substituted (1,2-diazine)2H(+) and (2-F-pyridine)2H(+), attractive interactions between the ortho (C-H) hydrogen atoms of one ring and the electronegative ortho atoms (N or F) of the other ring are stabilizing, and increase the protonated dimer binding energies by up to 4 kcal/mol. In all of the dimers, rotation about the hydrogen bond can involve a 2-4 kcal/mol barrier due to the relative energies of the rotamers.

Noncovalent Interactions of Organic Ions with Polar Molecules in the Gas Phase

The chapter is focused on noncovalent interactions of organic ions with small polar molecules in the gas phase. The organic ions studied include cyclic C3H3 + and the radical cations of benzene (C6H6 •+), pyridine (C5NH5 •+), pyrimidine (C5N2H4 •+), fluorobenzene (C6H5F•+), phenylacetylene (C8H6 •+), benzonitrile (C7NH5 •+) and naphthalene (C10H8 •+). In addition, protonated pyridine (pyridine.H+) and protonated pyrimidine (pyrimidine.H+) are also included for comparison with the radical cations. The solvent molecules include water (H2O), hydrogen cyanide (HCN) and acetonitrile (CH3CN). The results presented include experimental thermochemical data (ΔH° and ΔS°) for the stepwise association of the solvent molecules with the organic ions and theoretically calculated binding energies and structures. The four major topics discussed are: (1) Trends in binding energies and entropy changes, (2) Ionic hydrogen bonds with organic ions, (3) Internal vs. external solvation of the organic ions, and (4) Intracluster proton transfer and deprotonation of the organic ions.