Benzene Dimer Cation Research Articles

Water-accelerated \u03c0-Stacking Reaction in Benzene Cluster Cation

Single molecule electron devices (SMEDs) have been widely studied through both experiments and theoretical calculations because they exhibit certain specific properties that general macromolecules do not possess. In actual SMED systems, a residual water molecule strongly affects the electronic properties of the SMED, even if only one water molecule is present. However, information about the effect of H2O molecules on the electronic properties of SMEDs is quite limited. In the present study, the effect of H2O on the ON-OFF switching property of benzene-based molecular devices was investigated by means of a direct ab initio molecular dynamics (AIMD) method. T- and H-shaped benzene dimers and trimers were examined as molecular devices. The present calculations showed that a H2O molecule accelerates the π-stacking formation in benzene molecular electronic systems. The times of stacking formation in a benzene dimer cation (n = 2) were calculated to be 460 fs (H2O) and 947 fs (no-H2O), while those in a trimer cation (n = 3) were 551 fs (H2O) and 1019 fs (no-H2O) as an average of the reaction time. This tendency was not dependent on the levels of theory used. Thus, H2O produced positive effects in benzene-based molecular electronics. The mechanism of π-stacking was discussed based on the theoretical results.

Timescale of π‐Stacking Formation in a Benzene Trimer Cation Formed by Ionization of the Parent Neutral Trimer: A Direct Ab Initio Molecular Dynamics Study

AbstractThe π‐stacking interaction plays a crucial role in structural arrangements and molecular assemblies in several molecular systems. In the present study, π‐stacking formation in the benzene trimer cation, (Bz)3+, following ionization of the parent neutral benzene trimer, (Bz)3, was investigated using direct ab initio molecular dynamics to elucidate the timescale of π‐stacking formation and reaction mechanism in the ionized state. Cyclic (Bz)3 was chosen as the benzene cluster. After ionizing (Bz)3, the structure drastically changed to a double π‐stacking form. The (Bz)3+ product vibrated between two structural forms, (Bz‐Bzc)+—Bz ↔ Bz—(Bzc‐Bz)+, where Bzc is a benzene molecule located in the central position of (Bz)3+ and (Bzc‐Bz)+ represents a dimer core composed of the benzene dimer cation. The timescale of π‐stacking formation was calculated as 300–380 fs. The absorption spectrum of (Bz)3+ was blue‐shifted as a function of time. We discuss the reaction mechanism based on the theoretical results.

Density-fitted open-shell symmetry-adapted perturbation theory and application to π-stacking in benzene dimer cation and ionized DNA base pair steps.

Symmetry-Adapted Perturbation Theory (SAPT) is one of the most popular approaches to energy component analysis of non-covalent interactions between closed-shell systems, yielding both accurate interaction energies and meaningful interaction energy components. In recent years, the full open-shell equations for SAPT up to second-order in the intermolecular interaction and zeroth-order in the intramolecular correlation (SAPT0) were published [P. S. Zuchowski et al., J. Chem. Phys. 129, 084101 (2008); M. Hapka et al., ibid. 137, 164104 (2012)]. Here, we utilize density-fitted electron repulsion integrals to produce an efficient computational implementation. This approach is used to examine the effect of ionization on π-π interactions. For the benzene dimer radical cation, comparison against reference values indicates a good performance for open-shell SAPT0, except in cases with substantial charge transfer. For π stacking between hydrogen-bonded pairs of nucleobases, dispersion interactions still dominate binding, in spite of the creation of a positive charge.

Electromers of the benzene dimer radical cation.

The well-studied benzene dimer radical cation, which is prototypical for this class of species, has been reinvestigated computationally. Thereby it turned out that both the σ-hemibonded and the half-shifted sandwich structures of the benzene dimer cation, which had been independently proposed, represent stationary points on the B2PLYP-D potential energy surfaces. However, these structures belong to distinct electronic states, both of which are associated with potential surfaces that are very flat with regard to rotation of the two benzene rings in an opposite sense relative to each other. The surfaces of these two "electromers" of the benzene dimer cation are separated by only 3-4 kcal mol(-1) and do not intersect along the rotation coordinate, which represents a rather unique electronic structure situation. When moving on either of the two surfaces the title complex is an extremely fluxional species, in spite of its being bound by over 20 kcal mol(-1).

Direct ab initio molecular dynamics (MD) study of the ionization on a benzene dimer

The ionization dynamics of a benzene dimer have been investigated by means of a direct ab initio molecular dynamics (MD) method in order to elucidate the reaction mechanism. Following the ionization, the T-shaped neutral benzene dimer was gradually changed to a π-stacked benzene dimer cation. The structural change and time evolution of the electronic absorption spectrum were completely visualized for the first time. The time scale of the dimer formation was estimated to be 1.0–1.5 ps. First, the benzene molecule at the stem position (Bz′) was ionized, and the structure of Bz′+ was rapidly deformed due to the Jahn–Teller effects. Next, the rotation of (Bz′)+ gradually occurred relative to Bz. Finally, a π-stacked benzene dimer cation was formed. TD-DFT calculations indicated that the absorption spectrum of (Bz)2+ is blue-shifted as a function of time. The formation mechanism of the benzene dimer cation was discussed on the basis of the theoretical results.

Modeling Charge Resonance in Cationic Molecular Clusters: Combining DFT-Tight Binding with Configuration Interaction.

In order to investigate charge resonance situations in molecular complexes, Wu et al. (J. Chem. Phys. 2007, 127, 164119) recently proposed a configuration interaction method with a valence bond-like multiconfigurational basis obtained from constrained DFT calculations. We adapt this method to the Self-Consistent Charge Density-Functional-based Tight Binding (SCC-DFTB) approach and provide expressions for the gradients of the energy with respect to the nuclear coordinates. It is shown that the method corrects the wrong SCC-DFTB behavior of the potential energy surface in the dissociation regions. This scheme is applied to determine the structural and stability properties of positively charged molecular dimers with full structural optimization, namely, the benzene dimer cation and the water dimer cation. The method yields binding energies in good agreement with experimental data and high-level reference calculations.

Charge localization and Jahn–Teller distortions in the benzene dimer cation

Jahn-Teller (JT) distortions and charge localization in the benzene dimer cation are analyzed using the equation-of-motion coupled cluster with single and double substitutions for ionization potential (EOM-IP-CCSD) method. Ionization of the dimer changes the bonding from noncovalent to covalent and induces significant geometrical distortions, e.g., shorter interfragment distance and JT displacements. Relaxation along interfragment coordinates lowers the energy of the t-shaped and displaced sandwich isomers by 0.07 and 0.23 eV, respectively, whereas JT displacements result in additional 0.18 and 0.23 eV. Energetically, the effect of JT distortion on the dimer is similar to the monomer where JT relaxation lowers the energy by 0.18 eV. While the change in the interfragment distance has dramatic spectroscopic consequences, the JT distortion causes only a small perturbation in the electronic spectra. The two geometrical relaxations in the t-shaped isomer lead to opposing effects on hole localization. Intermolecular relaxation leads to an increased delocalization, whereas JT ring distortion localizes the charge. In the sandwich isomers, breaking the symmetry by ring rotation does not induce considerable charge localization. The optimization and property calculations were performed using a new implementation of EOM-IP-CCSD energies and gradients in the Q-CHEM electronic structure package.

Dicationic states of benzene dimer: Benzene dimer cation and benzene dication parenthood patterns

AbstractHigh density functional computational levels, B3LYP/cc‐pVDZ and B3LYP/cc‐pVTZ, are invoked to study the structural patterns exhibiting between a variety of the lower‐energy conformers of the benzene dimer cation and benzene dication, on one hand, and the obtuse and acute ground‐state conformers of the benzene cation radical, on the other. In particular, these “obtuse versus acute” patterns for the lowest‐energy stable sandwich structures of the benzene dimer cation, formed due to a strong charge‐transfer resonance interaction between the benzene cation radical as a hole donor and the ground‐state benzene molecule as a hole acceptor benzene dimer cation, are examined using the molecular orbital analysis and a two‐state model. It is shown the existence of homodimers of the benzene dimer cation composed of equivalent benzene rings, mimicking either the obtuse or acute conformers of the benzene radical cation, and representing saddle points on the potential energy surface. The computed potential energy surface of ionization of the benzene radical cation demonstrates a slight preference of the triplet state of the benzene dication originated from the obtuse benzene cation over the singlet which, in contrast, arises from the acute one. The parenthood features of the benzene dimer cation and benzene dication that appear in the dicationic states of benzene dimer are discussed. © 2007 Wiley Periodicals, Inc. Int J Quantum Chem, 2007

Mass-Selected Ion Mobility Studies of the Isomerization of the Benzene Radical Cation and Binding Energy of the Benzene Dimer Cation. Separation of Isomeric Ions by Dimer Formation

A mass-selected, ion mobility drift-cell technique has been used to study the isomerization of the benzene radical cation generated by electron impact (EI) ionization. Evidence is presented for the generation of both the benzene and fulvene cations, with a lower limit for the barrier of isomerization Ea(benzene+• → fulvene+•) estimated as Ea > 1.6 eV. The reduced mobilities of the benzene and fulvene cations are nearly similar, making it difficult to separate the two ions in pure helium. However, because of the large difference in the bonding of the benzene and fulvene cations with neutral benzene, the two isomeric ions are readily separated in the presence of neutral benzene in the drift cell. This suggests that isomer ions with similar mobilities can be separated on the basis of the degree of their associations with their neutral precursor in the drift cell. This separation concept allows us to discover the “hidden isomerization” process responsible for the generation of the fulvene cations via EI ioniz...

Structure and bonding of the (C6H6)+ 2 radical

To elucidate the relative stability of various structures of the benzene dimer cation radical, (C6H6)+ 2 in its ground and low-lying excited states, ab initio complete active space self-consistent field (CASSCF), multi-reference singly and doubly excited configuration interaction (MRSDCI), and multi-reference coupled pair approximation (MRCPA) calculations were performed. Full optimization was performed at the CASSCF level for various structures of the dimer cation, followed by MRSDCI and MRCPA calculations. It was found that the global minimum of the cation is at a slipped C2h sandwich structure but there are some other sandwich structures with almost the same stability, being within about kcal mol−1. T-shape structures are less stable than the sandwich structures, by more than 5 kcal mol−1 by MRCPA calculations. Low lying electronic excited states in various structures are also discussed.

New challenges in quantum chemistry: Quests for accurate calculations for large molecular systems.

As quantum chemistry plays a more and more central role in many complicated chemical problems, it has become necessary to obtain accurate results for large molecular systems. Conventional quantum chemistry methods are either too expensive to apply to large systems or too approximate for the results to be reliable, and they fail to satisfy this requirement. A variety of different approaches is being developed with the aim of achieving this goal: local correlation methods; divide-and-conquer methods; linear-scaling density functional methods based on the fast multipole and other approximations; effective potential methods; and hybrid methods. ONIOM (our N-layered integrated molecular orbital plus molecular mechanics method), developed by the authors, is a hybrid method in which a large molecular system is divided into onion-skin-like layers, and different quantum chemistry/molecular mechanics methods are used for different parts of the system; the results are combined to extrapolatively estimate the results of high-level calculation for the real system. Several applications of ONIOM will be discussed.

Comparative study of the binary clusters of acetic acid–water and acetic acid–benzene using electron impact and multiphoton ionization techniques

Electron impact and multiphoton ionization techniques are used for a comparative study of the acetic acid–water and acetic acid–benzene clusters generated by supersonic beam expansion. In acetic acid–water clusters, hydrogen-bonding interaction is the driving force in determining the structures of the clusters. The protonated and the methyl cation containing clusters are characterized by 6-membered cyclic and 8-membered bicyclic structures. The similar magic number patterns observed for the protonated and methyl cation containing clusters suggest that the cyclic structures are stabilized by the charge interaction. A remarkable periodicity in the ion intensity of benzene (acetic acid) n clusters is observed. The clusters containing an even number of acetic acid molecules exhibit enhanced ion intensities. This effect is attributed to the formation of multiple cyclic dimers as a result of clustering from a dimer-rich vapor phase. Protonated acetic acid clusters are generated following the three-photon absorption by the binary clusters, which leads to dissociative charge transfer followed by proton transfer within the ionized acetic acid clusters. Evidence is presented for the special stability of the B 2A 8 ion, which is proposed to consist of a benzene dimer cation entrapped between two acetic acid tetramers.

Reversible Reactions of Cycloalkane Solvent Holes. 2. Scavenging of cis- and trans-Decalin•+ by Benzene

Solvent holes in liquid cis- and trans-decalin form metastable complexes with benzene, which are probably solvation complexes of monomer radical cations of benzene. At 25 °C, the rate constants of formation of the complexes are, for trans- and cis-decalin respectively 1.1 × 1011 and 3.6 × 1010 M-1 s-1, and the natural lifetime of the complex is 7 ns for trans- and 50 ns for cis-decalin. Thermodynamic potentials for the complexation are determined. The heat of the complex formation is ≈−26 kJ/mol for trans- and ≈−18 kJ/mol for cis-decalin. In cis-decalin, the complexes are more stable because of near-zero entropy of the complex formation. The complex decays by proton or H atom transfer with activation energy ≈10 kJ/mol (which yields a benzonium cation) or in a reactive encounter with a second benzene molecule (which yields a benzene dimer cation). Laser excitation of the dimer cation with a 1.165- or 2.33 eV photon causes valence band electron transfer and injection of a free solvent hole.

Formation of benzene dimer cations in neat liquid benzene studied by femtosecond transient absorption spectroscopy

Photoionization of neat liquid benzene has been studied by femtosecond pump-probe experiments in the near infrared region. A transient species is formed within 1 ps following two-photon excitation of liquid benzene at 268 nm, which shows a broad absorption band with a peak at 920 nm and extended intensities at wavelengths longer than 1000 nm. This band is attributed to a charge resonance band of a localized benzene dimer cation. The transient species show essentially no evidence of the recombination with electrons in a time shorter than 50 ps.

Cation radical intermediates in benzene and toluene polycondensation on sulfated zirconia

Benzene dimer cation radicals were detected on sulfated zirconia at low temperature and benzene coverage. Two temperature regions for their transformations were observed. Below 473 K the radical concentration remained stable reflecting the amount of strong surface acceptor sites. Above 623 K it increased drastically.

The Role of Solute/Solvent Electronic Interaction in the Femtosecond Dynamics of the Bromide/Benzene Cation Contact Ion Pair in Benzene Solution

The bromide/benzene cation contact ion pair was prepared in benzene solution by ultrafast excitation of the bromine atom/benzene molecule charge transfer complex. Bromine atoms were prepared in situ by photolysis of [alpha]-bromoacetophenone. A simple first-order charge recombination reaction was observed to be the only significant decay process for the ion pair. Despite the apparent simplicity of the charge recombination kinetics, spectroscopic data show that the reaction involves a rapidly formed benzene dimer cation. Various issues of the dynamics involving the dimer cation and the ion pair have been investigated. 17 refs., 3 figs., 1 tab.

Ultrafast experiments on intermolecular electron transfer in the benzene - bromine atom charge transfer complex

Ultrafast pump - probe measurements have been made on the benzene - bromine atom charge transfer (CT) complex in CCl 4 and cyclohexane solutions. Ultrafast optical excitation of the CT band of the complex yields an ion pair, which is comprised of a benzene cation and a bromide anion. The rate of charge recombination between the bromide and the benzene cation in the ion pair has been observed to be much faster than the rate of diffusion apart. The charge recombination rate is accelerated at high benzene concentrations as a result of the formation of a benzene dimer cation - bromide ion pair which undergoes much faster charge recombination than the benzene cation - bromide ion pair.

Stability and structure of benzene dimer cation (C6H6)+2 in the gas phase

The gas-phase equilibria of the clustering reaction (C6H6)+n−1+C6H6 =(C6H6)+n were studied with a pulsed electron-beam high-pressure mass spectrometer. The enthalpy changes −ΔH0n−1,n for the reaction with n=2–4 were determined to be 20.6±1.0, 7.8±0.5, and ∼7 kcal/mol, respectively. The structure of dimer cation (C6H6)+2 was determined by ab initio spin-restricted open-shell Hartree–Fock molecular orbital (ROHF MO) calculations. Two isomers, ‘‘axial C2 ’’ and ‘‘parallel C2v, ’’ are calculated to be located at the potential-energy minima. The axial C2 is 3.0 kcal/mol more stable than the parallel C2v by ROHF/6-31G**//ROHF/3-21G.

ChemInform Abstract: Photodissociation of the Benzene Dimer Cation in the Gas Phase.

ChemInformVolume 21, Issue 33 Preparative Organic Chemistry ChemInform Abstract: Photodissociation of the Benzene Dimer Cation in the Gas Phase. J. T. SNODGRASS, J. T. SNODGRASS Dep. Chem., Univ. Calif., Santa Barbara, CA 93106, USASearch for more papers by this authorR. C. DUNBAR, R. C. DUNBAR Dep. Chem., Univ. Calif., Santa Barbara, CA 93106, USASearch for more papers by this authorM. T. BOWERS, M. T. BOWERS Dep. Chem., Univ. Calif., Santa Barbara, CA 93106, USASearch for more papers by this author J. T. SNODGRASS, J. T. SNODGRASS Dep. Chem., Univ. Calif., Santa Barbara, CA 93106, USASearch for more papers by this authorR. C. DUNBAR, R. C. DUNBAR Dep. Chem., Univ. Calif., Santa Barbara, CA 93106, USASearch for more papers by this authorM. T. BOWERS, M. T. BOWERS Dep. Chem., Univ. Calif., Santa Barbara, CA 93106, USASearch for more papers by this author First published: August 14, 1990 https://doi.org/10.1002/chin.199033093AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat No abstract is available for this article. Volume21, Issue33August 14, 1990 RelatedInformation

Photodissociation of the benzene dimer cation in the gas phase

The photodissociation of the benzene dimer cation, (C 6 H 6 ) 2 + , has been investigated by crossing a mass-selected ion beam with a linearly polarized laser beam and measuring the kinetic energy distribution of the charged photodissociation products with an electrostatic energy analyzer. The C 6 H 6 + +C 6 H 6 product channel was the only channel observed